Abstract
The existence of a planform vortex is considered to be the cause of development and sustention of large-scale local scouring in the Kiso River. In order to make clear the 3D flow structures in this deep local scour, we conducted a field observation and model experiments. From the result of ADCP set on the bottom of the scour hole, the velocity in the hole was increasing and the vertical vortex was developing, with an increase of river discharge. It is indicated that considerable magnitude of velocity was generated even near the bottom by the action of vertical vortex. This vortex was recognized in the model experiments. Consequently, the vertical vortex contributes to develop and maintain the large-scale local scouring.
Highlights
In the Kiso River, there exists a deep scour hole with 20m in depth where the sand layer was locally and severely scoured by peeling of the clay layer which covered over the lower sand layer
As to the erosion of the clay layer, it is estimated to be caused by a local increase of the bed shear stress due to the flow contraction involved by fixed sand bar
It is estimated from the process of scouring that the initiating scour occurred near the head of round projection at 36.8km point, and this caused over-all bed degradation and deep scouring in the upstream area
Summary
In the Kiso River, there exists a deep scour hole with 20m in depth where the sand layer was locally and severely scoured by peeling of the clay layer which covered over the lower sand layer. The clay layer was exposed by flushing of the upper sand layer due to river-bed degradation This deep scour is possible to be expanded to the upstream area and this may cause destabilization and damage to levee and hydraulic structures. There is a round projection on the right side of the river a little downstream from the most scoured region at 37 km point as shown in Fig.. It is estimated from the process of scouring that the initiating scour occurred near the head of round projection at 36.8km point, and this caused over-all bed degradation and deep scouring in the upstream area. Tominaga et al [4] investigated the flow and bed deformation around a narrowed section in an open channel but such a deep scouring was not reproduced at the similar location as the actual river
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