Heidi Moritz, Kate White, and Rod Moritz
The elevation, duration, and frequency of occurrence of coastal water levels are used in the evaluation of performance and reliability for existing coastal projects and in the planning and design of new coastal projects. The components of total water level (TWL) vary spatially and temporally and include mean sea level, seasonal water level changes, storm surge, and wave-related components. TWL can also be affected by long-term circulation and atmospheric processes, riverine flow, and geomorphology. The observed and expected sea level change occurring over near and long-term time periods has been the topic of a great deal of discussion and analysis recently because of its direct and indirect impacts to coastlines and its effects on total water levels over time. Understanding which component of total water level or combination of components controls vulnerability and performance (and at what time scale) is critical to the planning, design, and evaluation of a project. Project vulnerability and performance will vary depending on project type and location. Estimating future conditions over the project life must incorporate the uncertainty in global sea level rise due to observed and projected changes in the atmosphere, ocean, land, and ice sheets. This paper discusses how the U.S. Army Corps of Engineers (USACE) considers scenario-based approaches in sea level change and the inherent contribution to total water levels in project alternative development.
Scott Hayward, M. Reza Hashemi, Marissa Torres, AnnetteGrilli, Stephan Grilli, John King, Chris Baxter, and Malcolm Spaulding
Accelerated shoreline retreat due to sea level rise is a major challenge for coastal communities in many regions of the U.S. and around the world. While many methods of erosion mitigation have been empirically tested, and applied in various regions, more research is necessary to understand the performance of these mitigation measures using process-based numerical models. These models can potentially predict the response of a beach to these measures and help identify the best method. Further, because nearshore sediment transport processes are still poorly understood, there are many uncertainties in assessment of coastal erosion and mitigation measures. Hence,there is a need to better assess the capabilities and shortcomings of numerical models as a way to improve them. In this work, a suite of numerical models was used to assess coastal erosion and the performance of a number of recommended solutions, along a section of coast in southern Rhode Island, US, which represents a typical coastal barrier system. The coupled modeling system SWAN(Simulating Waves Nearshore), a third-generation wave model, and ADCIRC (ADvanced CIRCulation Model), a two-dimensional depth integrated circulation model, was applied over a regional grid encompassing northeastern U.S. to compute offshore sea levels and wave conditions for specified storm scenarios,both historical and synthetic. The coastal wave-circulation and morphodynamic model XBeach was then nested within this regional grid to simulate nearshore sediment transport processes and shoreline erosion. After validating the regional modeling system for a historical storm (Hurricane Sandy), Hurricane Irene(2011) was used to validate XBeach, on the basis of a unique dataset of pre-and post-storm beach profiles that was collected in our study area for this event. XBeach showed a relatively good performance, being able to estimate eroded volumes along three beach transects within 8% to 39%, with a mean error of 23%.The validated model was then used to analyze the effectiveness of three living shoreline erosion mitigation methods that were recommended in a recent study of coastal erosion in New England: beach nourishment, coastal bank (engineered core), and submerged breakwater. Further, the effect of an artificial surfing reef on sediment transport was also investigated. Conceptual designs were implemented in the model and the eroded volume were computed, with and without the presence of these solutions. Using two synthetic storms, it was shown that erosion mitigation methods that focus on reinforcing the beach face and dunes are more effective than those that reduce wave action. While this study showed how models such as XBeach can help examine the technical performance of erosion mitigation measures, more detailed assessments including cost-benefit analyses are necessary at the decision-making level.
R. Boudreau, R. Sloop, A. Holloway, and J. Rivera
The County of Maui was in trouble. A Coastal Hazard Analysis for the county demonstrated that the main wastewater treatment plant for the island was vulnerable to earthquakes, tsunamis, winter waves, flooding, and inevitable sea level rise due to its low-lying beachfront location on the world-renowned high-wind and big-surf North Shore. An economic analysis indicated that relocating the plant would cost upwards of $400 million and still require a pumping facility in the same location due to the gravity flow piping system, rendering portions of the system still vulnerable. An initial engineering feasibility study in 2005 concluded elements of the treatment plant could be retrofitted to protect against environmental hazards for $30 million to $40 million. Time was of the essence for the coastal protection aspects as the shoreline analysis indicated the coastal injection wells were vulnerable within a 1-2 year timeframe, and a critical chlorine storage facility would be at risk in an 8-10 year timeframe. Damage to and flooding of this building would pose a grave environmental disaster that could impact the entire North Shore. An alternatives analysis that included beach nourishment, sand retention structures, and shoreline protective devices demonstrated that a rock revetment was the preferred alternative for drawing a hard line in the sand. While the design team (including coastal engineers, geotechnical engineers, marine biologists,and planners), in conjunction with county staff, understood the primary focus of the project was to provide much-needed shore protection to critical public infrastructure,the consensus and instincts were also to push for a softer, more adaptable and environmentally conscientious approach using beach nourishment and dune enhancement to bury the revetment. Once that paradigm shift occurred, two immediate challenges emerged: (1) Chronic lack of sand resources throughout the state;and (2) severe restriction by a local regulatory agency of any placement of beach nourishment material seaward of the mean high tide line. After many long discussions, the team hit upon a revelation – if the county would be willing to“sacrifice” a portion of their property, a majority of the revetment could be“pulled back” and buried with the excavated sand and use the remaining sand volume to increase the dune height to reduce overtopping and flooding for present and future conditions.
Eileen S. Johnson, Jeremy M. Bell, Daniel Coker, Elizabeth Hertz, Nicole LaBarge, and Gavin Blake
Maine’s rural coastal communities face distinct challenges associated with the impacts of climate change, specifically sea level rise and coastal flooding. Limited road networks, longer travel times to area hospitals, and a more isolated and aging population contribute to communities’ social vulnerability. Adaptation planning at a local level requires the provision of data that is location specific, accessible, understandable for decision-makers and reflects place-based vulnerabilities. Our research presents a web-based tool for providing data to decision-makers in support of local and regional adaptation planning processes. Two key components of the Maine Coastal Risk Explorer include a customized social vulnerability index derived from variables that contribute to social vulnerability for Maine rural coastal communities and an analysis of impacts of sea level rise on road networks as lifelines critical for residents to access health and emergency response services.
Philip G. King, Chad Nelsen, Jenifer E. Dugan, David M. Hubbard, Karen L. Martin, and Robert T. Battalio
California’s coast is eroding, along with its iconic sandy beaches. As a result, the pressure for already extensive coastal armoring of these dynamic shorelines is increasing. Beach loss will accelerate with sea level rise, as will the rate of armoring, unless checked by major public policy initiatives. To conserve functional beach ecosystems for the public good, adaptation strategies need to include preservation of shorelines without armoring and the restoration of natural coastal processes. As in many other ecosystems, investigations of the full value of ecosystem services of intact dune-beach-surf zone systems and the development of protocols for robust measurement of indicators of those services are incomplete. Consequently, valuation of the ecological functions and services (except for storm buffering and recreation) of beaches are rarely applied to mitigate for armoring projects. To move forward in developing a viable approach for mitigating the increasing losses to sandy beach ecosystems associated with the multitude of coastal armoring projects on open coast sandy shorelines, we considered several economic valuation methods and suggest that ecosystem replacement cost should be considered as part of any mitigation strategy. Using this framework, we propose a mitigation system for shoreline armoring projects that is: 1) intended to minimize long term loss of the resources and services of intact beach ecosystems, 2) based on simple metrics, 3) easy to interpret and apply, and 4) capable of being used in conjunction with a mitigation banking system.