Tsunami Inland Structures Interaction and Impact of Floating Debris

Abstract

Civil engineers have long known that large forces from tsunamis can be imposed on coastal structures. This was made obvious by the recent 2011 Japan tsunami, which also emphasised that the risk of tsunami damage is no less significant than the risk of earthquake damage. New Zealand, due to its exposed location, is also at high risk of tsunami impact by local-, regional-, and distant-source tsunamis. Although some efforts have been made to determine tsunami loads on structures, there are still discrepancies among the limited number of published design guidelines. Data on actual tsunami loads on structures are rare, hence physical modelling of the interaction between a tsunami and a structure is vital to explore the problems associated with tsunami impact, and thereby to improve the existing knowledge of tsunami engineering. This study comprises an experimental investigation of tsunami bore interaction with structures located near coastlines. Specific aims of the study were accomplished by investigation of: (1) the impact of a tsunami bore on a square prism structure having different orientations to the flow direction; (2) the impact of a tsunami bore on a cylindrical structure; (3) the effect of structure stiffness on tsunami bore impact; (4) the impact of a tsunami bore on an elevated structure, in which a square prism structure base was raised at different elevations above the ground (‘base elevation’); (5) the tsunami-borne debris impact on structures through development of a new technique, utilising a smart debris device; (6) factors affecting debris impact forces. The experiments were conducted in a 14m long, 1.2m wide and 0.8m deep wave flume, separated from a reservoir using an automatic gate. The gate was designed to rapidly release water into the flume to simulate a tsunami bore. Measurements were made of the bore heights and velocities, based on which an empirical equation, with the basic form of the gravity wave velocity equation, is proposed for calculating the bore propagation velocity. For all structures, forces and moments were measured at the base of the structure, using a multi-axis load cell. For forces and pressures exerted on a square structure, a square prism with 300mm×300mm cross-section and 600mm height was made from acrylic sheets. The vertical distribution of bore induced pressure was measured on the front face of the square prism. The effect of structure orientation on the forces and pressures was investigated, with the structure’s front wall oriented at 30º, 45°, 60º, 90° and 135º to the flow direction. The bore induced forces were theoretically computed assuming that the total stream-wise force is due to the hydrostatic force plus the hydrodynamic force, and the upward force is due to the buoyancy only; the results were validated using the experimental data. Drag coefficients for the square prism at different orientations ranged from 1.15 to 1.65. For forces and pressures exerted on a cylindrical structure, a cylinder with 300mm outer diameter and 600mm height was made from acrylic. The vertical and angular distributions of bore induced pressure were measured on the front face of the cylindrical structure perpendicular to the flow direction, and at 30º, 45°, 60º, 90° and 135º to its original alignment. Similarly to the square prism, the bore induced forces were computed and validated using the experimental data, giving a drag coefficient of 0.65. For forces and pressures exerted on different structure stiffnesses, four different square prism structures, each with 300mm×300mm cross-section and 600mm height, were made from various thicknesses of acrylic sheets. Deflection of the top of the square prism structures was measured, with the deflection angles ranging from 0.01° to 6° with respect to the flume vertical axis. The structure deflection increased with decreasing stiffness of the structure. The vertical distribution of the pressure was not affected by varying the structure stiffness. Also, the bore induced forces measured at the structures’ bases had similar magnitudes for different stiffnesses. The results demonstrated that the bore induced pressure and forces are independent of the structure stiffness. However, this conclusion is valid only when the effect of P-delta is not significant (i.e. for small P value), and should be validated for more significant effect of P-delta (i.e. for large P value) by adding gravity load on the structure. For forces and pressures on elevated structures, the square prism structure was raised above the flume floor at base elevations of 50mm, 70mm and 90mm. Pressure exerted on the underside of the structure base was also measured and the horizontal pressure distribution was investigated. The magnitudes of the bore induced pressures on the underside of the structure base decreased with increasing base elevation. The stream-wise and upward forces due to the bore both decreased with increasing base elevation. Elevating the structure reduced the stream-wise force by up to 50% of that for a non-elevated structure. Also, the upward force for an elevated structure was significantly smaller than that for a non-elevated structure. Empirical equations are presented, to give guidance for the preliminary design of elevated structures under tsunami bore impact. For impact forces of tsunami-borne debris, an accelerometer-equipped disc-shaped smart debris device was used for direct measurement of the acceleration of the debris collision with the structure. An image processing technique was used to detect the angle of the debris at the instant of impact, and subsequently to resolve the acceleration data to horizontal and vertical directions. The measured acceleration data were converted to force using the impulse-momentum formula. The debris impact experiments were conducted for various bore velocities and various debris masses (550, 800 and 1000 g). The debris impact force was found to be a function of the debris mass, velocity, contact duration, and additional mass of entrained water. Additionally, the debris impact force was measured at the structure base, validating the results from the smart debris device. An equation was developed for the debris impact velocity, for a known distance between the location of debris pick-up by a tsunami and the structure. The debris velocity equation was validated using the experimental debris velocities obtained from integration of the measured acceleration data. For factors affecting the debris impact forces, evaluative experiments were carried out by varying debris shape (disc- and box-shaped debris), debris rigidity (rigid and deformable box-shaped debris), and structure stiffness. The results were used to modify the basic impulse-momentum formula by the addition of coefficients that take into account the added mass, the debris impact velocity, the debris shape, deformability of the debris, and stiffness of the structure. The results of this study give practitioners the ability to better analyse the tsunami bore impact on coastal structures. Most importantly, this study provides guidance for the preliminary design of structures that may be impacted by a tsunami bore, including estimation of the total stream-wise force, upward force, and floating debris impact force. Also, this study addresses a major knowledge gap in the design for tsunami impact on structures with different stiffnesses, and structures elevated above the ground.