All storm sewers, whether private of public, and whether constructed on private or public property shall conform to the design standards and other requirements contained herein.
(A) Manning Equation.
(1) The hydraulic capacity of storm sewers shall be determined using Manning’s Equation:
V = (1.486/n )(R 2/3)(s 1/2), where
V = mean velocity of flow in feet per second
R = the hydraulic radius in feet, A/P, cross sectional area / wetted perimeter
s = the slope of the energy grade line in feet per foot
n = 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 “n” values and maximum permissible velocities for storm sewer materials are listed in Table 3. Roughness coefficients (n) values for other sewer materials can be found in standard hydraulics texts 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 orifice plate or other devices, subject to approval of the Council, 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 Council. Uniform slopes shall be maintained between inlets, manholes and inlets to manholes. A minimum drop of 0.1 foot through manholes and inlets should be provided. 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.
(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. Manholes may be used as inlet or drainage structures and shall be provided at the following locations:
(a) Where one or more storm sewers converge.
(b) At the point of beginning or at the end of a curve, and at the point of reverse curvature (PC, PT, PRC).
(c) Where the pipe size changes.
(d) Where an abrupt change in alignment occurs.
(e) Where a change in grade occurs.
(f) At suitable intervals in straight sections of sewer.
(2) The maximum distance between storm sewer manholes, unless otherwise approved by the Council, shall be as follows:
Size of Pipe (inches) | Maximum Distance (feet) |
12 through 42 | 400 |
48 and larger | 600 |
(F) Inlets.
(1) (a) 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 Section 8-300 of the Illinois Department of Transportation Drainage Manual or other approved design procedure.
(b) The inlet grate opening provided must be adequate to pass the design 10 year flow with 50% of the sag inlet areas clogged. An overflow 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 7 inches. Inlets may be used as manholes at locations where the pipe sizes do not exceed eighteen (18) inches in diameter.
(2) Inlet design and spacing may be done using the Rational Method. Use of the HEC 12 computer program is also an acceptable method. Gutter spread on continuous grades may be determined using the modified Manning’s equation, or by using the attached Figure 3, Flow in Triangular Gutter Sections.
(Prior Code, Art. 20, § 20-11)