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HomeMy WebLinkAboutWPO200600066 Review Comments Erosion Control Plan 2009-10-30� 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: Briarwood; Phases IA-1, 1B -1, 4, and 8 Plan preparer: The Engineering Groupe, Inc. Owner or rep.: Woodbriar Associates Plan received date: 31 March 2009 (ESC, SWM, FSP) (Rev10) 21 July 2009 (full set: Rev. 11, Phase IV: Rev. 1) 09 October 2009 (full set: Rev. 12, Phase IV: Rev. 2) Date of comments: 24 April 2009 (Rev 10) 4 September 2009 (full set: Rev. 11, Phase IV: Rev. 1) 30 October 2009 (full set: Rev. 12, Phase IV: Rev. 2) Reviewer: Phil Custer A. Road Plan (SDP200600041) 1. VDOT approval is required. At the time of this letter, VDOT approval has not yet been received. (Rev. 1) VDOT approval has yet to be received. (Rev. 2) VDOT approval has been received. 2. The road bond amount which was previously quoted is no longer applicable. A new road bond amount will be completed once the applicant submits a Road Bond request form with an updated schedule of completion. A road bond is not necessary until a final plat is submitted. (Rev. 2) Comment remains unchanged. 5. Please show how the extension of St. Ives road will affect the RWSA site and provide a VDOT commercial entrance. The building and driveways in this area appear to be masked. (Rev. 1) Consent from RWSA is needed before this plan can be approved. (Rev. 2) The RWSA has been relocated into the VDOT ROW. In all sheets showing the generator in the VDOT ROW, please add a notation stating that the generator is to be removed. This is a VDOT requirement. 10. Please show VDOT standard cross - drains in the road profile. (Rev. 1) A CD -2 appears to be required at Inlet 411 and 422. (Rev. 2) Comment has been addressed. 11. (Rev. 1) Recently, there has been discussion to modify the intersection of Boulders Rd. and Route 29. This plan should reflect the improvements currently being proposed in this area. (Rev. 2) The latest design of the intersection improvements must be shown in this plan. If additional ROW dedication or a modification to the Briarwood Drive entrance is necessary, this plan will need to account for it. A note stating that intersection construction will be performed by others is acceptable. B. Final Site and Drainage Plan (SDP200600041) 2. Before this plan can be approved, ROW dedication and drainage easements are required on all offsite properties. This plat must be recorded before the site and road plan can be approved. Please also provide temporary construction easements for any work outside the ROW dedication and drainage easements. (Rev. 1) Comment remains unchanged. (Rev. 2) Comment remains unchanged. All necessary easement plats have been submitted Engineering Review Comments Page 2 of 5 and are in the process of being reviewed. L. .. .... �__ __ �_-__.a,�over is needed on all slopes steeper than 3:1. For a list of acceptable groundcovers, please see table 3.37C in the VESCH. Engineering review will also accept other equivalent groundcovers if proposed by the applicant. (Rev. 1) Please show as proposed on all plan view sheets within the set where 2:1 slopes are present. (Rev. 2) Comment has been addressed. C. Stormwater Management (WP0200600066) 1. Please submit a stormwater management facility maintenance agreement for TMP 32G - -A. Properties 32G -1, 32G - -C, and 32G -02 -97 already have recorded maintenance agreements. (Rev. 1) Comment has not been addressed. (Rev. 2) Comment has been addressed. 2. For all facilities treating runoff from Phase 4, please use the grass and gravel cells in the modified simple spreadsheet. (Rev. 1) Comment has not been addressed. Gravel and grass areas have not been included in the spreadsheets for ponds 3 and 4 on sheet 1OA. Please also be aware that ponds 3 and 4 are located upstream of the intake for a RWSA water supply facility. These ponds must meet the removal rate for the water supply protection areas. Both Pond 3 and 4 will need to be Type III basins and neither appear to have aquatic benches. Aquatic benches must have a specified planting plan [see checklist in the design manual]. (Rev. 2) The modified simple spreadsheets for ponds 3 and 4 appear to missing portions of the drainage area. Except for the water surface area of the pond, the post- development column should add up to the drainage area for each facility because no forested areas will remain with the development. The post development areas for Ponds 3 and 4 are short by 3.23 acres and 7.78 acres, respectively. Please update each spreadsheet and make modifications to the ponds, if necessary. Please make sure the proper aquatic bench grading shows up in all applicable sheets (site plan, ESC, and SWM sets). Please provide atypical planting detail for each aquatic bench for each facility. Please provide a planting schedule for each facility. typical 14'x 10' planting area specified for aquatic bench at least 3 species of aquatic plant used 5. Please eliminate the vegetated filter strip and extend channels for facility 3 and 5 to collect this water. (Rev. 1) Channels will not be needed at this time. However, please provide a note on the plan in this area that all rooftops must drain to stormwater management facilities. (Rev. 2) Comment has been addressed. 7. The minimum drainage area for wet ponds is 10 acres. Facility 5 cannot be a wet pond. (Rev. 1) Both ponds now possess drainage areas greater than the minimum required by the VSMH. "Dry weather stagnation may result in aesthetic and odor problems for adjacent property owners. Therefore, for residential applications, a minimum of 15 to 20 acres may be appropriate. " —VSMH Engineering review recommends using biofilters for facilities 3 and 4. This is not a requirement. 12. The SWM bond will need to be recalculated. The calculation will occur once the plan is closer to approval. (Rev. 2) The bond will be calculated with the next submittal. 13. (Rev. 1) Stormwater management facility maintenance roads must be IOft wide. [DM] (Rev. 2) Comment has been addressed. Engineering Review Comments Page 3 of 5 D. Erosion Control Plan (WPO200600066) 1. The ESC bond will need to be recalculated. The calculation will occur once the plan is closer to approval. (Rev. 2) The bond will be calculated with the next submittal. 2. Adequate channel calculations must be provided from the outlet of all stormwater management facilities in Phase 4, Pond 1, and Pond 2. For the outfall of Pond 2, the analysis may terminate once the road culvert is analyzed. The channel from this pond should be designed using proposed contours and not existing because this area will have been previously disturbed with the sediment basin. All other analyses may terminate once the river is reached. The analyses should meet all requirements specified in the latest edition of the design manual. (Rev. 1) The adequate channel analysis requires modifications before it is deemed satisfactory. In general, the following needs to be considered in all calculations: -One point of analysis for the outfall of each facility is not representative of the downstream conditions between the outlet and the river. For instance, before and after the cross - section A -A there is no semblance of a channel. [page V -122 of VESCHI -For all calculated channels, please specify for each segment of the channel cross - section the type of ground, n -value (Table 5 -16 through 5 -21 from VESCH), permissible velocity (Table 5 -22 from VESCH), and calculated velocity. Varying velocities should be encountered across the channel cross - section in natural channels. Specifically, the following changes must be made: pt)"d 1 -The cross - section used in the calculation does not appear to be representative of the path from the outlet to the river. From the topography shown on the plan and a visit to the site, there does not appear to be a channel suitable for the discharges from the pond. -A new trapezoidal channel from the pond to the river appears to be unavoidable unless an elaborate level spreader is designed. The county prefers a new channel. (Rev. 2) The channel provided is adequate. However, the crossing of the waterline must be approved by RWSA orACSA, depending on who owns that line. Right now, the channel is cutting 2ft from the cover of the pipe. I have forwarded a scan of the new channel to ACSA and RWSA and am awaiting a response. Please also note on sheet 31 of the ESC plan that the downstream channel construction is to occur in Phase I. Pond 2 - Section B -B appears to calculate the adequacy of the existing channel, but since this area will be inundated with water and disturbed by construction, it is reasonable to anticipate that it will be disrupted. When the sediment basin is removed, a new channel design will need to be specified. Please show an adequate new channel with proposed contours from the outfall of pond 2 to the Route 29 culvert that is properly lined with riprap or matting depending on what the calculations find is required. -My routing of pond 2 yielded 2 and 10 year discharges of 33.2 and 44.1 cfs. - Engineering review agrees that the existing road culvert is an adequate channel. (Rev. 2) Comment has been addressed. Pond 3 - Cross - section C -C is not representative of the entire channel from Pond 3. Between C -C and the outfall, the longitudinal and side slopes are much steeper. The topography is indicative of an existing Engineering Review Comments Page 4 of 5 eroded channel. Please provide photographs of this channel to confirm its condition and n- value. -This channel appears to be irregular and not trapezoidal as calculated. (Rev. 2) The technical portion of this comment has been addressed. However, please make sure the channel lining shows up on Phase I of the ESC plan. Please note that simply meeting pre - development flows of the channel does not meet minimum standard 19 because the volume of runoff increases with development. Pond 4 -The channel from the outlet down the 66% slope appears to be the necessary measure. However, please provide the dimensions of the channel and calculations to confirm it is correctly sized. Also, the slope of this hill is approximately 312:1 which is steeper than the maximum side slopes for a riprap lined channel. Because of this, the channel must be grouted or re- graded to lengthen the channel to reduce the slope. -A channel does not appear to be present past the riprap. If it exists please provide photographic confirmation and supporting calculations proving its adequacy. If it does not, please propose a channel with supporting calculations proving adequacy. (Rev. 2) The new proposed channel terminates at the base of the 2:1 slope and no calculations proving adequacy to the river were provided in the set. However, because of the topography of the floodplain, it may be possible that the additional discharge from the pond may not cause erosion problems. Please provide the following note on the plan: "The outlet of Pond 4 (Sediment Basin 6) in the floodplain will be monitored for erosion by county personnel during construction. Should the County determine that the area is eroding or likely to erode with development, the developer will be required to continue the channel to the river or construct a level spreader to the specifications approved by the county. The county will bond this potential downstream work with Pond 4 in the Stormwater bond." The concept for the ESC plan in phase 4 is problematic and should be amended. The transition from Phase 1 to Phase 2 has many issues that would leave portions of the site unprotected. Given the existing and proposed topography, it appears an intermediate phase is needed showing the traps and basins in place at the time the road and drainage system is constructed. Once the road and drainage system is constructed, the measures in the low points (traps 2 and 3) can be replaced with silt fence since the majority of the drainage area is diverted to Basin 5 and the trap/basin that will be permanent facility 5. This would require relocating several sections of the drainage system. We recommend setting up a meeting to discuss the ESC plan corrections prior to the next submittal. The use of the temporary slopes drain across the active construction area will not be permitted. Additional comments may be required. (Rev. 1) Since the drainage area to the diversion dike south of SB -5 is greater than 5 acres it must be called out as a diversion(DV) and specifically designed. [VESCH 3.091 In phase 2, please label this diversion as a fill diversion (FD). Please provide a Diversion (DV) north of St. Ives Extended to Sediment Basin 6 Provide a note on the plan that this diversion is to be maintained until the stormsewer system to SB -6 is installed. (Rev. 2) Comment has been addressed. 8. The construction entrance for the site does not appear to drain to the sediment basin because of the difference in elevations. Please provide a trap downhill of the construction entrance. (Rev. 1) Comment has not been addressed. (Rev. 2) Comment has not been addressed. The construction entrance is at an elevation of 409 and the top of embankment for the sediment basin is 412. A trap is needed between the construction entrance and the Route 29 culvert /storm - system until Briarwood is constructed and stabilized enough to move the washrack to the 413 elevation with a RWD placed there. Engineering Review Comments Page 5 of 5 9. Please provide a note on the plan at the four other entrances that access to the site will be limited to only the entrance at Briarwood Drive. (Rev. 1) Comment has been addressed. (Rev. 2) Please make sure this note is clear on the ESC sheets for St. Ives and Camelot Drive. (6A, 9, and IOA) 10. Show the erosion and sediment control measures needed around the St. Ives cul -de -sac. (Rev. 1) Diversions are shown going uphill in the set. Replace the diversion with silt fence in all areas that show the diversion running uphill. Add additional silt fence downhill of the road from the cul -de- sac to the existing portion of St. Ives Road. (Rev. 2) The diversion from the generator to SB -6 that existed in the set signed in June should have been kept. Please add this diversion. The silt fence shown on the south side of St. Ives Extension from the generator to the existing road is shown correctly. 12. The location of ST -1 is in the middle of construction. Please move the trap to the corner of the property at the start of the St. Ives extension. (Rev. 1) Please provide a Diversion (DV) north of St. Ives Extended to Sediment Basin 6. Provide a note on the plan that this diversion is to be maintained until the stormsewer system to SB -6 is installed. (Rev. 2) Comment has been addressed. 14. Pond 5 should be designed as a sediment trap for the ESC phase. A diversion should carry runoff from construction area around the St. Ives cul -de -sac. (Rev. 1) The diversion is shown going uphill. Replace the diversion with silt fence in all areas that show the diversion running uphill. Add additional silt fence downhill of the road from the cul -de -sac to the existing portion of St. Ives Road. (Rev. 2) The diversion from the generator to SB -6 that existed in the set signed in June should have been kept. Please add this diversion. The silt fence shown on the south side of St. Ives Extension from the generator to the existing road is shown correctly. 16. Please label the contours in all traps so that the volumes can be verified. (Rev. 1) Please show ST -I on the Phase II sheet. It appears the drainage area to the trap is larger than the lines indicate. Due to site constraints, a variance will be given by county engineering for the embankment greater than 5ft. (Rev. 2) Comment has been addressed. 18. (Rev. 1) Please note that a recent ordinance amendment [17 -207] will require permanent seeding on all phases of the project nine months from the date the grading permit is issued. It may be advantageous for the applicant to consider breaking the project into at least two sets of plans. (Rev. 2) Comment has been acknowledged. 19. (Rev. 1) Please clarify the note on sheet 12 of the ESC set about the conversion of the riser to Pond 4. It appears as though the crest of the facility will stay at the same elevation but the riser would need to be cut to allow for two 1'x4' weirs. (Rev. 2) Comment has been addressed.