Loading...
HomeMy WebLinkAboutWPO201300044 Review Comments Stormwater Management Plan 2014-02-18 Albemarle County Engineering Stormwater Management Plan checklist for plan reviewers 18 Feb 2014 (Use the latest checklist from the Team Services engineering forms site) A stormwater management plan is a plan detailing all measures and facilities to handle and treat stormwater runoff and protect downstream waters and channels. [17-303,304, and the Virginia Stormwater Management Handbook, and the BMP Clearing House] Reference key; [Square Brackets] are County Code references, {Curved Brackets} are policy references, and(regular parenthesis)are explanatory. Links to reference documents are provided where possible. Title information: (applicable to any type of plan) >/ Project title. Titles should be appropriate. It should be a stormwater management plan, not an erosion control plan, or site plan etc. Professional seal, with original signature and date. [18-32.6.1] C• ten\ The stormwater management plan, a permanent document, must not contain the - osion c introl plan, a temporary document. It should not be part of a site plan or road •lan. T •se must be separate documents,to be reviewed and bonded separately. If these • - ' eluded, they must be removed. ill X VSMP general consruction permit application made with DEQ. 0 A completed copy of the standard stormwater maintenance agreement and fee for recordation for any stormwater management facilities [17-304E] NA copies of manufacturer's approval of the plan for any manufactured stormwater management facilities (filterra, stormfilter, sandfilter, etc.) {Policy} M pro-rated share payment for any sites which drain to County regional stormwater management facilities. (This should have been identified with the preliminary plan or plat) [17-316] Exists9g conditions plan view information: (applicable to any type of plan) V accurate current existing topography at the time of submittal, including all existing site features, and any recent disturbances, all at a legible scale [18-32.6.21 V date and source of the topographic information: 118-32.6.21 All topography should be at least visually field verified by the designer within the last year {Policy} WPO buffer limits; 100' from stream or wetland bank, 200' from reservoirs, or floodplain limit if greater [17-3171 floodplain limits, including 100yr flood limits for any channel with a drainage area of 50+ acres [18-32.6.2d, 18-30.3] all existing easements (access, drainage, sight, sanitary easements, etc.) with deed book references, locations and dimensions [18-32.6.2] / all critical slopes (typically shaded) [18-32.5.2j Proposed plan view information: (applicable to final site plans, road and drainage plans) Grading: [18-32.6.2, 14-303] proposed topography at minimum 2' contour intervals—tied into existing contours, as well as all proposed site features. (Sites with less than 6' of grade change should consider using smaller contour intervals.) proposed slopes are all 2:1 (horizontal:vertical) or flatter {Design Manual, section 8} Albemarle County Engineering Stormwater Management Plan Checklist Page 2 of 5 • (..) proposed slopes steeper than 3:1 have low maintenance (not grass) ground cover specified on the plan {Design.Manual. Section 8} existing critical slopes are not disturbed, unless a waiver or exemption has been granted for the disturbance. [ 18-4.2, l4-3041 C) Retaining Wall Plans. Any walls supporting necessary infrastructure require engineered plans (not generic manufacturer's details) and computations. {Design Manual, section 8 This will also be required where walls are close to property lines and there is the danger of affecting neighboring property, either during construction,with later failures, or with pedestrian or vehicle safety. Easements: [Zoning Ordinance 18-32.7.4, Subdivision Ordinance, Article IV, Division 4] 0 all proposed permanent easements, dimensioned and labeled stormwater management easements over all facilities and associated structures and access - all drainage easements are a minimum 20' wide. Required width: 10'+(pipe dia. or channel width)+2'+2(depth-5'). The pipe, channel or structure must be within the center third of the easement. {Design Manual, section 61 .,,,/ no structures or trees within drainage easements {Design Manual. section 6} Facilies: stormwater facility and site layout as shown on the site or subdivision plan sheets, preferably at a scale of 1"=30' or 1"=20'. 90% or more of site development captured in facilities for water quality (ordinance requires as much as practical, 90% is a target) ,‘40 c Albemarle County general stormwater notes y• t7 facilities are not in the floodplain Kfacilities are not in buffer areas with Program Authority authorization mitigation plan. facilities meet or exceed requirement removal rates in concept • A25--- .. facilities are off-line of major streams ,/ facilities are not built over any other utilities (sanitary sewer, water, gas, fiber optic, etc.) ____. permanent stormwater management easement encompasses 100yr high water mark, all structures (embankments if present, and culvert release channels) and access flow is not short-circuited, flow patis 2:1 or greater in all water quality facilities, without the excessive use of baffles to extended dikes .11 Cross-sections and details of each facility: i embankments (between 3 and 25 ft in height) tz top width 8ft min., 10' for heights 15' to 20', 12' for dams over 20',heights measured from the outside toe z2 V' upstream face slope 2:1 or flatter, labeled � "vdownstream slope 3:1 or flatter, labeled v' V 10yr and 100yr high water elevations labeled with 1 ft freeboard for 100yr storm /1y, with emergency spillway or 2 ft freeboard without emergency spillway r/ an impervious core ,/ a cutoff trench of minimum 4' wide, 4' deep and 1:1 sloped sides principle spillway V riser crest 1' below emergency spillway if present riser crest stays within weir flow control during the 10yr storm Wig, () trash racks over all inlets and controls to prevent clogging ,...----"Trash rack over riser crest is not flat(no DI-7's)- e/ A out .. Albemarle County Engineering Stormwater Management Plan Checklist Page 3 of 5 - riser size and materials labeled barrel size, strength, material, inverts and slope labeled 6 all orifices, weirs or other control devices labeled with dimensions and inverts control orifices not more than 25% of riser perimeter at any point(does not affect riser structural integrity, 25% is a target) KOp ( riser base size and dimensions labeled mew._ tali, access to riser is provided (riser should be in dam, not free-standing without structural support and analysis) iV• emergency spillway dimensioned and labeled V invert labeled APO dimensions in control section S dimensions in channel section provided if different than control section linings labeled and shown where applicable A' spillway over native soils, and/or sufficient armoring provided. The spillway should never be over fill material. appropriate materials for controls (for example, a computation which relies on a flow depth in inches for flow control should not be specifying rip-rap, where this is impossible) V sediment forebays provided for all retention basins, constructed wetlands, and basin e r'.+.' of t/' biofi ters at all inlets y ' ' sized to 0.25" of runoff per impervious acre (can be included in WQV) �° 1 direct vehicle access provided biofilters with minimum floor dimensions labeled Y 1 does not receive any stream base flows, and; areas of sheet flow into facilities are protected from erosion by appropriate scour protection stone or energy dissipation measure plants are shown and labeled with height or caliper 3 plant species are specified (3 trees if present, 3 shrubs) with sizes for survivability biofilter floor is 2.5% of impervious cover for type I, or 4% of impervious cover for type II (This must be used with judgement. This assumes almost all of the area drainage to a facility is impervious. If there are large areas draining to a facility otherwise, this will not work. Also, if the water quality volume is not substantially captured, which is the intent, the water quality volume should I govern.) 2.5' depth of sand/soil media i.i 1'-1.5' stone drain in geotech fabric under media, with 6"perforated PVC pipe to daylight 0.5' ponding depth provided in facility at 5% imp., 1.0' at 2.5% (see volume comments above) overflow provided biofilter area is flat cleanouts are provided at the end of the underdrain pipe, and every 50' 17 in subdivisions, facilities are in open space where present. In subdivision with no open space, facilities are out of yards and away from residences. Facilities must not be a / nuisance in design or location. [17-304F] overland relief is provided for any facility in case of clogging. The failure of any system will not cause structures or yards to flood [17-304F] Albemarle County i Engineering Stormwater Management Plan Checklist �� \' Page 4 of 5 Retention basins/16e appropriate water quality volume; 3WQV for type I, 4WQV for type�II. good base flow, or 10 acres or more drain to the pond V normal pool elevation is labeled on plans `? 15% of surface area is a depth of 1.5' or less I 15% of surface area is a depth of 1.5' to 2' 170% of surface area is a depth of 2' to 6' (greater depths are counted in WQV) a 10'-15' vegetated strip/safety bench is provided around the perimeter for Type III, a 10-15' wide and 1-1.5' deep aquatic bench is provided - typical 10'x10' planting area specified for aquatic bench ,/ at least 3 species of aquatic plant used m extended detention basin II (enhanced)has marsh zones normal pool elevation is labeled on plans 20%pool areas 1.5'-4' deep 40% shallow marsh 0.5'-1.5' deep, contains 1WQV 40%high marsh 0.5' deep or less as-built data is provided for existing ponds and drainage structures and facilities existing ponds and drainage structures and facilities are brought up to current standards (the rest of this list) v vehicle access roads provided to all facilities 1/ access graded to 10' width and less than 20% grade V/ minimum 10' wide permanent easement over access / gravel surface or better for grades over 10% / underground facilities have adequate manhole access at upstream and downstream points. / manufactured pre-fabricated water quality facilities have a written approval of the plan from the manufacturer. minimum pipe sizes into and out of the facility are 15" diameter for maintenance Stormwater management computations: 11/s. Existing drainage area map: existing conditions for site and off-site areas draining to proposed stormwater facilities (The stormwater management analysis should not include off-site areas which do not drain to proposed facilities) drainage divides for all drainage areas to proposed facilities as used in computations acreage of each drainage area, labeled on the map area and matching comps coefficient used for each drainage area as used in comps time of concentration for each drainage area where applicable as used in comps Proposed drainage area map: Q proposed conditions for the site and any off-site areas draining to proposed stormwater facilities. The analysis should not include off-site areas which do not drain to proposed facilities, and should not include large off-site areas in relation to the site. drainage divides for total drainage areas (not sub-areas)to proposed facilities, matching the computations. Sub-areas may be shown on added exhibits or sheets. ) acreage of each drainage area, labeled on the map matching comps Albemarle County Engineering Stormwater Management Plan Checklist Page 5 of 5 coefficient for each drainage area, labeled on the map matching comps Vtime of concentration for each area labeled on the map where applicable, matching comps 0 for future development to be considered in the analysis, include assumed land uses, impervious areas, and hydrologic coefficients on the proposed drainage map 1 Existing drainage divides should be honored as much as possible. Compensation schemes and unnatural divides which may not hold up in extreme weather are discouraged. Target Values: War quality; removal rate computation summary for each facility agreement on target removal rates,required water quality Water uantity; existing flow release summary for each facility or release point from the site agreement on existing target flow rate for each release point from the site Compliance: riser floatation computations for risers which are not concrete, or are taller than 10' hydrology for each facility used to generate inflow hydrographs including; hydrologic land use coefficients (CN, C), areas and flow times 24hr storm hydrographas are used (the modified rational method is not accepted) as closely as possible,the drainage areas represent the site development, and not off-site areas computations are provided for composite coefficients _ --- '. - e - - . I • _ ) , -4, times of concentration are representative of watersheds. If the omission of small sheet flow areas yields a higher peak flow,the small sheet flow areas are not used. Sheet flow should not exceed 200'. With steep slopes it should not exceed 50'. times of concentration generally decrease with development areas over 50 acres make use of more than one hydrologic method to verify computation of peak flows contour areas and elevations used for storage, matching the plans a stage-storage (elevation vs. storage volume )table with water quality volume requirements computed for facilities with volume requirements the hydraulic dimensions and coefficients for each weir, orifice, culvert or other control structure used in the routing model, matching plans Water quality; drawdown computations for any extended detention facilities Water quantity; routing model and results for each detention facility flow control is maintained by the riser when present, not the outlet barrel pipe detention computation summary for each facility channel computations for erosion and capacity for emergency spillway channels detention storage and routings are over and above water quality volumes