All storm sewers subject to this chapter whether private or public, and whether constructed on private or public property, shall conform to the design standards and other requirements contained herein.
The hydraulic capacity of storm sewers shall be determined using Manning’s Equation:
(A) Manning’s Equation.
Q = (A)(V)
Where; V = 1.486 R(2/3) S(1/2)
n
n
A = Waterway area of conduit in square feet
Q = Discharge in cubic feet per second (cfs)
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 = roughness coefficient
(1) The hydraulic radius, R, is defined as the cross sectional area of flow divided by the wetted flow surface or wetted perimeter.
(2) Typical “n” values and maximum permissible velocities for storm sewer materials are listed in Table 3. Roughness coefficient “n” values for other sewer materials can be found in standard hydraulics texts and references.
(B) Minimum size. The minimum culvert size under all public roads shall be 15 inches. The minimum size of all other storm sewers and culverts shall be 12 inches. Where a 12-inch pipe will not limit the rate of release as required, the rate of release for detention storage shall be controlled by an orifice plate or other devices, subject to approval of the Board.
(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 the approval of the Board. Uniform slopes shall be maintained between inlets, manholes and inlets to manholes. Final grade shall be set with full consideration of 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. Velocity in excess of seven feet per second shall require the use of water-tight joints to reduce soil infiltration potential.
(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. Storm sewers smaller than 42 inches in diameter shall not be curved.
(E) Manholes. 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:
(1) Where two or more storm sewers converge;
(2) At the point of beginning or at the end of a curve, and at the point of reverse curvature (PC, PT, PRC);
(3) Where pipe size changes;
(4) Where an abrupt change in alignment occurs;
(5) Where a change in grade occurs; and
(6) At suitable intervals in straight sections of sewer (400 feet maximum distance).
(F) Inlets. Storm 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 1 or other approved design procedure. The inlet grate openings provided must be adequate to pass the design ten-year flow with 50% of the sag inlet areas clogged. An overflow channel from sag inlets to a channel or basin shall be provided at sag inlets, so that the maximum depth of water that might be ponded in the street sag or gutter shall not exceed 12 inches above the gutter elevation.
Table 3: Typical Values of Manning’s “n” | ||
Material | Manning’s “n” | Desirable Maximum Velocities* |
Table 3: Typical Values of Manning’s “n” | ||
Material | Manning’s “n” | Desirable Maximum Velocities* |
Circular corrugated metal pipe, annular corrugations, 2-2/3 x 1/2 in. | ||
Unpaved | 0.024 | 10 f.p.s.1 |
25% paved | 0.021 | 10 f.p.s.1 |
50% paved | 0.018 | 10 f.p.s.1 |
100% paved | 0.013 | 10 f.p.s.1 |
Circular corrugated metal pipe, helical, 2-2/3 x 1/2 in. unpaved corrugations | ||
12" | 0.011 | 10 f.p.s.1 |
18" | 0.013 | 10 f.p.s.1 |
24" | 0.015 | 10 f.p.s.1 |
36" | 0.018 | 10 f.p.s.1 |
48" | 0.020 | 10 f.p.s.1 |
60" or larger | 0.021 | 10 f.p.s.1 |
Closed conduits | ||
Brick | 0.015 | 15 f.p.s. |
Cast iron | 0.013 | 15 f.p.s. |
Concrete | 0.013 | 15 f.p.s. |
Vitrified | 0.013 | 15 f.p.s. |
Corrugated polyethylene smooth interior pipe (HDPE) | 0.012 | 15 f.p.s.1 |
Open channels | ||
Concrete, broom or float finish | 0.013 | |
Concrete, trowel finish | 0.013 | |
Dense growth of weeds | 0.040 | |
Dense weeds and light brush | 0.040 | |
Dense weeds and heavy brush | 0.060 | |
Existing earth (fairly uniform, with some weeds) | 0.030 | |
Gabion | 0.028 | |
Gunite | 0.018 | |
New earth (uniform, sodded, clay) | 0.025 | |
Riprap, hand-placed | 0.030 | |
Riprap, dumped | 0.035 | |
Swale with grass | 0.035 | |
NOTES TO TABLE: —*Velocity in excess of 7 f.p.s. require the use of water tight joints. —Values shown are for materials in good condition. —Values may increase as materials age or are damaged. | ||
(Ord. 8-2014, passed 7-21-2014)