Large-Scale Laboratory Experiments On The Wave Generation Due To The Collapse Of Partially And Fully Submerged Granular Columns

Abstract

Landslides that occur in coastal environments can drive cascading consequences such as wave forces, flooding, and infrastructure damage to coastal communities. It can be difficult to classify these slides as subaerial or submarine, and the mechanics of wave generation associated with partially submerged failures are not well understood. Limited physical modelling has been conducted that encompasses both the triggering of granular landslides and subsequent waves associated with partially and fully submerged mass movements. To date, laboratory work investigating tsunamis generated by submarine landslides has focused on the wave formed in the direction of the mass movement (seaward direction) for rigid block experiments (eg. Rzadkiewicz, 1997) and deformable slide masses (e.g. Grilli et al., 2017, Takabatake, 2020, Bullard et al., 2023). From these experimental data sets, predictive relationships connecting slide acceleration, mass, and initial submergence depth to the amplitude of the wave formed have been presented for the seaward wave. Such relationships have not been presented for the landward directed wave, which propagates in the opposite direction of the submarine landslide motion.

 Further, not all landslides are easily classified as either subaerial or submarine. Consider the 2018 Anak Krakatoa landslide in which the sliding surface was estimated to be 100 m below sea level (Pakoksung et al., 2020), resulting in one third of the total collapse being submerged. In comparison to the end-member conditions of subaerial and submarine failures, the mechanics of wave generation associated with partially submerged failures is much less clear. Granular column collapse experiments provide an idealized experimental framework to explore momentum transfer processes and the resulting waves generated in partially submerged and fully submerged conditions. Work by Cabrera et al., (2020) made use of granular collapse experiments of partially to fully submerged columns to derive a continuous momentum-based function to estimate the maximum seaward wave amplitude based on the initial column submergence ratio (H_w/H_o). However, these experiments were conducted at a small-scale (H_o = 0.15 m) with a width of one particle (2.4 mm diameter). To address this research gap, a series of 22 large-scale granular collapse experiments were conducted by releasing columns of river stone (0.75 m and 0.50 m high) into a laboratory flume reservoir with water depths ranging up to 1.10 m to explore the wave generation and runup processes in both seaward and landward directions. The columns were released by a rapid pneumatically actuated vertical rising gate designed to enable the near instantaneous loss of support of the source volumes resulting in granular collapse. The failure mechanics were captured with high-speed cameras (Figure 1a,b) and wave amplitudes were measured using wave capacitance gauges (Figure 1c,d). This work also provides the first experimental data set of the landward propagating wave and runup associated with submerged granular collapse experiments. Overall, the seaward wave amplitudes measured in these highly-instrumented, large-scale physical models agree with empirical relationships developed in a previous study using smaller-scale models.