U.S. Department of Transportation
Federal Highway Administration
TECHNICAL ADVISORY
T 5140.23
October 28, 1991
Par.
(1) Hydraulic studies should be prepared for bridges over waterways in accordance with Article 1.3.2 of the Standard Specifications for Highway Bridges of the American Association of State Highway and Transportation Officials (AASHTO) and the floodplain regulation of the FHWA as set forth in 23 CFR 650, Subpart A.
(2) Hydraulic studies should include estimates of scour at bridge piers and evaluation of abutment stability. Bridge foundations should be designed to withstand the effects of scour without failing for the worst conditions resulting from floods equal to or less than the 100-year flood. (See HEC 18, Chapters 3 and 4.) Bridge foundations should be checked to ensure that they will not fail due to scour resulting from the occurrence of a superflood on the order of magnitude of a 500-year flood. (See HEC 18,Chapter 3.)
(3) The geotechnical analysis of bridge foundations should be performed on the basis that all stream bed material in the scour prism above the total scour line for the design flood (for scour) has been removed and is not available for bearing or lateral support. In addition, the ratio of ultimate to applied loads should be greater than 1.0 for conditions of scour for the superflood. (See HEC 18, Chapter 3.)
(4) Data on scour at bridge piers and abutments should be collected and analyzed in order to improve existing procedures for estimating scour. (See HEC 18, Chapter 1.)
(1) An initial screening process should identify bridges susceptible to scour and establish a priority list for evaluation. (See HEC 18, Chapter 5.)
(2) Bridge scour evaluations should be conducted for each bridge to determine whether it is scour critical. A scour critical bridge is one with abutment or pier foundations which are rated as unstable due to:
(a) observed scour at the bridge site or
(b) a scour potential as determined from a scour evaluation study. (See HEC 18, Chapter 5.)
(3) The procedures in Chapter 5 of HEC 18 should be followed in conducting and documenting the results of scour evaluation studies
(1) The plan of action should include instructions regarding the type and frequency of inspections to be made at the bridge, particularly in regard to monitoring the performance and closing of the bridge, if necessary, during and after flood events. (See HEC 18, Chapter 7.)
(2) The plan of action should include a schedule for the timely design and construction of scour countermeasures determined to be needed for the protection of the bridge. (See HEC 18, Chapter 7.)
(1) The bridge inspector should accurately record the present condition of the bridge and the stream. At least one cross section at each bridge should be documented and compared with previously recorded cross section(s) at the site. Pier locations and footing elevations should be included.
(2) The bridge inspector should identify conditions that are indicative of potential problems with scour and stream stability.
(3) Effective notification procedures should be available to permit the inspector to promptly communicate findings of actual or potential scour problems toothers for further review and evaluation.
(4) Special attention should be focused on the routine inspection of scour critical bridges and on the monitoring and closing as necessary of scour critical and other bridges during and after floods.
(1) "The primary function of the bridge maintenance program is to maintain the bridges in a condition that will provide for safe and uninterrupted traffic flows. The protection of the investment in the structure facility through well programmed repairs is second only to the safety of traffic and to the structure itself." (p. 25.)
(2) "Determining an effective solution to a stream bed or river problem is difficult. Settlement of foundations, local scour, bank erosion, and channel degradation are complex problems and cannot be solved by one or two prescribed methods. Hydraulic, geotechnical, and structural engineers are all needed for consultation prior to undertaking the solution of a serious maintenance problem. In some cases, certainremedial work could actually be detrimental to the structure." (p. 155.)
Thomas O. Willett, Director
Office of Engineering