Measuring, Modeling and Projecting Coastal Land Subsidence: Implications for Relative Sea-level Rise and Flooding along US Coast

Abstract:

Land subsidence in coastal areas impacts ongoing and future projections of sea-level rise, and can exacerbate flooding hazards. The rate of land subsidence varies spatially and temporally due to natural and anthropogenic processes. However, spatially expansive (100s of km) observation of coastal land subsidence at management relevant resolution (10s of m) are scarce due to sparsely distributed monitoring stations.

Here, we report on measurements of high-resolution vertical land motion along the US coast obtained from multitemporal processing of large sets of Synthetic Aperture Radar (SAR), spanning period 2003-2020. We find that subsidence rates of up to several centimeters per year affect different parts of the US West and Gulf coasts, in particular, deltas, wetlands, artificial landfills, and Holocene mud deposits. For instance, a subsidence rate of less than 2-10 mm/yr affects the majority of coastal areas along San Francisco Bay. Furthermore, we estimate that between 4.3-8.7 million people in California’s coastal communities, including 460,000-805,000 in San Francisco, 8,000-2,300,00 in Los Angeles, and 2,000,000-2,300,000 in San Diego, are exposed to subsidence.

Next, we show that maps estimating inundation hazards solely based on a projection of sea-level rise underestimate the area at risk. In combination with future projections of sea-level rise under different climate warming scenarios, we estimate that in San Francisco Bay, the inundation hazard is underestimated by 13.5% - 40.1% through the 21^st century, compared with the revised maps that also account for projections of local land subsidence. However, we show that the estimates of future land subsidence and associated uncertainties are enigmatic, due to complex underlying physical processes, thus and can be a significant source of error in projections of relative sea-level rise

This study emphasizes the urgency with which the flood resiliency plans must adapt to scenarios in which coastal land elevation drops rapidly. Understanding the fundamental processes driving relative sea-level change enables policymakers to prioritize the risk reduction and adaptation interventions to better identify communities and ecosystems most vulnerable to flooding.