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HomeMy WebLinkAboutWPO201000045 Review Comments Erosion Control Plan 2010-09-10� OF AL ,. vIRGI1`IZP COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 Project: Briarwood; Phases 5 and 6 Plan preparer: Mr. John Matusik, PE; The Engineering Groupe, Inc. Owner or rep.: Plan received date: Date of comments: Reviewer: Woodbriar Associates 27 July 2010 10 September 2010 Phil Custer The WPO plans (ESC and SWM) for Briarwood Phases 5 and 6, received 27 July 2010, have been reviewed by county engineering. The review of the road, drainage calculations, and subdivision will be performed when a SUB application is received by the county. Please note that there will be an additional $250 fee for a public road plan review that must be submitted with the SUB application. Engineering review can approve the WPO plan after the following comments have been addressed. A. General Review 1. A grading permit cannot be given by the county until the applicant has an approved subdivision (preliminary or final) or final site plan for these phases. If a final- before - preliminary subdivision plat is submitted, the applicant will need to bond the road and WPO plan prior to the county signing the subdivision plat. I recommend submitting the preliminary subdivision application. 2. This set is not a site plan. Please modify the title on the cover sheet to refer to a construction set or similar name. 3. Please provide confirmation from the Army Corps of Engineers that the proposed work through the floodplain (construction of the jogging path and grading to establish an adequate channel) does not require a nationwide permit. Please forward communication from the Army Corps representative that references the latest plan's limits of disturbance. 4. The channel on the north side of Dickerson Road appears to be an intermittent stream and possesses a 100ft buffer on both sides of the stream bank. Please show this on all applicable sheets. Please also show where this stream terminates and whether or not the branch of the channel behind lots 20 and 21 is an intermittent stream. 5. Please provide an existing conditions sheet. On the existing conditions sheet, please also clearly indicate the approved the grading of the previous site plan, referring to SDP -2006- 00041. 6. On the existing conditions sheet, please include the date and source of topography. Please also show the benchmark elevation and note the datum. Because there is FEMA floodplain on the property, the datum must be the same one used to generate the base flood elevations specified on the maps. Also, when I visited the site, I noticed a few discrepancies between the site conditions and the topography shown on the plan. 7. On the existing conditions, grading, and ESC sheets, please lightly hatch or shade all slopes of 25% or steeper. Only disturbance to these critical slopes necessary to construct the roads may be authorized by the county engineer administratively. All other critical slope disturbance must be approved by the Planning Commission. Since there is significant disturbance to critical slopes that may or may not be approved, County Engineering will not review another revision to this WPO plan until the slope disturbance has at least been authorized by the Planning Commission. Engineering Review Comments Page 2 of 4 8. In the set, please reference the FEMA map number and date of the map. 9. Please remove fill from the floodplain to construct the emergency spillway for Pond 5. This channel should either be in cut or on existing grade. 10. The grading plan shows both filling and cutting onto Dickerson Road. Please correct the proposed contours to show the limits of work stopping at the edge of the roadway. B. Stormwater Management 1. Please submit a stormwater management facility maintenance agreement for all properties where a stormwater facility is proposed. 2. On the cover sheet, please specify that this development is within a Water Supply Protection Area. 3. The pre and post development maps must have the hydrologic coefficient and time of concentrations indicated for each drainage area. 4. The SWM plan does not treat as much of the development as practicable which is required by 17- 315.A. Additional smaller facilities are necessary to capture the rear of lots. The grassed swale and vegetative buffer do not provide anywhere close to the required removal rate for these two drainage areas (77% and 82%). The grading of the lots may need to be modified to provide area for these facilities to be constructed. The SWM plan also appears to rely on the front of every house to drain into the roadway, which is impractical in many cases and likely not to be constructed in this manner. Please revise the conceptual SWM plan to capture more of the development. Engineering review recommends this concept be submitted with the preliminary application before providing a resubmittal of the full SWM plan with all of the necessary calculations. 5. The modified simple spreadsheet for Pond 5 must be updated to include the entire drainage area to the facility. The total area at the top of the spreadsheet should be 10.48 acres. The pre - developed condition should include existing impervious areas in the left column, which will lower the required removal rate but increase the Water Quality Volume. 6. The embankment must have a 3:1 downstream slope, impervious core, and cutoff trench that is a minimum 4ft wide and 4ft deep with 1:1 side slopes. 7. The drainage area to Pond 5 is below the typical acreage needed to maintain a sustainable water quality wet pond. Please provide a water balance calculation to determine whether the wet pond will be drawn down after a 30 -day summer drought by an acceptable amount. Section 6.2 of the new proposed SWM pond standards (available online) explains this calculation in detail. 8. A sediment forebay is needed for the southern inflow point. 9. The emergency spillway in the detail does not match the plan. 10. It appears a detention pond west of Hummingbird lane was constructed to meet detention requirements when Phase IA (subphases I and H) was built. Therefore, the pre - development rates that must be met for Pond 5 must use the pre - development drainage area as if Phase IA (subphases I and H) were not built. Please clearly show the 2 and 10 year pre - development rates that Pond 5 will need to meet. Assuming a 10.48 drainage area, a time of concentration of 25min, and rational coefficient of 0.3, I computed the 2 and 10 year pre - development rates as 8.17cfs and 11.00cfs, respectively. 11. The HEC -1 detention routing output on Sheet 29 is very confusing. I could not find any place where inputs for the pond geometry, riser characteristics, or the drainage area coefficient were entered. If more output options are available, please provide a full printout with the next submittal so the detention calculations can be reviewed in detail. An independent analysis that I performed shows that detention requirements are not met. 12. Please provide the weighted hydrologic coefficient computation that will be used for each of the drainage areas. If the SCS method is used, please use the values from Table 4 -6a and b of the VSMH in the calculation. 13. The tables to the right of Sheet 29 appear to be related to the Modified Rational Method. If the Engineering Review Comments Page 3 of 4 Modified Rational Method is used to check detention compliance, the critical storm must be used. 14. The SWM bond will be computed at the time of plan approval. C. Erosion and Sediment Control 1. Please provide the updated County General ESC notes from the latest edition of the Design Manual, available online. 2. This ESC plan is reliant on the ESC plan of Phases 1A, 1B, 8, and 4 of Briarwood (WPO -2006- 00066). A grading permit associated with the Phase 5 and 6 project cannot be issued until Sunset Drive is sufficiently constructed. Throughout the plan set (cover sheet, ESC plan view sheets, and the construction sequence), please refer to the chronological relationship of the two plans especially in reference to Sediment Basin 5 of Phase 4. 3. The general concept of the ESC plan in Phase 2 relies too heavily on silt fence. Silt fence only works well when the fence is placed parallel to contour lines with only a drainage area 100ft wide ( -1/4 acre drainage area for a silt fence of 100ft length). This standard appears to be violated consistently throughout the phase 2 plan as traps and diversions are removed to be replaced with silt fence when the fill operations begin. Sediment traps must be located so that they can remain in place during both phases. I recommend that the applicant set up a meeting with engineering review to agree upon sediment trap locations and grading limits before the next submittal. To start, the following modifications should be made: -A trap is needed just above the flood plain limit behind lot 5. The retaining walls and grading of lots 3 -6 will need to be modified to accommodate this trap. A fill diversion will be required on these lots to direct runoff to this trap (or SB 5 from Phase 4). The diversion from lot 15 must remain. -In phase 2, show a diversion over the storm system from 616 to 614. - Provide a trap north of lot 20 with a diversion to the east and a fill diversion to the west. The outfall of this facility will be directed to the culvert underneath Dickerson Road, which must be cleaned out prior to construction. This trap can be converted to a SWM facility if needed. -In phase 2, show a fill diversion to the north of Sediment Basin 5 as far as possible. -Place a trap in the back half of lot 11 and move the adjacent retaining walls south so that a diversion can be placed to direct this runoff to the trap above the Dickerson Road cut. Add a fill diversion south of the trap to meet the fill diversion from Sediment Basin 5. -In phase H, show a fill diversion to Sediment Basin 5 of the Phase 4. Additional comments may be required based upon the amount of required changes. 4. Minimum Standard 19 has not been met for this site. The narrative on sheet 30 mentions the 1% rule and compares the acreage of the development to the total watershed of the North Fork of the Rivanna River. However, I have concerns about the lack of an adequate channel up to the river banks. At the very least, a channel from Pond 5 to the river will be needed to convey the 2 year storm without erosion and the 10 -year storm without overtopping. An analysis of the pre and post development runoff discharge and volumes of the 1.45 acre and 2.12 acre drainage areas must be performed to determine whether an adequate channel analysis is needed from these sectors of the development to the river. If either the peak discharge or the volume of runoff from these watersheds is increased, Minimum Standard 19 is applicable. Enlarging the roadside ditch on Dickerson Road is impractical given the topography. The drainage area to Sediment Basin 5 in Phase 2 is greater than 4.3 acres. If the stormsewer system from existing Phase 1A is routed to the facility as shown, 10.48 acres must be used. If this stormsewer system will be diverted until the final conversion to SWM facility 5, then 5.18 acres Engineering Review Comments Page 4 of 4 must be used. If this is the applicant's intention, the stormsewer system from structure 619 to structure 614 and the downstream channel (required by the MS -19 comment) must be constructed at the same time as the sediment basin is installed and before any other clearing and grubbing on the site. Please show this storm sewer system in the Phase I ESC plan and clearly indicate its construction in the sequence. At structure 615, a temporary pipe will need to be installed to divert the discharge into the spillway. The pipe from 615 to 614 must be temporarily sealed until the SWM pond is converted. This should also clearly be specified in the plan. In this case, the TSD can be removed. 6. In notes 31, 32, and 33, please include Albemarle County personnel as having the authority to require more measures in the field and allow the removal of unnecessary measures after stabilization. 7. Please include in ESC note #22 that signs will be posted around each basin and trap stating: "Danger. Quicksand. Do not enter." 8. Step 6 of the Phase H construction sequence should be included in step 8. The site must be stabilized in the opinion of the county erosion and sediment control inspector before the basin can be converted to a SWM facility. In this step, please also state that structure 615 should be diverted into the pond if the sediment basin was not designed for 10.48 acres. 9. Please provide calculations for the dewatering orifice. A dewatering orifice of 6 inches appears to be too large for a watershed of only 5 acres. In the calculation, please also be aware that when additional dry storage is provided in a sediment basin the dewatering orifice should be calculated from the height to the required volume, not provided volume. 10. All slopes steeper than 3:1 must have a low maintenance, non - grassed groundcover specified. 11. "Super Silt Fence" is not a state approved standard. Please either remove this item from the plan or submit a variance request to the County Engineer, Glenn Brooks. 12. All traps must have embankments no steeper than 5ft or a variation must be requested to the County Engineer, Glenn Brooks. The sediment trap sizing will not be checked until this geometric issue is resolved. 13. Please provide spot elevations on diversions flatter than 2% to assure positive drainage. 14. An anti -vortex device for the riser structure must be provided. It does not appear that the proposed trash rack will act as an anti -vortex device. 15. A baffle will be needed on Sediment Basin 5 for the flow from the south if the facility will be designed for the full 10.48 acres. 16. An erosion and sediment narrative is required. The ESC narrative must contain all sections required by the VESCH. The narrative must mention that any offsite borrow site must be operating under a county grading permit. 17. A soil map is required. 18. A second construction entrance is required on the north end of existing Sunset Dr. 19. Please provide Dust Control symbols throughout the phase 1 sheets. 20. Please show areas for soil stockpiles. 21. Please show areas for staging and parking. 22. Please refer to the sediment basin proposed in this plan as sediment basin 7 to avoid any confusion with the basin in Phase 4. 23. Please remove the silt fence from the centerline of Dickerson Road. 24. Please include the jogging /walking path within the limits of disturbance. The review of this path will be performed with the subdivision or site plan application. 25. The ESC bond will be computed at the time of plan approval.