Oregon Sea Grant

This thesis is comprised of two manuscripts based on a laboratory experiment conducted to examine realistic wave forcing on a vertical wall subjected to tsunami loading. The first manuscript examined tsunami force and pressure distributions on a rigid wall that was fronted by a small seawall. Six different seawall heights were examined, two of which were exposed to a range of solitary wave heights. The same experiment was done without a seawall for comparison. The measured wave profile contained incident offshore, incident broken, reflected broken, and transmitted wave heights measured using wire resistance and ultrasonic wave gauges. Results showed that small individual seawalls increased reflection of the incoming broken bore front and reduced force on the rigid landward wall. These findings agreed well with published field reconnaissance on small seawalls in Thailand that showed a correlation between seawalls and reduced damage on landward structures. The second manuscript examined cross-shore variation of tsunami loading as the loading scenario changed from an impulse to a quasi-steady bore. In this study tsunami force and pressure distributions on a rigid wall were determined experimentally in a large scale wave flume, and forces were examined experimentally at three different cross-shore locations. Incident offshore and incident broken wave heights measured using wire resistance and ultrasonic wave gages, and velocity was measured using acoustic-Doppler velocimeters. Force and pressure profiles were measured using load cells and pressure transducers. At each cross-shore location, the force and pressure profiles showed an impulse peak followed by a period of sustained force. This type of profile was seen for each wave height tested, and as expected as wave height increased the value of the maximum impulse force also increased. By examining force time histories, it was also found that while the hydrostatic pressure distribution accurately depicted the force profile during the period of sustained force it was significantly less reliable during the impact period. It was found that as the rigid wall was moved further offshore the peaks were less pronounced and the corresponding maximum impulse force values decreased. Thus, as the wall was moved onshore the loading scenario transitioned from impact to a quasi-steady bore-like loading condition. The sustained forces measured experimentally verified the empirical formula for steady state force presented by Iizuka and Matsutomi (2000). This theoretical formula was also presented by both the FEMA Coastal Construction Manual (2000) and the City of Honolulu Building Code (2003) as the “hydrodynamic force.”