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HomeMy WebLinkAboutWPO200700045 Review Comments Stormwater Management Plan 2008-11-12� 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: Stonewater Subdivision [SUB200700077, WP0200700045] Plan preparer: Collins Engineering [email: scott @collins -en ing eering com] Owner or rep.: Rio Road Holdings, LLC [973 -9120] Plan received date: 18 June 2007 06 Sep 2007 (Revl) 29 Jan 2008 (Rev2) 11 Apr 2008 (Rev3) 04 June 2008 (Rev4) 11 August 2008 (Revs) 27 October 2008 (Rev6) Date of comments: 10 July 2007 24 Sep 2007 (Rev 1) 12 Mar 2008 (Rev2) 20 May 2008 (Rev3) 09 July 2008 (Rev4) 08 September (Revs) 13 November 2008 (Rev6) Reviewer: Jonathan Sharp Phil Custer (Revs) Phil Custer (Rev6) (Revs) With this site plan submittal, the applicant has submitted an amendment to the WPO plan (WPO- 2007- 00045) that was reviewed with the preliminary subdivision plat and road plans. However, since that WPO plan has not yet been approved, an amendment cannot be submitted. The review of the WPO plan with has been performed under the original WPO application number (WPO- 2007 - 00045). Please contact Debi Moyers for a refund of the amendment application fee. A review of the road plan was not performed because it was not submitted in the site plan set. All road comments are still outstanding. No WPO plan can be approved without the approval of the road plans. (Revs) The plans can be approved once the following items have been addressed. Please note that since a full set was submitted this time, a review of the road plan discovered an error that needs to be corrected before approval can be granted. Please see comment 26A. A. Road & Drainage Plans (SUB200700077) 1. Please provide the date and source of the topographic information: All topography should be at least visually field verified by the designer within the last year. Rev]: comments addressed. 2. Please provide all necessary offsite easements needed for right -of -way improvements. Rev]: It appears that the easement for Stonehenge does not include an easement to construct the interconnection shown on the plans. Please provide all required easements on the Stonehenge property. Rev2: The pending easement plat must be approved prior to approval of the Road plans and WPO Albemarle County Community Development Engineering Review Comments Page 2 of 6 plans. Rev6: Engineering comments regarding the easement plat for the stormwater facility will be given in a separate document. It does not appear that the easements on Stonehenge property have been submitted for County review. 3. VDOT approval is required. Rev]: VDOT approval is required. Copies of the plans have been forwarded to VDOT for review. Rev2: VDOT approval is required. Copies of the plans have been forwarded to VDOT for review. Rev6: VDOT approval has been granted. 4. Portions of the grading for Lots 16 -18 is missing from the grading plan on sheet S -3. Rev]: comments addressed. 5. Portions of the proposed trails are missing from the plans. Rev]: comments addressed. 6. Please show the required grading to install the proposed stormwater facility access /trail on the plans. Rev]: comments addressed. 7. All drainage easements are a minimum 20' wide. Required width: 10'+ (pipe dia. Or channel width) + 2'+ 2(depth -5'). The pipe, channel or structure must be within the center third of the easement. Rev]: It appears that not all drainage easements meet the Design Manual requirements. For example, pipe S -3 appears to need up to a 47' wide easement, when only a 30' wide easement is provided. Please show all drainage easements meeting the Design Manual requirements. Rev2: comments addressed. 8. Proposed slopes steeper than 3:1 must have low maintenance (not grass) ground cover specified on the plan. Rev]: Sod is a type of grass. Please specify a non -grass low maintenance ground cover. Rev2: comments addressed. 9. Overland relief must be provided for Structure 10 and Structure 10 in case of clogging. The failure of any system will not cause structures or yards to flood. Engineering recommends placing the low point in the road between lots 22 and 23 and lots 11 and 12 and then placing a well defined ditch between the lots. Please clearly label overland relief ditches on the plans and provide adequate details for installation as they are critical to prevent the flooding of structures or yards. Rev]: comments addressed. 10. Please provide provisions and easements for drainage across 3 or more lots. Dense development where fencing, decking, etc is expected should provide yard inlets and pipes rather than ditches. Rev]: comments addressed. 11. Please provide typical sections for proposed channels with locations referenced from the plan view sheets. Rev]: comments addressed. 12. Please provide cross drain locations shown and labeled with VDOT designations (CD -1,2) at every major cut and fill transition or sag curve on the road profile sheets. Rev]: comments addressed. 13. Please show and label the station of intersections with the street names on the road profile sheets. Rev]: comments addressed. 14. Please provide a transitioning detail (20' minimum) for roll -top curbing in front of any inlets. Rev]: comments addressed. 15. Please provide a VDOT designation (MH -1, DI -313, etc.) for each structure on the drainage profiles. Rev]: comments addressed. 16. Please provide the throat length for each drop inlet on the drainage profiles. Albemarle County Community Development Engineering Review Comments Page 3 of 6 Rev]: comments addressed. 17. Please provide the grate type for each grate inlet on the drainage profiles. Rev]: comments addressed. 18. Please provide a note for concrete inlet shaping (IS -1) specified on any structure with a 4' or greater drop on sheets DP- 2,3,4. Rev]: comments addressed. 19. Please provide a not for safety slabs (SL -1) in any structure taller than 12' on sheets DP- 2,3,4. Rev]: comments addressed. 20. Please provide end sections (ES -1) or endwalls (EW -1) on all pipe outlets. Endwalls for culverts 48" or taller. Rev]: comments addressed. 21. Please provide scour outlet protection at all outlets, corresponding to computations. Rev]: comments addressed. 22. Please provide destination structure labels for each drainage area on the proposed pipe and inlet drainage area map. Revl: comments addressed. 23. Please show any necessary grading needed for construction of the proposed right -of -way improvements. Rev]: comments addressed. 24. Please verify that ponding will not occur from drop inlet structure S -8 on the neighboring property. An easement may be needed. Revl: comments addressed. 25. Additional comments may be necessary upon resubmittal. 26. A road bond will be computed once the plans have been approved. Rev2: Please submit a schedule of completion and road bond estimate request. The road bond has been set at $532,650. Once you have received VDOT approval, please submit 2 copies of the road plans for our files. 26A. Rev6. The road plan does not appear to meet our ordinance requirements. Many planting strips are shown as 4.5ft when the ordinance states that they must be 6ft. Additionally, the ROW must include all sidewalks. This requirement can only be waived by the Planning Commission. [14 -410 and 14 -4221 B. Stormwater Management Plan (WPO200700045) 27. Please provide adequate drainage measures to ensure that lots 26 -34 will drain to the proposed Stormwater facility. One possible solution is to provide a ditch along the proposed Stormwater facility accessway /trail to the facility. Rev]: comments addressed. 28. Please provide the following information on the existing drainage area map: a. coefficient used for each drainage area as used in the comps b. time of concentration for each drainage area where applicable as used in comps Rev]: comments addressed. 29. Please provide the following information on the proposed drainage area map: a. coefficient for each drainage area, labeled on the map matching comps b. time of concentration for each area labeled on the map where applicable, matching comps c. for future development to be considered in the analysis, include assumed land uses, impervious areas, and hydrological coefficients on the proposed drainage map Rev]: comments addressed. 30. Please provide a plan view of the Stormwater facility as shown on the site or subdivision plan sheets, preferably at a scale of 1 " =30' or 1 " =20'. Albemarle County Community Development Engineering Review Comments Page 4 of 6 Rev]: comments addressed. 31. Please provide the Albemarle County general Stormwater notes on the plans. Rev]: comments addressed. 32. Please provide cross - section details of each facility including: embankments, principle spillway, emergency spillway, and sediment forebay. Please make sure the details include all needed information from the Albemarle County Design Manual Engineering Final Plan Checklist, pages 7 -8 and are in accord with the Virginia Stormwater Management Handbook. Rev]: The downhill side of the embankment for the SWM facility must be a minimum of 3:1 in slope. Rev2: comments addressed. 33. The extended detention basin must be designed as follows: a. normal pool elevation is labeled on plans b. 20% surface area pool areas 1.5' -4' deep; 40% volume c. 40% surface area shallow marsh 0.5' -1.5' deep, 40% volume, contains 1WQV d. 40% surface area high marsh 0.5' deep or less; 20% volume I was unable to verify whether or not the proposed facility meets these criteria, as the plan view of the pond is not at a suitable scale for review. Rev]: comments addressed. 34. Vehicle access must be provided to all forebays for the facility. Grade cannot exceed 20 %. Revl: Please provide adequate access to the forebay below Structure S -1. Rev2: comments addressed. 35. Please provide a completed Stormwater Maintenance Agreement and fee. The agreement should reference TMP 61 -183 and TMP 61 -184 and should be signed by both property owners. Revl: We have not received a copy of the Agreement. Please submit the agreement to Pam Shifflett. Rev2: We have not received a copy of the Agreement. Please submit the agreement to Pam Shifflett. Rev3: We have not received a copy of the Agreement. Please submit the agreement to Pam Shifflett. Rev6: We have not received a SWM facilities maintenance agreement for either property. Please contact Pam Shifflett for questions regarding the submittal of the maintenance agreement. The SWM easement plat is currently under review. Engineering comments will be given for the SWM plat (SUB- 2008 - 00253). 36. Removal rate computations should not include offsite drainage areas (The Stonehenge property that will not be treated for Stormwater quality should not be included in removal rate computations.). Rev]: comments addressed. 37. Please clearly show the assumed impervious values for future development (both the Treesdale development and the proposed townhouses). Please clearly differentiate between proposed impervious areas and future impervious areas. Rev]: comments addressed. 38. Please provide channel computations for erosion and capacity for emergency spillway channels. Rev]: I cannot find any emergency spillway calculations. Several of the details propose an emergency spillway (3.07 2a and 2b on SWM -2), while other details do not show an emergency spillway. If no spillway is proposed, remove all details of emergency spillways. Rev2: comments addressed. 39. Please provide drawdown computations for the extended detention facility. Rev]: I cannot find these calculations. Please provide calculations to show that the ]xWQV is drawdown over a 30 hr period per the VSMH. Albemarle County Community Development Engineering Review Comments Page 5 of 6 Rev2: comments addressed. 40. The routing model is confusing. The tables are not to scale and do not label peak values. The routing model does not appear to use critical durations. Rev]: comments addressed. 41. Please provide critical durations for the peak basin volume events for the 2yr, 10yr, and 100yr storms. Rev]: comments addressed. 42. Please clearly show the assumed C- values for future development (both the Treesdale development and the proposed townhouses). Please clearly differentiate between proposed C- values and future C- values. Rev]: comments addressed. 43. Please provide computations for composite C- values. Rev]: comments addressed. 44. Please provide pre - development C- values. Rev]: comments addressed. 45. Please provide the pre - development time of concentration. Rev]: comments addressed. 46. Please provide a stage- storage (elevation vs. storage volume) table with water quality volume /surface area requirements for facilities with volume requirements /surface area requirements. Rev]: comments addressed. 47. Please provide the hydraulic dimension and coefficients for each weir, orifice, culvert, or other control structure used in the routing model, matching the plans. Rev]: comments addressed. 48. Additional comments may be necessary upon resubmittal. a. Rev3: The SWM facility has not been routed to the critical storm duration. If a storm of greater duration was routed through the facility, a higher rate would be discharged for each of the design storms. Please adjust the calculations. Rev6: Comment has been addressed. b. Rev3: The 15" perforated pipe system is not represented in the routing. The perforated pipe system shown in DCR's detail is generally used to protect the drawdown orifice from becoming clogged. Please adjust the site specific detail and, if necessary, routing calculations to place the perforated pipe system over the 2" WQv orifice. Rev& Please specify trash racks on all orifices. c. Rev3: Will the ESC riser be replaced with the SWM riser once stabilization has occurred? Engineering review recommends designing one riser for both ESC and SWM purposes. It appears the riser has different top elevations. Rev6: Comment has been addressed. d. Rev6: Two feet of freeboard for the 100 year storm is required on all facilities that do not possess an emergency spillway. 49. A Stormwater Management bond will be computed once the plans are approved. Rev2: The SWM bond is set at $75, 000. C. Erosion Control Plan (WPO200600045) 50. There are both limits of clearing and limits of disturbance line on the plan. This is confusing. Please provide one limits of clearing and grading encompassing all disturbances, entrances, staging and parking areas, areas where sediment laden runoff will cross, or any construction Albemarle County Community Development Engineering Review Comments Page 6 of 6 related activities. This must match any landscaping and conservation plans. Rev]: comments addressed. 51. Areas within the limits of clearing and grading are not shown on the erosion control plans. Revl: comments addressed. 52. Please show the existing drainage divides on the plans. Rev]: comments addressed. 53. Tree protection should be located at the drip lines of the trees. Tree drip lines should be field verified. No disturbance can occur within the drip lines of any trees to be saved. Rev]: comments addressed. 54. No erosion control measures can be located in the way of construction access or grading. Please remove any silt fence or diversion dikes located in the way of grading. Rev]: comments addressed. 55. Please provide erosion control measures for all off -site disturbances. Rev]: comments addressed. 56. Please show outlet protection (OP) at all outlets. Rev]: comments addressed. 57. Please verify adequate channels (MS -19) at the outfall of the stormwater management facility. Rev]: Please provide adequate channels from the outfall of the facility to the floodplain. Rev2. comments addressed. 58. The contour areas used in the sediment basin calculations are incorrect. Please revise the sediment basin calculations and design the basin in accordance with the Virginia Erosion and Sediment Control Handbook and the Albemarle County Design Manual. Rev]: The contour areas and elevations do not match the plans. Also, the bottom of the basin is shown at 383, and the top of wet storage is shown at 382. Rev2: comments addressed. 59. Please specify the type of pipe used for the sediment basin and specify a trash rack. Rev]: comments addressed. 60. The embankment of the sediment basin is at a higher elevation than the permanent facility. Rev]: comments addressed. 61. Please specify safety fence and signs stating "danger, quick sand, do not enter." Rev]: comments addressed. 62. The baffle calculations are incorrect. Please provide calculations in accordance with the Virginia Erosion and Sediment Control Handbook. Revl: The baffle calculations specify no baffles to be provided and baffles are proposed on the plans. Please provide adequate baffle calculations. Calculations should be provided for each concentrated flow into the facility that is 30% or more of the total flow into the facility. Rev2: comments addressed. 63. Please provide a detail of a paved construction entrance. Rev]: comments addressed. 64. Additional comments may be necessary upon resubmittal. 65. An Erosion Control bond will be computed once the plans have been approved. Rev2: The ESC bond is set at $104, 000. If you have any questions, you can contact me at 296 -5832 ex. 3072.