Flume experiments on bridge clogging by woody debris: the effect of shape of piers.

The accumulation and blockage of woody debris (WD) at rivers bridges is a relevant problem in flood hazard management. Blockage causes a reduction in the flow rate through the contracted opening and increases water level inducing upstream flooding. In this study, we focused on pier shape and cross section constriction influence on WD accumulation, in order to individuate the geometrical configuration more prone to developing blockages. To this aim, we carried out various flume experiments in a straight channel with three different pier shapes (rectangular, triangular, circular) and four constriction geometries (with and without pier in the channel). Driftwood elements were reproduced using wooden cylindrical dowels of three sizes (small, medium and large in relation to stream width). The relative size and the frequency distribution of the dowels was determined according to field measurements taken in the Arno river basin. In the experiments, 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. Preliminary results show that the triangular shape of bridge pier is the most critical for the accumulation of wood, and the class of large woody debris is the one more prone to be blocked.