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 Equation. The hydraulic capacity of storm sewers shall be determined using Manning's Equations:
V = 1.486 2/3 1/2
n R S
n R S
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, Manning's "n"
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 coefficient (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 twelve (12) inches, except for house connections which will be six (6) inches. Rate of release for detention storage shall be controlled by an orifice plate or other devices, subject to the approval of the Town Engineer, with the developer to have the right to appeal the determination of the Town Engineer to the Department of Planning and Building within thirty (30) days of such determination.
c. Grade. Sewer grade shall be such that, in general, a minimum of two (2) 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 Department of Planning and Building after review and recommendation of the Town 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 two and one-half (2-1/2) and fifteen (15) feet per second, respectively, when the sewer is flowing full.
TABLE 3 | ||
Typical Values of Manning's n | ||
Material | Manning's n | Maximum Velocities (feet per second) |
Closed Conduits | ||
Concrete | 0.013 | 10 |
Vitrified Clay | 0.013 | 10 |
HDPE | 0.012 | 10 |
PVC | 0.011 | 10 |
TABLE 3 | ||
Typical Values of Manning's n | ||
Material | Manning's n | Maximum Velocities (feet per second) |
Circular Corrugated Metal Pipe, Annular Corrugations, 2-2/3 x 1/2 in. | ||
Unpaved | 0.024 | 7 |
25% paved | 0.021 | 7 |
50% saved | 0.018 | 7 |
100% paved | 0.013 | 7 |
Concrete culverts | 0.013 | 10 |
HDPE or PVC | 0.012 | 10 |
Open Channels | ||
Concrete, trowel finish | 0.013 | 10 |
Concrete, broom or float finish | 0.015 | 10 |
Gunite | 0.018 | 10 |
Riprap placed | 0.030 | 10 |
Riprap dumped | 0.035 | 10 |
Gabion | 0.028 | 10 |
New earth (uniform, sodded, clay) | 0.025 | 4 |
Existing earth (fairly uniform, with some weeds) | 0.030 | 4 |
Dense growth of weeds | 0.040 | 4 |
Dense weeds and brush | 0.040 | 4 |
Swale with grass | 0.035 | 4 |
Source: HERPICC Stormwater Drainage Manual, July 1995
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 one hundred (100) feet for sewers forty-two (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. 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 (2) 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;
6. At suitable intervals in straight sections of sewer;
7. The maximum distance between storm sewer manholes shall be as follows:
Size of Pipe (inches) | Maximum Distance (feet) |
12 thru 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 Section 7-400 of the Indiana Department of Transportation Road Design Manual – Part IV or other approved design procedure. The inlet grate opening provided must be adequate to pass the design 10-year flow with fifty percent (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 (7) inches.
(Ord. No. 1708, 6-13-07)