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HomeMy WebLinkAboutWPO201300021 Review Comments Erosion Control Plan 2013-05-14�� OF A i" A COUNTY OF ALBEMARLE Department of Community Development 401 McIntire Road, Room 227 Charlottesville, Virginia 22902 -4596 Phone (434) 296 -5832 Fax (434) 972 -4126 Project: Plan preparer: Owner or rep.: Plan received date: Date of comments: Reviewer: Old Trail Village Block 14 Roudabush Gale & Associates Inc. March Mountain Properties, LLC 3 May 2013 14 May 2013 Michael Koslow (WP02013- 00021) [tel 434- 977 -0205] The first erosion and sediment control (ESC) and stormwater management (SWM) plan (WPO201300021) submitted 3 May 2013 has received Engineering Review and does not appear to meet Albemarle County minimum checklist items for approval. Please adequately address the following comments for final approval: A. Application Information 1) After plan approval but prior to grading permit issuance, a stormwater management maintenance agreement will be required. 2) After plan approval but prior to grading permit issuance, a pro -rated share payment to Lickinghole Basin will be required. B. Title Information 1) Title sheet of final plans will require a date and professional engineer signature prior to approval. C. Existing Conditions Information 1) Please indicate date of topographic information. All topography should be at least visually field verified by the designer within the last year. D. Erosion and Sediment Control (ESC) Plan 1) Please expand adjacent areas description of narrative on sheet 3 to include a description of the land cover and use for properties adjacent to proposed Block 14 (e.g. former pasture if applicable). 2) Narrative on sheet 3 appears to refer to an existing sediment basin approved with WP02012- 00013 in "existing site conditions ". Please update reference which currently indicates WP02011- 00013. 3) Please indicate locations for contractor parking, stockpiling (if proposed) and staging areas or comment if these are included within the proposed limits of disturbance. 4) Albemarle County requires a paved construction entrance (see Design Standards Manual for detail). Please revise proposed stone construction entrance detail on sheet 3. Engineering Review Comments Page 2 of 3 5) Please indicate dust control for this project. 6) Please include in report format or with the plans the existing soils boundaries. 7) Silt fence appears to be proposed crossing contour lines. Please propose diversion dikes instead of silt fence to divert runoff laden sediment to proposed sediment basin. 8) Please provide an adequate channel analysis for runoff from proposed construction. Analysis needs to be carried further downstream compared with submitted MS -19 computations shown on p. 28 of SWM Report dated 10/31/2012. Analysis could be proven given: a) adherence to VESCH MS -19 requirements (see 4VAC50- 30 -40, MS -19 sec. b (1)): The applicant shall demonstrate that total drainage area to the point of analysis within the channel is one hundred times greater than the contributing drainage areas of the project in question. A report or plan depiction with analysis to this point could prove channel adequacy. b) adherence to VA code 10.1- 603.4(7): essentially, the required condition for an on -site solution without looking downstream for a 24 hour 1. 5, 2, and 10 -year intensity event - (post - development) Peak flow rate <= Pf*VfNp Where Pf = peak flow rate in a forested condition Vf = runoff volume in a forested condition Vp = runoff volume proposed Since the proposed sediment basin drains to Lickinghole Creek, an analysis to that point is acceptable for adequate channel analysis for this project. 9) Sediment Basin Schematic indicates invert elevation for the dewatering device as 1637.85; please update this invert elevation. 10) Please confirm existing 60" diameter riser anchor embedded 18" is intended to be used in place for the expanded sediment basin or propose an alternate riser anchor. 11) Please confirm existing 42" diameter trash rack is intended to be used in place for the expanded sediment basin or propose an alternate trash rack. 12) Please provide dewatering device sizing calculations to confirm a 6" diameter orifice is sized to dewater the expanded sediment basin in 6 hours. 13) Please increase the length of the proposed baffle to ensure a length -to -width ratio of at least 2 to 1. E. Stormwater Management Plan 1) Please provide a plan view depiction of existing conditions proposed to drain to the proposed bioretention basin. Depiction should include: a) Drainage divides for all drainage areas to proposed facilities as used in computations b) Acreage of each drainage area, labeled on the map area and matching comps c) Drainage divides for total drainage areas to proposed facilities, matching comps Engineering Review Comments Page 3 of 3 2) Please provide a plan view depiction of proposed conditions proposed to drain to the proposed bioretention basin. Depiction should include: a) Drainage divides for all drainage areas to proposed facilities as used in computations b) Acreage of each drainage area, labeled on the map area and matching comps c) Drainage divides for total drainage areas to proposed facilities, matching comps d) For future development to be considered in the analysis, include assumed land uses, impervious areas, and hydrologic coefficients 3) Please provide a flow path > 2:1 for proposed bioretention basin witthout the use of baffles. 4) Please include 10 & 100 year high water elevations on Pond Schematic on sheet 6. Please include a minimum of 1' freeboard for 100 year storm. 5) Please include a minimum 4' wide, 4' deep and 1:1 slope sides cutoff trench by note or detail for bioretention basin berm. 6) Please include a trash rack over riser crest of principal spillway to prevent clogging. A DI -7 is not an acceptable trash rack. 7) Please label proposed (or used in place existing) riser base size and dimensions. 8) Emergency spillway must be in "cut" or be armored (appears to be in fill) per VSMH 3.03 -2. 9) Please label all existing and proposed contour elevations in bioretention plan on sheet 6 for plan clarity. 10) It appears that the modified bioretention basin is designed to treat impervious surfaces from all of Blocks 11 & 14 and parts of Blocks 12 & 15 as proposed on sheet 5 (383,983 sft). To meet 50% removal rate (0.5" x this impervious surface area) with a maximum ponding depth of 1' (as shown on `Bio- Filter Combined with Detention Basin" detail in Design Standards Manual), the floor of the modified bioretention basin would need to be 16,000 sft. Please reconfigure BMP or expand modified bioretention basin to include this amount of floor area. 11) Please include geotech fabric under "choking layer" in bio- retention section on sheet 6. 12) Please increase cleanout spacing to a minimum frequency of every 50' under floor area. F. Stormwater Management Computations 1) Routing calculations will be checked after bio- retention basin is reconfigured per comment E10. Current Development Engineering is available from 2:30 -4 PM on Thursdays to discuss these review comments. Please contact Michael Koslow at 434 - 296 -5832 ext. 3297 or email mkoslowgalbemarle.or to schedule an appointment. [17- 204.f1 An application for an erosion and sediment control plan that requires modifications, terns, or conditions to be included in order for it to be approved shall be deemed to be withdrawn if the owner fails to submit a revised plan addressing the omitted modifications, terms or conditions within six (6) months after the owner is informed of the omitted information as provided under paragraph (B).