# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 3 Results
### Benefits from Different Flood Mitigation Scenarios  
Losses due to floods of different return periods and under different flood mitigation scenarios are computed for each river sector in the Tamnava watershed by summing the losses over the four considered loss categories (building structures and their content, agriculture, and population displacement). The losses for all sectors are presented in Figure 9 in function of the flood return period. The losses under the grey scenario are lesser than for the baseline scenario up to the 100-year return period, which is the design criterion for the elevated levees in the grey scenario. The losses for the green scenario are smaller than the losses for the baseline and grey scenarios for all return periods. Finally, the losses for the green-grey scenario show just a marginal improvement in comparison to the green scenario. Similar graphs of damage distribution are plotted for each sector (not shown in the paper), and although they show somewhat different patterns depending on the relative improvement of protection for a particular sector, they generally show that the green scenario is superior to the grey scenario, while the green-grey scenario is just marginally better than the green scenario. Figure 8: Water depths due to 200-year flood event in downtown Ub, including part of sectors 8, 9, 12, and 13′, under existing flood protection measures (**left** panel) and under green scenario (**right** panel). The expected annual damages (EAD) for the four scenarios are presented in Table 6, together with their breakdown per loss categories considered. Share of the loss categories in EAD varies across the scenarios. For example, the largest share in EAD have damages to agriculture under all flood mitigation scenarios, except for the existing conditions. Combined losses due to damages to building structures and content are slightly higher than the losses in agricultural production under the baseline scenario. These results can be explained by the fact that Tamnava is predominately rural, agricultural watershed, with low percentage of urbanized and industrial areas. Losses due to displacement of the affected population represent the smallest part of losses due to floods, with up to 5% of total EADs, and remain relatively constant under all scenarios. Figure 9: Total flood damages due to flood events of various return periods under the four flood mitigation scenarios in the Tamnava watershed. Figure 10 shows EAD for individual river sectors under all flood mitigation scenarios. The greatest losses under the baseline scenario are in sectors 8, 9, 11, and 13', corroborating high flood hazard in these sectors (see Figure 6). In addition, these sectors are characterized by large areas (e.g., sector 8) or by presence of urbanized or industrial zones (e.g., sector 13', see Table 1). Sectors 8, 11, and 13 are not covered by grey measures, so EAD for the grey scenario is the same as for the existing conditions. Figure 10 also shows that measures of the green and green-grey scenarios result in lower damages than under grey measures, especially in the most downstream sectors 9 and 12. Under the grey scenario, the heightened levees in these sectors provide protection from the 100-year flood, so the EAD is the result of levee overtopping by more extreme events (200- and 1000-year floods). On the other hand, the detention basins in the green and green-grey scenarios efficiently reduce damages due to such extreme flood events in these sectors, and consequently result in lower EAD than under the grey scenario. As can be seen in Figures 9 and 10, the greatest benefits, i.e., reduction of losses relative to the baseline scenario, are obtained under the green and green-grey scenarios. Figure 11 shows the benefits for different return periods and indicates that the grey scenario has lower benefits than the remaining two scenarios for floods of 100-year return period and smaller.