Influence of Permeability of Rubble Mound on the Erosion Process in a Seabed under Breakwater due to a Tsunami

Abstract

Tsunamis cause scour and erosion on bearing grounds, resulting in ground instabilities that lead to damage to coastal structures. For example, with regard to the damage to breakwaters due to the tsunami from the Great East Japan Earthquake of 2011, scouring around these structures has been reported to be a cause of structural failure. However, the actual mechanism underlying such damage has not yet been clarified. Therefore, this study focuses on the scour and erosion of the seabed under a breakwater, specifically, as a result of bottom shear and seepage forces for different permeabilities of the rubble mound. In a flume channel experiment, seepage was investigated using point data measured on a pore water pressure sensor. To confirm the occurrence of spatial seepage in the mound and seabed, seepage analysis was conducted using finite-element analysis. The seepage was examined via a detailed comparison of experimental and numerical results. The results of the experiment show that scour and erosion of the seabed vary with respect to the permeability of the rubble mound. If the rubble mound is highly permeable, scouring of the seabed will occur because of seepage flow in the rubble mound. On the other hand, if the rubble mound is composed of a low-permeability material, seepage failure is the likely result. Seepage will also occur on the seabed following variations in water-level difference with a caisson. This suggests that the seepage will flow on the edge of the caisson on the outer harbor side and out to the edge of the caisson on the inner harbor side. Vertical motion of the seepage is generated on the seabed surface, and with increases in the permeability of the rubble mound, the upward hydraulic gradient in the inner harbor side of the caisson also increases. This study clarifies the relationship between normalized friction velocity and averaged hydraulic gradient using experimental data for erosion on a seabed surface under the rubble mound of a breakwater structure. As a result, a nondimensional index is proposed for the erosion initiation process on a seabed under a breakwater structure using the normal friction velocity and averaged hydraulic gradient.