For coastal freeboard considerations, federal requirements require freeboard when practicable for bridges (23 CFR 650.115). TxDOT recommends one foot minimum of freeboard for coastal bridge applications, barring specific exemptions. TxDOT recommends two feet of freeboard for coastal bridge designs that include assumptions associated with Level 1 analyses and more approximate Level 2 analyses. For roadway applications, TxDOT does not have a specific recommendation for freeboard above the design storm, but for nuisance flooding concerns, TxDOT recommends utilizing 0.5-1.0ft of freeboard whenever possible above these more frequent annual tide events to minimize issues. Variations from these recommendations should be documented in the Design Summary Report (DSR), drainage report, and/or plans for each project. Freeboard, when included, is added to the maximum wave crest elevation to determine the design elevation.

HDM Chapter 9 (Bridges), Section 3 (Bridge Hydraulic Considerations) indicates that navigational clearance and other reasons notwithstanding, the low chord elevation is established as the sum of the design elevation and freeboard.

For on-system bridges, the Department recommends a suitable freeboard based on the following criteria:

Generally, for off-system bridge replacements, the low chord should approximate that of the structure to be replaced, unless the results of a risk assessment indicate that a different structure is the most beneficial option. The design freeboard, when considered in a coastal context, should also account for the uncertainty in high waves in the larger areas of open water, since the uncertainty in coastal storm surge and wave analysis can be just as great as uncertainty in riverine conditions.

Historically, in some cases, bridges have survived during storms because their elevations were lower than the full storm wave height. In those cases, it has been observed that wave loads may have been small enough that they did not generate critical damage to the bridge and that the weight of the bridge span was sufficient to overcome wave forces. However, a study of the I-10 Bridge over Mobile Bay, conducted by Auburn University, suggests that it is the structural design of a bridge in conjunction with the wave height, rather than wave height alone, that will determine the impacts of wave forces on the bridge structure. More research is needed to further detail the critical elevation for wave height on bridges.

Complete clearance over the wave height might not be needed or be achievable due to roadway and other design considerations. Likewise, it may not be economical within the roadway geometry design constraints. When complete clearance is desired but cannot be achieved, then the wave and storm tide impacts on the structures can be addressed through erosion and structural countermeasures, such as shoreline armoring or structural retrofits to counter wave forces. An advanced approach is to set the deck elevations based on modeled wave loads analyzed over the deck and pile cap design, to ensure that both can withstand wave loads.