The Effect of Flood Protection Works on Flood Risk

We pose the following research question: “what is the effect of flood protection works on flood risk?” To answer this question, we developed a flood risk assessment method that combines the typical hazard assessment via integrated hydrological and hydrodynamic calculations using HEC-HMS and 1D/2D HEC-RAS, respectively, and an original procedure for vulnerability assessment at the building level, which we applied in the town of Mandra in Attica, Greece. We performed calculations for 15 scenarios—combinations of return periods (T = 20, 50, 100, 150, and 200 y) and rain durations (t = 6, 12, and 18 h)—for the conditions of the year 2017, when there were no flood protection works, and today with these works in place. We identified the regions with high flood risk and concluded that the presence of the works caused a decrease in the inundation areas by 53–89%, along with reductions in the maximum water depths, the maximum flow velocities, and the average flood risk in Koropouli Street—the main street of Mandra, which suffered severe damage during the 2017 flood—by 38–62%, 18–52%, and 27–74%, respectively. The effect of the flood protection works increased with the increases in the return period and rain duration, while for the same return period the effect of the rain duration was more pronounced for the smaller return periods.