SCOUR IN COHESIONLESS SEDIMENT BED UNDER SUBMERGED INCLINED JETS

Abstract

The Scour of beds of gravel, sand, weak rock is of significant importance in the field of hydraulic engineering so it is essential to control and predict excessive scour near hydraulic structures. Impingement of inclined submerged turbulent jet on beds in motion leads to a local scouring. The study of scour due to submerged inclined turbulent jet has many applications in Coastal Engineering. One such application is the prediction and prevention of localized scour of narrow ship channels due to ship thruster usage. Also, the study of cohesive incline scour can be applied to improving the efficiency of Jet assisted drag head dredging. Hydraulic structures such as culverts, spillways, gates, weirs, aqueducts and grade control structures built in rivers and channels are subjected to scour around their foundations and for its safety scouring pattern are important. l-ience, due to the practical importance, scouring process caused by submerged circular inclined jets has been a topic of continued interest to the design engineers as well as to the researchers. Even conducting larger number of experiment by researcher using different types of sediment. nozzle diameters and jet velocity there exist a large scope. In actual field conditions, the river bed can be composed of layers of sediments stratified which have different resistance to the scouring phenomena. The scour at stratified bed can be one of the reasons for the failure of the hydraulic structures, but this phenomenon has not yet been studied in the literature. Therefore. the present investigation has been conducted to fulfill this gap. The main objective of the present study is to determine the response of stratified sand and gravel bed under submerged circular inclined impinging water jets. To achieve the envisaged objectives, extensive set of experiments was conducted on scour in stratified cohesionless sediments under submerged inclined water jets. The laboratory experiments were performed in a circular steel tank of diameter 1.25 in and depth 1.25 rn. The tank was filled with the desired sediment up to a height of 0.70 m. The diameter of jet nozzle was 12.5 mm and centrally mounted in the circular tank and always submerged below water. Jet scour experiments were first conducted on saturated stratified gravel-sand beds, gravel of median particle size 5.2 mm and fine sand of size 0.24mm layers over gravel, jet heights, and jet velocity. Three jet velocities Iv of 5.98 m/s, 6.39m/s, 6.78ni/s were used with different angle of jet from horizontal at 360, 450, 550, 651, 75° in inclined position. Measurements for maximum dynamic, maximum static scour depth, dune height and scour hole volume were taken. The scour in stratification of cohesionless sediments has revealed interesting aspects about the process of generation of scour bed profile development. It has also revealed that armoring effect occurs when angle of inclination increases. The scouring process, temporal variations of scour depth and scour profiles have also been analyzed. Effect of inclination on the scouring process and on all dimensions of scour hole has been quantified. Equations for the estimation of maximum static scour depth in stratified Sand-Gravel bed have been proposed.