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Application of collars as scour countermeasure at various contraction ratios caused by two spill through abutments

Bridge abutments built on rivers cause local scour formation around these structures. Especially in narrow rivers the bridge abutments on both sides of the river, contract the cross section and affect the pattern and magnitude of scour. If the contraction ratio is sufficiently large in addition to the local scour, contraction scour also comes into picture. Within the present study the collar plates put around two spill through abutments, which are constructed at opposite sides of a rectangular channel, are investigated as a scour countermeasure. Experiments are conducted on a rectangular channel of 29 m length and 1.5 m width which has a sediment storage section where the abutment models are put. Scour formation is investigated at clear water scour conditions at different contraction ratios whereas different sized collar plates at three different levels. For the selected two contraction ratios scour evolution is investigated at two predefined sections by recording real time data for cases with and without collars. It is observed that collar plates are very effective in reducing the scour depth especially for large contraction ratios.