# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 3 Results
### Flood Hazard  
The flood hazard maps under the baseline and three alternative flood mitigation scenarios in the Tamnava watershed result from hydrodynamic modelling, as described in Section 2.3, for each considered return period (2, 10, 20, 50, 100, 200, and 1000 years). Table 5 presents peak flows at the locations of the three stream gauges under all the four flood mitigation scenarios, and for all considered return periods.  
The hazard map of the 100-year flood under existing flood protection system is shown in Figure 6. As described in Section 2.1, the existing levees already provide some flood protection level to certain parts of the Tamnava watershed, primarily to those in urban setting (e.g., sectors 7, 11\({}^{\prime}\) and 11\({}^{\prime\prime}\)). These parts of the watershed have lower flood hazard level under current conditions than the other sectors in the watershed. Flood hazard is particularly pronounced in the most downstream parts of the watershed (e.g., sectors 9 and 12, some parts of sectors 8 and 13; see Figure 6).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Return Period** **(Years)**} & \multirow{2}{*}{**Flood Mitigation** **Scenario**} & \multicolumn{3}{c}{**Peak Flood Discharge [m3/s]**} \\ \cline{3-5}  & & **Tamnava at** & **Tamnava at** & **Ub at Ub** \\  & & **Koceljeva** & **  
Figure 7 presents the 100-year flood in the Tamma watershed under the three considered scenarios of flood mitigation development. Their effects can be inferred by comparing these hazard maps to the map in Figure 6. The greatest hazard reduction is obtained under the green-gray scenario. Interesting examples are the most downstream sectors 9 and 12, which are not flooded by the 100-year event under all development scenarios shown in Figure 7. Although the existing levees provide protection from the 25- and 50-year floods (Section 2.1), the detention basins proposed within the green scenario provide sufficient reduction of the 100-year flood, so that it can be conveyed without overtopping in these sectors. These results are obtained also for the 200-year flood, while the 1000-year flood (not shown here) causes levee overtopping in sectors 9 and 12. Reduced flood hazard is also noticed in sector 13\({}^{\prime}\), where construction of the bypass canal is planned (see Figure 3). Comparison of the flood hazard maps for different return periods shows that the grey measures have effects in their immediate proximity, while the green measures (i.e., detention basins) reduce peak flows and pertinent flood levels along the downstream river sections, thus having farther-reaching effects.  
Figure 6: Flood hazard due to 100-year flood in the Tamma watershed under the baseline scenario (existing flood protection system).  
Figure 7: \(Cont.\)  
The flood hazard maps enable identification of the affected assets and the depth of flooding water. Figure 8 shows water depth in each building in downtown Ub due to 200-year flood event under baseline and green scenarios and illustrates how the fine-scale asset data and a fine-resolution hazard map facilitate the micro-scale assessment of damages.  
Figure 7: Effects of the three flood mitigation scenarios on flood hazard in the Tannava watershed: green (**top**), grey (**mid**), and combined green-grey (**bottom** panel). The figure shows hazard due to 100-year flood.