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Bank Erosion and Bridge Scour – NCHRP Practice Ready Solutions

by Airton Kohls – Source: NCHRP Research Topic Highlights July 2017 and FHWA EDC–4 Innovations

The National Cooperative Highway Research Program just released an 8 page document where it highlights some of the research on prediction and prevention of bank erosion and bridge scour. The document can be found at the following address (http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_researchtopichighlights_02.pdf)

The most common cause of bridge failure is scour, which occurs when moving water erodes the soil and rock from around bridge piers and abutments, or from the banks or bed of the river channel spanned by the bridge. The sediment surrounding piers and abutments supports their foundations and is vital to the stability of bridges. Consequently, one of the most important elements of bridge design is the ability to predict the effects of scour so that foundations can be designed to withstand these effects over the life of the bridge. Such predictions require the use of complex hydraulic equations, which are set out in several FHWA's Hydraulic Engineering Circulars (eg.: HEC-18, HEC-20 and HEC-23).

Predicting river channel migration and deploying bank erosion countermeasures, such as placing rocks and vegetation along banks to protect the soil from the flow of water, are critical to preventing bridge scour. Predicting channel migration has historically been difficult to do, even with the best computer models. In NCHRP Report 533, researchers provide engineers with a practical and empirical GIS-based methodology for predicting channel migration. Use of vegetation can be an environmentally sensitive and aesthetic way to prevent stream bank erosion. NCHRP administered research to evaluate existing guidelines for the design, installation, monitoring, and maintenance of environmentally sensitive stream bank stabilization and protection measures. The resulting NCHRP Report 822 provides updated design guidelines for three widely used treatments: live siltation and live staking with a rock toe, vegetated mechanically stabilized earth, and vegetated riprap.

Eroded Bridge Abutment
Abutments and embankments are the most commonly damaged bridge components during floods. Image courtesy of Lake County, MN.

Scour countermeasures are at once necessary and costly, making their selection, design, and construction important considerations for transportation agencies. NCHRP research has been critical in establishing practical guidance for using scour countermeasures. NCHRP Report 593 includes practical selection criteria for bridge pier scour countermeasures; specifications for their design and construction; and guidelines for their inspection, maintenance, and performance evaluation. The report is accompanied by a spreadsheet containing an interactive version of the countermeasure selection methodology. Bridge abutments and their approach embankments are the most commonly damaged bridge components during floods. NCHRP Report 587 provides validated selection criteria and guidelines for the design and construction of countermeasures to protect bridge abutments and approach embankments from scour damage. Solutions include flow-altering devices, such as guide banks and weirs, and nontraditional abutment scour countermeasures, including articulated or tied mats and blocks. Before NCHRP research, most states had differing specifications for classifying riprap size and gradation, and construction practices were sometimes ineffective. NCHRP Report 568 provides engineers with design guidelines; material specifications and test methods; construction specifications; and construction, inspection, and quality control guidelines for riprap at streams, riverbanks, bridge piers, and abutments.

Bridge scour analysis and scour countermeasure analysis are also some of the applications of CHANGE (Collaborative Hydraulics: Advancing to the Next Generation of Engineering), one of FHWA's Every Day Counts (EDC) 4 – Innovations – initiative. (https://www.fhwa.dot.gov/innovation/everydaycounts/edc_4/change.cfm) With CHANGE, the Federal Highway Administration (FHWA) continues to recognize the benefits of 2D modeling for the safety of the traveling public and in the resilience of transportation infrastructure. This next generation of hydraulic engineering tools, particularly two-dimensional (2D) modeling and graphical visualization features, allows users to create better representations of the often complex interaction between transportation assets and the riverine or coastal environments. These representations provide the planning and design team with better data with which project quality can be improved.

In Tennessee, the TDOT structures division has scour studies as one of their key responsibilities. In addition, the division is responsible for designing and developing plans and specifications relating to all highway structures and for the inspection of all bridges in Tennessee. These include the design of foundation conditions, tolerances and complex geometry; the design of major or unusual structures; resolution of hydrology and hydraulic problems; coordination of Geological Survey services; processing of federal disaster assistance, hydraulic studies involving legal action; inspection of refabrication plants, welding procedure review; on-site erection supervision; design and development of contract plans specifications; development of automated structural detailing systems; and bridge inspection. The Structures Division has over 100 employees to accomplish its objectives. Annually, it prepares hydraulic and structural designs for approximately 120 bridges for on-system and off-system routes. Additional information may be found at (http://www.tn.gov/tdot/section/structures-home).


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