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Habersack, H., Nachtnebel, H. P..
(1995): Short term effects of local river restoration on morphology, flow field, substrate and biota.
Regulated Rivers, Research and Management. Vol. 10, Nr. 2-4, 291-301

The objective of this paper is to analyse the short-term effects of restoration on river morphology, flow field, substrate and biota. One of the aims of the restoration measures is to arrest the degradation of the river Drau, which has caused technical (instability of bank protection measures) and ecological (loss of dynamic river reaches with alternate bars) problems. The restoration programme consists of several measures such as digging a new side channel on the right side, leaving an island and erection of groins on the left side of the main channel. These were made to keep the entrance open. After a two year monitoring programme the results of the analysis show that with respect to the river morphology, flow held and substrate, positive and negative effects were recorded, whereas for the ecology, positive effects were in general documented. Repeated morphological surveys using an echosounder showed that within a year without major flood events the main channel aggraded on average about 16 cm and the upper parts of the island degraded by up to Im. The groin pools became partially filled due to their function as sediment traps for finer material during low flows. Subsequent, measurements performed after several hoods showed that degradation reduced the amount of aggradation by 50%. Flow velocity measurements demonstrated the effects of groins and their inability to force the main flow path to the side channel. In contrast with the main channel, the side channel was characterized by highly variable sediment grain sizes and dynamic morphological changes. The lower shear stresses and higher growth of algae in the side channel were associated with higher numbers of species of macroinvertebrates. Population studies showed significantly higher densities of fish species in this reach with a side channel than in any other river reach. Forty species of birds were observed, seven of which were typical water-related species, and the number of plant species increased from 48 to 118.
Authors BOKU Wien:
Habersack Helmut
Nachtnebel Hans-Peter

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