Abstract

Low-lying Coastal Landfill Neighborhoods (CLaNs) often have a large aspect ratio, defined here as the coastline length divided by neighborhood width, due to the common practice of reclaiming fringing wetlands along tidal waterways. Flood risk reduction for CLaNs frequently involves elevated barriers, in the form of berms, seawalls, or levees, which reduce risk but cannot completely eliminate residual risk (e.g., due to overtopping during extreme events). Managed retreat is an alternative approach for flood risk reduction, the general idea of which is to strategically ban development in hazard zones, relocate structures, and/or abandon land. This study aims at exploring the tradeoffs between elevated barriers and managed retreat in terms of both CLaN aspect ratio and storm climate, for both short-term and long-term risk reduction with sea-level rise. Hydrodynamic flood modeling of an idealized CLaN protected by different adaptation plans is used to simulate flood conditions and mortality for a range of storm surge amplitudes for both the present-day and under different sea-level rise scenarios. Results show that for a berm and a case of managed retreat of an equal cost, retreat becomes more beneficial than the berm in terms of mortality risk reduction for neighborhoods with a larger aspect ratio. The study also shows that berms are generally less effective for reducing mortality in regions with less common but higher intensity storms. This study reveals the potential of idealized modeling to provide fundamental insights on the physical factors influencing the efficacy of different adaptation strategies for mortality risk reduction.

Highlights

  • Flood risk reduction for coastal floodplains frequently involves elevated barriers, in the form of berms, sea dikes, seawalls, or levees

  • Results by models F2 and F3 show that when a neighborhood has a narrower width, it can have a higher Expected Annualized Fatalities (EAFs) per capita (e.g., AR 3:1 verse 1:1 case, or AR 11:1 verse 1:1 case)

  • 3.3.1 An Alternative Setup by Replacing Houses With High Roughness The idealized setup of the model used in this study considers homes as impermeable elevated areas in the Digital Elevation Model (DEM)

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Summary

Introduction

Flood risk reduction for coastal floodplains frequently involves elevated barriers, in the form of berms, sea dikes, seawalls, or levees. The authors’ prior research (Zhang et al, 2020) on mortality risk during Hurricane Sandy (2012) on Staten Island, New York, showed that once overtopped, a protective berm can cause more dangerous “flash flood–like” conditions and higher mortality. These neighborhoods are similar in 1) their land elevation being low and relatively flat, bounded by increasing elevations on the inland side; 2) an elongated shape consisting of long along-coastline length and narrow cross-coastline width. As illustrated by the case of Hurricane Sandy and Staten Island, traditional hard defenses can sharply reduce but not eliminate flood risk for CLaNs. Once the defenses are overtopped, these basin-like neighborhoods are likely to be filled up with water quickly and dangerously, due to their large aspect ratio or length-to-width ratio. While much attention has been paid to storm surge physics in the coastal ocean, little attention has been focused on flooding processes in coastal neighborhoods and the role of neighborhood morphology

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