(A) Design storms. The selection of a design storm is the basis for all runoff calculations and facility design for a project site. The stormwater conveyance system shall be designed to adequately handle the runoff from storms having various frequencies of occurrence from different types of development in accordance with the general categories set forth below in this section. To ensure the adequacy of the stormwater conveyance system, the following minimum design storms shall be used, where applicable:
(1) The ten-year storm shall be used for all public storm sewer inlets and closed pipe systems. The ten-year storm shall also be used for all channels and ditches, which should be capable of conveying the ten-year storm flow within their banks.
(2) The 100-year storm shall be used as a check storm for storm sewer systems designed for a ten-year storm to confirm containment at levels below the rim opening to prevent surcharging.
(3) The two-year storm, ten-year storm, and 100-year storm shall be used to calculate pre-development runoff from a site for detention, retention, and sediment control basins.
(4) The two-year storm, ten-year storm, and 100-year storm shall be used to calculate post-development runoff from a site for detention, retention and sediment control basins.
(5) The 100-year storm shall be used for all detention, retention, or sediment control basins as a check storm to ensure against flooding or surcharging.
(6) The post-development peak flow rates shall be less than or equal to the pre-development peak flow rates for the two-year storm, ten-year storm, and 100-year storm events.
(B) Runoff calculation methods.
(1) The Rational Method, the Modified Rational Method and the Soil Conservation Service (SCS) Method are all acceptable. The calculations can be made either by hand or with computer software. The design engineer should choose the method that best fits the known values available and the size of the development.
(2) The rainfall intensity should be obtained from Rainfall Intensity-Duration-Frequency (IDF) Curves for the appropriate design storm. Since IDF Curves have not been specifically developed for Oldham County, then the IDF Curves developed for Metro Louisville shall be used and may be found in the Louisville Metropolitan Sewer District Design Manual or obtained from the Oldham County Engineer's office.
(C) Water quality treatment standard.
(1) The on-site water quality treatment standard is intended to improve the water quality of stormwater runoff before being discharged into the MS4. The standard is established so that stormwater management measures shall be sized to capture the stormwater runoff volume from the 80th percentile precipitation event, equivalent to a rainfall event of 0.6 inches, at a minimum. This volume is sometimes referred to as "the first flush".
(2) The on-site water quality treatment standard is a volume-based standard and is appropriate for sizing green infrastructure BMPs that provide their primary treatment function by storing and treating the water quality volume (WQv). Stormwater management measures, also known as structural best management practices (BMPs) or green infrastructure BMPs, are designed to treat a volume of runoff, which is detained for a certain period of time to allow for settling of solids and associated pollutants, as well as any biochemical treatment processes that may be provided for dissolved pollutants such as adsorption, precipitation, biodegradation, and plant uptake.
(3) The WQv used for sizing green infrastructure BMPs shall be computed as follows:
Rv = 0.009 (I) + 0.05
where:
Rv = volumetric runoff coefficient; and
I = the percent imperviousness of the drainage area.
WQv= 3,630 xRvxPx A
where:
WQv = water quality volume (ft3)
Rv = volumetric runoff coefficient;
P = rainfall depth associated with the 80th percentile precipitation event (in), equivalent to a rainfall event of 0.6 inches; and
A = drainage area to BMP (acres).
(4) Green infrastructure BMPs should be selected, sized, and designed to completely capture the WQv prior to discharging from the site. The green infrastructure BMPs shall be designed such that the drain time is long enough to treat the stormwater and release it at a rate that minimizes degradation of the water resources, but short enough to provide storage available for successive rainfall events and avoid the creation of nuisance conditions. The drain time for all green infrastructure BMPs shall be 48 hours at a maximum, such that there is no standing or residual water in the BMP following the WQv rainfall event.
(Ord. 16-830-348, passed 8-16-2016)