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چكيده انگليسي :

92 Armoring Effect on Local Scouring around the Bridge Pier under Unsteady Flow Safar Ebrahimi s ebrahimi@cv iut ac ir Date of Submission June 20 2011 Department of Civil Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisor Dr Abdorreza Kabiri Samani akabiri@cc iut ac ir AbstractScouring is a natural phenomena which is the result of the erosive action of flowing water excavating andcarrying away materials from the bed and banks of streams and from around the piers and abutments of thebridges Different materials scour at different rates e g loose granular soils are rapidly eroded while cohesiveor cemented soils are more scour resistant Failure of bridge due to scour at their foundations includingabutments and piers is a common occurrence Based on the mode of sediment transport by the approachingflow local scour is classified into two categories namely clear water scour and live bed scour Clear waterscour occurs when there is no transport of bed material upstream of the crossing or encroachment It is also inpart due to the transportation of the material from the upstream reach through the downstream reach at less thanthe capacity of the flow Live bed scour occurs where there is transport of bed materials from the upstreamreach into the crossing or encroachment River beds are composed mainly of nonuniform materials If the flowcondition is tend to the initiation of motion of the finer sediment particles a protective layer namely armorlayer will be formed and developed on the bed surface With formation of armor layer further erosion of thebed is inhibited In most rivers flow regime is commonly unsteady During a flood event the maximum scourdepth at peak flow may be smaller than the equilibrium scour depth which is commonly estimated using thepeak flow conditions When the flow unsteadiness is pronounced the difference between the maximum scourdepth and the equilibrium scour depth can be quite substantial and therefore should be addressed There are fewinvestigations for estimating the scour depth under unsteady flow Previous investigations demonstrated thatthe base time of the hydrograph plays a major role in the scouring process on one layered beds and the effect ofhydrograph shape is negligible Hence according to the literature river hydraulics involves both unsteady flowand armored bed conditions Sediment transport is a complicated problem and cannot be solved simply based onnumerical analytical approaches Hence most investigators studied such problem based on modelexperimentation In this study armoring effect on local scouring under unsteady flow condition was investigated based on modelexperimentation Experiments were performed in a 8 5 m long 0 405 m wide and 0 7 m high laboratory scaledflume Different combinations of uniform sediment of bed armor layer were employed Three circular pierswere used and ten triangular hydrograph considered For each experiment the armor layer thickness wasmaintained at 3da da is the armor layer material mean size The bed materials were relatively fine non cohesive sediment overlain by a thin armor layer of coarser sediment In this study the clear water scourcondition is considered The approaching flow velocity is restricted to the clear water scour condition withrespect to the armor layer particles u u ca 0 8 where u is the flow shear velocity estimated based on theShields diagram and u ca is the critical shear velocity of the armor layer particles Results showed that inarmored beds the effect of hydrograph shape on scour depth is negligible and the base time is the mostimportant parameter Observations illustrated that the effect of hydrographs which composed of repetition of ntriangular hydrographs with the same base time T is almost identical to that of a continues hydrograph with thebase time of nT Key WordsLocal scouring Bridge pier Armor layer Unsteady flow Triangular hydrograph

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