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HomeMy WebLinkAboutWPO200800059 Review Comments Stormwater Management Plan 2009-04-16ALg�,�� �'IRGINZ� COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 Project: WPO- 2008 - 00059, Airport Runway Extension SWM Plan Plan preparer: Delta Airport Consultants, Inc. Owner or rep.: Barbara Hutchinson, Executive Director Date received: (Rev. 1) 2 April 2009 29 January 2009 Date of Comment: (Rev. 1) 16 April 2009 27 February 2009 Engineer: Phil Custer The SWM plan for the Airport Runway Extension Project, received on 2 April 2009, has been reviewed. The plan can be approved after the following changes /corrections have been made: A. General Review Comments Safety Slabs should be specified as SL -1, not SL -7. (Rev. 1) Comment has been addressed. 2. Outlet protection calculations do not appear to be given for all necessary temporary and permanent outlets. A few of the outlets missing from the calculations package include D123, D109, slope drain on sheet 15, slope drain on sheet 13, and D114. The dimensions of the outlet protection should also be detailed in the drainage profiles. (Rev. 1) Comment has been addressed. 3. The INV In is missing from D105 in the profile. (Rev. 1) Comment has been addressed. 4. Many structures on Exhibit 1, "Phase 1 Design Drainage Map ", are not labeled with the same title as they are in the set. Please make sure the exhibit is coordinated with the sheet set. (Rev. 1) Comment has been addressed. 5. The grate inlet analyses for the DI -5 and DI -7 structures should use the inlet capacity charts in the VDOT Drainage Manual rather than the weir equation calculation. (For DI -5, the DI -7 capacity chart can be used since the grate options are the same.) (Rev. 1) Comment has been addressed. 6. The ditch design computations tables show EC -2 as proposed for some sections of the ditch but sheets 13 -16 specify only EC -3. Please clarify. (Rev. 1) Comment has been addressed. 7. It appears a few channels have been omitted from the calculations (from D 112, from D 114, and from D121A). Additionally, there is no information in the plan set on the channel in the calculation package labeled as "ERSA" (left and right). (Rev. 1) Comment has been addressed. 8. The package is missing the culvert profiles for the two box culverts at the perimeter fence and the two 36" culverts underneath Crickenberger Lane. (Rev. 1) The culvert at the perimeter fence should be countersunk 6 inches. B. SWM Plan Review Comments: 1. The portion of this SWM plan dealing with water quality treatment is unclear and will require greater clarification. a. The applicant must submit copies of the County's Modified Simple Spreadsheet for each SWM facility's drainage area. The spreadsheet used by the applicant to compute removal rates does not yield the same results as the county's spreadsheet. For the west, east, and north basins, I computed removal rates to be 44 %, 18 %, and 47 %, respectively. (Rev. 1) The county has been informed that all facilities will be biofilters due to an error in the delineation of Areas 2 and 5. All biofilters beds must be sized so that they are at least 2.5% of the impervious are draining to them, assuming a Ift of ponding depth. b. In the SWM narrative, it appears that the applicant wishes to provide water quality treatment with extended detention facilities. However, the removal rate of a standard extended detention facility is only 35% (VSMH). A 50% removal rate can only be achieved in an extended detention facility when a permanent pool wetland is provided (Enhanced Extended Detention). (Rev. 1) Comment has been addressed. All treatment facilities will be biofilters. c. I could not find in the package the VSMH sizing calculation for the drawdown orifices in the extended detention facilities. On the next submittal, please include these calculations. (Rev. 1) The county has been informed that all facilities will be biofilters due to an error in the delineation of Areas 2 and 5. Please remove all drawdown orifice calculations from the submittal package. d. All SWM treatment facilities require sediment forebays. Calculations are required for all sediment forebays. (Rev. 1) Sediment forebays have been provided for all SWM facilities. The sediment forebays appear to be oversized, but this is permissible. Please note that the sediment forebays for Basins A and B will impound water unless an underdrain is provided. Engineering review has no preference either way, but if an underdrain is proposed, it should be shown on the plan. e. (Rev. 1) The biofilter bed cannot be sloped. Please correct the standard detail and adjust the grading of facilities A, B, and C to show a flat bottomed biofilter bed. (For SWM C, it appears the biofilter bed must be the size of the 523 contour.) Ideally, the biofilter should be inline with the channel through the facility but in a position where higher flows can bypass the bed easily. The biofilter should not have raised knolls as shown in the details on sheet 43B. After the required grading modifications, please update the routings in Section 9. f. (Rev. 1) The standard biofilter details states that the underdrain is 6" but the basin details on Sheet 43A shows underdrains of varying diameters. The larger diameter underdrain is okay for Basin C because of the considerable size of the bed, but the large diameter underdrain does not appear necessary for A or B. g. (Rev. 1) Trashracks are needed on all orifices. The trashracks must meet VSMH standards. Please detail all trashracks on the stormwater facility embankment cross - sections on sheet 43A. h. Additional comments may be required based on the changes and provided calculations. (Rev. 1) No future comments are anticipated at this time. With the provided information, it is difficult to definitively conclude that detention requirements have been met, though it appears that the volume provided in the three facilities are sufficient. Please see below for the a list of comments regarding the detention calculations: a. The pre - development drainage area map should show the existing contours at the time that the basin at the edge of the runway was designed. The predevelopment drainage area map in the calculation package appears to be the existing conditions on site today. (Rev. 1) Comment has been addressed. b. It appears that the composite CN values used for each watershed are a bit low and may underestimate the discharge to each facility. Please provide more detailed calculations for each post and pre - development watershed. The soil types in each watershed have significant impacts on CN values so it would be helpful to overlay the soils map on the drainage area map exhibit. And, since the majority of the post - development drainage area will include fill soil from offsite, it is necessary to identify the borrow site for this project and identify the soil characteristics of it. (Rev. 1) Comment has been addressed. However, for analysis in future submittals, please do not use the CN reduction for slopes less than 6 %. 3. Please provide an embankment section and drainage structure /riser detail (similar to the ESC details found on sheet 36) showing all critical information regarding the permanent setup of each facility. The detail should show the embankment and note that it must be certified by a geotechnical engineer if an impervious core is not provided specifically on the detail. (Rev. 1) Please update these details as necessary if any grading, orifice, or underdrain pipe is adjusted. 4. All basins appear to impound some volume of water permanently. The lowest orifice is above the deepest contour in the provided routings. (Rev. 1) The county has been informed that all facilities will be a biofilter due to an error in the delineation of Areas 2 and 5. In each facility, there will be an impoundment of water that will gradually drain through the biofilter mix. All other underdrains entering the riser should be eliminated. If the applicant desires to keep the grass paver underdrain detail on sheet 38 of the plan, the only applicable use is to drain the sediment forebays. 5. Please use the latest values of the 24 -hour storm for Albemarle County found in the Design Manual for all runoff calculations. (3.7 ", 5.6 ", and 9.1" for the 2, 10, and 100 -year storms) (Rev. 1) Comment has been addressed. 6. The input for the routing of basin A shows a 48" manhole structure but there does not appear to be one provided. (Rev. 1) Comment has been addressed. 7. Please provide greater clarification regarding the elevation - discharge relationship of all of the anti - vortex structures on the top of all risers. I do not understand the input values in the routing provided by the applicant for each top of structure. (Rev. 1) Comment does not appear to have been addressed. For the riser tops, it appears that an orifice was entered into the routing program rather than user - specified routing curve provided by the riser -top manufacturer referred to in the letter. 8. Please provide vehicle access to all sediment forebays, structures, and embankments. The grade of the vehicular access can be no greater than 20% and must be surfaced (gravel, pavement, geo -grid, or other) in all places where the grade is greater than 10 %. (Rev. 1) Comment has been addressed. 9. From the forebay of basin C to the riser structure, please provide a low flow channel so the slope from the forebay and the bottom of the basin does not erode. (Rev. 1) Comment has been addressed. 10. On both the pre and post development maps, please provide the time of concentration and average CN for all drainage areas. (Rev. I) Comment has been addressed. 11. A bond will be computed once all comments are addressed. C. Erosion and Sediment Control Plan Update 1. We are still waiting for the analysis of the downstream channel. This issue must be resolved before a grading permit can be issued. (Rev. 1) Comments regarding the downstream channel analysis will be given in a separate letter. Once the modifications have been made to address these comments, please submit one full copy of the plan and calculations to Engineering for review. Please contact me at (434)296 -5832 ext. 3072 should you have any questions. File: E2_swm_PBC _ wpo- 2008 -00059 Airport Runway Extension.doc