A coupled hydrodynamic and discrete element method for modelling flash flood debris

Abstract

Floating debris transported during ash ooding damages structures, blocks bridges and alters channel hydraulics. In recent years, a number of high pro le ash ood events have exhibited these processes. Recreating ood events through hydrodynamic modelling is an essential means by which engineers understand ood risk. However, there exists relatively little research focused on oating debris as a ash ood process and until now there have been limited attempts to incorporate oating debris processes into hydrodynamic ood modelling. In this work, a new coupled oating debris modelling tool is developed for 1D and 2D applications. The new tool combines a nite volume Godunov-type hydrodynamic scheme that solves the governing shallow water equations with the discrete element method for solving object contact and motion. A balanced force coupling procedure is used to calculate the hydraulic forces acting on oating objects and the corresponding shear stress imparted to uid cells. Hydrodynamic and hydrostatic force components are derived from the uid momentum principle and overcome problems associated with an empirically derived drag force used elsewhere. Balanced force coupling enables the new tool to predict both the transport dynamics of oating objects and their resulting backwater e ects. Debris dimensions are approximated using the multi-sphere method for shape representation. This ensures collisions are realistically modelled and application is not restricted by debris shape and size. The new modelling tool is extensively validated for dam break experimental test cases performed in a hydraulic ume. Predicted values for water depth and oating object position compare well with their observed counterparts for both 1D and 2D validation cases. Additionally, the coupled numerical modelling approach is applied to investigate ash

ooding, including oating debris impacts in Boscastle, 2004. The Boscastle event was signi cant as 116 vehicles were washed downstream, some of which blocked bridges, altering

ood hydraulics. Model predictions of water depth, depth averaged velocity and Froude number demonstrate the localised e ects of two debris blockages during the ood. Predicted water levels compare well to evidence of maximum depths collected after the event. Application of the new debris modelling tool to investigate the transport of ooded vehicles predicts vehicle transport pathways consistently with eye witness and post event observations. Application of the oating debris modelling tool to the Boscastle event demonstrates that the new tool can perform well for real world applications. However, i high computational costs require further model development to accelerate the long simulation process. This work demonstrates that a combined nite volume, discrete element approach to hydrodynamic modelling provides a greater understanding of ood hazard than purewater hydrodynamic modelling alone. Model outputs are valuable for quantifying ood risk, assessing ood damage and planning remediation measures. Furthermore, the new tool will enable a multitude of future applications and improve understanding of oating debris processes. Though the coupled approach has here been applied to ash ooding, the modelling methodology is applicable to a number of other natural hazards. Object transport by tsunami inundation, storm surge and river ice may all be simulated using the modelling methodology presented in this work.