Bridge clogging caused by woody debris: experimental analysis on the effect of pier shape

The accumulation and blockage of woody debris (WD) at bridges is a relevant problem in flood hazard management. Blockage may cause a strong reduction in the flow rate through the contracted opening and therefore a strong increase in upstream water levels which may lead to flooding or nearby urbanized areas. Physical scale model tests on bridge clogging were conducted in the hydraulic laboratory of the University of Florence. We reproduced three different pier shapes (semi-circular nose, triangular nose and a cutwater of streamlined shape formed of two arches of circles) and four constriction geometries (with and without pier in the channel). Driftwood elements were reproduced using wooden cylindrical dowels of three sizes and two types of log transport were reproduced: congested and uncongested. A dimensionless index quantifying the capability to block the transport of WD was defined and calculated for the various geometrical configurations employed. First results suggest that the triangular nose is more effective in blocking WD