Modelling elevations, inundation extent and hazard risk for extreme flood events

Climate change is expected to result in more frequent occurrences of extreme flood events, such as flash flooding and large scale river flooding. Therefore, there is a need for accurate flood risk assessment schemes in areas prone to extreme flooding. This research study investigates what flood risk assessment tools and procedures should be used for flood risk assessment in areas where the emergence of extreme flood events is possible. The first objective was to determine what type of flood inundation models should be used for predicting the flood elevations, velocities and inundation extent for extreme flood events. Therefore, there different flood inundation model structures were used to model a well-documented extreme flood event. The obtained results suggest that it is necessary to incorporate shock-capturing algorithms in the solution procedure when modelling extreme flood events, since these algorithms prevent the formation of spurious oscillations and provide a more realistic simulation of the flood levels. The second objective was to investigate the appropriateness of the “simplification strategy” (i.e. improving simulation results by increasing roughness parameter) when used as a flood risk assessment modelling tool for areas susceptible to extreme flooding. The obtained results suggest that applying such strategies can lead to significantly erroneous predictions of the peak water levels and the inundation extent, and thus to inadequate flood protection design. The third and final objective was to determine what type of flood hazard assessment methods should be used for assessing the flood hazard to people caused by extreme flooding. Therefore, two different flood hazard assessment criteria were modelled for three extreme flood events. The predicted results suggest that in areas prone to extreme flooding, the flood hazard indices should be predicted with physics-based formulae, as these methods consider all of the physical forces acting on a human body in floodwaters, take into account the rapid changes in the flow regime, which often occur for extreme events, and enable a rapid assessment of the degree of flood hazard to be made in a short time period

Flood hazard assessment for extreme flood events

Climate change is expected to result in an increase in the frequency and intensity of extreme weather events. In turn, this will result in more frequent occurrences of extreme flood events, such as flash flooding and large-scale river flooding. This being the case, there is a need for more accurate flood risk assessment schemes, particularly in areas prone to extreme flooding. This study investigates what type of flood hazard assessment methods should be used for assessing the flood hazard to people caused by extreme flooding. Two flood hazard assessment criteria were tested, namely: a widely used, empirically derived method, and recently introduced, physically based and experimentally calibrated method. The two selected flood hazard assessment methods were: (1) validated against experimental data, and (2) used to assess flood hazard indices for two different extreme flood events, namely: the 2010 Kostanjevica na Krki extreme river flood and the 2007 Železniki flash flood. The results obtained in this study suggest that in the areas prone to extreme flooding, the flood hazard indices should be based on using the formulae derived for a mechanics-based analysis, as these formulations consider all of the physical forces acting on a human body in floodwaters, take into account the rapid changes in the flow regime, which often occur for extreme flood events, and enable a rapid assessment of the degree of flood hazard risk in a short time period, a feature particularly important when assessing flood hazard indices for high Froude numbers flows

Appropriate model use for predicting elevations and inundation extent for extreme flood events

Flood risk assessment is generally studied using flood simulation models; however, flood risk managers often simplify the computational process; this is called a “simplification strategy”. This study investigates the appropriateness of the “simplification strategy” when used as a flood risk assessment tool for areas prone to flash flooding. The 2004 Boscastle, UK, flash flood was selected as a case study. Three different model structures were considered in this study, including: (1) a shock-capturing model, (2) a regular ADI-type flood model and (3) a diffusion wave model, i.e. a zero-inertia approach. The key findings from this paper strongly suggest that applying the “simplification strategy” is only appropriate for flood simulations with a mild slope and over relatively smooth terrains, whereas in areas susceptible to flash flooding (i.e. steep catchments), following this strategy can lead to significantly erroneous predictions of the main parameters—particularly the peak water levels and the inundation extent. For flood risk assessment of urban areas, where the emergence of flash flooding is possible, it is shown to be necessary to incorporate shock-capturing algorithms in the solution procedure, since these algorithms prevent the formation of spurious oscillations and provide a more realistic simulation of the flood levels