# Evaluation of Green and Grey Flood Mitigation Measures in Rural Watersheds
## 2 Materials and Methods  
### Study Area and Flood Mitigation Options  
#### 2.1.1 Description of the Study Area  
The study area comprises upper watershed of river Tamnava and its tributaries Gracica and Ub, in central Serbia, approximately 60 km southwest of Belgrade, the capital. This area is a part of the larger Kolubara watershed, depicted in Figure 1. The Tamnava watershed covers 726 km\({}^{2}\) and is primarily rural, with 79.3% of the area being cultivated land [19] (Figure 2). Urbanized and industrial land constitutes only 1.2% of the area and is concentrated in two small population centers, the towns of Ub and Koceljeva. The terrain elevation ranges from 470 m a.s.l. in upper reaches of the watershed to 64.4 m a.s.l. at its mouth (Figure 2), with arable land located in wide floodplains of the three rivers. The agricultural landscape consists primarily of small farms, with emphasis on several main crops. The rest of the economy is based on service industry located in towns of Ub and Koceljeva. Figure 1: _Cont._Figure 1: The study area: location of the Kolubara River watershed in Serbia (**top**) and the subbasin of the Tamnava River (**bottom**). Figure 2: _Cont._  
Hydroclimatic regime of the Tamnava watershed is characterized by pronounced seasonal pattern, with the highest flows in early spring due to combined rainfall and snowmelt, and in early summer due to high-intensity convective rainfall events. The latter mainly occur in June, which is the month with the greatest precipitation amount. Mean annual precipitation amounts to 787.7 mm, while mean annual temperature is 11.4 \({}^{\circ}\)C for 1981-2010. Mean annual flow of the Tamnava River at Koceljeva is 0.99 m\({}^{3}\)/s, and of the Ub River at the Ub gauge is 1.01 m\({}^{3}\)/s. The highest flows at these gauges are 178 m\({}^{3}\)/s and 146 m\({}^{3}\)/s, respectively, and were observed during a great flood in May 2014. The study area has been affected by a number of historic flood events (1999, 2006, 2009, 2020, and earlier), including the catastrophic flood in May 2014. Flooding in the Tamnava watershed has often been caused by flooding of the river Kolubara, whose high-water levels would prevent efficient drainage from the Tamnava watershed and cause excessive backflow flooding in its interior. Historically, the floods along the Tamnava River have caused considerable damages, particularly in the vicinity of its confluence with the Kolubara River. The May 2014 flood was an unprecedented event in the hydrological record of the whole West Balkans region [20]. It was triggered by heavy rain falling during several days over near-saturated soils after several similar antecedent events. The extent of flooding exceeded historical records, with the Kolubara watershed being among the most affected ones. High water levels remained in the river valleys for weeks; the damages were substantial, and the casualties were reported. A special study on the 2014 catastrophic flood [21], herein referred to as the UNDP study, was intended for reconstruction of this flood event and a comprehensive evaluation of various proposed flood mitigation measures in the Kolubara watershed. This extensive study comprises the following:  
* A detailed description of the entire Kolubara watershed, including topography, stream network, soil types, geology, and other relevant features of the watershed, such as population, infrastructure, and economy (particularly agriculture and coal mining). * Thorough analyses of various aspects of flooding in the Kolubara watershed, including descriptions of the major observed floods in the study area. Figure 2: Maps of terrain elevations (**top**) and land cover (**bottom**) of the Tamnava River watershed. * Development of hydrological and hydrodynamic models aimed at reconstruction of the May 2014 flood event. * Comprehensive analyses of the existing flood protection measures in the watershed. * Macro-economic damage assessment in the watershed from the May 2014 flood, and for scenarios of the existing and planned flood mitigation measures.