In order to estimate the effective width, one must first estimate the direction of flow approaching the gap during the design discharge. Project the positions of the upstream noses of fixed elements parallel to this flow direction onto an imaginary plane perpendicular to the flow direction. A horizontal gap should be assigned to the most drift-prone location category occupied by the fixed elements that define the gap. A horizontal gap from a pier to a bank or abutment has the same location as the pier. Where one of the fixed elements is sheltered and the other is not, the gap should be regarded as unsheltered.
Most drift is transported at the surface.
When floating drift hits the superstructure, most pieces rotate to one side, remaining at the water surface. The drift then accumulates against the superstructure at the surface or is swept under the superstructure. However, some pieces of floating drift hit roughly endwise, and the upstream end rotates downward until it encounters the streambed. Some such pieces remain lodged against the superstructure and the streambed. The fixed elements defining this vertical gap are low steel and the streambed beneath it.