الفهرس 
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Abstract
When an old bridge pier is accompanied by another new one , it is of great importance to know the effect of the interaction between both new and old one on the scour hole around the old one.
The objective of this research is to investigate how the scour hole characteristics around the old bridge pier are affected by the construction of the new one, taking into consideration different hydraulic and geometrical parameters. These parameters are Froude number, longitudinal separate distance between piers, new pier dimensions which are taken as relative values from the old one, and the new pier position with respect to the old one in the long direction.
The bed material used in the experiments is sand with 0.12 m thick and 0.47 mm median diameter. A sum of 180 experiments were carried out under steady clear-water scour conditions in the irrigation and hydraulic laboratory at the faculty of Engineering, Mansoura University , Egypt.
Applying dimensional analysis technique, empirical relationships were obtained based on the experimental results relating the predominant scour parameters to the flow parameters and the new pier geometry.