(A) Applicability. In a conservation-designed subdivision as defined in the County Subdivision Ordinance, as codified in Chapter 154, a density bonus is granted for each acre (or fractional acre) of land utilized for stormwater infiltration. See § 154.268 of the subdivision regulations.
(B) Infiltration credit.
(1) To obtain the maximum infiltration credit, infiltration BMPs shall be designed, installed and maintained to infiltrate runoff to the maximum extent practicable in accordance with one of the following goals:
(a) Infiltrate sufficient runoff volume so that the post-development infiltration volume shall be at least 90% of the pre-development infiltration volume, based on an average annual rainfall for the region; or
(b) Infiltrate at least 25% of the post-development runoff volume from the two-year, 24-hour design storm with a Type II distribution. Separate curve numbers for pervious and impervious surfaces shall be used to calculate runoff volumes and not composite curve numbers.
(2) Natural conditions such as soil types and slopes will affect the ability of a site to meet these infiltration goals. Projects that are shown to meet these goals will be granted full (100%) credit (i.e., one acre of credit for one acre of land dedicated to stormwater infiltration). Projects that exceed these goals or partially meet these goals will be given credit according to the following table:
Goal #1 90% Average Volume % Calculated | Goal #2 25% 2-Year Volume
% Calculated | % Credit |
Goal #1 90% Average Volume % Calculated | Goal #2 25% 2-Year Volume
% Calculated | % Credit |
> 92.5% | > 26.0% | 110% |
90.0% to 92.5% | 25.0% to 26.0% | 100% |
85.5% to 89.9% | 23.8% to 24.9% | 95% |
81.0% to 85.4% | 22.5% to 23.7% | 90% |
76.5% to 80.9% | 21.3% to 22.4% | 85% |
72.0% to 76.4% | 20.0% to 21.2% | 80% |
67.5% to 71.9% | 18.8% to 19.9% | 75% |
63.0% to 67.4% | 17.5% to 18.7% | 70% |
58.5% to 62.9% | 16.3% to 17.4% | 65% |
54.0% to 58.4% | 15.0% to 16.2% | 60% |
50.0% to 53.9% | 14.0% to 14.9% | 55% |
< 50% | < 14.% | 0% |
(C) Pretreatment required. Before infiltrating runoff, pretreatment shall be required for parking lot and/or commercial area runoff that will enter an infiltration system. The pretreatment shall be designed to protect the infiltration system from clogging prior to scheduled maintenance and to protect groundwater quality. Pretreatment options may include, but are not limited to, oil/grease separation, sedimentation, biofiltration, filtration, swales or filter strips. To minimize potential groundwater impacts, it is desirable to infiltrate the cleanest runoff. To achieve this, a design may propose greater infiltration of runoff from low pollutant sources such as roofs, and less from higher pollutant source areas such as parking lots and commercial areas.
(D) Infiltration. Infiltration from areas requiring pretreatment and regional devices (facilities with a drainage area of five acres or greater) for infiltration from residential development shall not be installed within 400 feet of a community water system well or within 100 feet of a private well.
(Ord. 10-164, passed 6-17-2010)