Clear-Water Experimental Scour Depths at Abutments
Abstract
Laboratory experiments on local scour process at wing wall bridge abutments were conducted for different hydraulic conditions and compared with measurements on local scour depths around vertical wall abutments, under the same flow and sediment transport conditions. The study reports an extensive experimental investigation performed in a laboratory flume in the Technological Educational Institute of Thessaly, whose findings are used to describe the effects of different hydraulic parameters on local scour depth variation in the vicinity of the constructions. Three different sizes of wing wall abutment’s lengths, transverse to the flow direction, were used in order to investigate the impact of this parameter to local scour variation. The expected bed erosion and the maximum scour depth at the upstream edge of the wing wall abutments are satisfactorily simulated by the experimental procedure. All the experimental results are graphically presented and comparisons between clear-water scour depths around the vertical wall and the wing wall abutments show lower values of scour depths in the vicinity of the wing wall abutments, under the same hydraulic and sediment transport conditions and for the same abutment widths. The experimental data of clear-water scour conditions were used to determine an equation of maximum equilibrium scour depth through regression analysis. The estimated scour depths were in agreement with the experimental values for each abutment geometry.
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Introduction
Realistic estimation of scour depth around bridge abutments in alluvial rivers is important for safe and economic design of their foundations. The scour hole just downstream of the head of the abutment can endanger the stability of the structure and led to the failure of the construction. The basic mechanism causing local scour at piers or abutments is the formation of vortices at their base which removes bed material from around the base of the construction. From the engineering view point, an accurate quantitative estimation of local scour process around hydraulic structures is necessary for the prevention of severe environmental problems un natural rivers.Extensive research has been conducted to determine the depth and location of the scour hole that develops around abutmentsand numerous abutment scour equations have been developed to predict this maximum scour depth [5].
Numerous experimental investigations have been performed on the study of the flow, the bed level variation and mainly the scour mechanisms in rivers and especially around bridge piers and abutments. Laboratory investigations of local scour at bridge piers and abutments were presented [7] and [6]. Development of local scour depths at vertical wall bridge abutments of varying lengths was investigated in several series of experiments of ranges of uniform sediments and clear-water flow intensities[1].Local scour experiments were performed around a trapezoidal abutment [9] and experimental measurements to simulate bed variation near a trapezoidal bridge abutment and investigation of the impact of water depth and flow discharge on scour depths were also performed [3].
The objective of this research work is to investigate local scour process around wing wall abutments and compare the experimental results of equilibrium scour depths around vertical wall and wing wall abutments, under the same hydraulic and sediment transport conditions. For this purpose, a laboratory experimental procedure was established to simulate local scour around wing wall abutmentsin uniform sediments under clear-water scour conditions. The impact of flow discharge and width of the abutment, for each abutment geometry, is also investigated and the findings are used to describe the effects of various parameters on scour depth and to determine an equation of maximum equilibrium scour depth.
Conclusion
The current research work presented bed level experimental measurements to produce data base intended as support in the development of new and the refinement of existing codes for computing free-surface flows with movable beds specifically around bridge abutments and generally on bed level evolution due to sediment transport. All measurements were carried out in a laboratory open channel flume and the test case geometry is formed from wing wall abutments. Experimental measurements on scour depths around the wing wall abutments were obtained at various locations near the abutments, for different inflow discharges and abutment widths, in uniform sediments, under clear-water scour conditions and were compared with available measurements on vertical wall abutments. Analyzing the experimental data it is obviously that for each abutment width the maximum scour depth and the scour hole area are always greater in the region of the vertical wall abutments than around the wing wall abutments, for the same hydraulic and sediment transport conditions. The experimental results in clear-water scour conditions have been used to determine an empirical equation of maximum equilibrium scour depth and the estimated values are in good agreement with the experimental measurements.