(A) The low impact development practices provided in the BMP Manual and in Appendix B of this chapter shall be utilized for all regulated activities to the maximum extent practicable.
(B) Stormwater runoff volume controls shall be implemented using the design storm method or the simplified method as defined below. For regulated activity areas equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either method; therefore, the applicant may select either method on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and other factors.
(1) The design storm method (CG-I in the BMP Manual) is applicable to any sized regulated activity. This method requires detailed modeling based on site conditions.
(a) Do not increase the post-development total runoff volume for all storms equal to or less than the two-year 24-hour storm;
(b) For hydrologic modeling purposes:
1. Existing non-forested pervious areas must be considered meadow (good condition) for pre-development hydrologic calculations; and
2. Twenty percent of existing impervious area, when present within the proposed project site, shall be considered meadow (good condition) for pre-development hydrologic calculations for re-development.
(2) The simplified method (CG-2 in the BMP Manual) is independent of site conditions and should be used if the design storm method is not followed. This method is not applicable to regulated activities greater than one acre or for projects that require detailed design of stormwater storage facilities. For new impervious surfaces:
(a) Stormwater facilities shall capture at least the first two inches of runoff from all new impervious surfaces;
(b) At least the first one inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow, (i.e., it shall not be released into surface waters of the state). Removal options include reuse, evaporation, transpiration and infiltration;
(c) Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first half-inch of the permanently removed runoff should be infiltrated; and
(d) Actual field infiltration tests at the location of the proposed elevation of the stormwater BMPs are required. Infiltration test shall be conducted in accordance with the BMP Manual. A minimum 48-hour notification of the municipality shall be provided to allow witnessing of the testing.
(3) In cases where it is not possible or desirable to use infiltration-based best management practices to partially fulfill the requirements in either divisions (B)(1) or (B)(2) above, the following procedure shall be provided to allow witnessing of the testing.
(a) At a minimum, the following documentation shall be provided to justify the decision to reduce the use of infiltration BMPs:
1. Description of and justification for field infiltration/permeability testing with respect to the type of test and test locations);
2. An interpretive narrative describing existing site soils and their structure as these relate to the interaction between soils and water. In addition to providing soil and soil profile descriptions, this narrative shall identify depth to seasonal high water tables and depth to bedrock, and provide a description of all subsurface elements (fragipans and other restrictive layers, geology and the like) that influence the direction and rate of subsurface water movement;
3. A qualitative assessment of the site’s contribution to annual aquifer recharge shall be made, along with identification of any restrictions or limitations associated with the use of engineered infiltration facilities; and
4. The provided documentation must be signed and sealed by a professional engineer or geologist.
(b) The following water quality pollutant load reductions will be required for all disturbed areas within the proposed development.
Pollutant Load | Units | Required reduction (%) |
Total nitrate (NO3) | Pounds | 50 |
Total phosphorous (TP) | Pounds | 85 |
Total suspended solids (TSS) | Pounds | 85 |
(c) The performance criteria for water quality best management practices shall be determined from the state’s Stormwater Best Management Practices Manual most current version.
(C) The applicable worksheets from the BMP Manual must be used in calculations to establish volume control.
(Ord. 1508, passed 12-20-2010)