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سوراخ (سوراخ ایجاد شده با اب)
The scour hole formed due to the impingement of high velocity jet far away on the riverbed-composed of hard and sound rock-was of little concern, since it was at a considerable distance away from the structure and chances of retrogres- sion towards the structure were remote.
The process of rock scouring with trajectory buckets has three distinct phases: fracturing of rock into blocks of various sizes as a result of dynamic pressures in the fissures and discontinuities in the rock mass, pulling out, and disintegration and transport of rock blocks by the flow forces to outside the zone of fracturing, thus producing a scour hole and a down- stream mound.
There- fore, most model tests employ a downstream bed simulated with cohesionless fractured material that could atleast be transported by the flow, out of scour hole.
The difficulty with cohesionless bed is that the resulting dish-shaped scour hole in the model overestimates the areal extent and is unable to predict steep sides and secondary scour patterns near the structures, so important from designers' view point.
In a few cases, semi-erodible material composed of some weakly binding additive like gypsum, bentonite, etc. has been used to obtain steep side slopes of scour holes and secondary scour patterns.
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