Flow Distribution
Any stream crossing that uses a combination of fill and bridge within the floodplain disturbs flow distribution during some floods. However, the normal flow distribution should be preserved to the extent practicable in order to:
- avoid disruption of the stream-side environment
- preserve local drainage patterns
- minimize damage to property by either excessive backwater or high local velocities
- avoid concentrating flow areas that were not subjected to concentrated flow prior to construction of the highway facility
- avoid diversions for long distances along the roadway embankment.
Generally, the disturbance of flow distribution can be minimized by locating bridge openings at the areas of high conveyance.
For many situations, one-dimensional analysis techniques suffice for determining optimum bridge locations. When analyzing complex sites, such as those at a bend (Figure 9-4), and at skewed crossings (Figure 9-9), a great deal of intuition, experience, and engineering judgment are needed to supplement the one-dimensional analysis. Unfortunately, complex sites are frequently encountered in stream crossing design. The development of two-dimensional techniques of analysis greatly enhances the capabilities of hydraulics designers to deal with these complex sites. However, two-dimensional models required a great deal more data, intuition, experience and time than a one-dimensional model.
Figure 9-4. Highway Stream Crossing at a Bend