2000, Naderi Beni Mohsen Graduated

Control of local scour at bridge piers by using Slot

Abstract

Scouring IS a natural phenomenon which causes bed materials to erode and to be carried out by water flow. Local scour is a special kind of scouring that is the major cause of bridge collapses. Different methods are used to control and reduce local scour. Using riprap or collar and combination of them are studied for many years. The use of slot through a pier is one of the new proposed methods for controlling local scour at bridge piers.

In this study, controlling of local scour at bridge piers is studied by use of 20 physical models at clear water condition. The models consist of a cylindrical pier without slot, three rectangular round nosed piers without slot and 16 piers with slot. The slots have two heights, equal to width (diameter) of pier and twice of it. Furthermore, they are located at two positions, near the bed and above it. The scour parameters such as, upstream and downstream scour depths, radius of scour hole (parallel and perpendicular to flow) and downstream pattern of pier scour are investigated. The experimental arrangement is set at the Hydraulic Laboratory of Isfahan University of Technology.

It is found that reduction ratio of scour depths and radia varied between 100% to 0% and even negative amount. The results of experiment data indicate that the above bed slots (except the slots close to the water surface) are ineffective on reduce the scour depths. At high flow velocity, close to the water surface slots have reduction ratio of 5% for circular piers and 13% for rectangular piers. In the first hours of the experiments when 50% to 60% of scouring occurs, close to the bed slots for all piers, cause reduction of scour parameters up to 100%. These two slots at low velocity have caused reduction over 50%. The bed slots at rectangular piers are more effective than circular piers to reduce scour parameters. The bed slot height is important parameter in the reduction process; the bigger the bed slot height, the more reduction the ratio of scour parameters. The scour patterns at the downstream of piers don't reach an equilibrium conditions. The bed slots prevent the formation of deposited materials near the pier and move them to a location further downstream.