Wave energy dissipation in Great Lake harbor entrances.

Abstract

Many navigational harbors on the Great Lakes use parallel jetty entrances that were historically constructed of rock-filled timber cribs. Over time they have become deteriorated and have needed rehabilitation. The typical rehabilitation procedure was to encase the original structure with sheet pile and concrete. After rehabilitation, there was a perception of increased wave heights in the entrance channel. To remediate the more energetic wave climate, sections of sheet piling were recessed at selected harbors and then filled with sloped stone, thereby creating a pocket wave absorber. The goal of this research is to determine which physical parameters are important in determining wave energy dissipation caused by pocket wave absorbers. This was primarily accomplished by performing a hydraulic model study. Additionally, field data was collected at several harbors to quantify the performance of existing pocket wave absorbers and to validate laboratory results. Field measurements were found to be in good agreement with laboratory results. Experimental results from the hydraulic model study determined that a majority of the parameters evaluated had very little effect on wave energy dissipation. Uniform stone size, slope of stone interface, incident wave height, and water level all had minimal effects on dissipation when evaluated within practical design limits. Of the variables considered, only pocket geometry (length and width) and porosity had a significant effect on the amount of observed dissipation. The most important discovery from this investigation was the multiple effects that a pocket wave absorber has in dissipating wave energy. As waves travel past the offset pocket, energy is transported laterally into the pocket via diffraction thereby reducing wave heights in the channel. However, the pocket is more effective at dissipating wave energy than pure diffraction. The pocket dissipates additional energy from the fluid motion over the sloped porous surface. When the waves flow over the sloped stone interface, energy is being dissipated both externally (surface friction) and internally (turbulent flow through the pore space). Results indicate that both contribute to energy dissipation in about the same order of magnitude, but that internal turbulence has a higher degree of variability.