Tsunamis

Tsunami events are traditionally represented in the geological record by a sequence of fine-grained sediments, but increasingly coastal boulder deposits are being used as indicators of past tsunami events. The emplacement mechanism of many boulder deposits, however, is heavily debated and determining whether the inundation event was a tsunami or storm remains an unresolved challenge. Using physical experiments, we aim to achieve a better understanding of how tsunamis move coastal boulders. This knowledge will aid field geomorphologists in the identification of the emplacement mechanism for coastal boulder deposits and allow for the determination of wave parameters. In January 2023, physical experiments using the HR Wallingford Tsunami Simulator were completed as part of the MAKEWAVES collaboration. These experiments investigated the movement of a cuboid and irregular shaped boulder model when impacted by different tsunami waveforms. We propose new empirical formulae to describe relationships between transport distance and different tsunami waves.

Prior to the adoption of tsunami-specific design considerations in ASCE 7-16, research extensively examined tsunami debris transport and debris impact forces. Tsunami debris damming remains significantly less explored, with limited transient flow experiments conducted on this topic. Herein, 1:20 scale shipping containers transported via hydraulic bore freely accumulated against an instrumented column array representative of an elevated coastal structure. Resulting debris dams were analyzed through a validated photogrammetric method to estimate submerged frontal area during both accumulation and quasi-steady damming phases. A comparison with current ASCE 7-22 debris damming considerations yielded: (1) varying levels of conservatism among two different overall horizontal load prediction equations, (2) unconservative closure ratio estimates for open structures, and (3) underprediction of overall structure drag coefficients. It is proposed that bulk resistance coefficient may provide a more apt dimensionless measure of flow resistance for surface-piercing elevated coastal structure column arrays.

Due to climate change extreme events will become more frequent and more intense in the future. Hence, a better understanding of the physical processes associated with highly unsteady flows is essential. Among these, tsunamis, storm surges and flash floods are the most common. Due to their rarity and complexity, experimental approaches are required and dam-break waves are often use to reproduce tsunami-like flows. However, most laboratory tests are conducted on (unrealistic) smooth inverts, hence rising the question on how the bed roughness affects the hydrodynamic properties of these flows. Based on a large experimental campaign with multiple repetitions, these results show a clear dependence of the wave front celerity and inundation depth on the bed roughness, providing additional knowledge and new empirical expressions that will support researchers and practitioners in predicting the behavior of these unsteady flows during future extreme events.  

A series of large-scale experiments are conducted at HR Wallingford. Froude scaled tsunami waveforms are produced with periods (T) in the range of 40 – 230 s and wave amplitudes (a) between 0.03 - 0.14 m in water depths of 0.5 – 1.0 m.
The waves propagate over a 1:30 sloping bathymetry that extends onshore over which their runup is recorded. The obstacles are represented by circular cylinders (wooden dowels). Variation in diameters (D) from 0.05, 0.08 and 0.1 m and coverage inland (relative to the incident shoreline) of 1 – 2 m (Figure 1). 
Results show that relative to a smooth slope, these configurations can reduce normalised runup (R/a) by < 20% for T < 100 s. For T > 100 s R/a shows no appreciable reduction compared to the smooth slope. For certain combinations of obstacle geometry and T, a small increase in R/a is observed.

Landslides that occur in coastal environments drive cascading consequences such as large wave forces, flooding, and infrastructure damage in 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 is less clear. 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. The focus of previous studies has primarily been on the waves generated in the direction of failure (seaward) and not on the waves formed above and behind the failure (landward). To address this research gap, a series of 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 vertically rising gate designed to enable the near instantaneous loss of support of the source volumes resulting in granular collapse. The wave amplitude is measured using wave capacitance gauges and the failure mechanics are captured with high-speed cameras. 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. From these observations, a new analytical solution relating the initial column submergence to the magnitude of the landward trough wave is presented. The physical model results presented in this study accurately quantify the wave amplitude and runup observations in both the seaward and landward directions and could serve as a first order approximation for risk assessment for tsunamis generated by submerged and partially submerged mass failures.