Wave reflection, run-up, run-down and pressures on seawalls defended by an offshore breakwater

Abstract

The hydrodynamic performance in terms of wave reflection, run-up and run-down and wave pressures on plane seawalls protected by an offshore breakwater has been studied in a random wave field. Plane seawalls with different slopes (i.e. q = 90 degrees, 60 degrees and 30 degrees) and for different water depths relative to the height of the offshore breakwater covering both the submergence and emergence of the offshore breakwater are used. The hydrodynamic performance of a plane seawall without an offshore breakwater is compared with one protected. For 2% probability of exceedence, the run-up on the seawall can be reduced by about 50 - 60%, when there is a detached breakwater with a crest level at the still water level. For the same condition, the wave pressure on the seabed near the seawall can be reduced by about 35 - 40%. With a detached breakwater of 20% emergence in air, the wave pressure on the seawall can be reduced by about 80 - 85% when compared to the wave pressures without any protection. The present study reveals that when the crest of the breakwater is submerged by about 14% of the water depth, the configuration induces a water jetting effect over the detached breakwater and causes higher wave kinematics in front of the seawall and hence such conditions are recommended to be avoided in the field.