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Home My WebLink AboutWPO201300032 Review Comments WPO VSMP 2021-04-02$ County of Albemarle COMMUNITY DEVELOPMENT DEPARTMENT PYRGIN�l'. 401 McIntire Road, North Wing Charlottesville, VA 22902-4579 Telephone:434-296-5832 WWW.ALBEMARLE.ORG VSMP Permit Plan Review - Amendment #2 Project: Cascadia Blocks 4-7, Final ESC and SWM Plan Amendment #2 Project file#: WPO2013-00032 Plan preparer: Jimmy Taggart, PE, Dustin Greene, PE —Roudabush, Gale & Associates, Inc. lJTaggart(a),,roudabush.com. dweene(o)COUdabush.com ] 172 S. Pantops Drive, Charlottesville, VA 22911 Owner or rep.: Cascadia Development LLC [170 S. Pantops Drive, Charlottesville, VA 22911] Charlie Armstrong(crmstrongAsouthem-development.com ) Received date: 19 Feb 2021 Date of comments: 2 Apr 2021 Reviewer: John Anderson SWPPP, ESOP, Mitigation Plans not reviewed with this correspondence —NA (SWM Plan Amendment) WPO201300032 —Amendment 2, explained in cloud text image, below 1. Ensure proposed Amendment wet pond design and elevations yield post -developed Q1.5, Q2.0, and Q10 rates less than or equal to the approved post -developed release rate summary shown on pg. 7 of Calculation report d. 2/18/21 (PE -seal 2/19/21). Reasons: 9VAC25-870-488 includes specific requirements concerning "no increase in the volume or rate of runoff." [9VAC25-870-48.A.1. - hops://lis.virginia.gov/cg—i- bin/legp604.exe?000+reg+9VAC25-870-48 J. Also, significance of this SWM facility: it provides water quantity and quality control for 60.77 acre Cascadia subdivision, yet proposal reduces pond volume. Quoting VSMP Amendment #2 Narrative, sheet SWI: `The northeastern portion of the pond bottom and the eastern 2:1 pond upland slope was located in cut with depths of up to 30. Significant rock was encountered in these locations at the pond elevations, which was fairly hard rock, but was also fairly rippable. The contractor ripped as much of the rock as feasible using a hoe ram but encountered refusal at bedrock. For this reason, the provided pond volume is approximately 90% of the originally approved plans.' This is first notification of decision or circumstance concerning sediment basin excavation issues, which now affect permanent SWM facility wet pond design, we are aware of. There are implications, foremost of which: Any approved amendment to this SWM facility pond design cannot propose predictive /modeled Q1.5, Q2.0, or Q10 flows higher than flows approved with WP0201300032, 9/16/14, else it loses 9VAC25-870-48 (Grandfathering) eligibility. A SW24 Storm Sewer Calculations VSMP AMENDMENT A2 FOR WP02013ODD32 THIS AMENDMENT IS TO CONVERT THE SEDIMENT BASIN FACIUTY IMMEDIATELY ADJACENT TO ROUTE 20 NORTH INTO THE PERMANENT WET POND FACIUTY THAT WILL PROVIDE STORMWATER MANAGEMENT WATER OUANTITY AND OUAUTY CONTROL FOR THE 60.77 ACRE CASCADIA SUBDIVISION. THE NORTHEASTERN PORTION OF THE POND BOTTOM AND THE EASTERN 2.1 POND UPLAND SLOPE WAS LOCATED IN CUT WITH DEPTHS OF UP TO 30. SIGNIFICANT ROCK WAS ENCOUNTERED IN THESE LOCATIONS AT THE POND ELEVATIONS, WHICH WAS FAIRLY HARD ROCK, BUT WAS ALSO FAIRLY RIPPABLE. THE CONTRACTOR RIPPED AS MUCH OF THE ROCK AS FEASIBLE USING A HOE RAM BUT ENCOUNTERED REFUSAL AT BEDROCK. FOR THIS REASON, THE PROVIDED POND VOLUME IS APPROXIMATELY 90% OF THE ORIGINALLY APPROVE PLANS. THE PERMANENT POOL HAS BEEN LOWERED AND THE OUTLET STRUCTURES HAVE BEEN UPDATED SO THAT THE STORMWATER FACILITY PERFORMS WITH THE INTENT OF THE ORIGINAL APPROVED PLANS AND CALCULATIONS. PLEASE SEE SHEET 14 FOR THE UPDATED DESIGN DETAILS AND THE H&H CALCULATIONS FOR COMPUANCE WITH THE ORIGINAL APPROVED PLAN. 0 LU Cw L Q OIL 2 a) m 1 Q< FILE NUMBE 12.0065 SHEET SW1 Engineering Review Comments Page 2 of 5 2. Additional a. P. 2, Calc. report Predeveloped, Allowable Flows: Q1.5=0.52cfs 2.0=0.67cfs Ql 98 cfs Postdeveloped ted Flows: Q1.5 = 0.cfs Q2.0 = 0.60 60 cfs Q10=8.91cfs b. P. 7. Cale. report Allowable Release Rate: 01.5 (allowable) = 3.16(42,253/256,568) = 0.520 cfs Q2 (allowable) = 3.83(47,045/269.636) = 0.668 cfs Q10 (allowable) = 27.83(172,498 / 534,481) = 8.982 cfs Post -Development Release Rat Summary. 01.5 Post -Development Routed - 0.48 cfs < 0 20 cfs 02 Post -Development Routed 0.52 cfs < 0 68 cfs Q10 Post -Development Routed 8.83 cfs 8.982 cfs c. Please note that Amendment proposes Q1.5 that is 0.04cfs > Approved Q1.5 (0.48cfs), and Q2.0, Q10 peak flow rates that are each 0.08 cfs higher than 9/16/14 approved post -developed release rates. These increases, if not eliminated, void eligibility for Amendment review under SWM design technical criteria BC. [ Grandfathering] 3. Now is an appropriate time to provide geotechnical inspection reports for construction of the permanent wet pond embankment. Engineering seeks to confirm embankment construction (compaction, suitable material, cutoff trench, impervious core, etc.). Please submit relevant permanent geotechnical reports for the existing 13.3' high embankment with next submittal of the WPO Amendment application. 4. Emergency spillway elevation: a. Compare approved emergency spillway elevation, 356.10', with proposed, 356.00'. b. Compare approved 10-yr elevation 355.92' with proposed 10-yr elevation, 356.03' c. Revise design to ensure emergency spillway is not activated during the Q10 event. d. Provide at least 0.18' freeboard between 10-yr elevation and emer. spillway elevation. 5. Emergency spillway outfall: a. Approved 16' emergency spillway width at weir elevation 356.10 outfalls into a riprap channel with same section (16'). b. Approved riprap outfall emergency spillway design includes proposed grading intended to bend discharge away from adjacent property line, and toward a culvert beneath site access to Broadus Memorial Baptist Church. c. Approved design shows proposed and existing grades, to PL and beyond. d. Revise emergency downslope riprap spillway design to accommodate a threefold increase in weir width, from 16' to 48'. Ref images, below: e. f. g Engineering Review Comments Page 3 of 5 C �Y awe (sea U With revised emergency spillway riprap outfall design, provide detail to specify emergency spillway riprap keyed into embankment. Label riprap class and specify filter cloth underlay. Provide Note to clear and maintain the emergency spillway free of vegetative growth. See 6/l/20 satellite image, below. Vegetation covers lower reaches of Ex. sediment basin stone outfall. I 14 W) a Provide and label existing and revised proposed grades in vicinity of emergency spillway, to DroDerty line (PL) and bevond. Ref. unmoved Dian image. below: nd Y>� o• - �.i Engineering Review Comments Page 4 of 5 h. Provide design rationale as riprap outfall narrows by %: to the 48' width at weir. Consider velocity, momentum, and scour potential as riprap narrows, 48' to 16' (or other dimension). 6. Ensure primary spillway barrel slope is listed correctly, both approved and proposed principal riser pipe slopes appear incorrect. 7. Revise Detail 3/SW 14 since label reading `outfalls into rip -rip channel with same section' appears inaccurate. Embankment EIev.=358.00'% \ _-1 48' A2 Yr. EIev=357.00' r Grass Weir Pev.-356.0' J// Oress lInto Rip -Rap Channel wlh Some Section) � � EMERGENCY SPILLWAY DETAIL / Y 1(SECTION B—B) SW14 NOT TO SCA12 8. SWM facility wet pond sediment forebay does not yet appear to be constructed. Ref. 6/l/20 satellite No rationale is presented for change to SWM-1 forebay design details (proposed lower floor elevation, 10' aquatic bench vs. 15' approved aquatic bench), these changes are not approved. Please ensure SWM-1 forebay is constructed per approved SWM-1 forebay detail, below: Engineering Review Comments Page 5 of 5 Top of Dan Elev = 352.25 SWM-1 FOREBAY DETAIL o (SECTION B-B) "^°^�°•'a°+•°w''w mi"^'°r°`°I ouTWR .1, Ma *w, t° W wMi%E S�l�xowz. scue: faz' „yro�•.mKa "m- MT 3CAIL'. 1 -5 C. VSMP: SWPPP: Erosion Control Plan -NA D. Mitigation Plan -NA WP0201300032 Caseadia blocks 4-7 VSMP Amendment 2 040221