DEVELOPMENT OF FRAGILITY CURVES FOR REINFORCED DUNES TO IMPROVE THE RESILIENCY OF COASTAL COMMUNITIES

This body of work consists of four manuscripts regarding coastal vulnerability assessment methods, the use of geosynthetics to reinforce coastal systems including case studies, and the development of fragility curves to quantify the resilience of reinforced coastal systems. Coastal Vulnerability Assessment Methods Recurring hazards have the potential to continually degrade natural and constructed coastal protection systems especially if the time between subsequent hazards is shorter than a natural recovery cycle or an assess-scope-fund-construct recovery project timeline. The natural recovery process cannot be changed, so the efficiency of assessing hazard impacts and implementing resilient recovery efforts needs improvement to minimize the vulnerability of coastal communities. This paper focuses on three methods to conduct post hazard coastal assessments: Real Time Kinematic-Differential Geographical Positioning System (RTK-DGPS) surveys, Structure-from-Motion (SfM) photogrammetry surveys using close-range aerial photography, and boat-based mobile terrestrial laser scanner and interferometric sonar surveys. Each of the methods have strengths to produce detailed Digital Elevation Models (DEMs) to document where vulnerabilities exist and limitations that may reduce accuracies if not properly accounted for. A better understanding of each survey method will improve the quality of post hazard assessments and reduce the time associated with recovery projects, which can serve to improve a community’s resilience to subsequent hazards as highlights in the Montauk, NY socio-economic case study. Geosynthetic Reinforced Dunes and Bluffs: No Longer Just an Emergency Solution for Shoreline Protection and Improving Resiliency? “Soft” coastal protection systems, such as Geotextile Sand Containers (GSCs) and Geotubes, have been used in emergency or temporary applications for decades to stabilize erosion hot spots and protect homes from extreme storm events. These systems have the potential to protect coastlines, particularly when buried as a core of a dune from larger storm events (e.g. 25 or 50-year storms) while providing flexibility to account for uncertainty in rates of sea level rise and storm frequency. The objective of this paper is to present results of an on-going field study to assess the performance of GSC reinforced dunes in Montauk, NY. Performance of a geo-tube reinforced bluff in Nantucket, MA is also discussed. While laboratory experiments and numerical models have been developed to predict the hydraulic stability of coastal revetments made of GSCs, there is limited performance data of these systems, especially when they are used to reinforce the core of a natural dune system. Field Performance of Refined Dunes for Improving Coastal Resilience Increased coastal erosion rates have forced communities to rethink how to manage vulnerable coastlines. In many locations there is a trend towards implementing temporary engineering solutions, such as geotextile sand containers (GSCs) and geo-tubes, to stabilize erosion hot spots and assess the impact of these designs while long-term solutions are developed. GSCs and geo-tubes have the potential to increase the resilience of natural systems to protect coastlines from smaller storm events (e.g. 25-year storms) while providing flexibility in design considering the uncertainty regarding future rates of sea level rise and storm frequency. The objective of this paper is to summarize the performance of geotextile stabilized coastal sites and present results of on-going field studies to assess the performance of GSC reinforced dunes in Montauk, NY. The better-than-expected, resilient performance of GSCs and geo-tubes at most locations and recent reinforcement of dunes, bluffs, and shorelines in New York, Massachusetts, and Hawaii emphasizes the need for continued field research and in situ monitoring to collect high-quality performance data to better evaluate laboratory experiments and numerical models developed to predict the hydraulic stability of these systems. Fragility Analysis of Dunes Reinforced with Geosynthetic Sand Containers In many coastal communities, encroachment of infrastructure on the natural beach system constrains natural dune volumes and necessitates construction of structural and nonstructural mitigation measures for improved coastal resilience. Nature-based solutions, such as dunes reinforced with geosynthetic sand containers (GSCs), are increasingly being used to avoid the permanency of hardened structures like seawalls and revetments. Dunes reinforced with GSCs have the potential to protect communities from smaller storm events (e.g. 50-year storms) while at the same time providing flexibility in design considering the uncertainty regarding rates of sea level rise and the increasing destructive power of storm events. However, there are currently no accepted design standards for these geosystems, and there is a gap in the knowledge of designers, planners, and decision makers of how to assess the performance and understand the tradeoffs of reinforced dunes for coastal protection systems. The objective of this paper is to present a fragility analysis of a Federally-funded GSC-reinforced dune in Montauk, NY. This dune was constructed in 2016 and experienced significant erosion of the protective berm and sand covering the GSCs during a 1-year storm event that same year. To perform the fragility analysis, field observations of erosion/deposition made over a 3-year period were used to calibrate 2-D and 1-D erosion models using the morphodynamic model XBeach. Damage was quantified as erosion of the berm, exposure of the GSCs, and movement of the GSCs. Monto Carlo simulations of erosion at a critical transect were performed using distributions of surge level and significant wave height for storms with return periods of 1, 2, 5, 10, 20, and 50 years, and the results were binned for each damage state. Movement of the GSCs was estimated using published relationships between a stability number and the surf similarity parameter. The resulting fragility curves are consistent with the performance of the reinforced dune since 2016, including the amounts and frequency of annual replenishment needed at the site. This approach can be incorporated into various hazard mitigation and loss prevention tools, and can better inform all stakeholders about the benefits and drawbacks of adaptive, nature-based coastal protection systems