الفهرس 
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Abstract
The use of natural open channels for transportation of water presents the possibility of scour. Loose boundary
channels are constantly shifting alignment due to this phenomenon while man made channels through natural earth must be carefully designed so as to either keep local velocities below that causing scour or to maintain an equilibrium between deposition and entrainment at anyone point.
In the majority of hydraulic works, the flow is such as to form a clear hydraulic jump and a concrete floor is
provided which is usually longer than or equal the length of the hydraulic jump. Downstream of the concrete floor,
existing the bed, which can be eroded.
There are two major differences between the flow after formation of a jump and the flow in a long reach of channel.
The first difference is in the velocity distribution, which is high close to the bed after the jump, whereas in a channel the maximum velocity is close to the surface. The second
difference is the sudden ending of the rigid concrete floor followed by an erodible bed. This is the case of separation into flow lines resulting in the formation of eddies and
rollers. At this point erosion of the bed take place. To prevent this erosion a material of suitable diameter and
weight should be placed until ufficient length.
Rock has been used for river improvements, and below
hydraulic structures as a means of protection against the scouring action of flowing water. it is usually believed that very large boulders would prevent scour, but in order to reduce costs, it is desirable to know the smallest size rock