# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 2 Materials and Methods
### Outline of the Methodology  
The methodology for evaluating flood mitigation scenarios presented in this paper is based on assessment of flood risk for each scenario. The flood risk assessment follows a process well elaborated in the literature, with risk being quantified as a product of hazard and vulnerability [24; 25], and shown in Figure 4. The flood hazard is here represented by maps of flood depth for different hazard scenarios (i.e., flood return periods or probabilities of exceedance) and different mitigation scenarios. The flood hazard maps are obtained as a result of hydrodynamic modelling of flood flows along the Tamnava, Ub, and Gracica rivers, as described in Section 2.3. Therefore, combining the natural hazard and mitigation scenarios produces multiple scenarios that should assist in identifying the optimal solution for the watershed.  
Assessment of the vulnerability to floods is based on identifying the assets in the focus area that are at risk of flooding. The vulnerability assessment is the basis for flood damage assessment. Vulnerabilities from natural hazards are usually grouped as physical, social, economic, and environmental [26]. However, estimating the potential damages from floods in terms of these four categories can be a complex process associated with high uncertainties. In engineering applications, it is much more common to divide the damages into direct and indirect losses [17; 18]. In this study, four categories of vulnerabilities, which were identified as the most important ones and for which sufficient data was available, are considered. These include three categories of direct losses and one category of indirect losses:  
* Physical damage to the buildings (residential/public).
* Physical damage to building contents and equipment.
* Damage to agricultural crops.
* Population emergency displacement and relocation.  
There is an extensive literature on quantifying vulnerability for the considered categories [25; 27]. The direct losses are estimated in this study by applying depth-damage relationships, i.e., by calculating the damage in function of the flood depth for each scenario, which is a common approach [16; 28] particularly useful for physical assets. The indirect losses of population displacement are estimated on the basis of daily unit costs per person displaced. Details on the vulnerability and damage assessment are given in Section 2.4.  
Figure 4: General approach to flood risk assessment for a chosen mitigation strategy.  
Assessment of risk is performed in function of asset vulnerability and damages for a series of synthetic (design) flood events under each mitigation scenario. Additionally, the same methodology for damage assessment is also applied to the flood event of May 2014 and compared to official damage estimates for the selected losses categories to validate the methodology. The effectiveness of a particular mitigation approach under the specified hazard scenario is measured as the damage avoided by implementing that particular option, i.e., as the difference between the losses of the baseline scenario and the considered mitigation scenarios outlined in Section 2.1.2. This step is elaborated in Section 2.5.