Submerged Breakwater Modeling and Coral Reef Ecological Analyses for Harbor Protection
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2015-05
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[Honolulu] : [University of Hawaii at Manoa], [May 2015]
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This dissertation provides insight into the previously unexplored option of using submerged breakwater structures to provide harbor protection while simultaneously providing ecological and recreational value as coral reefs. Chapter 1 provides background on the concept of multifunctional reefs and how they can be used to protect the Kahului Harbor on the island of Maui, Hawai‘i. In Chapter 2, a numerical wave response model was developed to determine how a multifunctional reef of various dimensions and orientations would influence the wave energy inside the Kahului Harbor. The numerical model results were validated with a physical scale wave response model as discussed in Chapter 3. Further numerical modeling to explore the effect of various reef shapes are discussed in Chapter 4. In Chapter 5, the costs and benefits of various coastal engineering technologies for the design and construction of the submerged breakwater structure were considered. In Chapter 6, the relationship between substrate characteristics and coral colonization was investigated through coral recruitment experiments and study of field conditions in order to develop the biological information needed in the ecological engineering of the artificial coral reef. The significant findings from this dissertation research are summarized in Chapter 7.
The Kahului Commercial Harbor 2030 Master Plan recommends modernizing Maui’s only commercial harbor to accommodate projected growth in cargo and passenger traffic. The planned $345.1 million in harbor developments include the West Breakwater Harbor Development, which requires the dredging of an expanded turning basin and the construction and extensions to the west and east breakwaters to allow vessels to berth without risk of surge. Previously, the Kahului Harbor 2025 Master Plan proposed similar breakwater extensions and developments. A U.S. Army Corps of Engineers study modeled the effects that the proposed developments would have on the wave energy in the Kahului Harbor. The study indicated that none of the configurations of the breakwater extensions were ideal for reducing the wave energy in the harbor. The study especially argued against the landward extension of the west breakwater due to the likelihood that the structure would increase wave oscillations inside the harbor. The updated 2030 Master Plan proposed similar breakwater extension configurations that are likely to also create surge problems inside the harbor. The modernization plan will also have a significant impact on the marine habitat and recreational activates in the Kahului Harbor area. A portion of the coral reef inside the harbor will be removed by the dredging of an expanded turning basin. This reef is of significant value to the Maui community because of its environmental and recreational value. The reef forms three popular surfing waves that are within close proximity to large residential areas, schools, and the community college. The destruction of the surfing resource is likely to cause negative social and cultural impacts in the community. The modernization plan has been publicly protested by community organizations opposed to the removal of the reef.
This dissertation research introduces the concept of a multifunctional reef as a means to at least partially mitigate the significant impacts of the modernization of the Kahului Harbor. A multifunctional reef is an engineered structure that alters the bathymetry of the sea floor to supplement the benefits that naturally occurring reefs provide to a coastal area or partially mitigate reef damage. Benefits of building a multifunctional reef include coastal protection, marine habitat, and a venue for recreational activities such as surfing, fishing and diving. The proposed multifunctional reef located on the ocean side of the Kahului Harbor’s east breakwater would serve two primary functions. First, a sophisticated reef design would shoal and refract approaching waves before they enter the harbor, thereby reducing the amount of wave energy that disturbs harbor operations. In this sense, the multifunctional reef will function as a coastal engineering structure known as a submerged breakwater, so the terms are used interchangeably in this dissertation. Second, the multifunctional reef would provide significant ecological value by being specifically designed to provide habitat for coral reef species. This function is commonly known in marine biology as an artificial reef, so the multifunctional reef is also referred to as such in this document.
A numerical modeling investigation was conducted to evaluate the installation of a submerged breakwater near the entrance of the Kahului Harbor. The breakwater is intended to improve the harbor’s tranquility and, thereby, increase the harbor’s operational days. The objective of the numerical modeling investigation was to propose the optimal location, orientation and dimensions for the layout of a submerged breakwater outside the Kahului Harbor. To this end, the wave characteristics in the vicinity of the harbor were analyzed and the model was used to investigate the wave transformation as they propagate towards shore. The tranquility conditions at various locations along the harbor approach channel and inside the harbor basin were evaluated using phase-resolved nearshore wave modeling. The model results for tranquility inside the harbor and along the approach channel under existing conditions versus conditions in the presence of a submerged breakwater were compared.
Based on a comprehensive wave data analysis, the numerical study used a predominant wave direction of 17.5° (east from north), wave height of 1 meter (m), and a wave period of range 4 to 18 seconds (s). The study considered the orientations of a submerged breakwater with respect to the geographic north that varied from 105° to 150° in steps of 15°. The length, breadth and depth of submergence of the proposed breakwater in the model was varied from 100m to 150m, 7m to 21m, and 1m to 5m, respectively, and the results for the wave transmission coefficient inside the harbor and along the approach channel were calculated for each case. The investigation critically reviewed and compared the results obtained from all the combinations of orientation, length, breadth and depth of submergence of the breakwater structure. The results of the numerical model investigation indicated that the optimal Kahului Harbor submerged breakwater will have a length of 100m, width of 15m, submergence depth of 1m, orientation of 135°, and be placed 280m offshore from the existing eastern breakwater. A submerged breakwater of this design was found to reduce the average wave diffraction coefficient (kD) by 38% in the harbor basin and increase kD along the approach channel by 4% for long-period waves (T=11 to 18 seconds). For short-period waves (T= 4 to 10 seconds), the reduction in the average kD was found to be 0.9% and 16% along the approach channel and within the basin, respectively.
A scale physical model study on the wave penetration into the Kahului Harbor was carried out in a 15m wide, 17m long and 1m deep wave basin at the Department of Ocean Engineering, IIT Madras, India. The objective of the physical modeling exercise was to validate the results of the numerical modeling in terms of how the tranquility inside the Kahului Harbor is increased by placing a submerged breakwater structure offshore of the harbor entrance. The submerged breakwater location, orientation and size defined by the results of the numerical model were reproduced in the physical model basin. The bathymetry of the area was also carefully reproduced in the physical model. The conclusions drawn from this physical model study showed that there was a statically significant agreement between the results of the physical and numerical models, and that construction of the submerged breakwater can be expected to cause an average reduction in wave transmissions of between 3% and 15% in the navigation channel and between 20% and 35% in the harbor basin. Additional numerical modeling was carried out to determine how other options for the submerged breakwater design shape affect the ability of the structure to attenuate wave energy propagating into the harbor basin while simultaneously providing secondary functions of enhanced marine habitat and surfing recreation resources. The wave response of the additional submerged breakwater concepts were investigated using the CGWAVE finite-element model interfaced with the Surface-water Modeling System (Version 11.1). Three designs were investigated: the IIT Madras reef concept developed in a previous numerical modeling study, and two new concepts, reef alternative 1 and 2. The reef alternative 1 concept consisted of a shore-connected reef, linked by a sloping spur from the existing east breakwater head which descends to the reef crest elevation of -1m (MLLW). The reef alternative 2 concept is completely submerged and located offshore of the existing east breakwater head like the IIT Madras reef. Unlike the Madras reef, reef alternative 2 has a chevron-shape footprint and shallow slope on the offshore face (4:1, horizontal to vertical).
Based on the results of the calculated wave height responses, this numerical study found that out of the three alternative configurations, reef alternative 1 demonstrates the highest degree of effectiveness in reducing average wave heights within the harbor. The model results also showed that the IIT Madras reef and concept reef alternative 2 each provided average wave height reductions within the central and eastern portions of the harbor, but in the vicinity of the harbor mouth, these two designs produced increased average wave heights. These two designs also increased the wave heights at the tip of the existing east breakwater, while alternative 1 showed a reduction in wave heights all along the breakwater. The results of this analysis indicated that while alternative 1 is most beneficial, all three of the studied reef designs are effective at reducing average wave heights within the harbor area.
This dissertation also reviewed and analyzed the design alternatives for the installation of the submerged breakwater at the Kahului Harbor. The submerged breakwater size and location as defined by the results of the numerical and physical modeling studies informed how the structure should be constructed in the field. The conceptual designs of rubble-mound, caisson, floating and geotextile-tube breakwater structures were considered in this analysis to develop perspective on the suitably of these various design options. The analysis considered aspects of each option in terms of hydrodynamic performance, structural design stability, construction logistics and procedure, execution time, and rough cost. The results of this exercise concluded that compared to other alternatives, using geotextile tubes seems to be the less attractive alternative, because it would result in extended execution time and significant increases to project costs. For the given site-specific depth of installation (~15m) of the proposed breakwater layout, a caisson-type breakwater would be the most efficient alternative. A prefabricated concrete caisson structure is easier to install and more cost-effective because it requires a smaller body width and less material. The prefabricated caisson approach also fits the five-month window for installation dictated by the local wave climate.
While the numerical and physical modeling studies showed how the structure can be expected to mitigate operational problems caused by wave energy entering the harbor, the question remained of how a large marine structure can be designed to provide coral reef habitat in order to increase recreational and environmental value of the project. There is limited information on how the design of coastal structures can be manipulated to enhance the ecology of targeted coral communities. The relationship between substrate characteristics and coral colonization was investigated through coral recruitment experiments and study of field conditions in order to develop the biological information needed in the ecological engineering of the submerged breakwater. Three concrete compositions that differed by the use of basalt, limestone, or recycled aggregates were tested in field and laboratory experiments to determine the impact of each substrate on the recruitment of juvenile hermatypic corals. The concrete test plates were deployed in three environments for a period of about one year, after which the coral recruits on each plate were identified and counted. The field trials at two sites of different depths showed no significant difference in the average number of coral recruits on the concrete mixed with basalt, limestone and recycled aggregate (60 ± 9, 83 ± 17 and 76 ± 14, respectively). However, the laboratory results versus field results showed significant differences in coral recruitment between the laboratory tanks, deep water, and shallow water field tests environments (86 ± 11, 135 ± 15 and 4 ± 1, respectively). These results highlight the importance of environmental site conditions for the development of coral reef habitat. When designing the submerged breakwater for coral reef ecology, effort should be spent on enhancing the environmental conditions at the site so that they are favorable for recruitment and growth of hermatypic corals.
A field study was conducted in the vicinity of proposed artificial reef site at Kahului Harbor in order to relate the topographic features of the surrounding environment to the levels of live coral coverage. These data provide information on what types of communities will develop on the proposed submerged breakwater. The benthic zone was surveyed using a drop camera system and by SCUBA divers who recorded in-situ observations. Of the area surveyed, the highest density of coral coverage (>90% cover on 60% of the area) was found on an adjacent natural reef area that was characterized by spur and groove geomorphology with a high degree of macro- and micro-topographic complexity. In contrast, sparse coral cover was discovered on the concrete armor units of the existing east breakwater structure. No live coral cover was observed on the sand and carbonate rubble substrate at the proposed artificial reef location. The high coral coverage on the adjacent natural reef suggests that the artificial coral reef design should emulate the natural spur and groove structure with regards to topographic complexity on multiple scales, orientation with wave direction, and water depth.
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Ph.D. University of Hawaii at Manoa 2015.
Includes bibliographical references.
Includes bibliographical references.
Keywords
multipurpose artificial reef, multifunctional structure, coastal engineering, shoreline protection, wave energy mitigation, numerical wave response model, physical wave basin study, armor units, marine concrete mix design, coral recruitment, benthic survey, topographic complexity, marine construction, caisson breakwater, environmental enhancement
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Theses for the degree of Doctor of Philosophy (University of Hawaii at Manoa). Civil & Environmental Engineering
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