Various parts of a drainage facility must accommodate runoff water as follows:
(A) Street drainage. The street drainage system, such as inlets, catch basins, street gutters, swales, sewers and small channels that collect stormwater must accommodate peak runoff from a ten-year return period storm.
(1) The allowable spread of water on collector and arterial streets is limited to maintaining two clear ten-foot moving lanes of traffic. One lane is to be maintained on local streets.
(2) Open channels carrying peak flows greater than 30 cubic feet per second shall be capable of accommodating peak runoff for a 50-year return period storm within the drainage easement.
(3) Culverts shall be designed to carry, without overtopping the roadway, the 25-, 50-, and 100-year storms for local, collector, and arterial streets, respectively.
(4) Culverts shall be capable of accommodating a peak runoff from a 50-year return period storm when crossing under a road that is classified as an arterial or collector. In no case shall a new culvert be designed to carry less than the existing capacity of the channel.
(B) Overall drainage system. The overall drainage system shall be designed in accordance with the most current Indiana Department of Natural Resources standards. Storm sewers and swales/ditches shall be designed for a ten-year storm.
(1) Pre-developed runoff rates. Pre-developed runoff rates shall be based on the existing land use at the time the new development is being planned.
(2) General drainage system design standards. The drainage system shall be constructed and installed in accordance with plans and specifications approved by the Board of Public Works. All streets shall have a storm drainage system consisting of curbs, gutters and storm sewers, unless a waiver is granted to the applicant.
(3) Storm sewer design standards. All public storm sewers shall conform to the design standards and other requirements contained in this subchapter and per the standard detail sheets as approved by the Board of Public Works.
(a) Manning Equation. The hydraulic capacity of storm sewers shall be determined using Manning’s Equations:
V=(1.486/n) R2/3 Sl/2 where,
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
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 are listed in the table below. Roughness coefficient (n) values for other sewer materials can be found in standard hydraulics texts and references.
TABLE 3.1 TYPICAL ROUGHNESS COEFFICIENTS (MANNING’S N VALUE)
Type | Range |
Concrete Pipe | 0.010-0.016 |
Corrugated Metal Pipe | 0.019-0.025 |
HDPE Pipe | 0.011-0.015 |
PVC Pipe | 0.011-0.015 |
(b) Velocity. The minimum pipe flow should not be less than two and one-half feet per second. The maximum pipe velocity is to be 15 feet per second. However, outlet velocities in excess of five feet per second will necessitate energy dissipation measures, as approved by the City Engineer.
(c) Minimum pipe size. The minimum pipe size of all storm sewers shall be 12 inch diameter. An orifice plate, weir, or other devices, subject to approval of the City Engineer shall control the rate of release for stormwater storage, where the pond’s primary spillway pipe will not limit the required rate of release.
(d) Grade. Storm 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 City 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 and five feet per second, respectively, when the storm sewer is flowing full.
(e) 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.
(f) 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;
6. At suitable intervals in straight sections of the storm sewer. The maximum distance between storm sewer manholes shall be 300 feet for pipe 12 inches through 42 inches, and 500 feet for pipes larger than 42 inches.
(g) 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 INDOT’s Road Design Manual or other appropriate design procedures, but in no case may be spaced more than 500 feet apart. Inlets shall be sized and each individual inlet shown on plans. The inlet grate opening provided must be adequate to pass the design ten-year flow with 50% percent of the sag inlet areas clogged. An overload channel from sag inlets, so that the maximum depth of water that might be ponded in the street sag shall not exceed six inches.
(h) Protection of embankment. Erosion protection shall be provided for the primary outlet and emergency spillway so that the stormwater facility embankment will be adequately protected. The location of the emergency spillway shall be in undisturbed material, unless otherwise approved by the City Engineer.
(i) Outlet conduit. The minimum allowable size for the primary outlet conduit, from the stormwater facility, shall be 12 inches. If further restriction of the outlet conduit is required, the restriction shall be made to the inlet end of the conduit.
(j) Outlet velocity. In those instances where the discharge velocity from the primary outlet or emergency spillway is excessive in the opinion of the City Engineer, energy dissipation may be required. In general, outlet velocities in excess of five feet per second in earth/grassed channels are considered excessive.
(Ord. 2-2010, passed 3-8-10)