The most obvious design guideline is to avoid the imposition of hydraulic forces on a bridge superstructure by placing the bridge at an elevation above which the probability of submergence is small. Obviously, this is not always economically or physically practical.

One design alternative is to make the superstructure as shallow as possible. Box girders that would displace great volumes of water and have a relatively small weight compared to the weight of water displaced are not a good design alternative unless the probability of submergence is very small. Solid parapets and curbs that increase the effective depth of the superstructure can give increased buoyancy over that of open rail designs. If submerged, the increased effective depth of the superstructure causes increased general scour, and drag forces on the superstructure are much greater than with open rails.

Another consideration is to provide a roadway approach profile that will be overtopped prior to the submergence of the bridge superstructure. This will reduce the probability of submergence of the bridge and help to reduce the potential for scour at the bridge . The consequence may be the need for repairs to the roadway approach.

Where large volumes of debris are likely to occur, longer spans and high freeboards may be warranted. In extreme situations, debris racks may be installed to stop the debris before it reaches the structure. Bridge designers should consult with Design Division Hydraulics prior to specifying or installing debris racks.

For even a small probability of total or partial submergence, see the Bridge Division for guidance. If the dead load of the structure is not sufficient to resist buoyant, drag, and debris impact forces, the superstructure may need to be anchored to the substructure. Air holes may also be provided through each span and between each girder to reduce the uplift pressure.