Laboratory Experiments of Tsunami Inundation in Patchy Coastal Forest on a Steep Beach

Abstract:

Tsunamis are a leading natural threat to coastal communities, and events such as the 2011 Japan and 2004 Indian Ocean tsunamis caused widespread, crippling damages to coastal infrastructure. Yet, these events also called attention to the role of coastal forest as sustainable mitigation against tsunami hazard. Here, we present large-scale experiments of tsunami runup and withdrawal on a steeply sloping beach in the presence of patchy forest.

The forest is modeled using 1.2-m diameter macro-roughness patches of varying resistance were constructed from staggered arrays of 2.7-cm diameter rigid cylinders. Macro-roughness patches were affixed in a staggered arrangement with mean spacing of 3.2 m between patches (Fig. 1). The basin depth and wave height at the wavemaker were 0.73 m and 0.43 m, respectively, such that a broken roller formed offshore of the still-water line.

Point measurements of velocity and flow depth were made at twenty locations using co-located acoustic Doppler velocimeters and sonic wave gauges, respectively, in order to construct a flow field in the vicinity of three macro-roughness patches. Simultaneous, high-resolution video was also collected in order to track the runup bore position in time.

Analysis of mean flow conditions reveals that patchy roughness induces non-uniform changes in momentum flux throughout the patch array (Fig. 2). During runup, momentum flux is generally reduced in the lee of the patches. However, flow channelization between cross-shore rows of patches leads to an increase in momentum flux. During withdrawal, the strong gravity-driven flows that develop as a result of the steep 1:10 beach lead to an increase in momentum flux in areas behind the patches, which benefited from reduced momentum flux during runup.

The experiment findings indicate that flow interactions with the natural environment are indeed complex and that care must be exercised when considering the use of coastal forest as a tsunami bioshield.

Acknowledgements: This material is based upon work supported by the National Science Foundation under Grant Number CMMI-1206271. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.