Improved estimation of critical bed shear stress downstream of low-head weirs in streams with coarse bed material

Existing methods for estimating critical bed shear stress are limited and usually do not consider local scour processes; therefore, the accuracy of numerical modeling is usually compromised. This paper presents the results of physical experiments on scour downstream of low-head weirs and proposes new equations for estimating the critical bed shear stress in scour holes. The experiments were done using a camel hump weir and coarse-bed materials which are applicable to steep streams with coarse-bed materials (gravel, rocks, etc.). The critical shear stress was regarded as the bed shear stress within the scour holes, which was determined using the three-dimensional (3D) flow field and bed morphology measured at the equilibrium scour state. The influence of the sediment size also was investigated. The experimental results showed that a scour hole can be divided into three zones: Zone-I for the upstream scour slope, Zone-II for the downstream scour slope, and Zone-III for the downstream slope of the sediment deposit downstream of the hole. The relation between the critical shear stress of the bed surface in Zone-I, the flow rate, and bed position was established, and the new equations yielded better accuracy than existing methods. In addition, the relation between the critical shear stress and the slope of the bed and the ratio of the local water depth to the particle size in Zone-II and Zone-III were established. After verification, the calculation results of the newly proposed equations were in good agreement with the standard values of the dimensionless critical Shields parameter obtained through processing the experimental results in the current study. Further discussion is provided regarding the integration of the parameters calculated applying the new parameters in the numerical models.