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Mean and Turbulent Flow Structures in Two Consecutive Meander Bends

Author(s): Markus Foerst; Nils Ruther

Linked Author(s): Nils Rüther

Keywords: No Keywords

Abstract: Soil erosion along riverbanks has been an important issue due to economic and ecological damage. One cause for erosion is natural river migration. A natural meandering river in northern Norway (Troms) has been subject for this investigation. The first bend is a 90 degrees turn after a straight reach. The following two bends turn each 180degrees. Focus lies on the abundance of secondary currents and its influence on natural erosion processes within the first two bends. The river is non-regulated and is not exposed to scour protection on the river banks. It developed consecutive meander bends in Holocene fjord sediments. The meander bends are strong curved and show an upstream orientation. Climatic conditions cause an annual water high stand in spring. The hydraulic measurements have been conducted with a moving Acoustic Doppler Current Profiler (ADCP) along cross sections perpendicular to the main flow. The flow measurements were conducted in 72 cross sections with 8 crossing for each cross section. This density of measurements results in a detailed description of the mean flow in the meander bends. In addition, the secondary flow in the outer bank has been quantified with set of highly dense fixed position ADCP measurements. The investigation shows that the flow is highly influenced by the macro roughness elements of the banks, i. e. trees, submerged trunks and relatively small coherent local scours in the bank. Different flow pattern were analyzed and grouped based on statistical similarity measurement. Results show that the flow pattern in the subsequent meander is more susceptible to changes in water level than the first one where the water has a rather straight inflow. The impact angle dips steeper into the second outer bank the higher the water level is. During snowmelt Breidvikelva shows a rather constant high water level which is 1. 8 to 2. 0times above the water level during summer. A decrease of the occurrence and strength of secondary flow has been observed during lower water levels. Preliminary results show a stable pattern of the water flow though the water level alternates. Mayor changes occur in the water velocity.


Year: 2012

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