This section deals with underground storm sewer systems and appurtenances.
(A) Catch basins, inlets, and manholes. It will be the responsibility of the designer to locate catch basins or inlets in the street or in other locations using accepted empirical techniques, and falling within the criteria below.
(1) Storm frequencies. Surface run-off for each catch basin or inlet on a continuous grade in the street will be determined using a storm frequency of five years.
(2) Catch basin or inlet interception requirements. Each catch basin or inlet on a continuous grade in the storm water system should intercept no less than 80% of the gutter flow reaching it, except at intersections and low points where 100% of the flow must be intercepted.
(3) Size and spacing. The size and spacing of pavement catch basins or inlets with grates on continuous grades will be selected assuming the grate alone will intercept water from the gutter. Where feasible catch basins and inlets should be constructed upstream of crosswalks and at intersections. At any design point requiring more than two catch basins or inlets side by side, no more than two will be permitted. Others must be placed upstream at least 100 feet, at a point halfway between the high point and the subject dual catch basins or inlets, or at some other logical point approved by the Village Engineer.
(4) Manholes. Manholes shall be located at each horizontal change of direction, vertical change of grade, and generally no more than 400 feet apart.
(5) Street sags and other low points. Low point catch basins or inlets will be designed to accept the flow reaching them through the curb opening only. The depth of water cannot exceed the top of the curb opening of the catch basin or inlet for a storm with a frequency of 25 years. A backup system may be required at each low point in the street profile to accept overflow from the low point catch basin and to function independently of it to the main sewer or to eventual outlet.
(B) Storm sewer design.
(1) Storm frequencies. Surface run-off for design of the storm sewer shall be determined by using the following storm frequencies: pipe size and slope - ten years.
(2) Pipe capacities. Storm sewers shall be designed to carry peak flows as calculated by the method specified in § 156.04. Size of pipes shall be determined by Manning's Formula. Appropriate charts such as those found in "Design Charts for Open Channel Flow Hydraulic Design Series No. 3," published by the U.S. Department of Transportation, Federal Highway Administration may be used.
(3) Roughness coefficient. The roughness coefficient (n) used in Manning's Formula shall be:
(a) n = 0.013 for concrete pipe 30 inches in diameter or larger.
(b) n = 0.015 for concrete pipe less than 30 inches in diameter.
(4) Pipe size. The minimum diameter for storm sewer pipe shall be 12 inches. In no case shall the diameter of a storm sewer pipe leaving a structure such as a manhole be less than the largest pipe entering it.
(5) Minimum cover. Unless specifically requested and approved by the Village Engineer, the minimum cover for any storm sewer will be one foot measured from the bottom of the roadway base material to the top of pipe. Outside the paved roadway the minimum cover shall be three feet unless prohibited by topography or other restrictions impractical to change.
(6) Maximum and minimum velocities. Velocities in storm sewer pipes shall not be less than 2.5 feet per second nor more than 12 feet per second. Higher velocity than that allowed will be considered in rare cases where it can be shown that staying within the maximum would cause an unreasonable burden. In such situations, special provisions shall be made to protect the sewer pipe against erosion and displacement by shock or for checking the flow velocity.
(7) Free outlets. Where a storm sewer outlets to an open channel, erosion control will be determined by the same method as specified in these criteria for culvert outlets.
(Ord. C-604, passed 9-8-86)