# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 4 Discussion
Although previous research shows that enhanced hydrodynamic modelling of flood hazard does not substantially reduce uncertainties in damage assessments [43], there is also evidence that flood damages occur outside of the delineated floodplain boundaries corresponding to certain flood return periods [44]. This is important for agricultural watersheds considering that the damages of this type are generally related to large areas and could be affected significantly by the flooding extent. Uncertainties in the damage assessment can be generated by multiple causes, including vulnerability categories taken into account, spatial scale of the damage model, depth-damage relationships, and estimated asset values (e.g., [38]). In terms of the vulnerability assessment, this research can be characterized as the \"lower boundary\" analysis because it includes a limited number of key categories. For example, taking other crop cultures into account could increase agricultural losses [41]. Some of the additional categories that could be considered include transportation and energy infrastructure damages and loss of functionality, traffic delays, loss of critical facilities (water and wastewater systems, hospitals, communications, emergency service), lost productivity, business displacement, etc. [17]. More vulnerability categories could be taken into consideration if the adequate data were available, resulting in a more comprehensive flood loss estimates. Alternatively, using rough estimates or generalized (aggregated) data would lead to uncertain damage assessment, as shown by [45]. This is the reason for applying the \"lower boundary\" assessment approach in this study. If a particular scenario can be proven to be effective with the limited number of vulnerability categories, then it would be even more advantageous if the benefits from additional categories were included. Furthermore, including categories such as ecosystem services offered by green measures would also contribute to higher effectiveness of green mitigation scenarios. Despite the limited choice of vulnerability categories, the proposed flood mitigation scenarios for the Tamnava watershed are compared in relative terms, allowing their unbiased ranking. In this paper, a comprehensive micro-scale flood damage assessment is applied, with locally adjusted depth-damage relationships and locally estimated asset values. Previous research has shown that the micro-scale approach yields realistic damage estimates [46], and has also recommended that the asset values are adjusted to the regional economic situation and property characteristics [38]. Validation of the damage models is not a usual practice in their development [42; 47]. In this paper, the damage assessment approach in this paper is validated successfully for the case of the flood event in May 2014, leading to a conclusion that the approach yields plausible damage estimates. Therefore, it is expected that the uncertainties in flood hazard and limited number of vulnerability categories do not reflect the overall ranking of the scenarios. In this study, the effects of the detention basins on peak flow reduction provide a significant advantage for the green and green-grey scenarios. This is expected because the detention basins with considerable storage volume have a principal role in reducing peak flows. The same effects in terms of flood risk reduction would be possible with a grey detention basin of similar storage volume. However, the advantage of a green detention basin should be corroborated by its ecosystem services, the value of which should be included in the assessment of benefits of a flood mitigation scenario [13]. In our case study, the green-grey scenario provides slightly greater loss reduction than the green one. This difference may not suffice to provide its advantage over the pure green scenario if the construction cost of additional grey measures is high.