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چكيده انگليسي :

Invstigation of armoring effect on abutments scouring M Maherani mehrmah523@yahoo comEmail address Date of Submission 25 Feb 2009 College of Agriculture Department of water Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiSupervisors H Afzali Mehr and A R Kabiri SamaniAbstractScour is a natural phenomenon that is the result of the erosive action of flowing water excavating andcarrying away material from the bed and banks of streams and from around the piers and abutments ofbridges Different materials scour at different rates Loose granular soils are rapidly eroded by flowingwater while cohesive or cemented soils are more scour resistant Failure of bridges due to scour attheir foundations which consist of abutments and piers is a common occurrence Based on the modeof sediment transport by the approaching flow local scour is classified into two categories namely clear water scour and live bed scour Clear water scour occurs when there is no transport of bedmaterial upstream of the crossing or encroachment or the material being transported from the upstreamreach is transported through the downstream reach at less than the capacity of the flow Live bed scouroccurs where there is transport of bed material from the upstream reach into the crossing orencroachment River beds are composed mainly of nonuniform materials Sediment transport problemsinvolving nonuniform sediments cannot be solved alytically because the physical processes oftransport have not been yet described analytically If the flow conditions in tend to the motion of finersediment particles others will develop a protective layer develops on the bed surface With formationof an armor layer further erosion of the bed is inhibited but finer sediment particles from tributariesmay move downstream over the armor layer Experiments were conducted on local scour around shortsemi circular or semi eliptical abutments for different flow conditions armor layer and bed thiknesses The bed materials were relatively fine noncohesive sediment overlain by a thin armor layer of coarsersediment and the clear water scour condition is applied The abutments were aligned with theapproaching flow in a rectangular channel Different combinations of uniform sediment armor and bedmaterials were used In this study the approaching flow velocity is restricted to the clear water scourcondition with respect to the armor layer particles Depending on the approaching flow conditions local scour around the abutments for both cases of with or without armore layer were tested anddifferent influential parameters effect on equilibrium scour at depth are examined Results show that the scour depth around abutments with armor layer applying the stability of the surface particleslimitation is always smaller than that of without armor layer for the same bed material size Thedimensionless parameters affecting the maximum equilibrium scour depth scour depth to abutment length ratio d sa identified based on dimensional analysis including abutment Froud number Fr armor layer thikness to armor particle diameter ratio t and armor particle to bed particle diameter ratio d a The experimental data of clear water scour condition in thinly armored beds under thestability of surface particles limitation were used to determine an equation for equilibrium scour depth Finally two relations for forecasting equilibrium scour depth for both semi circular and semi elipticalabutments were achived Key WordsLocal scouring abutment armoring sediment

ماهراني، مهرنوش

بررسي تاثير لايه سپري در آب شستگي تكيه گاه ها

فرسايش , رسوب , گرداب نعل اسبي , گرداب برخاستگي