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Draft Supplemental Environmental Impact Report Page IV.G-20 Forward Inc. Landfill 2018 Expansion Project trucked for offsite disposal at the City of Stockton Municipal Utility Department wastewater treatment plant,or trucked to another offsite licensed treatment and disposal facility. Leachate stored in the temporary onsite tanks may be released back into the impoundment at a later date. The following measures are proposed as part of the project: • The proposed measures to address concerns about additional leachate generation as a result of the expanded landfill will be addressed in the JTD with the presentation of the updated EPA HELP model results based on the projected volumes of refuse, a historical analyses of actual leachate generation volumes(which were at significantly higher volumes than the model predicted for peak year rainfall) and the description of the leachate collection system designed to meet the maximum probable leachate generated. Engineering control systems (leachate collection system, drainage control, groundwater and gas controls),monitoring programs, and institutional controls have been presented in the JTD,which has been reviewed by the RWQCB. Reporting on leachate generation volume and quality is a requirement of the RWQCB-stipulated progress reporting through the various proposed landfilling phases. • The landfill cell anchor trenches would be elevated 2 to 3 feet above the surrounding land to minimize the possibility of water from major storm events draining into the cells and adding to the volume of leachate. Implementation of these procedures would reduce the impact to a less-than-significant level. 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 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 are 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