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HomeMy WebLinkAboutWPO200900061 Review Comments Erosion Control Plan 2010-01-12� 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: North Pointe Stream Crossing [WPO- 2009 - 00061] Plan preparer: Doug March, PE; WW Associates Owner or rep.: Neighborhood Investments, NP, LLC Developer: Charles Rotgin; North Pointe Charlottesville, LLC c/o Great Eastern Management Company Plan received date: (Rev. 2) 11 January 2010 (Rev. 1) 16 December 2009 6 November 2009 Date of comments: (Rev. 2) 12 January 2010 (Rev. 1) 6 January 2010 4 December 2009 Reviewer: Phil Custer The second revision to the Erosion and Sediment Control Plan for the North Pointe Stream Crossing project, submitted on 11 January 2010, has been reviewed. The technical ESC aspects of the plan are hereby approved. All items remaining in Section A of this letter must be addressed prior to the issuance of a grading permit. A. General 1. As previously indicated in an email from Amy Pflaum sent 30 October 2009, a road plan will be required within this set. The road plan must meet all of the conditions outlined in the approval letter of SP- 2006 -00034 including all detailed landscaping requirements. (Rev. 2) Comment has been acknowledged by the applicant. 2. Approval of the road landscaping plan by the ARB Design Planner is required per SP conditions. (Rev. 2) Comment has been acknowledged by the applicant. 3. VDOT approval of the road plan is required. (Rev. 2) Comment has been acknowledged by the applicant. 4. FEMA approval of the flood map revision is required before a grading permit will be issued. (SP condition #3) (Rev. 1) Before a grading permit is issued, the applicant will need to forward to the county approval of a Condition Letter of Map Revision by FEMA. (Rev. 2) Comment has been acknowledged by the applicant. 5. Please provide a copy of the Army Corps of Engineers Permit for this project. Please also supply a copy of the application submitted to ACE for county records. (Rev. 1) A copy of the Virginia DEQ permit for the stream crossing has been provided by the applicant. However, this permit is set to expire shortly. A grading permit will not be issued for the project after January 16'h, 2010 unless a Continuation of Coverage is granted. Please forward proof of this Continuation of Coverage once it is granted. (Rev. 2) Comment has been acknowledged by the applicant. 6. A mitigation plan will be required prior to the release of final ESC approval for this project. Please refer to the County's Design manual (pages 11, 12, and 25 of 42) for all mitigation plan Albemarle County Community Development Engineering Review Comments Page 2 of 4 requirements. (SP Condition #5) (Rev. 1) A mitigation plan has been submitted by the applicant. Review of this plan will be provided in a separate comment letter. CORRECTION: The mitigation plan must be approved and bonded prior to a grading permit, not tentative ESC plan approval. (Rev. 2) Comment has been acknowledged by the applicant. 7. The county will only be reviewing the culvert to confirm that the 100 -year storm will not overtop the road because this road will serve as the sole access to the residential community, if only temporarily. [18- 32.7.2.3] The culvert must be checked with the discharge number, adjusted for anticipated development in the watershed, used in the existing FEMA model. (Rev. 1) County staff is not satisfied that the 100 -year storm will not block the road to the residential community. [18- 32.7.2.31 a. The calculation provided by the applicant using the current hydrologic and hydraulic assumptions shows the 100 -year flood reaching an elevation of 397.82 which would inundate a significant portion of the roadway. b. The time of concentration appears to be too slow. The time of concentration should be calculated with post- development conditions in mind (quicker sheet flow and almost no shallow concentrated flow). Also, according to the FEMA study, the existing stream channel has an average velocity of 4.3fps. This number should be used to calculate the stream travel time unless a more detailed calculation is provided. c. West of the mobile home park, a regional service curve number should be used to match the approved rezoning plan. d. On the southeast corner of the development, use a townhouse CN to match the approved rezoning plan. e. The elevation of the 100 -year storm must be calculated using HEC software (HEC -RAS or HEC -HMS, depending on what was used in the 2005 study). The HEC file must be submitted to the county with the next plan revision. (Rev. 2)Comment has been acknowledged by the applicant. 8. Please note that the latest rainfall amounts for a 24 -hour ten and hundred year storms are 5.6in. and 9.1in., respectively, for Albemarle County. (Rev. 1) A calculation using the latest 24 -hour 100 year storm has been provided. 9. The stage - elevation data of the area upstream of the culvert must consider the proposed contours of the embankment. Because the inlet point is located farther upstream, the current computation overestimates the volume available uphill of the culvert's inlet. (Rev. 1) The latest stage -area data is acceptable. 10. Please provide a low maintenance, non - grassed groundcover on all slopes steeper than 3:1. For examples of acceptable groundcovers, please refer to Table 3.37C of the VESCH. [DM] (Rev. 1) Comment has been addressed. B. Erosion and Sediment Control Comments 1. The ESC measures provided do not appear to exceed the minimum state requirements. (Proffer 4.3.a) For a list of recommended measures above and beyond state minimum requirements, please refer to the County Engineer's Commentary . Incorporating the use of sediment traps into the filling operation is also something to be considered as a measure to exceed minimum state requirements. (Rev. 1) I have discussed the above - and - beyond erosion controls with the County Engineer. The county will accept the EC -2 lining material or hydroseeding with tackifiers on 2:1 slopes and the double barrier row of silt fence (though, see below) as a portion of the above - and - beyond ESC measures. The 5 -day seeding proposal would be difficult to track and enforce in the field and the wire - backed silt fence is a current VESCH standard (the minimum stake spacing is wider with the Albemarle County Community Development Engineering Review Comments Page 3 of 4 wire backing silt fence). Engineering review will conclude that Proffer 4.3.a is met if a 3 month timeline (from the time a grading permit is issued to the final seeding operation) is committed to by the applicant. If this timeline is not met, the plan will be opened up for re- review and additional measures may be required based on site conditions. The second row of silt fence must be move closer to the stream, ideally 5ft from the first row. If the applicant acquiesces to these three items for above and beyond measures, please provide a summary list /table on the cover sheet clarifying the agreement, including the re- review by county staff if the 3 -month timeline is not met. (Rev. 2) Comment has been addressed. 2. The diversion above culvert 2 appears to be acting as a `cleanwater' diversion which is not a state accepted standard. Please remove all diversions that are proposed to direct clean water around disturbed areas. (Rev. 1) The placement of the channel and its dimensions are acceptable. However, please extend it to the stream, terminate the silt fence before it runs across ditch, and provide a note that all disturbance is to be directed towards the silt fence and away from the riprapped ditch. I would also recommend turning the edge of the silt fence uphill when it meets the new channel so the silt fence doesn't wind up directing runoff into the channel. (Rev. 2 Comment has been addressed. 3. ESC measures are needed to treat runoff from the haul road. A sediment trap/basin just east of the box culvert outlets is necessary. (A sediment basin in this location will be needed for the development of the full site plan and engineering review recommends that the basin is constructed now if the site plan is anticipated to be constructed soon after this crossing. This basin could also be converted to a biofilter to help meet stormwater management requirements in Proffer 4.3.b.). The diversion shown on the plan uphill of culvert 2 can direct runoff into this trap as long the trap is designed to consider this area. Having two settling facilities in series (the other being the current proposed trap in the borrow area) would be a step towards satisfying Proffer 4.3.a. (Rev. 1) The current design of the trap technically possesses an embankment height of 16 -18 feet (though the tallest amount of fill required is slightly greater than Eft) which is taller than the maximum 5ft height required by the VESCH. The trap should be built closer to the stream. The location of the embankment should be placed where this slope transitions from cut to fill ( -378 contour). Size this trap for a 1.25 acre drainage area by extending the length as far north as necessary. Please also mention the construction of trap 2 into the construction sequence. This trap will need to be in place before the haul road is used. (Rev. 2) Comment has been addressed. 4. Please provide dewatering symbols (DS) on the ESC plan and detail for the installation of the box culverts. (Rev. 1) Comment has been addressed. 5. A stockpile and laydown area is needed just east of the proposed earthwork. Please adjust limits of disturbance and ESC measures accordingly. (Rev. 1) This comment is withdrawn. However, I still recommend a laydown area east of the crossing for the contractor to use. 6. With regard to the keeping construction activity out of the live stream, the construction sequence is acceptable. However, the removal of the existing culvert and embankment will limit access to the majority of site for a considerable amount of time. I recommend a more detailed sequence where the southern 50ft of the two box culverts are built and a crossing is established before removing Albemarle County Community Development Engineering Review Comments Page 4 of 4 the existing embankment. Again, this is only a recommendation and not a requirement. Please note that if the contractor wishes to establish a temporary crossing at another location along the stream, an amendment to the ESC plan will be required. (Rev. 1) Comment has been noted by the applicant. 7. Please specify in the plan where the existing embankment material will be deposited on site if it turns out to be suitable. Conversely, please also specify where the soil is to be deposited if it turns out to be unsuitable. If it is to be removed from the site, please state that the soil must be taken to a property with an approved erosion and sediment control plan. (Rev. 1) Comment has been addressed. 8. Please provide a note in the vicinity of the borrow area which states that at all times the cut operation must be carried out in a way that directs all runoff to the diversion and sediment trap. (Rev. 1) Comment has been addressed. 9. The ESC bond will be computed at the time of plan approval. (Rev. 1) Comment remains unchanged. (Rev. 2) Correction: The bond will be calculated at the time that all conditions for earth disturbance have been met. E3_esc_PBC_North Pointe Stream Crossingwpo200900061.doc