Experimental study on placement of toe protection elements of river bank protection works under live bed condition

Abstract

Bank protection works are essentially important parts of river training works. In recent times, out of all types of river training structures, bank revetments works are common in Bangladesh. The purpose of bank revetments is to prevent bank erosion as a structural measure. They have to be designed to resist the current and wave and to protect the river bed and river bank against erosion. Toe scour is probably the most frequent cause of failure of riprap revetments. This is true not only for riprap, but also for a wide variety of protection techniques. A flexible toe protection in the form of a falling apron finds many applications in Bangladesh. For designing of bank protection works in perennial rivers, where low water level prevails, the construction works have always been a difficult task. Placing of underwater apron materials requires a considerably higher skill, equipment demand and standard and practical approaches. Such scenarios become more challenging as the channel bed composed of sand. In the sand bed channel flowing under equilibrium condition may be considered as live bed channel. The present study has been undertaken to investigate experimentally two important aspect of underwater construction such as the placing behavior and incipient condition of toe protection elements under live bed condition. The experiments are conducted in the Hydraulics and River Engineering Laboratory of Water Resources Engineering Department, BUET. Three types of CC block and five types of geobag have been used to conduct fifteen experimental runs with two different hydraulic conditions to investigate placing behavior. For the incipient condition experimentation, four types of CC block and five types of geobag have been used to conduct nineteen experimental runs with various conditions. The measured data have been used to obtain various relationships at the incipient condition of the toe protection elements. Experimental results are analyzed to develop relationships between shear velocities, depth averaged velocity, relative depth, turbulent intensity, depth factor, boundary Reynolds number and Shield’s number. Developed empirical relationships can be used to predict the shear velocity, depth averaged velocity, turbulent intensity and depth factor at incipient condition for selected types and sizes of toe protection elements. The proposed relationships are also compared with the equations available in previous studies. Comparisons show that the predictive capacity of the proposed xviii relationships is found to be satisfactory. Both CC block and geobag shows greater shear velocity and shear stress in case of live bed than that of fixed bed. Shear velocity on a live bed is about 2% greater than that of fixed bed for CC Block. For geobag shear velocity is about 8% greater on a live bed than that of fixed bed. The shear stress on live bed for CC block and geobag is about 4% and 16% greater than fixed bed respectively. However, a comparison has been made between underwater and dry bed toe protection to observe the incipient parameters. It is suggested that the designer should consider the hydraulic parameters which should be commensurate with underwater constructional aspects. Therefore, this study will be helpful in designing of toe protection in case of underwater construction in real life practice.