To be conservative, assume that a drift accumulation extends from the water surface to the streambed. Also assume that an accumulation on a single pier will have a width equal to the design log length over its full depth, and that an accumulation across two or more piers will extend laterally half the design log length beyond them. These assumptions are consistent with the largest observed accumulations, and will probably maximize effective pier width and predicted forces and scour depths. In calculating scour, however, it may be more conservative to assume that an accumulation extends only part of the distance downward from the surface to the streambed. The maximum size of drift accumulations on superstructures cannot be estimated based on the few existing descriptions of such accumulations.
Abundant drift can close all apertures in trusses and parapets as they are submerged.
To be conservative, assume that these apertures will be closed. Where drift accumulates across the vertical gap from low steel to the streambed, the entire gap could ultimately be closed. Accumulations on the upstream side of a bridge may extend vertically beyond low steel and the top of the superstructure. The design length used for this vertical extension may have a large effect on estimates of the forces on the bridge and the upstream water level. Data on the distance of vertical extensions are scarce.