From the Guest Editor’s Desk:
Maximize preparedness, minimize impacts of increased coastal flooding
Reza Marsooli and Karina Johnston
Ocean City, New Jersey coastal flooding management: A case study
Nicholas Brown and Travis Merritts
Coastal flooding from both extreme storm events and sea level rise is impacting low-lying areas globally within the coastal zone. The intensity and frequency of these impacts are expected to increase as a result of climate change, thus requiring resilience planning to address both short- and long-term coastal change dynamics. Rates of actual and projected sea level rise in southern New Jersey are relatively high when compared to other portions of the eastern coast of the United States – partially due to glacial isostatic adjustment specific to that region. Numerous municipalities are attempting to adapt to these climate-driven coastal flooding hazards by adopting, or enhancing, integrated coastal resiliency programs. Located in southern New Jersey, the City of Ocean City has implemented numerous mitigation measures and policies focused on bolstering community resilience and managed through the implementation of various management plans. These adaptive plans’ purpose is to guide waterway maintenance; mitigate flooding through the implementation of numerous mitigation projects such as installing or upgrading pump stations and focused public engagement programs; and manage stormwater, while also protecting the environment through restoration-focused projects. Projects undertaken by the city as part of plan implementation include restoring tidal wetlands at Shooting Island, management of back bay waterways, and analysis of flooding impacts. The objective of this study is to examine one municipality that has been proactive in its climate adaptation and mitigation efforts and to disseminate the outcomes, solutions, and lessons learned for coastal managers and decisionmakers to allow them to utilize this information as the impacts of climate change continue to increase.
Probabilistic compound flood hazard analysis for coastal risk assessment: A case study in Charleston, South Carolina
Ferdinand Diermanse, Kathryn Roscoe, Maarten van Ormondt, Tim Leijnse, Gundula Winter, and Panagiotis Athanasiou
Coastal communities are susceptible to flooding due to flood drivers such as high tides, surge, waves, rainfall, and river discharges. Recent hurricanes such as Harvey, Florence, and Ian brought devastating impacts from combinations of high rainfall and storm surge, highlighting the need for resilience and adaptation planning to consider compound flood events when evaluating options to reduce future flood risk. Flood risk assessments often focus on a single flood driver (e.g. storm surge) due to the complexity of accounting for compound flood drivers. However, neglecting these compound flood effects can grossly underestimate the total flood risk. A probabilistic compound flood hazard analysis considers all compound events that lead to flooding, estimates their joint probabilities, simulates the flood response, and applies a probabilistic computation technique to translate flood responses and probabilities into probabilistic flood maps (such as the 100-year flood map). Probabilistic flood maps based on compound events can be used to assess risk more accurately for current and future conditions, with and without additional adaptation measures. In this paper we present an example of a probabilistic compound flood hazard analysis for the city of Charleston, South Carolina, considering tide, surge, and rainfall, for both hurricane and non-hurricane events. Charleston is regularly confronted with compound flood events, which are expected to worsen with sea level rise and more frequent tropical storms. Starting with an initial set of over 1,000 synthetic compound events, selection techniques described in the paper led to a final set of 207 compound events. The fast compound flood model SFINCS simulated the flood response for each event and, using numerical integration, compound flood return-period maps were created for Charleston, under current and future sea level rise conditions.
Protecting the built environment in a barrier beach and marsh system: A case study of the Hampton-Seabrook Estuary, New Hampshire
Paul Kirshen, David Burdick, Semra Aytur, Thomas Lippmann, Sydney Nick, and Chris Watson
Many barrier beaches in the U.S. are areas of high socioeconomic activity that some stakeholders want to maintain despite being increasingly vulnerable to tidal and storm surge coastal flooding due to climate change and associated sea level rise (SLR). Here we examine how this can be accomplished using a hybrid of nature-based solutions and grey infrastructure under present and short-term future climates. Our case study site of the Hampton-Seabrook Estuary in New Hampshire has a barrier beach that is densely developed with residential, tourist, and commercial facilities and buildings; it is a major economic engine for the state. It also has extensive back-barrier tidal wetlands. Given the few options available for flood protection with present land uses, it was found that at least in the short term primarily gray approaches would have to be used to maintain the socioeconomic activities of this barrier beach system, such as elevating key roads and fortifying (but greening) existing seawalls. In some locations, however, dune maintenance programs could be expanded, and in other locations living shorelines could be constructed to increase resilience to storm flooding. In addition, many assets would have to rely upon purely site-specific protection measures such as elevating and flood proofing. Socioeconomically vulnerable residents would be afforded some benefits due to the built environment and anchor institutions being protected, but the costs of flood-proofing individual homes (or choosing to relocate) would likely be borne by individual homeowners. Adding public greenspace and walkable areas may afford the greatest health benefits to people in lower socioeconomic groups who typically have the least access. The reduction in flood risk can be accomplished with minimal environmental impacts compared to those the region will face from SLR alone. In the longer term, more consideration may need to be given to the concept of managed retreat. Unfortunately, the environmental benefits of retreat would not be recognized if short-term gray actions were successfully implemented.
Research Letter: Influence of living shoreline elements on wave run up elevations
Ashley Ellenson, David Revell, Matt Jamieson, and Sam Blakesley
Nature-based coastal protection, also known as engineering with nature or living shorelines, is becoming increasingly popular due to its dual benefits of reducing coastal flooding and providing ecological and recreational opportunities. In many coastal areas experiencing chronic erosion, changes in sediment supply, composition, and grain size are significant contributing factors to shoreline recession. One living shoreline strategy to consider includes the application of cobbles over more traditional sand nourishments. On sandy beaches that experience high-energy wave conditions, the introduction (or reintroduction) of cobbles can mitigate backshore erosion. Cobble-backed beaches have been found to mitigate the effect of coastal erosion and flooding in laboratory settings and field observations, and they have recently been piloted in locations such as Cape Lookout State Park in Tillamook County, Oregon, and Surfers Point in the City of Ventura, California. However, there are no formal engineering guidelines stipulating the calculation of wave run-up on cobble-backed beaches. This study applies three different wave run-up equations on a living shoreline design (i.e. mixed sand and cobble berm-backed beach) in Malibu, California, and compares the predicted run-up levels with existing condition flood levels for typical and eroded conditions. The different wave run-up equations were designed for cobbles only, revetments, and composite beaches, respectively, where the composite beach equation was most applicable to project design. For typical beach conditions (higher levels of sediment accretion resulting in shallower beach face and berm slopes), all three equations showed a reduction in wave run-up values. When applied to worst-case conditions (i.e. scoured by a creek channel and steeper fronting beach slopes), the equation most applicable to the design showed the highest reduction of total water levels. A sensitivity analysis found that the cobble-backed beach equation predicted the most consistent values of run-up (run-up values changed the least), even when input parameters (slope and water depth) changed. This study shows that cobblebacked beaches hold promise to mitigate coastal flooding in appropriate areas, in addition to being a natural solution for areas experiencing erosion. This study also points to the need for more studies and field observations to validate the run-up levels determined here.
Quantification of coastal transportation network resilience to compounding threats from flooding and anthropogenic disturbances: A New York City case study
Julia Zimmerman, Sukhwan Chung, Gaurav Savant, Gary L. Brown, and Brandon Boyd
Technological advancements and management adaptations have improved the function of engineered systems in response to threats from coastal flooding. Other natural and anthropogenic disturbances such as pandemics, utility hijacking, infrastructure destruction, and biochemical releases can stress a system beyond acceptable limits or in ways not previously conceived. Such threats can be direct or indirect and often result in large-scale disruption to the critical functions of the system. Traditional risk management approaches, while effective for known and predictable threats, are not adequate preparation for compound disturbances that are often unpredictable and not well defined. Rather, a resilience-based approach is required. Resilience-based approaches acknowledge that multiple disruptions to the system will occur, such as the interaction of impending hurricanes and evacuation complications due to a pandemic, while focusing on the recovery and maintenance of critical functions. The system can be stressed with multiple disturbances to determine its capacity to resist and recover. Analysis of these capacities or subsequent failures can then be used to determine the resilience of the system and provide insight into remedial actions or improvements. A framework combining hydrologic modeling and network science can be used to determine critical weaknesses in transportation infrastructure due to compounding threats. This determination could be used to address pre-disaster staging by identifying areas that are likely to be isolated and to identify the characteristics of a resilient network to incorporate into future designs. To analyze resilience under compounded disturbances, coastal flood modeling is combined with hypothetical vehicle bridge failure in New York City, USA, and connectivity is analyzed through the use of ego networks. Our results show network analysis can be effectively used to identify areas of need to improve whole network resilience and is a valuable tool to quantify the compounding effects of multiple threats.