APPROPRIATE FLOOD RISK ASSESSMENT TOOLS FOR PREDICTING FLOOD INUNDATION EXTENT AND ASSESSING FLOOD HAZARD RISK FOR FLASH FLOOD EVENTS

Climate change is predicted to result in more frequent occurrences of extreme flood events, such as flash flooding. This being the case, there is now an increasing need for accurate flood protection design in the areas susceptible to flash flooding. However, existing flood risk assessment procedures and methods frequently used, including simplifications of the computational processes, i.e. a "simplification strategy", often do not adequately represent the complex hydrodynamic processes associated with flash flooding and therefore lead to inadequate flood protection designs. This paper examines what flood risk assessment tools should be used for flood risk assessment in areas where the emergence of flash flooding is possible. The 2004 Boscastle flash flood, in the U.K. was selected as a typical case study. Firstly, the appropriateness of the "simplification strategy" was investigated, as a flood risk assessment tool for areas prone to flash flooding. Three different configurations of the DIVAST model were considered in this study, including: (i) with shock-capturing (DIVAST-TVD), (ii) excluding shock-capturing (DIVAST-ADI), and (iii) excluding the inertia terms. The findings reported in this paper strongly suggest that applying the "simplification strategy" is not appropriate for areas susceptible to flash flooding. The results also show that this strategy can lead to significantly erroneous predictions of the main parameters, such as flood inundation extent. Secondly, two different flood hazard assessment methods were tested, including: (i) a widely used empirically derived method, and (ii) a new, experimentally derived, and physically based method. The results obtained suggest that the flood hazard assessment in areas prone to flash flooding should be conducted with physically based methods, which take into account: (i) all of the physical forces acting on a human body in floodwaters, and (ii) the rapid changes that often occur in the flow regime, particularly where flash flooding exists.