# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 2 Materials and Methods
### Hazard Assessment  
The hydrodynamic model used for flood hazard assessment in this paper was originally developed for reconstruction of the extreme flood event in May 2014 in the UNDP study [21], and it will be referred to as MODEL 2014. It is a 1D model specifically developed to simulate all leveee breaches and overtopping, and backwater effects that occurred during flood event in May 2014. This model version is calibrated within the UNDP study according to the observed water stages and surveyed flooding extents. An overview of the characteristics of this model and its calibration are presented in the Supplementary material. Details on the reconstruction of this flood event can be found in the literature [29].  
The MODEL 2014 was further modified to enable hydrodynamic simulations and flood hazard assessments under different mitigation scenarios described in Section 2.1. For the existing flood protection, leveee breaches and overtopping options were excluded from the MODEL 2014. Further modifications of the hydrodynamic model were aimed at simulations under the future mitigation scenarios by including the three proposed detention basins (green scenario), elevated levees, and the bypass canal (grey scenario) and combination of both types of measures (green-grey scenario).  
The hydrodynamic models of the Tamnava, Ub, and Gracica rivers were fed with the hydrographs as the upstream boundary conditions, and stages of the Kolubara River at the confluence with Tamnava as the downstream boundary condition [21]. Only MODEL 2014 was used for non-stationary simulations. The hydrographs used to run MODEL 2014 were either observed at the stream gauges or simulated by hydrologic models [29]. The remaining model versions were used for stationary simulations and were driven with design flood flows of various return periods. The approach of using only the peak flows instead of entire hydrographs was adopted as a conservative approach that maximizes the flooding extents and the flood damage.  
The design flows for hydrodynamic simulations were estimated in the UNDP study [21] at the locations of the stream gauges in the watershed by applying the flood frequency analyses of the observed annual maximum floods. These design floods were regionalized to estimate corresponding flows at ungauged locations in the watershed that were needed for hydrodynamic simulations.  
All hydrodynamic model versions were developed with the HEC-RAS software [30]. The models included all related structures, such as levees, reservoirs, detention ponds, and bridges. Terrain data were obtained by combining several Digital Terrain Models (DTMs) available for the Kolubara watershed. The combined DTM was further refined according to the structural lines, such as the levees and road network, river cross-sections placed at 500 m distance from additional geodetic surveys, as well as precise data on the bridge structures. The data exchange between HEC-RAS and GIS software was established via the HEC-GeoRAS tool [31].  
Simulations with the MODEL 2014 were performed in this paper to enable comparison of losses during the flood event in May 2014 to the official post-disaster estimates of flood damages in the Tamnava watershed, and, thus, to enable validation of the damage assessment approach used in this paper. The other model versions were used for flood hazard and damage assessments under the four analyzed flood mitigation scenarios. Flood hazards were assessed for 2-, 10-, 20-, 50-, 100-, 200- and 1000-year design floods. In all simulations performed, it was assumed that the protection system fulfills its design role completely (no leveee breaches).