Freeboard
Navigational clearance and other reasons notwithstanding, the low chord elevation is established as the sum of the design normal water surface elevation (high water) and a freeboard.
For on-system bridges, the department recommends a suitable freeboard based on the flowing criteria:
- Higher freeboards may be appropriate for bridges over streams that are prone to heavy debris loads, such as large tree limbs, and to accommodate other clearance needs.
- Lower freeboards may be desirable, because of constraints such as approach geometry. However, the design high water must not impinge on the low chord.
Generally, for off-system bridge replacement structures, the low chord should approximate that of the structure to be replaced unless the results of a risk assessment indicate a different structure is the most beneficial option.