Study ‘Assessment of the Environmental Flow of the Drina Downstream from the Visegrad HPS in an Urban Section in the City of Visegrad’
This Study defines environmental flow (EF) appropriate to this particular system and observes the EF with detailed adaptation to hydraulic, climate and ecological conditions of this river section of the Drina, particularly through the urban area of Visegrad.
Based on the available data from the Visegrad GS on the Drina, the environmental flow was calculated with different periods of hydrological data series, as follows:
- Hydrological data series for the period 1926-1975: natural flow regime of the Drina, i.e. undisturbed conditions before the construction of upstream dams on the Drina and its tributaries, which could significantly affect the discharge and stage at the Visegrad GS;
- Hydrological data series for the period 1977-2006: artificial flow regime of the Drina; data representative for this period are taken from the document ‘Processing and selection of a new hydrological data series of the Drina River Basin from the Visegrad HPS, HET, Trebinje’ (2008). This study presents data on average annual discharge for the period 1946-2006, without showing the daily discharge and stage at the Visegrad GS;
- Hydrological data series for the period 1926-2006: unified natural and artificial flow regimes of the Drina.
The environmental flow was calculated using three different methodological approaches:
- The approach from the RS Water Act, Article 65: until the adoption of the bylaw, the environmental flow will be adjusted on the basis of hydrological characteristics of the water body for characteristic seasons as a minimum average monthly discharge of ninety-five percent exceedance;
- The approach from the Draft Rulebook on the Determining the Environmental Flow in FB&H;
- The previously tested methods in RS - the Guaranteed Environmental Flow Method (GEF); Pursuant to the Article 65 of the RS Water Act, attempts have been made so far to propose an improvement of the existing legal regulations for determining the EF. Some methodologies were developed, among which the GEF method was evaluated as the one with the highest chance of success.
Given that the representative data for the artificial flow regime of the Drina are quite general and insufficient, we could not calculate the EF for the period 1926-2006 without taking data from the period 1926-1975, where observed daily discharges are available.
The EF calculated by different methods for different hydrological data series (by duration, flow regime, etc.) gave approximately similar results. Given that the daily discharge data for the period 1946-2006 were not available, we were not able to produce a more precise calculation of the EF for the period 1977-2006.
The figure below shows a comparative overview of the input data and environmental flow calculated using different methods at the Visegrad GS on the Drina for the period from 1926 to 2006: