Background: Because current scour prediction techniques are not applicable to a wide range of field conditions, scour at bridge abutments is often over-predicted. Over-predicting abutment scour may result in construction of unnecessary countermeasures or excessively deep foundations, adding significant costs to bridge construction and maintenance.

Most experimental studies of scour around abutments have been conducted in rectangular laboratory flumes with level streambeds. Real river channels are decidedly different from these idealized laboratory conditions. Idealized experiments are representative only of flow around abutments in well-defined incised channels and abutments in wide braided rivers. A limited number of studies have been conducted to consider interaction of flows in compound channels (i.e., channels consisting of both floodplain and main channel sections), and only a very limited number of experiments have been done on abutments that terminate on a floodplain near a main channel.

Accepted prediction equations for scour around abutments were developed from a narrow range of conditions and do not incorporate the effects of spill-slope embankment failure into surrounding scour holes. As scour develops at the toe of abutment embankments, soil and rock protection fall into the scour hole. Rock protection or embankment material tends to armor the scour hole, and scour tends to proceed horizontally instead of vertically. This process changes the geometry of the embankment and expands the waterway opening, and is likely to play a dominant role in the mechanics of scour. Inclusion of the spill-slope failure process in model studies will have a substantial effect on the location and depth of scour.

Specific information about the mechanics of scour around abutments is needed to substantially improve scour-prediction techniques. Data are needed for flow conditions in which the abutment (embankment) terminates near the main channel. In addition, scour depth and contracted flow velocity-distribution data are needed from experiments that model flow conditions in which the position of the abutment on the floodplain, the geometry and roughness characteristics of the floodplain, and the flow distribution in the main channel and floodplain are varied over the practical ranges of these parameters.

Objective: The objective of this research is to develop more accurate and comprehensive methodologies for predicting abutment scour in compound channels.

Tasks: Accomplishment of the project objective(s) will require at least the following tasks.

PHASE I (1.) Review the technical literature from foreign and domestic sources to assess the adequacy and extent of information on abutment scour prediction. The literature review should identify completed and in-progress experimental model studies, field data, and processes causing scour at unprotected bridge abutments. Conduct a critical evaluation of existing methodology for predicting abutment scour. (2.) Survey state DOTs and other transportation agencies to determine common abutment designs and case histories for bridge abutments experiencing significant scour and bridge abutment failures due to scour. Summarize the information in tabular form to include the depth and extent of scour, and the original design assumptions. (3.) Refine and revise the work plan. The revised work plan shall include, but not be limited to, experiments designed to assess the effects of the following parameters on scour at abutments:

• structure and roadway configurations that accurately reflect real-world conditions including the position of the abutment with respect to the main channel and the valley walls. Particular attention should be given to cases where the abutment is situated on the floodplain (i.e., compound channels), especially abutments sited near the edge of the main channel;
• the lateral distribution of approach flow in the compound channel;
• the relative width, depth, and roughness of the floodplain and main channel;
• the state of bed particle motion in the floodplain and main channel for live-bed and clear-water scour conditions;
• the shape of the abutment (i.e., spill-through abutments, vertical wall abutments with and without wingwalls or a combination of these), especially for longer approach embankments; and
• erodible approach embankments that are constructed on cohesionless materials.

(4.) Submit an interim report documenting the information developed in Tasks 1 through 3. The interim report shall contain a review of the literature and the Task 3 revised work plan as appendices to the main report. Meet with the NCHRP panel to review the interim report and the proposed revisions to the work plan. Work on Phase II will not begin until the interim report and the research work plan are approved by the NCHRP.

PHASE II (5.) Conduct appropriate laboratory experiments as described in the approved work plan. The experiments shall determine but not be limited to scour depth, scour distribution, velocity distribution, shear stresses around the abutment, and time rate of scour development for a wide range of common flow conditions at abutments. (6.) Develop a methodology for predicting abutment scour in cohesionless soils and develop parameters for extrapolating the methodology to predicting abutment scour in other than cohesionless materials. (7.) Validate the methodology and parameters developed in Task 6 against actual bridges that have experienced abutment scour, as well as bridges that have suffered no scour-related abutment damage. (8.) Submit a final report that documents the entire research effort. Include the scour-prediction methodology and extrapolation parameters as a stand-alone appendix.

Status: A continuation project, NCHRP Project 24-20(02), will compare this project equation’s predicted scour, computed using input data from an actual event, with the measured scour caused by that same event.  In this way, we can evaluate the performance of this equation under field conditions.  The data and measurements for this comparison will come from a USGS database.  The results of this comparison will become part of this project final report.

Product Availability:  Here is the link to an unedited copy of the Final Report for NCHRP Project 24-20.   NCHRP Research Results Digest 334 summarizes key findings from a workshop sponsored by this project panel and NCHRP Project Panels 24-15(02) and 24-27.