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Administrative Draft Environmental Impact Report Gill Medical Center Project i. Result in substantial erosion or siltation on- or off-site; ii. Substantially increase the rate or amount of surface runoff 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;or iv. Impede or redirect flood flows. The Project would include conversion of existing agricultural land to an institutional use, which would change the existing drainage pattern and add impervious surfaces at the building footprints and the parking areas.As discussed in Section 3.5.6.3, connection to the nearest stormwater sewer system is not feasible due to the distance from such facilities and the site. Therefore, onsite stormwater retention is proposed as part of the Project. Storm water runoff volumes were calculated based on state water quality requirements and related San Joaquin County improvement standards (Siegfried Engineering, Inc. 2020). These calculations indicate that full project buildout (Phases 1 and 2) would generate the need for approximately 15-acre feet (AF) of onsite storm water storage. Storm water runoff from impervious surfaces would be collected via drop inlets and underground piping and conveyed to onsite retention basins where it would undergo pre-treatment and be allowed to infiltrate and evaporate. Terracon (2021 B) conducted percolation tests to verify that the soils at the retention pond locations met applicable standards for infiltration of stormwater. Figure 3-7 shows a full buildout conceptual retention basin plan that employs multiple sized basins with 3:1 side slopes (min) occupying 9.5 acres of the Project site. All basins in the concept plan are located down gradient from development areas allowing for a gravity flow system. Like water and wastewater, stormwater improvements would be constructed and sized consistent with development phasing. The onsite underground piping and retention basins would prevent erosion and siltation onsite, while eliminating any offsite stormwater discharges.The retention basins have been sized to retain the runoff from design storm events and will, therefore, prevent flooding due to increased runoff from the impervious surfaces that would be added as part of the Project. The capacity of the onsite stormwater system has been designed to convey and retain the required stormwater volumes onsite and would not contribute any runoff to existing or planned offsite stormwater drainage systems. The Project site is not within a floodway or a floodplain that conveys flood flows, so the Project improvements would not impede or redirect any flood flows. With the stormwater system improvements incorporated as part of the Project, the alteration of existing onsite drainage patterns and the addition of impervious surfaces would result in a less than significant impact to erosion, siltation, and flooding. Mitigation Measures None required. Hydrology and Water Quality 4.12-24 October 2021