Hydraulic performance of bridge rails based on rating curves and submergence effects

Abstract

The Texas Department of Transportation (TxDOT) is required by the Federal Highway Administration (FHWA) to use crash tested bridge rails on all new bridge construction and for existing bridges scheduled for safety rehabilitation. In general, crash tested bridge rails have a greater height and less open space than bridge rails that have failed crash testing. In the case of safety rehabilitation of existing bridge railing systems that have failed crash testing, the change to successfully crash tested rails would likely result in a rail design with greater height and less open space. This design could result in poor hydraulic performance of the new bridge rails during flood events, which may increase the upstream water surface elevation. Such a change could impact the floodplain for the 100-year return period flood. In the event that the floodplain changes by more than one foot, the Federal Emergency Management Agency (FEMA) requires a floodplain map revision. This can be costly and result in the delay and complication of rehabilitation projects. The objective of this research is to evaluate the hydraulic performance of different bridge rail designs that have been crash tested and found acceptable for TxDOT use. Physical modeling experiments are conducted in order to determine rating curves for various rail systems. The rating curves describe the upstream water surface elevation as a function of the flow rate passing over the rail. A simple three parameter model is developed in order to describe the rating curves based on obtained experimental data. In addition, the effects of the submergence of bridge rails by an increase in downstream water surface elevation are also evaluated. Submergence can occur on a rail located on the upstream side of the bridge due to the backwater produced by the rail on the downstream side of the bridge. This will increase the upstream water surface elevation predicted by the rating curves for each rail. Two different models are used to approximate and characterize the effects of rail submergence.