Loading...
HomeMy WebLinkAboutWPO202100063 Correspondence 2022-10-28ROUDABUSH, GALE & ASSOCIATES, INC., ENGINEERS, SURVEYORS AND LAND PLANNERS P October 20`h, 2022 Mr. John Anderson Albemarle County Engineering Department 401 McIntire Road Charlottesville, VA 22902 Project Title: Dunlora Farm — Dunlora Village WPO Plan (WPO 2021-00063) Mr. Anderson, Please find attached the revised plans for Dunlora Village WPO 2021-00063. The plans have been revised to address the comments received from the County of Albemarle's Engineering Department dated June 30`h, 2022. A. Stormwater Pollution Prevention Plan (SWPPP) The SWPPP content requirements can be found in County Code section 17-405. A SWPPP must contain (1) a PPP, (2) an ESCP, (3) a SWMP, and (4) any TMDL measures necessary. SWPPP 1. PI. ensure SWPPP cover includes reference to WP02021-00063. (Rev. 2) Not Addressed, but county approval stamp will identify project. No follow-up required. (SWPPP will be approved once all plan revi- sions complete, and SWPPP 11" x17" ESC/SWM plan sheet inserts are revised, as needed). 2. Submit SWPPP using county template located at: (Rev. 2) All other SWPPP review comments addressed. [remaining 30-Dec comments removed.] B. Pollution Prevention Plan (PPP) —see SWPPP item 2.b., above (Rev. 2)Addressed. The PPP content requirements can be found in County Code section 17-404. C. Stormwater Management Plan VSMP Regulation 9VAC25-870-108 requires the VSMP authority to approve or disap- prove a 5WMP. This plan is disapproved for reasons listed below. The stormwater management plan content requirements can be found in County Code section 17-403. 1. Please provide a separate VaRRM.xIs for phased development indicating (by phase IA404~0119esWnses tions) that minimums (forest -open space easement) shown on plans are sufficient. Sheet 2 lists Ph%e 2 of III, IV (4.0, 4.0, 4.0, 2.5 Ac.). These areas were referencing the zoning requirements, and not relating to stormwater compliance. For compliance, Phase 1, requires no open space but dedicating the a slope areas (-2 ac); open space dedicate rear of Ph 2 lots- 2.1 ac. ;w/ Ph3, —4ac w/ rear of Ph3 lots, and remainder of dedication in Ph 4. Since Level Spreaders in treatment train w/ Ext'd detention during Ph.1, sand filters in Ph 2, a removal surplus will exist until the final phase, requiring the remainder of FOS dedication. Nutrient summary provided in narrative. 2. Engineering evaluates filtering practices in comparison with VA DEQStormwater Design Specifica- tion No. 12 (strict evaluation against): a. Maximum contributing CDA of 5 Ac. for surface sand filters, and a maximum CDA of 2Ac. (recommended for perimeter or underground filters). Revise design per Spec. 12. Applicant response (5/12/22 letter): 'Plunge pools and demising walls have been introduced to reduce the maximum drainage area to under 5 acres to each filtration bed' As follow-up: 5 Ac. CDA is a design maximum for a surface sand filter. Please revise design. —Also, see Ad- ditional review detail. b. Please note that "filters have been used on larger drainage areas in the past, but greater clogging problems have typically resulted." VA DEQ Tech Spec No. 12,.44. (Rev. 2) Re- minder. Filters have been independently sized and placed for under 5 acre total cda's c. Soil testing requirements: Ref. 6.2, Soil testing requirements, p. 7. BMP specification No. 12. (Rev. 2) Persists. Applicant: 'Soil testing will be provided as available.' d. Overall sizing requirements, Ref. 6.1. — see Additional review detail. e. Depth to water table and bedrock, Ref. p. 4. —see Additional review detail. f. 7.3, Steep Terrain, several key design criteria, including (p. 11): —see Additional review detail. g. 7.4, Cold climate and winter performance (p. 11). —see Additional review detail. h. 6.5, Filter media and surface cover, Impervious drainage area: —see Additional review detail. i. 6.4, Conveyance and overflow (p. 8): —see Additional review detail. 3. It is unclear if filtering practices with associated upslope forebays and distilling ponds are capa- bleof meeting energy balance at release points to natural conveyances: Point of Analysis for each concentrated outfall is summarized on Outfall Compliance Sheet. a. Each discharge to a natural channel or terrain feature is required to meet 9VAC25-870- 66.13.3, Energy balance. (Rev. 2) Partially addressed. Applicant:'Shown for each concen- trated outfall proposed. In addition, all capturable sheet -flow is to be directed to the SWM devices. Perimeter terrain offers some limitations to achieving this goal, and is depicted on the Outfall Summary Sheet' As follow-up: i. A combined calculation packet with water quality /quantity tabulated Q1 ,rdata for each of the 8 outfalls depicted, with narrative (numeric /summary) information is required. Initial submittal included a series of individual routings. Please combine individual routings to create a single calculation packet with compliance narrative for each outfall. ii. Provide /show (in calc. packet) energy balance equation for each outfall to a natural receiving stream using equation listed at state administrative code 9VAC25-870- 66. B.3. Provide calculations for flood protection for each outfall; ref. 9VAC25-870-66.C. Summarized on Outfall Compliance Sheet; correspondence with the calculation packet was improved/labeled and num- bered to coordinate with plan references.. iv. Perform analysis to limits of analysis (9VAC25-870-66.B.4/9VAC25-870-66.C.3.). v. Sheet 12 depicts a series of Level Spreaders (ELS); for each ELS: 1. Provide discrete design: plan /profile (at a scale small enough to discern detail). 2. Reference ELS design criteria at VA DEQ Stormwater Design Specification No. 2, Sheet Flow to Filter or Open Space, Table 2.2. 3- ELS blow-ups provided Sheets 20-22 b. Can RGA provide detail or at a conceptual outline of how proposed SWM concept meets en- ergy balance at each discharge point? (Rev. 2) Persists. Applicant: 'Runoff Reduction Calcula- tions including pre/post mapping provided with this submittal.' Unless overlooked, calcula- tions (single document) not attached to May submittal. Please see item 3.a. for information requested. c. —see Additional review detail. d. —see Additional review detail. RGA Comment Responses Page 3 of 8 Overall site shown w/outfall locations & stormwater runoff labels to support routings, design and runoff reduction requirements. SWM Compliance sheet compares the preceding Pre & Post independent analysis sheets for concentrated runoff not controlled by on ELS 4. Given BMP design criteria, Engineering evaluates concept plan (as presented): —see Additional review detail. a. Likely requires adjustment /concept revision to ensure compliance with: —see Additional review detail. i. Max. CDA ii. Impervious CDA In. Steep terrain design criteria iv. SWM quality/quantity requirements (likely not met with limited number of filtering devices proposed) v. Note: Numerous, smaller, strategically placed filtering practices maybe required. vi. Engineering cannot approve, per ordinance (17-408.C.2.), BMP design that does not attain BMP Clearinghouse Website design criteria. Design that deviates from BMP Clearinghouse design criteria is an unapproved BMP (design). vii. —see Additional review detail. 5. —see Additional review detail. As follow-up: Engineering anticipates FOS easement will be proposed with phased final subdivision plats. Also, related comments, elsewhere. Revised to less than 5 acre cda to each filter. An exhibit for FOS is included in the WPO plan with recordation taking place on the final plat as suggested. Compliance in narrative illustrated for compliance of each Phase, proceeding from 1 to 4. Although, dedication of the FOS is not re- quired for compliance until Phase 4, FOS portions within each Phase will be dedicated as the phases proceed. 6. Rooftop runoff not captured /conveyed to SWM facilities: SWM /WPO plan design (and later, inde- pendent homebuilders) may not propose release of concentrated downspout runoff to fill or cut (Ref. 18- 4.3.3.C.4). (Rev. 2) Persists. Applicant: 'Noted in plans. Physically capturable rear yard drainage is being directed via rear yard swales toward stormwater inlets of facilities.' As follow-up: ... Proposed grading (unless overlooked) does not clearly define Swale conveyance to stormwater in- lets of facilities. Sheet 17 includes detail titled sheet flow measure-uncaptured, non-SWM areas with caption that reads 'Note: Utilize measure as necessary or required to prevent re -concentration of runoff from adjacent impervious surfaces. Rear yards shall be directed to an approved SWM facility when possible. Concentrated runoff leaving the rear yard toward steep slopes without sheet -flow management or storm- water management controls is prohibited' Sheet 17 note that reflects ordinance require- ments, prohibition, measures as needed, while helpful, lacks detail required to prevent rooftop runoff (concentrated storm runoff) release without capture /treatment. Rear yard swales have been designed to capture and convey all disturbed areas to either a SWM facility or a concentrated runoff location defined by a specific POA. Uncontrolled runoff to POA locations are controlled via Outlet Protection since contributing area or runoff is reduced from pre -developed conditions. 7. Please exercise caution when representing uncollected runoff as sheet flow, in post -developed condi- tion; for example: rooftop runoff requires transition to sheet flow, if not conveyed to a dedicated SWM facility. (Rev. 2) Persists. Applicant: 'Since the uncollected runoff areas were not included /treated as a portion of the necessary stormwater compliance strategy, areas of concentration are being directed to Level Spreaders, so as to achieve sheet -flow. Smaller, more remote and uncapturable areas —due to terrain constraints —shall have graded yards to initiate the sheet -flow conditions.' As follow-up: Please see comments related to ELS, elsewhere. Revised rear yards to capture & convey all impervious runoff from exiting the disturbed area onto steep slopes and have been directed with ditches to controlled outfall (POA locations) or storm sewer inlets and treated and detained to meet runoff requirements. 8. Items above relate to compliant filtering practice design (Engineering evaluates against BMP specifications), compliance with 9VAC25-870-66.B.3, and 9VAC205-870-69. 9. Additional comment may be possible with additional detail. RGA Comment Responses Page 4 of 8 best, J. Anderson [ Items 1.-10. below, based upon 30-Dec 2021 review comments —Also, see Additional review detail.] 1. General (Rev. 2) Items below persist. a. Provide access easement to each permanent SWM facility. Access, SWM and drainage easements have been added across the site. b. Provide easement for each SWM facility. c. Provide forest -open space easement. See Open Space Forest Exhibit d. Provide public drainage easement downstream of SWM facilities to the point discharge reachesa natural receiving stream, or to the property line [see 14-428]. SWM & Drainage Easements provided Comment noted. e. Note: Easements La, b, c, d may be recorded with a final subdivision plat. Off -site ease- ment (or written agreement) needed to construct /maintain temporary ESC measures, sedi- ment basins or outfalls, for example, must be recorded (or copy of the written agreement provided to county) priortoWPO plan approval. f. Nutrient credit purchase is required prior to receiving a grading permit, but is not re- quiredfor WPO plan approval. Comment noted. 2. Provide Calculation packet: (Rev. 2) May persist. —see Additional review detail. a. Combine multiple routing documents submitted 12/6/21 into a single document. Routings are now easier to follow as of software upgrade/reorganized/named/sequenced Added, see narrative b. Provide stormwater quantity and quality narrative that indicates compliance for overall development, and at each discharge point. (Rev. 2) Persists. c.— see Additional review detail. (Rev.2)Addressed. d. Include VaRRM.xIs. (Rev. 2) Partially addressed. As follow-up: Comments elsewhere re- quest VaRRM.xIs to show post -developed forested condition for phased development, with minimum post -development land cover (forest) values that correspond with intent to de- velop Dunlora Village in phases. These min. values are likely to be 4,4,4,2.5 Ac. for Phases I, II, III, and IV. See narrative These above acreages were the zoning mins for open space and are not relative to VRRM/FOS. Intend to have FOS provided within each Phase it co -exists within. Not required for nutrient compliance until Phase 4. e. Discuss runoff where plan indicates post -developed condition is sheet flow. Please note re- quirements for sheet flow analysis at 9VAC25-870-66.D. (Rev. 2) Persists. Also, seefollow-up at (email) review items 3.a.v. and 6. above. —see Additional review detail. D. Increased volumes of sheet flow resulting from pervious or disconnected impervious areas, or from physical spreading of concentrated flow through level spreaders, must be identified and evaluated for potential impacts on down -gradient properties or resources. Increased volumes of sheet flow that will cause or contribute to erosion, sedimentation, or flooding of down gradient properties or resources shall be diverted to a stormwater management facility or a stormwater conveyance system that conveys the runoff without causing down - gradient erosion, sedimentation, or flooding. If all runoff from the site is sheet flow and the conditions of this subsection are met, no further water quantity controls are required. f. TFRRgfeF!;hppt 16 FRIA!Rg data to the Galen lade.. packet (Rev 2) 0....4.a.. g. —see Additional review detail. (Rev. 2) Withdrawn. Ref. email to RGA, June 24, 2022 8:25 AM. Capture and conveyance of disturbed areas to each outfall has been provided so as to avoid any impervious conveyance toward steep slopes and only rely on sheet -flow condition for remote, managed turf slopes. RGA Comment Responses h. Revise VaRRM.xIs to reflect post -developed forest -open space land cover available to be Page 5 of placed in permanent deeded easement, that meets DEQ concept of FOS. (Rev. 2) Persists. Please see comment request for individual VaRRM.xIs tied to each phased plat, since plan indicates a minimum area to be platted as FOS easement, with each of (the 4) final subdivi- sion plats. 3. —see Additional review detail. (Rev. 2) Addressed; also, see 3/9/22 email section, above. FOS exhibit to preserve 18.5 ac. in total. See narrative for Phased breakdown. 4. —see Additional review detail. (Rev. 2) Withdrawn —Engineering accepts design is consistent with Steep Slopes Exhibit. Every attempt has been made to avoid any impacts to steep slopes resulting from grading or SWM facilities. 5. Sheet 10 a. —see Additional review detail. (Rev. 2) Withdrawn. See item 2.e., this page. b. Please label lots. (Rev. 2) Persists. Overall Phasing & Lot #'s provided c. Show roof leader line runoff discharge to stormwater conveyance system. —see Additional review detail. Please ref. email review item 7. above, for follow-up comments. Rear yard ditches were implemented to provide full capture of impervious surfaces and conveyance to managed outfall locations d. Label SWM facilities. (Rev. 2) Partially addressed. As follow-up: Please review plan sheets that display SWIM facility design, and ensure facilities are labeled in plan view: SWM 'A','R', etc. SWM facilities clearly labeled e. At each SWM facility outfall, indicate discharge is to an adequate receiving channel. SWIM facility discharge must be to an adequate receiving channel. (Rev. 2) Persists. Please ref. email review item 3.a.iii-iv., above, for follow-up comments. See Sheet 12, Stormwater Compliance & Sheet 11 Post-Dev DA Map f. All SWM facilities appear to discharge to natural terrain. Provide energy balance equations for each SWIM facility discharge to natural terrain. Also, please see Calculation packet com- ment, below. (Rev. 2) Persists. Please ref. email review item 3.a./3.a.ii., above, for follow- up comments. See Sheet 12, Stormwater Compliance g. Delineate Forest/Open Space Easement for SWM purposes. (Rev. 2) Partially addressed. Please ref. item 2.d., this page, for follow-up comments. FOS Exhibit provided h. —see Additional review detail. (Rev. 2) Addressed. 6. Sheets 12-15 (Rev. 2) Partially addressed. Please see email item 2�i. above, for follow-up comments. a. Label primary spillway outfall elevation on profile. Additional Specifications, inverts and details provided b. Label primary spillway pipe type, length, slope, INV IN/OUT on profile OK 7. —see Additional review detail. (Rev. 2) Addressed. 8. —see Additional review detail. (Rev. 2) Addressed. 9. —see Additional review detail. (Rev. 2) Addressed. Ref. sheet 17. 10.— see Additional review detail. (Rev.2)Addressed. B. Erosion and Sediment Control Plan (ESOP) Virginia Code §62.1-44.15:55 requires the VESCP authority to approve or disapprove an ESOP. This plan is disapproved for reasons listed, below. The erosion control plan content requirements can be found in County Code section 17-402. 1. Provide Notes, sheets 6 and 7: a. Preserved steep slopes (>25%) to be marked in the field and reviewed by A199A gWX'6@pWesponses ESC inspector prior to land disturbance. (Rev. 2) Partially addressed. As follow-up: PleasePage 6 of revise Note 3 sheet 6, 7, to read: "Preserved steep slopes (>25%) to be marked in field by contractor and reviewed by Albemarle County ESC inspector prior to land disturbance." That is: Albemarle does not mark slopes, but reviews contractor /engineer flagging. Added/revised on ESC, Phase 1 Plan Sheet 6 and Construction Narrative on Sheet 5. b. —see Additional review detail. (Rev. 2) Addressed. c. No permanent or temporary disturbance of preserved steep slopes is permissible. (Rev. 2) Partially addressed. As follow-up: Please revise Note 5 sheet 6, 7, to read: "No permanent or temporary disturbance of preserved steep slopes is permissible unless related to storm or utility line installation, and only if: No alternative exists; impacts are minimized; slopes are immediately stabilized; and pipe location across steep slopes is shown on an approved WPO or site plan" That is: extremely limited temporary disturbance is permissible for utility line or storm pipe installation only, where no other utility corridor exists, or is impracticable. Every attempt has been made to avoid any impacts to steep slopes resulting from grading or SWM facilities. 2. Sheets 6, 7 a. Label limits of disturbance, x.xx Ac. (Rev. 2) May persist. Please direct reviewer to label (x.xx Ac.) on plan sheet 6 or 7, if overlooked. LOD limits are more clearly defined b. —see Additional review detail. (Rev. 2) Addressed. c. —see Additional review detail. (Rev. 2) Addressed. Note: SB#3 emergency spillway removed. d. — see Additional review detail. (Rev.2) Addressed. e. — see Additional review detail. (Rev.2) Addressed. f. — see Additional review detail. (Rev.2) Addressed. g. — see Additional review detail. (Rev.2) Addressed. h. — see Additional review detail. (Rev.2) Addressed. i. — see Additional review detail. (Rev.2) Addressed. j. Sediment basin and sediment trap outfalls are problematic: i. Provide off -site permanent public drainage easement for SB#4 outfall (OP4 is off - site). (Rev. 2) Persists. Space for future maintenance problematic: limited or non- existent where detail shows SB#4 emergency spillway touching property line. SB#4 & SWM'E" was pulled back in avoidance of offsite encroachments. ii. —see Additional review detail. (Rev. 2) Addressed. FF9Fn ^ 2 of DEG deeu eRt [image removed with Rev. 2 comments]: Revise SB#3 Emergency Spillway: (Rev. 2) Persists. Emergency spillway eliminated with revision. Plan now shows sediment trap sP "''llway weir at property line, which does not address concern expressed with initial review comment (reprinted as 1.,2.,3., below). 1. Eliminate curved exit. Design using straight exit channel, per DEQ ref. doc. From p. 3, DEQ Emergency Spillway document: The topography must be carefully considered when constructing an emergency spillway. The alignment of the exit channel must be straight to a point far enough below the embankment to insure that any flow escaping the exit channel cannot damage the embankment. This may result in additional clearing and/or grading requirements beside the abutments, property line, etc. 2. Current design overlaps emergency spillway and fill slope at base of fill slope. There is a relatively high likelihood that flow may escape the exist channel and erode the embankment. Revise design. 3. Insufficient space: Emergency spillway is coincident with development prop- erty line. There must be adequate space to construct /maintain the spillway on the development parcel. Alternatively, off -site easement is required prior to WPO plan approval. Easements in connection with ESC measures may not be deferred until final subdivision plat, but any easement (or written agree- ments) required to implement VESCP measures must be in place prior to WPO plan approval.ZGA Comment Responses SB#3&4, including spillway designs were revised to address concerns. Oversized Page 7 of 8 iv. SB#4 Emergency spillway: (Rev. 2) Not addressed. 4. Revise to avoid undercutting fill slope SB embankment. Provide straight exit. SB#4 revised to eliminate need for Emergency Spillway. Over -sized iv. Provide armored emergency spillway design for any SB emergency spillway proposed to be in fill rather than cut. Consider velocity, channel dimensions, riprap classification, etc. Provide calculations for armored emergency spillways. (Rev. 2) May persist. Please see item 2.0.iii, above. All specified spillway sections remain in cut locations. v. SB#1 Emergency spillway: (Rev. 2) Not addressed. 1. Revise, since armored spillway terminates at top of fill slope; extend armor to base of fill slope. Adhere to DEQ Vegetated Emergency Spillway design criteria, and guidance a —see Additional review detail. (Rev. 2) Addressed. i. Revise bottom trap dimensions to L x W, for each ST. ii. Provide weir length, each ST. Spillway Rip -rap and overflow rip -rap channel section from SB#1 extended to existing channel is located in cut. 4. Sheets 12-15 a. Since each SB is destined for permanent service as a SWM facility, include relevant geotech- nical design, construction, inspection, and maintenance notes on the plan. Ref. VA DEQ Stormwater Design Specification, Appendix A, Earthen Embankment. Without these notes, there is risk or likelihood of inadequate geotechnical material testing, inspection, etc. during construction of embankments that transition from VESCP measure to permanent SWM facil- ity. (Rev. 2) Not addressed. Geotechnical considerations are critical for earthen embank- ments. Geo-technical notes added, including soil testing specifications. b. Provide sediment basin profiles. Do not combine SB and SWM detention as a single profile. SBs are temporary ESC measures, bonded, built, inspected, and maintained independent of future use as a SWM facility. Segregate ESC profiles from SWM profiles. (Rev. 2) Partially ad- dressed. As follow-up: i. SB#1. sheet 13 (comments (may) also apply to SB#2, SB#3, SB#4): 1. SB #1 profile indicates 1,380.2 CY storage required, 749.0 CY storage pro- vided. SB#1 appears undersized. If so, revise SB#1 to provide sufficient stored volume. Design change, Pond regraded and re -designed to meet requirements. 2. Provide profile consistent with VESCH Plate 3.14-2, to include these eleva- tions (in addition to top of dam and riser crest top elevations provided): a. design high water b. t9p Of m c. dewatering device d. sediment clean -out point e. bottom of base of riser structure SWM a is currently Level 1 extended detention w/forebay and micro -pools. Dewatering added for maintenance. Grading and outfall structure revised. 3. Show continuous proposed grade across floor of sediment basin #1. 4. Show clay core, cutoff trench, collars, as needed. Sheet 17 typical pro- vides inadequate detail. Facility is currently extended detention and does not permanently impound large quantities of water. Detail/notes revised accordingly. Clay core and earthen dam construction notes added. 5. If collars required, show in profile, and label collar dimensions. N/A 6. Show /label 24" pipe end section (VDOT ES-1, for example). Structures and pipes labeled with size, type, material & inverts. Storm sewer schedule added for SWM facilities 7. Permanent riser ht. >12' requires safety slab (VDOT SL-1) RGA Comment Responses Noted for access on structures deeper than 8', where appropriate. Page 8 of 8 Dimensioned N/A 8. Label SB#1 floor dimensions (L x W). 9. Provide baffles, as needed. 10. Label 24" pipe INV OUT. Pipe inverts labeled throughout plan -set. ii. SB#2, sheet 14 1. Label ST#14 in plan view. Numbered all traps & basins, some renumbered Ell Added Added Revised as appropriate. 2. Apply SB#1 comments to SB#2, as needed. 3. WQV (storage) required /provided not listed on profile. iii. SB#3 & Outfall to SB#4 (sheet 1S) 1. Recommend compaction note for MH /pipe nearly entirely in fill section. 2. Comments for SB#1 above may also apply. 3. Label is confusing: 'Sediment basin 3 shall be a sediment trap until the storm sewer outfall into sediment basin 4 is constructed (limit contributing drain- age area to less than 3 Acres). Note: Divert all runoff to SB#4 until basin & SWM 'D' outfall is operational'. Clarify proposed sequence /interdepend- ence of SB#3 (basin/trap), SB#4, SWM 'D'. Attempted to clarify. Issue is associated with phasing. SWM'D' will likely be converted to permanent Filtration prior to conversion of SB#4 into Ext'd detention. In addition, the phasing would not 'construct' the storm sewer outfall system from SB#3, until Phase 4. Ultimately SB#4 is large enough to accommodate the entirety, but needed the sediment trap initially to 'capture' the graded slope w/o escaping the site or attempting the use of silt fence; primarily due to lack of an adequate or manageable outfall location. iv. SB#4, sheetl6: Apply SB#1 comments, as needed. Ell c. Provide Sediment basin design (tables), per VESCH, 3.14 (size, baffles, collars, spillways, etc.). (Rev. 2) Not addressed. OK, Sediment Basin Dewatering added to Sheet 23 We thank you for taking the time to review these plans and hope to have adequately addressed your comments. Should you need any additional information, please let me know and I'll respond as quickly as possible. Sincerely, Mj Chris Mulligan, P.E.