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Home My WebLink AboutWPO201600009 Plan - Stormwater Stormwater Management Plan 2016-04-25 Stormwater Master Plan Update APPROVED FINAL PLAN APRIL 25, 2016 Old Trail Village (4.4 c Prepared for: Mr. Dave Brockman March Mountain Properties, LLC Prepared by: Glenn Muckley, PE Andrew Tiefenback, EIT 5209 Center Street Williamsburg, Virginia 23188 April 22,2016 Table 7 note revision April 4,2016 Original: April 14,2015, Revised February 5, 2016 STORMWATER MASTER PLAN UPDATE Table of Contents ABBREVIATIONS 1.1 1.0 INTRODUCTION 1.2 1 .1 PROJECT DESCRIPTION 1 .2 1.1.1 Project Location 1.2 1.1.2 Project History, Guidelines, and Approach 1.5 1.1.3 Applicable Technical Design Criteria 1 .9 1.1.4 Relevant Guidance by Albemarle County 1 .9 1.1.5 Understood Compliance and Tracking Resolution to Original Guidance 1.12 1.2 PURPOSE OF STUDY 1.12 2.0 CONSTRAINTS AND CONSIDERATIONS 2.14 2.1 PROJECT SOILS 2.14 2.2 JURISDICTIONAL FEATURES 2.16 2.3 STREAM BUFFERS 2.16 2.4 100-YEAR FEMA FLOODPLAIN 2.16 3.0 SUITE OF BMPS FOR PROJECT IMPLEMENTATION 3.17 3.1 LICKINGHOLE REGIONAL BASIN 3.17 3.2 BIORETENTION FACILITIES 3.17 3.3 RETENTION BASINS 3.17 3.4 GRASS CHANNELS 3.18 4.0 PROPOSED SWM STRATEGY 4.18 4.1 WATER QUANTITY-VSMP WATERSHED SUMMARY 4.18 4.1.1 Drainage Area Development 4.18 4.1.2 Proposed SWM Facilities 4.23 4.1.3 Quantity Compliance Summary 4.25 4.2 VSMP WATER QUALITY COMPLIANCE 4.25 4.2.1 Overall Project Removal Requirements 4.25 4.2.2 Proposed Water Quality Treatment Facilities 4.26 4.3 REVIEW OF PROPOSED TREATMENT PER COUNTY GUIDANCE 4.48 4.3.1 Albemarle County Capture Definition 4.48 4.3.2 Response to Original Guidance & Understood Resolution 4.49 4.3.3 Proposed Submittal, Review, and Tracking Process 4.51 5.0 STREAM BUFFERS ENCROACHMENT SUMMARY 5.52 6.0 PROJECT SUMMARY AND REQUESTS 6.53 LIST OF TABLES Table 1 Land Use Breakdown 1 .7 Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx STORMWATER MASTER PLAN UPDATE Acceptance Signature Blocks Print Name /J c C/ `9-0.---\ Date:S L S ��. Signature "W /j/I re) March Mountain Properties, LLC, Authorized Official � Print Name ��(-1-�. T�,,fij -" ;72 , Date: q 2-5-11 16 Signature �,cw��/ c /// (signature) Albemarle County Community Development, Responsible Official Stantec mg u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx V STORMWATER MASTER PLAN UPDATE Table 2 Soil Infiltration Suitability 2.14 Table 3 Existing Drainage Area Summary 4.21 Table 4 Proposed Drainage Area Summary 4.23 Table 5 Quantity Compliance Summary 4.25 Table 6 Project Removal Requirements 4.26 Table 7 Overall Water Quality Summary 4.47 Table 8 Albemarle County Area Quantification Summary 4.49 Table 9 Conceptual Buffer Encroachment Summary 5.52 LIST OF FIGURES Figure 1 Project Vicinity Map 1.3 Figure 2 Project Location Map 1.4 Figure 3 General Development Plan (2005) Sheet 3 1.6 Figure 4 Land Use Map 1.8 Figure 5 Albemarle County Treatment Area Guidance Sketch (11-21-2014) 1 .11 Figure 6 Soils Map 2.15 Figure 7 Pre-development Drainage Areas 4.20 Figure 8 Post-development Drainage Areas 4.22 Figure 9 Proposed SWM Facility Layout 4.24 Figure 10 L-1 4.29 Figure 11 L-2 4.30 Figure 12 L-3 4.31 Figure 13 S-1 4.33 Figure 14 S-2 4.34 Figure 15 S-3 4.36 Figure 16 S-12 4.37 Figure 17 S-4 4.39 Figure 18 S-5A and S-5B 4.40 Figure 19 S-6 and S-13 4.42 Figure 20 S-6A and S-7 4.44 Figure 21 S-7A 4.45 Figure 21 Albemarle County Treatment Area Identification 4.48 LIST OF APPENDICES APPENDIX A RELEVANT CORRESPONDENCE A.1 APPENDIX B GENERAL DEVELOPMENT PLAN B.2 APPENDIX C HYDROLOGY SUPPORT INFORMATION C.3 APPENDIX D WATER QUALITY COMPLIANCE WORKSHEETS & TRACKING LEDGER....D.12 Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 5 STORMWATER MASTER PLAN UPDATE Executive Summary This summary will serve to provide quick reference to key elements of the Master Plan more thoroughly discussed in following sections. Regarding Part IIC Technical Criteria: • Per Albemarle County(County)guidance letter of August 6, 2015, (provided under Appendix A) Part IIC technical criteria shall apply to Old Trail for two permit cycles,or until 2024,provided CGP coverage is maintained. • Per-BMP peak flow analysis provided based on our understanding of the County's wishes. • A comprehensive project-wide approach to water quality centering on bioretention filters in accordance with refinements to the General Development Plan(GDP) Sheet 3 Plan. Compliance with Part IIC technical criteria is to be achieved primarily through on-site treatment in order to reduce treatment burden on Lickinghole Regional Basin (LRB). Regarding compliance with 9VAC25-870-96: • Old Trail Village falls under Situation 2,which requires that pollutant discharge after disturbance not exceed existing pollutant discharge. • Compliance will be achieved through appropriate on-site BMPs(see Table 7) supplemented by LRB,which Old Trail Village understands the resolution of County guidance and the acceptable methods to demonstrate overall project water quality compliance may best be summarized by the following(understood from March 17, 2016 meeting: o To meet Albemarle County guidance and meet VSMP compliance requires Old Trail Village to capture: ■ Bioretention: 92%,or 51.15, acres of the total maximum Master Plan target across the project by bioretention facilities (0.5 inch,50% efficiency) as a minimum. ■ Retention Basins: l00%of total maximum Master Plan target, or 92.58 acres across the project to Retention Basins as defined herein. o Remainder treatment required for VSMP compliance will be provided by LRB under a treatment train approach for bioretention capture only. As such, if for some reason total Retention Basin capture is problematic as shown herein, alternative Retention Basin capture would be proposed, OR additional bioretention capture could be proposed as a net benefit to LRB. o As long as approved plans meet overall minimum total area capture noted under Table 7,the project shall be deemed to remain in compliance by Albemarle County to the stormwater management(SWM) Master Plan, County guidance, and VSMP water quality requirements. o Proof of overall compliance will be provided under each WPO submittal with an updated Water Quality Compliance Tracking Ledger(See Appendix D). 4111 Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx �� STORMWATER MASTER PLAN UPDATE Nkaito- Regarding compliance with 9VAC25-870-97, 98: • Compliance with 9VAC25-870-97 will be provided in detailed design; however, demonstrations of peak flow analysis on a per-BMP basis are provided in Table 5. Regarding compliance with WPO ordinance, including stream buffers: • Infringements of grading of development activities including SWM facilities have been forecasted to the extent possible at this time. Note: It is the intention of Old Trail to further minimize these encroachments under detailed design, and it is our understanding that it will be acceptable to provide more detail per the typical WPO submittal process at detailed design. • It is understood that planting or offsetting water quality measures will be required. These aspects will be addressed in detailed design and quantified based on actual proposed encroachments,which may be minimized beyond master plan levels shown herein. Initial planting areas within context of proposed BMPs have been provided. • Additional planting areas are likely to be required and will be identified through detailed buffer mitigation plans will be submitted with WPO detailed design submittals. Regarding compliance with Zoning,including variance to ZMA200400024: • The overall capture and treatment theme of the ZMA is intended to remain intact. • Per email and graphic of November 21, 2014, regarding departmental guidance from Zoning and Planning(provided below, Section 1.1.4), no less than 75%of areas showing on-site stormwater management and no less than 75%of areas shown draining to ponds must be captured on-site. See Section 1.1.5 for more detail on resolution. Regarding compliance with General Development Plan: • The General Development Plan depicted basic stormwater management layout and several potential water quality and quantity facilities.The SWM Master Plan is a refinement of the GDP inclusive of quantifiable water quality and quantity compliance targets. Regarding required coordination with Albemarle County Parks Department: • It is understood that if any County-owned property is desired for use for a stormwater facility(for instance, L-3),this will need to be coordinated with the Albemarle County Parks Department. '4411,0- Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx IV STORMWATER MASTER PLAN UPDATE Acceptance Signature Blocks Print Name Date: Signature (signature) March Mountain Properties, LLC,Authorized Official Print Name Date: Signature (signature) Albemarle County Dept. of Parks, Responsible Official Print Name Date: Signature (signature) Albemarle County Community Development, Responsible Official Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx V STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4,2016 ABBREVIATIONS AC ALBEMARLE COUNTY CN CURVE NUMBER DEQ THE VIRGINIA DEPARTMENT OF ENVIRONMENTAL QUALITY GIS GEOGRAPHIC INFORMATION SYSTEM LBS TP/YR POUNDS OF TOTAL PHOSPHORUS PER YEAR LID LOW IMPACT DEVELOPMENT IMP INTEGRATED MANAGEMENT PRACTICE MEP MAXIMUM EXTENT PRACTICABLE METF MAXIMUM EXTENT TECHNICALLY FEASIBLE MS4 MUNICIPAL SEPARATE STORM SEWER SYSTEM MTD MANUFACTURED TREATMENT DEVICES NRCS NATURAL RESOURCES CONSERVATION SERVICE OT OLD TRAIL ROW RIGHT-OF-WAY SCS UNITED STATES DEPARTMENT OF AGRICULTURE- NATURAL RESOURCES CONSERVATION SERVICE (FORMERLY KNOWN AS SOIL CONSERVATION SERVICE) SWM STORMWATER MANAGEMENT TMDL TOTAL MAXIMUM DAILY LOAD TP TOTAL PHOSPHORUS VDOT THE VIRGINIA DEPARTMENT OF TRANSPORTATION ZMA ZONING MAP AMENDMENT -4416100, Sita ntec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1 STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4, 2016 1 . 0 INTRODUCTION Stantec Consulting Services Inc. (Stantec)has been retained by March Mountain Properties, LLC to perform an overall refinement of the stormwater management(SWM) Master Plan for the Old Trail Village development, specifically the Zoning Map Amendment(ZMA-o4-o24) property(237+/-acres)that was previously approved under a rezoning effort(2005). Over time,portions of the Old Trail Village have been developed with Albemarle County approvals and with March Mountain's general understanding that the spirit of the development was in keeping with the prior approved ZMA documents. More recently this SWM Master Plan was submitted to Albemarle County by Stantec in April 2015 and February 2016. Albemarle County, Old Trail Village, Roudabush, Gale, &Associates, and Stantec have worked together since that time towards progress related to the future development of this parcel,particularly with regard to Virginia Stormwater Management Program(VSMP)technical criteria. The project will employ Part IIC technical criteria for the current and next 5 year permit cycles, provided that Construction General Permit(CGP) coverage is maintained. As this has been resolved,the following document has been simplified to focus on water quality and quantity compliance on a conceptual per-BMP basis per our understanding of the County staffs desires. Note that the east side,or blocks 10, 16, 17, 18 and 31,have advanced under planning and design efforts. Accordingly,this affords an opportunity to have relatively higher resolution in this area as more detailed thinking regarding design of this section of the property have come together over the second half of 2015 and early 2016. While other areas of future development may be more unclear,we envision this document as a framework for on- site stormwater management treatment which also provides reference future design review and approvals,particularly with regard to an understanding total drainage area capture tracking defined herein. 1 .1 PROJECT DESCRIPTION 1.1.1 Project Location The project is located in the Crozet community in western Albemarle County outside of Charlottesville,Virginia. The site is northeast of the intersection of Route 25o (Rockfish Gap Turnpike)and Interstate 64,within eight-digit Hydrologic Unit Code(HUC) 02080204 (VAHU6: JR02). The site drains to Slabtown Branch and Lickinghole Creek,both tributaries of Meechums River,which drains to the South Fork Rivanna River,the Rivanna River,the James River, and ultimately drains to Chesapeake Bay. The project site is served by Lickinghole Regional Basin, an Albemarle County Regional Water Quality facility. The Project Vicinity Map and Project Location Map are provided on the following pages as Figures 1 and 2. +tirrr Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1.2 . i( Y \ I A � n 7" . Vin) or t Whin Ha'I Vin ywds 4 u' . C 4 ,„,,C,'" M�111ngta�� F ndi t nv (.hve-ntmlir. t SITE LOCATION u..li`.d �r- le ' , ;_ fk i ;, a r C 4.1)). Y IJOw W5 % k.it l ormc, M� g x 4 C ,2N ;,,,. -5'tP"""' to ., -,- 1 t _ - ...gym Mw, ,>a �`,�O 'i.,, ... ,,' . : „. N 4i:%1 SITE v 1.2 0 1.2 2.4 3.6 4.8 6 7.2 8.4 n L l"I Imil rmiims 1 VIRGINIA SCALE IN MILES SOURCE:VA.ATLAS&GAZETEER FOR: FIGURE: 4. MARCH MOUNTAIN PROPERTIES,LLC OLD TRAIL SWM MASTER PLAN Stantec__ ALBEMARLE COUNTY,VA SITE VICINITY MAP 5209 Center Street Williamsburg,VA 23188 JOB NUMBER: DRAWN BY: CHECKED BY: APPROVED BY: DATE: PHONE:(757)220-6869 FAX:(757)229-4507 203400430 AJT JGM JGM 02/05/2016 FILEPATH:U:t203400430\05_report_deliv\dwgs_design\cad\00430-LOC-VIC_20160203.dwglatiefenbacklFeb 03,2016 at 14:05ILayout:SITE VICINITY MAP ,.._ ..7:11 41„ et z.,,,.71145.,. liiii i. , „II _ ..1r, ‘ ,,,,,44,t _____ ;„_______,;:-.7,„"7,0711117,- ....\`.e4, - „itt.,...„._,r, 1044(\•_. 2.....„.37r,,,. ..: ,, , . ,4.A. .... .. 1 ..\\,.........‘„,.. .. .1.....,,,r-0, . ., -- _ iii,, :,..„ ...,„....:,...,...,:,,....: ,,,,,,44,,,__,,,, ..,7 ..,:..,,,v_. ..)14.*:41„.h.,, iiimimill , __....c.___I .-2.:. /e4 ,...0.- Agior ,,-- -, ‘ 4100,s, lio ... .....„ . , . . �:� .••:Vi_e;"-r4 )r, \ �� •• e Parr, ,....: ,,. .,,...•. \ ao( 1:::, „A...4w . _ ..,....„.....u.st .is, ,&:".. • *____,_.•=11,-,9 -- NP-- • N "..tio. - rets 011Iv 1;77; ... \,..iiiik..--, it kii. 7)71I° ) ...r....iipprotamL. ,, w , -.:-., : HOIralw" l - -44 .L.,4154•1•1 li ye. ' C ' i •`` • •.: ,,,,,• 7/:•;:...--. ... TSL 0 . •• : 4 m ssY ,� • p di wet 4.H tn, I a - " a. .. t:° � a i P 4,,,s_ it a . -"'' " s..4N11111PirAIIP - ' ''''' ''' rM►pp. /+ma �yy A✓r�+^� tp,1/4W4,...$17-1.'•VW 1°_ Pak � ►.I.t6' /,� o�� .rev � `' �• `ryl ....i4..._ _ to 506 4'SITE LOCATION ��!% �i? 3477 ,�� WitiVAVI e 44 �jJ I ' . ),1/4iii....a, $ . 46. ...... . ,_..,y: , ,,,,,, ,,,,r,„7„// .___, 1. tu-.... -...t.,,,y....,,,,ropirAl „ . ". bliVI. . N$1,14),,,,-----4,-1,07 It ',..rt,,.55b ..-4 • *:".,e-, \ - 1 (2- ..*Wzo- . .1:-. ,:,:tdot .for .11.Wr l• ,/ ** ( e iirrillIW,,, ��_� }. • •'. •n�.� ,r-� A** � - Vito• oka. L ,..,...-- ,N,..4./ri__ c_ .7.kiwi.. 4 / ir • �� .+ ; :.+ ��,,� ^ ", • - tit r 0( \ti it CAION.„....1 ') l'A'-',T-"!,,.. .i ! Q Wit ' „ , , 144, N EI 1ITE 1 1/2 0 1 V p�Illlll. IIIII♦.IIIIIIIII— -- SCALE IN MILES 1000 0 1000 2000 3000 4000 5000 6000 7000 VIRGINIA n LJ r1 imi' INNENL SCALE IN FEET LATITUDE:38°03'28.74"N LONGITUDE:78°42'34.42"W SOURCE:DELORME MAPPING CO., 1995. FOR: /+ ALBEMARLE FIGURE: Stante4 - MARCH MOUNTAIN PROPERTIES, LLC OLD TRAIL SWM MASTER PLAN COUNTY,VA SITE LOCATION MAP 2 „......1 4 5209 Center Street Williamsburg,VA 23188 JOB NUMBER: DRAWN BY: CHECKED BY: APPROVED BY: DATE: PHONE:(757)220-6869 FAX(757)229-4507 203400430 AJT JGM JGM 02/05/2016 FILEPATH:U.\203400430\05_report_deliv\dwgs_design\cad\00430-LOC-VIC_20160203.dwglatiefenback(Feb 03,2016 at 14:05(Layout:SITE LOCATION MAP STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4,2016 1.1.2 Project History, Guidelines, and Approach Stantec has been retained by March Mountain Properties, LLC to perform an overall refinement of the stormwater management(SWM) Master Plan for the Old Trail Village development, specifically the ZMA(ZMA-o4-o24)property(237+/-acres)that was previously approved under a rezoning effort(2005). At that time, a set of plans entitled the Old Trail Village Rezoning ZMA-o4-o24 General Development Plan(GDP) accompanied the Old Trail Village ZMA-o4-o24 Rezoning Application and Code of Development(September 12, 2005) that was approved by Albemarle County. Specifically within this set of plans, Sheet 3, or the Stormwater Management and Stream Conservation Plan(henceforth noted as the GDP Sheet 3, or Original SWM MP)provided for a basic stormwater management layout,depicted several potential water quality and quantity facilities in bioretention filters, and through the use of modified on-site or adjacent pond facilities established a conceptual layout to address stormwater management. See Figure 3 for the GDP Sheet 3. This SWM Master Plan Update is intended to serve as a refinement of the GDP Sheet 3 Plan inclusive of quantifiable water quality and water quantity compliance targets for the benefit of all parties as future development phases move forward. Based on the November 17, 2013 meeting with Albemarle County staff, and Roudabush, Gale and Associates(RGA), Stantec understands that the client and Albemarle County both wish to consolidate prior planning and design efforts into a comprehensive stormwater management (SWM) master plan for the ZMA(ZMA-o4-o24)property(237+/-acres)that was previously approved under a rezoning effort(2005). Accordingly, Stantec has worked with Albemarle County, Old Trail, and RGA to assess the overall plan of development in effort to more concretely define the SWM Master Plan and what conceptual water quantity and quality treatment is attainable on-site. While the detailed design process still requires flexibility based on specific constraints, design codes, and more critical review at heightened levels of design submittal,this document is intended to provide a roadmap for future discussions,submittals, review, and approvals. It may be valuable to review overall project compliance should deviations from this plan be required on a phase-by- phase basis. Specifically, drainage patterns may need to be altered minimally based on design requirements and updates to the land plan but the overall capture and treatment theme is intended to remain intact. It is understood that this Master Plan establishes a project baseline for water quality and water quantity compliance. As such,if any given section exceeds the levels of treatment, it should be generally deemed a net benefit to local water quality and may provide design flexibility in other regards. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1.5 STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4, 2016 Figure 3 General Development Plan (2005) Sheet 3 LEGEND I -4: nn * ORNIKO siax f t.j rcnrcw�rr..uerw.�.ea.. wti� 'f0,.eir..i+ar�`%�umuc�nw TMmwe.ri.rm _ w +rrn. .. rm.rmirve 4no-xao.nm• _ ..,+.i --- .e � ="'"'7..1.1.70....... ��re cF.rf � -.. rmowrwrtltU@t¢Irt10M dow.rff+wlh�r cusmia...c.ro n»«,w $ , 1:II r 1 i t iI�i w�suc:,wwroWW1.. ■omxsww , {}t NIP,' _ sn. .. , 4/ z r, / 1 r�nil,1R^ 1 IN V11 .; . ,....... ..,, „ ,, g G p (9 P f , uBr r rfoz. � .,, j 1 Y m 0 , urEit a j r Te x t FED / x / ) Di - t44 x VR }is i c\s„, (.\\., ' / ' t, 11 ' ' ''. 4 ! f; T4 * 4.-C' .V4t;”'° ` # , ' :a ° , t ! i ° (j f ( '111 ' / '"" t_7 r'�` f' f% C.. .: r 1.1.2.1 Refined Land Use One of the primary goals is to more quantitatively define and refine the original(by Timmons) Master Plan layout in in terms of both water quality and water quantity compliance targets. This remains a conservative effort on a project-wide basis to the extent practicable so that future planning and design will continue to have reliable compliance benchmarks under Part IIC criteria. To accomplish this,Stantec utilized historical master plan information provided by RGA to assess the relevant approximate phosphorus loadings and applicable design flows intended by the proposed development. For establishment of these targets,the earliest obtained CIO Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1.6 STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4,2016 Now- digital master plan layout was utilized as Stantec defined the planning areas and associated impervious areas. These areas and land use estimates, used later for quantifying water quality compliance targets, are detailed in the figure and table below. Table 1 Land Use Breakdown Land Cover Area (SF) Area CN I Impervious Impervious (AC) Percentage Area (AC) Open Space 3,993,280 91 .67 61 0% 0.00 Townhomes 3,650,408 83.80 85 65% 54.47 Roads 1,192,193 27.37 98 100% 27.37 Commercial 830,895 19.07 92 85% 16.21 Single Family 580,791 13.33 75 38% 5.07 Homes Existing Ponds 91,346 2.10 98 0% 0.00 Total Site 237.35 103.12 It should be noted that while this Master Plan update establishes the compliance targets based on historical master planning,the future compliance strategies take into account some greater level of thought for design, consistent with on-going discussions. For instance, certain planning of BMP facilities(on the east side of the project, actively under design)was incorporated with more current design thinking in these areas to best facilitate future review. No densities are planned greater than those from historical master plan levels. Similarly,it also should be noted that market trends indicate potentially increased demand for single family units within the context of the same basic development layout over townhome units (which dominate the original layout). This may ultimately place incrementally less pressure on future design compliance in context of the original master plan densities. tantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1.7 .4- - - _ (> 2 i-,7 0 i 1-, 0 4 r Li' 5 ,-, ¢ I )1L,, ! 5 ,11 3 i, 0) .- -e; g g g 1 i 11 i 4,/1/4/ E 2 C"5 0 'Th-1 I 1, // , dl: 5 1ll '',-, ,,S • -g L'il2 .). 2 k..-s. --• - __-- s--- ----.\' -')":-,;-:-%---'[[-:;'-V.,'%,//''.:--:------ KI i; -- --. ;?..-et' . -5----- ....-..., [7-.-,-.;?-_-------- C 1,.,,8 . / '',' Pr?-;':---<-.±.--,;';'-;.,'--,-;--- "I'`--'''''''%/i'4 in4a VA i ,, ‘5‘.',;,\', i ,, [. -z,;....[...----..r.;---...„--,,,iNe:-......-,,If,[,:,",,- .v . 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Based on more recent discussion and guidance delivered via letter from Albemarle County dated August 6, 2015(provided under Appendix A) Part IIC technical criteria shall apply to Old Trail for two permit cycles, or until 2024, provided CGP coverage is maintained. The following summarizes the overall SWM Master Plan approach,with the following areas of focus by the Old Trail Team in accordance with VSMP Part IIC technical criteria: 1. Water Quantity: The prior project-wide nodal analysis which reviewed flow peaks at various property locations beyond BMP outfalls has been substituted based on our understanding of the County's wishes. This change in approach has been implemented conservatively such that flow attenuation leaving areas of development for the 2-year and 10-year storm events was compared on a per-BMP basis. 2. Water Quality: A comprehensive project-wide approach centering on bioretention filter placement and design in accordance with refinements to the GDP Sheet 3 Plan. Focus was placed on compliance with VSMP Part IIC technical criteria primarily through proposed on-site treatment in effort to reduce the treatment burden on Lickinghole Regional Basin to the extent practicable. The performance-based approach was used to provide a succinct approximate accounting of master planned crediting of treatment (in pounds of total phosphorus per year or lbs TP/yr) for on-site practices versus that which may be needed from off-site treatment options. Note the design team has targeted an approximate minimum of 75% of the required water quality compliance treatment on- site, which is consistent with current VSMP regulation regarding use of off-site compliance options without a Maximum Extent Practicable (MEP) demonstration to the VSMP Authority. We feel we can achieve greater on-site treatment than this level on an overall project basis. Overall Master Plan water quality compliance worksheets for the targets are provided in Appendix D. More discussion on these aspects is provided below. 1.1.4 Relevant Guidance by Albemarle County In addition to state project compliance,Albemarle County has provided additional clear guidance related to project goals. Per an email from Mr. Mark Graham(Friday, November 21, 2014 4:36 PM)to Mr. Dave Brockman,the Old Trail Team noted the following(including Figure 5,which was attached to the email): Dave, To follow up on our Monday meeting, Glenn and I have met with Zoning and Planning to make sure we are staying consistent with the approved zoning plan and stormwater requirements. In doing this, we've tried to set the limit on what is expected and what we can Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1.9 STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4, 2016 Notioi approve. Please do not take this as a negotiation offer, it is not. Instead,we've realistically looked at what is needed to comply with the VSMP and without first amending the current property zoning. From that,I am offering the following as a set of guidelines to follow in development of a stormwater master plan that avoids the need to amend the zoning. To assist in understanding,I have attached Glenn's illustration at the end of this message. The following are what I believe are the important points: • Those areas that show onsite stormwater management(red on the illustration) must have facilities to capture at least 75%of this drainage area. That said, they do not need to be biofilters as indicated on Timmons plan, but could be another approved type of stormwater management facility. • Those areas that were shown as draining to the ponds within the approved zoning plan(blue on that illustration),but were not constructed to drain to them or have other reasons the ponds cannot be used, issues, must either have the storm sewer constructed so they correctly drain to the ponds as shown or may have other facilities within Old Trail to treat that runoff before it enters the stream. The same as the areas in red, we expect no less than 75%of the drainage area to be captured on-site for treatment. Obviously, if this requires you to propose changes to the approved layout plan, this will likely require either a variation to the plan or a zoning amendment. In that case, we should talk with the planners about what changes staff will be comfortable in supporting. • Those areas that were shown as not having facilities to address the runoff(white on this plan)are allowed to have flow directly into the stream, but the water quality impacts must still be addressed. For all the areas not being routed through BMPs(this area plus uncaptured areas with the red and blue areas), a go%reduction credit can be provided by Lickinghole. • With the above,our initial analysis indicates it should be possible to meet the phosphorus reduction required with the VSMP that apply to this property(2C). If it turns out the onsite facilities and the Lickinghole credit cannot meet the entire pollutant reduction, we will consider using the offsite ponds to address the incremental difference. That would need to be verified as genuinely providing a reduction above and beyond what was previously approved. • Finally,if there is an interest in using any of the County owned property for a stormwater facility,please recognize this will need to involve a number of people outside of Community Development. This will require additional time and effort to coordinate with Parks and Recreation, the County Attorney's Office, the County Executive's office and ultimately the Board of Supervisors to assure the County's interest in the property is being protected. Past experience suggests it will likely take 4-6 months to finalize an easement agreement with the County once everyone has agreed upon the recommendation. I hope this simplifies things and gives Stantec the guidance needed to develop a good plan. Have a great Thanksgiving. Mark Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_rap_v3_20160404.docx 1.10 STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4, 2016 Figure 5 Albemarle County Treatment Area Guidance Sketch (11-21-2014) ,t., *if w04a.., « . ..au t1 ..., = 1 Ill ..,,,�._... . I : KMR#IJi##�f Af Pl R._ w .� .+..4 .101•14,7 ..w.1 ,,,, ....V.'ml w y ,a. e .....tee • Morn k f1' xrr*rr e k Y.41.r^M W.rM'.4 i ": .. C ponos Fi s us t�4'x t �: .�p l WI* 4.7 acres j } i V IN 1 ,` m. t 1 ,: 1- I $ .,7 ' i t a, ;- /4 F,-....,." Sri > \\\_, '`\`� y,�'f Areas shown by outline are approximate areas Z, I.{ to discharge points used for compliance computations 0 n;i iz ; , . a E � E i iiI MIMI 4.d t i 1 ., This email has served as the basis for subsequent productive conversations and is the primary foundation for the overall SWM Master Plan approach, with the following additional areas of focus by the Old Trail Team in accordance with VSMP Part IIC technical criteria for water quantity on a per-BMP outfall basis assessing post-development flow attenuation leaving the property for the 2-year and 10-year storm events to pre-development levels. For water quality, a comprehensive project-wide approach centering on biofilter placement and future design in accordance with refinements to the GDP Sheet 3 Plan. Focus was placed on compliance with VSMP Part IIC technical criteria primarily through proposed on-site treatment in effort to reduce the treatment burden on Lickinghole Regional Basin or maximization of on-site treatment to the extent practicable. More discussion on these aspects is provided below. kid COStantec to u:\203400430\O5jeport_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1.1 1 STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4,2016 1.1.5 Understood Compliance and Tracking Resolution to Original Guidance On March 17, 2016,a meeting was conducted between John Anderson of Albemarle County, Bill Ledbetter(RGA),and Glenn Muckley(Stantec),which sought to clarify a final acceptable compliance range for on-site treatment and resultant LRB treatment. The following points are understood to be in agreement: 1) To meet Albemarle County guidance and meet VSMP compliance requires Old Trail Village to capture: a) Bioretention: 92%,or 51.15, acres of the total maximum master plan target across the project by bioretention facilities (0.5 inch, 5o% efficiency) as a minimum. b) Retention Basins: loci%of total maximum master plan target, or 92.58 acres across the project to Retention Basins as defined herein. 2) Remainder treatment required for VSMP compliance will be provided by LRB under a treatment train approach for bioretention capture only. As such,if for some reason total Retention Basin capture is problematic as shown herein, alternative Retention Basin capture would be proposed, OR additional bioretention capture could be proposed as a net benefit to LRB. 3) As long as approved plans meet minimum overall minimum total area capture noted under Table 7,the project shall be deemed to remain in compliance by Albemarle County to the stormwater management Master Plan, County guidance, and VSMP water quality requirements. 4) Proof of overall compliance will be provided under each WPO submittal with an updated Water Quality Compliance Tracking Ledger(See Appendix D). 1 .2 PURPOSE OF STUDY The purpose of this study is to address compliance with stormwater quality and quantity criteria for the proposed development improvements.Additionally,the Master Plan has three underlying goals which are: 1. To provide an overall demonstration of how the project will satisfy part IIC technical design criteria for use in subsequent detailed design submittals and approvals; 2. To serve as an overall guidance for subsequent design, review, and approval, understanding that details regarding the development may be refined but this over- arching master plan approach may be updated for reasonable implementation in future years; and 3. Establish a reliable"proof of theory"planning-level document that comprehensively addresses conceptual SWM compliance so that a zoning variations(which may include key excerpts from this document) may be pursued that will provide clear,reasonable guidelines for all parties to review and reference under detailed SWM plan reviews for future development phases. This is inclusive of an understanding to use of Lickinghole Regional Basin to provide the balance of water quality compliance on a per phase basis. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1.12 STORMWATER MASTER PLAN UPDATE INTRODUCTION April 4,2016 Accordingly, as detailed designs are submitted, more detailed hydrologic and hydraulic calculations will be provided per individual development block. Similarly,water quality calculations will be provided and it may be prudent to provide a project ledger per detailed design so that the County and Old Trail may have an understanding of where the overall project compliance stands. The documentation will serve as a means for the plan reviewer to verify that the compliance is in accordance with the Master Plan, or should slight deviations be required, how they are to be addressed in greater context. Detailed design compliance and supporting calculations will be provided at detailed design submission, including verification of downstream receiving channel adequacy. Stantec to u:\203400430\35_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 1.13 STORMWATER MASTER PLAN UPDATE Constraints and Considerations April 4,2016 2.0 CONSTRAINTS N CONSIDERATIONS A general review of the project constraints was performed by Stantec based on information provided by RGA. In context of SWM, several considerations are important in this regard, and while many will be addressed during detailed design, several have been noted here for background and understanding. 2.1 P ROJ ECT SOILS Soils within the project area are characterized as being generally suitable for SWM infiltration practices. Infiltration suitability was defined using the permeability(Ksat) and depth to water table as specified by the NRCS soil survey for Albemarle County,Virginia. Based on review of the project site,it appears that infiltration could be a component of the overall SWM strategy; however,based on the slopes found on-site and the general earthwork required to address project infrastructure and supporting SWM facilities,fill depths and disturbance may preclude the reliance on infiltration as a primary component of SWM compliance. At this time, all bioretention practices have been envisioned with underdrains to remain conservative in this regard. Considerations for enhanced infiltration aspects of proposed BMPs may be reviewed, as applicable, during the detailed design phases of individual village blocks. The defined ranges and their related qualitative infiltration suitability rating are shown in Table 2. Table 2 Soil Infiltration Suitability Nosey Infiltration Suitability Soil Slopes Depth to Water Depth to Restrictive Permeability (%) Table(FT) Layer(FT) (IN/HR) Suitable Infiltration <20 >5 >6 >0.6 Moderate Infiltration <20 >5 >6 <0.6 Not Suitable Infiltration >20 <5 >6 - A figure depicting the on-site soils,a graphical representation of the infiltration suitability rating,and the soils with high water table has been included as Figure 6. 4 Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 2.1 4 110 kfidi v,,,..„,,... ,,.., .. ,,, ,,,,.. „,.,.„:„.\,, ,,,,„_„:,,,,. 7 ,-- , 0 , ,,. • ",r/ l- \--'-' 1,1i ,4',A; ( - ` // 'Z'r r• —•-------47--„,, Cil �M r,�\ l^>r� l•/,\�'Fr Irrr`l ,y}mat. � 1 1i � `_ 1• \t,/ f r ,�( f'r ^ n 72 n v -.1"‘ r /) !11 o,� c , \' / 1 -�r r \ 'J\ rI `\J )1^V-n ( L� "r z(..1. ��' n�� ! \a i f ? ?i\ fi/ ` Q , $ % • ,. r. -.._.✓ -_�-_.}fir<.` /'- , t`.� \ p rte! �,`:- 1`�,f �r�� tr %1'� ,,,-„ ,/-4',',. // Gi 1,\`I' '° •,, '�r \G ,� �)��_.,�.J \ l< �••^� C 1 \ yo,,,,,I, l'., t c f•<, \ ,` ' ' moo` i nY .> ;I 2 C__�_ - �/02-1 n\`rte t l �s�/� 0` l r 'Fv„4,, _ - �s\ :�U i'� _. _ _ — i ,!-. 1,41 \ `'f .T �\` i ? \bt ,i-• , .. ,/ /v^ )/ r( ) �t �\ y \rr/ I P \ / ' `..r ��. /, 1'/ - ...''''4."..-..':-..--� ? \ i3r ,ate+`--;-/- -- 11)- - ,I=='''''',-,•= 0A1,,,,,;„,.,.,`", ! �, I \• = i \ ter-' , 4, k` i, ''`_ < H ~ _ - p,\1 1 _)1..., ;pr e,\ \ tom' - •'. #;,''' r! - '71y )` , I ' Ir! \\;�,,\ ` , r %- �:°' .4k .` _ 1 &_- dam'.- Ya \\�: .- - r'i ..i.,;./. r '�%rr m ') r ��\ yniy�li \'1-�, !,/!r ',.#-.i-_-- ,rf"--) , ^\:"' \y'A r �l�' ''� \ f!j ',' ')...t-_ - {_\Wei `t l //2/•\'''//J\\Z".\‘'..\\1 #t�i I r'r, __r''• r. - -. 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Avoidance and minimization of impacts to these features to the extent practicable is an on-going project initiative. The site contains several areas of jurisdictional features, primarily isolated to the main reaches of Slabtown Branch and Lickinghole Creek. Other areas of wetlands are more predominant within the proposed park area; however, no facilities are proposed in these areas. 2.3 STREAM BUFFERS Consideration was included for the(ioo)hundred foot stream buffers in accordance with the Albemarle County Water Protection Ordinance(WPO). Certain items have been historically proposed within these buffer areas, such as practical outflow points for SWM facilities or outfall pipes,proposed passive recreation facilities, and infringements of grading of development activities including SWM facilities. These practical infringements have been forecasted to the extent practicable, are intended to be minimized,and are shown within the first, or landward, 5o feet of stream buffer where applicable. It is understood that plantings or off-setting water quality measures may be needed. At this time, it is envisioned that encroachments maybe addressed through post-construction plantings. These aspects would be addressed at detailed design submission in more detail; however, some planting projections have been conceptually shown within the context of the proposed BMPs. More detail can be found under Section 6 below related to buffer encroachment and future avoid and minimization efforts are envisioned under detailed design of future phases. 2.4 100-YEAR FEMA FLOODPLAIN According to FEMA Flood Insurance Rate Map (FIRM) Panel 51003Co237D (February 4, 2005), portions of the property reside within the 100-year floodplain zones AE and X. Proposed activities near the floodplain have been considered preliminarily herein, and will be addressed in detail at the time of design submission as applicable. No floodplain study has been performed, although it should be noted that floodplain elevations were reviewed and it is believed that all grading envisioned can be accomplished above floodplain limits to the extent practicable. Conceptual footprints have been shown for Master Plan modeling purposes, although alternative layouts or orientations may be desirable based on project constraints and goals. At detailed design, it may be shown that outfalls or grades extend into the floodplain. As such, supporting data will be provided as necessary. Consistent with approved zoning, no residential development is proposed within the floodplain. tantec fa u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 2.16 STORMWATER MASTER PLAN UPDATE Suite of BMPs for Project Implementation April 4, 2016 3.0 ITE OF BMPS FOR PROJECT IM E T TI 3.1 LICKINGHOLE REGIONAL BASIN Consistent with GDP Sheet 3 and prior implementation,the water quality strategy for the project has consistently included the use of Lickinghole Regional Basin(LRB)treatment to the extent available under the Albemarle County pro-rata share program. Based on County of Albemarle Department of Community Development Memorandum: Lickinghole Regional Basin Board policy implementation(September 19, 2014), it is our understanding that an approximate 3o%treatment is typically available for by-right zoning developments within the watershed. Based on discussions over time with the County, March Mountain Properties is committed to lessening the burden on the LRB facility to the extent reasonably practicable. As such, Stantec has evaluated the property to optimize on-site treatment in a consistent fashion with the GDP Sheet 3 and subsequent guidance from Albemarle County. These efforts establish upper targets for on-site treatment which greatly exceed County guidance. More discussion is provided below related to minimum drainage area capture. 3.2 BIORETENTION FACILITIES Bioretention facilities were noted conceptually on the GDP Sheet 3 as a potential means of addressing offsetting water quality impacts prior to entering LRB. As such,Stantec has evaluated the layout plan in an effort to site more refined proposed bioretention facilities in keeping with the GDP SWM layout plan. These facilities, discussed in greater detail below, have been footprinted conceptually in an attempt to optimize water quality treatment from denser areas of development,in approximate locations noted on the original GDP, and to systematically address water quantity compliance in tandem with water quality goals. Previously,several facilities were proposed with larger footprints capable of treatment to a 65%level of pollutant removal efficiency in accordance with the Virginia Stormwater Management Handbook (VSMH),First Edition 1999 ("Bluebook"). However,these have been reduced in concept to 50% efficient facilities,and final design footprints will be optimized based on the 2.5%sizing criteria under the Bluebook. Also, note that the bioretention concepts have been advanced from preliminary bulk storage routings previously provided and now provide consideration and detail to demonstrate: 1. No stacking of water greater than 0.5 feet above the bioretention bed by way of access to spillway crest elevations,and 2. Conceptual methods by which to provide high flow bypass during larger storm events for facilities with drainage areas greater than approximately 3 acres. These components and configurations will be further evaluated and refined under detailed design to a much greater degree, but are intended to provide relative assurance of the function of the facilities. 3.3 RETENTION BASINS Certain areas as shown under the GDP Sheet 3,were envisioned to be treated for both water quality and quantity,including facilities noted as L-1 and S-2. These existing facilities have been included in the approach consistent with the GDP and subsequent discussions with Albemarle Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpf_swm_mp_v3_20160404.docx 3.17 STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4,2016 County. As such, more discussion is provided on the conceptual structure improvement and conversion of these facilities to address water quantity and quality control. Further, some opportunities may exist to use retention basins in key development areas with concentrated drainage patterns and heighted project density and constraints. Existing approved facility L-2 is one such instance, as well as facility S-2 (a plan-approved BMP),which is currently serving as an approved erosion and sediment control device. More discussion regarding these facilities is provided under Section 4 below. March Mountain Properties, LLC retains the option of utilizing retention basins as 50%efficient facility type that is understood to be reasonable should drainage area considerations become problematic for implementation of other practices envisioned herein. 3.4 GRASS CHANNELS Initially it was believed that grassed channels may be included in the overall SWM strategy in a more prominent fashion. Following more thorough site review,it appears that many locations may be infeasible given several factors,including likely future development grades required. The east side planning was used as an evaluation process in this regard. At this time only a few grass channels are considered for use as pretreatment for bioretention and any crediting associated with them has been removed to remain conservative. However,the implementation of grass channels as a water quality enhancement should remain an option during detailed design should constraints or opportunities suggest their use as stand-alone facilities or in combination with other practices. 4.0 PROPOSED T T - While detailed design will bear out whether fewer or additional measures are required,the proposed overall SWM strategy includes the conceptual use of fifteen(15) previously or newly proposed facilities to address water quantity and quality requirements. These facilities will require modification and advancement from the conceptual plans shown here. Additionally, fewer or additional facilities maybe required, depending on site constraints, and remain in keeping with the spirit of this plan; however, it is envisioned that this plan be the general road map for future development. For instance, one larger bioretention area may be broken into two or more cells depending on final development layout, effort to optimize treatment,and reduce impacts to existing intact stream buffer. A summary of water quantity and quality analyses is reported in the following sections. 4.1 WATER QUANTITY - VSMP WATERSHED SUMMARY As noted,the project approach for water quantity is to address the whole of the ZMA property providing a series of controls that work in concert to address post-development flow attenuation to pre-development levels for the 2-year and 10-year storm events. 4.1.1 Drainage Area Development The scope of this report addresses an overall watershed modeling approach to understand the impact of the project on specific outfall points of compliance, or per-BMP envisioned within individual phases, and to provide flow attenuation to pre-development levels for the 2-year and Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.18 STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4,2016 Nttiopv 10-year design storms. More detailed information will be provided on a per phase basis, including addressing channel adequacy; however,this section is meant to demonstrate conceptual overall project compliance. Curve number computations for pre-development and post-development conditions were originally computed using WMS (Watershed Modeling System)software. Curve numbers were developed using Albemarle County soils mapping and land use boundaries derived from US Geological Survey(USGS) National Land Cover Database(NLCD)for pre-development conditions (2001). Post-development conditions were updated using proposed land use mapping provided by RGA. However, note that in some instances Stantec has used curve numbers from approved plans and additionally manually adjusted some original post- development WMS curve numbers up to remain conservative for purposes of water quantity compliance. These do not represent final design hydrology,but resulting curve numbers are presented in tabular form below. Lag times for each subwatershed were computed using WMS. The proposed lag times are similar to pre-development values. Lag times were computed using the SCS curve number,the average watershed slope,and the hydraulic length. Lag times were converted to time of concentration values for modeling purposes using TR-55 methodology software(PondPack). Similar to the above, some subwatershed times of concentration were adjusted down to represent more conservative preliminary flows given lack of design clarity at this stage. PondPack was utilized to compute peak discharge and runoff volumes for pre-development and post-development conditions. Pre-and post-development PondPack modeling summaries are provided in Appendix E. Stantec has performed preliminary BMP routings for purposes of comparing pre-development and post-development flows using PondPack, and routed preliminary BMP facility footprints to present a relative understanding of the level of service for the proposed controls on-site. The facility types and function will be further refined in detailed design submittals. The overall intent is to provide peak flow attenuation of typical design storm events(2-year and 10-year) relative to BMP outfalls. One hundred year storms flows have been addressed to the extent practicable. The existing and proposed drainage areas can be seen graphically in Figures 6 and 7 and Tables 3 and 4. Requirements for confirmation of channel adequacy will still be required to be met on-site and will be provided under detailed design. While a general site walkover of most portions of the property were performed by Stantec, no detailed study or survey of individual outfall channels was performed. This assessment does not address other forms of permitting that maybe required for the development or the implementation of the greater SWM strategy. 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STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4,2016 Table 3 Existing Drainage Area Summary Pre-Existing Conditions Watershed Area Tc Curve Number Watershed Area Tc Curve Number ID ac hr CN ID ac hr CN 100A 22.46 0.250 74.00 38B 26.62 0.397 58.30 lOB 20.53 0.550 61.00 40B 32.10 1.460 40.80 118 7.03 0.229 55.20 41B-1 3.09 0.283 61.00 12B 9.78 0.598 61.00 41B-2 2.92 0.337 61.79 15B 1.93 0.256 61.00 42B 4.44 0.287 57.60 16B 11.73 0.416 61.00 43B 3.39 0.280 56.00 18B 4.11 0.175 55.40 44B 8.63 0.529 59.58 1B 467.57 2.353 62.20 45B 14.08 0.482 56.10 21B-1 1.92 0.083 66.90 468.1 6.69 0.083 61.70 21B-2 6.43 0.083 66.90 46B-2 5.36 0.083 61.70 218-3 1.21 0.083 66.90 47B 4.63 0.238 59.90 22B 5.17 0.160 63.00 48B 9.98 0.088 88.58 23B 8.73 0.193 58.0049B 12.23 0.210 65.70 24B 9.71 0.452 58.30 4B 0.86 0.083 61.00 25B 5.17 0.137 57.60 50B-1 118.96 1.420 64.10 27B 5.82 0.163 60.40 50B-2 13.11 0.200 64.10 2B 81.56 0.630 59.60 51B-1 7.33 0.083 63.80 31B 1.76 0.127 62.40 51B-2 39.51 0.596 60.15 36B 3.40 0.173 61.00 528 1.13 0.103 65.30 37B 6.02 0.233 84.32 Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.21 T F ♦' > 0 ' • ' ? 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Z !.'.77.'y M% V.�"'�1 - 'lig ��, w�1�i � Q�.r t �sy#" `i'ts t v '� p ter+ g. do -..,,,-5,-:;(;,,,,,\.,-.\--,,,-- s 1 1 '''‘,*1'. t 'f' '``w�� i W v J" r +}y r .fib'' r _ .(" 1.„/Y!. it. . . 4 A 1,ry•' .,.;• ` fc I -, f tr. - W ..'\1 _ Mme, » ',1• � `, Y/ 1 �t F 1 W { SA,•' 101 N :I 'e,�, ;e- 1 �,15\\1- ` '!/ a"ib ,i 'S`w°� \ °40.. 1 qa I �'_";`�.� ,��. „4y. i `�t \1 ',-,,,.: �,\ Vis". ., .r. �x ,� t _ r IF 1�- mo*___.- �,: ' /-, ' o 1 --- 1 •�,��N s \(Il t }�. ,.. -4.,ya, auk'` ►: `4 •- ili -o75 � D 1 ::), y w ys P W 7 P (," • c, --,,,-8(-if..4 ‘ D -� L [I ilt J sl ai W F W . `p s' �� I � D m W'i+w ill "� r > j• r a v m kll\ 1 ? vim. v � `yti\I\\ aVu1f1� i '4 .0 m i c '^ N \ Z r r \--z-,..-_-•-,,,,,-,;,-,..,-..� ,a 1 \ \ 3 o , rTh o m A G p T -1 r /) O� U '"1 Q STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4, 2016 Table 4 Proposed Drainage Area Summary Proposed Development Watershed Area Tc Curve Number Watershed Area Tc Curve Numtser ID ac hr CN ID ac hr CN 100A 22.460 0.250 74.00 36B ' 1.651 0.167 76.80 10B 23.600 0.267 87.65 37B 4.380 0.083 92.69 11 B 8.960 0.34084.38 388 26.620 0.397 58.30 12B 6.890 0.256 82.95 40B 31.970 1.215 44.90 15B_I 4.620 0.36188.87 41B-1 1.250 0.359 87.59 15B-3 2.230 0.083 73-50 41B-2 1.140 0.083 77.01 166 1.030 0.083 87.70 42B 4.44 0.285 64.44 (Remainder) 18B 3.520 0.18389.60 436 3.230 0.233 66.60 18B-1A 0.550 0.083 88.86 44B 14.710 0.286 85.00 1B 467.570 2.353 62.20 456 12.370 0.482 61.50 21B-1 1.810 0.083 76.15 46B-1 9.390 0.203 81.20 21B-2 6.730 0.160 77.04 466-2 0.870 0.083 81.20 218_3 1.890 0.083 83.00 47B 4.520 0.238 60.00 22B 4.990 0.160 63.00 488 10.130 0.088 93.60 23B-1 1.250 0.08383.60 49B 12.230 0.210 65.70 236-2 5.940 0.16087.43 4B 0.860 0.083 83.18 23B-3 0.670 0.083 64.10 50B-1 118.370 1.420 85 23B-4 3.390 0.083 66.93 50B-2 13.110 0.200 64.10 24B 5.460 0.452 62.20 51B-1 4.310 0.093 77.70 25B 9.680 0.132 85.00 51B-2 42.420 0.267 77.70 278-1 6.015 0.16376.50 526 1.130 0.098 80.50 2B 81.560 0.630 59.60 8B 4.860 0.188 60.40 31B 1.750 0.207 74.20 Golf Dr. 1.050 0.083 98.00 4.1.2 Proposed SWM Facilities As noted above,the stormwater management facilities are conceptually proposed as shown on Figure 9 below. tantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.23 ♦., ..r F Zj m m O v = D m m N (n y y U U U y U U fn N r r r O W N D V D P D W A W N W N O O D o X_ 6 D Im cn m $ Z o 73 a D z - r ws m A o ml N T. 73 °f 0 O°y q m o m o mm m Z _2. w co. w m w m o. co. w = 4 ce P -1 OOOOaOOOOO jajp A A a 0 NID (D N (D CD (D (D (D 0 (D m (D w D' p po000oOo00oO . oOQo s s v v r n U v s U < u 5N O £ E <zsss7- 7- ssssssXs > f7 f � D z ° O = Ii ` co co Q m D OQ N w0 U — P N — A W U P (.7, L' N WNnn m WP UO N U WN OOc' O A 'P O A (3 i3 flD /') v A r (p O_ N n MI 3 P A A Co O W N W A V U p, N n NJ A W A N •N V <co W W P 0 0Ni.,(TOr. 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D :; 1,,_;--,,,--i\--,,,, :{ li \� \' \, ` � \ �y • ,! \ �f 'r. .".:::.4.,,'f,2,;,:171,:,41111114-, / i O \'\v ; •1/ J { / ��'� (' -/;/ k\�b � 4 •_,./ ----- ffl \ * N \j'4 •' •4'4' .-- \ "..I;,.'''''1.' --->------ •'Sf,lti I'.....""11'—'' • •Itity-i': ''''tc:,, ,.,„ ,:,,,„:„. ,. , \ .„,, ,,.„,„„,. ,, ,.,„...,,,,,,„,„„,.,„„,..,,,,. .. .,,A4 iit ,-'1•-- ;.---4,, ,•4) it m D O n '� �L.....,--- — r 4 1 �p e� �I;1 q k3F,; a `. '�i� a�� ll i Y « X414to �,�- a._-.yam r � P V A V•��ir _�, âi;j V 1i - 0 m c CO o m -.. .. .....—, R: t Q A O A fl N - N O STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4,2016 rr.. 4.1.3 Quantity Compliance Summary Collectively,the BMPs noted above address provide attenuation on a per-BMP, or envisioned outfall,basis for master planning purposes as shown in the table below for the 2-year and 10- year storm events. The 10o-year storm is managed to the extent practicable. Minor outfalls or alterations in land plan based on design may be addressed under individual package or phase submittals. Table 5 Quantity Compliance Summary Existing Peak Discharge Proposed Peak NODE (cfs) Discharge (cfs) Percent Difference ID 2-YR 10-YR 100-YR 2-YR 10-YR 100-YR 2-YR 10-YR 100-YR L-2 8.25 26.25 69.07 7.68 25.47 117.95 -6.9% -3.0% 70.8% L-3 2.12 9.16 27.48 1.84 7.77 52.26 -13.2% -15.2% 90.2% S-1 9.60 28.04 180.16 2.01 27.75 202.08 -79.1% -1.0% 12.2% S-2 2.23 8.53 24.69 2.10 8.09 87.81 -5.8% -5.2% 255.7% S-3 3.70 11.68 31.07 3.64 11.60 43.68 -1.6% -0.7% 40.6% S-4 4.32 13.03 33.71 3.83 12.74 41.41 -11.3% -2.2% 22.8% S-5A 8.14 21.44 51.09 6.02 16.86 33.75 -26.0% -21.4% -33.9% S-5B 1.66 6.59 19.78 1.63 6.39 27.62 -1.8% -3.0% 39.6% S-6 4.75 16.16 44.49 4.48 7.93 14.40 -5.7% -50.9% -67.6% S-6A 4.80 13.22 32.24 3.71 12.79 51.41 -22.7% -3.3% 59.5% S-7 7.97 19.38 44.06 7.53 19.12 44.12 -5.5% -1.3% 0.1% S-7A 2.68 6.40 14.56 2.64 5.48 15.23 -1.5% -14.4% 4.6% S-12 1.25 3.83 9.93 1.14 3.41 24.98 -8.8% -11.0% 151.6% S-13 9.28 27.36 71.94 1.75 26.15 58.86 -81.1% -4.4% -18.2% 4.2 VSMP WATER QUALITY COMPLIANCE The document addresses water quality compliance under Part IIC across the entire proposed project. Accordingly, this report addresses the ability for the project to meet water quality on- site and additional measures required(i.e. Lickinghole Regional Basin), as necessary. 4.2.1 Overall Project Removal Requirements Based on the land use numbers provided, impervious area estimates for the Master Plan were established to assess overall project pollutant removal requirements. Pre-existing ZMA Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.25 STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4,2016 impervious percentages were relatively low, approximated at roughly 5 acres(2%)for planning purposes,or below the average watershed condition impervious percentage of 16%. Based on most recent calculations, approximately 43% (or 103.12 acres) of the 237.35 acre planning area is encompassed in proposed impervious area considering the layout of roads and approximated impervious areas noted within Table 1. Carrying forward the design parameters noted above for the ZMA property under VSMP compliance at 16%average land cover condition, Stantec estimates that the average pollutant removal requirement under Situation 2 equates to 133.68 lbs TP/yr.The worksheet calculations are included in Appendix C and are summarized below in Table 6. Table 6 Project Removal Requirements Planning Est, Ex Ex Prep Prop Prep Avg.Land Average t o Redux Remov. Req'd Areae Imp Imp. P Load Imp Imp Load Cover Watershed from Ex' VSWMH ', Rate Pollutant (ac) {ac) (°%) (Ib/yr) (ac) (%) (Ib TP/yrj (%)3 (lb TP/yr) (lhi TP/yr) (Sit.) (Ib=TP/yr) Rem. Eff. A lemfiog `' :. IP°5t "st Iwotershed ) . .r RR EFF 237.35 5.00 2 37.32 1 03.i 2 43 238.66 16 1 @4.98 33.59 2 133.68 56.01 `Water Quality performance based calculations using Simple Method based on planning area for ZMA Boundary. 2 Wafer Quality Calculations remain conservative. Proposed Roadway has been calculated with land bays estimated at 65%it 3 Average Land Cover Condition has been set at 16%. 4.2.2 Proposed Water Quality Treatment Facilities The facilities noted on the Proposed SWM Facility Layout(Figure 8) are discussed in more detail below,primarily in relation to water quality compliance but also regarding the overall manner in which they serve the development from practical perspectives. It should be noted that only areas on-site are shown to be treated, or credited,in overall water quality compliance numbers, regardless of the facilities location or acceptance of off-site drainage. Note the BMP facility structures have been conceptualized for water quality purposes as well as for 2-year and 10-year storm flow attenuation. Additionally, one hundred year flows have been routed to preliminarily forecast measures required; however, additional detailed routings will be provided at detailed design. Prior approved facilities are included for the sake of overall Master Plan accounting. Revisions to existing facilities are proposed only to address conceptual water quantity compliance based on information to date and to identify these potential issues. Should these not be necessary upon final design,the upgrades may be eliminated,while the water quality benefits and tracking may remain in relation to ZMA-wide compliance. Given many of the micro-scale of the bioretention features, minimal berm freeboard is anticipated in larger design events such as the loo-year storm.Additionally, any bulk storage routing through bioretention previously provided has been revised to include bioretention footprints with bypass or offline ability,where applicable. Reduction in footprints shown here are intended to aid in future buffer encroachment reduction in detailed design. These areas remain conservative, and may be reduced further. While these pre-treatment and bypass features are shown in concept at this time,they represent a general nature of the proposed solution and may be altered and optimized during detailed design. Optimization and minimization of media footprints may be n Stantec 4.26 to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4, 2016 Natio performed at the design phase. Based on discussions over time and prior comments, please note the following general items: • Many of the development areas have no updated development concepts,layout, or grading. As such,the facilities in these areas may be more conceptual. While lot conflicts have been attempted to be avoided or updated,this document does not represent a planning effort for the development components Residual lot conflict has been attempted to be noted as both required sacrifice of lots and/or adjustment that is required of the final land plan/design in the future. • Maintenance Access has been considered to the extent conceptually practicable at this time. Further evaluation of access will need to be considered and included at detailed design of the development and subsequent stormwater management facilities. • Drainage along the periphery of many blocks has been reviewed. At this time,much of the areas appear feasible to be picked via grassed channel or pipe networks and this has been planned for the extent practicable at this time. As such,moderate changes have been made to post-development drainage areas based on prior comment; however, upgrades to facilities, including the splitting into smaller facilities to maximize capture, may be explored as more land plan information become available and stormwater management features are integrated. Regardless,based on our current review it does not appear problematic to meet the 75%minimum capture guidance provided by Albemarle County. While the pre-development to post-development comparisons note herein are representative,much more detailed sub drainage area analyses may be warranted and provided with detailed design plans,particularly to demonstrate channel adequacy. 4.2.2.1 L-1 Facility L-1(Figure 9), or Upper Ballard Pond,is an existing Retention Basin II (4*WQv)that provides both water quality and quantity control as previously approved under the Upper Ballard Field Pond Conversion Plan(RGA,Approved 8-26-2014). It remains a core component to the overall SWM strategy per the GDP Sheet 3 plan,treating post-development drainage area 51B-2 under this plan. The facility is included for completeness to understand its approximate relationship to overall site compliance. No changes are proposed to L-1. 4.2.2.2 L-2 The existing facility L-2(Figure ro)was reviewed for ultimate water quality benefit. It should be noted that preliminary modeling suggests that this facility provides significant water quantity and quality control; however, preliminary modeling under the final layout condition upstream suggests that the facility may not meet attenuation of the 10-year storm event. To aid in overall water quality compliance and to provide heightened attenuation,the facility has been conceptualized as a wet pond with an aquatic bench. Given the surrounding slopes the bench may provide an added safety measure as a shared safety bench. As such,the proposed facility yields a higher pollutant removal efficiency of 65%. Note that the current concept has an embankment height slightly greater than 25 feet;however,the impoundment volume is less Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.27 STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4, 2016 Now.- than 15 acre-feet at the top of dam. As such,the proposed facility would not be subject to Virginia Dam Safety regulations. Should additional volume be required,the dam would be reconfigured to maintain unregulated status. Based on current drainage patterns and proposed future alterations, L-2 is proposed to treat post-development drainage area ioB. The manner in which drainage is forced to the facility maybe evaluated during detailed design. To accomplish this, several improvements are proposed,including: • Re-grading of the facility floor, • Raising the berm height, • Installing a new riser structure, • Adding an emergency spillway(maybe optimized through primary spillway in detailed design), • Installing a safety/ aquatic bench, and • Redefining sediment forebays and entry pipes. • Provide for an approximate maximum ponding normal pool depth of ten feet,with an average facility depth of an estimated average depth of less than five feet. 4.2.2.3 L-3 The proposed bioretention facility L-3 (Figure ii)is proposed to treat significant portions of blocks 10 and 16 and provides water quantity and quality control to outfall 3. L-3 is proposed to treat post-development drainage area iiB. The facility may serve to transition the future Greenway Trail from the development down to the existing stream buffer. Currently,the bioretention facility is broken into three smaller cells that flank a high flow channel,to emulate a natural stream system. The high flow channel will be constructed with low flow diversions to direct first flush flows into the bioretention"floodplain"benches. Once the initial treatment flow level is exceeded bypass will be provided by the channel,which is envisioned to be constructed and planted to emulate a natural channel section. To meet development goals and optimize treatment,the facility will include approximately 6,423 s.f. of bioretention footprint, exceeding the 2.5%bioretention sizing requirements for 50% pollutant removal efficiency. The proposed facility will also include a stormwater detention area downstream of the bioretention area,which is embedded amongst the development grades,to provide water quantity control.An optimized design will be provided under WPO submittal so that trail,maintenance access,development grades, and aesthetics can be thoroughly considered. A substantial planting component is also envisioned. N Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.28 — / ` ; E ) ' 7377co O0NO OmOC / n mm ' H • cntn r � x73 0 0 ' G DDOo › - -I' -I c � E � z -1 / - /� �, � c rDDv � DDm c —I (/) o i' ' / _ji' E, ;� Er � 00m mzrz '0 / D I ;, �% ;! -��-- - I '7 �F E • / D 0 jn) rm -I D rn `e n V 4 F, , O -; < (� O `:/ z V 14 mw o e Z / F j m Fr O II •� / 0 < Q r m t "� ,7- ., // I � r Q p o N 0 m O N r r Q \ -I I iPi • • * / i Zp - � ,,,. o < . 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AAN cn D> n rn cmi� p 0 -1D p p X -ID 0 r Z xi x w oowco n 0 ,ZQj D D n Dr 0 c`5 (-^) m is, t O -( r HH 0 m p F > Q 0 K -i > m corn 0 D Cu -< D H m U) OJ c C -I m m r c D Z 0 m m c W m O cn c1, -J 0 p II m T C n AH8 ooA TI rrn 0 m mzz 0 —1 m W m N IT co cn , �1 z Q N N w X m 0 m STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4,2016 4.2.2.4 S-1 Facility S-1, shown on Figure 12, is an existing retention basin that was previously envisioned to be converted to a shared water quality and quantity facility for the benefit of local waters. It was noted by County staff that the pond has previously been refurbished and that it should be left alone. No issues with respect to water quality treatment have been noted with the facility, rather the upgrades were proposed primarily to address aggressive upstream water quantity peak shaving under the prior nodal approach and the additional detention would provide enhanced water quality treatment. We have reviewed the available data, and have updated the conceptual plan in this area to remain as sensitive as possible in light of County comment. Additional disturbance of the embankment would appear preferable to all parties, if possible. Note that pre-development flows at the S-1 outfall were based on an understood outlet structure configuration with a 24-inch riser crest at the current normal pool 689.5o ft with an 18 inch diameter barrel. The existing,or refurbished outlet structure, is understood to have an approximate configuration of a 5-feet diameter riser(approximate SWM-1)with a crest of 691.54 feet,two 6-inch orifices at 689.5o feet, and a 15-inch barrel. Both configurations are understood to include a consistent emergency spillway section with a crest of approximately 691 feet. Upon further review, Stantec assessed the ability of the existing riser structure configuration to maintain peak flow attenuation for the 2-year and 10-year storm events. It would appear that the 10-year storm events would cause an increase in the 10-year storm; however,with a minor increase in emergency spillway crest elevation(raise from—691 feet to 691.40 feet)it appears that substantive attenuation may be met. Accordingly,this concept is proposed, although based on this relatively small difference in spillway height and lack of recent data we recommend that the existing S-1 spillway structure be verified through survey in detailed design to further assess if this action is necessary. If results of further study and survey suggest this strategy is infeasible, additional detention prior to discharge into the water quality facility S-1 may be explored, as necessary within Block 32. Preliminary routings suggest that increases in design storm flood elevations do not affect properties not under the control of the client. It should be noted that much of the existing data is preliminary. As such,further review and documentation may be required at detailed design. 4.2.2.5 S-2 Facility S-2 represents an important water quality and quantity structure, Retention Basin III (65%removal),which has recently gained approval for construction to primarily serve blocks 12 and 15. See Figure 13 for a representation,though final approved documents should be referenced for more detail. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.32 m , , -- , . ---- - ---.-- . 'Amor' -_______ - _______>-<..".••... . -- 12,,-/„. /,,, -"r: ----/-i . M / 77/ li 0 / /--------------'- -I..--------- > ______,-- -I i 2 ...------- ----- - Z / > _ MI __ ) / --/-' -0 0 (..> -.) 8 0 , ) ---- 0 x 4,. ..-----<:- .. - E E 6 > I 13 m //7---- / a > .-/ ..• : :: : • .'7'.-:-* :: H:H•::H : : s* '/ / ' 5 M k / ,------;,...4009' ! ' ,,..77 7 / / • . . : I m ..,0". a / 2 M ) ____---- _.„--- / / ----.....- • • : 2 0 .--7 / / ,...--- 0 o . . .. .' //// /* .. 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Z * T) H li qi 17) H X M (1) a) .r H007/ 0 0 0 z0 z M 0 X a) cn cn 0 L< 0 21 m 0 m z m m m - 0 , 0 0 0 cn z 8 7, 0) > 0 0 zi co c =1 co 0 H 30-I -1 0 0 00, op C,1) >cP, " 'x' ' (i) M -I o -1 m ° • a' -n m l'-' -o 5 0 0 > > m ---I cn > Li Z cn co > x 0 r Cn M Ci) C H Ci) c„ < ....a) 8 OD --. 1\3 c) --, '-'. m © -,1 (A) •11. 0 c:, -'s 1 0 rn z -n r7 c -n 5 - _1 ; a] -0 x —1 m 0 m ea FT<, E° -.1 g ci) - 2 a -I • 7:1 > Cr) 30 Z X 0 R3 -n 8 STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4,2016 4.2.2.6 S-3 Bioretention cell S-3 is proposed to treat areas of blocks 16, 17, and 18 currently under design development(Figure 14).The drainage area served is 12B under this Master Plan.The area is important for several reasons,which include the need for adequate flow transitions to the adjacent existing pond and ultimate flow attenuation.Additionally,given its location along the roadway, aesthetics will be a consideration.To provide heighted water quality treatment and attenuation,the facility will incorporate an 8,941 s.f.bioretention area along with a downstream detention area. The bioretention sizing criteria are based on 2.5% (0.5 inch capture)of the contributing impervious area.Accordingly,the facility is noted with a removal efficiency of 50%. As with most bioretention cells proposed,pretreatment will be provided by a cell prior to transition into the media basin floor. 4.2.2.7 S-12 Similarly,bioretention cell S-12 is proposed to treat areas of blocks 17, 18, and 31 currently under design development(Figure 15).The drainage area served is 15B-1 and 41B-1 under this plan.To provide heighted detention and water quality treatment,the facility will incorporate a sizeable offline bioretention facility footprint of 5,288 s.f. to aid in water quality compliance. A downstream detention area will be provided to address water quantity compliance.The bioretention sizing criteria are based on 2.5% (0.5 inch capture) of the contributing impervious area.Accordingly,the facility is noted with a removal efficiency of 50%.As with most bioretention cells proposed, pretreatment will be provided by a pretreatment cell prior to transition into the media basin floor and high flow bypass will be incorporated into the final design via weir structures,box bypass structures,or similar methods. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.35 / i f •%,0 \ / \ / eD / '\ \ \ / / i / v \ I / \• • / / /// / \_ "Y / li „�A W " �\ o m /� / / 670 \ \ 0 W y/ i '/ �`, o / A Frill= O se cn 8 m 00 et, /r— \\14000....,..._. / cnm0m � , ` rn . N. con, r C7 Z it ��► ♦O�� i cn � � '� / �•7Albai:t m J WY A v �1 11-6 11' ".-.0 ♦ / \ \ g /,,,,__________ \ \ \ m / /cb tilr !e'-=tl�1h11� . 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S-4, shown on Figure 16, is proposed to treat block 22, which is represented under post- development drainage area 27B-1. This facility is proposed as a 50% (0.5 inch capture, 2.5% sizing)bioretention cell of 4,63o s.f. (oversized from a sizing target of 4,12o s.f.)along with a downstream dry detention component. Considerable planting is envisioned in this area to reforest buffer encroachment and likely re-establish non-forested stream buffer following grading operations. Conceptual planting areas in this regard have been noted on the concept plan for S-4 as a representative approach of how buffer encroachment maybe mitigated through planting. S-5A is proposed to address treatment for portions of blocks 5, 6, 20, and 26 which are represented under drainage areas 51B-1 and 18B-1A. Although revised land plans are required in this area to facilitate final planning, it is proposed that the majority of proposed drainage be directed to a landscaped pretreatment forebay,which is then directed to a bioretention cell along the proposed road periphery. The facility will incorporate a high flow bypass channel leading to a small detention cell.The facility requires encroachment into the existing stream buffer, many of which may be reforested,for berm construction and to adequately transition outfall flows. This facility is proposed as a 50% (0.5 inch capture, 2.5%sizing)bioretention cell of 5,542 s.f.,which will be bypassed in times of high flow and directly forwarded to the downstream detention area. The facilities have been incorporated with anticipated development grades to limit concerns over embankment slopes from prior discussions. Note that pre- development drainage areas include portions that have been approved to be directed to facility L-1 and portions of the Golf Drive roadway drainage currently outfall in this area. These areas have been considered for inclusion in the pre/post comparative analysis. See Figure 17 for more detail. S-5B is proposed to address treatment for portions of blocks 20 and 21, which are represented under drainage area 188. The facility will need to be integrated into the final layout design as areas previously identified as buildings and parking will need to be repurposed. As these planning efforts have not commenced,we note this currently See Figure 17 for more detail. This facility is proposed as a 50% (0.5 inch capture, 2.5%sizing)bioretention cell of 2,894 s.f. and includes the ability to bypass to a downstream detention area that allows for maintenance access and pedestrian Greenway Trail integration. 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See Figure i8. S-6 is proposed to address treatment for limited portions of blocks 24 and 25, and has been conceptualized as a small,bioretention cell making use of a swale transition to direct limited flows to the facility. No additional water quantity control measures are expected to be necessary at this time. S-6 is proposed to treat post-development drainage area 23B-1. This facility is proposed as a 50% (0.5 inch capture, 2.5%sizing)bioretention cell and has been sized at approximately 1,244 s.f. (greater than the 991 s.f. required for water quality control)for quantity purposes. Much of the post-development flow in this area is anticipated to be addressed by facility S-6A; however, a more refined land plan will need to be reviewed in the future when these efforts commence. S-13 represents a water quality enhancement that seeks to maximize treated. The floodplain is an important consideration in this area. Accordingly, planning efforts do encroach into existing stream buffer out of some necessity to provide drainage into the floodplain areas while avoiding encroachment into floodplain with proposed berms,which have been reduced from bulk planning efforts prior. Currently,it is anticipated that upstream and swale drainage networks may be designed to flow into one large bioretention area with the ability to bypass the large majority of high flows directed a this facility. It is also possible that several more discrete bioretention areas be implemented in a similar fashion once more detailed land planning efforts commence and these facilities may be integrated. It is also envisioned that the future Greenway Trail may be sensibly incorporated into this feature,furthering its effectiveness in dual use for stream buffer encroachments. Should minor berm fill be required on the periphery of the floodplain to accommodate the berm and Greenway Trail, documentation may be provided that support no net rise in floodplain elevations. Once again, a significant landscaping component is anticipated post-construction to offset buffer encroachment and enhance the design. Currently, this facility is proposed as a 50% (0.5 inch capture, 2.5%sizing)bioretention facility although detailed design review will be required to assess the feasibility of final bioretention footprinting. U. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.41 y', �\ „ � '' 1 . 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Per the original GDP and in keeping with County capture guidance,this facility is sited to accept substantial portions of this development area through piping and grading improvements and treat incidental upstream areas, including areas likely to be conserved along the Route 250 corridor. Future land planning and design efforts will be required to adequately plan this facility; however,at present this facility has been oversized to accommodate significant impervious areas and high levels of flow attenuation,which includes footprinting to provide 50%removal efficiency. Note that this facility has been broken into multiple bioretention cells conceptually based on County comments on Master Plan version two. Current drainage area assumptions include the direction of the majority of this development area to this location, including pre-development areas from the S-6 area. Optimization of this facility and its final location will need to take place under more detailed planning efforts such that roadway and lot changes maybe integrated appropriately. Similarly, S-7(Figure 19)will provide for substantial water quality treatment for the southern portion of proposed block 19. The exact configuration may require alteration based on final design; however,this facility is envisioned as a series of cells that offer combined pre-treatment, treatment, and detention of design storms. Currently the outfall for S-7 is shown tying in to the existing roadway network for Old Trail Drive. This is requires further evaluation under design. Also note that this 5o%bioretention facility network is assumed to accept some development from the pre-development S-7A area. 4.2.2.11 S-7A Bioretention facility northern portion of proposed Block 19 (Figure 20), and is envisioned to treat roughly 1.81 acres of proposed residential area.The facility is composed of a single 50% efficient bioretention cell with a small detention area downstream. The ample pretreatment cell anticipated upstream and limited drainage area of the facility appears to allow for an online cell in this instance. ta ntec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.43 <O / F m r m Noir / W i v p mn � � �� O \ / " 2 / r l O / \ \ �\ \\. 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These are estimates and it should be noted that the backside of some lots and incidental impervious areas will remain uncaptured. • Treatment of the majority, or greater than 90%, of the red and blue areas noted under County guidance. Note most of the blue areas appear to be feasibly captured at present, which would provide some flexibility in final layout drainage design and still meet guidance. Based on the above evaluation of what we believe represents the estimated maximized on-site treatment, approximately 121.93 lbs TP/year(or approximately 91%of the compliance target)of the 133.68 lbs TP/year removal required may be met through the noted facilities and their associated conceptual efficiencies,leaving only roughly 11.74 lbs TP/year required through off- site compliance options. As has been discussed over time, it may be that site constraints and planning considerations hamper this ability to achieve on-site compliance. Unknown issues at this time might include geotechnical or other physical constraints related to the development. As such,final treatment may be further reduced. Accordingly,this represents the Master Plan target treatment and/or capture area, or upper target. Less capture as defined under Section 1.1.5 would also constitute compliance. See Appendix D for Bluebook water quality compliance worksheets for the development under the master plan target and the minimum estimated required drainage area scenarios. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.46 161 * * * * * —I * * A CO > N a TyD _ Q Q Q C- oa D CD Q 00 a a 0m • = 7 (D(D C T CO SD J 7 W 0 N Ei p C Q Q .• V=Q C rsa Q m H iN NNn n QQ 0CrO Q CA N CA N Cn N r r r '< <O n Q QD JJ Ww N D V D T D A W N -, Ca N .. a7 co n O >. 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V v N ACD a Do ° Q 00 Q n -- ON A CO w w Os sO CO W 0 A N) CilNQ CS w m w n A N N NV v O Q O a O 7 • p7 a C. cH c 7 Q_ (D a a CD O __ W a .-� 3 3 3 3 o ^ * c _ Q _ Q D Q CD O 0. V Os Os Os Os Os a a. Os Os Os Os Os cn D N CD 0 f . N (D k 7 0 (n V CA Cn CA (!, Cn CA Cn CI, V V, CP '-O N A a N O' Q D o is x 0 0 0 0 0 0 0 0 0 0 •V -. Q < a "_' Q 3 O Q a O Q a co C 3 a - ° 3 O O c0 x A 90 a m N a o a = co 3 ^ CD 0 _ _ _ CT O Q cD 0' 3 .0 7 0 O A (o U O� O O O A (O N O, N P O a p O W • CD v O 'O N A 0 w VI 'O w CO coVG) a (Ci, C 7 N vC ' m O Q c .-., co a Q .. x C 3 m x CD O 0 N 5 a 3 3 O m -0 (.7, 7 = QCn N-• 3 C O C C CD 3 O O O O O CO O O O O T O O en' O C-• -. 3 _ ° o 0 Q0np 2Q .......< Nv O c.,- = J Q 6 0 n 0 77 .7 rn 0 W -O ^ c < CD a C V A A sO w N A A A CO Os W _i CD v _ O O `O w N 00 O N CM N Cb a -. o, A 3 A D '.o w < 0 7, -o O a N O 71 7, Q - �' - j a 3 (D - -« .O C. v -0. N c n D Co cn 0 Q n w N O O 5- 5- a (D Q Vf v, (A) ;O 7 A OO O' N A A �` N w ^ 9 V COco 00 V CO -. '0 N N VV O- IV w V V v O. O OA CO A CO 0 V CO A -I. 7 G -n 0 O < O --. -aQ n xi c D . 7 . - V A C., sO w N A Cn o, Q Q_ 7 •- W N a O OcriM P U COCCO A Q v 3 Q. 0 ( 7 O Q ( a Q O STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4, 2016 4.3 REVIEW OF PROPOSED TREATMENT PER COUNTY GUIDANCE As noted above,guidance was provided in November of 2014 by Albemarle County regarding the advancement of SWM Master Plan strategy. The following represents a summary of the approach the development team proposes to meet guidance. 4.3.1 Albemarle County Capture Definition For purposes of more specifically defining the capture areas under County guidance(Figure 5 under Section 1.1.4 above), Stantec estimated the total red and blue under version one of the SWM Master Plan. For sake of documentation and summary Figure 22 and Table 8 below graphically identifies and quantifies these areas,respectively. Figure 22 Albemarle County Treatment Area Identification I11 # 11 _1 ii \ `` 1.....t 4,4 , • • S,`� �\ \ L3 t • • \ ,-.', .v-, .....rtcy I . . a)y i i 4 i 1 �i ..,,,, 4M _,_ , ... , , , . , s,_ - ... _t _ _,,...„ ., t 1 ... , _ , . .4 \r r ' 1 � ; - jr. _ 1. , - r , i lk '* to • i .w 1 I. OFF-SITE i,' _AS SNOT INCLUDED IN WATER QUALITY - ;;,.. ar" 0►15.. COMPLIANCE C A LCULATIO";S ""` 1 711A Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.48 STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4,2016 Table 8 Albemarle County Area Quantification Summary Capture Estimates from Version 1 of SWM Master Plan Area Area ID'd by 75%Capture Area AC Estimates Difference ID ac ac ac 14 50.30 37.73 12.58 3 4.90 3.68 1.23 4 15.80 11.85 3.95 5 10.70 8.03 2.68 6 7.40 5.55 1.85 7 2.70 2.03 0.68 8 5.40 4.05 1.35 9 4.40 3.30 1.10 10 27.60 20.70 6.90 11 9.20 6.90 2.30 12 10.90 8.18 2.73 Total 149.30 111.98 37.33 Capture Estimates - Red Area Area ID'd by 75%Capture Area AC Estimates Difference ID ac ac ac All Red 55.60 41.70 13.90 Capture Estimates - Blue Area Area ID'd by 75%Capture Area AC Estimates Difference ID ac ac ac All Blue 93.70 70.28 23.43 4.3.2 Response to Original Guidance & Understood Resolution The following is provided to address specific points of guidance. The Albemarle County point of guidance is noted below in italics while the response by the development team is provided in regular text. 1.o AC: Those areas that show onsite stormwater management(red on the illustration) must have facilities to capture at least 75%of this drainage area. That said, they do not need to be biofilters as indicated on Timmons plan, but could be another approved type of stormwater management facility. 4446.. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.49 STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4, 2016 '44610.- Response: Our calculations indicate that based on the current maximized plan the development may reach greater than go%capture of the areas noted in red above. Understanding that site constraints and final layout may affect this final capture,the development team remains confident that the minimum estimated required drainage area under Table 7(understood to be in agreement based on the March 17, 2016 meeting) may collectively be captured on-site. 2.0 Those areas that were shown as draining to the ponds within the approved zoning plan(blue on that illustration), but were not constructed to drain to them or have other reasons the ponds cannot be used, issues, must either have the storm sewer constructed so they correctly drain to the ponds as shown or may have other facilities within Old Trail to treat that runoff before it enters the stream. The same as the areas in red, we expect no less than 75%of the drainage area to be captured onsite for treatment. Obviously, if this requires you to propose changes to the approved layout plan, this will likely require either a variation to the plan or a zoning amendment. In that case, we should talk with the planners about what changes staff will be comfortable in supporting. Response: Based on the maximized SWM strategy defined and the supporting information above,the majority of blue area may be captured. See Table 7. 3.o Those areas that were shown as not having facilities to address the runoff(white on this plan)are allowed to have flow directly into the stream, but the water quality impacts must still be addressed. For all the areas not being routed through BMPs (this area plus uncaptured areas with the red and blue areas), a 30%reduction credit can be provided by Lickinghole. Response: While some portions of development shown in white will remain untreated, the plan addresses these areas aggressively through water quality and quantity measures. This is in an effort to both maximize the proposed treatment by the development and reduce the downstream burden by Lickinghole Regional Basin. See Section 1.1.5 and Table 7 for summary of understood compliance resolution between the development team and Albemarle County. 4.o With the above, our initial analysis indicates it should be possible to meet the phosphorus reduction required with the VSMP that apply to this property(2C). If it turns out the onsite facilities and the Lickinghole credit cannot meet the entire pollutant reduction, we will consider using the offsite ponds to address the incremental difference. That would need to be verified as genuinely providing a reduction above and beyond[sic]what was previously approved. Response: With the facilities proposed the development team feels the spirit of guidance and water quality treatment may be reasonably met through measures proposed to the minimum collective capture areas defined under Table 7. We do not currently propose usage of off-site facilities not identified under the GDP. These facility modifications are envisioned to increase design storm detention and enhance the pollutant removal potential. Further discussion of final disposition of SWM management strategy is discussed below in Section 4.3.3 below. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.50 STORMWATER MASTER PLAN UPDATE Proposed SWM Strategy April 4, 2016 5.o Finally, if there is an interest in using any of the County owned property for a stormwater facility,please recognize this will need to involve a number of people outside of Community Development. This will require additional time and effort to coordinate with Parks and Recreation, the County Attorney's Office, the County Executive's office and ultimately the Board of Supervisors to assure the County's interest in the property is being protected. Past experience suggests it will likely take 4-6 months to finalize an easement agreement with the County once everyone has agreed upon the recommendation. Response: In context of the GDP Sheet 3 Plan,facilities S-1, L-1, and the existing County pond noted here are considered"on-site"ponds in terms of applicability to SWM plan and zoning compliance. Use of the existing County facility as noted is not proposed at this time;however,the developer will provide updates to Albemarle County regarding any advancement of discussions or agreements for additional benefits identified with Parks and Recreation or others that are unknown at this time. 4.3.3 Proposed Submittal, Review, and Tracking Process Supplemental to the requirements of the WPO submittal, review,and approval process,we understand the following will serve as a helpful general outline for Old Trail Village and Albemarle County to gauge overall project compliance in accordance with the requirements detailed under Section 1.1.5. 1) The minimum 92% Bioretention area(51.15 acres) capture per Section 1.1.5 will be met. 2) The total maximum Master Plan target, or 92.58 acres across the project to Retention Basins as defined herein will be captured on aggregate. 3) Water Quality Compliance Tracking Ledger will be provided at each WPO submittal. 4) Additionally,project review checklists,provided to streamline review and conformance with guidance, master plan,and VSMP requirements at each detailed WPO submittal. See draft checklist under Appendix D. Checklists may be updated to provide better clarity over time as the process is refined. 5) Water Quantity compliance is intended to be provided to show attenuation per outfall for the 2-year and 10-year storm events. 6) Detailed stream buffer impacts quantification and mitigation design will be provided at each WPO submittal. Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 4.51 STORMWATER MASTER PLAN UPDATE Stream Buffers Encroachment Summary April 4,2016 5.0 STREAM Consideration was included for the loo-hundred foot stream buffers in accordance with the Albemarle County Water Protection Ordinance(WPO). Certain items have been historically proposed within these buffer areas, such as practical outflow points for SWM facilities or outfall pipes,proposed passive recreation facilities,and infringements of grading of development activities including SWM facilities. These practical infringements have been forecasted conservatively to the extent practicable, are intended to be minimized, and are shown within the first 5o, or landward,feet of stream buffer where applicable. Encroachment into the seaward 5o feet of the buffer will be avoided to the extent practicable unless required for Greenway Trail, outfall adequacy, or similar reasons. It is understood that plantings or off-setting water quality measures will be needed under WPO to address encroachments. At this time, it is envisioned that encroachments maybe addressed through post-construction plantings, and where applicable and practicable. It is our understanding that this will be acceptable per the typical WPO submittal process at detailed design. The encroachments noted below are intended to be worst case at the current planning level and are based on the footprints shown. We strongly believe that through development layout design evolution in tandem with integrated SWM facility planning and optimization,that stream buffer encroachments can be greatly reduced. Further avoidance and minimization of stream buffer encroachment will be pursued during detailed planning and design of these future blocks,but note that it is anticipated that replanting of encroachments in these areas,inclusive of currently non-forested areas,will be part of the overall strategy. Table 9 Conceptual Buffer Encroachment Summary 100 ft Buffer Facility ID Encroachment - Landward 50 feet(SF) L-1 2,020 L-2 10,070 L-3 21,488 S-1 - S-2 18,999 S-3 - S-4 17,731 S-5A&S-5B 67,029 S-6 5,112 S-6A 751 S-7 8,973 Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 5.52 STORMWATER MASTER PLAN UPDATE Project Summary and Requests April 4,2016 S-7A 3,364 S-12 35,433 S-13 44,496 Total 235,466 Note: It is the intention of Old Trail to further minimize these encroachments under detailed design, and it is our understanding that it will be acceptable to provide more detail per the typical WPO submittal process at detailed design. 6.0 PROJECT SUMMARY AND REQUESTS Several stormwater design alternatives have been reviewed during this Master Plan refinement process, including numerous options on facility types,locations, and layouts. The preferred alternative noted herein is a culmination of many iterative planning efforts and discussions with Albemarle County. The development team is extremely appreciative of the effort on behalf of County staff and willingness to coordinate schedules, embrace discussion, and share goals as we navigate to a comprehensive solution together that will ultimately benefit all involved. Based on our current water quality and quantity compliance estimates and the project history to date, Stantec respectfully requests Albemarle County review of this SWM Master Plan and concur with the overall strategy for implementation at a higher detail level through upcoming plan submittals. It is the intent of the development team to provide comprehensive detailed design submittals under the WPO process consistent with Section 4.3.3. Stantec ntec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx 6.53 STORMWATER MASTER PLAN UPDATE Appendix A RELEVANT CORRESPONDENCE April 4, 2016 Appendix A RELEVANT RESP N Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx A.1 h,o�c witwasNtokie COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road,Room 227 Charlottesville,Virginia 22902-4596 Phone(434)296-5832 Fax(434)972-4126 6Aug 201S Mr.David A.Brockman 1005 Heathercroft Suite 100 Crozet,VA 22932 Mr. Glenn Muckley,P.E. Senior Engineer/Stantec 5209 Center Street Williamsburg,VA 23188-2680 Mr.Bill Ledbetter,L.S. Roudabush, Gale&Assoc.,Inc. 914 Monticello Road Charlottesville,VA 22902 Dear Sirs; This letter addresses applicability of design criteria at Old Trail relative to 9VAC25-870-47. Applicant and County have discussed technical criteria that apply to Old Trail for months. Scott Collins,Collins Engineering,has submitted material prepared under his direction contemporaneous with ZMA2004-00024(Old Trail). Items submitted August 4-5,2015 include: plans titled Old Trail Village Stormwater Management Plan with initial or revised dates: 06/24/05; 08/09/05; 11/15/05; 01/11/06; and.PDF routings for bio-filters(bio-filter#4,#5,#7). Routings include 2-, 10-, 100-yr events using Modified Rational Method. We are trying very hard to interpret the new data in your favor with regard to permit continuation under 9VAC25-870-47 B and using the Part IIC Technical Criteria. While the computations and plans submitted by Collins are not all dated,those that are predate the VPDES general construction permit re-issuance on August 15,2014,and while acreage specified on the permit does not cover August 6,2015 Page 2 of 3 the entire acreage,and the plans have no County approval,we are still willing to work with them, as they are the best evidence we have received to date. Similarly,while the computations and plans do not appear to specifically provide calculations for required pollutant loadings and removals,a general process of technology-based criteria per Chapter 2 of the Virginia Stormwater Management Handbook,effective at the time,appears as though it may have been followed. Given these findings,and the evident fact,that DEQ was not in the practice of checking any plans or calculations on any permit,these would appear to suffice for demonstrating continuing coverage under the DEQ guidance. Stated in relation to the regulation and guidance,Material submitted by Scott Collins substantiates 9VAC25-870-47,Applicability of other laws and regulations; time limits on applicability of approved design criteria,B.,applies to unbuilt portions of Old Trail Village Development. Plans, design data,and runoff routings include"description of,and necessary calculations supporting,all development-wide post-construction stormwater management measures that are to be installed prior to the completion of construction ..."(Ref.DEQ Guidance Memo No. 14-2014). County staff(John Anderson)has sent email to David Brockman,August 4-5,2015,on this topic. Part II C technical criteria will apply to all unbuilt portions of Old Trail Village_development through 2024, or two permit cycles(Ref. 9VAC25-870-47)provided the Construction General Permit(VAR100043)issued to Old Trail is renewed prior to expiration. The general permit will expire on June 30,2019. Please see Coverage Letter dated August 15,2014 sent to March Mountain Properties LLC from Frederick K. Cunningham,Director, Office of Water Permits, DEQ. Letter reads in part: "The conditions of the general permit require that you submit a new registration statement at least 90 days prior to that date if you wish to continue coverage under the general permit,unless permission for a later date has been granted by the Board. Permission cannot be granted to submit the registration statement after the expiration date of the general permit." Note: General Permit renewal is vital to Part II C eligibility: 1-Jul2019—June 30,2024. Please accept this letter as assurance that Part II C technical criteria applies to the entire Old Trail development. This letter does not address or deal with plan review,stream buffers,or other issues. This letter addresses 9VAC25-870-47. Significant issues difficult to resolve remain, issues that relate to capture and treatment expectations within any given block at Old Trail Village.As of • this writing,we have not reached agreement on a Stormwater Management Master Plan,but County and Stantec continue to work toward such an agreement. Email sent to David A.Brockman requests Old Trail revise General Permit Registration Statement(VAR100043). Revising the General Permit Registration Statement does not alter Part IIC eligibility or applicability relative to current or future development,to as-yet unbuilt portions of Old Trail,provided current and future development fall within ZMA2004-00024 boundaries. Detailed review is required in context of individual VSMP/WPO applications. SWPPP imposes specific and continuing obligations. Zoning special exceptions are required where proposed water quality BMP facilities depart from Approved ZMA2004-00024,Old Trail Code of Development,or ZMA2004-00024 Application. August 6,2015 Page3of3 9VAC25-870-48,Grandfathering is not discussed as it does not appear to apply to any area within ZMA2004-00024 boundaries. v4166.- ncerely, john • derson,P.E. Civil Engineer II Glenn Brooks,P.E. County Engineer VSMP Program Administrator GEB/ Copy Stantec Memo To: Mr. Mark Graham From: Glenn Muckley Director of Community Development Williamsburg, Virginia Albemarle Count, Virginia mgraham@albemarle.org File: Old Trail SWM Master Plan Update-- Date: March 6, 2015 #203400430 Reference: Old Trail Village Stormwater Management Master Plan Status Update The following is provided as a brief update regarding the most recent progress of the Old Trail Village Stormwater Management (SWM) Master Plan. Executive Summary Based on a very productive meeting on January 29th, 2015 between Albemarle County staff, Dave Brockman (Old Trail), Bill Ledbetter (Roudabush, Gale, and Associates, or RGA), and Glenn Muckley (Stantec), Stantec has pursued the completion of an overall SWM master plan for overall site compliance under Virginia Stormwater Management Program (VSMP) Part IIC technical criteria. It is our intention to provide much greater detail within a SWM master plan document that will serve as guide for future detailed design development on-site for the benefit of Albemarle County, Old Trail, and various consultants-addressing compliance issues including water quality compliance and water quality control on a holistic site basis. At the January 29th meeting, an aggressive effort to submit a SWM Master Plan document for County review by late February/early March was discussed. More recently, detailed planning efforts are underway for portions of the project which are intended to be submitted in the near future. It is our belief that the incorporation of these design components into the master plan would greatly benefit future consistency of plan documents and streamline subsequent County review. As such,we ask for your understanding while we take additional time of approximately thirty days as we coordinate these items within the project team. In the meantime,we are happy to meet in person or via conference call to maintain our productive dialogue at your convenience. January 29th Meeting Summary: 1. Stantec presented a conceptual SWM approach, largely focused on water quality compliance on-site to the maximum extent practicable. The approach is based on developing strategic BMP's of differing type in series, seeking to maximize nutrient removal prior to stormwater leaving the site on a project-wide basis,while taking no credit under the new runoff reduction method. Practices under primary consideration, depending on location and site development context, include grass channels, bioretention facilities, and wet ponds. 2. Stantec noted the primary goal of the master plan effort is to ultimately come to consensus regarding any "above and beyond" treatment that will be provided by the developer. It was noted that our understanding of "above and beyond" has evolved during recent discussions to be defined as effectively 70% (on-site to the extent practicable) of the total Design with community in mind mg u:\203400430\02_correspondence\county\mem_swmmp_stat us_20150306.docx Stantec March 6,2015 Mr.Mark Graham Page 2 of 2 Reference: Old Trail Village Stormwater Management Master Plan Status Update project water quality reduction requirement, such that 30%treatment be provided by the Lickinghole Basin Regional BMP per typical pro rata share payments within the watershed. 3. Old Trail prefers to provide more than the minimum water quality treatment required on-site if practicable; however, given the many project constraints, the developer retains the option of finding off-site treatment options in accordance with VSMP regulations should all opportunities be exhausted on-site. Stantec can provide justification under VSMP regulation should that be needed based on final water quality strategy. 4. Water quantity compliance presents additional challenges, but the over-riding philosophy includes a pre-development/ post-development comparative analysis to demonstrate that BMPs work in concert to attenuate flows on-site or leaving the property. While subsequent design submittals will be modified to suit the phase-specific site design requirements, they will remain tailored with general conformance with the master plan to a reasonable degree. Additionally, channel adequacy will be addressed on a per submittal basis. 5. The County was generally accepting of the overall approach, noting that the future detail of the plan discussed would be helpful in technical review and approval moving forward. 6. The County may request Virginia Department of Environmental Quality review, including: a. The use of existing ponds for conversion and use as SWM control practice,which is consistent with the original zoning documents. RGA and Stantec note that this has been acceptable in other instances. b. The potential use of off-site treatment options,which may include nutrient credit trading or off-site BMPs, should on-site options be exhausted. c. Possible clarification on the manner in which Construction General Permit (CGP) coverage updates may be acquired. Stantec noted that this is a relatively straightforward permit modification process based on experience and prior discussions with DEQ related to other projects and/or regulatory panel involvement. 7. The master plan will summarize how minimum levels of capture on-site detailed in the guidance email/sketch from Mark Graham (November 21st, 2014) are met or exceeded. 8. Potential efficiency decreases of some BMPs in series were noted in a follow-up email by John Anderson. Stantec will address this in more detail; however, at this time we do not intend to propose like facilities, particularly wet ponds, in series on-site. Additionally, our approach is consistent with our understanding of the "above and beyond" noted above. STANTEC CONSULTING SERVICES INC. Glenn Muckley Senior Engineer Phone: (757) 220-6869 Fax: (757) 229-4507 glenn.muckley@stantec.com c. Glenn Brooks, John Anderson (Albemarle County), Dave Brockman (Old Trail), and Bill Ledbetter (RGA) Design with community In mind mg u:\203400430\02_correspondence\county\mem_swmmp_stat us_20150306.docx From: Dave Brockman <dave@oldtrailvillage.com> 'Sent: Wednesday, January 21, 2015 4:09 PM To: 'Mark Graham'; 'Bill Ledbetter'; Muckley, Glenn; Boyd, Ron Cc: 'Amelia McCulley'; 'David Benish'; 'Glenn Brooks'; 'Rebecca Ragsdale'; 'Doug Walker'; 'John Anderson' Subject: RE: DRAFT- FOR INTERNAL REVIEW ONLY I'll look forward to seeing you then. Dave Brockman From: Mark Graham [mailto:mgraham@albemarle.org] Sent: Tuesday,January 20, 2015 12:40 PM To: Dave Brockman; 'Bill Ledbetter'; glenn.muckley@stantec.com; ron.boyd@stantec.com Cc:Amelia McCulley; David Benish; Glenn Brooks; Rebecca Ragsdale; Doug Walker; John Anderson Subject: RE: DRAFT- FOR INTERNAL REVIEW ONLY For us, 9 AM on Thursday,Jan 29th works best. Still have conflicts with some folks but this time has the fewest conflicts. From: Dave Brockman [mailto:dave@oldtrailvillage.com] Sent: Friday, January 16, 2015 4:20 PM To: Mark Graham; 'Bill Ledbetter'; glenn.muckley@stantec.com; ron.boyd@stantec.com Cc: Amelia McCulley; David Benish; Glenn Brooks; Rebecca Ragsdale; Doug Walker; John Anderson Subject: RE: DRAFT- FOR INTERNAL REVIEW ONLY HI Mark, Looks like best availability is the January 29 or 30th. Can you let us know the best time and day between those?We are holding each day open for you. I met with Stantec/WEG today and they are making very good progress. Thanks,. Dave Brockman From: Dave Brockman [mailto:dave@oldtrailvillage.com] Sent: Monday, January 12, 2015 9:32 AM To: 'Mark Graham'; 'Bill Ledbetter'; 'glenn.muckley@stantec.com'; 'ron.boyd@stantec.com' Cc: 'Amelia McCulley'; 'David Benish'; 'Glenn Brooks'; 'Rebecca Ragsdale'; 'Doug Walker'; John Anderson (ianderson2@albemarle.orq) Subject: RE: DRAFT- FOR INTERNAL REVIEW ONLY Hi Mark and all, hope all of your Holiday's were good. We have been working with Stantec/WEG on the next progression of a storm water master plan for Old Trail and doing our best to follow your guidance.Thank you again for provide this right before the Holiday break. Our team would be ready to meet and review a preliminary draft prepared by Stantec/WEG that attempts to address these issues and to get your feedback.The first proposal for dates are Jan 29-30 or Feb 2-6. Can you let me know what works with yourself and the relevant players from your side. If they don't work please provide the next earliest for us to ,,.-consider. Thanks and looking forward to getting together. Dave Brockman From: Mark Graham [mailto:mgraham@albemarle.ora] Sent: Friday, November 21, 2014 4:36 PM To: Dave Brockman; Bill Ledbetter; glenn.muckley@stantec.com; ron.boyd@stantec.com Cc: Amelia McCulley; David Benish; Glenn Brooks; Rebecca Ragsdale; Doug Walker. Subject: FW: DRAFT- FOR INTERNAL REVIEW ONLY Dave, To follow up on our Monday meeting, Glenn and I have met with Zoning and Planning to make sure we are staying consistent with the approved zoning plan and stormwater requirements. In doing this,we've tried to set the limit on what is expected and what we can approve. Please do not take this as a negotiation offer, it is not. Instead,we've realistically looked at what is needed to comply with the VSMP and without first amending the current property zoning. From that, I am offering the following as a set of guidelines to follow in development of a stormwater master plan that avoids the need to amend the zoning. To assist in understanding, I have attached Glenn's illustration at the end of this message. The following are what I believe are the important points: • Those areas that show onsite stormwater management( red on the illustration) must have facilities to capture New at least 75%of this drainage area. That said,they do not need to be biofilters as indicated on Timmons plan, but could be another approved type of stormwater management facility. • Those areas that were shown as draining to the ponds within the approved zoning plan (blue on that illustration), but were not constructed to drain to them or have other reasons the ponds cannot be used, issues, must either have the storm sewer constructed so they correctly drain to the ponds as shown or may have other facilities within Old Trail to treat that runoff before it enters the stream.The same as the areas in red, we expect no less than 75%of the drainage area to be captured onsite for treatment. Obviously, if this requires you to propose changes to the approved layout plan, this will likely require either a variation to the plan or a zoning amendment. In that case,we should talk with the planners about what changes staff will be comfortable in supporting. • Those areas that were shown as not having facilities to address the runoff(white on this plan) are allowed to have flow directly into the stream, but the water quality impacts must still be addressed. For all the areas not being routed through BMPs (this area plus uncaptured areas with the red and blue areas), a 30% reduction credit can be provided by Lickinghole. • With the above, our initial analysis indicates it should be possible to meet the phosphorus reduction required with the VSMP that apply to this property(2C). If it turns out the onsite facilities and the Lickinghole credit cannot meet the entire pollutant reduction,we will consider using the offsite ponds to address the incremental difference. That would need to be verified as genuinely providing a reduction above and beyond what was previously approved. • Finally, if there is an interest in using any of the County owned property for a stormwater facility, please recognize this will need to involve a number of people outside of Community Development. This will require additional time and effort to coordinate with Parks and Recreation,the County Attorney's Office, the County Executive's office and ultimately the Board of Supervisors to assure the County's interest in the property is being protected. Past experience suggests it will likely take 4-6 months to finalize an easement agreement with the County once everyone has agreed upon the recommendation. 2 I hope this simplifies things and gives Stantec the guidance needed to develop a good plan. Have a great Thanksgiving. Mark P.M.!ANNA MOW. vr`r vniM ,s.�.a..,..«... ""' 1 is ~•�• R�w.4s�M .w.a wmwwrwr 'uwwaw.••• i rut rsgmanign.*** M a....,... - 1 1.11011.1111141.011¢110. ,„" _a” 14 1 1 ponds OA /09 acres ,p i w..i�.w i A ii,I - 1 4.7 acres , •••••••�rlYi•ww'ww t t'I. 1111 swam ma b/r.pl� '‘','T qr MNIrlrl�l t.V , -- � l v , op110 /0.$ \ ‘ 1 i i A ,,, ,4,.......„ x ....r , 444 L.......,44 7 IIILS g `� 1St 7- " v cw ;" 0 t7 i Areas shown by outline are approximate areas '„,Z ' :a to discharge points used for compliance computations 0 G s' 1 , Ix s 1 , 1 .,.y 3 Imo fir ' County of Albemarle Department of Community Development Memorandum To: Engineering Division,Mark Graham From: Glenn Brooks,County Engineer Date: 19 Sep 2014 Subject: Lickinghole Basin Board policy implementation In light of the recent county implementation of the Virginia Stormwater Management Program, I am re-evaluating our implementation of the policy on Lickinghole Basin for developments within its watershed. The policy implementation now needs to reflect the use of runoff reduction methods (RRM) and on-site facilities. Furthermore, considering how stormwater management has evolved, it is not valid anymore to assume that all water quality requirements in the watershed are met by this basin. Lick! hole Basin r% rp• t r s. + rA r 1 11 �:w 4 00. IN'et etg It has been the practice at the county, based on a Board of Supervisors policy established in 1991 and later amended in 2001, to allow by-right development in this watershed to proceed without on-site water quality treatment. The board policy, attached for reference, states a pro-rated share cost,based on the increase in runoff leaving the property during a 10 year storm, shall be paid to the county in order to recover some (a projected 34%) of the cost of constructing the basin. This policy had two major implementation hurdles; (1) The policy contains no technical discussion of Albemarle County Community Development Lickinghole Basin Policy Implementation Page 2 of 5 whether on-site facilities will or will not be provided, or what happens if they are. (2) It contains no instruction on how the increase in runoff is to be computed, or how this correlates to the state code requirements. The Board of Supervisors minutes for the policy in 1991 contained a brief exchange by board members of on-site facilities, item (1). It was noted that this would be a possibility, on a case-by- case basis. The point of the question was to avoid the county expense in building the basin if on- site facilities would be cheaper. The basin was a proposed project at the time. (There was no discussion of how the case-by-case approach would fall short of the target cost recovery, or how it could be administered.) Subsequent practice has been that the pro-rata share cost is paid in full regardless of on-site facilities. Initially, this meant no on-site facilities were provided. As regulations became more stringent, requiring localized detention, adequate channel protection, and mitigation for buffer disturbances, on-site facilities were often necessary anyway. In addition, the Board began to get additional facilities with rezonings. However, the Lickinghole Basin implementation never changed, and credit has not been given for such facilities. Computing the pro-rata payment was the main emphasis of the 2001 amendment to the policy, which gives direction on item(2). I instigated this amendment, in response to inconsistencies in plan review and pro-rata share payments. The new computation procedure is what we use now. It looks like this; Cpost=0.45 from table 4-3 on p.4-20 of VSMH Vol.2 Cpre=0.32 uniform for entire watershed,not based on individual site condition A=2 acres Ipre=5.2 tc of 12 min.App.4C VSMH Vol.2 Ipost=6.4 tc of 7 min. Qpre=Cpre*Ipre*A Qpost=Cpost*Ipost*A Qdiff=Qpost-Qpre Qdiff= 2.4 [Qdiff/A= 1.2, should average around 1.25cfs/acre 1 R=456.96$/cfs BOS Policy recovery rate(last updated 16 Jan 2014) R*(Qdiff)__1111.33$ This computation is performed by county reviewers. With the use of tabulated values, and a consistent approach, it provided better consistency, and better cost recovery, than was originally achieved. There are some things it does not do. It does not claim to provide more, or less, hydrologic accuracy. It does not directly compute velocity or volume as referenced by the policy. Instead,it computes the related value of a theoretical peak discharge. It does not compute pollutant loadings. Even in 2001, staff knew there were significant flaws in the Lickinghole Basin approach to water quality. Correspondence at the time shows water resources staff had established that there was not enough nutrient or sediment removal to protect the reservoir. ' study by F.X. Browne, Inc., of July 1999,was mentioned to indicate 37% to 46%pollutant and sediment removal. This was when the basin was new. In its present silted state, it is likely lower. Also in 2001, the state Albemarle County Community Development Lickinghole Basin Policy Implementation Page 3 of 5 programs were beginning to emphasize that facilities should be built on a scale to protect local streams and wetlands, such that tributary streams, and Lickinghole Creek leading to the big basin, were not sacrificed. This trend has continued, such that now facilities in streams are prohibited and discouraged by state and federal agencies. Furthermore, the South Fork Rivanna Reservoir is significantly silted in, and a new reservoir has been constructed at Ragged Mountain, rather than refurbishing the South Fork Rivanna. Through all these changes, the Lickinghole Basin policy implementation has continued unaltered. The county is now at a point where it may be easier to develop within the drinking water watershed and Lickinghole Basin than elsewhere. Outside the watershed, the full state program is applicable. Inside the watershed, when we exempt development from water quality treatment, and instead only collect the pro-rata share, the requirements are much less. This has been the case for some time. However, now the practice must be examined under new code provisions. The previous code was explicit in its exemption; "Sec. 17-316 Contribution to regional stormwater management program. Each stormwater management/BMP plan shall require that the owner contribute to a regional stormwater management program,as provided herein: A. If the land development is located within the watershed of a regional stormwater management program established by the county which requires pro rata share contributions,the owner shall pay a pro rata share of the cost of the facility in accordance with any ordinance of the county establishing the program. B. An owner's payment pursuant to paragraph(A)shall relieve the owner of the requirements of section 17-314,if the regional program is designed to control the peak rate and velocity of runoff,and/or the requirements of section 17-315,if the regional program is designed to provide best management practices.An owner's payment pursuant to paragraph(A)shall not relieve an owner of his responsibility to comply with any other requirement of this chapter,except as provided in this section. " Section B relieved the owner of on-site requirements, as far as they are covered by the regional facility. I'm not sure how it came about that Lickinghole Basin was expected to take care of all water quality compliance, other than common practice. The documentation I can find on Lickinghole Basin never made that conclusion. Under the new code, it is stated differently; "9VAC25-870-69.Offsite compliance options. A. Offsite compliance options that a VSMP authority may allow an operator to use to meet required phosphorus nutrient reductions include the following: 1. Offsite controls utilized in accordance with a comprehensive stormwater management plan adopted pursuant to 9VAC25-870-92 for the local watershed within which a project is located; 2. A locality pollutant loading pro rata share program established pursuant to § 15.2-2243 of the Code of Virginia or similar local funding mechanism; 3. The nonpoint nutrient offset program established pursuant to § 62.1-44.15:35 of the Code of Virginia; 4. Any other offsite options approved by an applicable state agency or state board;and 5. When an operator has additional properties available within the same HUC or upstream HUC that the land-disturbing activity directly,discharges to or within the same watershed as determined by Albemarle County Community Development Lickinghole Basin Policy Implementation Page 4 of 5 Nikiwe the VSMP authority,offsite stormwater management facilities on those properties may be utilized to meet the required phosphorus nutrient reductions from the land-disturbing activity." Development within the Lickinghole Basin watershed falls under section A.2. This in turn references this section of state code; § 15.2-2243.Payment by subdivider of the pro rata share of the cost of certain facilities. A. A locality may provide in its subdivision ordinance for payment by a subdivider or developer of land of the pro rata share of the cost of providing reasonable and necessary sewerage,water,and drainage facilities,located outside the property limits of the land owned or controlled by the subdivider or developer but necessitated or required,at least in part,by the construction or improvement of the subdivision or development;however,no such payment shall be required until such time as the governing body or a designated department or agency thereof has established a general sewer,water,and drainage improvement program for an area having related and common sewer,water,and drainage conditions and within which the land owned or controlled by the subdivider or developer is located or the governing body has committed itself by ordinance to the establishment of such a program. Such regulations or ordinance shall set forth and establish reasonable standards to determine the proportionate share of total estimated cost of ultimate sewerage,water,and drainage facilities required to adequately serve a related and common area,when and if fully developed in accord with the adopted comprehensive plan,that shall be borne by each subdivider or developer within the area.Such share shall be limited to the amount necessary to protect water quality based upon the pollutant loading caused by the subdivision or development or to the proportion of such total estimated cost which the increased sewage flow,water flow,and/or increased volume and velocity of storm water runoff to be actually caused by the subdivision or development bears to total estimated volume and velocity of such sewage,water,and/or runoff from such area in its fully developed state.In calculating the pollutant loading caused by the subdivision or development or the volume and velocity of storm water runoff, the governing body shall take into account the effect of all on-site storm water facilities or best management practices constructed or required to be constructed by the subdivider or developer and give appropriate credit therefor. I have added the italics above for emphasis. In this section of code, and in the original board minutes of 1991, I find a basis for providing, and giving credit for on-site facilities. Code references to volume and velocity and/or pollutant loading are not new. However, they have not been directly used in the context of Lickinghole Basin. Historically, velocity and volume have been computed as they relate to a peak discharge rate (cubic feet per second of flow). With the new state stormwater program, a pollutant loading computation is required, as are discharge rates. Volume is also directly reduced in the runoff reduction method through infiltration practices. These need to be accounted for in the implementation of the Lickinghole Basin policy. Volume reductions will be accounted for by multiplying the cost in the original calculation by the ratio of"runoff reduction (cf)" to "treatment volume (cf)" in the runoff reduction method. For example, the two acre site had a pro-rata cost of$1,111.33. It might have 1 acre of turf and 1 acre of impervious cover, giving a treatment volume of 4,175cf. With 0.5 acres of impervious cover draining to a grass channel, and 0.5 acres in permeable pavement(type 2), the runoff reduction is computed to be 345 cf and 1293 cf. This is a ratio of 0.39. So the final cost is Albemarle County Community Development Lickinghole Basin Policy Implementation Page 5 of 5 $1,111.33(1-0.3923) = $675.32. (Refer to the DEQ Runoff Reduction Method, RRM, spreadsheets for more detail.) The fact that Lickinghole Basin will not meet current water quality requirements must also be considered. For purposes of calculation, Lickinghole should be computed at 30% phosphorous removal. This would apply to the remaining runoff at discharge. There is no runoff reduction, and no water quantity compliance contribution, directly attributable to Lickinghole Basin. Furthering the above example, the RRM computes a total phosphorous load (TP) of 2.62 lbs/yr for the 2 acre site, with a load reduction requirement of 1.80 lbs/yr. Lickinghole could potentially take care of 30% of the site load but on-site treatment is required first. With the permeable pavement and grass channel example above, the water quality compliance summary shows that 1.23 lbs/yr has been treated on-site. 1.40 lbs/yr of the original site load is discharging. Lickinghole can treat 30% of this, bringing the total treatement volume to 1.23+0.3(1.4) = 1.65 lbs/yr. This is still not quite at the required 1.80 lbs/yr, but getting closer. The 30% treatment credit from Lickinghole cannot be used for banking credits. Keep in mind that it is likely 60-70% of the remaining load sent to Lickinghole Basin will not be adequately treated, but passed on to the reservoir. This is just one way to account for the actual basin contribution, and give credit for required on-site treatment. The majority of benefit is expected to come from runoff volume reductions. 1460., I do recognize that there may be much less cost recovered in future when considering on-site measures as the code directs. However, it is important to consider this in view of the current program, and the state of Lickinghole Basin, which will not provide water quality protection equivalent or close to the new state program. On-site facilities may easily provide better treatment under the new state computation methods, and Lickinghole Basin provides only a partial treatment as backup for difficult to capture areas. This was arguably its intended purpose, as added protection in allowing a development area within a drinking water watershed. Summary 1. Lickinghole Basin pro-rate cost share amounts will be computed with the rational method peak discharges as before. This is based on the procedure devised in 2001. 2. Credit will be given for on-site facilities by multiplying the cost by the ratio of runoff reduction to total treatment volume. 3. On-site facilities will be required to meet state water quality pollution (phosphorous load) treatment requirements, and Lickinghole Basin may be used to account for 30% of pollutant load leaving the site. STORMWATER MASTER PLAN UPDATE Appendix B General Development Plan April 4,2016 endix B GENERAL DEVELOPMENT PLAN Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx B.2 _ _ 0 , w ti p, 0 ® 00000000 0 O 0 0 0 0 0 0 0 0 0 0 0 :b b ggl; gy � r >ggWH 'gaqs=^y s x �a, q� Nq., > > €�",l2iia .Igi A 1 m Ni"i D'Ill'ilr-'"11 H k 9 s t t$ i r " I E ' g$ _ `> ! wg * g ; g, -.1,, . °>° � lgi vs z €1 "ea§i > .... n "' 0 � " m' a i g r> l>ii 1121 a i m i5 1OS �t1 mil g i a . ') i HJ:! 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STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4,2016 Appendix C HYDROLOGYSUPPORT INFORMATION Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx C.3 STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4,2016 Curve number computations for pre-development and post-development conditions were originally performed using WMS(Watershed Modeling System) software. Curve numbers were developed using Albemarle County soils mapping and land use boundaries derived from US Geological Survey(USGS)National Land Cover Database(NLCD)for pre-development conditions(2001). Post-development conditions were updated using proposed land use mapping provided by RGA(Figure 4). Following revision of several drainage areas during master plan refinement,new curve numbers were calculated manually; however, note that in some instances Stantec has used curve numbers from approved plans and additionally manually adjusted some original post-development WMS curve numbers up to remain conservative for purposes of water quantity compliance. These curve numbers do not represent final design hydrology. WMS CNs Pre-development Drainage Areas: Drainage Area Curve ID Number 100A 74.0 11B 55.2 18B 55.4 1B 62.2 Agiuto- 21 B-1 66.9 21 B-2 66.9 22B 63.0 23B 58.0 24B 58.3 25B 57.6 27B 60.4 2B 59.6 31B 62.4 36B 61.0 38B 58.3 40B 40.8 42B 57.6 43B 56.0 45B 56.1 46B-1 61.7 468-2 61.7 47B 59.9 49B 65.7 4B 61.0 Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx C.4 STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4,2016 50B-1 64.1 50B-2 64.1 51B-1 63.8 52B 65.3 Post-development Drainage Areas: Curve Drainage Area ID Number 100A 74.0 15B-3 73.5 1B 62.2 21 B-3 83.0 22B 63.0 23B-1 83.6 23B-3 83.6 23B-4 66.9 24B 62.2 27B-1 76.5 2B 59.6 Name 31B 74.2 36B 76.8 37B 79.5 38B 68.1 40B 44.9 43B 66.6 44B 85.0 45B 61.5 46B-2 81.2 47B 60.0 49B 66.6 4B 66.8 50B-1 64.1 50B-2 63.9 51B-2 77.7 52B 80.5 tca ntec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx C.5 STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4,2016 Calculated CNs Pre-development Drainage Areas: f� 5® 1tit' t�TD , G A - w 1E HSC ,AREA(AG) C lOB FORESTED B 0 55 0.00 OPEN SPACE B 20.529 61 1252.27 WATER B 0 98 0.00 SUM 20.529 1252.27 CN 61.00 .x �' ..-fes_•`tiw,�v`rg,_.'�.`**r` m .. ..a, . .,. .fr. ,t 1» .,.= >. FORESTED B 0 55 0.00 OPEN SPACE B 9.782 61 596.70 WATER B 0 98 0.00 SUM 9.782 596.70 CN 61.00 FORESTED B 0 55 0.00 OPEN SPACE B 1.930 61 117.73 WATER B 0 98 0.00 SUM 1.930 117.73 CN 61.00 8��Wf 04?* s" FORESTED B 0 55 0.00 OPEN SPACE B 1 1.725 61 715.23 WATER B 0 98 0.00 SUM 1 1.725 715.23 C N 61.00 FORESTED 2.226 61 B 0 55 0.00 OPEN SPACE B 135.79 WATER B 3.796 98 372.01 SUM 6.022 507.79 CN 84.32 ttgistay 4., Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx C.6 STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4, 2016 FORESTED B 0 55 0.00 OPEN SPACE B 3.217 61 196.24 WATER B 0 98 0.00 SUM 3.217 196.24 CN 61.00 FORESTED B 1.028 55 56.55 OPEN SPACE B 1.658 61 101.13 WATER B 0.229 98 22.44 SUM 2.915 180.12 CN ri 61.79 FORESTED B 2.049 55 112.69 OPEN SPACE B 6.585 61 401.69 WATER B 0 98 0.00 SUM 8.634 514.38 CN 59.58 FORESTED B 0.000 55 0.00 OPEN SPACE B 2.539 61 154.85 WATER B 7.431 98 728.21 SUM 9.969 883.06 CN 88.58 51 B- FORESTED B 5.589 55 307.41 OPEN SPACE B 33.918 61 2068.98 WATER B 0 98 0.00 SUM 39.507 2376.39 CN 60.15 `h tartcc to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx C.7 STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4,2016 Post-development Drainage Areas: r .., 4'' �P LEVE : ' . INAGE AREAS,€ �, , � ,e.v.::. `; y DA <z . . x' .LANs , � 1 ,,.AREA(AC) ;. CIV , t :'c" ,' "; ,gym' i40 ``�'. exe. ._,. i .. '€`',x '.� x �' , x '; ASPHALT B 3.454 98 339 TOWNHOMES B 7.252 85 616 SINGLE-FAMILY 1.197 B 75 90 COMMERCIAL B 9.212 92 848 OPEN SPACE B 1.819 61 1 1 1 WATER B 0.671 98 66 SUM 23.605 2069 CN 87.65 ASPHALT B 1.622 98 159 TOWNHOMES B 3.736 85 318 SINGLE-FAMILY B 1.836 75 138 OPEN SPACE B 0.306 61 19 WATER B 0.000 98 0 SUM 7.500 633 CN 84.38 ASPHALT B 1.461 98 143 TOWNHOMES B 3.742 85 318 SINGLE-FAMILY B 0.523 75 39 OPEN SPACE B 1.163 61 71 WATER B 0.000 98 0 SUM 6.889 571 CN 82.95 '`1 5B' a ee " ,.. ASPHALT B 1.450 98 142 TOWNHOMES B 3.130 85 _ 266 SINGLE-FAMILY B 0.000 75 0 OPEN SPACE B 0.040 61 2 WATER B 0.000 98 0 SUM 4.620 411 CN 88.87 "t .',•,0 e �+aI eivativety high at interim estimate;-8147-1::-:6( ASPHALT B 0.000 98 0 TOWNHOMES B 0.739 85 63 SINGLE-FAMILY B 0.000 75 0 /erk Stantec C.8 to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4, 2016 OPEN SPACE B 0.288 61 18 WATER B 0.000 98 0 SUM 1.027 80 CN 78.27 ., ..e ,t ��a` 's= it#46; 1...., . %:4-4,1,',;-;,'4:„.1":?'_ .-mac.., i. .. ASPHALT B 0.448 98 44 TOWNHOMES B 1.826 85 155 SINGLE-FAMILY B 0.000 75 0 OPEN SPACE B 1.240 61 76 WATER B 0.000 98 0 SUM 3.514 275 CN 78.19 '"' �... c.A.... .� a:: r rim. l88 1 A� .; - �'r` ., .e'0.4: ASPHALT B 0.164 98 16 TOWNHOMES B 0.388 85 33 SINGLE-FAMILY g 0.000 75 0 OPEN SPACE g 0.000 61 0 WATER B 0.000 98 0 SUM 0.552 _ 49 CN 88.86 :: t,_. . ...:, .. . r21B 1.,: A . ASPHALT B 0.000 98 0.00 TOWNHOMES B 1.145 85 97.30 SINGLE-FAMILY B 0.000 75 0.00 OPEN SPACE B 0.669 61 40.82 COMMERCIAL B 0.000 92 0.00 WATER B 0.000 98 0.00 SUM 1.814 138.12 CN 76.15 2 B 2 M l..Pr.,, oto a... wel iii hat interim es im• a4 .._ ASPHALT B 0.730 98 _ 72 TOWNHOMES B 2.890 85 246 SINGLE-FAMILY B 0.000 75 0 COMMERCIAL B 0.000 92 0 OPEN SPACE B 3.110 61 190 WATER B 0.000 98 0 SUM 6.730 507 CN 75.32 ._. r ; i 4' .. 1is DA LAND USE HSG AREA CN CN x A ASPHALT B 1.312 98 127 Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx C.9 STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4,2016 TOWNHOMES B 4.523 85 368 SINGLE-FAMILY B 0.000 75 0 COMMERCIAL B 0.000 92 0 OPEN SPACE B 0.109 61 0 WATER B 0.000 98 0 SUM 5.944 520 CN 87.43 € tuftW,a r.- ter. :;- r ASPHALT B 0.248 98 24 TOWNHOMES B 0.999 85 85 SINGLE-FAMILY B 0.000 75 0 OPEN SPACE B 0.000 61 0 WATER B 0.000 98 0 SUM 1.247 109 CN 87.59 p ASPHALT B 0.000 98 0.00 TOWNHOMES B 0.091 85 7.78 SINGLE-FAMILY B 0.000 75 0.00 OPEN SPACE B 0.616 61 37.58 COMMERCIAL B 0.000 92 0.00 WATER B 0.435 98 42.63 SUM 1.142 45.35 CN 77.01 Hr�., .. A# g 426, P ', ASPHALT B 0.000 98 0.00 TOWNHOMES B 0.636 85 54.10 SINGLE-FAMILY g 0.000 75 0.00 OPEN SPACE B 3.807 61 232.20 WATER B 0.000 98 0.00 SUM 4.443 286.30 CN 64.44 --466 lr toy .(; w ASPHALT B 0.827 98 81 TOWNHOMES B 8.402 85 714 SINGLE-FAMILY B 0.000 75 0 COMMERCIAL B 0.000 92 0 OPEN SPACE B 0.158 61 10 WATER B 0.000 98 0 SUM 9.387 805 CN 85.74 Sita ntec C.1 0 to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx STORMWATER MASTER PLAN UPDATE Appendix C Hydrology Support Information April 4, 2016 f ASPHALT B 0.000 98 0 TOWNHOMES B 0.000 85 0 SINGLE-FAMILY B 0.000 75 0 COMMERCIAL B 0.000 92 0 OPEN SPACE B 2.505 61 153 WATER B 7.621 98 747 SUM 10.126 900 CN 88.85 A ° v&!A4. > r- r' 0 " - aps- .: 71 7,. 514 .� .4:41#06 ...p,911 ,higher,85) , > ASPHALT B 0.585 98 57 TOWNHOMES B 2.847 85 242 SINGLE-FAMILY B 0.000 75 0 OPEN SPACE B 0.884 61 54 WATER B 0.000 98 0 SUM 4.316 353 CN 81.85 The following drainage areas were adjusted to conservative curve numbers: Post-development: Drainage Area Curve ID Number 25B 85.0 Golf Dr. 98.0 .,, Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx C.1 1 STORMWATER MASTER PLAN UPDATE Appendix D Water Quality Compliance Worksheets &Tracking Ledger April 4,2016 ix D WATER QUALITYCOMPLIANCE R TS TRACKING LEDGER Stantec to u:\203400430\05_report_deliv\deliverables\swm_mp_v3\rpt_swm_mp_v3_20160404.docx D.12 PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDA:: Worksheet 1 Page 1 of 3 STEP 1 Determine the applicable area(A) and the post-developed impervious cover ('post)* Applicable area(A)* = 237,55 acres Post-development impervious cover: structures = acres parking lot = acres roadway = acres other: acres acres Total = 1 0112- acres 'post=(total post-development impervious cover±A) X 100= Lf 3t145 % * The area subject to the criteria may vary from locality to locality. Therefore, consult the locality for proper determination of this value. STEP 2 Determine the average land cover condition(Iwat„sw)or the existing impervious cover(Texisting). Average land cover condition(I ): watershed.‘ If the locality has determined land cover conditions for individual watersheds within its jurisdiction,use the watershed specific value determined by the locality as Iwatershed• Iwatershed = 14 Vo Otherwise,use the Chesapeake Bay default value: Iwatershed 16°/0 5D-5 PERFORMANCE-BASED 8VATER QUALITY CALCULATIONS APPENDIX 5Dil Worksheet 1 Page 2 of 3 Existing impervious cover(I xisting : Determine the existing impervious cover of the development site if present. Existing impervious cover: structures = acres parking lot = acres roadway = acres other: = acres acres Total = 5 acres 'existing=(total existing impervious cover A*) x 100 % * The area should be the same as used in STEP 1. STEP 3 Determine the appropriate development situation. The site information determined in STEP 1 and STEP 2 provide enough information to determine the appropriate development situation under which the performance criteria will apply. Check(4, )the appropriate development situation as follows: Situation 1: This consists of land development where the existing percent impervious cover(fexisting) is less than or equal to the average land cover condition (1watershed)and the proposed improvements will create a total percent impervious cover clpc,$)which is less than or equal to the average land cover condition ( watershed) Ipost % 4watershed 5D-6 PERFORMANCE;-BASED WATER QUALITY CALCULATIONSAPPENDIX 51) Worksheet 1 Page 3 of 3 Situation 2: This consists of land development where the existing percent impervious coverl7 (iexist;,,g)is less than or equal to the average land cover condition ( liwatershed)and the proposed improvements will create a total percent impervious cover(Ipost)which is greater than the average land cover condition(I A) 'existing 2- % * "watershed 'C %; and 'post 43.4f5 %5'watershed /D % Situation 3: This consists of land development where the existing percent impervious cover('existing) is greater than the average land cover condition(Iww,atcrshed)• o ? o 'existing /0 'watershed /0 Situation 4: This consists of land development where the existing percent impervious cover(Iexisting)is served by an existing stormwater management BMP(s) that addresses water quality., If the proposed development meets the criteria for development Situation 1, than the low density development is considered to be the BMP and no pollutant removal is required. The calculation procedure for Situation 1 stops here. If the proposed development meets the criteria for development Situations 2,3,or 4,then proceed to STEP 4 on the appropriate worksheet. 5D-7 PERFORMANCE-BASED WATER QUALITY CALCULATIONS APPENDIX 5D Worksheet 2 : Situation 2 Page 1 of 4 Summary of Situation 2 criteria: from calculation procedure STEP 1 thru STEP 3, Worksheet 1: Applicable area (A)* = 23735 acres mast —(total post-development impervious cover_A) x 100= 43r115 % (watershed — 0 or Iwatershed 16% (existing,=(total existing impervious cover_A*) x 100= 'existing % 4watershcd %; and 1post %> Inatershed df0 STEP 4 Determine the relative pre-development pollutant load (LpCJ. Lpre(watershed)= [0.05 +(0.009 x (watershed)] x A x 2.28 (Equation 5-16) where: Lpre(watersi,ed) = relative pre-development total phosphorous load(pounds per year) 'watershed — average land cover condition for specific watershed or locality or the Chesapeake Bay default value of 16% (percent expressed in whole numbers) A = applicable area(acres) Lpre(wateshed)— [0.05 + (0.009 x 16 )] x 2.3A 35 x 2.28 = 104.Q$ pounds per year 5D-9 PERFORMANCE-BASED WATER QUALITY CALCUJLATIONS APPENDIX 5D Worksheet 2 : Situation 2. Page 2 of 4 STEP 5 Determine the relative post-development pollutant load (Lpost). Lpost = [0.05 + (0.009 x 'post)] x A x 2.28 (Equation 5-21) where: Lpost = relative post-development total phosphorous load(pounds per year) Ipost = post-development percent impervious cover(percent expressed in whole numbers) A = applicable area(acres) Lpost = [0.05 + (0.009 x 43t115 )] x 2-3t. 35 x 2.28 238, 66 pounds per year STEP 6 Determine the relative pollutant removal requirement(RR). . . RR past pre(watershed) RR = • •. pounds per year STEP 7 Identify best management practice (BMP) for the site. 1. Determine the required pollutant removal efficiency for the site: EFF = ( RR Lpost) x 100 (Equation 5-22) where: EFF = required pollutant removal efficiency (percent expressed in whole numbers) RR = pollutant removal requirement (pounds per year) Lpost = relative post-development total phosphorous load(pounds per year) EFF = ( 133. b8 - 23 .66 ) x100 3'6,01 5D-10 PERFORMANCE-BASED WATER QUALITY CALCULATIONS; APPENDIX 5D Worksheet 2 : Situation 2 Page 3.of4 2. Select BMP(s) from Table 5-15 and locate on the site: BMP 1: L ? BMP 2: BMP 3: 3. Determine the pollutant load entering the proposed BMP(s): LBnmr, = [0.05 +(0.009 x IBS)] x A x 2.28 (Equation 5-23) where: LBmp = relative post-development total phosphorous load entering proposed BMP(pounds per year) IBMP = post-development percent impervious cover of BMP drainage area (percent expressed in whole numbers) A = drainage area of proposed BMP (acres) LBripr — [0.05 +(0.009 x )1 x x 2.28 = pounds per year LBmp2 = [0.05 +(0.009 x )] x x 2.28 pounds per year LB,rp3 = [0.05 + (0.009 x )1 x x 2.28 pounds per year 3L3r fbsrely, /C� 5D-11 1 e` 3 t r it3 1 3 ' I j t P i�Ki , 't PERFORMANCE BASEli WA�TE*QUA r,��ITYBASEli1 111 ,WNTE*QUAPPENIAX 51) Worksheet 2 Situation 2 Page 4 of 4 4. Calculate the pollutant load removed by the proposed BMP(s): Lremoved = Effe1in, x LBMP (Equation 5-24) where: Lremoved = Post-development pollutant load removed by proposed BMP (pounds per year) EffBmp pollutant removal efficiency of BMP(expressed in decimal form) LBS relative post-development total phosphorous load entering proposed BMP (pounds per year) L = Xpoundsper year removedBhlPl = Lremoved/BMP2 = x pounds per year Lremo ed/BMP3 X = pounds per year Rertotnee =, l 21.`13 f bs 'TP/�,, Ef=f; 1.Q % 5. Calculate the total pollutant load removed by the BMP"(s)` -'removed/total LremovedtBMPi +I-'removed/BMP2+LremovedBMP3+ • • • (Equation 5-25) where: L =totalpollutant load removed by -BMPsj +� removed,total !� �'1 I6 !/�� ► LremovediBmpi pollutant load removed by proposed BMP No. 1 S fy Lremoved/BMP2=pollutant load removed by proposed BMP No. 2 LremovedBMP3=pollutant load removed by proposed BMP No. 3 Reviovid j? Lremoved/total = + + +. . • =T1 V all pIkMc.. i/ pounds per year ^' 13 3, d Q 1 b S ?P/y r 6. Verify compliance: O Lremoved/total *RR • • 5D-12 ^ C)o a N 0 c 0 �" Planned Q G 0 — 4- 0' ' n .43 0 a 3 a. n ti p 0 tr o a 0 a t0 �'Q Tp p t=iia 0 Cl" a 0. n r n r f7• Q ! Nncn y n n N r n.(p ONN Q W WA DV D os D WNW WyQ 'v 0 :°3 Co 7 2 co o N 0 cep a. A �, Q Q N • Q n — -, N C 7 co co C W O _ _ 3 m = — ° 3 ; -�+ 74=<- -I (D C W Co co. W W co. 07 O co. W W 0• 5. D o' CO 0 0 0 - 0 0 0' 0 00 0 - Q p p Q m 3 N N (D (D CD ,--e- (D N CD N CD `^ g 'C N - 0 0 0 o O o o O o 0 0 ® a 9' , 0 _ N Q Q u, -. L, u, 'v, u, L, L, Cr, u, 01 N Q m -4C > D x y a n n n 5. n n H y rt .<,->< 4 S ^,� Q 3 D- ''''- 3 3 3 3 3 3 3 �^ N co o 05.- 0 a N 0 n Q C 7 3 , 0 0 O ro Q_ 0 � N � C � -v ' v 0 c Q a fl -. '< N W V, N �i n (Q a• '� co `0 Cn N A W Cn O� 07 0 a "'O y: -&,,,, rn w v G �p cp o b o� b� rn cr, :tom cn ba b� o p o A O O A n y 3 Q N O� O O v N V �O O, fD N Q o a co y O C �. 0 to 3 _ rt 8. co (p n, C C.S3 a _ p 3 N •.-. -, 'Ic V Cn •N 0 O V A Cr, W tto N Cn O� H a Q. 3 a OJ W Cn �` • • IV W (,J O A O O� O� A • V "0 R1 P o y a �„ 0) w rn cn rn 0, N w .p y 0 v +✓ �. Q N Q O Q A .* CD a (D a. K v 3 =. 0 0 o n a �, fl Q o 0 a N 0 0 o to 3 a Q o +a N 0 0 0 N c o -, N c A ..- n3 n 0 a � o m m o �, Q a m Q v c -• N = a c (} , 7 'C to ..1C-.) . 0 3 0 0 0 0 n H n 0 n a 2• D o i o 0 N - O o Q 3 m.3 .�': 030 m � Q I, 0 0 c) 0 oN c '� 0i 0 o O - 5 a G 3 0 0 c 0 ., o H 13 0 ID + F, 3 N o S. o . a 0 ccA S 0 v � 3. � fov v .v 0 a Old Trail Village Stormwater Management Master Plan Reference Checklist (Draft-T©be'Provided at Design Submittal) Plan Number: Plan Name: Legend:✓ Complete Inc,-Incomplete N/A- Not Applicable Item Plan Preparer Notes Reviewer Reviewer Comments Preparer Underdrains included on all biofilters. Maximum ponding depth of 6"on all biofilters Safety/Aquatic bench provided on the two retention basins Steep slopes are not implicitly approved in the master plan— Impacts steep slopes? Floodplain impacts are not plicitly proved in the master plan— Encroachment into Floodplain? Wetld impacts are notanimplicitly approved in the master plan— Impacts to Jurisdictional Features? Stream Buffer impacts for SWM BMPs,BMP access,and Greenway trails within landward 5o'are approved with MP Stream buffer impact(Yes/No?) If so,stream buffer impact mitigation is included? Outfall protection included in design Media -necification )vided in sign(biofilters) Emergency spillway provided. Forebay or pretreatment provided. High flow bypass included for individual bioretention cells with greater than icres of direct Ittaittrainage? Connectivity between facility outfall and existing downstream channel provided. A 10'wide vehicle access graded at less than 20%, unless paved or graveled provided Necessary lot adjustments provided— reference site plan Design storm(2- year and io-year) attenuation to predevelopment flow levels provided per BMP outfall? Water Quality Compliance Tracking See WQ Compliance Tracking Ledger- Is there sufficient 'nning mpliance 'rapture surplus (prior approved plans+plans submitted under this review) across the project for the development to remain in compliance?