Effect of Duration and Pattern of Hydrographs on Scour Around Pier in Sharp Bend Under Incipient Motion and Live Bed Con
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RESEARCH PAPER
Effect of Duration and Pattern of Hydrographs on Scour Around Pier in Sharp Bend Under Incipient Motion and Live Bed Conditions Saman Solati1
•
Mohammad Vaghefi1
•
Abdol Mahdi Behroozi1
Received: 11 March 2020 / Revised: 8 July 2020 / Accepted: 28 July 2020 Ó Iran University of Science and Technology 2020
Abstract In the present study, the effect of peak occurrence time and flood duration on the scour around the bridge pier installed in a sharply bended channel was investigated. For this purpose, several simple single-peaked hydrographs with the same durations and different peak occurrence times were produced in laboratory. In addition, hydrographs having two consecutive peaks with a steady flow discharge at the first peak and an unsteady-flow discharge at the second peak and vice versa were investigated. The flow velocity range began from incipient motion conditions and steeply increased to live bed conditions. In each experiment, the temporal variations of the maximum local scour depth were measured. In single-peaked experiments, by increasing hydrograph duration from 25 to 100 min, the maximum scour depth increased by 20%, and the progression of sediments along the downstream straight path increased by 150%. For hydrographs with two non-equal and the same duration peaks, at a constant discharge for the first peak and a decrease in the second peak discharge from 70 to 60, 50, and 45 (L s-1), the volume of the scour hole around the pier was decreasing 15, 40, and 60%. Its area fell by 10, 40, and 55%, respectively. When the rising and falling limb time of hydrographs were equal, the maximum sedimentation height increased by 2.7, 3.5, and 5 times, respectively by increasing test duration from 25 to 50, 75, and 100 min. Keywords Local scour Live bed Flood hydrograph Peak number Flood durations List of Symbols Vm The velocity of the flow (m s-1) Vmc The competent mean velocity of the incipient motions (m s-1) Dg The average sediment particle diameter (m) d The flow depth (m) c0s Submerged specific weight of grains (kg m-2 s-2) The sediments density (kg m-3) qS q The water density (kg m-3) g The acceleration of gravity (m s-2) D Pier diameter (m) Tr The peak occurrence time (min) TD Duration of hydrographs (min) QP1 The maximum discharge in the first peak (L s-1)
QP2 A V b1 ds dsmax Hmax – O b2 L
& Mohammad Vaghefi [email protected] Saman Solati [email protected] Abdol Mahdi Behroozi [email protected] 1
Department of Civil Engineering, Persian Gulf University, Shahid Mahini St, 7516913817 Bushehr, Iran
The maximum discharge in the second peak (L s-1) Area of the scour hole at the bridge pier (m2) Volume of the scour hole at the bridge pier (m3) Location across of the scour hole at the upstream face of the bridge pier (m) Scour depth (m) Maximum depth of the scour hole at the upstream face of the bridge pier (m) The height of sedimentation over the bend (m) Position of the height of sedimentation over the bend (0) Location across o
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