748.3 Freeboard
Bridge freeboard should be based on a 50 year flood frequency. When the drainage area produces unusually large debris, additional freeboard to protect the structure is desirable. If the drainage area produces very little debris the freeboard criteria may be reduced.
Freeboard is the required clearance between the lower limit of superstructure and the design high water surface elevation. An appropriate amount of freeboard allows for the safe passage of ice and debris through the structure. The required structure grade elevation is obtained by adding freeboard and superstructure depth to the design high water elevation.
| Structure Type | Minimum Freeboard |
|---|---|
| Headwater: | |
| Bridges with Drainage Area ≥ 20 mi2 | 2.0 ft |
| Bridges with Drainage Area < 20 mi2 | 1.0 ft |
| Temporary Bridges | 1.0 ft |
| Culverts | 0.0 ft |
| Backwater from another stream: | |
| Bridges | 1.0 ft |
| Culverts | 0.0 ft |