Environmental Declaration WPCP 20240409

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Iroject Description The current basin elevation might lead to water backing up during high tides. Raising the bottom elevation to 6.5 feet (NAVD88) using about 41,000 cubic yards of soil addresses this, which reduces the area available for riparian/wet meadow habitat around the perimeter. b) Planting Design The Proposed Project involves creating three types of plant communities, each suited to different hydrologic conditions. The plant selection includes native riparian, wetland, and upland species: Wet Meadow and Riparian Scrub: These species will be planted along slopes fitted with crates and effluent distribution pipes, creating a wetland -like environment similar to the OLSD project, without extra irrigation. Emergent Wetland: These plants will mainly establish at the base of slopes and in select areas, with floating vegetation expected to develop over time, requiring drainage of the basin and maintenance to ensure floating mats do not shade out the open water segment. Uplands ; Portions of the slopes that do not feature seepage slopes will support upland native plants and seeds, irrigated with effluent in a controlled manner to avoid wetland conditions. Open water areas will remain largely unplanted to aid UV penetration and treatment of effluent. Plant species selection will focus on water quality benefits, adaptability to water depths and climate, habitat suitability, and maintenance needs. The initial plant list was based on the OLSD Horizontal Levee Project, with modifications to avoid species that could obstruct bioreactor media. Planting is vital for wetland success. Establishing an on -site nursery for mature plants is recommended to ensure robust growth. Proper soil substrate and horticultural practices will be followed, and the planting approach will vary based on site conditions, including spacing, irrigation needs, and methods suited to wet or dry soil. Draining the wetland occasionally will aid plant establishment, weed control, and mosquito management. c) Outfall The Proposed Project involves modifying the existing outfall structure connecting the basin to San Francisco Bay to regulate water levels within the basin (Figure 2.14, Outfall Plan & Profile). A recent inspection confirmed that the structure allows tidal water and debris from the bay to enter the vault during high tides. The structure includes a concrete vault with a weir -style inlet and a 48-inch corrugated metal pipe leading to the bay. The current weir inlet elevation would result in water depths exceeding the preferred 18 to 24 inches, necessitating modifications to adjust the water depth based on flow conditions. Modifications involve installing an adjustable weir plate in the vault. A 30" gate valve shall be installed in the existing outfall pipe at a location upstream of the tidal extent to prevent tidal backflow into the outfall pipe during high tides and storm surges and to avoid any construction - phase work in the outboard area of the levee. Sediment build-up within the outfall pipe is also a concern, with the outlet pipe currently half-filled with sediment. Regular flow is expected to prevent complete blockage, but existing sediment might need removal at construction time. Preliminary visual inspections indicate the existing pipe liner is intact. San Leandro Treatment Wetlanu IS/MND February 2024