# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 2 Materials and Methods
### Study Area and Flood Mitigation Options
#### 2.1.2 Flood Mitigation in the Tamnava Watershed  
To reduce flood hazard and consequent damages, the flood protection system in the Tamnava watershed has been set up since 1950s. Flood protection system in 2014, which is the existing system used as reference in this study, is primarily based on the levees along several sections of the Tamnava \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sector** & **Stream** & **Area [ha]** & **General Description** & **Agricultural Land Use [\%]** \\ \hline
7 & Tamnava & 573.20 & Rural & 99.86\% \\
7\({}^{\prime}\) & Tamnava & 75.28 & Urban & 35.18\% \\
8 & Tamnava & 3184.30 & Rural & 91.22\% \\
9 & Tamnava & 1429.02 & Rural & 92.72\% \\
11 & Ub & 1384.33 & Rural & 85.25\% \\
11\({}^{\prime}\) & Ub & 103.75 & Urban & 82.07\% \\
11\({}^{\prime\prime}\) & Ub & 101.16 & Urban & 43.99\% \\
12 & Ub & 859.49 & Rural & 94.28\% \\
13 & Gracica & 173.23 & Rural & 100.00\% \\
13\({}^{\prime}\) & Gracica & 244.58 & Semi-urban & 71.09\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: River sectors in the study area. Figure 3: Division of the Tamnava, Ub, and Gracica rivers into river sectors. Figure also shows locations of levees and planned detention basins. and Ub rivers (Figure 3). The levees that protect the populated areas along the Tamnava River are designed for 50-year or 100-year floods, while the remaining levees provide protection of agricultural land from 25-year floods. The mitigation measures that are proposed in the UNDP study [21] and analyzed in this study include:  
* Three detention basins with earthen dams at the upper reaches of Tamnava, Gracica, and Ub that should serve as flow regulating and erosion prevention structures during floods and would otherwise retain a minimal amount of water. Considering that detention basins can provide various ecosystem services [22; 23], they are considered as green measures in this study. Additionally, natural anti-erosion measures are planned in the UNDP study to complement the detention basins. * Reinforcement and crown elevation of the existing levees along the Tamnava and Ub, in sectors 7', 9, and 12, respectively (Figure 3). Additionally, construction of a bypass canal connecting Gracica and Ub rivers (sector 13') is proposed. All of the above measures are referred to as grey flood protection measures in this study. Locations of the proposed mitigation measures are depicted in Figure 3. The existing levees that already provide certain level of flood protection are planned to be elevated as follows:
* Sector 7', bank protection and levee protecting downtown Koceljeva: increasing flood protection level from return period of 50 years to 100 years;
* Sector 9, agricultural levee on both banks of river Tamnava: increasing flood protection level from return period of 50 years to 100 years;
* Sector 12, agricultural levee on both banks of River Ub: increasing flood protection level from return period of 25 years to 100 years. Technical details of the levee system in the watershed and the proposed detention basins are given in the Supplementary material. To enable comparison of benefits from implementing green and grey measures, and, consequently, identification of optimal flood protection setup for small, predominately rural watersheds, four alternative scenarios of flood protection in the Tamnava watershed are considered in this study:  
1. **Baseline scenario (existing conditions, \"no action\" scenario)**. This scenario includes only existing flood protection measures and assumes that flood protection level in the watershed would remain the same in the future. 2. **Green scenario** includes the three proposed detention basins along with the existing grey infrastructure (i.e., levees) with the flood protection level that they currently provide. 3. **Grey scenario** implies raising the existing levees and constructing the bypass canal, as proposed in the UNDP study. 4. **Green-grey scenario** includes implementation of both green (detention basins) and grey measures (heightened levees and the bypass canal) in the watershed.