A laboratory flume characterization of log jams at bridge piers

In natural rivers the transport phenomena of floating debris involve a large number of problems that are relevant to both environmental and technical aspects. A large part of the research developed has been motivated by linkages between fish habitat and geomorphologic processes and forms influenced by large woody debris, but drift reduces also the capacity of bridge openings, contributes to scour around piers and abutments, and increases lateral forces on bridges. Drift accumulates at bridges when it encounters structural components that trap it. During flood events, relevant drift accumulations can reduce the size of inadequate bridge openings and this can cause a severe backwater phenomenon and the inundation of surrounding lands. Moreover, dam break phenomena can derive from the sudden collapse of debris accumulations, increasing the power of flood with devastating effects. Spaces between piles can clog with drift, increasing flow contraction and local scour depth due to piles and bridge abutments. Drift contributes to more than one-third of the bridge failures in the United States, but methods for estimating a maximum drift accumulation size for use in bridge design have been recommended for Australia and New Zealand only. The potential scour depth as well as the backwater phenomena associated with drift depend on the maximum size that drift accumulations can reach. The features of observed accumulations give a broad idea of the maximum size related to some specific investigated sites, but studies looking for more general insights are limited. This presentation reports the evidence deduced from an extensive series of flume experiments developed to identify the general characteristics of drift accumulations at bridges in terms of size and probability of occurrence. To reach this goal a large number of experiments was developed in a flume 1.0 m wide and 21.7 m long, where the section was modeled with a central channel and two symmetrical banks for a 14 m length and slope of about 0,31%. Three different single pier shapes were considered: square, rounded and triangular nose and end, and different multiple piers configurations (two and three piers of different spacing). At each run about 500 logs of prescribed statistical distribution in terms of length and diameter were injected in the flume with a frequency of 150 - 180 logs/min, and the (possible) formation of the accumulation was video-recorded. Each run, characterized by the same discharge, geometric configuration of piers, and boundary condition (downstream water level) was repeated 10 times to obtain a broad probability of the log jams formation for a whole number of more than 500 runs. The digital analysis of the flume experiments in conjunction with the water level and discharge measurements gives a measure of both the shape and the size of accumulations at bridge piers and of their probability of occurrence in different geometric and hydrodynamic conditions.