Experimental Investigation Effect of Geometric Parameters and Hydrograph Shape of a Single Unsubmerged Spur Dike on the Temporal Development of Scouring Around the Structure Under Unsteady Flow

Background and Objectives: A spur dike is one of the structures that play a fundamental role in reducing the shear force on the river bank. The confrontation between this structure and the water flow causes strong eddies in both horizontal and vertical directions around the spur dike, which is the main cause of the scouring phenomenon around the spur dike structure and a result of its failure. Determining the depth of flooding is important because it is an indicator of the amount of flow destruction potential around the structure and is also an important parameter in the design of the foundation dimensions of the structures along the flow path. The findings of steady flow tests, in which the quantity of flow rate is equal to the peak flow rate of the flood hydrograph, are used to establish the maximum scour depth in the design of spur dikes (with a specified return period). The flow characteristics, and therefore the factors causing the scour, change with time in flood waves, and the scour depth after the hydrograph is less than the comparable peak flow rate’s equilibrium scour depth (link et al. 2017). The results demonstrated that because the non-steady flow and flow conditions vary in nature during floods, the temporal variations of scouring dimensions around structures under unsteady flow would be fundamentally different from those under steady flow. However, because no study has been performed on scouring around the spur dike under unsteady flow, there is no definite and recorded information in this field, and the magnitude of flood currents in nature makes the need for research in this sector even more pressing. Enhancing our understanding of scouring conditions and their temporal variations over time in the hydrograph will help us build better hydraulic structures.
Methodology: Experiments were carried out at the Hydraulic Laboratory of the Shahid Chamran University of Ahvaz (Iran) in a flume 10 m long, 0.74 m wide, and 0.60 m deep. In the present study, a single unsubmerged spur dike was considered for three percent permeability of 0% (i.e., impermeable spur dike), 33%, and 66%. Moreover, three spur dike alignment angles  equal to 60° (repelling alignment), 90° (deflecting alignment), and 120° (attractive alignment) were considered.  is the angle between the spur dike and the upstream wall. Totally, 27 experiments were performed in the flow rate range of 15 to 50 LS-1.
Finding: The experiments were designed to examine the impact of widely accepted geometric parameters of the spur dike (as an important and general structure used in river engineering projects to preserve river walls or other important structures such as bridges), such as its permeability (closed and open spur dike) and placement angle relative to the wall in time changes, as well as the maximum scouring depth around the spur dike in unsteady flow conditions. Furthermore, the influence of the shape of the hydrograph as a variable on the scouring process was explored. The comparison of scour depth variations between various scouring angles shows that the scour depth changes at different angles are nearly identical, and the distinction between scour depth changes in the test angles is small, indicating that the angle has little impact on scour depth changes. The spur dike permeability parameter plays an essential role in the maximum scour depth surrounding the spur dike and its value drops dramatically as permeability rises. Scouring in this area is caused by horseshoe vortex and rising in the spur dike nose. The movement of water through the open spur dike rods minimizes or reduces the intensity of vortices that occur behind the spur dike and near the nose. The process of scouring depth changes caused by all skewed and normal hydrographs has many differences. Since hydrographs skewed to the left (hydrograph with a ratio of peak time to hydrograph continuation time of 0.33) the time of the ascending branch is shorter and the discharge reaches its maximum value quickly, so the slope of the graph of the scour depth changes over time. It is very intense at first and then become insignificant. In hydrographs with a skew to the right (hydrograph with a ratio of peak time to hydrograph continuation time of 0.66), scour depth changes occur in more time.
Conclusion: By comparing the scour depth changes between different angles of the impervious spur dike, it shows that the scour depth changes are the highest at 90 degrees and the lowest at 120 degrees. While in spur dike with the permeability of 33% and 66%, the most changes in scouring depth occur at an angle of 60 degrees. The highest percentage of changes in the maximum scour depth compared to the scour depth in the peak hydrograph is related to the hydrograph with the ratio of the peak time to the duration time of the hydrograph 0.5 (normal distribution). The temporal development of scour depth in all three angles of 90, 60, and 120 degrees and all three hydrographs with the ratio of peak time to hydrograph continuation time is 0.33, 0.5, and 0.66, which is such that with the increase in the permeability of the scour, the scour depth It decreases significantly. So, On average, in the spur dike with permeability of 33% and 66%, respectively, compared to the impermeable spur dike, 48% and 88% reduction in scour depth is observed. The process of scouring depth changes caused by all skewed and normal hydrographs has many differences.