Experimental investigation scouring around spur dike under unsteady Flow

Estimation of maximum possible scour depth around spur dike is an important step in the design of spur dike foundations To determine the maximum scour depth in the design of spur dike foundations, the equilibrium scour depth, which is commonly estimated using peak-flow conditions for engineering design of spur dike foundations, are used.. In fluvial rivers, significant transport of bed materials often takes place during peak-flow discharge in a flood event. For large rivers, the duration of a flood event may last for a few months, but for others the unsteadiness of a flood can be pronounced. The general practice of employing peak-flow discharge to evaluate the maximum scour depth for design may be questioned because the maximum scour depth occurring under a flood hydrograph can be much smaller than the calculated value using peak-flow discharge. In other words, using the peak-flow discharge for design can greatly overestimate the maximum scour depth in comparison to the actual conditions under the flood hydrograph. Therefore, when the flow unsteadiness is pronounced, investigation of temporal variation of clear water scour at spur dike is important to estimate the possible extension of the scour hole. This would provide useful information for safe design of footing and the selection of scour counter-measure to be implemented. The degree of severity of the problem is dictated by the magnitude of this scour hole. In this experimental study, the scour around the single spur dike was investigated under unsteady flow. These tests have been done in terms of clear water and non cohesive sediments. In order to investigating the scour around the spur dike, experiments have been done by changing the properties of normal hydrograph such as peak discharge and durability time. In order to producing the hydrograph, we used a device for adjusting the speed of pump motor and generating the variable discharge according to actual discharge time series. In the inlet pipe of pump, a magnetic flow meter is located that Measures the discharge in per one tenth of a second. Thus, the system verifies of requested discharge at any time. The results show that one of the influential parameters that affect the scour around the spur dike is the durability time of hydrograph. We concluded that when the durability time increases 6-times, scour depth will increase 40 percent, because with increasing durability time, the time that spur dike exposes tensions increases. Also, the depth of scour has increased 35 percent, by increasing 25 percent of peak discharge of hydrograph that is caused by increasing stresses imposed on the bed. Finally, the resulted scour under unsteady flow was compared with the resulted scour under steady flow (With flood peak discharge), with a significant difference between the scour under steady and unsteady flow. Then, an imperial equation was proposed for calculating the scour depth under hydrograph. This equation was obtained by adding the unsteady coefficient of flow in the equation of equilibrium scour depth under steady flow. This unsteady coefficient of flow includes effective parameters of hydrograph such as peak flow of hydrograph, peak time, durability time of hydrograph and equilibrium time of scour.

Experimental investigation scouring around spur dike under unsteady Flow Keywords:

Experimental , Scour , hydrograph , Spur dike