Earth Observatory – Sediment Transport Processes under Tsunami Waves: A Case Study of Tsunami Deposits in a Coastal Cave in Aceh, Sumatra

Prof Philip Liu

The project aims to develop coupled hydrodynamic and morphological models to numerically simulate sediment erosion, transport, and deposit under tsunami waves from the tsunami generation region to inundated coastal areas. The sediment deposits discovered in a cave at Aceh, Indonesia are used a case study for understanding the complex process and constructing numerical models.

Using the existing 2004 earthquake slip distribution models, we first carried out numerical simulations to examine tsunami hydrodynamics in the vicinity of the cave.  Not surprisingly, simulation results indicate that slip value and fault geometry control the resulting tsunami wave heights. The maximum slip of these models in offshore Banda Aceh ranges from 11 m to 35 m, resulting the maximum wave heights near the cave between 15 m to 30 m.  The waveform time series show that most tsunami sources generate 4-5 wave peaks with variable wave heights in front of the cave. It is interesting to point out that three fining-upward sequences were observed in the 2004 tsunami deposits. These fining-upward sequences could be related to the three incoming tsunami waves.

We also simulated the sediment transport process using an existing model, COMCOT-SED, which has integrated a variety of existing empirical sediment transport formulas. Our simulations assume well-mixed sediments with grain size of 0.2 mm (50%) and 0.5 mm (50%) are available in the entire coastal and offshore simulation domain. Based on the simulated results, the cave could be flooded by the first tsunami wave over a duration of 10~20 minutes. The sediment concentration in the vicinity of the cave entrance is 50~100 kg/m3 with a flow velocity of 2~5 m/s. Using a very simplified cave geometry, we have a preliminary estimation that 0.125 ~ 0.25 m thick sediment could be deposited during each tsunami wave if the cave is fully filled. This estimated deposit thickness is quantitatively reasonable as compared with 0.2~0.43m measured deposit thickness.

The cave inundation has been further simulated with the 3D CFD model olaFlow. Using a time series of free surface elevation and velocities calculated from the tsunami models (COMCOT) the flooding processes inside the cave are simulated.  It is observed that the cave gets full of water within a couple of minutes of the leading wave, but the emptying is a progressive process that takes longer times than filling. Using the simulated time histories of the free surface elevations and the calculated sediment concentration carried by the tsunamis, a simplified one-dimensional sediment deposit model is developed. The model is capable of producing the fining-upward sequences in the sediment deposits inside the cave.

Higuera, P., Liu, P. L.-F., Lin, C., Wong, W-Y. and Kao, M-J. (2018) Laboratory-scale swash flows generated by a non-breaking solitary wave on a steep slope. J. Fluid Mech., 847,  https://doi.org/10.1017/jfm.2018.321.

González-Ondina, J. M., Fraccarollo, L., and Liu, P. L.-F. (2018) Two level, two phase model for intense, turbulent sediment transport. J. Fluid Mech., 839, 198 – 238.  https://doi.org/10.1017/jfm.2017.920

Barranco, I.; Higuera, P.; Liu, P. L.-F. (2019) Physical and numerical modelling of tsunami-like bore generated surf and swash flows. In: Goseberg, Nils; Schlurmann, Torsten (Hg.): Coastal Structures 2019. Karlsruhe: Bundesanstalt für Wasserbau. S. 545-554. https://doi.org/10.18451/978-3-939230-64-9_055 .

Barranco, I. and Liu P. L.-F. (2020) Swash flows generated by bores (under review)

González - Ondina, J. M., Liu, P. L.-F. and Fraccarollo, L. (2020) Entrainment and adaptation processes in the evolution of collisional bedload layers. (under review)