THEME 2: FLOOD MANAGEMENT

Title: Flood Modelling and Hazard Assessment for Extreme Events in River Basins
by Prof. Roger A. Falconer
Professor of Water Management, Hydro-environmental Research Centre,
School of Engineering, Cardiff University, Cardiff, UK

In recent years there has been an increasing awareness of the impact of extreme flood events associated with climate change etc., particularly with regard to higher peak flows and increased inundation extent along steep river basins. Such hydrodynamic features commonly occur across parts of the world where many rivers flow in the trans-critical regime during high flood events, and where river banks are also often protected by levees. The main difficulty in modelling levee breeches and trans/supercritical flows is that conventional numerical models do not accurately model steep water level gradients and artificial damping can be introduced, either indirectly through the numerical scheme or through increased friction. To overcome this inaccuracy an efficient shock capturing TVD (Total Variation Diminishing) numerical model, specifically focused on accurately predicting flood elevations and inundation extent for such events, has been developed and tested for idealized cases. The model was then applied to an extreme flood event occurring at Boscastle, UK, where extensive damage to property and infrastructure occurred in a storm in 2004. The predicted water elevations along the centreline of the main channel and the inundation extent were compared with observed data, with good agreement generally being obtained between both sets of results for the shock capturing model.

For the case of the Boscastle flood event, one of the main factors exacerbating the increase in flood risk was the fact that approximately 100 cars were picked up by the flood from an upstream car park and transported downstream to the open sea. One car blocked a small bridge on the downstream side of the town, causing considerable blockage by debris and leading to a significant increase in the water elevations and flood risk upstream. This led to refinements being made to the model to assess the flood hazard risk to people and vehicles during extreme flood events. In this model refinement a new mechanics based formulation was derived and experimentally calibrated, for scaled model children, adults and vehicles, to assess the degree of stability of people and vehicles for extreme floods. The model was then applied to two case studies, including: Glasgow and Boscastle in the UK. The analysis based on the model simulations showed that: (i) predictions for the Glasgow flood highlighted that children would be in danger of standing in the flooded streets in a small urban area; and (ii) simulations for the Boscastle flood indicated that vehicles in the town car park would be flushed away by the flow, which confirmed the accuracy of the incipient formula for vehicles.

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