(A) Design standards.
(1) All storm sewers, whether private or public, and whether constructed on private or public property, shall conform to the design standards and other requirements contained herein.
(2) The minimum diameter of all storm sewers shall be 12 inches. The rate of release for detention storage shall be controlled by an orifice plate or other device, subject to approval of the Drainage Board, when the minimum 12-inch diameter pipe will not limit the rate of release to the required amount.
(3) Sewer grade shall be such that, in general, a minimum of one and one-half feet of cover is maintained over the top of the pipe. If the pipe is to be placed under pavement, then the minimum pipe cover shall be two feet. Pipe cover less than the minimum may be used only upon approval of the Drainage Board. All pipe crossings involving a county highway, street, or road shall require the approval of the County Engineer. Uniform slopes shall be maintained between inlets, manholes, and inlets to manholes. Final grade shall be set with full consideration of the capacity required, sedimentation problems, and other design parameters. Minimum and maximum allowable slopes shall be those capable of producing velocities of between two and one-half and 15 feet per second, respectively, when the sewer is flowing full.
(4) Storm sewers shall be straight between manholes.
(5) Manholes shall be installed to provide access to continuous underground storm sewers for the purpose of inspection and maintenance. Manholes shall be provided at the following locations:
(a) Where two or more storm sewers converge;
(b) Where pipe size changes;
(c) Where a change in horizontal alignment occurs;
(d) Where a change in grade occurs; and
(e) At intervals in straight sections of sewer, not to exceed the maximum allowed.
(6) The maximum distance between storm sewer manholes shall be as follows:
Size of Pipe (Inches)
|
Maximum Distance (Feet)
|
12 through 42 | 400 |
48 and larger | 600 |
(7) Inlets or drainage structures shall be utilized to collect surface water through grated openings and convey it to storm sewers, channels, or culverts. The inlet grate opening provided shall be adequate to pass the culverts. The inlet grate opening provided shall be adequate to pass the design ten-year flow with 50% of the sag inlet areas clogged. An overload channel from sag inlets to the overflow channel or basin shall be provided at sag inlets, so that the maximum depth of water that might be ponded in the street sag shall not exceed seven inches. Inlet design and spacing may be done using the hydraulic equations by manufacturers or orifice/weir equations. Gutter spread on continuous grades may be determined using equations or methods approved by the County Surveyor.
(B) Workmanship and materials.
(1) The specifications for the construction of storm sewers shall not be less stringent than those set forth in the latest edition of the State Department of Transportation “Standard Specifications.” Additionally, ductile iron pipe shall be laid in accordance with American Water Works Association (AWWA) C-600 and clap pipe shall be laid in accordance with either American Society of Testing Materials (ASTM) C-12 or the appropriate American Association of State Highway and Transportation Officials (AASHTO) specifications.
(2) (a) Storm sewer manholes and inlets shall be constructed of masonry, cast in place concrete, or precast reinforced concrete. Material and construction shall conform to the State Department of Transportation “Standard Specifications,” § 720.
(b) Pipe and fittings used in storm sewer construction shall be extra-strength clay pipe (ASTM C-76). Other pipe and fittings not specified herein may be used only when specifically authorized by the Drainage Board. Pipe joints shall be flexible and watertight and shall conform to the requirements of § 715.02 - Materials, of the latest edition of the State Department of Transportation “Standard Specifications.”
(c) Special hydraulic structures required to control the flow of water in storm runoff drainage systems include junction chambers, drop manholes, inverted siphons, stilling basins, and other special structures. The use of these structures shall be limited to those locations justified by prudent planning and by careful and thorough hydraulic engineering analysis.
(Ord. 1997-16, passed 12-8-1997; Ord. 2005-01, passed 2-17-2005) Penalty, see § 153.999