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HomeMy WebLinkAboutWPO201200070 Review Comments Stormwater Management Plan 2012-09-12ALg�,�� �'IRGINZ� COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road, Room 227 Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 Project: Whitewood Road Day Care Center; SDP - 2012 -00043 and WPO- 2012 -00070 Plan preparer: Mr. Frank Pohl, PE; Pohl Consulting, LLC Owner: Global Country of World Peace Date received: SDP: 16 July 2012, WPO: 8 August 2012 (Rev. 1) 5 September 2012 Date of Comment: 17 August 2012 (Rev. 1) 12 September 2012 Engineer: Phil Custer The E &SC, SWM, and Final Site Plans for Whitewood Road Day Care Center have been reviewed. The plans can be approved after the following comments are addressed. A. Final Site Plan Comments (SDP- 2012 - 00043) 1. Please provide a low maintenance, non -grass groundcover on the southside of the entrance off of Oak Forest Drive where 2:1 slopes exist. [DM] (Rev. 1) Comment has been addressed. 2. Please provide a guardrail above the retaining wall for the travelway on the adjacent property. [18- 32.7.2] (Rev. 1) It does not appear there is enough room to fit the guardrail between the sidewalk and the back of the retaining wall. To help clarify, please provide another retaining wall detail showing the guardrail and sidewalk instead of the light pole and fence. Given the width of the wall and the batter, the sidewalk may need to go on the other side of the travelway. 3. Please show the existing grade in the drainage profiles. (Rev. 1) Comment has been addressed. 4. Please show the grading necessary to establish the proper sight distance on the adjacent property. A vertical profile of this sight line is necessary to understand the grading limits. [18- 32.6.6] (Rev. 1) Comment has not been addressed. 5. (Rev. 1) Approval from Charlottesville Gas is required prior to site plan approval. Please forward their approval letter once it is received. B. Stormwater Management Plan Review Comments (WPO- 2012 - 00070) 1. This comment is only advisory. The applicant is proposing infiltration throughout the plan. The use of infiltration on such a dense site is not prohibited by the county but there are inherent risks if the soil is damaged during construction or choked after a few years. If this were to happen, it will be the sole responsibility of the property owner to design and construct an alternative SWM Plan. 2. The primary underground infiltration facility is in close proximity to two sanitary sewer lines and the foundation of the building. The construction of both of these things will result in gravel backfill even closer to the infiltration facility. To prevent water escaping the infiltration trench and avoiding treatment, please make sure there is at least loft of undisturbed soil between the outside of the primary infiltration facility and the excavation for the sewer line and building foundation. Please expand the typical cross - section detail on page C -5 to show the minimum distance to these areas of excavation and backfill. (Rev. 1) Please locate the gas line in the detail. Otherwise, this cross - section is approved. 3. Chapter 3. 10 of the VSMH requires that the bottom of the infiltration facility be greater than 2 -4 above the water table or a rock layer. The report provided to the county does not seem to investigate this. An addendum to this report is necessary to get this confirmation. (Rev. 1) Comment has not been addressed. Before the plan can be approved, the depth of the highest point of the water table must be confirmed by afield examination to be an appropriate distance from the bottom of the infiltration facility. 4. Chapter 3.10 of the VSMH requires that permeability tests are needed every 5Oft of an infiltration test. The county will not require that many for the "exfiltration" trench, but at least three are needed at the elevation of the bottom of the stone. (Please also see the following comment.) (Rev. 1) Infiltration trench has been eliminated. This comment is no longer necessary. 5. Please relocate the exfiltration trench so that its edges are outside of the existing storm drain and gas easements. (Rev. 1) Infiltration trench has been eliminated. This comment is no longer necessary. 6. Please provide a profile of the exfiltration trench clearly showing the bottom elevations and pipe inverts. The trench should be stationed in plan view to match the profile. (Rev. 1) Infiltration trench has been eliminated. This comment is no longer necessary. 7. The sod proposed over the top of the infiltration trench will not allow infiltration into the facility and most of the water from the drainage area will bypass. Please propose at least a lft wide stone opening to allow overland flow to infiltrate into the gravel trench. (Rev. 1) Infiltration trench has been eliminated. This comment is no longer necessary. 8. Many drainage areas on Sheet C -9 do not match the calculations. Please make the necessary corrections. (Rev. 1) Comment has been addressed. 9. Please specify the elevation of the roofdrain pipes into the detention elevation. The inverts must be close to the top so there is infrequent flooding of the roofdrain pipes. Also, please clearly indicate in the drainage area map that the roof, except for the northern entrance overhangs, must drain to the detention facility. (Rev. 1) Comment has been addressed. 10. Please show and specify a trash rack on the orifice in the detention facility that meets all of the standards of VSMH 3.02. (Rev. 1) Comment has been addressed. 11. Please show the RCP to CMP fittings that are needed off of the detention facility. (Rev. 1) Comment has been addressed. 12. In the detention facility detail, please show that the pipe from behind the orifice plate must be built at a particular angle, not perpendicular. (Rev. 1) Comment has been addressed. 13. Two observation wells located out of the pavement are needed for the primary infiltration facility. Three observation wells are needed in the exfiltration trench. (Rev. 1) This comment has been withdrawn. The trench has been eliminated and the manholes will act as the access points into the storm water facility. 14. The calculations were very difficult to interpret by themselves. Please provide a Stormwater Narrative that succinctly summarizes the SWM strategy, results of the routings, and implications with regard to treatment, detention, and adequate channel concerns. (Rev. 1) The narrative and routing calculations have been reviewed. I concur that detention requirements have been met. In addition, the total volume of runoff for the two and ten year storms has been reduced to below pre- development values. Therefore, no adequate channel analysis is required. Comment has been addressed. 15. Please provide the infiltration calculations located on pages 14 and 15 of VSMH 3.10 for each infiltration facility. (Rev. 1) Comment has been addressed. 16. Please explain how no runoff from the 20ft buffer in the post - development scenario exists. (Rev. 1) Comment has been addressed. 17. Please provide approval letters from the manufacturers of the fabricated stormwater facilities. (Rev. 1) Comment has been addressed. 18. Please submit a completed Stormwater Facility Maintenance Agreement and $17 fee to Ana Kilmer after reviewing the instructions online. (Rev. 1) This comment has been acknowledged by the applicant. 19. Once the plans are approved, please provide a completed Bond Estimate Request Form to the County Engineer to receive a SWM bond computation. Cost estimates from each manufacturer will be needed after approval of the SWM plan. (Rev. 1) Comment remains unchanged. 20. (Rev. 1) Changes in direction in a storm system must be done at an inlet or manhole. Please provide a manhole at the 90degree bend of pipe 200 or rearrange the system so a new manhole is not needed. C. Erosion and Sediment Control Plan Review Comments (WPO- 2012 - 00070) 1. Please show all the grading necessary to establish sight distance at the entrance to onto Whitewood Road. (Rev. 1) Comment has not been addressed. 2. Protection is insufficient after the sediment trap in Phase I is removed and Phase H is begun. The accessway grading directs runoff out into the Oak Forest Drove without treatment. A sediment trap is needed at the eastern boundary of the property to be utilized throughout Phase 11, A curb cut between the sidewalk ramp and inlet should be opened to drain to the sediment trap. A RWD is needed at the entrance to direct runoff to this curb cut and sediment trap. The construction sequence should state that trap 1 can only be removed once this new trap is constructed. (Rev.]) Sediment trap 2 is not allowed to rely on a pump. Please design this facility to drain into pipe 101 or a new structure placed on pipe 101 with a rim elevation at the area where 134cy /acre is provided (refer to Plate 3.07 -4). Please draw the drainage area used in the calculation for this trap. It seems the drainage area used in the calculation is smaller than what will actually reach this trap. A drainage area closer to 0.75acres seems more reasonable. 3. Please show the grading necessary to upgrade sediment trap 1 to the proposed design in Phase 1. (Rev. 1) Please label the contours. 4. Please show the rerouting of the sewer line in Phase I. (Rev. 1) Comment has been addressed. 5. All inlets must be sealed until the site is stabilized so as not to clog the infiltration facility. Please add notes throughout the plan to this effect. (Rev. 1) Comment has been addressed. 6. Please include in the plan the paved wash rack detail from the design manual. (Rev. 1) Comment has been addressed. 7. The applicant is attempting to reduce the volume and peak discharge in the post - development scenario to pre - development levels so as to not deal with MS -19. This concept is acceptable, but MS -19 approval cannot be given until the SWM plan is approved that conclusively shows that this is the case. (Rev. 1) Please refer to Comment B.14. 8. Once the plans are approved, please provide a completed Bond Estimate Request Form to the County Engineer to receive a ESC bond computation. All parcels disturbed with this plan must sign this bond request form and be party to the bond unless easements that allow for the count, to enter the property to complete the work are recorded. (Rev. 1) Comment remains unchanged. Any property disturbed to establish sight distance will need to be party to a bond unless a new easement is recorded that allows the county to enter the property if the bond must be called. The current sight distance easement on file does not do this.