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HomeMy WebLinkAboutWPO201100103 Review Comments Stormwater Management Plan 2012-01-26� 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: Boars Head Sports Club Expansion ESC and SWM Plans (WPO- 2011 - 00103) Plan preparer: Mr. Alan Franklin, PE; Water Street Studios Owner: University of Virginia Host Properties, Inc. Plan received date: 14 December 2011 Date of comments: 26 January 2012 Reviewer: Phil Custer The first submittal of ESC and SWM plans for the Boars Head Sports Club Expansion (WPO -2011- 00103), received 14 December 2011, has been reviewed. Engineering can recommend approval to the plan after the following comments have been addressed. A. General Review Comments On 24 January 2012, I visited the site to perform a stream assessment of the wetland areas within the vicinity of this project. With close to a 150acre watershed, the stream running through the Boars Head compound was suspected to be perennial. On this site visit, it was confirmed that these water features have many perennial characteristics. However, it was also clear that the channel downstream of Pond 2/Wellington Drive is a manmade and not a natural stream. After researching the history of the site, I believe this manmade channel was created between 1992 and 1994 as the improvements proposed in SDP - 1992 -036 were constructed. Without any information to the contrary, I must assume this work was done legally and, therefore, these streams are not subject to any new stream buffer per Section 104 of the Water Protection Ordinance. If the definition of a perennial stream was to be modified by a future Board of Supervisors to include manmade modifications, Boars Head Inn may be limited in their development within close proximity to these water features. After my research and site investigation, it is relatively unclear where the primary outlet of Pond 2 is directed. I strongly suspect that the riser on Pond 2 outfalls into Pond 3 (downhill of the employee parking lot). On site Tuesday, there was no water flowing out of the spillway underneath Wellington Drive, but the depth of Pond 3 was to the top of the riser and embankment. The lack of freeboard and almost permanent saturation at the top of the embankment of Pond 3 is a concern. B. ESC Plan Review Comments 1. The drainage area for the silt fence protecting the employee parking lot is deeper than the 100ft requirement stipulated by the VESCH. Since a biofilter is being constructed here, please design this facility first as a trap with diversions flanking it. 2. Please remove the silt fence adjacent to the existing Racquet Club building. Because the fence is not placed parallel to contour lines, it will act as a diversion and will direct dirty water out of the site. Please provide a berm downhill of the drainage structure so that water doesn't bypass the inlet protection measure already specified. Albemarle County Community Development Engineering Review Comments Page 2 of 3 3. In phase 1, when the diversion crosses the travelway, please call it out as a RWD or an asphalt diversion. Please also call out a I % positive slope on this diversion. It is currently proposed going uphill in at least one location. 4. Please maintain the grading of the sediment trap in Phase 2, remove all references to a biofilter, and provide a RWD across the entrance so as much of the construction area can be diverted to this measure. 5. Please provide Permanent (PS) and Temporary (TS) seeding symbols throughout the plan. 6. Please replace the stone construction entrance detail with the county detail for the paved wash rack found in the design manual online. 7. In both phases, please move both construction entrances so that they drain into the sediment trap. I understand that for a few weeks work will need to be performed beyond these entrance locations, but it's important that the wash off area of the entrances are able to drain to a trapping measure for the rest of the time. 8. The two existing drop inlets within the construction area must be sealed for the life of the project. Please provide notes to this effect on sheets 1 and 2. 9. In phase 2, please provide a note stating that the inlet in biofilter B is to be sealed. Please place inlet protection on all other inlets in parking islands. 10. Please replace all references in the plan and narrative to the waste area on the golf course with "a site with an approved and valid grading permit." The county is in the process of closing out all old permits and the golf course waste area may not be available to this project by the time a grading permit is issued. 11. Please provide an adequate channel analysis for the channels downstream of the developed areas of the site. With the employee parking lot, I'm concerned with the flooding of the pond 3 embankment during the ten -year storm. Downstream of the parking lot, I'm concerned about erosion issues with the two year storm until the main stream is reached 12. This comment is only advisory. The construction sequence does not seem to acknowledge that the employee parking lot must be constructed first before anything else is done on site. 13. No changes are required by this comment; it is only advisory. The construction sequence mentioned that the proposed work will take 12 months to perform. Please note, however, that the county requires all areas not necessary to construct any building to be permanently seeded after 9 months from the date the permit is issued. 14. A grading permit cannot be issued for this project until the ESC and SWM bonds are posted for this project. Bonds will not be computed until after plan approval. To receive a bond computation, please submit a Bond Estimate Request Form to the County Engineer after the plans have been approved. All TMPs disturbed with this application will need to be listed on the request form and party to the WPO bonds when they are posted. C. SWM Plan Review Comments 1. A water quality spreadsheet is needed for a watershed to each BMP, not for the overall site. Please provide one spreadsheet for each of the three biofilters and a total of proposed impervious area not being treated. The new impervious area not being captured by this plan should be subtracted from the pre - development totals used in the spreadsheets for Biofilter A and B. 2. The sizing methodology of providing 2.5% of the total impervious area of a watershed as the biofilter footprint is only acceptable if a Ift ponding depth is provided. The goal of a biofilter facility is to treat the first 1/2" of runoff over the impervious areas. For facility A, this works out to be 1333cf. With a 800sf facility and 6" of ponding, the provided volume treated is a little over 400cf, about 33% of the VSMH intention. 3. Please provide individual biofilter planting plans for each of the three facilities in the WPO set. Albemarle County Community Development Engineering Review Comments Page 3 of 3 The biofilters should be planted with a variety of species of trees and shrubs as well as at a rate of at least 1 plant every 100sf. 4. The stormwater narrative mentions the use of an equation to obtain intensities used in the calculations. The applicant should use the intensities detailed on page 33 of the latest edition of the county's Design Manual. Are these values equivalent? 5. The removal rate attributed to nearly all stormwater facility assumes that forebays are provided. Please provide forebays at all areas of concentrated discharge with a gravel ring (similar to a sediment trap weir), or equivalent, between 10% and 20% of the biofilter floor area. 6. Regarding the level- spreader biofilter proposed in the second submittal of the site plan, but not shown in the WPO application, please make the following changes to the plan: a. Please provide a modified simple spreadsheet for this watershed. The pre - development scenario should use what was last approved on a county site plan, not what is currently in place. In 1994, a plan was approved (SDP- 1994 -010) for this area for a tent on a gravel lot for special events. The total area of gravel in this plan was 10,270sf. b. I estimate, with the above information and the current proposal, the required removal rate to be 50% with a Water Quality Volume of 631cf. The current biofilter seems to only have a volume of close to 400cf because of the 6" depth. The biofilter will likely need to expand. c. The outlet of the underdrain at elevation 534 will likely be below the pond water elevation. Because of the grade challenges, I recommend that the applicant investigate using Internal Water Storage within this facility and raising the outlet of the underdrain. The ponded water inside the biofilter cannot be closer than 18" from the biofilter bed elevation. d. The minimum required media mix depth in the county is 2.5ft. e. After reviewing page 3.11 -11 of the VSMH, I cannot justify the waiving of the curbing requirement for SWM purposes. Please direct runoff to one or two curb cuts and line the opening to the facility to prevent erosion. f. If the planning department grants a curb waiver, engineering review will require a curb on the west side of the parking lot or a modification to the grading plan so runoff cannot bypass the facility if there is a slight grading error. g. This facility must be designed for detention unless a waiver is granted. 7. In every place it is referenced, please replace "Luck Biofilter Mix" with "State- Approved Mix." 8. Please provide spot elevations around the plan view and cross - section detail of biofilter B. 9. Biofilter B appears to have been routed with a 48" diameter riser, rather than the 30" that was proposed in the detail. Please correct this discrepancy. 10. A Stormwater Facility Maintenance Agreement must be recorded for this project. Please submit a completed agreement document and $17 fee to Ana Kilmer after reading the instructions online. The title of the plan must refer to WPO- 2011 - 00103. This agreement must refer to each parcel a stormwater facility is being proposed on. 11. A grading permit cannot be issued for this project until the ESC and SWM bonds are posted for this project. Bonds will not be computed until after plan approval. To receive a bond computation, please submit a Bond Estimate Request Form to the County Engineer after the plans have been approved.