Scale Effects in Flume Experiments on Flow around a Spur Dike in Flatbed Channel

This paper presents the findings from a series of flume experiments conducted to determine scale effects in small-scale models of flow around a single spur dike (wing-dam, groyne, or abutment) placed in a channel whose bed is fixed and flat. The flow features of primary interest are flow-thalweg alignment (line of maximum streamwise velocity) around a dike, and area extent of the flow-separation region (wake) immediately downstream of the dike. Those features are of practical concern in the deployment of dikes for various channel control purposes. The scale effects of concern herein are those attributable to use small length scales together with a bed-shear stress parameter (e.g., Shields parameter) as the primary criterion for dynamic similitude. Small-scale models, especially micromodels, often are used for investigating channel-control issues. Also, the shear-stress criterion is commonly used for models of flow in loose-bed channels, whereas Froude number commonly is the primary similitude criterion for models of fixed-bed open-channel flows. The experiments show that use of a shear-stress parameter as the primary criterion for dynamic similitude influences the flow thalweg and flow separation region at a dike. It does so by distorting pressure gradients around the model dike and by affecting turbulence generated by the dike. It also is shown that, for a range of small models, thalweg alignment and extent of separation region do not scale with model length scales. These findings are important for interpreting results from small hydraulic models, especially micromodels, of flow in loose-bed channels.