Sea-Level Change Adaptation
Sea-level change has been the focus of intense interest by the U.S. water resources science agencies (NOAA and USGS), along with other agencies contributing to the U.S. Global Change Research Program (USGCRP, 2009) and the third US National Climate Assessment (2014), as well as the Fourth National Climate Assessment released in 2017 (Volume 1) and 2018 (Volume 2). Among the recent science reports supporting the development of new guidance are the NRC's 2012 report Sea-Level Rise for the Coasts of California, Oregon, and Washington: Past, Present, and Future and the expert report published by NOAA-USGS-SERDP-USACE on Global Sea Level Rise Scenarios for the United States National Climate Assessment, which was written in support of the 2014 National Climate Assessment. Finally, agency reports and peer review literature contain more than 10,000 citations in the area of sea-level change (or sea-level rise).
Because of the importance of coastal areas to the missions and operations of the USACE, the agency has considered sea-level change in its planning activities since 1986, beginning with a memorandum in 1986 and with EC 1105-2-186: Planning Guidance on the Incorporation of Sea Level Rise Possibilities in Feasibility Studies (pdf, 609 KB) in 1989. In 2000, USACE incorporated sea-level change considerations in its Planning Guidance Notebook, and in 2009, released an Engineer Circular (EC) 1165-2-211, Incorporating Sea-Level Change Considerations in Civil Works Programs (pdf, 457 KB), superseded by Engineer Circular (EC 1165-2-212 Sea-Level Change Considerations for Civil Works Programs). Engineer Circulars have a two-year lifespan, so these were superseded by Engineer Regulation 1100-2-8162, Incorporating Sea Level Change in Civil Works Program, released in December 2013. In July 2014, USACE published guidance on how to adapt to changing sea levels, Engineer Technical Letter 1100-2-1, Procedures to Evaluate Sea Level Change: Impacts, Responses and Adaptation (pdf, 4.75 MB). This was transitioned to Engineer Pamphlet (EP) 1100-2-1, also named Procedures to Evaluate Sea Level Change: Impacts, Responses and Adaptation, in 2019. An existing probabilistic tool used to assess vulnerability of non-developed natural coastlines or beach protection projects (Beachfx) has been updated for use with the new sea-level guidance, and a second tool for life-cycle analyses of developed coastlines, the Generation Two Coastal Risk Model (G2CRM) is currently under development at the U.S. Army Engineer Research and Development Center.
Adaptation to changing sea level is addressed along with examples in Engineer Pamphlet (EP) 1100-2-1. A key concept in this guidance is the identification of tipping points and thresholds. Tipping points are levels or conditions at which an irreversible change occurs. For example, if salt-water intrusion cases a particular wetland marsh to transition from primarily brackish to saltwater. Thresholds are often expressed in terms of elevation, but can take a wide range of forms, including physical, economic, social, and environmental. For example, a barrier island may use the elevation of the roadway which allows evacuation as a threshold: when sea level rise causes the roadway to be unusable during surge events, adaptation will be required to maintain the ability to evacuate. This adaptation could be structural (e.g., change the road configuration) or nonstructural (e.g., improve warning systems and mandate evacuation before the roadway will be impacted).
Identifying thresholds beyond which performance is adversely affected is an important way to understand current and future vulnerability. The USACE (or stakeholder) sea level change scenarios are used in a “When, Not If” approach to estimate the time at which these thresholds or triggers will be exceeded. Mapping the scenarios against one or more thresholds or triggers helps the planner and engineer to decide when adaptation is needed, or when one method is no longer adequate and a different adaptation pathway might be needed. It is especially important to understand any tipping points, because the performance of the system can deteriorate rapidly once these tipping points are exceeded. Understanding thresholds can inform the urgency of action, the range of feasible actions, any necessary transition points from one type of measure to another, and the selection of extreme conditions for design, as well as larger system effects (Environment Agency 2009).
USACE continues to work with Federal science agencies and other experts to learn more about coastal climate change impacts and how to adapt to these changes that can cause damages to human life and property as well as ecosystems. USACE considers the full array of coastal risk reduction measures, including natural or nature-based features (e.g., dunes), nonstructural interventions (e.g., policies, building codes and land use zoning, and emergency response such as early warning and evacuation plans), and structural interventions (e.g., seawalls or breakwaters), and combinations of these features. Natural and nature-based features can attenuate waves and provide other ecosystem services (e.g. habitat, nesting grounds for fisheries, etc.), however, they also respond dynamically to processes such as storms, both negatively and positively, with temporary or permanent consequences. Nonstructural measures are most often under the jurisdiction of State and local governments (and individuals) to develop, implement and regulate, and cannot be imposed by the federal government. Perhaps more well-known are the structural measures that reduce coastal risks by decreasing shoreline erosion, wave damage and flooding.
Together with its partners and stakeholders, USACE can apply science and engineering to configure an integrated approach to risk reduction through the incorporation of natural and nature-based features in addition to nonstructural and structural measures that also improve social, economic, and ecosystem resilience. To clarify our commitment to using the full array of measures for coastal risk reduction, USACE published a report in 2013 called "Coastal Risk Reduction and Resilience: Using the Full Array of Measures." This work identified the need for focused research is needed to reduce the uncertainties involved in evaluating and quantifying the value and performance of natural and nature-based measures for shoreline erosion and coastal risk reduction. Federal investments supporting erosion mitigation and coastal risk reduction and resilience could benefit from more consistent integration of natural and nature-based infrastructure. The USACE, along with other agencies, has made progress in addressing the need through its Engineering with Nature ® program, among other activities.
Incorporating social sciences along with physical sciences and engineering is another gap area where improved knowledge and understanding could help improve understanding of measures that encompass social (technological, institutional, and behavioral) responses (Kates et al. 2012) and legal issues (e.g., Craig 2010). Progress in this area would help to better inform investments in coastal systems and result in longer-term benefits for coastal risk reduction and an array of societal needs.