As sea levels rise, coastal communities will experience more frequent and persistent nuisance flooding, and some low‐lying areas may be permanently inundated. Critical components of lifeline infrastructure networks in these areas are also at risk of flooding, which could cause significant service disruptions that extend beyond the flooded zone. Thus, identifying critical infrastructure components that are exposed to sea level rise is an important first step in developing targeted investment in protective actions and enhancing the overall resilience of coastal communities. Wastewater treatment plants are typically located at low elevations near the coastline to minimize the cost of collecting consumed water and discharging treated effluent, which makes them particularly susceptible to coastal flooding. For this analysis, we used geographic information systems to assess the exposure of wastewater infrastructure to various sea level rise projections at the national level. We then estimated the number of people who would lose wastewater services, which could be more than five times as high as previous predictions of the number of people at risk of direct flooding due to sea level rise. We also performed a regional comparison of wastewater exposure to marine and groundwater flooding in the San Francisco Bay Area. Overall, this analysis highlights the widespread exposure of wastewater infrastructure in the United States and demonstrates that local disruptions to infrastructure networks may have far‐ranging impacts on areas that do not experience direct flooding.

Coastal communities rely on levees and seawalls as critical protection against sea-level rise; in the United States alone, $300 billion in shoreline armoring costs are forecast by 2100. However, despite the local flood risk reduction benefits, these structures can exacerbate flooding and associated damages along other parts of the shoreline—particularly in coastal bays and estuaries, where nearly 500 million people globally are at risk from sea-level rise. The magnitude and spatial distribution of the economic impact of this dynamic, however, are poorly understood. Here we combine hydrodynamic and economic models to assess the extent of both local and regional flooding and damages expected from a range of shoreline protection and sea-level rise scenarios in San Francisco Bay, California. We find that protection of individual shoreline segments (5 to 75 km) can increase flooding in other areas by as much as 36 million m³ and damages by $723 million for a single flood event and in some cases can even cause regional flood damages that exceed the local damages prevented from protection. We also demonstrate that strategic flooding of certain shoreline segments, such as those with gradually sloping baylands and space for water storage, can help alleviate flooding and damages along other stretches of the coastline. By matching the scale of the economic assessment to the scale of the threat, we reveal the previously uncounted costs associated with uncoordinated adaptation actions and demonstrate that a regional planning perspective is essential for reducing shared risk and wisely spending adaptation resources in coastal bays. Significance As sea levels rise, coastal cities will rely on shoreline protection strategies such as levees and seawalls to mitigate flooding. Although these strategies provide local flood-reduction benefits, they can increase inundation along other shorelines within the same estuary or bay. Using hydrodynamic and economic models, we quantify previously unmeasured regional economic damages (up to $723 million per flood event) that result from the protection of individual shoreline segments in San Francisco Bay, CA. We also highlight and quantify opportunities to alleviate regional flood damage through strategic floodwater storage in low-lying areas. Integrating the findings into coordinated planning efforts that account for the regional impacts of local shoreline actions could provide opportunities to reduce shared risk in coastal regions globally.

When storm surge, river discharge, and rainfall occur simultaneously or in succession, they may cause compound flooding and exacerbate impacts in coastal communities. In traditional modeling approaches, these driving processes are simulated separately, leading to discrepancies between simulated and observed data and highlighting the need for coupled modeling frameworks. Here, we develop a coupled coastal–fluvial–pluvial model for the Sabine–Neches Estuary in southeastern Texas using Delft3D to simulate flood extent and depth during Hurricanes Rita, Ike, and Harvey. For each hurricane, four scenarios (i.e., coastal, fluvial, pluvial, and compound) are modeled to determine the relative contribution of each driving force to water levels across the domain. Our results reveal that tropical cyclones such as Harvey that are accompanied by extended periods of rainfall produce the largest extent and longest duration of compound flooding. Such storms also produce highly dynamic and evolving patterns of compound flooding over their duration, with multiple flood peaks that can inhibit response and recovery efforts. When simulating these storms, a coupled model can provide substantial improvements over coastal‐, fluvial‐, and pluvial‐only models, decreasing root mean square error by 69%, 69%, and 79%, respectively. In contrast, storms with short‐duration rainfall and moderate to severe storm surge produce less extensive compound flooding, which is typically dominated by coastal–pluvial interactions. Our results further highlight the potential for compound flooding at the locations of critical water infrastructure assets, suggesting the need for flood adaptation approaches that are robust to multiple types of flooding.

Sea level rise increases the risk of storms and other short‐term water‐rise events, because it sets a higher water level such that coastal surges become more likely to overtop protections and cause floods. To protect coastal communities, it is necessary to understand the interaction among multiday and tidal sea level variabilities, coastal infrastructure, and sea level rise. We performed a series of numerical simulations for San Francisco Bay to examine two shoreline scenarios and a series of short‐term and long‐term sea level variations. The two shoreline configurations include the existing topography and a coherent full‐bay containment that follows the existing land boundary with an impermeable wall. The sea level variability consists of a half‐meter perturbation, with duration ranging from 2 days to permanent (i.e., sea level rise). The extent of coastal flooding was found to increase with the duration of the high‐water‐level event. The nonlinear interaction between these intermediate scale events and astronomical tidal forcing only contributes ∼1% of the tidal heights; at the same time, the tides are found to be a dominant factor in establishing the evolution and diffusion of multiday high water events. Establishing containment at existing shorelines can change the tidal height spectrum up to 5%, and the impact of this shoreline structure appears stronger in the low‐frequency range. To interpret the spatial and temporal variability at a wide range of frequencies, Optimal Dynamic Mode Decomposition is introduced to analyze the coastal processes and an inverse method is applied to determine the coefficients of a 1‐D diffusion wave model that quantify the impact of bottom roughness, tidal basin geometry, and shoreline configuration on the high water events.

This paper translates Ostrom’s “diagnostic approach” for social-ecological systems to identify the collective action problems and core governance barriers for sea-level rise adaptation in the San Francisco Bay Area. The diagnostic approach considers variables related to the resource system, the resource units, the users, and the governance system. Coupled ecological-infrastructure models identify two core collective action problems: vulnerability interdependency and adaptation interdependency. Qualitative social science case study methods identify the key structural governance and behavioral barriers to cooperation and ongoing activities to address them. The diagnostic approach is potentially applicable to any coastal regions that are vulnerable to sea-level rise and also other climate adaptation issues where vulnerability and adaptation interdependencies require overcoming governance challenges to collective action.