Any development within the Town of Winfield shall be provided with a storm sewer system capable of handling, at a minimum, a 25-year storm event based on the Rational Method for post- development conditions. Two (2) copies of the drainage calculations, signed and stamped by a professional engineer in good standing with the state of West Virginia, with drainage area map(s) shall be provided to the Town Engineer. The design of the system must take into account off-site runoff that flows onto the development as well as future expansion of the development and meet, at least, the following requirements: (The Town Engineer may make additional requirements to insure the development’s storm sewer/drainage system meets minimum acceptable engineering standards and requirements).
(a) Culvert and Pipe Material.
(1) Corrugated Metal Pipes (CMP), bends and fittings shall be a minimum of 16-gague and be bituminous coated with a paved invert.
(2) Reinforced Concrete pipe meeting the requirements of ASTM.
(3) High Density Polyethylene (HPDE) pipe shall be corrugated on the outside and have a smooth interior wall. All bends and fittings shall be of the same material as the pipe.
(4) Polyvinyl Chloride (PVC) with push on joints meeting the requirements of ASTM for SDR-35 pipe.
(5) The Town Engineer must approve alternative pipe material.
(b) Inlets and Storm Manholes.
(1) Inlets shall be placed at all low points within the development.
(2) Inlet grates shall be constructed ½” below finished grade.
(3) Inlets shall be placed so that runoff does not exceed ¼” in depth at any point within the development.
(4) Inlets shall be sized to handle a 25-year storm event.
(5) Inlets located in traffic areas or parking lots or have inlet and/or outlet pipes made of Corrugated Metal or Reinforced Concrete shall be made of concrete and have a metal grate. Inlets in pedestrian or landscape areas can be PVC.
(6) Storm manholes shall be made of concrete and have a metal frame and cover.
(7) Any inlet or storm manhole over three (3) feet in depth shall have plastic reinforced steps spaced 18-inches vertically.
(8) All inlets and storm manholes shall have at least a 6-inch sediment zone below the outlet pipe.
(9) An inlet or storm manhole shall be placed at least every 400 feet for access and cleaning of the pipe.
(10) Whenever two (2) or more storm pipes confluence, an inlet or storm manhole shall be provided. No wyes or tees will be allowed, unless otherwise approved by the Planning Director.
(11) A minimum of 6-inches of crushed stone shall be placed under all inlets and/or storm manholes.
(c) Construction Methods.
(1) No pipe shall have less than 2.5 feet of cover over the crown.
(2) Culverts or pipes installed in traffic or loading areas shall have a minimum of 3.5 feet of cover over the crown, unless approved by the Town Engineer.
(3) For all inlets and storm manholes, if an outlet pipe is larger than an inlet pipe the crowns of the pipes, at a minimum, must match elevations.
(4) No culvert or storm drain shall be installed with less than a 0.40% slope, unless approved by the Town Engineer.
(5) Culverts and storm drains shall be bedded with crushed rock or stone, sand or suitable earth material as approved by the Town Engineer. The material shall have 95% passing a ¾” sieve, with no particles larger than 1½”.
(6) The trench width shall be a minimum of the nominal pipe diameter plus 24- inches (12-inches each side of the pipe).
(7) The bottom of the trench shall be excavated 6-inches below the pipe invert to provide for bedding material. Additional excavation will be required for pipes with bells. Bedding shall extend to 6-inches above the pipe crown.
(8) Suitable backfill shall be placed over the pipe and bedding in 6-inch layers. Only natural material may be used for backfill, as approved by the Town Engineer. No particles larger than 4-inches in diameter in any direction will be allowed in the backfill.
(d) Ditches and Swales.
(1) Where practical, the use of ditches and swales may be used with the approval of the Town Engineer.
(2) Ditches may be grass, rock or concrete lined. Grass lined ditches will have minimum side slopes of 4:1 to allow for mowing.
(3) The maximum allowable velocities in feet per second (fps) for ditches are as follows:
0 to 8 - Type A Matting (per WV DOH)
8 to 15 - Type B Matting (per WV DOH)
15 or Greater - Concrete or Rock Lined
(4) If rock or concrete lined ditches are proposed regardless of the velocities, matting is not required. However, rock lined ditches will require a double weed mat to prevent vegetation from growing up between the rocks.
(e) Storm Water Management Facility.
(1) In areas that have experienced or it is anticipated will experience storm water problems, any new development shall provide adequate control of storm water by means of a detention basin to accommodate a post development storm with a 2-year, 24-hour frequency, a 10-year, 24-hour frequency, and a 25-year, 24-hour frequency to be released at a rate not to exceed the predevelopment discharge for the same storm event. The facility shall be constructed with an overflow designed at a 50-year frequency. A storm water management facility will not be required if the discharge is directly to the Kanawha River and has been approved by the US Army Corps of Engineers and/or the Public Lands Corporation.
(2) Any redevelopment project that increases the existing runoff by more than 5% shall accommodate all of the runoff above the 5% increase.
(3) Unless otherwise approved by the Town of Winfield Planning Commission, all detention basins will be underground.
(4) For sizing the storm water management facility, flow rates will be determined by the Rational Method and volumes will be determined by the Abt and Grigg Method. Two (2) copies of the engineering calculations, signed and stamped by a professional engineer in good standing with the state of West Virginia, shall be provided to the Town Engineer.
(5) If approval is given for an above grade facility, it shall not be constructed in an area that would devalue or impair the use of other properties or be aesthetically displeasing. The facility will be screened on all sides with a privacy fence or slated wire fence. A lockable gate will also be provided. Some form of insect control, approved by the Town Engineer, will also be required.
(6) The use of retention basins/ponds are prohibited.
(7) The maintenance of the storm water management facility will be the sole responsibility of the owner/developer of the property.
(Ord. 2005-06-02. Passed 3-20-06.)