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Home My WebLink AboutWPO201300032 Review Comments WPO VSMP 2021-05-06$ County of Albemarle COMMUNITY DEVELOPMENT DEPARTMENT �tBGIN��' 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. dgreene(o)roudabush.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 (Rev. 1) 23 Apr 2021 Date of comments: 2 Apr 2021 (Rev. 1) 6 May 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. - https:Hlis.virginia.gov/cgi- bin/legp604.exe?000+re2+9VAC25-870-48 ]. 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 SW 1: `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. (Rev. 1) Addressed. Applicant response (letter d. 4/23/21): `An exception to the orifice size has been requested with this submittal. We are showing that a 2.8" orifice provides flows that are less than originally approved post -developed flows. A 3.0" orifice has been provided. See the narrative for the differences in these flows.' Note: A separate exception request approval will follow. A Engineering Review Comments Page 2 of 5 SW24 Storm Sewer Calculations SW25 Storm Sewer Calculations VSMP AMENDMENT 62 FOR WPO2DlMW2 THIS AMENDMENT IS TO CONVERT THE SEDIMENT BASIN FACIUTY IMMEDIATELY ADJACENT TO ROUTE 20 NORTH INTO THE PERMANENT WET POND FACILITY THAT WILL PROVIDE STORMWATER MANAGEMENT WATER QUANTITY AND QIIAUTY 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 HBH CALCULATIONS FOR COMPLIANCE WITH THE ORIGINAL APPROVED PLAN. FILE NUMBE 12."5 SHEET swi 2. Additional (Rev. 1) SUDDOrrive: see previous item. No design revision required 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. R 7- Calc. report Allowable Release Rate: Q1.5 (allowable) = 3.16(42,253 / 256,568) = 0.520 cfs Q2 (allowable) = 3.83(47,045 / 269.636) = 0.668 cfs Q10 (allowable) = 2T83(172,498 / 534,481) = 8.982 cfs Post -Development Release Ra Summary: Q1,5 Post -Development Routed - 0.48 cfs < 0 20 cfs 02 Post -Development Routed 0.52 cfs < 0 68 cfs 010 Post -Development Routed 8.83 crs 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 IIC. [ Grandfathering] (Rev. 1) Supportive: see item 1., above. 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. (Rev. 1) Not addressed. Applicant: `This is an Amendment submittal. We are not asking for final as -built review or trying to get off of any bonds. I believe this is a little early for inspection reports at this time and not required for amendment approval.' Asfollow-up: This Amendment plan proposes modification to existing conditions that already deviate from design, request for As -built information for a SWM facility that does not obtain design parameters before applying or approving additional modification is reasonable. For Engineering Review Comments Page 3 of 5 example, equipment mobilized to modify the emergency spillway could at the same time effect adjustments that As -built information may identify are needed. Please respond to initial comment for embankment construction /other information. 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. (Rev. 1) Addressed. d. Provide at least 0.18' freeboard between 10-yr elevation and emer. spillway elevation. (Rev. 1) Addressed. 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: (Rev. 1) Partially addressed. Applicant: `The emergency downslope riprap spillway design has been revised to include the 40' increase in the emergency spillway. A permanent diversion will be used at the new end of the emergency spillway to create a non -erodible channel down to the original 16' spillway near the outside toe of the dam.' As follow-up: Please add section arrows, notes, or labels to plan view to clearly indicate placement and purpose of modified permanent diversion dike. Proposed grade must redirect flow in the spillway, provide spot elevations or 0.5' proposed contours to emergency spillway so that grade redirects flow in the spillway. Design may rely on a modified permanent diversion dike to redirect flow, but without grade change, proposed modified diversion dike may experience inertial force not yet analyzed, or force it cannot withstand. e. f 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. (Rev. 1) Addressed. g. 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. (Rev. 1) Addressed. Engineering Review Comments Page 4 of 5 h. Provide and label existing and revised proposed grades in vicinity of emergency spillway, to property line (PL) and beyond. Ref. approved plan image, below: (Rev. 1) Partially addressed. As follow-up: Please see_follow- u at item 5.d., above. i. 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). (Rev. 1) Partially addressed. As follow -up -up: Please see oolow- at item 5.d. above. 6. Ensure primary spillway barrel slope is listed correctly, both approved and proposed principal riser pipe slopes appear incorrect. (Rev. 1) Addressed. 7. Revise Detail 3/SW 14 since label reading `outfalls into rip -rip channel with same section' appears inaccurate. (Rev. 1) Addressed. [Image removed with Rev. 1 comments.] 8. SWM facility wet pond sediment forebay does not yet appear to be constructed. Ref. 6/l/20 satellite image, below: [Image removed with Rev. 1 comments] 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 [Image removed with Rev. 1 comments]: (Rev. 1) Addressed. Engineering Review Comments Page 5 of 5 9. New: Propose barrier with warning on HOA property at top of steep 2:1 cut slope that falls to SWM wet pond. There are multiple risks: 2:1 slope 44' vertical interval between elevation 400f and 356f is not traversable. Design proposes permanent pool depth of not less than 5.60' ft. with depth approaching 13' during event that raises pond elevation to that of emergency spillway (355.93'). At top of 2:1 slope, single- family residences stretch from the lot south of and adjacent to 1849 Marietta Drive to 1891 Marietta. For the moment, satellite imagery indicates eight occupied SF dwellings: 1849, 1855, 1861, 1867, 1873, 1879, 1885, and 1891 with back lawns immediately adjacent to 44' vertical 2:1 cut slope ending in 5.6-13' deep permanent pool. Barrier must be permanent and include placard warning (text/symbols) of hazardous risk to residents. Not required with the initial plan, comment is justified, then and now, to ensure reasonable safety and to protect public welfare. Cascadia BOA is not required to fiord this expense, rather, this is a development expense. Landscaping alone is not sufficient at the top of the slope immediately adjacent to residential lawns. Elsewhere, for the entire perimeter of the wet pond, provide either hard structure barrier (fencing subject to ARB approval) or impenetrable thom-bearing landscaping to discourage human trespass with all but minimal gap or break in hard structure Aandscaping at pond embankment elevation/s. There shall be no gap in barrier at top of slope immediately adjacent to residential back lawns for the currently built eight, and future -assumed ninth SF structure that may be built on parcel south of 1849 Marietta Drive. C. VSMP: SWPPP: Erosion Control Plan —NA D. Mitigation PlanNA "'P0201300032 Caseadis blocks 4-7 VSMP Amendment 2 05062IreN I