Investigating Time-Scale Effects and Maximum Scour Depth around Structures during Tsunami-Like Wave Propagation

Tsunami waves propagating overland and in coastal rivers damage structures such as buildings or bridge piers due to impact forces and scouring phenomena. Prior studies evaluating the scour depth around the structures do not consider time-scale effects because of the laboratory limitations in generating long tsunami-like waves. In this study, experiments were conducted in a 12-m-long channel using the dam-break technique, with the scour compared for two durations of the flow. The dam-break wave was followed by a sudden opening of the inlet valve of the flume, which resulted in a continuing tsunami-like wave-induced flow. In the first case, the duration of the wave flow using this technique was 300 s, which considers time-scale effects when following Froude scaling. Six consecutive tsunami waves of 300 s duration each were generated in Test 1 since a tsunami wave train usually consists of 3–11 waves. In this test, the scour depth around a circular pier model (of 0.05 m diameter) was measured to be 2.52 times the pier diameter (D), which is much higher than the previously reported scour depths in the literature for either tsunami or breaking waves in the surf zone. This scour depth was compared with the second experiment (Test 2) when the duration of each of the six tsunami waves was only 30 s. The scour depth measured in Test 2 was 65% lower than in Test 1, highlighting the time-scale effects and the significant influence of the tsunami wave duration on the scour process. The influence of a pre-existing scour hole was also investigated. Thus, the present results can help improve the scour design criteria for coastal structures like bridges and vertical evacuation towers.