Scour Analysis in High Vertical Drop Shallow Using Numerical and Physical Models
The "vertical drop Structure" is one of the basic types used in channel system flow. Drops are used as slope breakers in some parts of channels where we encounter supercritical slopes. Scour is a dynamic phenomenon that is visible in vertical drop tail water depth. Because of complexity of Scour process and simultaneous testing of variables' effective compliance, studies are still underway to determine the accuracy of the equations and provide possible explanation for this phenomenon. These investigations include field or laboratory experiments under special conditions where mathematical models have been often used to provide an explicit result. Using the laboratorial physical modeling, this research explores the impact of a vertical drop flow rate on the balance of forces at the scour depth below a high vertical drop . Satisparametery results have been obtained by changing several of the parameters like sediments grading, tail water depth and flow rate. In order to specify a set of effective parameters, dimensional analysis has been used to show the scour hole dimensions. Also, computer software FLOW-3D was used to simulate the numerical modeling of Scour below a high vertical drop. The data from numerical simulation is then compared with physical modeling resulting in the great compatibility and correlation.
Scour below a high vertical drop
Experimental results on scour below a high vertical drop (drop height/critical depth >1) in uniform sands and gravels are presented. The experimental results are used to describe the effects of important parameters, identified from the dimensional analysis, on equilibrium scour depth. The important observations are that the equilibrium scour depth increases with increase in densimetric Froude number, whereas the scour depth decreases with increase in sediment size and tailwater depth. The time scale of scour depth that follows an exponential law is determined. The nondimensional time scale decreases with increase in densimetric Froude number.
Scour in Long Contractions
Experimental results on local scour in long contractions for uniform and nonuniform sediments (gravels and sands) under clear-water scour are presented. An emphasis was given to conduct the experiments on scour in long contractions for gravels. The findings of the experiments are used to describe the effects of various parameters (obtained from dimensional analysis) on equilibrium scour depth under clear-water scour. The equilibrium scour depth increases with decrease in opening ratio and with increase in sediment size for gravels. But the curves of scour depth versus sediment size have considerable sag at the transition of sand and gravel. The scour depth decreases with increase in densimetric Froude number, for larger opening ratios, and increases with increase in approaching flow depth at lower depths. However, it becomes independent of approaching flow depth at higher flow depths. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to the formation of armor layer in the scour hole. Using the continuity and energy equations, a simple analytical model for the computation of clear-water scour depth in long contractions is developed with and without sidewall correction for contracted zone. The models agree satisfactorily with the present and other experimental data. Also, a new empirical equation of maximum equilibrium scour depth, which is based on the experimental data at the limiting stability of sediments in approaching channel under clear-water scour, is proposed. The potential predictors of the maximum equilibrium scour depth in long contractions are compared with the experimental data. The comparisons indicate that the equations given by Komura and Lim are the best predictors among those examined.