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• • • Draft Supplemental Environmental Impact Report Page IV.G-22 • Forward Inc. Landfill 2018 Expansion Project Mitigation Measure GA:None required. • Impact G.5:The re-routing of the South Branch of South Littlejohns Creek could result in • flooding if the new alignment is not designed to accommodate peak flows. • The updated (April, 2018) design report for the relocation of the South Branch of South • Littlejohns Creek includes objectives to provide adequate flood control(i.e., has capacity to • carry the 100-year flow within its banks) in the realigned section of the creek; and provide a • stable channel design that meets or exceeds the functions and values of the existing creek. The realigned channel has been designed to carry the 100-year flood flows within its banks. Erosion • protection would be provided in areas with high velocities or sharp bends.The U.S. Army • Corps of Engineers HEC-RAS hydraulic model was used to determine design water surface • elevations and estimate channel velocities and other pertinent flow parameters for stable • channel design. • The following measures are proposed as part of the project, as described in the Project • Description and design study for the proposed creek realignment: • • The channel must function as a natural riparian corridor, require little or no maintenance • once the vegetation is established, and should provide 100-year flood protection. • • The channel slope and depth will be appropriate to the 100-year flood protection. The • channel slope and depth is based on the invert elevations of the existing channel at the start • and end of the new channel. The slope between these two points along this alignment is designed for 0.00055 ft/ft which translates into a ground surface profile along the alignment • a channel depth between 10 and 12 feet. • The appropriate responsible agencies must review and approve the updated April 2018 • design for the relocation of the South Branch of South Littlejohns Creek. Implementation of these procedures would reduce the impact to a less-than-significant level. • Mitigation Measure G.S: None required. • Impact G.6:The project of adding significant new landfill volume could potentially • contribute to known VOC-contaminated plume and other groundwater contamination. The new expansion would add approximately 8.2 million CY over the additional 17.3 acres of landfill in the stages proposed, which would allow for more leachate generation over time. This • additional leachate could potentially contaminate the underlying groundwater. This is because additional waste accumulation could result in some additional risk of potential spills, leaks, and • leachate control failures, despite the controls in place. Thus, the project could contribute over • time to the known VOC-contaminated plume extending north and northeast from the northern edge of the landfill border, and associated potential to contaminate groundwater. That could • result in the need to continue to supply water to affected offsite users for a longer time period. The following measures are proposed as part of the project to minimize this potential impact: • • • •