PhD Defense: Innovative Coastal Defense in an Idealized Barrier Reef System

Shijie Huang

PhD Candidate
Department of Ocean and Resources Engineering
University of Hawai’i at Mānoa

**This defense will be held both in person (Holmes Hall 287) and over Zoom**
Meeting ID: 862 6019 2680
Passcode: ShijiePHD

Climate change has dramatically exacerbated coastal erosion, primarily through the effects of sea level rise, the intensification of tropical storms, and the increased frequency of high-wave events. These processes pose a severe threat to the tourism-based economy and the densely populated coastlines of Hawai‘i, where coastal erosion has become an urgent issue. Compounding the problem, traditional coastal defense structures—such as seawalls, breakwaters, and revetments—have proven ineffective for the unique environmental conditions of Hawaiian waters. Given the economic and environmental stakes, there is an immediate need for coastal engineers and researchers to propose and implement innovative, site-specific defense strategies to safeguard Hawai‘i’s valuable shorelines.

The Hawaiian shoreline is generally well protected by extensive coral reefs in nearshore waters. These reefs serve as natural breakwaters, forming shallow water barriers that dissipate wave energy through wave breaking and by increasing friction against wave propagation. However, not all of Hawai‘i’s coastlines benefit from continuous reef protection. Gaps between reef patches, known as channels, leave certain beaches exposed to high-energy waves and offshore currents. The combination of high wave energy and offshore current can mobilize beach sediment and transport it offshore, accelerating beach erosion in vulnerable areas. 

Together with the lagoons frequently found in front of Hawaiian beaches, the reef, channel and the lagoon forms a typical topographic characteristic of the barrier reef system. This dissertation aims to deepen the understanding of hydrodynamic processes within a barrier reef system and to elucidate the beach erosion mechanisms associated with these processes using Computational Fluid Dynamics (CFD) simulations. This detailed analysis will inform the development of tailored, innovative solutions to mitigate beach erosion in channel-fronted areas.

Two specific solutions are explored and tested. The first one is a pile breakwater system, consisting of an array of cylindrical piles strategically placed to dissipate and reflect wave energy. This method offers a cost-effective alternative to conventional hard structures, particularly in the deep waters found within reef channels, where traditional approaches are both expensive and less effective. The second proposed solution is a dual-functional wave energy farm that provides both coastal protection and renewable energy generation. This system will utilize an array of Oscillating Water Columns (OWCs)–one of the most extensively studied wave energy harvesting devices—to form an OWC-pile breakwater. By combining shoreline protection with energy production, the OWC-pile breakwater system is well-suited for application in Hawai‘i, where there is an urgent need for effective coastal defense and a growing demand for sustainable energy solutions.