Effect of flow unsteadiness on local scour around bridge pier and application of collar and riprap

In the present dissertation, experimental and theoretical approaches were carried
out on the effect of flow unsteadiness on local scour around bridge piers and
application of collar and riprap. Experiments were conducted on two horizontal
flumes in Hydraulic lab of Amirkabir University of Technology. These experiments
can be divided into 3 different sections. First, experiments for detecting the effect
of flow unsteadiness on scour depth and dimension around unprotected pier.
Second, experiments for determining the performance of collar in protection of
stream bed around the pier in unsteady flow condition. Finally, experiments for
investigation of riprap stability around bridge pier in an unsteady flow condition.
Totally, around 110 sets of experiments for unprotected pier, 50 sets of experiments
for protected pier with collar and 40 sets of experiments for protected pier with
riprap were carried out in present study. Both circular and rectangular pier shape
were used and the ratio of collar to pier width or diameter was 3. In each section, to
detect the effect of flow unsteadiness, some experiments were preformed under
steady flow condition.
Experimental results showed that, hydrograph parameter time to peak has
considerable effect on the rate of scouring and little effect on final scour depth. In
addition, though the shape of scouring rate curve in unsteady flow differs to steady
flow condition, the calculation of the curve by stepped model hydrograph leads to
acceptable results. In the next step, mechanism of scour hole formation in case of
circular and rectangular pier protected with collar in steady and unsteady flow
condition were deeply studied. New empirical equations were developed for
calculating time at which scouring penetrated below the collar, final scour depth
and rate of scouring. Finally, in the case of protected pier with riprap, a
modification factor based on flow unsteadiness was determined for riprap size
calculation.

Effect of flow unsteadiness on local scour around bridge pier and application of collar and riprap Keywords:

Bridge pier , local scour , hydrograph , protective collar , riprap