A Laboratory Study on the Effects of Submerged Vertical and Semicircular Breakwaters on Near-Field Hydrodynamics and Morphodynamics.

A laboratory study is presented herein that investigates the effects of submerged vertical and semicircular breakwaters on near-field hydrodynamics and morphodynamics. Breakwaters are employed worldwide in order to reduce destructive wave forces often imparted on vulnerable shorelines; a task partially completed by reflecting incident wave energy back out to sea. However, two common side-effects of their function are the occurrence of scour at the breakwater, which can lead to structural problems; and offshore ripple formations, which are influential in scour protection failure and sediment transport patterns. This study aims to determine a relationship between breakwaters and the wave reflection coefficient (percentage of incident wave energy that is reflected out to sea), resultant scour along the base of the breakwater, and consequential offshore morphology. The initial phase of the investigation was a dimensional analysis study that yielded important parameters. The wave reflection coefficient was determined to rely only on a relative submergence parameter, defined as aHi, where a represents the depth of water above the crest of the breakwater and Hi represents the incident wave height. Parameterizations for the reflection coefficient were derived for each breakwater type (vertical and semicircular) and share the same functional dependency on the relative submergence parameter, yet include different constants. Therefore, an efficiency factor was developed in order to compare the semi-circular breakwater reflection coefficients to those of a vertical breakwater. It is important to note that as aHi goes to zero, the two breakwater types reflect identical percentages of energy, and therefore, the efficiency factor goes to unity. Even though semi-circular breakwaters possess a clear advantage in terms of stability in the wave field, vertical breakwaters are more efficient at reflecting wave energy. Due to tidal variations, engineers will need to determine which breakwater is superior for specific coastal conditions. The next phase of the study included measuring onshore breakwater-induced scour. However, prior to measuring scour geometry, it was determined that two scour regimes occurred: attached and detached. Attached scour connects directly to the onshore face of the breakwater while detached scour lies separate from the breakwater. The Keulegan-Carpenter number (KC) is critical in regime placement and it is concluded that for KC values less than p, attached scour occurs; for KC values larger than p, detached scour occurs. Dimensional analysis also led to the findings that onshore scour depended only on the mobility number (psi) and (KC). Scour depth, S max, was concluded to rely on psi and KC and was not regime dependent while scour length (Ls) and the distance of Smax from the onshore breakwater face (Ds) relied only on KC and were regime dependent. Another important conclusion is that scour characteristics were not breakwater shape dependent. The final aspect of the study was to qualitatively assess the offshore bedforms, which illustrated the partial standing wave system created by the breakwater. Offshore morphology, which is dependent on the nodal and antinodal near-bed velocities, occurred as plateaus in low near-bed velocity areas and ripples in high near-bed velocity areas. The presence of the breakwater limited onshore ripple migration which was evident from the lack of accumulation on the offshore breakwater face. Scour and ripples have caused breakwaters and scour protection to fail due to overturning, sliding, and undermining. Therefore, knowledge of these processes is crucial in the design of efficient...