Data-Driven Prioritization and Empirical Predictions for Bridge Scour in Nebraska

Bridge scour is a leading cause of bridge closures and failures in the country and Nebraska. Over the last few years, high-profile bridge closures in Nebraska have been widely publicized in the media citing scour as the primary issue. The indirect economic impacts of bridge closures can be substantial on the Nebraska economy, particularly in the agricultural and rural sectors. According to a recent TRIP report, trucks carry 91% of the ton-miles for the movement of perishable agricultural items highlighting the need for critical rural routes to remain open. Bridge scour is the gradual removal of sediment and soil from the areas surrounding bridge piers and abutments, resulting in significantly reduced capacity and safety of the bridge. In general, scour is controlled by three parameters, the hydraulic flow (of the channel), erodibility of the soil, and the geometry of the site (e.g., constrictions due to a bridge pier and angle-of­ attack). Bridge scour can be broadly classified into three categories: (1) Degradation - the general and progressive lowering of the channel bed due to erosion. (2) Contraction - the removal of material from the bed and banks across all or most of the channel width associated with a contraction of the flow area at the bridge which causes an increase in velocity and shear stress on the riverbed at the bridge. This can be the natural narrowing or human made narrowing of a stream channel. (3) Local - the removal of material around piers, abutments, spurs, and embankments caused by an acceleration of flow and resulting vortices induced by obstructions of the flow. Clear-water scour occurs when there is no movement of the bed material upstream of the bridge crossing at the flow causing a bridge to scour. Live-bed scour occurs when the bed material in the channel upstream of a bridge is moving at the flow causing a bridge to scour. Bridge inspections are performed to assure safe and continued operations. Due to the unpredictable stream behavior, scour may occur globally over the entire channel area or locally at an individual pier or abutment. The underwater inspection of foundations is included due to the continual changes in the stream/river and its floodplain and to determine the underwater member's condition with certainty. The goal of any inspection is to ensure that the changes in the channel conditions are within an acceptable tolerance or if a trend is evidenced that may compromise that stability of the bridge and therefore require action (or countermeasure). Scour critical bridges are typically inspected at more frequent intervals than the typical biennial schedule, including during and immediately following flooding events. In these cases, personnel and resources are required to perform these inspections, and sometimes bridge structures are closed for unsafe or unknown conditions. Significant scour holes may develop during a flood event, but will not be observed during inspection as the hole fills in with sediment as the water recedes. In this case, field monitoring of scour can improve confidence in the inspection of scour critical bridges. Field scour evaluation can be carried out using either portable monitoring devices or fixed instrumentation. FHWA outlines two categories for scour evaluation equipment including physical probes (e.g., sounding weights, extensible rods) and sonar instrumentations (e.g., echo sounders and fathometers). Both types can provide more accurate measurements of the depth of scour near the bridge piers and abutments. Numerical scour evaluation is generally conducted using the Hydraulic Engineering Circular No. 18, which is commonly known as HEC-18. This evaluation scheme is used for all new bridge construction as well as existing bridges identified as scour susceptible. HEC-18 encompasses a seven-step specific design approach to evaluate the total scour which includes more detailed hydrologic and hydraulic analyses. To align with the risk-based and data-driven approaches, the FHWA Scour program outlines the risk-based factor for the importance of the structure and the targeted frequency flood event. The general design procedure for scour to determine the type, size and location (TS&L}, as outlined in Chapter 2, is outlined briefly here: (1) select flood event(s) that are expected; (2) develop hydraulic parameters to estimate scour; (3) use the six-step specific design approach to estimate total scour; (4) plot total scour depths; (5) evaluate the results for reasonableness; (6) evaluate and modify the proposed bridge size, configuration, foundation elements; and (7) perform bridge foundation analysis. Within the FHWA specified process, two critical steps rely on site-specific details. This includes step 2 - to develop hydraulic parameters and step 5 - to evaluate the results for reasonableness. Different materials will scour at various rates. Loose granular soils can rapidly erode by flowing water, whereas cohesive soils, which are common to specific areas of Nebraska, are more scour-resistant (61. However, HEC-18, in section 3.1, conservatively assumes that the ultimate scour in cohesive soils can be as deep as the scour in loose granular soils (or sands). While this assumption is expected to be conservative because of the increased critical shear stress in cohesive soils, this can lead to highly improbable scour estimates and the potential for over-designed and costly bridge foundations. However, significant challenges arise in order to verify the magnitude of scour for these varying soils. This is primarily due to the cyclic nature of the scour process where scour is deepest during the peak of a flood but may be hardly visible as floodwaters recede and scour holes fill with sediment. Therefore, there is a critical need to develop improved hydraulic parameters and to provide guidance on reasonableness for scour estimates that reflect Nebraska soils. The first objective of this project is to reduce the uncertainty in the scour prediction equations specific to Nebraska soils and hydraulic conditions using empirical field data collected in this project. Particular attention will be paid to the scour predictions of clayey and cohesive soils, which are currently presumed to be overly conservative in the existing FHWA HEC-18 approach. The second objective of this project is to evaluate and provide guidance on reasonable scour estimates for Nebraska soil and hydraulic conditions. This objective is done to address engineering judgment on whether the numerical scour predictions are "conservative" or "over-conservative". Guidance will be provided using real field measurements to benchmark and clarify the ranges of acceptable scour in this area from the highly detailed, high-fidelity site assessments.


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    • Status: Completed
    • Funding: $115662
    • Sponsor Organizations:

      Nebraska Department of Transportation

      1500 Nebraska 2
      Lincoln, NE  United States  68502
    • Project Managers:

      Halsey, Lieska

    • Performing Organizations:

      University of Nebraska, Lincoln

      1400 R Street
      Lincoln, NE  United States  68588
    • Principal Investigators:

      Wood, Richard

    • Start Date: 20190701
    • Expected Completion Date: 20211231
    • Actual Completion Date: 20220630
    • USDOT Program: Transportation, Planning, Research, and Development

    Subject/Index Terms

    Filing Info

    • Accession Number: 01705857
    • Record Type: Research project
    • Source Agency: Nebraska Department of Transportation
    • Files: RIP, STATEDOT
    • Created Date: May 24 2019 11:09AM