Date of Award

2022

Degree Type

Thesis

Degree Name

Master of Science in Ocean Engineering

Department

Ocean Engineering

First Advisor

Annette Grilli

Abstract

Coastal erosion poses increasing risks for coastal communities and natural habitats due to the rising sea level and extreme weather conditions. The Ninigret Trustom Pond (NTP) beach barrier system, located along the southern shore of Rhode Island, USA, is exposed to tropical storms, hurricanes, and Nor’easters, which result in recurrent dune breaching, beach house destruction, and a general increase in vulnerability of the coastal community located landward of the beach barrier system. Therefore, assessing the feasibility of hypothetical mitigation strategies to protect the vulnerable local coastal communities is a priority. Among the favored Natural and Nature Based Feature (NNBF) strategies, artificial reefs or submerged breakwaters (SBWs) are used to reduce the incident wave energy; they aim to protect the shoreline by shifting wave-breaking processes further offshore, thus reducing erosion during storm events while also providing ecological benefits. Based on the findings of Ranasinghe et al. (2010), van der Baan (2013) developed a design criterion for multiple shore-parallel SBWs that includes geometrical, hydrodynamic, and morpho-dynamic parameters. However, testing modified hydro-dynamic boundary conditions and field investigations are necessary to validate the criterion. Thus, the objectives of this thesis are to assess (1) the ability of SBWs to protect the NTP beach barrier system and (2) the applicability of van der Baan’s (2013) design criterion in the field.

Based on a review of the physical processes and functional design criteria of SBWs and a study site analysis, nine SBW concepts are selected, primarily focusing on van der Baan’s (2013) design criterion. The concepts are tested against average and storm wave climates and sea level rise scenarios using the numerical model XBeach to evaluate wave energy reduction, the hydrodynamic response, and the short-term and long-term morpho-dynamic response.

Since the results imply circulation patterns around the SBWs that differ from the ones documented in previous research for controlled conditions (regular bathymetry and incident waves normal to the shoreline), we cannot conclude on the applicability of van der Baan’s (2013) design criterion in the field. However, we identify two characteristic circulation patterns for oblique wave incidence and a correlation between these patterns and the mode of shoreline response, which corresponds to the correlation van der Baan’s criterion is based on. The results identify an optimal SBW mitigation concept that significantly reduces overwash and beach erosion during storm events, including a potential SLR up to 1 foot, resulting in an accretive long-term mode of shoreline response and maintaining the original shape of the coastline. While these results are encouraging in affirming the concept’s protective ability of the shoreline, further validation of the design is in process using additional physics-based numerical simulations with a phase-resolving wave model. In addition, it is important to note that using such a concept as a potential acceptable mitigation strategy at the site would need to expand the study to a full ecosystem services cost-benefit analysis.

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