Abstract: River bank erosion is commonly counter measured by placing protecting elements. The placement of protecting elements in under water condition has always been a crucial task. In particular the construction works of toe protection like falling apron cannot be properly implemented in a perennial river, where low water level prevails. Placing of underwater apron materials requires a considerably high skilled manpower, equipment and appropriate practical approaches. The present paper aims to undertake experimental investigation on incipient (threshold) condition of toe protection elements under live bed condition. The aim is to provide a simple guideline for the ease of construction. The experiments have been conducted in the Hydraulics and River Engineering Laboratory of Water Resources Engineering Department, BUET. Four types of CC block and five types of geobag have been used. A total of sixteen experimental runs with two different hydraulic conditions have been investigated. Experimental results are analyzed to develop empirical relationships that can be used to predict the shear velocity at incipient condition of toe protection elements. The proposed relationships are also compared with the equations available in earlier studies. Comparisons show that the predictive capacity of the proposed relationships is found satisfactory. It is suggested that the designer should consider the hydraulic parameters which should be commensurate with underwater constructional aspects.

Keywords: Flume study, Incipient motion, protection elements, threshold velocity, falling apron.