Coastal Road Slope Collapses Behind a Retaining Wall Due to Scour and Erosion

Abstract

The present study investigates the coastal road slope disasters that took place in December 2014 and in November 2021 in Hokkaido, Japan. The coastal disasters represent collapses behind the retaining walls under high wave conditions. The results of the field investigations demonstrate that the extent of the damages was significantly larger in the 2014 disaster than in the 2021 disaster, and this difference was closely related with the presence and absence of overtopping. Based on the estimated offshore waves, a series of hydraulic wave flume experiments were conducted so as to gain insights into the disaster mechanisms. The combined field and experimental results show that the collapses were the consequence of coupled surface erosion, scour, and internal erosion beneath and behind the retaining walls. The overtopping enlarged the extent of the collapses. The paper also discusses an effective countermeasure meant to suppress such coastal road slope disasters behind retaining walls.