Floating debris as a design and analysis factor for bridges and culverts

Over the past ten years, the scientific community has focused a significant amount of attention on the accumulation and effects of floating debris at bridges and culverts. Two bridges failed catastrophically due to floating debris during the Upper Mississippi flooding of 1993 (Parola, et al, 1994). Various professional groups recognize the importance of Large Woody Debris (LWD), the primary component of floating debris, in riverine networks, yet no solid methodology exists for the quantification of floating debris and its hydraulic effects.

This thesis aims to investigate the extent and hydraulic effects of floating debris at bridges and culverts throughout the United States, summarize the procedures for quantifying floating debris in rivers, and make recommendations for the establishment of a protocol for incorporating floating debris into the design and analysis process of bridges and culverts. State bridge engineering representatives were recently surveyed to determine opinions and data on drift-related problems, maintenance programs, and economic factors.

Literature indicates three major steps are common in the analysis of floating debris: 1) evaluation of the potential quantity of floating debris delivered to the bridge or culvert site, 2) approximation of the quantity of floating debris accumulating at the site, and 3) hydraulic representation of the site incorporating the potential floating debris accumulation. The methods and models reviewed were the Diehl (1997) qualitative method of potential drift accumulation, the Debris at Bridge Pier Prediction Program (DBP3) (Wallerstein, 1999) for both quantification of potential drift accumulations and scour, and HEC-RAS (U.S. Army Corps of Engineers, 1998) for quantitative hydraulic values.

A common opinion of state bridge engineers was formed from the survey results, indicating that the most efficient way to approach floating debris accumulations at bridges and culverts is to operate on a case-by-case basis at the local level. This approach, however, has led to a general failure to observe the costs associated with floating debris removal, repair of damages, and maintenance.

The adoption of a consistent protocol for the quantification of floating debris accumulations has been hampered by the many site dependent variables associated with floating debris accumulations and their hydraulic effects at bridges and culverts, but one should be undertaken in order to guide engineers and modelers in the consideration of drift in the design and analysis process of hydraulic structures. An analysis protocol considering both qualitative and quantitative factors is presented to evaluate the potential hydraulic effects of floating debris, implementing flowcharts and computer models to guide the process.