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HomeMy WebLinkAboutWPO202200038 Review Comments WPO VSMP 2023-01-141/14/23 —Breezy Hill Subdivision A. Please use VRRM.xls 2011 BMP standards and specs, since 2013 is draft. Albemarle has not adopted 2013 draft VRRM.YJs B. Consider VRRM.YJs post -development project land cover values, beginning p. 1,155 of calc. packet. Current SWM configuration does not reduce TP Load by 26.88 lb. (plan sheet 24, calc. p. 1,171); please ref. 2011 VA DEQ Stormwater Design Specification No. 2: a. [Attached review comments, 21-Oct. 2022, SWM review item 12k.,1. and 14: It was not until a complete VRRM.YJs was submitted that total reliance on VA DEQ Stormwater Design Specification No. 2 became evident. (sheet flow discharge to conserved open space)] b. Ref. VA DEQ Spec. 2, p. 13, Engineered Level Spreaders, which reads, in part: `Figure 2.3 above represents a configuration that includes a bypass structure that diverts the design storm to the level spreader, and bypasses the larger storm events around the Conserved Open Space or Vegetated Filter Strip through an improved channel.' c. This design does not appear to provide bypass for larger storm events around the Conserved Open Space through an improved channel. Similar to Dunlom Village design, present clear graphic design and calculations that show improved channel bypass of ELS to direct larger events (less frequent than 1-yr energy balance /less frequent than 1-in./hr. event: ELS to treat larger of 1-yr. EB Eq., or 1-in./hr. Rational Eq. flow) across stream buffer, without erosion, to receiving stream. d. VA DEQ Design Spec. No. 2, p. 14 third (solid) fifth (open) bullet reads: `For the purposes of determining the Level Spreader length, the peak discharge shall be determined using the Rational Equation with an intensity of 1-inch/hour.' Using rational eq. Q ='iA, with low single-family residential runoff coefficient of 0.33 (there is no ref to runoff coefficient in USDA NRCS TR-55 Hydrology Guidance- but 0.33 is not overly -conservative) yields Q=0.33(1 in)(21.80 Ac. PA A]) =7.26 cfs, for purposes of determining ELS length. Given 40 If /cfs (forest), ELS L=290.7' Max. ELS L =130'. This literal reading is reviewer's interpretation of VA DEQ Design Spec. No. 2; please advise if error in calc. or reasoning. e. Overall: i. Please ensure every concentrated discharge meets energy balance, whether discharge is to stream to NW, or other feature in other direction. ii. Make every effort to correlate calculations to plan design. iii. Provide summary tables on the Ulan for each discharge to a receiving stream, or drainage feature. iv. Provide `pop-up' detail for disconnected (rear -facing roof) discharge to rear yards. Sheet 22 Note is accurate, and Engineering commends this plan note. NOTE: CONCENTRATED RUNOFF LEAVING THE REAR YARD TOWARD STEEP SLOPES WITHOUT SHEET -FLOW MANAGEMENT OR STORM -WATER MANAGEMENT CONTROLS IS PROHIBITED. v. All POAs should clearly identify whether analysis is quantity or velocity or some other analysis. POAs most often refer to a single point of SWM quantity analysis. vi. Ensure post -developed DAs are identical with VRRM.YJs DA A, B, C, D, as they must be. VRRM.YJs DAs must be shown on the WPO plan. Utilize identical labels. Drainage areas other than A, B, C, D (VRRM.xIs DAs) may be shown, but note that all discharge from development must meet stormwater quantity requirements, which, for channel protection, requires either sheet flow across rear lawns, or energy balance for point discharge (1-yr storm event). vii. For VA DEQ Stormwater Design Specification No. 2 Sheet Flow to Conserved Open Space (soils A/B): 1. Provide soil maps for areas below ELSs. Show A, B soils for this BMP. 2. Ensure ELS length (L) is calculated based on Rational Equation: 130' limit. 3. Revise VRRM.xls post -developed land use table to list post-dev conserved open space soil type A or B, that is non -zero. If 75% VRRM.xls credit claimed for discharge to soil type A, B, then, within DA, there is a measurable area below ELS that is either soil type A or B. 4. Ensure improved channel/s for event/s > 1-in./hr. bypass ELSs and extend across stream buffer /floodplain to receiving stream. Improved channel may not discharge storm event flows > 1-in./hr. to unimproved floodplain given erosion (MS-19) concern (Note Q10 value=55cfs, for example), but must extend to receiving stream, and discharge there without causing erosion of stream bed or channel. Alternatively, ELS maybe designed to transition Q10-yr to sheet flow, with ELS Max. L =130'. viii. Design that meets Min. VESCP NSW standards will be approved, but consider what is being proposed: 1. VRRM.xls is Virginia Runoff Reduction Methodology. 2. VRRM.xls indicates 26.01 lb./yr. TP load reduction is required. 3. The single SWM measure proposed is a series of ELS to discharge to stream buffer: VA DEQ Stormwater Design Specification No. 2, Sheet Flow to Conserved Space, A-B soil, 75% phosphorus reduction. 4. Project is not required by VAC to consider pre-dev land cover in the new dev VRRM.xls, but existing forested condition limits to maximum degree storm runoff. Impervious and managed turf release higher volumes of runoff according to VRRM and every other hydrologic methodology. 5. Existing site is forested /rural with a few scattered homes /outbuildings. 6. Project proposes 60.93 Ac. post-dev managed turf. 7. Project proposes 11.20 Ac. post-dev impervious area. 8. Project proposes 10.20 Ac. post-dev forest /open space. 9. Project requires 26.01 lb./yr. TP load reduction. 10. No BMP other than VA DEQ Spec. #2 is proposed. 11. Development will increase storm runoff, prior to control. 12. Development is subject to quality (phosphorus), quantity (volume), erosion (velocity) and other considerations. 13. Complete reliance on ELSs to achieve TP load reduction is subject to requirements at DEQ Spec. #2, particularly table 2.2, and p. 14. Length of ELS is subject to specific design criteria: must accommodate 1-in /hr. event across turf -impervious watershed that routes to it. Larger events may bypass via improved channel. ELS Max. L =130'. 14. Engineering cannot for the moment accept the idea that reliance on ELSs at development perimeter is a viable approach to reduce TP 26.01 lb. 15. Proposed design does not meet VA DEQ Stormwater Design Specification No. 2. C. We hope to discuss options for meeting required 26.01 lb./yr. reduction via combination of VA DEQ Stormwater Design Specification No. 2 discharge to conserved open space, post -developed forest -open space land cover unrelated to ELS (both Design Spec. and other forest -open space require FOS easement), additional on -site BMP practice/s (no BMPs proposed apart from ELS discharge to open space), and /or, nutrient purchase. Proposed ELS design does not meet VAC requirements. D. Remove ELSs from stream buffer: Article VI, Stream Buffers, WPO ordinance (chapter 17) does not permit SWM facilities in stream buffers. Hemico Level Spreaders #1 and #2 and #3A DEQ ELS are located in buffer, or, in case of outfall #3A, requires grading at edge of buffer to install #3A. E. Comments reflect view that 26.01 lb. phosphorus reduction cannot be achieved via VA DEQ Spec. #2 discharge to conserved area, A/B soils. VA DEQ Spec. 42, p. 13-14: `Engineered Level Spreaders. The design of engineered level spreaders should conform to the following design criteria based on recommendations of Hathaway and Hunt (2006), or a locally approved standard that meets the intent of these criteria, in order to ensure non -erosive sheet flow into the vegetated buffer area. Figure 2.3 above represents a configuration that includes a bypass structure that diverts the design storm to the level spreader, and bypasses the larger storm events around the Conserved Open Space or Vegetated Filter Strip through an improved channel.' Pg. 14 continues: `An alternative approach is that utilized by Hemico County, where pipe or channels discharge at the landward edge of a floodplain or stream (Resource Protection Area or RPA) buffer. The entire flow is directed through a stilling basin energy dissipater and then a level spreader such that the entire design storm for the conveyance system (typically a 10-year frequency storm) is discharged as sheet flow through the buffer. (Refer to Henrico County's Environmental Program Manual; Chapter 9, Minimum Design Standard 9.01 "Energy Dissipator": hftp7//www.co.henrico.va.us/works/eesd/.) Please note the Hemico alternative approach requires that the entire flow of larger storm events (10-yr) be directed through a stilling basin energy dissipator and then a level spreader such that the entire design storm for the conveyance system is discharged as sheet flow through the buffer. This cannot happen, given Hemico spreader # 1 or #2 Q(10): 14.0, 55.0 cfs, respectively. VA DEQ Spec. table 2.2 stipulates 40 If length of ELS per Icfs, if discharge is to forested /reforested area, as is the case at Breezy Hill. If Q(10) =14cfs and this is one of the Spec. 2 DAs (assume it is), then for 14cfs, ELS L = 560' , for Q(10)=55cfs, ELS L=2,200', or 0.4 mile. DA in Ref blue line, F. Plan sheet 11 (may repeat certain comments, above): a. Label DAs A, B, C, D such that the four DAs identified in VRRM.xIs correspond clearly with plan sheet post-dev DAs. b. Calculations i. Provide summary that discusses (compliance) significance of 15 points of analysis (POAs) listed among 26 items listed in computational index summary, submitted as 1-p. doc to accompany 1,181 p. calc packet. 1,181 pp. of undistilled data without summary has limited value. For example: Review cannot interpret significance of ditch #1,2,3 for POA# 1, or ditch 1,3,4,5 for POA #3. POAs typically consider a single point. POAs may refer to velocity analyses in multiple ditches, but then, each would be a single POA. Nomenclature is confusing. Design intent should be clear. Narrative context is missing. Demonstrate compliance clearly for SWM quantity and quality, for ditch velocities, though ditches may be removed as surface ditch conveyance of storm runoff is routed across lots via yard inlet /pipe system. ii. VRRM.xls: 1. Revise DA A, B, C, D to report a nonzero value for A or B soil, as required, if VA DEQ Spec. #2 discharge to conserved space is SWM control method. c. ELS i. Remove ELS from stream buffer. ii. Ensure VA DEQ Spec. 2 table 2.2 overall width below ELS lip exists. iii. For each of 3 ELS details, sheet 22, label width below ELS lip. iv. Any bypass of ELS must be via improved channel to stream. v. Show complete bypass details, show how flow splits, is diverted, etc. vi. Provide SWM facility access easement to each ELS that is a quality or quantity measure. vii. Provide SWM facility easement to each ELS that is a SWM control facility. viii. Provide ELS bypass improved channel design, V2-yr, with channel cross-section, with channel lining specified. G. Show lot lines to aid review of surface swale /ditch runoff across lots: 3 lot Max. Similar to Dunlom Village: if surface runoff proposed to cross > 3 lots provide yard inlet capture /pipe conveyance.