A Coastal Resilience Analysis of a Heterogeneous Landscape

Abstract

This paper develops a fine-scaled analysis in order to determine the cost and benefit of flood protection using hardened coastal structures within a large coastal segment. The probability distribution of surges and the relative rate of sea level rise are estimated from local tidal data and combined with detailed GIS data of all buildings to compute flood damage. Examining a heterogeneous suburban coastline of 110 km length (Branford, Connecticut), the paper defines a complete set of small segments along the coast between high elevation points. For each segment, the study determines whether the benefit of seawalls exceeds the cost and the optimal height for each wall. The analysis compares a uniform wall across the entire town, a uniform wall across only the low lying parts of the coastline, and a unique wall in each micro segment that maximizes net benefits. The uniform wall across the entire town fails a benefit cost analysis. By simply restricting the wall to the 30% of the coastline that is low lying, the flood benefits begin to exceed the cost of the walls. By carefully identifying just the low lying segments where the benefit exceeds the cost, the overall benefit to cost ratio can be increased to 3 to 1. The optimal flood protection program builds walls along only 10% of the coastline. These optimal micro segments are dispersed throughout the entire town including inland along a coastal river. The optimal elevation of the top of the walls is 2.3 m which is well below the 1/100 year storm height of 3.2 m. The benefit versus cost does not justify protection against rare but locally catastrophic storms such as hurricanes. Sea level rise increases the benefits of protection but plays only a small role in current protection decisions.