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HomeMy WebLinkAboutWPO201100019 Review Comments Stormwater Management Plan 2011-12-07� 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: Whittington Phase A SWM Plan (WPO- 2011 - 00019) Plan preparer: Mr. Mike Myers, PE; Dominion Engineering Owner or rep.: Stonehaus Plan received date: 18 March 2011 (plan date 14 September 2007) (Rev. 1) 25 August 2011 (Rev. 2) 7 November 2011 Date of comments: 26 April 2011 (Rev. 1) 9 October 2011 (Rev. 2) 7 December 2011 Reviewer: Phil Custer The second resubmittal of the SWM Plan for Whittington Phase A (WPO- 2011 - 00019) received 7 November 2011, has been reviewed. Engineering can approve the SWM plan after the following comments have been addressed. D. Stormwater Management Plan Review (WPO- 2011 - 00019) 1. The drainage areas and hydrologic calculations will need to be updated once the overlot grading plan is provided that shows in more detail the housing locations, proposed grading, and driveways. In the post - development drainage area map, please show the new tree line. The hydrologic calculations rely heavily on minimal clearing being performed on site. This has a positive benefit with the stormwater calculations. But if more land is cleared than what was shown (either by the contractor during construction or by the homeowner in the future), the stormwater plan will be insufficient. Please either use more conservative estimates for lawn sizes or find a way to protect homeowners from permanently clearing their property to establish a larger lawn. Please also note that it looks as though the driveway assumptions are also overly optimistic with regard to total impervious area. Note that the VSMH shows that the average 1 acre -lot development has an overall impervious area of 20%. (Rev. 1) Please see the latter comments. 2. The application plan was approved showing stormwater facilities in -line with streams in the middle of the property and at one of the southwestern corners. The latest plan appears to shift these facilities out of the streams and closer to the roadways. The drawback to this modification is that less of the development is treated in stormwater management facilities and many lots will be forced to rely on individual lot BMPs (a cistern and infiltration trench). This concept is extremely difficult to analyze with regard to its removal rate and almost impossible to effectively enforce in a lot by lot basis. The individual lot scheme should be removed and the facilities relocated to the approved locations as shown on the application plan. State permits will be required for all disturbance to construct these ponds. Stormwater ponds are exempt from county stream buffer requirements if the county engineer determines the facility is in a practical location. Since these ponds were approved on the streams by the Boards with the ZMA, they can be constructed in the buffer without mitigation requirements. (Rev. 1) A variation for a shift of the stormwater facilities is currently under review by the Planning Department. I will forward a response to you once I receive it. The removal of the Albemarle County Community Development Engineering Review Comments Page 2 of 4 individual lot facilities has been approved. (Rev. 2) Comment has been addressed. 3. In each modified simple spreadsheet, please enter square footage of the grass areas in the cell titled "actively grazed pastures & yards and cultivated turf." This will increase the required removal rate and water quality volume for each of the facilities. (Rev. 1) Comment has been addressed. 4. If you are designing for future development in these facilities, show the new lots, grading, house footprints, etc. on Sheet S 16. Biofilter 1 appears to be designed for future development with regard to water quality but detention for this watershed was not calculated. (Rev. 1) Biofilter 1 is too small for the future impervious area assumed in this plan. Please adjust the size of this facility. Biofilters 2 and 3 will likely be undersized but are backed up by a stormwater detention facility which will act like an extended detention facility. Please label SWM - 2 as an extended detention facility throughout the plan and no adjustments to biofilters will be necessary. (Rev. 2) The footprint for Biofilter 1 is acceptable. However, the embankment must be at least 6ft wider. The current design of this embankment shows a 2ft width when the 3:1 slopes to 507 are considered. Also, to reduce construction steps, I recommend (this is not a requirement) designing Biofilter 1 so that the embankment for sediment basin 3 is left untouched. 5. This development is located in a county "rural land" comprehensive plan area. Therefore, SWM -1 requires a 65% removal rate. To achieve a 65% removal rate, a pond must have at least a loft aquatic bench around the perimeter of the pond with the proper ratio of pond volume in the aquatic bench. Please refer to Chapter 3.06 of the VSMH when adding the aquatic bench to the plan. Please also include a sample planting plan for the aquatic bench showing at least three different species of wetland plans. (Rev. 1) The aquatic bench should be installed along the embankment as well. A sample planting plan for the aquatic bench will three different species of wetland plants must be included in the sheet set. (Rev. 2) The design of the aquatic bench is acceptable except for the discrepancy with regard to the water elevation. The outer edge of the aquatic bench must match the pool elevation. The outer edge of the aquatic bench is designed at 487.5 which matches the routing input. However, Detail 71E13 shows the pool elevation at 488.75 which sets the aquatic bench too deep in the pond. My assumption is that this detail is incorrect and the review of this entire submittal used this assumption. Please clarify and correct this detail or the routings, depending on which was in error. 6. Please provide approval from VDOT to have the access road for SWM -1 come off of Old Lynchburg Road. (Rev. 1) Comment has not been addressed. (Rev. 2) Comment has been addressed. 7. Since the access roads to both SWM -1 and SWM -2 are greater than 10 %, please provide gravel surface on both roads. The road to SWM -1 will also need a properly -sized culvert where it crosses the road Swale. Please extend the access road of SWM -1 to the sediment forebay. (Rev. 1) The access road of SWM -1 was not extended to the sediment forebay. (Rev. 2) Comment has been addressed. 8. Please modify the lot lines so that stormwater facilities are located on property owned by the Homeowners Association as much as possible. (Rev. 1) Comment has not been addressed. (Rev. 2) Comment has been addressed. 9. Please remove all references to the Luckstone Biofilter mix and replace with "State- Approved Mix." Albemarle County Community Development Engineering Review Comments Page 3 of 4 (Rev. I) Comment has been addressed. 10. Please specify a trashrack on the orifices of each of the stormwater ponds. (Rev. I) Comment has been addressed. 11. Please clarify the elevations in detail 4/S 10. The elevation of A subtracted from the elevation of B does not equal 3.5 as indicated in the detail. (Rev. I) Comment has been addressed. 12. Please modify the biofilter detail(s) so that it is clear that lft of ponding will occur in it. The detail used by the applicant appears to indicate that the weir is pervious, like a sediment trap weir. I know this detail appears in the design manual, but it still needs to be clarified. (Rev. I) Comment has been addressed. 13. Emergency spillways cannot be located on fill. Please move the emergency spillway in each of the ponds to either end of the embankment in existing ground. (Rev. I) Comment has been addressed. 14. Both SWM -1 and SWM -2 requires significant adjustments to the embankment when transitioning from ESC to SWM. The riser in SWM -1 is also being modified. The facilities should be redesigned so that only orifices need to be plugged and opened to convert each basin to the permanent SWM pond. If a variation for ESC requirements of Plate 3.14 -2 is needed to allow this, the program authority (county engineer) will provide it as long as the lft freeboard for the 25 -year storm requirement is met in each sediment basin. (Regarding SWM requirements, a Ift freeboard is still needed for a 100 -year storm however.) (Rev. I) Comment has been addressed. 15. In the detail for each pond embankment, please show the properly sized cutoff (aka key) trench and show an impervious core. (Rev. I) Comment has been addressed. 16. A sediment forebay is needed in Biofilter 1. A sediment forebay on a smaller biofilter can simply be a lft tall, stone horse -shoe shaped wall around the inlet (forebay area =l0% of the biofilter footprint). (Rev. I) Comment has been addressed. 17. Please specify the size of the plants to be used in each biofilter. (Rev. l) Planting sizes have been specified. However, the landscape specifications for Biofilter 3 appear to be copied from Biofilter 2. (Rev. 2) Comment has been addressed. 18. It does not appear the critical duration storm was used in the routing of SWM -2. A higher discharge will likely be experienced downstream if a storm of lower intensity but greater duration occurs. Please adjust the calculations. (Rev. 1) Comment has not been addressed. The calculation method used on page 5 of the stormwater narrative is only an approximate estimate. The critical storm duration is a function of not just the watershed, but the characteristics of the stormwater facility. The routing should be performed in such a matter than the maximum discharge for each storm (2, 10, and 100) is routed. For example, storms of about an hour in length provide the greatest discharge for SWM - l. When the critical storm is routed, the 100 -year discharge for SWM -1 has less than lft of freeboard. (Rev. 2) Comment is no longer applicable. The applicant is currently using SCS hydrology to analyze basins. 19. The calculations package has the title of the each sheet at the bottom of the previous page of the report which is very confusing. For instance, page 34 shows the 100 -year routing of DA -O, but the title appears at the bottom of page 33. Please correct. (Rev. I) Comment has been addressed. 20. Please provide an adequate channel from Biofilter -1 to the stream that is currently being assessed Albemarle County Community Development Engineering Review Comments Page 4 of 4 with the MS -19 analysis. (Rev. 1) Comment has been addressed. 21. After the plans have been approved, please provide a completed Bond Estimate Request Form to the County Engineer to receive a SWM Bond. (Rev. 1) Comment remains unchanged. (Rev. 2) Comment remains unchanged. 22. (Rev. 1) The width of the embankment for SWM -I must be 8ft wide. The detail for this facility shows a width of 27ft. (Rev. 2) Comment has been addressed. 23. (Rev. 1) Time of concentrations should decrease with development. (Rev. 2) Comment has been addressed.