Accumulations will not necessarily form on all piers with high potential. If an accumulation does form, its width may be as great as the design log length based on channel width, and it may extend vertically to the depth of flow, or it may be much smaller. The accumulation of drift on a single pier begins with a single piece of drift at the water surface, and the width of the accumulation may reach the length of the design log. If initially narrow, the accumulation can grow to the maximum width through the accretion of a single log. Where drift accumulates across a horizontal gap, the initial accumulation extends the full width of the gap at the water surface, and may ultimately extend beyond the vertical elements defining the gap.
Additional accretion causes the drift accumulation to grow downward toward the streambed.
The accumulation can continue to grow downward until it extends over the full depth of flow, with about the same width over its full height. 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.