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HomeMy WebLinkAboutWPO201100097 Review Comments Erosion Control Plan 2012-02-13� 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: Rivanna Club Rifle Range ESC and Mitigation Plans; WPO- 2011 -00097 Plan preparer: Mr. Scott Collins, PE; Collins Engineering Owner: Rivanna Rifle and Pistol Club, Inc. Plan received date: 22 November 2011 (Rev. 1) 18 January 2012 Date of comments: 4 January 2012 (Rev. 1) 13 February 2012 Reviewer: Phil Custer The first revision to the ESC plan for the Rivanna Club Rifle Range (WPO- 20 1 1 - 00097), received 18 January 2012, has been reviewed. With this submittal, a mitigation plan was submitted for the disturbance to the stream buffer. Engineering can approve the plans after the following comments have been addressed. A. ESC Plan Review Comments There are several issues with stream buffers in this plan. a. Stream buffers in the rural area are the limits of the 100 -year floodplain or 100ft from the bank of a stream, whichever is greater. Sheet 4 mislabels the stream buffer in the area of construction. (Rev. 1) Comment has been addressed. b. A portion of the construction work is shown within the stream buffer (see previous comment). County engineering will authorize the proposed construction in the outer 50ft of the buffer per 17 -321.A considering the grandfathered use, but a mitigation plan and fee must be provided. (Please note that for part of the project the limits of construction is about 35ft from the centerline of the stream and this must be moved.) It appears that the area along the travelway west of the rifle shooting table is an acceptable place for mitigation measures. Mitigation areas must be twice as much as the area disturbed. (Rev. 1) A mitigation plan has been received. Please see comment section B. c. A significant portion of the rifle range is located within the stream buffer. According to the Water Protection Ordinance, stream buffers can only be maintained in the manners listed in Section 318. The maintenance needed to allow the range to be completely unobstructed is not permissible in any way by ordinance. After several years without maintenace the buffer will obstruct the range, making it unusable. Engineering review recommends that with this submittal the applicant submit a request for an exception from this aspect of the WPO through the process described in 17 -308. The Board of Supervisors is the only entity that can grant permission for the maintenance of this buffer to sustain the rifle range. At this time, it's not clear what staff's recommendation to the Board would be; if approval was recommended, we would also recommend that mitigation (2sf planted: Isf disturbed) be required and that the range within the buffer be planted with a woody groundcover. If the applicant does not request an exception to keep the buffer clear for the range, county engineering will need to schedule a visit to the site and record Albemarle County Community Development Engineering Review Comments Page 2 of 3 the condition and limits of the buffer as it exist today. It is staff's preference to have this issue resolved with this plan. (Rev. 1) The applicant has opted not to have this issue addressed with this application. Before this plan can be approved, a county inspection must occur. 2. Given the large area, amount of earthwork, and the concentration of water, silt fence is not an appropriate erosion and sediment control measure. Please design a sediment trap at the base of the cut but as far out of the buffer as possible. The silt fence should remain on the northeast side of the construction. A diversion dike northeast of the new cleanwater channel should be provided to the trap. (Rev. 1) The proposed sediment trap is located too far in the construction area and would be a nuisance to the contractor. Please place it in the outer 50ft of the stream buffer. Please make sure the wet storage is not deeper than the state maximum of Oft, as currently designed, and that the bottom of the stone weir is the same elevation of the toe of the embankment so a wide channel does not need to be constructed to the stream to outlet the trap. 3. It is critical that the 20 acre watershed is bypassed around the construction site. Please make the following changes regarding this bypass channel: a. Please continue the channel to the stream. The channel construction does not require mitigation per 17- 318.B.5 and 17- 318.C.6, but please reduce the size of the channel to what is needed to pass the 10 -year storm to minimize buffer impacts. (Rev. 1) Comment has been addressed. b. Please provide a culvert sized for the 10 -year storm in the channel so that construction vehicles can cross the clean water diversion. (Rev. 1) The culvert overtops during the 10 year storm. Please enlarge the pipe so the 10 year storm passes safely. Please label the size of this pipe on sheet 4. c. The channel requires instant stabilization, otherwise the clean water from the 20 acres will be polluted with the sediment from the unstable channel. Please address this concern. (Rev. 1) Comment has been addressed. d. Please label this channel as a diversion (DV), not a diversion dike, because a diversion dike has a 5 acre maximum drainage area. (Rev. 1) Comment has been addressed. e. Please provide more detailed channel calculations for the new grass -lined ditch. With the same givens, I'm getting different results for 10 year depth and 2 year velocities. (Rev. 1) Comment has been addressed. f. Was this channel intended to be lined with EC -2? Previous grass -lined channels submitted by the applicant used a manning coefficient of 0.03 and permissible velocity of 3fps. (Rev. 1) Comment has been addressed. g. The channel has slopes much higher than 6.7 %. For the channel slope calculation, it appears the applicant is assuming 2 -ft contours. Resulting velocities will be greater than permissible values listed in VESCH Table 5 -14. A riprap lining seems appropriate for much of this channel considering the steep slopes and concern about instant stabilization. (Rev. 1) Comment has been addressed. 4. After meeting on site with RRPC President Bob Bossi and members of the excavating company performing the earthwork on site, it was my understanding before reviewing this application that all spoils from this project would be used on site to create earthen berms around the target. After looking at the proposed contours and reading the ESC narrative and notes provided by the applicant, it seems this is not the case. The path south of the construction area is not included in the ESC plan and is likely not adequate for a haul road that will be significantly used. If this is to be a net export project, please include the haul road in the limits of disturbance, provide more silt Albemarle County Community Development Engineering Review Comments Page 3 of 3 fence along this road, and provide stone stabilization on it. If the project will be balanced, design the grading plan to show where the excess soil is to be placed to make it balanced. The soil must be placed outside of the floodplain /stream buffer and adequately protected by the ESC plan. (Rev. 1) Please provide construction road stabilization (CRS) on the path and silt fence on the downhill side of the travelway from the area of cut to the area of fill. Please terminate the limits of construction just south of the fill area or extend silt fence and construction road stabilization to the proposed limits of construction. A properly sized and lined channel must be proposed so that the concentrated water uphill of the fill area will not wash it out over time. [I8- 5.1.281 5. Please update the standard county notes. (Rev. 1) Comment has been addressed. 6. After the plan has been approved, please provide a completed Bond Estimate Request Form to the County Engineer to receive an ESC Bond. All property owners from the construction site to Old Lynchburg Road along the haul road must sign this document and be party to the bond unless construction and/or access easements have been granted. (Rev. 1) Comment has been acknowledged by the applicant. 7. (Rev. 1) Given the large area, amount of earthwork, and the concentration of water, silt fence is not appropriate for use below the fill area. Please design a properly -sized sediment trap (or basin, if necessary) at the base of the fill. Please provide diversions flanking the fill area making sure that the access point to the fill area from the path is directed to the trap. 8. A construction area in the middle rifle range has been added with this second submittal. It is my understanding that the earthwork in this area has to do with the clearing of vegetation, removal of stumps, and the construction of some retaining walls on the berms around this range. If this is the only work being proposed in this area, the silt fence as shown is acceptable. If additional disturbance is necessary, please provide more detail on the plan. B. Mitigation Plan Review Comments 1. Please include the access path in the disturbed area calculation. 2. The proposed mitigation area is currently in a section of the site that is already heavily wooded. Please propose the necessary mitigation where vegetation is sparse or absent.