Seite - 1143 - in Book of Full Papers - Symposium Hydro Engineering

By analysing the differences in the water levels any potential adverse impacts during the construction are determined, so as not to excessively increase flood risk, deteriorate or even disable the functions of infrastructure facilities, or even put the stability of levees at risk, etc. 2D velocity analyses allow for assessing (the changes in) erosion risk in the wider flood area. 4.1. IMPACT OF THE HPP BREŽICE CONSTRUCTION PIT AND THE DIVERSION CHANNEL ON THE RUNOFF REGIME Prior to HPP construction, the occurrence of a 100-year return period event (3750 m3/s) would result in a quarter of the entire discharge (approx. 900 m3/s) flowing along the right floodplain between the Sava River and the Ljubljana–Zagreb motorway. The location of the construction pit and the diversion channel considerably change the velocity field; as the area of the construction pit is excluded from the flow, throughput is decreased, and the currents are redistributed, causing also a different distribution of depths, velocities, erosion, etc. To minimise the costs of dredging the diversion channel, the best location for the disposal site would be as close as possible to the channel, but the proposed location for the disposal of the material on the right bank would further narrow the already-narrow cross-section. The increase in water levels would increase flood hazard upstream, while, in the narrow cross-section and downstream part, the increased velocities or shear stresses would increase the erosion hazard on agricultural land and the motorway. At the discharges investigated, the mere construction of the construction pit would cause a rise in water levels in the left and right floodplains by up to 45 cm, the effects of which would diminish about 2 km upstream; this would require the construction of dikes near some settlements even in the initial stages of constructing HPP Brežice. The overall impact of the construction pit and the disposal sites would be sufficient to cause flooding of the motorway at a 100-year return period event. The disposal of the material dredged from the diversion channel at the right floodplain would partially obstruct the return of water from the right retention toward the confluence of the Sava and Krka rivers, which would increase the flood hazard vulnerability of the settlements upstream, along the Krka River. 1143