Flume experiments test grain-size distribution of onshore tsunami deposits

Abstract

We conducted flume experiments to examine the effects of grain size on the distribution of tsunami deposits, using well-sorted quartz sand with median diameters (D₅₀) of 0.064, 0.134, 0.215, and 0.250 mm. In these experiments, we investigated deposits transported onto a flat terrestrial area by a single tsunami-like bore without a strong backwash flow. The post-tsunami distributions of the coarser (D₅₀ = 0.215 and 0.250 mm) and finer (D₅₀ = 0.064 and 0.134 mm) sands differed markedly. The amount of the deposited coarser sand decreased landward across the entire terrestrial area, whereas that of the deposited finer sand was either approximately uniform across the terrestrial area or increased gradually landward. Time-series of flow depth and sediment transport observed via composited video frame images showed that suspension of the coarser sand did not reach the height of the maximum flow depth, and decreased with distance from the sediment source, and that the sediment settled gradually across the terrestrial surface with the amount deposited decreasing landward. Conversely, the finer sand was densely suspended up to the maximum flow depth and was transported over longer distances across the terrestrial area with the amount deposited being relatively uniform or increasing slightly landward. Advection lengths that we calculated on the basis of our experimental parameters supported the dependence on grain size of the distribution patterns of onshore deposits shown by our flume experiments. Moreover, advection lengths that we calculated for the 2011 Tohoku-oki tsunami from previously reported tsunami parameters were consistent with a deposit distribution similar to that observed in the field after the tsunami. Although we ignored some key factors (e.g., geomorphology and sediment availability), our flume experiments showed that the distribution patterns of identical tsunami flows differ for sediments of different grain size.

Introduction

Understanding the inundation areas of paleotsunami waves is of crucial importance for assessments of modern tsunami risks because these areas can be used to estimate tsunami magnitudes and, in some cases, the magnitude of the triggering subduction-zone earthquake (e.g., Kelsey et al., 2005; Nanayama et al., 2007; Sawai et al., 2012; Sugawara et al., 2014). The extent of inundation areas, especially the spatial distribution of sandy tsunami deposits, have been a focus of previous paleotsunami reconstructions because such deposits are relatively easy to identify in environments where formation of organic soil or peat predominates (e.g., Cisternas et al., 2005; Jankaew et al., 2008; Fujino et al., 2009).

Previous studies of the 2011 Tohoku-oki tsunami have revealed that when the runup was more than 2.0 km, the extent of recognizable tsunami sand deposits was considerably less than the inundation distance (e.g., Goto et al., 2011a; Abe et al., 2012; Shishikura et al., 2012). For example, the maximum inland extent of recognizable sandy deposits of the 2011 Tohoku-oki tsunami was about 2.5 km (57%–76% of the 4.5-km inundation distance of the tsunami) on the Sendai Plain of northeastern Japan, where coastal lowlands of less than 5 m elevation extend continuously up to 5 km inland (Abe et al., 2012). Although the distribution of sandy tsunami deposits is influenced by other factors, such as geomorphology, topography, and nearshore bathymetry, the above observation suggests that previous paleotsunami inundation distances that were determined on the basis of the landward extent of sandy tsunami deposits may have been underestimated, at least in areas where flat coastal plains extend more than 2.0 km inland. Indeed, on the basis of findings for the 2011 Tohoku-oki tsunami, Namegaya and Satake (2014) revised upward the previously estimated magnitude of the 869 CE Jogan tsunami and the earthquake that triggered it.

Studies of the 2011 Tohoku-oki tsunami deposits reported that the distribution patterns of the muddy and sandy deposits differ (e.g., Goto et al., 2011a; Abe et al., 2012; Chagué-Goff et al., 2012; Putra et al., 2013). These studies indicate that the maximum extent of the muddy tsunami deposits on the Sendai Plain approximately matched the inundation limit. Moreover, the thickness of the muddy tsunami deposits was either uniform or increased slightly landward, whereas the sandy tsunami deposits thinned landward (Goto et al., 2011a; Abe et al., 2012; Chagué-Goff et al., 2012; Putra et al., 2013). These results suggest that the distribution and sedimentation processes of tsunami deposits are influenced to a considerable degree by grain size, and that the extent of muddy tsunami deposits might provide a better representation of tsunami inundation areas than that of sandy tsunami deposits. In contrast, numerical simulations by Cheng and Weiss (2013) suggest that the difference between the maximum inundation distance and the extent of sand deposits is controlled mainly by wave amplitude and onshore slope, not by sediment grain size. Further research to improve our understanding of the influence of grain size is needed to allow precise reconstruction of tsunami inundation areas.

The distributions of onshore tsunami deposits are generally determined by factors such as topography, sediment availability, and sequential tsunami flow, as well as by sediment grain size (e.g., Costa and Andrade, 2020). Because tsunami events are infrequent, and because there are inherent difficulties in acquiring detailed real-time field measurements of tsunami hydraulic and sedimentological conditions, it is difficult to distinguish the effects of the individual factors that influence the distribution of tsunami deposits. However, flume experiments can be used to investigate these factors. Sediment transport, tsunami flow, and topography can be readily controlled in flume experiments, and sedimentary processes can be observed closely. Previous studies have used flume experiments to examine the influences of terrestrial topography (Yamaguchi and Sekiguchi, 2015, Yamaguchi and Sekiguchi, 2018), the position of sediment sources (Harada et al., 2011), sedimentary structures (Yoshii et al., 2017, Yoshii et al., 2018), and grain-size distributions (Johnson et al., 2017) on tsunami deposits. A few laboratory studies have focused on the effect of grain size on the distribution of tsunami deposits (Harada et al., 2011, Harada et al., 2017; Johnson et al., 2016); however, they employed relatively coarse sediments (median diameter D₅₀ > 0.11 mm).

For this study, we designed a simple flume apparatus to simulate a cross-shore setting in which a single tsunami-like bore without a strong backwash flow transports sediments from a nearshore terrestrial sediment source onto a coastal lowland. We then used this apparatus in experiments with sands in the grain-size range from D₅₀ = 0.064 mm to D₅₀ = 0.250 mm to investigate the effect of grain size on the sedimentary processes and distribution of onshore tsunami deposits.