Army Corps North Atlantic Coast Comprehensive Study – Main Report
This US Army Corps of Engineers (Corps) study, released in January 2015, assesses flood risk from coastal storms for the North Atlantic coast from Virginia to Maine – the states and communities affected by Hurricane Sandy – and recommends strategies for reducing those flood risks. The study was designed to help Sandy-affected states and communities better understand how flood risks will change as a result of climate change and sea-level rise. It captures the latest scientific information on sea-level rise, and provides state, local, and tribal governments with tools to help them prepare for changing flood risks and apply the lessons learned from the study.
The Main Report (summarized here) provides an overview of the process used by the Corps to develop the North Atlantic Coast Comprehensive Study (NACCS or the study). The Corps also released a range of reports, products, and tools developed as part of the study including engineering analyses (Appendix A), economic and social analyses (Appendix B), and planning analyses (Appendix C). In Appendix D, the report provides state-level analyses for the 9 states and District of Columbia within the study area (31,200 miles of coast). Also included in this appendix are more granular analyses that were conducted for 9 areas within each state (“Focus Area Reports”), these include analyses of the Rhode Island Coastline, Connecticut Coastline, New York-New Jersey Harbor and Tributaries, Nassau County New York Back Bays, Delaware Inland Bays and Bay Coast, City of Baltimore, District of Columbia, and City of Norfolk.
Coastal Storm Risk Management Framework: The main report details the Coastal Storm Risk Management Framework (Framework) that was developed as part of the NACCS to provide a replicable model to help other communities conduct similar assessments of risks and develop solutions for managing coastal flood risks. The Framework walks decisionmakers through a process for developing an adaptation plan for coastal risks and provides information on data and tools that decisionmakers can use at each stage of the process. In Table IV-8 (pp. 68 -69), a chart summarizes the different phases of analysis and data and tools that are available to help decisionmakers at each step.
- Step 1 – Initiate Analysis: set study parameters, establish geographic and spatial scale, set study purpose and goals, and determine stakeholders.
- Step 2 – Characterize Conditions: At the regional scale, inventory existing conditions and future forecast conditions for the study area, collect data, select a planning horizon (25, 50, 100 years), utilize existing plans and studies, consider environmental conditions and cultural resources, consider changes in population and supporting infrastructure, consider climate change and sea-level rise scenarios. To refine the analysis to a state or local scale, collect refined geographic data (including bathymetry, topography, land use, environmental and habitat data); collect refined coastal hazard data (storm, surge, waves, rainfall), and consider local policies and other local data.
- Step 3 – Analyze Risk and Vulnerability: At the regional scale, identify problems and opportunities through exposure and risk assessments by mapping inundation and exposure and multiplying exposure by the chance of inundation to present risk. At the state or local scale, the alter exposure index metrics and weight where appropriate, consider existing projects and their efficacy at reducing risks, conduct more detailed analysis and modeling of coastal responses to sea-level change and storms, and evaluate existing and planned risk management approaches.
- Step 4 – Identify Possible Solutions: At the regional scale, identify possible solutions based upon the shoreline types characteristic of the study area. At the state or local scale, refine solutions in consideration of feasibility and adaptability. Consider combining different measures to develop a comprehensive coastal risk management strategy.
- Step 5 – Evaluate and Compare Solutions: To determine the most viable strategy, use evaluation criteria to compare and choose among options including: efficacy, cost, adaptability to long-term forecasts, and ability to combine with other solutions. At the state and local scale, consider how projects can be enhanced of replicated, how projects can work together, and site-specific variables, among other factors.
Characterizing conditions: Using this Framework the Corps assessed risks in the study area from Maine to Virginia. To characterize conditions in the study area, they used evaluation scenarios for the years 2018, 2068, and 2100 to consider how climate change would result in accelerated sea level rise and to develop inundation maps. Future population forecasts were also used for a 50-year planning horizon. At page 19 of the report, a list of the data sets used to perform the different tiers of analysis is provided. The report describes the range of expected effects from climate change and sea-level rise in the study area, including: shoreline retreat from erosion and inundation, increased frequency and magnitude of storm-related flooding, temperature changes, saltwater intrusion into estuaries and aquifers, changes in habitat and species distribution, and changes in sea surface temperatures. For example, for Sandy Hook, NY the study used an estimated rise in sea-level of 1.8 feet under an intermediate scenario and 2.9 feet under a high scenario by 2068 and 2.2 feet (low scenario) and 5.5 feet (high scenario) by 2100. The intermediate scenario is based primarily on ocean warming and the high scenario accounts for melting of land-based ice sheets. The study used FEMA floodplain maps (that determine areas that have a 1% chance of flooding based upon historical flood data) and SLOSH modeling (Sea, Lake, Overland Surges from Hurricanes) to determine what areas of the study area would have a 1% chance of flooding if you added 3 feet of sea-level rise. The 10% chance floodplain was also modeled. The details of the scenarios, models, and data used to complete the study are described in the report (at pages 29-42).
Risks and Vulnerabilities: To assess vulnerabilities in the study area, three exposure indices were developed: population density and infrastructure, social vulnerability characteristics, and environmental and cultural resources. Inundation mapping was used in combination with the exposure indices to determine areas with the greatest flood peril (i.e., areas where flooding would have the greatest consequence). The social vulnerability indices looked at age, income, and non-English speaking populations. The National Wetlands Inventory, Nature Conservancy Ecoregional Assessments, and data provided by the US Fish and Wildlife Service was used to assess at-risk environmental and cultural resources.
Solutions: The report examines a range of potential solutions for responding to increasing flood risks including non-structural (land-use approaches), structural (engineered approaches), and nature-based approaches (green or hybrid green-grey approaches). Each of these approaches can be used to achieve different community goals—avoid, accommodate, and preserve—based upon local site and shoreline conditions and community values. Additionally, the report stresses that coastal resilience will require a combination of measures creating multiple lines of defense that are resilient, redundant, robust, and adaptable.
The types of resilience approaches discussed in the study include.
- Nonstructural approaches include acquiring or relocating flood-prone properties, retrofitting buildings to be more resilient to impacts, enhancing flood warning and evacuation systems, and promoting better floodplain management and land-use practices.
- Structural approaches include floodwalls, deployable floodwalls, levees, seawalls, revetments, bulkheads, and storm-surge barriers.
- Nature-based approaches include beach restoration, drainage improvements, living shorelines, reefs, and wetlands. The study also provides analysis on the flood risk reduction benefits that can be provided by natural ecosystems like wetlands noting that inundation flooding can be reduced by 5 to 40 percent by wetlands, oyster reefs, and other aquatic vegetation.
The study also developed conceptual cost estimates for various approaches, including estimates for operation and maintenance costs for some measures (Table IV-7 and Appendix C).
Systems Approach: The study encourages use of systems approaches for managing coastal flood risk defined comprehensive, integrated strategy implemented at a landscape or watershed scale (at page 83). By taking a systems approach, decisionmakers can minimize negative unintended consequences, maximize benefits for the most number of people, and preserve natural assets. Several examples of how a systems approach has been used to reduce coastal flood risk are described including nature-based approaches (beach nourishment, dune grass planting, and salt marsh enhancement) along the New Jersey coastline (at page 85) and Regional Sediment Management in Northeast Florida connecting federal dredging activities with coastal risk management needs at adjacent beaches, estuaries, and bays (at page 87).
The Coastal Policy Landscape and its effect on coastal risk management is describe at Section VI of the report including: zoning codes, building codes, evacuation plans, preservation of open space, nature-based approaches, structural measures, floodproofing of homes and businesses, and insurance. Table VI-I, lists the variety of executive orders, federal laws, programs, and policies that affect coastal risk management. This sections also describes “Opportunities for Action” – ways that federal agencies can reduce or eliminate barriers or encourage better state and local practices and federal programs that show success. A table of barriers and opportunities for action is included at pages 99-101. Recommendations include the following:
- Risk and Resilience Standards: Standardize definitions, and develop design standards and performance metrics; conduct a national vulnerability study; develop a national strategy for coastal storm risk management; develop regional and watershed-based plans.
- Communication and Outreach: conduct risk management visioning sessions with the public; and develop information to educate the public about flood vulnerabilities, flood risk, residual risk, blended solutions, and evacuation planning; develop a community of practice for nature-based approaches.
- Risk Management: strengthen and enforce floodplain management policies and simplify coastal programs.
- Science, Engineering, and Technology: improve research, coordination, and collection of pre- and post-storm data; develop design guidelines for nature-based approaches; compile information and ecosystem good and services provided by nature-based approaches/
- Leadership and Institutional Coordination: reevaluate projects using a comprehensive systems approach; increased coordination across all levels of government; and support national adaptation planning.
- Local Planning and Financing: Apply lessons learned from other disaster recovery processes (such as Katrina, Rita, and Sandy) to improve coastal risk management approaches; create new tax and market based incentives to encourage local resilience actions; explore innovative financing options to sustain long-term operations, maintenance, monitoring, and adaptive management of resilience approaches; leverage public private partnerships as part of a community financing strategy.
Data Gaps: Section VII of the study identifies data gaps and activities needing additional analyses, including:
- The effect of sea-level change on existing stormwater management systems. Sea-level rise will alter the ability of streams and rivers to convey rainfall to coastal bays and estuaries and it may increase the frequency and severity of rainfall-driven flooding in inland and coastal areas.
- The performance, timing and scale of nature-based approaches need to reduce coastal flood risks and the ecosystem goods and services provided by these approaches.
- Understanding of coastal dynamics of watersheds including impacts in back bays and the interrelationship of coastal features.
The study was paid for by funding from the Hurricane Sandy Disaster Recovery Supplemental Appropriation (PL 113-2) authorized by Congress in January 2013. The Appropriation Act called for the Corps to conduct a “comprehensive study to address the flood risks of vulnerable coastal populations in areas that were affected by Hurricane Sandy within the boundaries of the North Atlantic Division of the Corps.”
Publication Date: January 2015
- Assessment guide
- Planning guides
- District of Columbia
- New Hampshire
- New Jersey
- New York
- Rhode Island