Flood Risk Assessment of the Clear Creek Watershed considering compound events

Abstract

Tropical cyclones generate risk of compound flooding in coastal watersheds due to both precipitation and storm surge. For regions prone to this phenomenon, it is clear that in order to properly quantify flood hazard and flood risk to later use these estimates in the implementation of mitigation, adaptation and prevention measures that are effective on decreasing the flood risk, it is necessary that compound flooding is taken into account since not contemplating the simultaneous occurrence of different flood drivers and their joint dynamic interaction, will immediately leave out the worst case scenario.

Nonetheless, this is not the regular practice carried out by risk agencies as is the case of the Federal Emergency Management Agency (FEMA) of the United States, which delineates floodplains considering only one flood driver at the time and has ignored the co-occurrence of both mechanisms. Recent studies have shown that in the United States, the currently estimated flood hazard generally underestimates the actual flood losses in coastal areas (Blessing et. al. 2017). This is highly problematic since the delineation of the flood hazardous zones drives policy decisions such as urban planning, flood mitigation measures, and most relevant for the U.S case, the decision whether or not to take flood insurance policies which are all important strategies to effectively reduce and distribute flood risk.

This thesis aims to delineate flood hazard and flood risk in a coastal watershed including the effect of compound flooding using the SFINCS model; a semi-advanced 2D model which was developed to solve all relevant processes in coastal catchments with computational efficiency (Leijnse, 2018). The research is divided into four phases: (1) an overview of the Clear Creek coastal watershed and validation of the SFINCS model for two major hurricane events triggering compound flooding in the area, (2) delineation of boundary conditions for a compound flood analysis based on synthetic data, (3) new estimation of flood hazard and flood risk for the Clear Creek watershed, and (4) discussion on how relevant it is to include compound flooding in a flood risk assessment and how the results can be used to improve actual floodplain delineation and insurance rate estimates in the area.