How much does driftwood accumulation at a bridge deck increase backwater and hydrodynamic load? An experimental study

Driftwood, as other floating materials transported by rivers, can accumulate at bridges, increasing the water elevation upstream of the structure and the hydrodynamic force exerted onto the latter by the flow. Not much is known on the accumulation process. This manuscript presents some preliminary experimental results obtained in a laboratory flume with smooth rectangular cross section. Driftwood, in the form of stick-like natural branches, was supplied into the flow and let accumulate at a rectangular bridge deck without piers. The Froude number of the incoming flow was around 0.3, ensuring the formation of significant accumulations at the bridge. An original experimental setup was setup to measure, with adequate temporal resolution, some quantities of interest. The deck was hanging on a trolley that could slide along the stream-wise direction but with the downstream motion impeded by a force sensor that thus thus returned the total hydrodynamic load onto the bridge. The backwater induced by the obstacle to the flow was measured through an array of piezometers. The wood transport rate was measured upstream and downstream of the bridge, in order to estimate the volume of the accumulation applying the mass conservation principle. The results of the preliminary experiments presented in the manuscript demonstrate that, in the tested conditions, the driftwood accumulation is mostly upstream of the bridge; furthermore, both the backwater rise and the hydrodynamic load increase with the accumulation size, and can be much larger than the values for the deck without accumulated driftwood.