Experimental study on tsunami induced debris-laden flow
Date of Issue2018
School of Civil and Environmental Engineering
Institute of Catastrophe Risk Management
Tsunami induced debris-laden flow is a destructive process in a tsunami event, since the high speed and energetic tsunami waves could destroy items in its path and carry debris or any items they could move. A key limitation in understanding tsunami debris-laden flows is the lack of both quantitative experimental data and numerical models. In this PhD study, a series of laboratory-based studies had been conducted on debris-laden tsunami waves modelled as solitary waves. The maximum and final debris positions, along the maximum inundation were measured via video recordings for various experimental conditions. The objective was to study the inundation/runup reduction, final and maximum debris location, and the motion of debris due to a solitary wave incident on a composite slope. This included (i) a row of debris located at different positions on the slope, (ii) two rows of debris with different separation distances, and (iii) a single of debris with different sizes and orientations. The composite slope used is a 3-slope setup to be representative of the continental shelf and slope and the nearshore configuration. In the one and two debris-row experiments, the results showed that the presence of the debris reduced the maximum inundation. The values of maximum inundation, maximum and final debris positions were shown to be more sensitive to the incident wave conditions than the original position of the debris row under the tested wave conditions. The percentage of the debris that was further washed back into the deeper part of the flume (i.e. seaward) was obtained, but the chances of being washed back was small, random and unpredictable. Analysis of the experimental results showed that both the measured inundation limit as well as the position of debris had significant variability, and with the variability increasing with the solitary wave height. Probability distributions and normalized Kernel Density functions were used to characterize the spread and shape in the maximum and final debris positions. A single equivalent slope was further defined for the composite slope to characterize the wave runup as part of the data analysis. Together with a newly proposed the non-dimensional debris mass loading, the reduction in inundation due to the debris is then characterized via an empirical fit. This fit would allow future predictions of the inundation reduction as dependent on the amount of debris being carried by the solitary wave under such a composite slope arrangement. Such prediction would find use in post tsunami field surveys where the surveyed final debris locations are used to infer the incident wave heights. For experiment on a single piece of debris, the maximum inundation/runup was unaffected by the piece of debris due to its light weight and small size. However there was a high degree of variability in the maximum and final debris positions as dependent on small changes in debris size or even the same-sized debris but at different initial orientations. The equation of motion for single debris was used to estimate coefficients for the drag, added mass and frictional force based on the observed debris trajectory as captured via video recordings. This PhD work addresses a research gap in the study of a tsunami event as relating to the effects of the generated floating debris. The work, besides providing detailed experimental data for further analytical work, demonstrated the effect of the debris mass on the inundation/runup reduction and with its quantification over the mass loading range tested. Therefore, the obtained experimental results should be useful to support future calibration or validation of numerical approaches, more extensive experiments over a wider parameter range (e.g. different composite slopes and mass loadings), and to guide field estimations of tsunami wave condition via post-event surveys on the inundation extent.