Piers on bank slopes are not significantly more likely to accumulate drift than piers on the flood plain. In the Channel Drift can be transported anywhere in the channel, and drift accumulations are more common in channel locations than on banks or flood plains. USGS studies of scour potential have been conducted in humid regions. Anywhere below the top of the bank, judgment is involved in deciding whether an element of the bridge is in the channel. In most streams, secondary circulation currents converge at the surface, causing floating material to be transported along a relatively narrow drift path within the channel.
Piers located in the drift path are the most common sites of drift accumulation.
Such a drift path can be defined in most streams based on observations of small floating material if logs are absent. If such observations are not available, estimate the location of the drift path based on channel characteristics. The width of the drift path is variable, but for the purpose of design, assume a width one-third the channel width unless observations are available. In a curving reach, the middle of the drift path generally lies between the thalweg and the outside bank of the bend. At a sufficiently high flood stage, the drift path no longer remains confined to the channel.