Modelling the Dynamics of Multiple Floating Vehicles Driven by Transient Flood Waves

Many post-event field investigations of water-related hazards suggest that debris-enriched flow is much more destructive than water flow alone. However, the role of floating objects is rarely considered in the modelling or risk assessment approaches in practice. Existing modelling approaches are mostly focused on a single or limited pieces of debris. The interactions between the flow and multiple floating objects are not well explored and understood, and few modelling tools have been developed with the capability to simulate and predict these complex interactive processes.

This work aims to present a two-way coupling numerical model for simulating the full-process dynamics of floating debris driven by flood waves, based on a finite volume shock-capturing hydrodynamic model solving the 2D shallow water equations and a 3D discrete element method (DEM) model. A multi-sphere method (MSM) is introduced to the DEM model to better capture the shape and size of floating objects. The coupled model estimates the hydrostatic and dynamic forces acting on debris directly using the high-resolution water depth and velocity predicted by the hydrodynamic model, efficiently and automatically capturing the interactive dynamics between transient water flow and floating debris. The model is able to simulate the full-process dynamics of floating debris, including vertical displacement, initiation, horizontal transport, depositing, interaction with and impact on structures.

After being validated against experiment tests, the model is applied to reproduce a flash flood event in the coastal village of Boscastle, UK, in 2004. During the event, over 100 vehicles were carried by extreme water flow, which blocked and damaged a couple of downstream bridges, changed the pathway and extent of flooding, and finally moved with the flood water to the river mouth. The coupled model well predicts the flood dynamics, transport processes of floating vehicles, and their final locations. Further numerical experiments are caried out to discover and understand the process of floating debris blocking a bridge and the transport process and spatial distribution of different number of floating vehicles during the flood event. This model potentially provides a new and robust tool to more realistically assess flash flood risk and inform planning and design or urban buildings and infrastructure.