Effektstudier af vandløbsrestaureringsprojekter: Hydrauliske og geomorfologiske betragtninger

Lund Jensen, Anders

10. semester

Civilingeniør i Miljøteknik

2011

2011-06-30

75 pages

The effects of two river restorations in Denmark are investigated in order to assess, whether the restoration measures had the wanted effects. In the river Gudenå at Vilholt Mill, a weir was remo- ved in order to ensure the free passage for fish and macroinvertebrates. The weir removal initiated erosion of sediment in the upstream reaches which led to deposition of sediment in the downstream reaches. Today, about two years later, the reaches upstream the former weir are still being eroded, but they are considered stable. The physical quality of spawning gravel in the riffles located downstream the former weir is difficult to assess due to lack of documentation of the quality before the weir removal. The deposition of sediment above the spawning gravel is negligible. In the stream Rakkeby Å in Vendsyssel spawning habitat rehabilitation has been performed by reintrodu- cing spawning gravel and construction of a sediment trap. The physical quality of spawning gravel located upstream and downstream the sediment trap is investigated by measurements of the infilt- ration rates of fine sediment in a period in autumn/winter 2010/2011. The measurements show that the sediment trap generally sorts all sediment coarser than 0.5 mm from the sediment transport in the river. Consequently less sediment is infiltrating in the spawning gravel located downstream of the sediment trap. The effect of the sediment trap is negligible around 1500 meter downstream of the sediment trap. The sediment trap is only functioning during periods of high flow, where the bed load transport of sediment is high. Local hydraulic conditions and the morphology of the reaches have some effect on the infiltration rates. It is generally found that the total transport of sediment above the spawning gravel is controlling the infiltration rates of fine sediment. Although the sedi- ment trap is reducing the total sediment transport, the physical quality of the spawning gravel downstream of the sediment trap is low due to a high amount of sediment finer than 0.5 mm. The supply of sediment to the stream has to be reduced in order to improve the physical quality of spawning gravel in the stream.

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