Seite - 1147 - in Book of Full Papers - Symposium Hydro Engineering

the excavated material, in this construction phase the water levels would still remain above the previous state only in the lower part of the right retention. In stage 3 the middle third of the embankments is built. After the construction of the embankments throughout the length the outflow of water from the channel to floodplains is restricted to the section above the NPP Krško weir, the outflow openings in the built embankments (of a length of approx. 100 m) in the middle part, the inlet into the diversion channel, and the discharge over the relief system. In the latter, the spillway crest elevation of a fully open relief system was considered in the calculations, i.e. in the state where the concrete structure is built, while the hydromechanical installations are still lacking. In this construction period, retention areas during high waters would be activated to a similar degree than upon the completion of the Brežice HPP, meaning that in the retentions lower discharges than prior to HPP construction would occur, and as a consequence also lower water levels in the most part of the retention areas. To relieve the lower part of the right retention and reduce the water mass spilling across the area at the construction pit, there are still two 100-m long openings in the middle part of the reservoir envisaged (Figure 9). The location of the opening on the left allows for as much outflow as possible (outer part of the curve) to the left retention, while the opening on the right allows for a more favourable flow field in the part between the construction pit and the motorway. The increased mass of the water current from the middle to the lower part in the right retention considerably weakens the transversal water current which is, due to the shape of the construction pit and the diversion channel, directed transversally, towards the motorway. The analysis of the velocity field of the final variant confirmed this effect; at the same time, local velocities (or erosion hazard) and local water levels along the motorway are not increased. The final construction stage, which was critical mostly because of the potential locally increased velocities and thus erosion hazard, comprised the closing of openings in the embankments in the middle part, the closing of the diversion channel, and the redirection of the flow through the spillways of HPP Brežice. It was also necessary to determine the impact of redirecting the Sava River through HPP spillways on flood safety and the impact on the ongoing works at the reservoir (to estimate the possibility of damage) in the phase when the diversion channel is partially or completely backfilled. Figure 10 shows the velocity fields in the case of still active 100-m outflow openings in the levees for the case with backfilled diversion channel (left) and the still open diversion channel (right). As evident, the lower part of the right retention between the HPP weir and the Ljubljana–Zagreb motorway was the most affected by erosive action due to maximum flow velocities in retention areas. The calculations have shown that erosion risk can be reduced if backfilling of the diversion channel starts at a later stage, to preserve its discharge capacity as long as possible. In the case of backfilling the diversion channel before completing the embankments the erosion processes would be strongest at downstream ends of the already finished embankments (openings in the middle part and before the inflow into the diversion channel). Water current velocities up to 3 m/s would occur, which can transport gravel and rubble up to a grain size of approx. 100 mm. By preserving the diversion channel open it was possible, due to higher discharge 1147