Modeling Tsunami Sedimentation

The inundation of a tsunami, whether generated from a submarine landslide or an underwater earthquake, often results in beach erosion, landward sediment transport, and deposition of a tsunami deposit. If the deposit is preserved, then it becomes a record of the tsunami, and may be useful in interpreting a region's tsunami hazard. In addition to the knowledge that a tsunami had previously inundated an area, is there more that can be learned from paleo-tsunami deposits? To address this question we are developing models of tsunami inundation and sediment transport and using them to relate sedimentological characteristics of the deposit, such as grain size and thickness, to hydrodynamic characteristics of the tsunami, such as maximum velocity and flow depth. Model results are tested and verified using field data from the 2004 Indian Ocean tsunami. Data on tsunami flow depth, inundation distance, nearshore bathymetry, topography, tsunami deposit thickness, and sediment samples for grain size analysis were collected along several cross shore transects in Sumatra. These data are compared to predictions of erosion and deposition from a high resolution tsunami inundation and sediment transport model constructed over a 4-km cross shore section of coast. The Delft3D model, which solves the non-linear shallow water equations and calculates sediment transport with van Rijn (1993), is implemented with a flooding algorithm proven accurate for dam-break scenarios with rapid flooding. Forced with an offshore water level boundary condition taken from an Indian Ocean propagation model, the high-resolution inundation model predicts tsunami shoaling at the coast, with the wave-form steepening, wavelength decreasing, and wave height increasing. Maximum flow at the shoreline is directed offshore during the drawdown phase and is enhanced by the draining of water off the coastal plain. This offshore flow erodes the beach, transporting a substantial volume of sediment to a depth of approximately 12 m (msl), precisely the location of an offshore bar observed in the bathymetric data. Onshore flow, reaching a maximum velocity of approximately 8 m/s, also erodes the beach and carries sand onto the coastal plain. The tsunami deposit on land varies in thickness, with thinner deposits on topographic highs and thicker deposits in topographic lows. Model results suggest that variations in thickness results from divergences and convergences in the sediment flux as the flow slows over the topographic lows and speeds up over the topographic highs. Model results also indicate that the deposit thickness is sensitive to sediment grain size. Since the grain size of the deposit is poly-modal, multiple grain sizes should be included in future simulations.