Appendix A Water Quality Memo - Laserfiche WebLink

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19 Activities above and within the river are anticipated to include demolition of the existing concrete bridge, saw cutting and removal of the concrete invert (i.e., the channel lining below the bridge), excavation (3 feet deep at the channel bottom and 6 feet deep at the abutments), pile driving, and installation of concrete for the pile caps, columns, and reconstructed invert. A potential temporary bicycle detour route may be constructed within the Santa Ana River channel. This potential detour route would be constructed and deconstructed during dry-season work within the channel. The detour route would have a dirt base with an asphalt surface and would be entirely removed prior to completion of construction. Diversion of flows within the Santa Ana River is not anticipated to be required because construction activities would not take place within the low flow portion of the channel. However, sandbags or concrete k-rails with plastic sheets may be required upstream of the work area to ensure any water that escapes the low flow channel is diverted back to the low flow channel before reaching the construction area. A staging area would be located within along the riverbank (see Figure 2). No materials or equipment would be stored within the river channel. Projects that disturb more than 1 acre of soil are subject to the requirements of the CGP. However, projects that disturb between 1 and 5 acres are potentially eligible for a Small Construction Rainfall Erosivity Waiver, which would exempt the project from coverage under the CGP. To obtain a waiver, a project would need to demonstrate that there would be no adverse water quality impacts, because construction activities would only take place when there is a low erosivity potential (i.e., the rainfall erosivity value in the Revised Universal Soil Loss Equation [R value] for a project is less than 5). Based on a construction start date of spring 2020 and construction end date of spring 2022, the R factor for the proposed Project would be approximately 38. Because of the long construction schedule, the R factor is well above 5, and the proposed Project would not qualify for a CGP waiver. Therefore, the proposed Project would be required to obtain coverage under and comply with the requirements of the CGP. Based on the Risk Determination methodology outlined in the CGP, the project has a low Sediment Risk (the relative amount of sediment that can be discharged, given the project location and construction schedule) and a low Receiving Water Risk (the risk sediment discharges pose to the receiving waters), which results in a combined Risk Level of 1 (low risk to water quality). Risk Level 1 projects are subject to the best management practice (BMP) and visual inspection requirements of the CGP. In compliance with the CGP (Measure PF-WQ-1), a SWPPP would be prepared and construction BMPs that comply with the requirements of the Construction Runoff Guidance Manual for Contractors, Project Owners, and Developers (County of Orange Stormwater Program 2012) would be implemented to reduce pollutants of concern in the stormwater runoff. Construction BMPs would include, but not be limited to, Erosion Control and Sediment Control BMPs designed to minimize erosion and retain sediment on site and Good Housekeeping BMPs to prevent spills, leaks, and discharge of construction debris and waste into receiving waters. Construction BMPs around the work area within the Santa Ana River are anticipated to include a gravel bag or fiber roll perimeter barrier to contain spills and potential runoff, to be installed and maintained year-round. Additional Construction BMPs would be determined during preparation of the SWPPP. When Construction BMPs are properly designed, implemented, and maintained to address pollutants of concern, as required in Measure WQ-1, pollutants of concern would be retained on site so they would not reach receiving waters; therefore, no adverse water quality impacts are anticipated during construction.