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.
(A) Manning’s equation.
(1) The hydraulic capacity of storm sewers shall be determined using the Manning’s equation to determine velocity.
V | Mean velocity of flow in feet per second |
R | The hydraulic radius in feet |
S | The slope of the energy grade line in feet per foot |
N | Manning’s “n” or roughness coefficient |
(2) The hydraulic radius, R, is defined as the cross-sectional area of flow divided by the wetted flow surface or wetted perimeter. Typical V values and maximum permissible velocities for storm sewer materials are listed in Table 3 in division (C) below. Roughness coefficient (n) values for other sewer materials can be found in standard hydraulics tests and references.
(B) Minimum size. The minimum size of all storm sewers shall be 12 inches. Rate of release for detention storage shall be controlled by an office plate or other devices, subject to approval of the Plan Commission, where the 12-inch pipe will not limit rate of release as required.
(C) Grade. Sewer grade shall be such that, in general, a minimum of two feet of cover is maintained over the top of the pipe. Pipe cover less than the minimum may be used only upon approval of the Plan Commission office. 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 two and one-half and 15 feet per second, respectively, when the sewer is flowing full.
Typical Values of Manning’s “n” | ||
Material | Manning’s “n” | Maximum Velocities (feet/second) |
Circular CMP, Annular Corrugations, 2-2/3 x 1/2 inch | ||
25% paved | 0.021 | 7 |
50% paved | 0.018 | 7 |
100% paved | 0.013 | 7 |
Concrete culverts | 0.013 | 15 |
HDPE or PVC | 0.012 | 10 |
Unpaved | 0.024 | 7 |
Closed Conduits | ||
Concrete | 0.013 | 15 |
HDPE | 0.012 | 15 |
PVC | 0.011 | 15 |
Vitrified clay | 0.013 | 15 |
Open Channels | ||
Concrete, broom finish | 0.015 | 15 |
Concrete, trowel finish | 0.013 | 15 |
Dense growth of weeds | 0.040 | 3-5 |
Dense weeds and brush | 0.040 | 3-5 |
Existing earth (2) | 0.030 | 3-5 |
Gabion | 0.028 | 10 |
Gunite | 0.018 | 15 |
New earth (1) | 0.025 | 3-5 |
Riprap dumped | 0.035 | 10 |
Riprap placed | 0.030 | 10 |
Swale with grass | 0.035 | 3-5 |
Source: HERPICC Stormwater Drainage Manual, July 1994 (1) New earth (uniform, sodded, clay soil) (2) Existing earth (fairly uniform, with some weeds) | ||
(D) Alignment. Storm sewers shall be straight between manholes insofar as possible. Where long radius curves are necessary to conform to street layout, the minimum radius of curvature shall be no less than 100 feet for sewers 42 inches and larger in diameter. Deflection of pipe sections shall not exceed the maximum deflection recommended by the pipe manufacturer. The deflection shall be uniform and finished installation shall follow a smooth curve.
(E) Manholes.
(1) Manholes shall be installed to provide access to continuous underground storm sewers for the purpose of inspection and maintenance.
(2) 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.
(3) 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 |
(F) Inlets. Inlets or drainage structures shall be utilized to collect surface water through grated openings and convey it to storm sewers, channels or culverts. Inlet design and spacing shall be in accordance with § 7-400 of the State Department of Highways’ Road Design Manual: Volume One or other approved design-procedure. The inlet grate opening provided must 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.
(Ord. 2011-03, passed 4-4-2011)