Storm Water Pollution Prevention Plan (SWPPP) - the City of Lompoc!

proposed drainage swale. Run-on also occurs along the length of the transmission water main location,through sheet flow heading towards the Santa Ynez River. At the north end of the proposed transmissionwater main alignment, run-on occurs from a storm drain which terminates in a large, relatively steep gully,through which the transmission water main will be placed.The site is relatively flat to very gently sloping and drains by sheet flow. There is no run-on to the well site.Soils at the well site consist of layered sands that were moist to wet with a medium dense to denseconsistency. A isolated layer of silt soil exists at 14 -19 ft below the ground surface and the silt was moist andvery stiff. Groundwater was encoutered at a depth of 38 ft below the ground surface.The project is subject to requirements of the California Department of Fish and Game. The contractor shall beresponsible for ensuring compliance with all CDFG requirements and with the specific mitigation measures ofthe project. The letter from the California Department of Fish and Game, the Notification of StreambedAlteration Agreement and its attachments are provided in Appendix J2.4 SWPPP ObjectivesThis SWPPP has been designed to address the following objectives:1. All pollutants and their sources, including sources of sediment associated with construction,construction site erosion and all other activities associated with construction activity are controlled;2. Where not otherwise required to be under a Regional Water Board permit, all non-storm waterdischarges are identified and either eliminated, controlled, or treated;3. Site BMPs are effective and result in the reduction or elimination of pollutants in storm waterdischarges and authorized non-storm water discharges from construction activity to the best availabletechnology economically achievable (BAT) / best conventional pollutant control technology (BCT)standard;4. Calculations and design details as well as Best Management Practices (BMPs) controls for site run-onare complete and correct, and5. Stabilization BMPs installed to reduce or eliminate pollutants after construction are completed.2.5 Stormwater Run-on and Runoff from Offsite AreasRun-on is considered insignificant to the linear project and is received on site as sheet flow. The well site isapproximately 2 feet above existing grade and will not have any run–on. Runoff will also be minimal. The wellsite is less than 0.5 acre and runoff is directed to an existing roadside ditch. The disturbed area for thetransmission water lines site is approximately 30 ft wide in unpaved areas and approximately 3’ wide in pavedareas. On unpaved areas, it is adjacent to stable vegetation and low gradients. On paved areas, water will beconveyed by the existing municipal storm drain system. There are no calculations.2.6 Risk AssessmentThe General Permit establishes three levels of risk possible for a construction site. Risk is calculated in twoparts: 1) Project Sediment Risk, and 2) Receiving Water Risk. The site's sediment risk and receiving water riskwas evaluated for the period of soil exposure (i.e. grading and site stabilization) using the methodology inAppendix 1 of the General Permit. This project is a combined traditional and linear project. The higher risklevel of Risk Level 2 (Type 2 LUP) was chosen to represent the entire site.Municipal Water Well No. 11Sediment Risk: MediumThe RUSLE equation was used through the Rainfall Erositivity Factor Calculator and the GIS Map Method todetermine soil risk. The online rainfall erosivity factor calculator (R) is based on soil erosivity and constructionscheduling. It was determined that the R factor is 24.62. Using the GIS Map Method and soil characteristicsdescribed in the Soils Engineering Report (2009), the project site lies in the KLS risk category of 1.0.Therefore, the project will contribute less than 75 tons/acre of sediment and has a medium sediment risk.Municipal Water Well No. 11 and Transmission Water Main – SWPPP July 20, 2010Page 7