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DRAINAGE DESIGN
(Amended 7/18/88)


        

Drainage Design:

The following general criteria shall be employed in the design of surface water drainage systems for subdivisions:

Minimum inside pipe diameter - 12".

Type of pipe - Class IV, reinforced concrete.

Maximum distance between catch basins - 300 feet.

Minimum ground cover over pipe - 2.5'.

Minimum size on catchbasin sumps - 2.5'.

Maximum change in the direction of flow - 90 degrees.

Pipe ends - headwall or flared end.

Generally, catchbasins should be placed three feet (3') before the P.C. of curb returns at intersections.  Where roadway grades exceed seven percent (7%), basins should be spaced closer than the above noted 300 feet as determined by the City Engineer.  Manholes shall be provided at all points along the main line where changes in horizontal and vertical alignment are proposed.  At structures where pipe sizes increase along the main line the crown of the pipes shall match in elevation.  Cross-drain inverts should, where possible, enter the structure a minimum of one foot (1') above the outgoing invert.

Computations:

The following methods shall be used to perform the hydraulic and hydrologic calculations for the subdivision:

Watershed parameters - "TR55, Urban Hydrology for Small Watersheds", U.S. Soil Conservation Service, USDA.

Existing and proposed runoff hydrographs - "TR20 - Computer Program for Project Formulation Hydrology", U.S. Soil Conservation Service, USDA.





Design Storm Criteria

Generally post development flows must be equal to or less than pre-development flows measured at each analysis point.

Hydrologic analyses:  10 year and 100 year recurrence intervals, Type II, 24 hour, rainfall distribution.

Hydraulic analyses:  25 year storm recurrence interval, rational formula.

 

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