Factors Affecting Bridge Length
Bridges over waterways are not always limited to the length of the hydraulic opening required.
- The roadway alignment is at a skew to the streambed, and normalizing the alignment would require unsafe or undesirable curves on the approaches to the bridge.
- Embankments may be limited to a certain location due to local soil instability or permitting requirements.
- Bridge costs might be cheaper than embankment costs.
- Matching the highway profile grade line.
- High potential for a meander to migrate, or other channel instabilities.
These and other aspects are valid considerations that affect bridge waterway openings. However, hydraulic computations are necessary to predict the performance and operation of the waterway opening at flood stages. Do not neglect hydraulic design. The design decisions, including the reasons for any excess opening, must be documented.