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4 Environmental Analysis 4.10 Hydrology/Water Quality i) Result in substantial erosion or siltation on- or off-site? ii) Substantially increase the rate or amount of surface run off in a manner which would result in flooding on- or off-site? iii) Create or contribute runoff water which would exceed the capacity of existing or planned stormwater drainage systems or provide substantial additional sources of polluted runoff? iv) Impede or redirect flood flows? (Less than Significant with Mitigation Incorporated) The Project site straddles the drainage divide for three small watershed areas and contains ephemeral drainage courses, none of which could be considered a perennial or even an intermittent stream. There is a very short segment of the PG&E part of the gen-tie line that falls outside of this local drainage area and within the adjacent Patterson Run watershed. All of these drainage areas are within the Lower Old River hydrologic unit; and their drainage courses do not have any FEMA-identified flood zones. There will be up to 32 acres of surface disturbance during construction and a potential increase of 18.8 acres of impervious surface across the Project site after construction (including foundations and other paved surfaces). Construction The portions of the Project site to be disturbed by construction will flatten an area of rolling hills and produce a stable platform for placing battery storage units and a Project substation. Project grading will create a large area with little to no slope, surrounded by smaller areas of cut or fill surfaces with 2:1 (horizontal to vertical) slopes. This design will reduce the potential for any concentrated runoff to occur. Construction will involve the use of bulldozers, graders, semi-trucks, and various other types of heavy equipment for vegetation removal, grubbing, grading, and installation of roads and other facilities. These construction activities will disturb the ground surface and involve minor changes to onsite topography, increasing the potential for erosion during storm events. Water used for dust suppression also has the potential to generate runoff that could transport sediments and dissolved solids. No increase in surface runoff potential is expected to be caused by construction activities, and there are no stormwater management facilities on the Project site or in proximity downstream. The Project would be subject to compliance with the NPDES Construction General Permit, as required by Mitigation Measure GEO-1, as well as with Mitigation Measure HYDRO-1 (Hydrology and Hydraulics Study). The Construction General Permit would include development and implementation of a SWPPP. The objectives of a SWPPP are to identify pollutant sources that may be delivered offsite (in the form of runoff and associated sediment load) and affect the quality of stormwater discharge; to implement site controls and practices to reduce stormwater pollution; and to protect water quality of receiving waters. The SWPPP would include site-specific BMPs to minimize erosion onsite and reduce or otherwise prevent conditions of erosion and stormwater runoff. The Hydrology and Hydraulics Study would provide for the development of adequate data and an appropriate Project drainage plan to protect against upland and channel erosion. PG&E will develop a specific SWPPP for the short segment of the gen-tie line that extends outside of the local drainage area. Activities during construction would not substantially alter the existing drainage patterns at the Project site. Construction activities could increase the potential for erosion and sediment transport Griffith Energy Storage Project 4.10-15 Tetra Tech/SCH 2022120675 Draft Environmental Impact Report August 2023