A. Treatment of the first one-half inch (1/2") of stormwater runoff from the impervious surfaces set forth in this section shall be required prior to discharge of the stormwater overland or to ground or surface waters. Stormwater shall be mitigated utilizing bioinfiltration swales, as referenced in DEQ’s catalog of best management practices, or treated on site with existing natural vegetation if the characteristics of the parcel provide treatment.
1. Subdivisions: Site disturbance management plans will be required for subdivision utilizing calculations for detention and conveyance that include runoff from the proposed improvements.
2. Commercial Or Industrial Development: Site disturbance plans are required for commercial or industrial development utilizing calculations for detention and conveyance that includes runoff from the proposed improvements. Stormwater treatment of the first one-half inch (1/2") of runoff from all access and parking lot impervious surfaces shall be requested prior to discharge of the stormwater overland or to ground or surfae waters. Stormwater runoff from rooftops and other similar impervious surfaces is not required to be mitigated with treatment BMPs, but shall be properly managed to infiltrate on the property or to otherwise comply with the detention requirements of this article.
3. Common Driveways And Private Roads: Whenever a site disturbance plan is required for common driveway or private road development, calculations for detention and conveyance must be utilized that include the runoff from the proposed improvements.
4. Residential Development On Individual Parcels:
a. Whenever a site disturbance plan is required for residential development on an individual parcel, calculations must be utilized for detention and conveyance that include runoff from the proposed improvements. Stormwater treatment of the first one-half (½") inch of runoff from all access, parking areas, and other similar impervious surfaces shall be required prior to discharge of the stormwater overland or to ground or surface waters. Stormwater runoff from rooftops and other similar impervious surfaces is not required to be mitigated with treatment BMPs, but shall be properly managed to infiltrate on the property or to otherwise comply with the detention requirements of this article.
b. For replacement, or additions or alterations to existing site improvements where no stormwater system has previously been required, stormwater shall be managed utilizing a combination of stormwater treatment and erosion control BMPs to produce no net increase in the pollutant export from the site’s previously existing conditions.
B. BMP Efficiency Testing Not Required: On-site post-construction testing of BMP treatment efficiency will not be required by the County. The stormwater management plans must show that the proposed BMPs are anticipated to meet or exceed the treatment efficiencies listed above. Expected treatment efficiencies shall be included in the County's Manual of Best Management Practices or the Plan Criteria manual. The development of the BMP list and required range of removal effectiveness is not intended to limit the use of new or innovative treatment procedures that may be developed through the creativity of the design professional preparing the stormwater management plan. New approaches and procedures will be considered and approved with the submittal of the appropriate support data that confirms the effectiveness of the proposed new treatment method, its use related to site constraints, and the maintenance burden it will produce if adopted and utilized. (Ord. 493, 6-9-2016; amd. Ord. 546, 10-17-2019)