Application of Bridge Pier Scour Equations for Large Woody Vegetation

Abstract : Existing bridge pier scour prediction equations exclude the influence of tree roots and the cross slope of levee embankments. Developed for specific conditions, these equations do not include modeling of scour at trees near or on levee embankments. Therefore, existing bridge pier scour models must be carefully evaluated and possibly modified before being applied to tree scour. The research conducted by the U.S. Army Engineer Research and Development Center (ERDC) included review and evaluation of the Sheppard-Melville and the Federal Highway Administration Hydraulic Engineering Circular No. 18 (FHWA HEC-18) bridge pier scour equations and validation flume experiments. The research objective was to provide guidance for predicting maximum scour depths near trees on or near levee embankments. The Sheppard-Melville and the HEC-18 methods of bridge pier scour prediction were evaluated. Results from the flume experiments indicate that both methods consistently over-predict scour depth by as much as 25 to 75 percent. Although other bridge scour equations can be used, both the Sheppard-Melville and the HEC-18 equations are sufficient in assessing tree scour potential to conservatively estimate maximum scour that may occur.