Flood Hydrograph Generation for Predicting Bridge Scour in Cohesive Soils

The method currently used by South Dakota Department of Transportation (SDDOT) for designing bridge foundation assumes that the bed material is sand and designs for a single (worst-case) flood event such as the 100-year or 500-year flood using the peak flow magnitude. Many bridges in South Dakota are founded on cohesive soils consisting of silts and clays (Niehus, 1996). Since silts and clays scour more slowly than sands, using the traditional methods for evaluating scour at bridges may over predict the extent of scour. This could result in over design of new bridge foundations or installation of unnecessary scour countermeasures at existing bridges. Furthermore, bridges that are classified as scour critical may in fact be safe. With reliable methods for predicting scour in cohesive soils, SDDOT could save significant time and money on bridges built over waterways. The Scour Rate In COhesive Soils (SRICOS) method (Briaud et al., 1999, 2001) is included in the current edition of Hydraulic Engineering Circular No. 18 (HEC-18; Arneson et al., 2012) as an alternative approach for predicting bridge scour in cohesive soils. The SRICOS method uses site-specific measurements of soil erosion rate to predict scour depth as a function of time. For soils that erode slowly, the final scour depth predicted by the SRICOS method could be significantly less than the equilibrium scour depth in sand. However, a hydrograph is required in order to use the SRICOS method. Various models have been developed by researchers to generate a continuous hydrograph over the design life of bridges (e.g., Brubaker et al., 2004; Brandimarte et al., 2006). These time series models require a great deal of effort to construct and are difficult to apply routinely by engineers. There is no guidance in HEC-18 on how to generate a hydrograph for using the SRICOS method. It is unclear what level of detail (e.g., a continuous hydrograph for multiple years with an annual flooding cycle, a series of design floods) is required in the temporal record of flows to correctly predict the final scour depth in cohesive soils. It is also unclear how to apply the SRICOS method to small watersheds and ungauged streams where historical flow records are lacking. SDDOT needs guidelines to define the site conditions in which the SRICOS method is more appropriate and more cost effective than the traditional methods, and to select a hydrograph generation method for using the SRICOS method.

Language

  • English

Project

  • Status: Active
  • Funding: $139672
  • Contract Numbers:

    DTRT13-G-UTC38

  • Sponsor Organizations:

    Office of the Assistant Secretary for Research and Technology

    University Transportation Centers Program
    Department of Transportation
    Washington, DC  United States  20590
  • Project Managers:

    Kline, Robin

  • Performing Organizations:

    South Dakota State University, Brookings

    Department of Civil and Environmental Engineering
    P.O. Box 2219
    Brookings, SD  United States  57007
  • Principal Investigators:

    Ting, Francis

  • Start Date: 20170328
  • Expected Completion Date: 20180731
  • Actual Completion Date: 0
  • Source Data: MPC-531

Subject/Index Terms

Filing Info

  • Accession Number: 01648621
  • Record Type: Research project
  • Source Agency: Mountain-Plains Consortium
  • Contract Numbers: DTRT13-G-UTC38
  • Files: UTC, RiP
  • Created Date: Oct 20 2017 10:41AM