Calculating Tsunami Flow Speed from Tsunami Deposits

Tsunami deposits are commonly described as normally graded. Although created in different ways, normal grading is usually formed by sediment settling velocity sorting as particles fall out of suspension through the water column. The particles with faster fall velocities, those that are either larger or denser, arrive at the bed sooner than those with slower settling velocities, which results in a deposit that fines upward (normally graded). A simple model that exploits the normal grading of tsunami deposits was developed to predict the tsunami flow speed from characteristics of the deposit. Flow speed, in combination with flow depth, determines the deadliness and destructiveness of a tsunami. The model assumes suspended load dominates bedload and that sediment transport at the maximum tsunami flow speed can be approximated by steady, spatially uniform flow. The amount of sediment in suspension is assumed to be in equilibrium with the maximum flow speed during the uprush portion of the tsunami. Spatial flow deceleration is assumed to be small and not contribute significantly to the deposition. The subsequent return flow causes little erosion of the deposit because of slower flow speeds and being concentrated in topographic lows. Sensitivity analysis of the model shows that grain size of the deposit has more effect on calculated tsunami flow speed than deposit thickness. The model is tested using field data of deposit thickness and laboratory analysis of grain size of samples collected at Arop, Papua New Guinea (PNG) shortly after the 1998 tsunami. Speed estimates of 14 m/s near the shoreline compare favorably with those from a 1-D inundation model and from application of Bernoulli's principle to water levels on buildings left standing after the tsunami. The model calculated an inland decrease in flow speed in PNG and is able to reproduce observed normal grading and vertical variation of the sorting and skewness of the tsunami deposit. Tsunami flow speeds calculated from deposits in Indonesia created by the 2004 Indian Ocean tsunami are presented.