Date of Award

2025

Degree Type

Thesis

Degree Name

Master of Science in Ocean Engineering

Department

Ocean Engineering

First Advisor

Annette A. Grilli

Abstract

Coastal vulnerability along Block Island’s east coast motivates the need to evaluate effective, nature-based shoreline protection. This study uses a process-based numerical model, validated with field measurements, to assess the hydro-morphodynamic response of this highly exposed coastline and to evaluate the performance of artificial reefs/submerged breakwaters (SBWs) under storm and mean-wave conditions. First, site dynamics are examined using Hurricane Sandy and one-month mean-wave simulations. Then, eighteen multi-segment SBW configurations - varying in offshore distance (300-500 m), crest elevations (-0.75 m-0.75 m), and widths (300 m, 500 m), with a constant gap-to-width ratio of 0.4 - were designed to protect Crescent and Scotch Beaches and tested under both forcing conditions. Hurricane Sandy simulations underscore the strong influence of the site’s concave bathymetry on wave transformation, with pronounced energy dissipation across the 5-10 m surf zone and wave-surge-tide interactions shaping erosion timing and hotspot development. Under one-month mean-wave conditions, breaking and tidal variation modulate bed-level changes and shape the shoreline. Nearshore flow fields exhibit transient four-cell circulations under moderate forcing that collapse into two-cell structures at peak storm energy, illustrating the limitations of simplified theoretical design frameworks for this site. The effectiveness of SBWs depends strongly on design and local wave climate. Submerged structures dissipate storm-wave energy through breaking and promote lee-side accretion under moderate waves, whereas emerged structures provide greater energy reduction but can intensify gap flows and local scour. Under mean-wave climate, partially emerged designs more effectively reduce wave transmission, while deeply submerged structures under-perform. Overall, SBWs reduce peak erosion and dampen short-term variability, though benefits may be offset by gap-induced scour and alongshore sediment redistribution. Optimal performance requires careful tuning of crest height, length, and placement, supported by complementary sediment-management strategies and adaptive monitoring.

Creative Commons License

Creative Commons Attribution-No Derivative Works 4.0 License
This work is licensed under a Creative Commons Attribution-No Derivative Works 4.0 License.

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