Mitigation of Local Scour by Using a Training Structure Placed Upstream of a Spur Dike.

Akihiro Tominaga, Kazuki Tomida

Monday 29 june 2015

15:05 - 15:20h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 2A. Sediment - Erosion

The substantial function of spur dikes is to train the main flow away from river bank. Recently, spur dikes are broadly used for environmental and ecological purposes. However, excessive scouring around spur dikes is likely the biggest obstacle to its successful performance. In order to use spur dikes for a long time, it is necessary to investigate the method to prevent local scour around spur dikes. In this study, a lower-height structure placed upstream of the spur dike is considered as a measure of mitigating local scour. This hydraulic structure is called as a training structure. The relative height and location distance of the training structure with the major spur dike were changed and their effects were investigated in a movable-bed condition. Then, the mechanism of reducing local scour by the training structure was evaluated from the flow structures on a fixed bed experimentally and numerically. The experiments were conducted in a 12m long and 0.6m wide movable bed rectangular flume with a 0.002 slope. The sand diameter was roughly uniform, and d50 = 0.61 mm. Spur-dike model was rectangular column made of concrete and 5cm wide, 10cm long and 4cm high from the initial flat bed. The training structures were the similar column with different height of 1cm, 2cm, 3cm and 4cm from the bed. The interval length between the training structure and the spur dike was set from 5cm to 30cm with 5cm interval. The scour around main spur dike becomes smaller with an increase of the height of training structure, but the local scour around the training structure increases. Therefore the height of the training structure is adequate for 0.5 times of the height of main spur dike. The vorticity is concentrated in the gap when the interval length between training structure and spur dike is too short, but the reduction effect decreases when the interval length is too long. The location interval is recommended to as wide as the vorticity does not concentrate between the training structure and the target spur dike. The mechanism of reducing the scour around the spur dike is not attributable to the flow structure on the flat bed but it is subjected to the bed deformation process.