# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 3 Results
### Benefits from Different Flood Mitigation Scenarios
For more extreme floods, there are no benefits under the grey scenario, what is in accordance with designed heightening of the levees to the 100-year protection level. Figure 10: Expected annual damages (EAD) under different flood mitigation scenarios in different river sections across the Tannava watershed. Table 7 presents the expected annual benefits (EAB, Section 2.5) for the considered scenarios. This table also suggests that the greatest benefits are obtained under the green and green-grey scenarios, which include detention basins that generally have greater effects on reduction of flood hazard and losses than the levees. Hence, the benefits from implementing measures within the grey scenario are considerably smaller, and reflect primarily in reduction of losses to building structures and contents. This can be explained by the fact that the levees in the grey scenario are intended for protection of populated areas in which the buildings represent the most valuable assets. Since the levees make local impact on flood hazard and loss mitigation, reduction in the remaining, more remote, agricultural parts of the watershed are substantially smaller compared to the other two scenarios. The costs of population temporary displacement are considerably lower than the losses in other categories considered, and this pattern is also exhibited in the benefits. Difference in the benefits between the green and green-grey scenarios is minor. This is because the detention basins reduce flood peaks efficiently in the downstream sections so that there is no overtopping of the existing systems of levees. In other words, the levee heightening included in the green-grey scenario has minor effect in combination with implementation of the detention basins. \begin{table}
\begin{tabular}{c} 0.20 \\ (6\%) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Expected annual benefits (EABs) under three flood mitigation development scenarios and their breakdown per loss category. Figure 11: Benefits (loss reduction relative to baseline scenario) in the Tamnava watershed under the three proposed flood mitigation scenarios depending on the flood return period. The benefits achieved by implementing green, grey, and green-grey measures, computed relative to the baseline scenario are also presented for each sector in Table 8 in terms of benefits over the 50-year planning horizon (NPV), and illustrated in Figure 12 in terms of EAB. Four river sectors (7, 8, 11, 13) are not protected by the levees, nor would be under the grey scenario, so there are no benefits under this scenario in these sections. There are also no benefits for sector 11, which is already protected up to the 100-year flood and would not be additionally protected in the grey scenario. The benefits under the green and green-grey scenarios are the same in all sections, except for section 13, where the bypass canal is proposed (see Figure 3). As already discussed, levee heightening is not so beneficial if detention basins are implemented in the watershed.