Influence of ice-cover on local scour at circular bridge piers : an experimental study

Sammanfattning: The objective of this work has been to investigate the effect of a simulated ice cover on the local scour depth around a cylindrical pier. Experiments were performed in a laboratory flume for different flow velocities with both a smooth and rough cover. The bed material consisted of uniform ripple-forming sand. The experiments took place at the Hydraulic Laboratory of Clarkson University, USA. Comparisons were made with scour at free surface flow; the emphasis being on conditions with sediment transport on the bed, so called live bed conditions. The scour depth was continuously measured during each test and the result was presented as an average scour depth. A total of 30 test-series were made, 20 with a cover and 10 without. Two kinds of covers were used, smooth and rough. The experiments included both clear water conditions, i.e. no sediment transport, and live bed conditions. The results show that an ice-cover may increase the local scour depth by as much as 25-35%, or 0.3-0.5 pier diameters, at live bed conditions. The results in summary were: Covered tests gave larger scour depths for all flow velocities. These included both clear water scour and live bed scour. The difference in scour depth (between covered and free surface flow) was most pronounced for five bed conditions and most noticeable at relative flow velocities between 1.5 and 2. There the covered tests gave up to 35% larger scour depths, which in this range is equal to 0.5 pier diameters. The rough cover gave slightly larger scour depths than the smooth cover. The variation in scour depth during testing was larger for covered flow than free surface flow with approximate relative velocities between 1.8 to 3. At other velocities the variations were similar. The scour depths velocity-dependence and magnitude for free surface flow were in agreement with the findings of Melville (1984). The increased scour depth at clear water conditions, with covered flow, was in agreement with the findings of Bacuta and Dargahi (1986). The velocity-dependence of the scour depth followed a similar pattern for both covered and free surface flow conditions.

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