Section 343.20 of the City of Hamilton Subdivision Regulations, "Requirement for Storm Drainage System", stipulates that "every development shall be provided with a storm drainage system that provides adequate, complete and satisfactory drainage service for the entire area being platted for all projected land uses and otherwise meeting the approval of the officials having jurisdiction." This section stipulates that the City Engineer must finally approve the drawings, plans and specifications for the drainage system.

It is the responsibility of the developer's engineer to design this drainage system and it is the responsibility of the City Engineer to judge the adequacy of the design. The following has been written to clarify the standards that will be judged by the City Engineer as adequate. These should be considered minimum standards and certain conditions may require exceeding these standards.

A. Every development shall have an adequate drainage system to provide complete drainage for the entire development.

B. The development shall be graded so that each building site is protected from damage due to storm water run-off from adjacent lots.

C. The development shall be constructed so that the new construction does not increase the risk of flooding to downstream property.

Storm Sewer Design Policy

A. Storm sewers and open ditches are to be designed for a 10-year storm.

B. Roadway culverts are to be designed for a 25-year storm. Minimum floor elevations shall be shown for any lot which may be subject to flooding from the headwater upstream from a culvert.

C. In all locations where the potential exists for flooding and property damage, drainage facilities shall be designed for 50-year storms.

D. The Rational Method shall be used to estimate run-off for drainage areas of 250 acres or less. The "rainfall intensity-duration-frequency curves" Plate No. 1 attached to this document shall be used for determining the rainfall intensity factor in the Rational Method formula.

E. The State of Ohio, Department of Natural Resources publication "Floods in Ohio" shall be used to estimate run-off from larger areas.

F. When using the Rational Method for estimating storm water run-off, the following run-off coefficients are typical for the City of Hamilton. The coefficients may be reduced for those areas of the City with flat slopes that are underlain with porous sand and gravel.

Type of Area	Run-off Coefficient
Business	.70 - .80
Residential (single family)	.50 - .60
Residential (multi-family)	.60 - .80
Industrial (light)	.70 - .80
Industrial (heavy)	.70 - .90
Parks & Cemeteries	.30 - .40
Woodland	.20 - .40
Grassland	.25 - .45
Cropland	.40 - .50

G. Pipe used for storm sewers may be concrete pipe or corrugated metal pipe. Pipe will have a minimum inside diameter of 12 inches and a minimum slope of 0.5 percent. The capacity of the pipe shall be determined using the Manning Formula. The following Roughness Coefficients (n) shall be used with the Manning Formula.

Concrete Pipe	0.015
Corrugated Metal Pipe   (2 2/3" x 1/2") Unpaved	0.024
Corrugated Metal Pipe Paved Invert	0.021

H. Culverts shall cross under streets at an angle which is most compatible with the upstream and downstream channels. Culverts shall be designed using approved nomographs. Outlet velocities shall be checked on all culverts and erosion control measures provided when velocity exceeds 5-feet per second.

I. Open ditches shall be avoided wherever possible in residential subdivisions. Ditches may be permitted when the required pipe size exceeds 36 inches. Natural streams and water courses should be left as undisturbed as practical. Open ditches, when permitted, shall be constructed with 4:1 or flatter side slopes. The minimum grade for all ditches shall be 1.0 percent except for natural streams or paved bottom ditches. The Manning Formula shall be used for determining the capacity of open channels. The following coefficients will be used:

Rock Lined Channels	0.08
Grassed Channels	0.03
Paved Channels (concrete or asphalt)	0.015

Open channels shall be properly lined to prevent erosion. All ditches having a velocity of 5-feet per second or less shall be sodded, except that ditches may be seeded if the velocity is under 2-feet per second. All ditches with a velocity over 5-feet per second shall be lined.

A. In general, no development shall discharge a greater peak stormwater run-off than the peak rate of run-off from the same area prior to development. In order to control this peak discharge it will be necessary to detain or retain a portion of the storm water run-off temporarily on the site.

B. For the purposes of this section a development will be defined as a subdivision, a major condominium project, a major commercial or industrial facility or any other new construction which the City Engineer judges as appropriate for inclusion.

C. A plan shall be submitted to the City Engineer for approval for every development showing the method for detaining storm water unless a written waiver is approved as stated below.

D. All plans submitted must show the location and size of detention facilities and must be accompanied by calculations used to determine the design of the facilities. All calculations shall be based on the methods described in the United States Soil Conservation Service Publication TR-55. In using this method, the 50-year post development peak discharge shall not exceed the 10-year pre-development peak discharge. Emergency spillways shall be designed for 50- year storms.

E. Detention facilities will typically be dry-bed basins. Fencing will be required around all basins which are within lots to be deeded to the City or within easements to be granted to the City.

F. Retention basins which are partially filled with water during dry weather will be permitted where future ownership and all maintenance responsibilities rest with an individual, a private company, a Condominium Association or some other private organization.

G. A waiver of the requirements for detention facilities may be granted by the City engineer for the following:

1. Very small increases in run-off due to the development.

2. The presence of adequate existing downstream storm sewers and channels capable of handling the increased run-off.

3. Unusual and abnormal conditions making it impossible to construct the proper facilities.

In order to control pollution of streams and public waters the design engineer shall include in the construction plans sufficient requirements to control erosion and sedimentation during development. These requirements may include a designated sequence of operations, limits on area of surface to be disturbed, and temporary or permanent erosion control devices such as settling ponds or silt traps. The developer and his contractor(s) shall abide by the requirements set forth in the Ohio Department of Natural Resources Division of soil and water districts "Urban Sediment Pollution Abatement Rules" chapter 1501: 15-1-03, 15-1-04 and 15-1-05 of the Ohio Revised Code (Dated 11-1-79)

Click here for Plate 1, Rainfall Intensity Chart
 

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City Of Hamilton . 345 High Street . Hamilton, OH 45011 . (513) 785-7000