Offices of the Water Resources Division (WRD) are currently engaged in a number of bridge-scour projects in cooperation with State Highway Departments and, in some cases, the Federal Highway Administration (FHWA). These projects are of two principle kinds reconnaissance, where bridges are visited to evaluate the potential for scour, and data collection, where scour is measured at the bridge during floods. Some geophysical work is also being done.
This memorandum deals with reconnaissance projects. It is not, however, intended to encourage participation in this kind of project in preference to data-collection, or other, kinds of scour projects.
Personnel of the Office of Surface Water (OSW) and the FHWA met on August 9, 1990, to review the objects and approach of the bridge scour reconnaissance-type studies. Attached is a new generalized proposal which reflects the perspective of OSW and FHWA on reconnaissance-type projects. It is recommended that the existing reconnaissance-type proposals be changed or augmented to reflect this newer generic proposal. This memorandum discusses five topics where revision is recommended.
1) Important clarifications are needed in the existing proposals to prevent misunderstanding of the scope and product. The scope and product of these studies should be related to elements of FHWA's Technical Advisory 5140.20 Scour at Bridges (1988, copy attached).
2) Proposed methods for evaluating channel stability should have ongoing review to verify their effectiveness. The methods should be expanded as appropriate for the channel characteristics where they are applied.
3) Additional quantitative data should be collected to improve individual bridge evaluations and overall ability to transfer and use the data base.
4) The projects' scientific merit and relation to the WRD mission must be insured.
5) Plans for projects using sediment-transport models should provide for adequate model calibration and verification.
The broad objective of the reconnaissance-type projects is to assess the channel stability and scour potential of a stream site at a bridge crossing. The prominent type of scour projects being conducted has the objected of improved understanding and scour design techniques by real-time data collection and analysis. Scour-monitoring or other scour projects are also probable activities in which the WRD might assist cooperators. This memorandum is not intended to recommend or limit a particular type of project.
Scour processes include local scour, contraction scour, and various forms of channel evolution and instability. These processes are functions of many interrelated factors which must all be examined and analyzed in a complete analysis of scour potential at a bridge site.
Technical Advisory (TA) 5140.20, copy attached, was issued September 1988 by FHWA to State Highway Departments. The TA is focused on specific objectives addressing the widespread concern over risk from scour. The U.S. Geological Survey (USGS) is cooperating with Federal and State highway agencies in identifying conditions that are indicative of potential problems with scour and stream stability, as set forth in that Advisory. More than 10 USGS Districts have cooperative projects with State highway agencies to collect and analyze real-time bridge-scour data to improve our understanding of scour processes and bridge-scour design equations. About five USGS Districts have cooperative projects with State highway agencies to make a reconnaissance of channel-stability conditions at bridges, to rank bridges according to scour potential from channel instability, and to make complete scour evaluations at selected sites. The first of the served as a pilot for the other four. The five existing projects plan to do reconnaissance an additional 2900 bridges in one of the Districts. Total funding for the 5 existing projects is about $4.2 million.
The Tennessee study developed an inspection form which serves as the basis for the examination of channel stability at bridge sites. Selected variables from this form are weighted according to their relative importance to scour potential and summed to produce a channel-instability index for each bridge. Proposals for the active projects refer to this number as a "scour-potential index." The Tennessee inspection form as some similarities to Figure 1 in the FHWA Hydraulic Engineering Circular No. 20 (FHWA 1991). FHWA suggest at Figure 1 can be used as a screening tool to evaluate stream stability. The Tennessee inspection form provides a basis for evaluating channel stability and stage of channel evolution. Thus, the relative potential for damage to a bridge from channel instability can be evaluated using information from this form and bridge foundation data. Channel instability may be the dominant process causing bridge failure in some regions, such as west Tennessee. The form also includes items on piers, abutments, debris, and rip-rap by which an observably scour-critical condition from processes other than channel instability may be identified. The form does not include quantitative information on hydrology, hydraulics, or pier and abutment foundations. In general, existing reconnaissance-type projects have similar approach, scope, and format to the one in Tennessee.
Important clarifications are needed to prevent misunderstanding of the scope and product, as worded in the existing proposals of bridge-scour reconnaissance studies. The scope and product of these studies also should be related to elements of FHWA's TA on Bridge Scour (1988). Clarification is needed principally because of similar terminology used in both the FHWA TA and the reconnaissance-type proposals but with different meanings. The terms screening, scour evaluation, and scour critical hold special meanings in the FHWA TA. Paragraph 3 of the TA is titled "Recommendations for developing and implementing a scour- evaluation program." Part (c) of paragraph 3 is reprinted as follows:
FHWA has requested the initial screening process to be completed by March 1991 (L. Harrison, personal communication). The scour-evaluation process is described in chapter 5 of the TA 5140.20 attachment, "Interim procedures for evaluating scour at bridges." This interim guide is being replaced by Federal Highway Administration Hydraulic Engineering Circular (HEC) No. 18 "Evaluation Scour at Bridges" (FHWA 1991). The analytical step in this process begins (Step 3. a, page.69): "The recommended evaluation procedure is to estimate scour for a superflood, a flood exceeding the 100-year flood, and then analyze the foundations for vertical and lateral stability for this condition of scour."
The terms scour evaluation and scour critical also are used to describe the reconnaissance projects. The overall objectives of the existing proposals is
"to evaluate the potential for scour at bridges and to identify those structures that could potentially be endangered by the scour process."Given the methods used in the reconnaissance projects, this evaluation is not equivalent to that recommended in the TA. The field reconnaissance will help screen out some sites for which scour need not be further evaluated. The field assessment also will identify those bridges at which scour-critical conditions could be observed when visited. These observable scour-critical conditions could be from any of the scour processes. For the general case, the channel stability assessment provides an essential element of the overall scour evaluation, but a scour evaluation requires information on hydrology, hydraulics, and pier foundations for each site.
The reconnaissance studies may include a detailed scour analysis of selected sites based on scour potential rating due to channel evolution processes, or selected by the Highway Department. At these sites the level of detail of scour analysis would apparently equal or exceed that required in-the TA. Results from a detailed analysis and estimate of scour and channel stability would be followed by a foundation analysis (by State, Highway officials) for critical and lateral stability to determine if the bridge is scour critical.
To avoid problems, proposals and other documents for reconnaissance studies should not use the term scour evaluation to describe objectives, because this term encompasses analysis of bridge foundation stability. The term channel-stability assessment is recommended when referring to the field assessment. Descriptions of objectives to identify unstable bridges should use observably scour critical or equivalent because a scour evaluation and foundation analysis is required to determine that a bridge is not scour critical. In general, the use of terms included in the TA, should be equated with their meaning in the TA. These changes in wording are part of the attached generic proposal.
The channel stability inspection form is basically qualitative in content; however, its product is a channel instability rating or index by which bridges might be ranked to support decision processes such as selection for detailed scour evaluation or remediation. Thus, some verification is needed. The selection and weighting of index variables should be analyzed to optimize the sensitivity and representativeness of the resulting index. Verification of inspection variables and indices is complicated by the time scale of channel changes, by the random occurrence of floods impacting this process, and by the cross correlation and poor definition between explanatory variables and different scour processes. Project personnel are encouraged to develop and transfer verification schemes. A comparison of past, scour related maintenance with the channel stability index at a representative group of bridges may provide one form of verification. The objectivity of the form may be verified by comparing independent evaluations of the same group of bridge sites made by several qualified persons.
The inspection form developed for west Tennessee streams may not be applicable to all streams everywhere. The form should be reviewed, and parameters should be added or modified as necessary. This is step one in the work plan for the generic proposal. Changes to parameter definitions may improve the accuracy of the results for a given region, but will seriously inhibit comparisons of results between States where a different parameter definitions were used. Adding new parameters or changing the composition of index variables would not affect transferability.
The National Transportation Safety Board (NTSB) report (1988) on the catastrophic Schoharie Creek-bridge failure in April 1987 cites an inadequate bridge inspection program and inadequate over-sight as factors contributing to the probable cause of failure.
These factors were cited despite annual or biennial inspections since 1968 of the bridge site. The inspections included rating scour at each pier but not measurements of channel or scour-hole geometry. This finding highlights the need for review of inspection methods to improve and define the limits of their usefulness.
Additional quantitative data should be collected to substantiate observed scour-critical conditions deduced from the inspection and to improve the potential transferability of the project results. At a minimum, the cross-sectional geometry in the main channel should be defined, especially where pier footings are submerged in t@-e active channel section. if this is not done, observable scour critical situations will be missed in an inspection designed to include their identification. Observations of channel changes, scour, and fill require quantitative measurements, especially over the time scale of these projects. In addition to channel geometry, a bed material sample should be collected at each site, perhaps using simplified methods. The transferability of the qualitative data is uncertain. The measurement of channel geometry and bed material will improve the spatial and temporal transferability of the data base, an important scientific component of these investigations.
The public service merits of these investigations are obvious. The earth science merits lie in the collection of transferable data, in expanding our understanding of channel stability, in testing, calibration, and verification of detailed scour-evaluation techniques. It is important that these scientific components be included and highlighted in current and future investigations.
Some of the inspection form parameters describe structural characteristics of the bridges as related to scour susceptibility. These items of the inspections must not be represented as any assessment of structural integrity of the bridge. Apparent defects in the structural integrity of bridges observed by USGS personnel are reported to State highway agencies as a professional responsibility and not as part of a work plan.
One objective of the investigations is to model selected streams to make detailed scour analyses. Most of the existing proposals specify the model BRI-STARS for modeling some of the selected sites. Testing and verification of sediment-transport models for bridge-scour investigations has particular scientific merit. Detailed data collection for model calibration and verification should be a stated objective because of the scientific merit of this component and because of the substantial work element it represents. A list of general data requirements for calibrating and applying a sediment-transport model is forthcoming.