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AbstractWaves overtop berms and seawalls along the shoreline of Imperial Beach (IB), CA when energetic winter swell and high tide coincide. These intermittent, few-hour long events flood low-lying areas and pose a growing inundation risk as sea levels rise. To support city flood response and management, an IB flood warning system was developed. Total water level (TWL) forecasts combine predictions of tides and sea-level anomalies with wave runup estimates based on incident wave forecasts and the nonlinear wave model SWASH. In contrast to widely used empirical runup formulas that rely on significant wave height and peak period, and use only a foreshore slope for bathymetry, the SWASH model incorporates spectral incident wave forcing and uses the cross-shore depth profile. TWL forecasts using a SWASH emulator demonstrate skill several days in advance. Observations set TWL thresholds for minor and moderate flooding. The specific wave and water level conditions that lead to flooding, and key contributors to TWL uncertainty, are identified. TWL forecast skill is reduced by errors in the incident wave forecast and the one-dimensional runup model, and lack of information of variable beach morphology (e.g., protective sand berms can erode during storms). Model errors are largest for the most extreme events. Without mitigation, projected sea-level rise will substantially increase the duration and severity of street flooding. Application of the warning system approach to other locations requires incident wave hindcasts and forecasts, numerical simulation of the runup associated with local storms and beach morphology, and model calibration with flood observations.

• Sea-level rise (SLR) may impact inland sewer infrastructure kilometers away from the coastline. • More than 1/3 of the sanitary sewer length in Imperial Beach is subject to groundwater intrusion. • Defect flows may place $0.5−$2.7 M additional cost on the sanitary sewer system every year. • The elevated hydraulic loading can increase the potential of sanitary sewer overflows. The variation of the coastal groundwater table and the vulnerability of sanitary sewer infrastructure under a changing climate is considered for Imperial Beach (CA, USA) by incorporating the compound impacts of Sea-Level Rise (SLR), groundwater shoaling, and precipitation intensification. For 2 m of SLR, marine inundation is expected to impact only 2% of the urbanized area; however, SLR-driven groundwater shoaling is projected to impact 36% of the subterranean sewer system. Due to GroundWater Infiltration (GWI) and Rainfall-Derived Inflow and Infiltration (RDII), the sanitary sewage flow increases by 21% and 49% during dry- (i.e., consecutive days without precipitation) and wet-weather conditions (i.e., 24-hour rainfall with a 25-year return period), respectively. At SLR = 2 m, defect flows (GWI + RDII) can be elevated by 84% and 120% in dry- and wet-weather conditions, respectively. Such elevated hydraulic loads may place $0.5−$2.7 M additional cost on the collection system and treatment facilities every year. Moreover, pressurized junctions due to the above-mentioned hydraulic loading are likely to expose the community and the environment to raw sewage pollution. By involving structural, hydrological, and hydraulic criteria, a holistic approach is presented and implemented for prioritizing sewer system rehabilitation. [ABSTRACT FROM AUTHOR]

• Adverse impacts of sea-level rises are not limited to a landward shift in shoreline. • Shallow groundwater can impact inland infrastructure kilometers from the coastline. • About 2/3 of the stormdrain length is subject to seawater & groundwater intrusion. • Coastal stressors can increase flood magnitude, frequency, and duration. • Compound flooding volume increases up to 6 times by a 2 m sea-level rise. Low-lying coastal areas are susceptible to multiple types of flooding from marine, subsurface, and surface sources. The co-occurrence of a rainfall event with high sea level, which raises coastal groundwater tables conjointly, may result in a compound flooding event, which could be much more widespread and dangerous than that of an individual flooding source. Focusing on Imperial Beach as a low-elevation coastal community in California, this paper provides a generalized methodology to determine the vulnerability of coastal stormdrain systems to compound seawater, groundwater, and stormwater flooding under a changing climate. Although marine inundation by itself is expected to flood only 7% of the study area by 2100 under the most pessimistic scenario (i.e. 2 m rise), our results show that more than 20% of the study area's subterranean stormdrain system is already at the risk of subsurface flooding at current sea levels. While marine inundation is a concern near the coastline as sea level rises, the results show that seawater intrusion into the stormdrain system can impact areas kilometers away from the coastline. The consequences of sea-level rise (SLR) can be exacerbated by an under-resourced stormdrain system, caused by groundwater infiltration through system defects. As such, by a 2 m rise in current sea-level, the flooding volume may double in an ideal system (i.e., no defects) while this ratio can increase six fold in a slightly defective stormdrain system (with 0.25 % porosity systemwide). The continuous simulations of the stormdrain system performance indicate that flood events will be more destructive and frequent under these circumstances. [ABSTRACT FROM AUTHOR]