For example, a high-potential blockage across the channel may cause skewed flow through bents in the flood plain, raising the trapping potential at these bents from medium to high. If so, accumulation at these bents should be regarded as having high potential. Deflection of flow from drift accumulations changes the angle of approaching surface flow at nearby piers. If adjacent piers include multiple columns, assume they are no longer aligned. Also, decrease the effective width of adjacent horizontal gaps as needed to reflect increased skew.
Drift constricts the bridge opening and increases the backwater effect of the bridge.
If backwater from drift causes immersion of the superstructure, evaluate the potential for accumulation on the superstructure and across vertical gaps. Mid-channel piers in the drift path may retain drift for a long time after it accumulates. This problem is greatest where potential for accumulation is chronic and where the pier is difficult or expensive to clear. Factors impeding removal include a location in mid-channel beyond the reach of cranes on either bank, a pier nose overhung by the deck impeding removal from above, hammerheads with drift wedged under the top of the pier, and multiple columns with entangled drift. The potential for drift accumulation at a bridge is the maximum of the potentials estimated for each pier, superstructure section, or gap between fixed elements.