Experimental modelling of local scour phenomenon at a series of repelling emergent spur dikes

A spur dike is an elongated artificial structure with one end on the bank of a stream and the other end projecting into the current, and it is the most cost-effective river training structure that can be built at the channel’s banks. A series of spur dikes are usually more efficient in stabilizing the alluvial shores, whereas single spur dikes alter the local field. Thus, analyzing the local scour phenomena surrounding hydraulic structures in rivers is crucial to minimize the hazard of foundation collapse.

Therefore, experiments have been conducted to study the phenomenon of local scouring around the series of repelling spur dikes under clear water conditions, analysis of flow behavior & alterations in the morphology of sediment bed, and turbulent fluctuation. The inclination angle of the non-submerged spur dike with the vertical wall was kept 60⁰ during the study in the straight rectangular flume of length, width, and depth are 15 m, 0.91 m, and 0.70 m. While the projected length of spur dikes was 1/5 of the width of the channel, and the spacing between spur dikes was 2.5 * the projected length of spur dike. In laboratory experiments, the flow velocities and bed deformation around the series of repelling spur dikes were measured using an Acoustic Doppler velocimeter, a high-resolution laser displacement meter, and a point gauge.

The test section consists of uniform sediment particles, the experiment was initiated with a leveled sediment bed, and a scouring phenomenon was observed throughout the experiment at the head, middle, and end of each spur dike in the u/s and d/s. The 3D velocity measurement is done at the head of the spur dike from u/s of the first spur dike till downstream of the third spur dike. Velocity measurements provide information on dominant agents responsible for the local scour.

It was concluded that the maximum depth of the scour hole 14.47 cm at 1^st spur dike head. Digging and siltation was a cyclic process till equilibrium was achieved during the experiment, and the flow was classified as subcritical and turbulent. The approaching flow has less strength between the 1^st and 2^nd spur dike as it moves upward mostly in the top section.  The negative values of  over some length was observed in the scoring zone near the bed. While comparing the value of non-dimensional Reynolds Shear Stress -u^’v^’/u_*²,  -u^’w^’/u_*²,  -v^’w^’/u_*², it was observed that -u^’w^’/u_*², had a much greater both positive and negative value compared to the other. The Turbulent Kinetic Energy distribution shows that there is relatively more turbulence surrounding the 1^st spur dike.