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HomeMy WebLinkAboutWPO200900065 Review Comments Erosion Control Plan 2010-04-02OF ALg a® COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 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 (Rev. I) 18 February 2010 Date of comments: 8 January 2010 (Rev. I) 2 April 2010 Reviewer: Phil Custer The first revision to the ESC and SWM plans for the The Reserve (Belvedere Block 2), received on 18 February 2010, 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] (Rev. I) The drainage area from Sta. 29 +00 to Sta. 32 +00 is shown correctly, but only a fraction of the drainage area west of Belvedere Blvd. from Sta. 32 +00 to Sta. 43 +00 drains to the facility. The drainage map on DP-1 appears to be a closer approximation of the drainage area. By my calculations, the drainage area shown on Sheet SWM-1 west of the Belvedere Blvd. curbline is 4.60 acres. Of this area, only 130acres drains to the stormwater facility. Please update all calculations accordingly. 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] (Rev. I) Comment has been addressed. 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. (Rev. I) The spreadsheet must be updated to match the drainage area per comment #1. The drainage area in the table is based off of the pre- development area. This update should not affect the Water Quality Volume used to check the volume of the facility, only the RR required of the development. 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. (Rev. 1) Considering comment 1 and the tc provided by the applicant (0.33hr), the 2 and 10 year pre- development rates that I calculated are 19.8cfs and 46.4c/s, respectively. Please confirm in calculations submitted with the next revision. 5. The volume of the pond must not consider any contour below a depth of 6ft. [VSMH] (Rev. 1) The volume of the pond is not large enough. The Water Quality spreadsheet on sheet SWM -1 calculates a Water Quality Volume of 869.2cy from the impervious areas provided by the applicant. This water quality volume requires a pond with a volume of 3477 cy to treat at 65% (with an aquatic bench). The input data from the routing software shows that from 393 to 399, the volume of the pond is only 3158.7cy. Also, the contours are mislabeled in the forebay. 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] (Rev. 1) Comment has been addressed. 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] (Rev. 1) Comment has been addressed. 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) (Rev. I) The 2 and 10 year storms have been routed and the basinflow data has been provided. 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. (Rev. 1) The applicant is proposing a riser top with a stage- discharge relationship the county already has on record. The routing should be updated to replace the 30" weir top with the 36" Hanson Antivortex Device. Please contact me if you are having problems setting up the file. 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] (Rev. 1) I do not think most of the proposed plants are appropriate for use in an aquatic bench. Arrowwood viburnun is the only proposed plant specified for Zone 2 in Figure 3.05 -4 of the VSMH. Please propose an even distribution of at least two other species for plants specified for Zone 2 of the VSMH, or a similar source. 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. (Rev. 1) Calculations have been provided. However, the curve number calculation will need to be updated per comment 1. Using the adjusted data from TR -55 worksheet for post - development drainage area 1, I found the CN to be 86.6. [CN = 74 *(7.08121.79) + 98 *(11.93121.79) + 70 *(2.78121.79) = 86.61 The time of concentration used in the routing has doubled. In the routing, use a time of concentration that matches the design calculations for the pipe systems (0.18 hours) With the above changes, I anticipate that the BMP will meet detention requirements (2 and 10 year routed flows of 14.9 and 17.2). However, I noticed that if the top was raised to 402.5, the output of the 2-year storm would be reduced to 8.33 with only an increase of the 10 year storm to 17.6. This may be beneficial to the applicant when analyzing the downstream channel. 12. Please use an orifice coefficient of 0.6 in the routing. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Comment has not been addressed. 15. The SWM portion of the WPO bond will be calculated at the time of plan approval. (Rev. 1) Comment remains unchanged. 16. (Rev. 1) This facility is a Type III retention basin, not Type IL Please modify all incorrect references. 17. (Rev. 1) The upstream slope of the embankment is graded at 2:1 but is labeled on the plan and in the detail as 3:1. Please correct this discrepancy. D. Erosion and Sediment Control Plan [WPO- 2009 - 00065] 1. The tree protectio,. .- ..Ling 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) (Rev. 1) A dripline has been shown. County inspection will confirm in the field that tree protection fencing is placed at the correct locations. 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. (Rev. 1) The drainage area for the facility as shown is adequate. 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. (Rev. 1) The riser for the ESC facility will be used for the permanent SWM facility. 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. (Rev. 1) The sediment basin volume as designed is adequate. 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. (Rev. 1) Trap l has been removed. 6. In the January 6h 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. (Rev. I) There does not appear to be any sump provided downstream of inlet 82. A considerable amount of discharge from the existing sediment basin will likely bypass the inlet without the sump condition, especially if inlet protection is provided. It appears that grading offsite will be necessary in order to provide this freeboard. I recommend proposing a temporary connection from the outlet of the basin to structure 82 to reduce the amount of freeboard needed. Please also place the diversion on the downhill side of the storm sewer line. 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. (Rev. I) The dewatering orifice calculations are correct. However, final results must be rounded down rather than up. Please specify an 8" diameter orifice with a 10 "flexible pipe. 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. (Rev. I) Comment has been addressed. 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. (Rev. I) The permanent off site easements must be recorded prior to the approval of the site plan. The temporary construction easements should be graphically shown on the same documents of the permanent easements when possible. 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] (Rev. I) The design of the basin outlet does not appear to be adequate. The exit velocity of the barrel (- 17fps) will likely displace the 6" diameter riprap outlet protection. This is especially a concern because the slope of the outlet is close to 25% [ (386- 380)/241. Please reevaluate the stone size at the outlet (use Plate 3.19 -3 from the VESCH). The 3.75ft bottom width should be maintained and not widened as the riprap approaches the stream. Engineering review recommends dropping the invert out of the pipe so that a flatter channel with less energy is created as it enters the stream. A drop in the invert out would also require a drop in the invert in of the barrel so that the outlet velocity can be kept at an acceptable level (<15fps). 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. (Rev. I) Comment has not been addressed. The downstream channel analysis and any proposed improvements must be approved with the ESC plan prior to site plan approval. 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. (Rev. I) The watershed has been shown correctly, but the time of concentration of the watershed is not accurately estimated. If the applicant wishes to model the effect of the upstream culvert, a separate hydrograph will need to be generated and added to the two other hydrographs (from the Block 2 pond and the remainder of the watershed) with an appropriate lag time. Please contact me to discuss this further. 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. (Rev. I) Comment has not been addressed. The downstream channel analysis and any proposed improvements must be approved with the ESC plan prior to site plan approval. 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). (Rev. I) Comment has not been addressed. The downstream channel analysis and any proposed improvements must be approved with the ESC plan prior to site plan approval. 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. (Rev. I) Comment remains unchanged, though engineering review acknowledges that the outlet has been altered to provide a more natural convergence with the stream. 12. The ESC portion of the WPO bond will be calculated at the time of plan approval. (Rev. I) Comment remains unchanged.