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Redwood City Stormwater Roadmap Current Stormwater Challenges September 2025 4-1 Schaaf & Wheeler 4 Current Stormwater Challenges Redwood City faces a range of stormwater system challenges driven by aging infrastructure, known drainage issues, capacity limitations, regulatory requirements, and increasing climate impacts. While many of these problems have been identified in previous studies and through maintenance reports, others require further investigation to understand their extent and risk. Addressing these challenges proactively will enable the City to shift away from a reactive model toward a strategic, data-driven program that prioritizes long-term system resilience. 4.1 Flooding Sources 4.1.1 Within City Limits Redwood City is subject to several types of flooding generated by local conditions, including tidal influences, stormwater runoff, and infrastructure limitations. One of the most significant internal flooding mechanisms is tidal backflow from the San Francisco Bay. During high tide events, particularly king tides, Bay water can push upstream into the City's drainage system, impeding gravity-based outflows and elevating flood risk in low-lying neighborhoods, such as Friendly Acres, Redwood Shores, and the industrial waterfront (Sea-Level Rise Vulnerability and Adaptation Planning Study, ESA, 2024). Localized flooding also results from inadequate stormwater capacity. Flat topography, undersized or deteriorated pipes, and limited hydraulic grade differentials reduce conveyance efficiency. This is especially evident in older areas, such as downtown Redwood City, where shallow slopes and aging infrastructure constrain runoff discharge. Residential neighborhoods with incomplete pipe networks are similarly vulnerable, as documented in maintenance records and staff reports (Eastern Low-Lying Area Drainage Master Plan, Wood Rodgers, 2017). Other recurring sources of localized flooding include sediment and debris accumulation in inlets, channels, and pump stations. These obstructions limit flow and reduce system storage, especially during the first flush of seasonal storm events. Additionally, emergent shallow groundwater, particularly in areas like Redwood Shores, is expected to intensify surface flooding as sea level rise elevates the water table over time (Sea-Level Rise Vulnerability and Adaptation Planning Study, ESA, 2024). 4.1.2 Outside City Limits Cordilleras Creek flows into Redwood City from unincorporated San Mateo County. As the creek enters the City, it is constrained by culverts at El Camino Real and the Caltrain corridor, which create hydraulic bottlenecks that limit flow rate capacity at this location. These constrictions contribute to upstream flooding and lateral spillover into surrounding developed areas (Flood Insurance Study, FEMA, 2017). Moreover, the downstream part of the creek - towards Highway 101 - is tidally influenced, which creates challenges during high/king tides in combination with heavy rainfall. Additionally, the Lower Emerald Lake Dam, located upstream in Emerald Hills, discharges into the Redwood Creek watershed. Although the dam is privately owned and located outside City limits, its position within the same drainage basin means it could contribute to downstream flooding in the event of an emergency release or failure. The dam is designated as a high hazard structure by the California Division of Safety of Dams, emphasizing the need for regional 11.A. - Page 18 of 53 449