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HomeMy WebLinkAboutWPO201100103 Review Comments Stormwater Management Plan 2012-04-04� 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: 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 (Rev. 1) 12 March 2012 Date of comments: 26 January 2012 (Rev. 1) 4 April 2012 Reviewer: Phil Custer The first revision to the ESC and SWM plans for the Boars Head Sports Club Expansion (WPO -2011- 00103), received 12 March 2012, 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. (Rev. 1) Comment has been addressed. 2. Please remove the silt fence adjacent to the existing Racquet Club building. Because the fence is Albemarle County Community Development Engineering Review Comments Page 2 of 4 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. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Comment has been addressed. 5. Please provide Permanent (PS) and Temporary (TS) seeding symbols throughout the plan. (Rev. 1) Comment has been addressed. 6. Please replace the stone construction entrance detail with the county detail for the paved wash rack found in the design manual online. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Comment has been addressed. 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. (Rev. I) Comment has been addressed. 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 (Rev. 1) The channel design downstream of biofilter A is acceptable. However, adequacy concerns downstream of the employee lot have not been addressed. A routing or analysis of the 10 year storm for this pond is necessary to prove that all manmade structures are adequate per MS -19. Other than clearing debris, modifying the riser structure should only take place after thorough consideration because the pond was likely designed to meet a certain hydrologic function previously. Unless the history of this pond's design and construction is well - documented and it can be determined with a high degree of confidence the pond is meeting no specific stormwater requirement during a previous application, the pond riser should remain intact or only modified in a way that maintains 1) the permanent wet volume and 2) the current release rates. Opening the structure up, as currently proposed by the applicant, would nullify any prior design requirements, considerably increasing release rates from the pond as well as decreasing the treatment capacity of the pond. Albemarle County Community Development Engineering Review Comments Page 3 of 4 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. (Rev. 1) A completed bond request form has been provided. WPO bond amounts will be calculated after plan approval. 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 bioiilters 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. (Rev. 1) Comment has not been addressed. A modified simple spreadsheet is needed for each facility's drainage area. The remainder of county review will assume that the required removal rate for facility A and B is 50% and facility C is 65 01c, as indicated by the applicant. 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 lft 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. (Rev. 1) Comment has been addressed. 3. Please provide individual biofilter planting plans for each of the three facilities in the WPO set. The bioiilters 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. (Rev. 1) A greater variety of species are required. When possible, the VSMH states at least three species should be provided for both shrubs and trees. Each species should be equally specified on the plan. Please also locate the plantings in the bed material. 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? (Rev. 1) Comment has been addressed. 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. (Rev. 1) A forebay is needed in biofilter B. The current note is not acceptable for this facility. 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. Albemarle County Community Development Engineering Review Comments Page 4 of 4 (Rev. 1) Comment has not been addressed. b. I estimate, with the above information and the current proposal, the required removal rate to be 50% with a Water Quality Volume of 63lcf. 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. (Rev. 1) Biofilter Cis sized appropriately for a 65% removal rate. Comment has been addressed. 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. (Rev. 1) The design of the underdrain is acceptable. Comment has been addressed. d. The minimum required media mix depth in the county is 2.5ft. (Rev. 1) Comment has been addressed. 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. (Rev. 1) Curb has been added. Comment has been addressed. 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. (Rev. 1) Grading plan is acceptable. Comment has been addressed. g. This facility must be designed for detention unless a waiver is granted. (Rev. 1) Comment has been addressed. In every place it is referenced, please replace "Luck Biofilter Mix" with "State- Approved Mix." (Rev. 1) Comment has been addressed. Please provide spot elevations around the plan view and cross - section detail of biofilter B. (Rev. 1) Comment has been addressed. If there is a long term issue with the strength of the make shift walls around Biofilter B, the applicant will be responsible for correcting it in the future. 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. (Rev. 1) Comment has been addressed. 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. (Rev. 1) The stormwater facility maintenance agreement has been received and is currently being processed by Ana Kilmer. 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. (Rev. 1) A completed bond request form has been provided. WPO bond amounts will be calculated after plan approval.