Dynamics of water waves over fringing reefs

• Main Author: Luk, Max Sau Chiu
• Other Authors: Lo Yat-Man, Edmond
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering
• Online Access: http://hdl.handle.net/10356/60022
• Institution: Nanyang Technological University

Coral reefs contains a rich mixture of marine organisms and are natural habitat for more than 2 million various species and it mount up to a quarter of percentile of all marine life on Earth (Hearn, C.J., 1999). Example of a coastal region of the Belize Shelf (Jordan, 2002) is shown in Figure 1 for a clearer illustration of the various features of reefs. Coral reefs are commonly found in shallow tropical coastal regions and provide locations where a substantial proportion of surface wave energy is dissipated through wave breaking and bottom friction processes. As such, knowledge of wave processes is an essential part of coral reef hydrodynamics (Yao, 2012). Waves proves to be the primary energy source for the movement of water throughout reefs area. Also, waves are vital for the relocation of the sediments. Wave-driven circulation drives food over the reef, eradicates metabolic wastes of reef-building organisms and plays a key part in the mechanical cleaning of the coral polyps. As there are more studies and researches being carried out, there is a growing significance of wave as the main energy source of the reef ecosystem and it plays an important role in the consistency as well as the development in coral reefs. Waves breaking on a reef create a radiation stress gradient that drives wave setup and wave-induced currents (Yao, 2012). These phenomena demonstrated the impact on the hydrodynamics of shallow submerged coral reefs. The wave-induced processes are heavily influenced by the tidal evolution of water depth over a reef. (Yao, 2012). The pioneer study of waves in coral reefs was led by Munk and Sargent (1948). From wave height observations over an atoll they estimated 95% wave-energy dissipation for waves breaking and traveling across the reef flat. More recent field studies found values of 75% to 86% wave energy reduction. At low tide the energy dissipation increased, by as much as approximately 10%. Dissipation of wave energy was not uniform across the spectrum: higher frequency waves lost more energy than those with lower frequencies (Yao, 2012). Other major factors that influence energy reduction were reef geometry, length and morphology. As waves imposing on reefs are modified, they induce changes in reef morphology and on the coral themselves (Yao, 2012). Individual coral colonies align with the direction of incoming waves and the morphology of the reef adjusts to the prevailing wave conditions (Munk and Sargent 1948). These alterations allocate and scatter wave energy to levels that diminish mechanical damage. Wave refraction and energy dissipation produce wave height and energy gradients that facilitate the segregation of organisms into zones and habitat development in the reef (Yao, 2012). Wave-energy intensity is vital for sustaining the reef community. Different reef profiles and the characteristics of the incident waves, see Figure 2, do play an important part in the hydrodynamics of wave breaking over coral reefs (Yao, 2012). A distinct differentiation between a submerged fringing reef and an emerged plane beach is that the fringing reef are commonly made up of a reef flat with constant water depth. It slows down the process of wave breaking. Also, it tends to shift the incipient breaking point towards the shore or cause wave breaking and resulting near-shore currents to differ from those of a plane beach (Yao, 2012). Reef flat functions like a semi-infinite broad weir which imposes hydraulic control on the amount of water leaving the reef flat if the water above it is very shallow (Yao, 2012). Unlike beaches, coral reefs contains a steep rapid drop. Not much studies have been worked on wave-breaking over reefs, thus a report done by Smith and Klaus (1991) proved to be rather valuable. In his research, he utilised immersed bars as well as man-made rides of various dimensions to mimic wave-breaking over reefs. The results are apparent, waves of identical characteristics tend to behave or break in a different way on plane beaches and reefs. Significant differences were found in properties such as breaker type, breaker height index, breaker depth index, plunge distance and the splash distance. In particular, it was found that the transition values of the surf similarity parameter for the classification of breaker type suggested by Battjes (1974) were invalid for barred profiles, and waves of moderate steepness that plunge on plane slopes tended to collapse when breaking on a bar/reef structure (Yao, 2012). Wave transmission past both submerged and low crested breakwaters has been examined by many researchers including Ahrens (1987), Seelig (1980), van der Meer (1990), Allsop (1983) and Friebel and Harris (2004), and was found to be dependent on the relative water depth over the reef crest, incident wave conditions, relative crest width and the nominal diameter of the armour stone used for breakwater construction (Yao, 2012). It is worth mention that the transmitted wave height particularly demonstrated a strong relation on the relative submergence of the crest. At the same time, it drops as the water depth over the reef crest decreases too. Wave height decay are commonly caused by energy dissipation during wave breaking, thus such phenomena indicates that due to the change in reef submergence, there will be a difference in the landscape of the breaking event. A more recent study was done by Blenkinsopp and Chaplin (2008) on the effects of relative reef-crest submergence on wave breaking over a submerged, truncated 1/10 slope. The results derived indicated that the reef-crest submergence was a leading factor affecting wave breaking, along with transmission and reflection characteristics (Yao, 2012).