Horizontal and Vertical Grading in a Tsunami Deposit

High resolution sedimentological data can be collected from paleo-tsunami deposits, where the patterns of grading presumably reflect the hydrodynamics of the flow. To understand pre-historic events where the only record is the deposit, a way of relating grain size grading to models for tsunami hydrodynamics is needed. Here we compare vertical and shore-normal grading in the 2004 Sumatra tsunami deposit near Khaolak Thailand to hydrodynamic trends from a runup model. A first-principles, physics-based, approach to modeling tsunami sedimentation remains impractical. The interplay of flow competence and varying sediment capacity is not fully understood. Also sediment sorting depends on boundary conditions, such as spatially varying sediment source, that are rarely known with sufficient fidelity. However, we do have some basic conceptual tools for understanding sedimentation. In general, as a volume of water slows it tends to lose sediment capacity and deposit sediment. Also, slower moving water tends to deposit finer sediment. By themselves, these two properties of sedimentary systems can explain complex grading in tsunami deposits, where spatial and temporal trends in speed interact across topographically variable surfaces. This approach to interpreting hydrodynamic models in terms of sedimentation makes a number of predictions. We expect that more asymmetric waves will leave only normally graded sediment, while symmetric waves will leave deposits with basal inverse grading in local depressions. Waves that form a reflected bore will include a section of abrupt normal grading. At Bang Lut near Khaolak, Thailand, we collected over 600 sand samples along with detailed field descriptions that document vertical and lateral grading structure in this deposit. We will compare this to runs of a depth- integrated tsunami runup model, and with eyewitness observations, photographs, and video.