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HomeMy WebLinkAboutWPO201100019 Review Comments Stormwater Management Plan 2011-05-05� 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 Road and SWM Plans (WPO- 2011 -00019 and SUB - 2011 - 00029) Plan preparer: Mr. Mike Myers, PE; Dominion Engineering Owner or rep.: Stonehaus Plan received date: 18 March 2011 (plan date 14 September 2007) Date of comments: 5 May 2011 Reviewer: Phil Custer The first submittal of the SWM and Road plans for Whittington Phase A (SUB- 2011 -00029 and WPO- 2011- 00019) received 18 March 2011, has been reviewed. Comments from the ESC plan were provided in a separate letter. Engineering can approve the plans after the following comments have been addressed. A. General Review (Also provided in the ESC letter with comment #12 added.) 1. In VDOT's review of the plan, the engineer noted that the current layout of this development does not meet VDOT's latest Subdivision Street Acceptance requirements, which requires multiple connections in multiple directions to adjacent parcels and the reconfiguration of internal streets to meet a specific interconnectivity index. Under my current understanding of state law, the applicant/owner is not required to make any modification to the development plan that would require "significant government action ". In this case, a significant government action would be a required action by the Board of Supervisors; a modification subject to the variation procedures outlined in 18- 8.5.5.3 of the County Ordinance would not be significant enough to automatically grandfather the previous road layout. The Zoning Department has informed me that many, if not all, of the road alignment modifications that would allow the development to meet current VDOT acceptance requirements can be varied by the Planning Director and will NOT need to go back to the Board of Supervisors. I will forward an official response from the Zoning Department once I receive it. Please prepare a variation request to the Planning Director to alter the road network to meet VDOT's current acceptance guidelines. I recommend discussing this with county staff and VDOT during one of our Monday afternoon "pre -app" meetings before making any of the alterations to the road plan. 2. VDOT has also commented on the lack of pedestrian facilities and stated that a sidewalk or path will be needed on at least one side of the streets within this subdivision. I believe there are provisions in the state code that allow the developer to provide these paths or trails outside of the ROW, but it is unclear what the standard and proximity to lots these paths would need to be. In other words, the primitive trail in the few locations indicated on the application may not be adequate for the replacement of a path parallel to the road. At a minimum, the primitive trail shown on the application plan behind lots 48, 50, and 77 will need to be constructed in this phase. Additional paths around and behind nearly all lots and of an greater standard may be required. All modifications to the trail or road sections will require that a variation be approved by the Planning Department. Albemarle County Community Development Engineering Review Comments Page 2 of 2 An overlot grading plan must be provided before any subdivision plat is approved. It would benefit all parties if this plan was included in this application so: -all disturbances could be protected with one ESC plan - post - development stormwater management plan and ditch calculations could be based on more accurate estimations of lot disturbance, yards (affecting the hydrologic coefficient, and watershed boundaries The overlot grading plan must include all of the requirements of Proffer 4c -4h. Please also provide more likely estimates for final yard, driveway, and house dimensions. The current estimates are optimistic and will inhibit the house construction process if more disturbance is desired by a builder than what was shown on the grading plan. 4. The lot layout may need to be modified when the subdivision plat is reviewed by a Current Development Planner because it does not match the layout shown on the application plan. 5. Please number the lots in the same manner as the application plan. 6. Lot 82 currently violates proffer 2. 7. Please update all note lists with the latest available standard county notes found in the current edition of the design manual, available online. 8. Please make sure all critical slopes are shaded. For instance, there is a slope on Lot 1 that changes 8ft vertically in 20ft horizontally. 9. The water main is proposed as being constructed now in the alignment of Road A to nearly the highest elevation of the subdivision. I recommend that the applicant submit this road segment for review by the county and VDOT so the waterline will not need to be adjusted later based on road geometry issues. 10. Is there an existing easement for the offsite sanitary sewer connection? If so, please clearly show its limits and label it with the deedbook and page number. If not, this easement must be recorded prior to issuance of a grading permit or approval of the final subdivision plat. 11. There are solid black squares and rectangles throughout the set containing the ESC and SWM plans that seem to be a graphical error. Additional comments may be necessary on all applications for the areas and proposals obstructed by these shapes. 12. (NEW) In order for the plan to move forward, a variation must also be given by the Planning Director for the modifications shown to the stormwater management plan. Engineering review prefers the stormwater concept shown in the application plan because nearly all of the lots can be treated by the in -line facilities. C. Road Plan Review (SUB- 2011 - 00029) 1. VDOT approval is required. 2. Please use the full buildout ADT estimates for the design of the pavement cross - sections. For instance, road A will likely have close to 900 vehicles a day travelling on it and Road E will have more through traffic than just the lots shown on this plan. Please update the pavement design based on these new estimates. 3. The private sanitary sewer laterals across the VDOT roads may be an issue. Please provide confirmation from VDOT that these laterals are acceptable within their ROW. 4. Please show sight distance triangles at all intersections including the sight distance line looking southwest from the road yet to be built between lot 59 and 86. The crest of Road E may create a problem vertically at this intersection. 5. Please show all street name, stop, and all other necessary traffic control signs. 6. The street cross - sections do not match the cross - sections of the approved application plan. Please request a variation to deviate from the approved cross - sections. 7. The median is poorly aligned with the approach of the eastbound lane. The median should be centered on the centerline of the roadway and the south side of the road should be flared similar to Albemarle County Community Development Engineering Review Comments Page 3 of 3 the north side so this transition isn't so abrupt. The ROW should be widened where the median was added so that the clear zone is controlled by VDOT. 9. Please label all edge -of- pavement radii at intersections and turnarounds /culdesacs. 10. Many more crossdrains are needed throughout the set. At least two CD -1s are needed on the loop road. At least two CD -ls and one CD -2 are needed on Road E. 11. Show all utility line crossings in the drainage profiles. 12. Show outlet protection tables and graphics /details in the road plan set. 13. Please clearly label the sump and callout its depth at ST -7. 14. Please provide a summary table for the driveway culverts. It seems most culverts can be 15" in diameter, but some may need to be larger to pass the 10 -year storm. 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 I acre -lot development has an overall impervious area of 20 %. 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. 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. 4. If you are designing for future development in these facilities, show the new lots, grading, house footprints, etc. on Sheet S 16. Biofilter I appears to be designed for future development with regard to water quality but detention for this watershed was not calculated. 5. This development is located in a county "rural land" comprehensive plan area. Therefore, SWM -I 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. 6. Please provide approval from VDOT to have the access road for SWM -1 come off of Old Albemarle County Community Development Engineering Review Comments Page 4 of 4 Lynchburg Road. 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. 8. Please modify the lot lines so that stormwater facilities are located on property owned by the Homeowners Association as much as possible. 9. Please remove all references to the Luckstone Biofilter mix and replace with "State- Approved Mix." 10. Please specify a trashrack on the orifices of each of the stormwater ponds. 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. 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. 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. 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 lft freeboard is still needed for a 100 -year storm however.) 15. In the detail for each pond embankment, please show the properly sized cutoff (aka key) trench and show an impervious core. 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 =10% of the biofilter footprint). 17. Please specify the size of the plants to be used in each biofilter. 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. 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. 20. Please provide an adequate channel from Biofilter -1 to the stream that is currently being assessed with the MS -19 analysis. 21. After the plans have been approved, please provide a completed Bond Estimate Request Form to the County Engineer to receive a SWM Bond.