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HomeMy WebLinkAboutWPO201500037 Correspondence 2015-08-04 oAI*4%f . 41111,1 County of Albemarle Department of Community Development Memorandum To: Ana Kilmer,Management Analyst 'Greg Harper,Water Resources Manager From: Mark Hopkins Division: Inspections—E&S Control Date: 24 August 2018 Subject: Conversion to Permanent Stormwater Management Facility The requirements of the stormwater management and water quality component of the water protection performance bond have been satisfactorily completed and this component of the bond may be released at this time. The permanent stormwater management facility(ies) should now be included in the Stormwater Inspection Program. PROJECT NAME: Chesterfield Landing TMP: 05600-00-00-03600 SITE PLAN NUMBER: 2015-00037 FACILITY DESCRIPTION Type(s) and number of BMPs 2—manufactured BMP (incl.non-structural) #— [click to select type] Description of Outfall Drains to floodplain (Ex:24"RCP from 36"CMP riser) Water Quality ❑ Yes ®No Design Storm ❑ 10yr ❑ 2yr Any Additional Information 2 level spreaders (incl. stream buffer mitigation) A copy of the site plan and details of the BMP(s) are attached. I:\DEPT\Community Development\Forms\Inspections\SWM Bond Release Fonn-conversion to permanent swm facility REVISED 3 8 2010.doc PROJECT MXNAGEMENT SH' IMP CIVIL ENGINEERING LAND PLANNING ENGINEERING C- August 4, 2015 John Anderson Albemarle County Department of Community Development Regarding: WPO 2015 00037 Chesterfield Landing VSMP Plans Dear Mr. Anderson, We have submitted revisions to the VSMP Plans for Chesterfield Landing. Your comments for the first submittal and the changes we have made in response are below. A. Stormwater Pollution Prevention Plan (SWPPP) 1. Sec. 1, Registration Statement: list construction activity operator tel., email, and check whether willing to receive VGP correspondence electronically—check yes for quickest response. A new registration statement has been prepared,with contact information for the construction activity operator and the "yes" box checked for electronic correspondence. 2. Show section 6 PPP measures on section 3, ESCP exhibit; for example: concrete washout, sanitary portable facilities, dumpster, paint/solvent storage, on-site fueling, wash waters, vehicle and equipment washing. Each must drain to downslope trapment. Portable sanitary facilities should have downslope SF; on-site fueling/tank must have secondary containment (double-wall tank not sufficient); if shallow pit, then should be lined with 10 ml liner, and sized to contain stored fuel volume plus 10-yr storm event(5.6 in). —ref. VAR 10 Part II A 4 (e) As per your email dated 0810612015,the PPP has been updated to list section 6 measures and show preliminary locations and protections,to be updated by the contractor. 3. Sec. 5 —VaRRM .xls indicates C soils, but VRMM map shows A, B, B/D—please revise VRRM Map Legend. Also,please transfer land use Ac. values liste on .xls to VRRM Map (6.86, 11.04, 3.67). VRRM Map, incidentally, is excellent. As per your email dated 08/06/2015,the VRRM spreadsheet still shows C soils, as an approximation of the average condition of the soils listed as HSG BID in the NRCS soil report. 4. Sec. 5 —Revise per SWMP comment B.3. below The stormwater management plan has been revised to comply with DEQ requirements. 5. Sec. 6 Exhibit: show PPP measures listed in table on next 2-pg of SWPPP. See item#2 above. The exhibit has been updated to tabulate the PPP measures listed in the table and to summarize the protective measures applied. 6. Sec. 8—Designate qualified personnel, by name—ref. VAR10 Part II A 6. Justin Shimp is now designated in Section 8. 7. Sec. 12—Inspection Log should include inspection frequency. There are no details on back of sheet. General direction or inspector guidance is a minimal expectation for inspection logs. Explain or ref. prohibited discharges; failed controls. Please revise for clarity, completeness. The Inspection Log has been updated to show detailed requirements and inspection frequency on the back of the sheet, per the General Permit and guidance from your email dated 0810612015. 8. Update Sec. 4, ESCP, in response to comments The ESCP has been updated in response to comments. 9. Sec. 6.F. —As supplement, include: [page] The attached page is now included in the SWPPP. B. VSMP: SWPPP: Stormwater Management Plan 1. File Floodplain Development Application. On Application, list 1%base flood elevation at rear lot lines, Lots 10-15. Also, list BFE for ea. structures on these lots (604.3'-608.1'). At a minimum, document lowest structural floor elevation, including basement, relative to FEMA FIRM Zone 'A' elevation. List on the four blank lines reserved for description of development activity. —ref. CFR Title 44, §60.3 (b)(2), (b)(5)(i) [Appendix E,NFIP Regulations] Provide hydrologic and hydraulic computations used to establish the base flood elevations on each lot. A Floodplain Development Application is included with this submittal. The basement floor elevation and FEMA Zone "X" flood elevation are listed for each of Lots 10-15 on the four blank lines.The base flood elevations are from FEMA flood insurance rate study profile of Lickinghole Creek, Map 28P, Section U-V. 2. FEMA FIRM—compare New MH-1 top elevation (583.5') with base flood elevation, 585'±. Revise top of MH elev. relative to base flood elevation. Ensure proposed sanitary line is protected against floodwater intrusion, and damage due to floodwaters. —ref. 44CFR §60.3 (a)(4)(ii). MH-1 has been re-located to be above the base flood elevation. 3. Remove and eliminate conveyance channels (SCC) from stream buffer. SCCs are not listed as a structure, improvement or activity which may be allowed in a stream buffer by program authority (ref. 17-604A.-F.). Beyond this, proposed conveyance structures essentially skirt channel protection requirements (9VAC25- 870-66.B.) by introducing 500' and 225' ± v-notch stone channels (SCC 2, 15' wide; SCC B1, 12'; SCC Cl, 12') across floodplain/floodway/stream buffer to Lickinghole Creek. This design may be non-erosive, but without these channels, true 24"DIA storm line discharge is: OUTFALL Cl: 12.18 cfs/6.4fps; OUTFALL Bl: 16.42 cfs/6.8 fps. Design must comply with 9VAC25-870-66.B.3. Points of discharge release to protected stream buffer/floodplain. Provide energy balance equations at discharge outfalls: Cl and B 1. From a practical standpoint, erosion control and stormwater management should be provided before discharging to the stream buffer area. (The erosion control downhill of the fill slope for the lots appears difficult to protect without pulling back or collecting water with diversions.) This approach stands a better chance of preserving the buffer, and making use of its filtering benefits. Conveyance channels have been removed from the buffer. However,the energy balance equation applies to natural stormwater conveyance systems. Per field inspections,there are no natural channels within approximately 70' upland of the floodplain. Since there are no existing natural channels to which the site can discharge, and since construction of a man-made channel is prohibited in the floodplain, the site must discharge via sheet flow, under 9VAC25-870-66.D. This would be required regardless of the presence of upgradient quality treatments. Per the requirements of the DEQ specification for sheet flow, flows must discharge to areas with slopes less than 6%, and less than 2%within the first 10 feet. The areas immediately upslope of the floodplain are between 7% and 16% in grade,so are unsuitable for sheet flow discharge. Per 17.604A, improvements may be authorized within the landward 50 horizontal feet. Since stormwater discharges are not allowed without these improvements,they are necessary for adequate stormwater discharge. 4. Restore on-site SWM facilities shown(in concept form) on Approved preliminary plat. Ref. image below: The stormwater plan as currently submitted provides measures to comply with all DEQ regulations. 5. Sheet 3, hatch or otherwise delineate 9,650 SF and 6,536 SF `buildable' space in open space 2 and 3. Please consider impact, if any, on VaRRM .xls. A brick hatch has been added to the buildable open space. This should not have any impact on VaRRM.xls, as no buildings or other developments are currently proposed on these areas. If the HOA decides to develop these areas in the future,supporting documentation can be submitted at that time. 6. Provide documentation of purchase of nutrient credits: This re-submittal has provided nutrient removal on-site, and nutrient credits will no longer be purchased. 7. For 6.86 Ac. forest, post-developed site land cover,provide and show preservation or conservation easements. While stream buffer ordinance should protect forest or open space within developments, it does not, and is insufficient assurance of preservation. Show easements on sheets C4, C5. Also, aerial photography indicates much of this stream buffer does not have forest, so that would need to be established. In the past, stream buffer and 100-yr flood plain have been considered adequate protection to guarantee that open space areas will not be developed. In this current project,stormwater conveyance channels have been deemed impermissible development, so it seems unlikely that a heavier use would ever be approved. The DEQ's 2013 Stormwater Management Handbook counts undisturbed field space, among several other categories of non-wooded area, as "Forest/Open Space" in the VaRRM. Table 12.1 has been attached for reference. C. VSMP: SWPPP: Erosion Control Plan 1. Sediment basin design data sheet—dry storage, item 8: crest of riser 585.0' elev. does not match crest of riser elev. 581.4', p. 2, data sheet. Reconcile. The "Crest of Riser" in the Preliminary Design Elevations has been updated to match the crest of riser shown on page 1. 2. SB design data sheet,p. 2, design high water elev. 587' does not match 586.6' high water elevation, ESCP, sheet 20. Reconcile. The Design High Water under Preliminary Design Elevations has been updated to match that shown in Final Design Elevations and in the ESCP,sheet 20. 3. Sheet 20, SB detail—label principal (crest of riser) elev.; label dewatering device elevation. Noire Nisi The SB detail has been updated to clarify that the Dry Storage is the same as the Crest of Riser and the Wet Storage is the same as the Dewatering Device. 4. Sheet 16,Narrative, Phase 3 —Include text that Lots 1, 14, 15 may not be graded until all areas draining to sediment trap and sediment basin are stabilized. This text has been added to the narrative, Phase 3, note 3. 5. Sheet 16,Narrative and Plan must direct that paved CE will remain at Phase 1 PCE location until all but remnant grading remains at intersection Rte. 240 and Watervale Drive. A RWD will likely fail with harsh consequence for Rte. 240. Phase 1 PCE must remain active until mass/fine grading for Watervale Drive are all but complete. Plan and VSMP application will not be approved unless plan and narrative impose and clarify this condition via sequence of construction note. Notes have been added to the Phase 2 Narrative,the Phase 1 plan view, and the Phase 2 plan view indicating that the Phase 1 PCE must be used until Watervale Drive is nearly complete. 6. SB—Provide armored emergency spillway,provide dimensions. The emergency spillway is now specified to be grouted Class I rip rap extending to the limits of disturbance. Dimensions are provided for the bottom,the top, and the total length. 7. Two sheets C4 may be misprint,please renumber as necessary. The two sheets labelled C4 were indeed a misprint,thank you for this catch. D. VSMP: Mitigation Plan 1. No structure, improvement, or activity is allowed in stream buffer except connection with existing sanitary sewer easement. Only permitted improvements are proposed in the stream buffer. 2. Estimate/report and label Area(SF) of stream buffer impact associated with sanitary connection, #1 above. The area of stream buffer impact associated with sanitary connection has been estimated and labelled. 3. Provide mitigation for sanitary sewer line connection impact at 2:1 ratio. Per 17.602B,the installation of sewer lines for dedication to ACSA or RWSA is exempt from duties to manage a stream buffer. If you have any questions, please do not hesitate to contact me at your earliest opportunity. I may be reached at: Justin anshimr -engineering.com or by phone at 434-953-6116. a Justin Shimp, P.E. Shimp Engineering, P.C. • Virginia Stormwater Manaattrment Handbook,Chapter 12 4410r July 2013 Table 12.1. Land Cover Guidance for VRRM Compliance Spreadsheet IMPERVIOUS COVER • Roadways, driveways, rooftops, parking lots, sidewalks, and other areas of impervious cover. • This category also includes the surface area of stormwater BMPs that: (1) are wet ponds, OR (2) replace an otherwise impervious surface (e.g., green roof, pervious parking)) MANAGED TURF Land disturbed and/or graded for eventual use as managed turf: • Portions of residential yards that are graded or disturbed, including yard areas, septic fields, residential utility connections • Roadway rights-of-way that will be mowed and maintained as turf • Turf areas intended to be mowed and maintained as turf within residential, commercial, industrial, and institutional settings FOREST&OPEN SPACE Land that will remain undisturbed OR that will be restored to a hydrologically functional state: • Portions of residential lots that will NOT be disturbed during construction • Portions of roadway rights-of-way that, following construction, will be used as filter strips, grass channels, or stormwater treatment areas; MUST include soil restoration or placement of engineered soil mix as per the design specifications • Community open space areas that will not be mowed routinely, but left in a natural vegetated state (can include areas that will be bush hogged no more than four times per year) • Utility rights-of-way that will be left in a natural vegetated state (can include areas that will be bush hogged no more than four times per year) • Surface area of stormwater BMPs that are NOT wet ponds, have some type of vegetative cover, and that do not replace an otherwise impervious surface. BMPs in this category include bioretention, dry swale, grass channel, ED pond that is not mowed routinely. stormwater wetland, soil amended areas that are vegetated, and infiltration practices that have a vegetated cover. • Other areas of existing forest and/or open space that will be protected during construction and that will remain undisturbed. These include wetlands. Operational & Management Conditions for Land Cover in Forest& Open Space Category: • Undisturbed portions of yards, community open space, and other areas that will be considered as forest/open space must be shown outside the LOD on approved E&S plans AND demarcated in the field (e.g., fencing) prior to commencement of construction. • Portions of roadway rights-of-way that will count as forest/open space are assumed to be disturbed during construction, and must follow the most recent design specifications for soil restoration and, if applicable, site reforestation, as well as other relevant specifications if the area will be used as a filter strip, grass channel, bioretention, or other BMP • All areas that will be considered forest/open space for stormwater purposes must have documentation that prescribes that the area will remain in a natural, vegetated state. Appropriate documentation includes: subdivision covenants and restrictions, deeded operation and maintenance agreements and plans, parcel of common ownership with maintenance plan, third-party protective easement, within public right-of-way or easement with maintenance plan, or other documentation approved by the local program authority • While the goal is to have forest/open space areas remain undisturbed, some activities may be prescribed in the appropriate documentation, as approved by the local program authority: forest management, control of invasive species, replanting and revegetating, passive recreation (e.g., trails), limited bush hogging to maintain desired vegetative community, etc. 1 Certain stormwater BMPs are considered impervious with regard to the land cover computations. These BMPs are still assigned Runoff Reduction and/or Pollutant Removal rates within the spreadsheet, so their "values" for stormwater management are still accounted for. The reason they are considered impervious is that they either do not reduce runoff volumes (e.g., wet ponds) OR their Runoff Reduction rates are based on comparison to a more conventional land cover type(e.g.,green roofs, pervious parking). 12-18 PROJECT MANAGEMENT SHIMP CIVIL ENGINEERING LAND PLANNING ENGINEERING ' November 12, 2015 John Anderson Albemarle County Department of Community Development Regarding: WPO 2015 00037 Chesterfield Landing VSMP Plans Dear Mr. Anderson, Please find attached a digital revision to the VSMP Plans for Chesterfield Landing. Your comments for the second submittal and the changes we have made in response are below. B. VSMP: SWPPP: Stormwater Management Plan 4. Please provide standalone SWM plan sheet, titled SWM plan ... with site-specific ELS details, plan view of 6.86 Ac. forest/open space, BMP Spec. #2 design details, Spec. #2 construction/ maintenance detail (text/verbatim; cut/paste); SWM Narrative. In addition, provide narrative for ELS Maintenance. A standalone SWM plan sheet is now provided as sheet C16,with site-specific ELS details, a plan view of the open space, BMP Spec.#2 design details, Spec.#2 construction and maintenance details,a SWM Narrative, and a narrative for ELS Maintenance. 6. SWPPP Sec. 5 states "Stormwater shall be discharged in the east through level spreaders to ensure sheet flow."And "Flood protection: stormwater shall be discharged to manmade stormwater conveyance systems that confine the 10-yr 24-hr storm within the system." The system is a network of subdivision inlets and pipes. Where systems end (B1, C1), though theses points are coincident with limits of analysis, no defined channels and distances of 360' and 260'+ separate system discharge points from Lickinghole Creek. Compliance with 9VAC25-870-66 should rely on Energy balance (floodplain is a natural stormwater conveyance system), or, alternatively, convert pipe system concentrated flow to sheet flow. If water quantity compliance relies on sheet flow, compare LD-229 storm drain computations (Tot Flow/cfs)with ELS design criteria(Spec. #2/Table 2.2): Length of ELS Lip = 13 fin. Ft. per each 1 cfs of inflow if area has 90% cover. LD-229 cfs rates do not match ELS length provided. 16.42 cfs and 12.18 cfs require lengths -- 213.5' 213.5' and 158', respectively. Provide 13 lin. ft. / 1 cfs of inflow, while observing 100' max. Length. In August, we discussed this, Justin explained design would be altered to reflect relationship between flow(Qio/cfs) and length, in recognition of a practical upper limit. Also, please revise concrete footer detail, C15, so dimensions are easily readable. Computations are now provided to demonstrate that the engineered level spreaders distribute the 2-year storm without erosive velocities. 7. For 6.86 Ac. forest, post-developed site land cover,provide and show preservation or conservation easements. While stream buffer ordinance should protect forest or open space within developments, it does not, and is insufficient assurance of preservation. Show easements on sheets C4, C5. Also, aerial photography indicates much of this stream buffer does not have forest, so that would need to be established. (Rev. 1)Applicant response noted; we are grateful for copy of DEQ 2013 Stormwater Management Handbook, Table 12.1, which appears to support comment: "All areas that will be considered forest/open space for stormwater purposes must have documentation that prescribes that the area will remain in a natural, vegetated state. Appropriate documentation includes: subdivision covenants and restrictions, deeded operation and management agreements and plans,parcel of common ownership with maintenance plan, third-party protective easement, within public right-of-way or easement with maintenance plan, or other documentation approved by the local program authority." The developer will provide a permanent deed restriction to be held by Albemarle County or the Thomas Jefferson Conservation District. 8. Provide physical access, Dedicated Access Easement, and Private SWM Facility Easements for Engineered Level Spreader. The trail to access the greenway has been re-designed to be truck accessible, and a stormwater management access easement has been added to provide physical access.The Engineered Level Spreaders are encompassed by SWM easements. D. VSMP: Mitigation Plan 3. Provide mitigation for sanitary sewer line connection impact at 2:1 ratio. (Rev. 1) Please provide mitigation for permanent SWM BMP. SWM facilities are not listed as an eligible structure, improvement, or activity within stream buffers (17-604). A mitigation plan is now shown on the new SWM Plan sheet. If you have any questions, please do not hesitate to contact me at your earliest opportunity. I may be reached at: Justinna shimp-engineering.com or by phone at 434-953-6116. 4.40Be 0Mds, ustin Shimp, P.E. Shimp Engineering, P.C.