Extreme Flow Conditions Interacting With Coastal Structures: Large-Scale Physical Model Tests

Extreme flow conditions occur frequently in the context of flash floods, storm surges and tsunamis; some of these natural hazards are strongly exacerbated by climate change, either through more energetic storms, more intense precipitation or increased sea levels around the global coastlines. Most of these extreme flow conditions manifest through a series of transient flow stages or transitions from transient to steady flows. Typically, the onset of these extreme flow conditions is rapid, characterized by a steep gradient of surface elevation and depth-averaged flow velocity, followed by less rapid rise of flow depth, and eventual plateauing with a steady-state condition of depth and velocity. In some cases, these flow characteristics are a combination of background extreme flow conditions, with overlapping singular bore-type waves riding on top of it. It has remained a challenge to obtain a good understanding as to which of these stages is the most severe with respect to marine or terrestrial ecosystems and infrastructure, such as residential houses, bridges, culverts or road dams. This work will hence address the challenge by utilizing a unique, and large-scale experimental facility, the large wave-current flume (LWCF) of Coastal Research Center in Hannover, Germany, to demonstrate the use of breaking solitary waves climbing up a compound beach (von Häfen et al. 2022), which eventually led to a broken-bore, emblematic of the early stage of extreme flow conditions addressed herein. The study aims at illustrating the hydrodynamics, and flow-structure-interaction, with the compound beach and with single beach front houses, approximated by geometric primitives, ultimately providing accurate benchmark datasets and insights into the flow dynamics, both for further analysis and as a training dataset for numerical modelling. Two large-scale physical model studies were conducted in the LWCF (300 × 5 × 7 m). Solitary waves, generated by a piston-type wave generator, propagate across a water body of 3.4 m water depth, subsequently breaking over a 1:15 slope and impacting a simplified coastal structure on a horizontal platform in a height of 3.6 m. In total, three simplified coastal houses (1 × 1 × 0.7 m) with varying levels of structural elevation are utilized to model the impact of structural elevation on flow dynamics (Krautwald et al. 2022). Few test runs on hydraulics-only conditions were also recorded. Using large-scale particle image velocimetry (PIV), insights into flow phenomena, vortex shedding at pile structures below elevated buildings, and distributed velocities are obtained. The study demonstrates that the recirculation zone for slab-on-grade structures extends up to 2 m (with a building length of 1 m) at a Reynolds number (Re) of up to 10⁷. Furthermore, flow velocities increase for elevated structures compared to slab-on-grade structures up to 100% at a distance of 1 m downstream. Therefore, structural elevation serves as a method to decrease structural loads but should be carefully considered as a disaster mitigation strategy due to reduced flow sheltering effects in the light of requirements from local evacuation strategies, hydrodynamic loads on adjacent/downstream buildings and protection requirements of those buildings.