InTrans / Aug 14, 2018
Bridge Scour Evaluation: Screening, Analysis, and Countermeasures
Scour, defined as ÛÏthe erosion or removal of streambed or bank material from bridge foundations due to flowing waterÛ is the most common cause of highway bridge failures in the United States. Virtually all of the 7,650 bridges on National Forest lands are administered by the U.S. Department of Agriculture (USDA) Forest Service. Scour is also the single most common cause for bridge damage and failure on National Forest System lands (figure 1). Many bridges will experience floods that can cause damage each year. Minimizing future flood damage to bridges and ensuring public safety requires engineers to develop and implement improved procedures for designing bridges and inspecting them for scour. ÛÏEvery bridge over water, should be assessed as to its vulnerability to scour in order to determine the prudent measures to be taken for that bridge and the entire inventoryÛ (Richardson and Davis 1995).
To meet this need, the Federal Highway Administration (FHWA) issued a Technical Advisory in 1988 revising the National Bridge Inspection Standards (NBIS) to require evaluation of all bridges for susceptibility to damage from scour. In accordance with the Memorandum of Understanding between the Forest Service and the FHWA, the Forest Service is required to implement the Technical Advisory, establish a scour evaluation program, and submit progress reports of the evaluation program to FHWA.
Prior to 1998, the Forest Service had not implemented a scour evaluation program. In 1998, an Engineering Technology Development Proposal was funded to develop a scour evaluation program, specifically for the Forest Service, that all regions of the Agency could implement. The project was to outline a single process and establish criteria, methods, and guidelines that would ensure consistency throughout the Agency and eliminate duplication of effort.
The project was completed in cooperation with the regional bridge engineers and was organized into three phases.
Phase 1. Review the FHWA guidelines and existing public road agency scour programs.
Phase 2. Develop a scour evaluation program specifically for the Forest Service based on the information from Phase I.
Phase 3. Provide support for the program during implementation by the regions.
This document is the culmination of Phases 1 and 2.