BACKGROUND

Waterborne debris, composed primarily of tree trunks and limbs, often accumulates on bridges during flood events. Debris accumulations can obstruct, constrict, or redirect flow through bridge openings resulting in flooding, damaging loads, or excessive scour at bridge foundations. The size and shape of debris accumulations vary widely, ranging from a small cluster of debris on a bridge pier to a near complete blockage of a bridge waterway opening. Debris accumulation geometry is dependent on the characteristics and supply of debris transported to bridges, on flow conditions, and on bridge and channel geometry. Little information is available to state highway agencies for predicting the size and geometry of typical debris accumulations.

The effects of debris accumulation can vary from minor flow constrictions to severe flow contraction resulting in significant bridge foundation scour. In order to make prudent decisions in designing bridge-pier foundations, state highway agencies need reliable, accurate methods of predicting pier scour caused by debris accumulations. During flood events, decisions also have to be made about the stability of existing bridges and public safety, but current methods for assessing bridge-pier scour caused by debris accumulations are inadequate to make these decisions with confidence. There is a need for accurate methods of quantifying the effects of debris on scour at bridge-pier foundations, for use by highway agencies in the design, operation, and maintenance of highway bridges.

OBJECTIVES

The objectives of this research are to develop (a) guidelines for predicting the size and geometry of debris accumulations at bridge piers and (b) methods for quantifying scour at bridge piers resulting from debris accumulations.

Accomplishment of the project objectives will require at least the following tasks.

TASKS

PHASE I (1.) Review the technical literature from foreign and domestic sources to assess the adequacy and extent of existing information on debris accumulations and effect of these accumulations on bridge scour. The literature review should identify research in progress as well as completed work. ( 2.) Determine typical debris accumulations by surveying state DOTs and other agencies and by site reconnaissance of debris accumulations at bridge piers. The site reconnaissance should include field measurements, photographs, geomorphologic information, channel types and flow patterns, associated bridge and pier geometrics, and historical scour depths where available. (3.) Based on the Task 1 literature search and the empirical data collected in Task 2, develop preliminary guidelines for predicting the size and geometry of debris accumulations at bridge piers. (4.) Develop a detailed description of the Phase II laboratory experiments proposed for assessing the effects of debris accumulations on scour at bridge piers. Categorize typical configurations of debris accumulations identified in Tasks 1 and 2. Prioritize and select typical debris configurations for use in the Phase II laboratory experiments. Refine and revise the work plan as needed. (5.) Submit an interim report documenting the information developed in Tasks 1 through 4. The interim report shall contain a detailed discussion of the typical debris configurations to be used in the Phase II laboratory experiments and the preliminary guidelines for predicting the size and geometry of debris accumulations. The Task 4 revised work plan for completing Phase II should be included as an appendix to the interim report. Meet with the NCHRP panel to review the interim report and revised work plan. Work on Phase II will not begin without approval of the NCHRP panel.

PHASE II (6.) Based on panel comments and guidance during the interim meeting, finalize the Task 3 preliminary guidelines for predicting the size and geometry of debris accumulations. (7.) Using the typical debris configurations agreed on during the interim meeting, conduct the laboratory experiments according to the approved work plan. (8.) Based on the results of Task 7, develop methods for predicting the depth, shape, and lateral and longitudinal extent of scour at bridge piers resulting from debris accumulations. The methods shall be suitable for incorporation into FHWA Hydraulic Engineering Circular 18, "Evaluating Scour at Bridges," 4th Edition. (9.) Submit a final report that documents the total research effort. The guidelines and methods should be included as stand-alone appendices.

Status: The NCHRP panel met with the research team for the Task 5 interim panel meeting in October 2005. Research on Phase II is currently underway.

Product Availability: NCHRP Report 653: Effects of Debris on Bridge Pier Scour and NCHRP Web-Only Document 148: Debris Photographic Archive and Supplemental Materials for NCHRP Report 653.  Reference Document that contains detailed laboratory testing results from Colorado State University; click on the link to download an ISO image that can be burned to a CD.