Background: Most of the techniques and guidance that are available for mitigation of scour at abutments have been developed from scaled laboratory experiments and limited empirical data from field sites. Two typical approaches for protecting bridge abutments from scour are (1) mechanically stabilizing the abutment slopes with riprap, gabions, cable-tied blocks, or grout filled bags or (2) aligning the upstream flow by using guidebanks, dikes or spurs, or in-channel devices such as vanes and bendway weirs. Neither of these approaches has been totally successful; bridge abutments and their approach embankments are the most commonly damaged bridge components during floods. Therefore, better design and construction guidelines need to be developed to protect bridge abutments and approach embankments from scour damage and to reduce the need for expensive deep foundations. In addition, guidelines and selection criteria that address limitations imposed by environmental regulations, relative cost, availability, serviceability, constructability, and design constraints are needed to assist practitioners in preventing, reducing, or mitigating erosion damage.

Objective: The objective of this research is to develop and validate selection criteria and guidelines for the design and construction of countermeasures to protect bridge abutments and approach embankments from scour damage. The countermeasures considered shall include but not be limited to: (1) riprap at abutments, including the type and extent of filters that could be used under the protective riprap layer, the size of the riprap, and the extent the riprap should be placed up an abutment slope, into the channel, and on the approach embankment; (2) flow-altering devices such as guide banks, dikes, spurs, bendway weirs and other in-channel devices, including the size, type, placement, and feasibility of using these devices; and (3) nontraditional abutment scour countermeasures such as articulated or tied mats and blocks including the size, type, placement, and feasibility of using these devices.

Tasks: Accomplishment of the project objective 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 countermeasures. The literature review should define ranges of experimental parameters that have been investigated and critically evaluate the design technique or guidance presented. The literature review should contain sources of information and assess failure modes of protected abutments from as many documented bridge failures resulting from abutment scour as possible. The review should include a survey of existing state practices and an evaluation of current installations. (2.) Using the findings of the literature review outlined in Task 1, develop an approach to evaluate the most promising countermeasures for protection of bridge abutments under a select number of flow and channel-geometric conditions. The proposed approach should address how to correct deficiencies in the state of the existing practice through application of laboratory experiments, numerical modeling, or monitoring field sites and shall consider the effects of the following factors: flow velocity distribution and depth; compound channel geometry; abutment geometry; material properties of the channel or flood plain; filter requirements; lateral and vertical extent of protection; placement, length, and orientation of countermeasures; and effectiveness of countermeasures with debris and ice. (3.) Develop a prioritized list of research alternatives, including cost estimates, for developing new or enhancing existing selection criteria and design and construction guidelines for each countermeasure considered in Task 2. The costs associated with the research alternatives may extend beyond the available budget. Propose at least one research alternative that accomplishes the project objective within the remaining budget. Develop a research work plan that describes in detail how the remainder of the research will be accomplished for the proposed alternative. The research work plan should address important parameters that are to be studied in subsequent experimental analysis, numerical modeling, or field studies. (4.) Submit an interim report describing the information developed in Tasks 1, 2, and 3. Meet with the NCHRP panel to review the interim report, the proposed research work plan, and the prioritized list of proposed research alternatives. Work on Phase II will not begin until the interim report and research work plan are approved by the NCHRP.

PHASE II (5.) Conduct appropriate studies as described in the approved research work plan. (6.) Develop selection criteria and concise guidelines for application of each countermeasure investigated in Task 5. Include examples of the application of the guidelines. If possible, validate selection criteria and guidelines using field experience where countermeasures have been successful and unsuccessful. (7.) Submit a final report that documents the entire research effort and presents the selection criteria and guidelines for design and construction as a separate appendix.

Status: This project was initiated in July 2001 as NCHRP Project 24-18 with the University of Mississippi and with Dr. Brian Barkdoll as the principal investigator. Dr. Barkdoll left the faculty at the University of Mississippi in August 2002 to join the faculty at Michigan Technological University. The NCHRP oversight panel requested that the research remain with the original research team headed by Dr. Barkdoll. Therefore, at the completion of Phase I (August 2002), the contract with the University of Mississippi was terminated and Project 24-18 was closed-out. A contract to complete Phase II of the research was awarded to Michigan Technological University for $363,116 on August 29, 2003 with Dr. Barkdoll as the principal investigator. The research for Phase II of this project will be completed as NCHRP Project 24-18A.

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