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HomeMy WebLinkAboutSDP200600041 Review Comments Final Site Plan and Comps. 2009-09-04� 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) Date of comments: 24 April 2009 (Revl0) 4 September 2009 (full set. Rev. 11, Phase IV. Rev. 1) 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. 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 The road bond amount which was previously quoted is no longer applicable.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. 1) Comment remains unchanged. 3. Please show 200ft of the existing profile for St. Ives. If the road isn't already sloped at 6.27 %, a vertical curve must be provided. (Rev. 1) Comment has been addressed. 4. Please show the 18, 20, and 22 contour lines on sheet 20A. (Rev. 1) Comment has been addressed. 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) Consentfrom RWSA is needed before this plan can be approved. 6. The low point of the cul -de -sac must be carried to the stormwater management facility. Please show either a channel or pipe. (Rev. 1) Comment has been addressed. 7. Please provide traffic control and street name signs on Sunset Drive. (Rev. 1) Comment has been addressed. 8. Please label the curb radii at intersections and the cul -de -sac. (Rev. 1) Comment has been addressed. 9. In the road profiles, please label the existing grade at the 50ft stations. (Rev. 1) Comment has been addressed. 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. 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. B. Final Site and Drainage Plan (SDP200600041) 1. A critical slope waiver is necessary for the disturbance of natural slopes in Phase 4. Only critical slopes disturbed for the roadway construction can be disturbed by the county engineer. Please submit a request for critical slope disturbance and we will schedule it for a Planning Engineering Review Comments Page 2 of 6 Commission Meeting. (Rev. 1) Comment has been addressed. The sediment trap located in the critical slopes will be considered a utility to be consistent with current county engineer policy. 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. 3. Low maintenance, non - grassed groundcover 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. 4. Please provide drainage computations for the existing drainage structures on St. Ives which will have their drainage areas increased by this development. (Rev. 1) Comment is no longer applicable. 5. Please place a drop inlet to capture the concentrated runoff at Sta. 10 +00 on the St. Ives Extension. (Rev. 1) Comment is no longer applicable. 6. Please provide a note on the drainage profile sheets stating that all pipes deeper than 12ft require safety slabs (SL -1). There are several structures that currently do not have the safety slab specified. (Rev. 1) Comment has been addressed. 7. Please provide a note on the drainage profile sheets stating that all inlets must have inlet shaping (IS -1) if the drop from the surface (or pipe) to the bottom is 4ft or greater. (Rev. 1) Comment has been addressed. 8. Wall details are needed in the site plan set. (Rev. 1) No walls are currently proposed in Phase 4. 9. Provide more spot elevations around the proposed walls. Any wall 4ft or taller will require a handrail. (Rev. 1) Comment has been addressed. 10. Inlets that do not overflow to a stormwater management facility must be sized for a 6.5in/hr storm. (Rev. 1) Comment has been addressed. 11. Please design and specify on the plan the size of the ditch to the northwest side of Sunset Drive. (Rev. 1) Comment is no longer applicable. 12. Structure ST -415 appears to be a culvert on the plan, but the calculations sheet indicates it is a DI -7. Please clarify. If the structure will be a culvert, please provide a headwater calculation and provide an adequate sump area. Regardless of whether this structure is a DI -7 or an end section, the computation needs to take into account the drainage area to existing structure in the Camelot subdivision. (Rev. 1) Comment has been addressed. 13. Please specify a small berm on the downstream side of all DI -7's so that the headwater shown in the calculations has the chance to accumulate without bypassing the inlet. (Rev. 1) Comment has been addressed. 14. Please provide a detail for the pipe from structure 406 under the retaining wall. (Rev. 1) Comment is no longer applicable. 15. Please update all four standard Albemarle County Construction notes in the sheet sets. The latest set of notes can be found in the current edition of the design manual, available online. Engineering Review Comments Page 3 of 6 (Rev. 1) Comment has been addressed. C. Stormwater Management (WPO200600066) 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. 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 10A. Please also be aware that ponds 3 and 4 are located upstream of the intake for a R WSA 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]. 3. The flow path in Stormwater management 3 from structure 4 -406 should be longer. I recommend moving the outfall to the eastern corner and providing a 1Oft wide access path to the forebay. (Rev. 1) Comment has been addressed. 4. All water deeper than 6ft should not be considered as part of the 4xWQv. Please adjust the calculation for SWM facility 3 in this regard. (Rev. 1) Comment has been addressed. 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. 6. Facility 5 must have a 3:1 downstream slope. (Rev. 1) 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. 8. If a biofilter is sized to provide a 65% removal rate, it must have a bed equal to 4% of the impervious area draining to it. The biofilter shown in the plan possesses a removal rate of 50 %. (Rev. I) No biofilters are currently proposed. This comment is no longer necessary. 9. Please show the cutoff trench and impervious core in the embankment detail for all ponds. (Rev. 1) Comment has been addressed. 10. The biofilter detail is hard to read. Please clarify. Additional comments may be necessary after the change. The biofilter characteristics must match those listed in the engineering review checklist. To simplify design, review, construction, and inspection, I recommend combining this facility with Facility 5. (Rev. 1) No biofilters are currently proposed. This comment is no longer necessary. 11. The ponding within a biofilter must be lft when using the 2.5% or 4% calculation method specified in the design manual. If a 6" ponding depth is desired, the applicant should provide a volume calculation showing that the water quality volume is provided. (Rev. 1) No biofilters are currently proposed. This comment is no longer necessary. 12. The SWM bond will need to be recalculated. The calculation will occur once the plan is closer to approval. (Rev. 1) The bond will be calculated at a later date. 13. (Rev. 1) Stormwater management facility maintenance roads must be IOft wide. [DM] Engineering Review Comments Page 4 of 6 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. 1) The bond will be calculated at a later date. 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 VESCH] -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: Pond I -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. 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. 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 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. 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 Engineering Review Comments Page 5 of 6 channel with supporting calculations proving adequacy. 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 3.09] 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. 4. Sediment Basin 5 should be designed using the maximum drainage area to it. (Rev. 1) Comment has been addressed. 5. Please lightly hatch all critical slopes on the ESC plan. (Rev. 1) Comment has been addressed. 6. Please update the disturbed area in the ESC narrative. (Rev. 1) Comment has been addressed. 7. Please show the ESC measures for the Route 29 improvements. It appears as though check dam, ditch matting, and inlet protection are needed. (Rev. 1) 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. 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. 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. 11. Work is shown in a wooded area close to the property line of many existing lots in the Camelot subdivision. Though the limit of construction line is within the subject parcel, there have been cases throughout the county where, because of damage to trees and root systems, disturbance has crossed property lines. Please provide a note on the plans stating that damage to trees close to the property line is a critical area and the contractor must be careful not to disturb adjacent parcels. (Rev. 1) Comment has been addressed. 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. 13. ST -2 should be located farther downhill so the retaining walls can be constructed without disturbance of the sediment trap. Otherwise, a more detailed phased plan may be required. (Rev. 1) Comment has been addressed. Engineering Review Comments Page 6 of 6 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. 15. Please provide matting calculations for all proposed channels. (Rev. 1) All channels have been removed from this set. This comment is no longer necessary. 16. Please label the contours in all traps so that the volumes can be verified. (Rev. 1) Please show ST -1 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. 17. Please provide DC, TS, and PS symbols on the plan where applicable. (Rev. 1) Comment has been add- ' 18. (Rev. 1) Please note that a recent ordinance amendment [17 -2071 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. 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.