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HomeMy WebLinkAboutWPO200900065 Review Comments Stormwater Management Plan 2010-01-08OF ALg a® COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax Project: The Reserve (Belvedere Block 2) [WPO- 2009 - 00065] Plan preparer: Mr. Scott Collins, PE; Collins Engineering Owner: Mr. Robert Hauser and Stan Manoogian; Belvedere Station Land Trust Developer: Mr. Todd Dofflemeyer; Cathcart Properties Plan received date: 23 November 2009 Date of comments: 8 January 2010 Reviewer: Phil Custer 972 -4126 The ESC and SWM plans for the The Reserve (Belvedere Block 2), received on 23 November 2009, have been reviewed. Comments for these plans are provided in this letter. The review of the site and road plans has been provided in a separate comment letter. C. Stormwater Management Plan [WPO- 2009 - 00065] 1. Belvedere Blvd. from Sta. 29 +00 to Sta. 47 +50 should be captured by the SWM facility. Currently, the whole road is not shown within the drainage area. Additionally, the cut sideslopes adjacent to the road also eventually drain to the Belvedere Blvd. drainage system and should be included in the facility's drainage area. [17- 312.C] 2. Please run roof drainages from the eastern half of Building 6 to the stormwater structure in the southeast corner of the parking lot and adjust all drainage and SWM computations accordingly. [17- 312.C] 3. Please provide a modified simple spreadsheet for the pond. In the table, include all grassed or landscaped areas on the plan as "lawn ". The pre - development impervious area for the project should be zero because the site was forested before clearing. 4. The pre - development rates for the 2 and 10 year storm that must be met for this pond should consider the whole watershed to be forested. Because 90% of the soils onsite are type C, the pre - development curve number is 70. The time of concentration used for the pre - development condition of 0.2 hours is acceptable. The pre - development area of 19.58acres is also acceptable. 5. The volume of the pond must not consider any contour below a depth of 6ft. [VSMH] 6. The 1, 2, 10, and 100 year 24 -hour storms are 3.5in., 3.7in., 5.6in., and 9.1in. Please adjust all calculations accordingly. [DM] 7. The slope of the Stormwater access path is greater than 20% measured between many contours. Please adjust the slope so that no portion of the access road is steeper than 20 %. [DM] 8. Detention calculations must be provided for 2 and the 10 year storms, not 1 and 2. Also, please provide all of the input and output basinflow data on the plan. (see below) 9. Please provide a stage /discharge curve for the proposed concrete anti - vortex trashrack so the routings can be independently checked. If you are proposing to use the Hanson Anti -vortex top, I have a copy of that stage discharge curve already and will use it in the routing. 10. Please provide a typical 10'x10' planting detail for the aquatic bench. The aquatic bench must have at least three species of aquatic plants. [DM] 11. Use a post - development CN of at least 90 (VSMH- Townhouse /Condominiums) or else provide a detailed curve number calculation. The current calculation in the plan is for the rational method coefficients while the routing is performed using the SCS method. 12. Please use an orifice coefficient of 0.6 in the routing. 13. The emergency spillway must either be relocated so it is placed on existing grade or armoring needs to be provided. Engineering review recommends that the spillway be moved 130ft to the west along the embankment so that it is close to existing grade with only a short flow path to the stream. If the general location of the embankment is to remain in its current location a private drainage easement will be needed on the downstream property. 14. A SWM facility maintenance agreement will need to be recorded before the site plan can be approved. Please submit this document with $17 fee directly to Ana Kilmer after consulting the guidelines available on the county website. 15. The SWM portion of the WPO bond will be calculated at the time of plan approval. D. Erosion and Sediment Control Plan [WPO- 2009 - 00065] 1. The tree protection fencing provided on the current plan has been placed at the preservation line which is not congruent with the Code of Development. The Code of Development requires the tree protection fencing to be placed "no closer than the dripline of any tree growing inside the preservation areas." If a tree survey stamped and signed by a licensed surveyor is not provided, place the tree protection fencing 20ft off the tree preservation line of the approved application plan. The fence must also be called out as chainlink and not orange construction fence. (pg. 28 COD) 2. Belvedere Blvd. from Sta. 29 +00 to Sta. 47 +50 should be captured by the ESC facility. Currently, the whole road is not shown within the drainage area. Additionally, the cut sideslopes adjacent to the road also eventually drain to the Belvedere Blvd. drainage system and should be included in the facility's drainage area. 3. A metal anti - vortex device is being proposed in the SB -1 detail. The concrete anti -vortex device seems to work for both ESC and Stormwater and engineering review recommends that it be used for both so the ESC to SWM facility retrofit is simpler. 4. Based on comment 2, the drainage area to this sediment basin may increase slightly and the dewatering orifice elevation may need to be raised. Engineering review also recommends that some of the excess volume in this facility be used in the wet volume for greater basin efficiency and less frequent maintenance. 5. Trap 1 appears to be unnecessary as long as the diversions to Existing Basin 1B and Proposed Basin 1 are provided. The facility may remain on the plan but can be removed in the field with the authorization of the inspector. 6. In the January 6th meeting between the applicant and the County, the drainage system from the VDOT road was discussed and it is my understanding that a storm sewer will be provided north of Flat Branch between the development and the stream. The construction of this line should occur immediately after the construction of Sediment basin 1. The outfall of existing basin SB 1 must be temporarily connected to this drainage system or an inlet with adequate headboard should be provided at the basin's outfall so that the abnormal diversion as specified in detail on sheet ESC -4 is not necessary (otherwise a variance to the county engineer should be provided). At the same time the storm sewer is constructed, the diversion directing runoff to the basin should be installed. Please call this out in the construction sequence. 7. The dewatering orifice must be sized using the height and volume required for the dry storage, not the volume provided. Calculating the required orifice diameter with the current proposed dimensions of the dry volume results in an oversized orifice and substandard basin efficiency. Please provide this calculation as outlined on pages III -107 and III -108 of the VESCH with the next submittal. 8. Mention in the narrative that any offsite property used for cut disposal or fill acquisition must have an approved erosion and sediment control plan. 9. Please provide a copy of the temporary construction easement negotiated with the property to the south referred to in the ESC narrative. An easement on the property to the north is also necessary. 10. A properly sized channel from the outlet of the pond to the stream is needed. Provide this channel cross - section and a calculation of the 2 and 10 year storms in the resubmittal. [MS -19] 11. Engineering review does not agree with the applicant's conclusion in the adequate channel analysis. Please see the list below for the county's comments and expectations of the analysis. a. The channel analysis seems to be missing graphics of the cross - sections for each of the three chosen locations of analysis. I also could not find any accompanying calculations with these cross - sections either. For each channel cross - section segment, please provide a separate n- value, permissible velocity (Table 5 -22), and computed velocity. A uniform n- value, permissible velocity, and computed velocity should not be used in the calculations. Water flowing within the banks of the channel flows much quicker than the water that flows above the banks. b. The hydrology and peak discharges to the channel cross - sections do not appear to be accurate. After looking at a printout from the county's GIS system, it appears the drainage area to sections A, B, and C is close to 95 acres. The drainage area appears to be bounded by Rio Road (between Huntington Road and Chapel Hill Road), a ridge 500ft north of Wakefield Rd, and approximately Free State Road. c. The soils in the area of the stream analysis are 39B, Hazel Loam, and 95, Wehadkee Silt Loam. The permissible velocities for each segment of the stream cross - section should be adjusted accordingly. Some segments of channel cross - sections are Cobbles and Shingles (bottom), Graded- Loam to Cobbles(bottom), Ordinary Firm Loam, Silt Loam, as well as Alluvial Silts. d. For the portion of the stream adjacent to the pond and downstream of the outlet, please remove all non - natural debris. There are several sections of old pipe obstructing stream flow. e. State guidance stresses that adequate channel analyses should be provided downstream until the developed site is 1% of the total drainage area. In this project's case, please provide an analysis up to and including the culvert to Shepherd's Ridge Rd. The locations of the analyzed cross - sections should be areas with high erosion concerns (bends or changes in cross - section). f. The volume and peak discharge to the channel on TMP 62F -F from Block 2 will both be reduced with development. Therefore, adequate channel regulations do not apply to the reach north of the Shepherd's Ridge pond. Engineering review recommends moving the location of the facility outlet 120ft farther downstream so less analysis of the channel is required and so that the new channel from the facility will enter the stream at a more natural angle, rather than perpendicular. (However, this would require a permanent private drainage easement to the stream on the adjacent property.) After walking the channel from the Shepherd's Ridge Rd. culvert to the outfall of the basin, I found several locations where erosion appeared to be occurring rapidly, particularly west of the confluence of the two channels from the ponds from the Shepherd Ridge Development and Block 1. Engineering review recommends providing a detailed streambank restoration plan to help with proving the adequacy of this channel by providing greater bank stabilization and more erosion tolerant cross - sections. 12. The ESC portion of the WPO bond will be calculated at the time of plan approval.