An Examination of Compound Flood Hazard Zones for Past, Present, and Future Low-Gradient Coastal Land-Margins

Félix L Santiago-Collazo,Scott C Hagen,Matthew V Bilskie,Peter Bacopoulos

doi:10.3389/fclim.2021.684035

Félix L Santiago-Collazo, Scott C Hagen + Show 2 more

Open Access

https://doi.org/10.3389/fclim.2021.684035

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Abstract

Recent events worldwide demonstrate how coastal communities of integrated natural and human systems are exposed to hydrological and coastal flooding processes. Standard flood hazard assessment practices account independently for rainfall-runoff, tides, storm surge flooding and not the non-linear combination commonly defined as compound flooding. This research evaluates compound flood hazard zones for past, present, and future (c. 1890–2090) conditions of the Mississippi River Delta Plain (MRDP). The MRDP provides a low-gradient coastal land-margin representing similar landscapes around the world that are experiencing relative sea-level rise and serves as a warning beacon for our coastal settlements. A set of plausible synthetic storms and rainfall events, which account for antecedent rainfall-runoff, tropical cyclone-driven rainfall, and tropical cyclone-driven surge, are employed in a tide and surge hydrodynamic model that integrates rain over the mesh. This study demonstrates the evolution of the compound flood hazard zones from the 1890s, before major western settlement and alterations to the Mississippi River and deltaic system, to the present day and out to 2090. Furthermore, near-future projections of the compound flood hazard zones suggest that the coastal flood zone will suffer the most significant changes in coverage area due to a combination of increasing eustatic sea-level rise and alterations to the coastal land-margin during low flood events. Our results emphasize the need to establish evolution trends of compound flood hazard zones to enable more descriptive future projections under a changing climate. Such projections will aid policy-makers, stakeholders, and authorities as they pursue enhanced coastal resilience to compound flooding.

Highlights

The origins of the Mississippi River Delta Plain (MRDP) can be traced to the end of the last ice age (i.e., ∼22,000 years before the present time) when glacial melt increased sediments deposited in coastal Louisiana (Keddy et al, 2007)
The results suggest that the Barataria watershed is dominated by coastal flood mechanisms from 1970 to 2090
A total of 23 synthetic tropical cyclones were employed to provide model forcing to each compound flood simulation

Summary

Introduction

The origins of the Mississippi River Delta Plain (MRDP) can be traced to the end of the last ice age (i.e., ∼22,000 years before the present time) when glacial melt increased sediments deposited in coastal Louisiana (Keddy et al, 2007). The Mississippi River brought sediment-rich water to its deltaic plain during high flow events, maintaining its ecosystem, and transforming marsh regions into swamps. During the 1930s, multiple levees were constructed throughout the MRDP While these levee systems protected communities from high flood events, they hydraulically disconnected the river from its deltaic plain. These river management actions reduced the inputs of riverine freshwater, sediment, and nutrients, which initiated the environmental downfall of the MRDP. Longer-duration flooding, lack of recruitment in cypress wetlands, reduced accretion rates, subsidence, and diminished wetland productivity, including marsh collapse (Hiatt et al, 2019), are some complications to the resilience of the coupled natural and human system facing the sediment-starved MRDP. The MRDP has lost about 5,000 km of coastal Louisiana wetlands, including submergence of most barrier islands (Batker et al, 2010; Nienhuis et al, 2017; Hiatt et al, 2019)

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