Laboratory Measurements of Wave Forcing and Reactions on a Model Submerged Mesh Breakwater
Knoll, Alex Baxter
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The purpose of this thesis is to design the test setup, perform the model tests, and process the data to find the wave forces on a submerged breakwater. Breakwaters are coastal protection structures used to help prevent the erosion of the shoreline. Submerged breakwaters are a more recently developed type of offshore detached breakwater that does not interfere with the ocean view and still helps prevent the longshore transport of sediment. For submerged breakwaters that are made of a single structure and placed on the seafloor, the forces must be estimated to ensure proper anchoring. The prototype submerged breakwater that was examined was a 40 ft long 14 ft diameter half cylinder. The model tested in the laboratory was a 4:1 scale, with a model diameter of 3.4 ft and a length of 10 ft. The estimated forcing on the structure came from measurements during model testing in the Haynes Coastal Engineering Laboratory wave tank. Both regular sinusoidal waves and irregular waves were generated. The significant wave heights ranged from 0.75 ft to 1.0 ft and wave periods varied from 2.0 sec to 3.0 sec using a JONSWAP wave spectrum with a peak enhancement factor of 3.3. Theoretical calculations were completed to help size the instrumentation needed to conduct the experiments. Theoretical calculations were based on the drag law to obtain the force, and Airy as well as Stokes 2nd Order wave theory to obtain the orbital velocities. It was found that the maximum expected load for all cases at the anchor without any factor of safety (FOS) is 38 lb in shear and 9.9 lb of uplift force. This equates to prototype forces of 2,433 lb of shear and 633 lb of uplift force. Using a FOS of 2.0 the prototype horizontal reaction force required is 4,866 lb and the prototype uplift reaction is 1,266 lb. A total prototype reaction force of 5,028 lbs per anchor is needed. These anchors could be gravity anchors, driven piles or screw anchors.