All new development and subdivision of land shall be designed to adequately collect, control and dispose of storm water runoff from the site, including where necessary, but not limited to storm sewers, culverts, detention or retention basins, roof-top storage, parking lot ponding, porous pavement, grassed waterways, lined channels, diversions, underground storage, or high delay vegetative strips and other related storm water management facilities. Design criteria for watershed districts shall include, but not be limited to the following:
(a) All sites shall limit the rate of storm water runoff so that no greater runoff is permitted during and after site development than that of the site prior to development. If the volume of storm water runoff from a site during or after development shall be greater than that of the site prior to development, it shall be compensated so that:
(1) The peak rate of storm water runoff for the critical storm and all more frequent storms occurring on the development site does not exceed the peak rate of runoff from the one-year twenty-four hour storm event occurring on the same site under predevelopment conditions.
(2) Storms of less frequent occurrence (longer return periods) than the critical storm up to the 100-year storms have peak runoff rates no greater than the peak runoff rates from equivalent size storms under predevelopment conditions. Consideration of the one, two, five, ten, twenty-five, fifty and 100-year storms shall be considered adequate in designing and developing a site to meet this standard.
(b) All predevelopment calculations shall be based upon existing land use features, except where farm field or disturbed earth is the existing natural condition, meadowland shall be used as the starting base for such calculations instead of the existing condition.
(c) The critical storm for a specific development site is determined as follows:
(1) Use of procedures outlined in the U.S. Department of Agriculture, Soil Conservation service Publication, "Urban Hydrology for Small Watersheds," latest edition (Technical Release No. 55) or other professionally accepted methodology approved by the City Engineer, to determine the total volume of storm water runoff occurring on the development site before and after development;
(2) From the volumes determined in subsection (c)(1) hereof, determine the percent increase in volume of storm water runoff due to the development.
(3) Determine the watershed district from the map entitled "Watershed Districts", which is on file at the office of the City Engineer, and the corresponding watershed type from Table 1 below:
Table 1
Watershed District | Type |
I. Cross Creek Watershed District | B |
II. Permars Run Watershed District | A |
III. Wills Creek Watershed District | B |
IV. Ohio River Sand Bar Watershed District | C |
(4) Using the volumes determined in subsection (c)(2) hereof, and the watershed type determined in subsection (c)(3) hereof, select the critical storm from Table 2 Below:
Table 2
If the percentage of increase in volume of runoff is: | The critical storm year for discharge limitation shall be: | |||
Watershed Type | ||||
Equal to or greater than | and Less than | A | B | C |
--- | 10 | 1 | 1 | 1 |
10 | 20 | 2 | 1 | 1 |
20 | 50 | 5 | 2 | 1 |
50 | 100 | 10 | 5 | 1 |
100 | 250 | 25 | 10 | 2 |
250 | 500 | 50 | 25 | 5 |
500 | -- | 100 | 50 | 10 |
(d) Maximum permitted velocities for water carrying structures shall be determined by using the U.S. Department of Agriculture, Soil Conservation Service Publication, "Water Management and Sediment Control for Urbanizing Areas", latest edition.
(e) The Storm Water Management Plan shall include calculations indicating velocities of flow, grades, sizes and capacities of water carrying structures and detention and retention basins, and sufficient design information to construct such facilities.
(f) Where a site is traversed by an important watercourse, channel or stream, there shall be provided a drainage easement conforming substantially with the line of such important watercourse, channel or stream, and of such width as shall be adequate to preserve the unimpeded flow of natural drainage or for the purpose of widening, deepening, improving or protecting such drainage facilities.
(g) Storm water management facilities and easements shall be designed such that development as proposed shall not adversely effect or cause hazards to existing use of adjacent properties.
(Ord. 1991-122. Passed 11-18-91.)