PNNL

Abstract

In low-lying estuarine regions worldwide, compound flooding (CF) is caused by the co-occurrence of extreme precipitation, river flooding and storm surge. In recent decades, there has been a rise in the frequency and intensity of pluvial-coastal CF events in different parts of the U.S. due to sea-level rise (SLR) and an increase in the frequency of intense precipitation and storm surge events. However, in estuarine and deltaic regions, the CF characteristics depend mainly on the storm tide and river flow interaction. Understanding how the sensitivity of fluvial-coastal CF may respond to a changing climate by considering the changes to watershed and estuarine characteristics is essential for flood mitigation, risk reduction, and future CF joint hazard prediction. This study examined two critical processes: 1) the interplay between antecedent soil moisture conditions (AMC) and peak river flow, and 2) SLR’s impact on storm surge and river flood distribution – on altering the CF in complex estuaries. We selected a shallow and convergent estuarine system in the US mid-Atlantic region – Delaware Bay and River, where flood hazards during a CF can become more significant than the surge and river flood processes occurring in isolation. Also, we selected Hurricane Irene (2011) as the focal event for the study that reportedly produced the most extreme CF over the past two decades in the same region. The first portion of the study demonstrated the importance of systematically resolving different key physical processes using a higher-resolution integrated modeling approach. Then, using the fully evaluated models, we conducted a sensitivity analysis with the future AMC and SLR conditions to show the flood water depth’s response along the estuary. Ultimately, our results illustrated that the potential changes to the catchment and bay characteristics from the global temperature increase and SLR could significantly modulate the fluvial-coastal CF variability. Studies that focus on large-scale future CF hazard assessment need to adequately address the uncertainties from land-surface processes.