• Anders Lund Jensen
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.
Publication date10 Jun 2011
Number of pages75
ID: 53256042