The URL can be used to link to this page

Home My WebLink AboutSDP201000020 Review Comments Major Amendment, Final Site Plan 2010-04-18ALg�,�� 'IRGINZ� COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 Project: Airport Runway Extension- Phase 1B SDP - 2010 -00020 and WPO- 2010 -00016 Plan preparer: Mrs. Susan Winslow, PE; Delta Airport Consultants, Inc. Owner or rep.: Charlottesville - Albemarle Airport Authority Date received: 4 March 2010 (plan dated March 2010) Date of Comment: 18 April 2010 Engineer: Phil Custer The minor site amendment, ESC, and SWM plans for the Airport Runway Extension (Phase 1B) have been reviewed. Engineering review can recommend approval to the plan once the following comments are addressed. A. Site Plan Comments 1. Per a determination by the Chief of Current Development, a critical slopes waiver will not be required. 2. Please provide watershed limits and hyrdrologic data for the existing 30" storm sewer system that feeds into structure D200. 3. In the profile sheets, please specify the height of the berms immediately after many of the structures around the runway extension. 4. The county typically reviews stormsewer structures to verify that the system can safely pass the 10- year storm. However, the applicant has provided computations for the 25 and 100 -year storms. These computations show many of the pipes operating under pressure which is normally something that requires approval from the County Engineer. However, it appears that the pipes can adequately carry the 10 -year flow which is the parameter county engineering will use for the review of this system. County Engineering requires no changes to the proposed drainage system. 5. Basins D, E, and F have been placed on channels that have topography that appear to possess characteristics that are indicative of a stream. Has the applicant performed an assessment of these channels to determine whether intermittent streams or wetlands are present? Because this project is within the Water Supply Protection Area, all intermittent streams and contiguous wetlands are protected with 100ft buffers. If these are indeed streams or wetlands, please provide the county with a copy of the Army Corps of Engineers approval for the project. If buffers do exist within the limits of disturbance, I will need to determine whether the proposed work falls under the exemption contained in 17- 319.D. 6. Similarly, there is a 100ft buffer surrounding the wetland adjacent to Gate 4, if the wetland layer in the applicant's drawings is correct. Again, I will need to determine if disturbance of this area is exempt per 17- 319.D. [17- 317.B] Current Development Engineering Review Comments Page 2 of 3 B. Stormwater Review Comments 1. Detention requirements for this project have been satisfied. [17 -314] 2. In the typical biofilter detail, please replace "Minimal Compaction" with "No Compaction ". 3. In the typical biofilter detail, please label the difference between the bed and spillway elevations as 1 ft. 4. Are elements of the Phase 1A stormwater plan being amended with this plan? If not, to avoid any confusion, this sheet should be removed. 5. Please show the maintenance path to and around each permanent facility. The path must be able to access all critical elements of the facility. The path must be no greater than 20% and must be graveled (or equivalent) on all sections steeper than 10 %. If the path crosses a channel, a culvert sized for the ten year storm must be proposed. 6. The low flow channel in the bottom of facilities D, E, and F must start at the outfall from the biofilter, not the center of the facility. The channel should be more clearly defined; a 6" depression with 20:1 side slopes is not a satisfactory channel. 7. Two feet of freeboard is required in each basin for the 100 -year storm because emergency spillways are not provided. The DI-3's do not qualify as emergency spillways. 8. An additional Stormwater Facility Maintenance Agreement may need to be recorded that specifically references this plan. If this form must be completed, please make sure the Name of Plan/Development refers to the application title and refers to the WPO application number (WPO- 2010- 00016). Please contact Ana Kilmer at 296 -5832 (x3246) if you have any questions regarding the submittal of the stormwater facility maintenance agreement. 9. The SWM bond will be calculated at the time of WPO plan approval. C. Erosion and Sediment Control Review Comments 1. On sheet 26, please adjust the elevations of all dewatering orifices to the levels shown in the calculations. It seems that each facility will have at least one dewatering orifice which will be sealed when converted to the permanent facility. The orifice at the bottom of the facility will be plugged until the drainage area is stabilized. Currently, the basin details on sheet 26 indicate that the ESC dewatering orifice and the permanent detention orifice are one and the same. 2. Engineering review recommends designing a dewatering orifice that works for all stages in each basin. If two different designs of the dewatering orifices are required for each basin, please clearly show the design elevations in the basin details on sheet 26. Please make sure the sizes and elevations shown in the details match the calculations. 3. Two feet of freeboard is required in each basin for the 25 -year storm because emergency spillways are not provided. The DI -3's do not qualify as emergency spillways. 4. A sediment trap is required at Gate 4. Diversion dikes must flank this facility to direct as much disturbed earth to the trap. 5. In the construction sequence, the contractor is directed to construct diversion dikes before the construction of Basins 1A, D, E, and F and all traps. These basins and traps must be constructed prior to any diversion dikes are installed. Please correct the construction sequence to reflect this. 6. Sediment trap 1 does not appear to be necessary. Please extend the diversion dike to Sediment Basin lA to simplify the construction and inspection of the project. 7. The applicant has not provided a detailed analysis of the channels downstream of Basins D, E, and F that is normally required for the review of development. Instead, the applicant has claimed the design of each basin satisfies the alternate standard as specified in 10.1 -561 in the Virginia Erosion and Sediment Law and Regulations. After further review, this plan does meet these standards and, therefore, the plan has satisfied all adequate channel requirements for the final design of the site. Because the post - development watersheds are mostly grass, the volume Current Development Engineering Review Comments Page 3 of 3 captured by each biofilter satisfies items i and ii for each watershed. Independent routings of the post - development scenario confirms that the release rates in the post development scenario are below the allowable rates per the equation specified in item iii (please see the accompanying tables). 8. This comment is advisory and requires no changes to the plan or calculations. For the post and pre development computations, I found similar results as the applicant (please see the accompanying tables). However, for the routing of each of the basins during Stages B and C, the discharges that resulted were much higher than those documented by the applicant. (A reason for the discrepancy could be that the applicant's routing considered the basins as completely empty and did not include the dewatering orifice.) Though the increase to peak discharges to downstream properties is expected when land is denuded and does not appear to be considered in state regulations, County engineering will consider the temporary increase to peak flows as a concern that we will monitor during construction. When renewal of the grading permit is sought [17- 207.B.2 -3], the County engineer may impose conditions to the project which would help alleviate the problem of increased peak discharges during the lifetime of the project. I recommend excavating Basins D, E, and F to the Stage C volumes as soon as possible. By specifying Basins D and E (in addition to Basin F) in note 4 of the Stage C construction sequence, a lower discharge would be felt by all downstream channels for the many months the site is open. 9. Please provide the following note on the cover sheet: A portion of disturbance shown in this plan, namely the filling of the runway area, will be treated by basins and measures in the previously approved plan, WPO- 2008 - 00059. The previously approved application must remain active for the duration of the current application, WPO- 2010 - 00016. 10. Please provide a graphic that clearly shows the total limits of disturbance for Stage C. If possible, show the limits of disturbance of the previously approved plan as well. 11. Please show silt fence downhill of the construction of the storm drain system around the snow removal facility. 12. The ESC bond will be calculated at the time of WPO plan approval. File: E1_mia swm esc_PBC_sdp20100002O wpo201000016.doc