Evaluating Flood Resilience Strategies for Coastal Megacities

Abstract

Recent flood disasters in the United States (2005, 2008, 2012); the Philippines (2012, 2013); and Britain (2014) illustrate how vulnerable coastal cities are to storm surge flooding ([ 1 ][1]). Floods caused the largest portion of insured losses among all catastrophes around the world in 2013 ([ 2 ][2]). Population density in flood-prone coastal zones and megacities is expected to grow by 25% by 2050; projected climate change and sea level rise may further increase the frequency and/or severity of large-scale floods ([ 3 ][3]–[ 7 ][4]). ![Figure][5] CREDIT: NEONJELLYFISH/ISTOCKPHOTO Despite trillions of dollars of assets located in coastal flood-prone areas, investments in protection have often been inadequate ([ 8 ][6]), postponed for short-term economic reasons, for lack of consensus on how to properly evaluate the return on investment, or from the fear of making irreversible choices that become suboptimal over time. To help inform policy decisions, we have developed a multidisciplinary scientific approach to evaluate flood management strategies. It combines probabilistic risk assessment of hurricanes and storm surge with vulnerability determination of exposed assets at a census level, accounting for sources of uncertainty and the timing of investments in storm-surge flood-risk protection. We applied this methodology to New York City (NYC)—one of the most exposed coastal megacities—working with local policy-makers. A wealth of ideas about protecting NYC from floods has been proposed ([ 9 ][7], [ 10 ][8]), including barriers, levees, wetland restoration and beach strengthening that are effective in reducing flood occurrence in large parts of the city. However, as in other cities, some of these large-scale engineering options have been criticized because they are costly or may harm the environment. Other measures, such as reducing exposure and vulnerability (e.g., by enacting zoning regulations and enhancing building codes), may considerably reduce flood damage and entail lower investment costs, but they do not prevent flood waters from entering the city. We present three main classes of strategies that focus on reducing vulnerability or avoiding flooding or a combination of both [see the figure and supplementary material (SM)]. The Resilient Open City strategy (S1) is a cluster of measures to enhance building-code strategies in NYC ([ 11 ][9]) by elevating, or dry or wet flood-proofing, both existing and new buildings. Storm surge barrier strategies (S2, a, b, and c) aim to lower flood probabilities in NYC and parts of New Jersey (NJ), with barriers, levees, and beach nourishments. “Environmental dynamics” (S2a) consists of three barriers to close off parts of NYC and NJ that preserve wetland dynamics of Jamaica Bay. “Bay closed” (S2b) expands on S2a by adding a fourth barrier that closes off Jamaica Bay. “NJ-NY connect” (S2c) replaces three barriers from S2b with one large barrier in the outer harbor to protect a larger area (see the figure). The barrier systems are designed to withstand an extreme surge of 8 to 10 m (25 to 30 feet). The “hybrid solution” (S3) (see the figure), reflecting many measures in ([ 9 ][7]), combines building code measures of S1 that turned out to be cost-effective according to our analysis (SM) only in high-risk 100-year return flood zones (defined by the U.S. Federal Emergency Management Agency), with protection of critical infrastructure to reduce economic loss due to business interruption. S3 includes moderate local flood protection measures, such as levees and beach nourishment that are also part of S2c. The local protection measures and building codes for new structures are adjustable to future climate change, as they can be upgraded if flood risk increases in the coming decades. The heart of the method is a probabilistic flood-risk model developed for the city ([ 12 ][10]–[ 14 ][11]) (SM §1). We simulated 549 storm-surge simulations, varying from extremely low probability events to more frequent storms, using a new coupled hurricane–hydrodynamic–inundation model ([ 15 ][12]) (SM). Then we applied flood depth–damage curves to calculate potential damage to buildings and vehicles at the census block level. In addition to flood risk to buildings, the risk to other categories (like infrastructure), the risk to parts of NJ, and indirect economic effects were added, based on observed consequences of Hurricane Sandy in 2012 ([ 13 ][13]). We estimate the average annual expected flood loss for NYC alone at $174 million/year, if no flood management measures are implemented. Flood losses with a 100- and 1000-year return period are $2.2 billion and $25.4 billion, respectively. Our loss estimates for an extreme event of return period similar to Sandy are very close to the actual damages it triggered (SM §1.11). The future risk in 2040 and 2080 is also calculated, accounting for estimated changes in surge probabilities ([ 15 ][12]) and projected sea-level rise under future climate change scenarios, as well as the increase in urban exposure due to new construction in flood zones (SM §1.3). Flood defenses in the storm surge barrier strategies (S2, a, b, and c) are assumed not to fail. A benefit-cost analysis (BCA) of flood risk–management strategies was conducted for NYC over a 100-year period to evaluate the benefit (avoided risk) of each strategy and its cost ([ 13 ][13]), under future scenarios (SM §2). We tested the robustness of the BCA by considering various scenarios of climate change and different assumptions about the discount rate (SM §2) and by propagating uncertainty in water depth and damage estimation (SM §1.9) into the final risk assessment. For the barrier strategies, often seen as an irreversible choice, we also tested different investment timings. ![Figure][5] Strategies for protection vs. reducing vulnerability. ( Left )...