The bridge is integrated into both the stream and the roadway and must be fully compatible with both. Therefore, the alignment of the roadway and the bridge are the same between the ends of the bridge. Hydraulically, the complete bridge profile includes any part of the structure that stream flow can strike or impact in its movement downstream. If the stream rises high enough to inundate the structure, then the bridge and all parts of the roadway become the complete bridge profile.

It is not allowable for the design AEP flow to impinge on the bridge low chord or to inundate the roadway because it violates the definition of design frequency. However, flows exceeding the design AEP flow, including the 1% AEP flow, may inundate the structure and roadway. Unless the route is an emergency escape route, it is often desirable to allow floods in excess of the design flood to overtop the road. This helps minimize both the backwater and the required length of structure.

Several vertical alignment alternatives are available for consideration, depending on site topography, traffic requirements, and flood damage potential. The alternatives range from crossings that are designed to overtop frequently to crossings that are designed to rarely or to never overtop.

In Figure 9-6, the bridge is at the low point in a sag-vertical curve profile. An extreme example of this configuration is a bridge in rolling terrain on a low-traffic road which frequently overtop. Another example is a high bridge in rugged terrain that probably will never be threatened by floods. A distinctive feature of the sag-vertical profile is the certainty that the bridge structure will be submerged when any overflow of the roadway occurs.

If accumulation of drift in the superstructure is likely, placement of bridges on sag-vertical curves should be avoided. Trapped debris can increase the potential for scour by creating eddies and turbulence. The accumulation of debris on the structure can also increase the effective depth of the superstructure, which would impose larger hydraulic forces on the superstructure and possibly cause structural failure, especially if scour has affected the foundations.

If a sag-vertical curve design has even a small probability of overtopping, open-type railing should be used and the use of curbs should be avoided to minimize damage from high velocity flow around the ends of the parapets.

Figure 9-7 illustrates a profile that may be used where the valley width is sufficient for a crest profile that allows the roadway to be overtopped without submerging the bridge superstructure. Use variations of this profile in locations where the stream channel is located on one side of the floodplain (i.e., an eccentric crossing) and the profile allows overtopping of the approach roadway only on one side. However, perching the structure any higher than required for freeboard offers no economic or hydraulic advantage unless other clearance requirements control the vertical position of the structure.

You can vary the difference between the lowchord and the design water surface elevation, within geometric constraints, to meet requirements for maintaining free surface flow and to accommodate passage of debris and drift. However, perching the structure any higher than required for freeboard offers no economic or hydraulic advantage unless other clearance requirements control the vertical position of the structure.

Figure 9-8 illustrates a profile alternative. Variations of the level profile include a slight crest vertical curve on the bridge to establish a camber in the superstructure. With this profile, all floods with stages below the profile elevation of the roadway and bridge deck will pass through the waterway opening provided.

The disadvantages of the near level profile are similar to those of a sag profile. With either profile configuration, severe contraction scour is likely to occur under the bridge and for a short distance downstream when the superstructure is partially or totally submerged.

Because no relief from these forces is afforded, crossings on zero gradients and in sag-vertical curves are more vulnerable than those with profiles that provide an alternative to forcing all water through the bridge waterway.