(a) To control pollution of public waters by soil sediment from accelerated stream channel erosion, and to control floodplain erosion caused by accelerated stormwater runoff from development areas, the increased peak rates and volumes of runoff shall be controlled such that:
(1) The peak rate of runoff from the critical storm and all more frequent storms occurring on the development area does not exceed the peak rate of runoff from a two-year frequency, twenty-four hour storm occurring on the same area under predevelopment conditions.
(2) Stores of less frequent occurrence (longer return periods) than the critical storm up to the 100-year store have peak runoff rates no greater than the peak runoff rates of equivalent size storms under predevelopment conditions. Consideration of the one, two, five, ten, twenty-five, fifty and 100-year storm will be considered adequate in designing and developing to meet this standard.
(3) The critical storm for a specific development area is determined as follows:
A. Use the SCS TR-55, Urban Hydrology for Small Watersheds, or its replacement, to determine the total volume of runoff from a two-year frequency twenty-four hour storm occurring on the development area before and after development. Other design methods approved by the approving agency can also be used.
B. From the volumes determined in paragraph (a)(3)A. hereof, determine the percentage of increase in the volume of runoff due to development, and, using this percentage, select the twenty-four hour critical storm.
(b) Methods for controlling increases in stormwater runoff peaks and volumes may include, but are not limited to, the following: (Listed in order of preference)
(1) Retarding flow velocities by increasing friction. (For example, using grassed road ditches rather than paved street gutters where practical, and discharging roof water to vegetated areas or grass and rock-lined drainage channels.)
(2) Grading and construction of terraces and diversions to slow runoff and use of grade control structures to provide a level of control in flow paths and stream gradients.
(3) Induced infiltration of increased stormwater runoff into the soil where practical. (For example, constructing special infiltration areas where soils are suitable, retaining topsoil for all areas to be revegetated, or providing good infiltration areas with proper emergency overflow facilities.)
(4) Provisions for detention and retention. (For example, utilizing permanent ponds and lakes with proper drainage; multiple use areas for stormwater detention, recreation, wildlife, fire protection and aesthetics; or subsurface storage areas.)
(Ord. 140-96. Passed 4-4-96.)