Preliminary Results of Hydrologic Reconstruction of the May 2014 Kolubara River Flood - page 4

Similar results were obtained for the hydrologic station Beli Brod on the Kolubara River downstream from Slovac (Figure 5). At Beli Brod, only flow through the main river channel was registered, while a significant amount of floodwater flowed through two embankment breaches on the right banks of the Kolubara River, forming a flow path parallel to the main river channel.

 

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Figure 5: Computed and observed hydrograph for the hydrologic station Beli Brod.

 

The largest "disagreement" was noted at the Draževac hydrologic station on the Kolubara River (Figure 6). The primary reason for this is that, during the flood, the entire downstream reach of the Kolubara River was under the effect of backwater from the Sava River. The observed discharge data obtained from RHMSS was derived by extrapolating the stage-discharge curve, which was not developed to account for this amount of backwater effect. The limnigraph was completely submerged and did not continually register water levels; instead, only manual readings from staff in the field were provided, shown as points on the hydrograph.

 

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Figure 6: Computed and observed hydrograph for the hydrologic station Draževac.

 

To quantitatively assess the results of the hydrologic model of the reconstruction of the May 2014 Kolubara flood, the parameters of the computed and observed hydrographs are shown in Table 1. Values are given for all input, control and output profiles, i.e. hydrological stations where observed discharge data were available. The following basic parameters of the catchments and hydrographs are shown: drainage area (km2), total rainfall (mm and 106 m3), peak discharge (m3/s), total runoff (mm and 106 m3), and runoff coefficient.

 

Table 1: Characteristics of computed and observed hydrographs
Tab01

 

Based on the results, it is apparent that very large amounts of rainfall fell uniformly over the entire area of the Kolubara River basin (immediately before and during the course of the flood), in a short period of time. According to the delineation of subcatchments, the least amount of rainfall fell in the upper parts of the Tamnava River basin upstream from Koceljeva (195.6 mm), while most rainfall was registered in the neighboring catchment of the Obnica River upstream from Belo Polje (294.6 mm). A comparison of the computed and observed hydrographs that resulted from this rainfall reveals significant differences.

When analyzing the dimensionless runoff coefficient n, the hydrologic model gave more uniform values throughout the Kolubara River basin, ranging from 0.59 (Ub) to 0.86 (Belo Polje), with the exception of Draževac, located downstream of the open-pit mines, which lowered the runoff coefficient down to 0.44 due to the 200 million cubic meters of floodwater that filled the mines "Tamnava - West Field" and "Veliki Crljeni". The runoff coefficients for the observed hydrographs, on the other hand, range from 0.44 (Bogovađa) to 1.019 (Belo Polje) and 1.446 (Mionica), which are unreasonable values given that there was no snowmelt in May 2014 in the region. The remaining values of the runoff coefficient for the observed hydrographs range from 0.51 to 0.91.

From the perspective of total flood volumes, the observed hydrographs show a significant disbalance of + 164.6 106 m3 in the section between Beli Brod, Zeoke, Ub, Ćemanov Most and Draževac.

 

The hydrographs computed using the hydrological model show a minimal disbalance of about -7.5 106 m3, which amounts to less than 2%.

 

Conclusion

The hydrologic reconstruction of the flood wave formation and propagation in the Kolubara River basin in May 2014 yielded satisfactory results from a water balance perspective, considering the issues concerning flood protection, particularly overtopping and breaching of embankments, Kladnica Dam failure, and floodwater intrusion into the open mine pits "Tamnava - West Field" and "Veliki Crljeni". Hence, for most of the hydrologic stations in the Kolubara River basin, the maximum discharges calculated with the developed hydrologic model are more acceptable than the official values published in the Hydrological Yearbook of the Republic Hydrometeorological Service of Serbia (RHMSS).

 

Acknowledgement

The research presented in this paper is funded by Serbia's Ministry of Education and Science as a part of a scientific project "Assessment of Climate Change Impact on Water Resources in Serbia" (TR-37005) for the period 2011-2015.