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Redwood City Stormwater Roadmap <br />Current Stormwater Challenges <br />September 2025 4-2 Schaaf & Wheeler <br />coordination on downstream flood risk (Emerald Lake 1 Dam Break Inundation Study, Schaaf & <br />Wheeler, 2018). <br />Redwood City is also vulnerable to regional tidal flooding originating from San Francisco Bay. <br />High tides, storm surge, and wind-driven surface elevations from adjacent jurisdictions, such as <br />Foster City, San Carlos, and East Palo Alto, can propagate inland, raising base water levels at <br />Redwood City’s stormwater outfalls. These backwater conditions reduce outflow efficiency and <br />increase inland flooding risk (San Francisco Bay Tidal Datums and Extreme Tides Study, <br />AECOM, 2016). <br />Finally, the City shares shoreline levees and coastal buffers with neighboring jurisdictions, <br />particularly around Redwood Shores and Bair Island. A levee failure or overtopping event in an <br />adjacent area could divert floodwaters into Redwood City, especially in zones protected by non- <br />accredited or low-elevation levees (Regional Shoreline Adaptation Plan, OneShoreline, 2024). <br />4.2 Types of Stormwater Issues <br />There are four main stormwater challenges that Redwood City faces: condition, capacity, climate <br />resiliency, and water quality. <br />4.2.1 Condition Issues <br />Condition issues relate to the physical integrity and remaining service life of the City’s stormwater <br />infrastructure. Many of Redwood City’s storm drain pipes, pump stations, open channels, and <br />levees are decades old, with limited documentation on their current condition. Materials like <br />corrugated metal pipe (CMP) are particularly prone to corrosion and structural failure over time. <br />Without a comprehensive inspection program, underlying deterioration may go undetected until a <br />failure occurs. Regular assessment of system conditions is necessary to identify assets in need <br />of maintenance, rehabilitation, or replacement before they become liabilities. <br />4.2.2 Capacity Issues <br />Capacity challenges occur when stormwater infrastructure is undersized or hydraulically limited <br />relative to the volume of runoff it must manage. Redwood City’s flat topography, combined with <br />areas of high impervious cover, places stress on older systems that were not designed for current <br />or future rainfall intensities. Drainage networks, pump stations, and natural waterways can <br />become overwhelmed during large storm events, such as atmospheric rivers, particularly when <br />tides or downstream conditions restrict outflow. Systemwide hydraulic modeling is needed to <br />understand where capacity limitations exist and to prioritize upgrades that will reduce flooding <br />risk. <br />4.2.3 Climate Resiliency Issues <br />Climate resiliency refers to the stormwater system’s ability to withstand and adapt to future <br />environmental conditions, including sea-level rise, more frequent extreme storms, and rising <br />groundwater. Low-lying areas near the shoreline are already experiencing tidal influence in <br />drainage systems, and future projections show that groundwater will continue to rise in tandem <br />with sea levels. These changes threaten the reliability of gravity-based drainage, expose <br />infrastructure to corrosion and uplift, and increase the potential for compound flooding during <br />11.A. - Page 19 of 53 <br />450