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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Chapter 2 Plan of Action and the Countermeasures Matrix
The National Bridge Inspection Standards (23 CFR 650, Subpart C) requires bridge owners to maintain a bridge inspection program that includes procedures for underwater inspection. A national scour evaluation program as an integral part of the National Bridge Inspection standards was established in 1988 by Technical Advisory T5140.20 (USDOT 1988).
Technical Advisory T5140.20 was superseded in 1991 by Technical Advisory T5140.23, to provide more guidance on the development and implementation of procedures for evaluating bridge scour to meet the requirements of 23 CFR 650, Subpart C). Specifically, Technical Advisory T5140.23 provides guidance on:
The Technical Advisory suggests that scour evaluations of both new and existing bridges should be conducted by an interdisciplinary team comprised of hydraulic, geotechnical and structural engineers. The recommendation for new bridges is to design the bridge foundation for potential scour by assuming that all streambed material in the computed scour prism has been removed and is not available for bearing or lateral support. Bridge foundations should be designed to withstand scour during floods equal to or less than the 100-year flood, and should be checked to ensure they will not fail during a superflood (on the order of the 500-year event). The procedures for computing the scour prism, which represents calculated scour conditions, are detailed in HEC-18 (Richardson and Davis 2001).
The recommendation for existing bridges is to evaluate every bridge over a waterway for scour to determine if it is scour critical or low risk. For a scour critical bridge, prudent measures should be taken for its protection. A scour critical bridge is one with abutment or pier foundations that are coded as unstable due to (1) observed scour at the bridge site, or (2) scour potential as determined from a scour evaluation study. A bridge that is not scour critical was defined as low risk, generally considered to have little potential for scour or stream instability problems. Results of the scour evaluation study for existing bridges are coded in Item 113 of the 1995 Recording and Coding Guide for the Structure Inventory and Appraisal of the Nations Bridges - FHWA Report PD-96-001 (more commonly known as the Coding Guide) (FHWA 1995, 2001).
Technical Advisory T5140.23 specifies that a plan of action should be developed for each existing bridge found to be scour critical. The two primary components of the plan of action are instructions regarding the type and frequency of inspections to be made at the bridge, and a schedule for the timely design and construction of scour countermeasures . The Technical Advisory further recommends appropriate training and instruction for bridge inspectors in scour issues. These include issues such as collection and comparison of cross section data, identification of conditions indicative of potential scour problems, and effective notification procedures when an actual or potential problem is identified at or in the vicinity of the bridge.
Information in this chapter provides direction for developing a plan of action. A "standard" template for preparing a plan of action is referenced and discussed. Issues related to the type and frequency of inspections are described, followed by the range of scour countermeasures available that could be incorporated in the plan of action. Finally, the countermeasures matrix is introduced, which provides a concise summary of the available countermeasures in categories classified as hydraulic, structural, biotechnical, and monitoring. The remainder of HEC-23 details the various scour countermeasures available in each category, which might be implemented through a plan of action.
Revisions to the Coding Guide for Item 113 (FHWA 2001) provide new guidance for coding bridges for observed and assessed scour conditions. HEC-23 is referenced for guidance on countermeasures for the protection of bridge foundations. These revisions to Item 113 codes allow bridge owners to consider countermeasures when coding (or re-coding) a bridge as stable, low risk, or scour critical based on the results of a bridge inspection and/or a scour evaluation.
As noted, responsibility for the National Bridge Inspection Standards (NBIS) is established in the Code of Federal Regulations (23 CFR 650, Sub-part C). Updates to 23 CFR 650, Sub-part C, underscore actions required for bridges that are determined to be scour critical . These include preparation of a plan of action to monitor known and potential deficiencies and to address critical findings, and monitoring of bridges in accordance with the plan for bridges that are scour critical (USDOT 2004, effective January 2005).
Inspection procedures for bridge owners are delineated in the CFR (650.313). Regarding follow-up on critical findings, the regulation requires that: (1) a state- or federal agency-wide procedure be established to assure that critical findings are addressed in a timely manner, and (2) FHWA be notified periodically of the actions taken to resolve or monitor critical findings.
As described above, when a bridge is found to be scour critical, either by inspection or by evaluation (i.e., assessed or calculated), a plan of action must be developed and implemented for that bridge. While many bridges may be found to be scour critical, the severity of the problem and the risk involved to the traveling public can vary dramatically. As a result, the management strategy for the plan of action, including factors such as the urgency of the response, the type and frequency of the inspection work, the redundancy in the plan, and amount of money and resources allocated to countermeasures (including monitoring), can vary from one scour critical bridge to the next.
For example, a bridge found to be scour critical by inspection, such as during an underwater inspection that finds a substantial scour hole undermining the foundation, would obviously be a greater concern than a bridge that is currently stable, but rated scour critical based on calculations of conditions that might develop during the 100-year flood. In the first case, the bridge has already experienced scour and is at risk of failure, whereas in the second case the bridge is not presently at risk, but might develop a scour problem in the future when it is subjected to the 100-year flood. The resulting management strategy for developing and implementing the plan of action would be much more urgent in the first case.
The management strategy may also vary according to the importance of the roadway to the transportation network and may require a risk-based analysis. For example, a bridge with high average daily traffic (ADT), or one that provides the only access in and out of a given area would be a greater concern than a low ADT bridge, or one for which alternate routes or detours were available. Similarly, a bridge that provides access for a hospital or fire station would be very important and might justify more resources or concern in developing and implementing a plan of action. A bridge that is along an evacuation route or provides access to an airport might also require a different level of response in developing a plan of action.
The management strategy might vary as a result of other repair or replacement plans. For example, a bridge found to be scour critical but already programmed for replacement in the near future might be treated differently from another bridge that was newer, or not considered for replacement for many years. In the first case, the use of monitoring as a countermeasure until replacement can occur might be reasonable, whereas in the second case, a structural countermeasure, at substantially greater cost, would probably be necessary.
As noted in Section 2.1.2, updates to Item 113 codes allow bridge owners to consider countermeasures when coding a bridge as stable, low risk, or scour critical, based on the results of a bridge inspection and/or a scour evaluation. Hydraulic or structural countermeasures that have been selected and designed by the interdisciplinary team, and properly installed can change a scour critical coding under Item 113 to a low risk code. Also, the updates allow bridge owners to consider mitigation measures installed during and/or immediately after a flood event in determining the appropriate Item 113 code. For this case, a plan of action must include specific instructions for monitoring the countermeasures to reduce the risk to the public users from a bridge failure.
The type and frequency of inspection work called for in the plan of action can also vary dramatically based on the management strategy. Bridges that are more important, or at higher risk, may justify more intense inspection efforts. Factors such as when to begin the inspection work, how often to visit the bridge during a flood, and when monitoring is no longer necessary must be addressed in the plan of action.
If a bridge foundation is determined to be unstable for the assessed or calculated stream stability or scour condition, and field inspection shows no evidence of a scour problem, the inspection requirements may not be any more than those required by the NBIS. For example, a bridge that is rated scour critical by calculations, but has relatively deep piles in an erosion-resistant material and has been in place for many years with no sign of scour, might adequately be addressed through the regular inspection cycle and after major flood events.
If more frequent inspections are required, the plan of action should to describe when to begin monitoring efforts. Initiation of inspection work can be based on discharge or stage measurements. While discharge is used to define or analyze scour conditions (e.g. scour during the 100-year flood), it is typically not the best criteria for triggering flood monitoring and inspection work. The primary limitation of a discharge based criteria is that the inspector often does not have a way of determining the discharge in the river, such as gaging station or flood forecasting results.
A more viable approach to define when to begin scour measurements has been to use the stage data corresponding to a critical discharge condition. However, even stage data must be specified in a manner that is easily understood and measurable by bridge inspection crews. For example, defining the initiation of scour measurements based on flood stage is only practical if stage information is readily available, and/or a gaging station is located at or near the bridge. Alternatively, if the critical water surface elevation is defined based on the distance from the guard rail or curb line of the bridge, the inspector could measure that distance and know when to begin data collection. An even more direct approach is to mark a line on a pier or abutment that defines when data collection or monitoring should be initiated.
On a basin wide basis, it may be possible to define flood watch requirements based on flood forecasting information. A simplistic approach is to implement monitoring after a given amount of rain has occurred. For example, the criteria might be to begin monitoring after a cumulative rainfall of 10 inches (25 cm) in 24 hours. A general criteria such as this might require the bridge inspection crews to immediately begin monitoring all scour critical bridges in that basin. Alternatively, in a more instrumented watershed with extensive flood warning systems, the use of GIS data and flood forecasting models could define in advance which bridges will need to be monitored at what times during the flood.
Once the flood inspection program is underway, the inspector needs to know exactly what constitutes a critical scour condition, and what to do when this condition has been detected. Specifically, a scour critical water surface elevation should be defined in the plan of action for each pier or abutment to be monitored. Information on who to call and what action to take once that elevation has been reached should also be detailed in the plan of action. This could extend as far as discussion of emergency repair measures and/or bridge closure instructions.
Closure instructions can range from load restrictions, lane closures or complete bridge closure, again depending on the severity of the problem and the risk involved. The method of closure should also be described. In some cases barricades may be adequate, while in other cases it may require, or justify based on the risk involved, the posting of a law enforcement officer at the bridge to insure that no one attempts to cross the structure. The availability and description of detour routes should be included in the plan of action, so when a bridge is closed an alternative route has already been defined to minimize traffic disruption. The scour vulnerability of bridges along the detour route should be known and evaluated when developing detour alternatives.
Instructions on the criteria for re-opening the bridge or traffic lane, or removing the load restriction, should also be provided. In many cases, the act of closing is easier than re-opening. Virtually anyone who detects a problem, such as an inspector, law enforcement officer, or bridge owner could make the decision to close a bridge, but the decision about when it is safe to re-open may require more information and engineering analysis by the interdisciplinary team. The person authorized to make the decision to re-open should be identified in the plan of action.
The two primary components of the plan of action are instructions regarding the type and frequency of inspections to be made at the bridge, and a schedule for the timely design and construction of scour countermeasures. Developing a schedule for the timely construction of countermeasures first requires defining the preferred countermeasure alternative. It is typical that several different alternatives might be appropriate countermeasures for a given scour or stream stability problem at a bridge. A comprehensive plan of action should provide enough information that an independent reviewer could arrive at the same conclusion regarding the preferred alternative.
In order to evaluate alternatives a conceptual design should be developed for various alternatives. This facilitates evaluation of the engineering feasibility of the alternative, and allows developing preliminary cost estimates. The various alternatives developed should be presented in the plan of action, and a narrative provided describing why the preferred alternative was chosen.
Once the preferred alternative is selected, a schedule should be developed for the timely design and construction of the preferred alternative. It may be that a more intense monitoring alternative is recommended as a measure to reduce the risk from scour, prior to design and construction of countermeasures to make the bridge safe from scour.
The plan of action can include other information to the inspector, including special conditions to watch for such as debris build-up and associated problems. It might include instructions on communications with the media, such as who is authorized to make statements and what information should be provided. Actions such as bridge closures and/or bridge failures generate a lot of interest and concern from both the media and the public. Developing a communications plan ahead of time can minimize confusion and mis-communication. The plan of action might also describe emergency action countermeasures, such as what type of riprap is adequate, local sources, and installation methods during a flood situation.
The development of a POA means that bridge owners have held meetings involving the appropriate personnel from internal units within their corresponding agency (design, construction, inspection and maintenance, districts, and others as applicable) and with external entities (local authorities such as a commissioner, police department, fire department, and others as needed) to identify and document:
Additional guidance on developing a POA is presented in Section 2.2 and Appendix B .
The implementation of a POA means that bridge owners have completed disseminating POAs to the appropriate personnel within their internal offices/units and external entities and have met with these offices/units and with external entities to communicate:
Bridge owners should make sure that responsible parties identified in the POA understand their roles and responsibilities that they are provided with periodic training on the implementation of selected components of a POA such as bridge closure/opening procedures. Implementation also includes establishing the frequency to conduct periodic reviews and updates of the information presented in a POA.
In order to facilitate the development of a POA, the FHWA has created a "standard" template for bridges that are scour critical . The standard template and instructions for completing a POA are provided in Appendix B. This template includes the minimum information recommended by FHWA for a POA.
The template is intended to be a guide and tool for bridge owners to use in developing their POAs. The template provides the program manager with a summary of the type of information that should be part of a plan of action for a bridge identified as scour critical. Bridge owners may also want to consider using the POA standard template for bridges which have unknown foundations (i.e., foundation characteristics such as width, depth, and length may be unknown).
All the fields in the template may be modified so that local terminology is employed, unique information may be added regarding local and site-specific scour and stream stability concerns, and local sources of information may be included. The electronic Microsoft Word document template can be downloaded from the FHWA website:
All blocks in this template will expand automatically to allow additional space. Where check boxes are provided, they can be checked by double-clicking on the box and selecting the "checked" option.
To provide guidance and training on preparation of POA for scour critical bridges, the FHWA developed an on-line module which includes the POA standard template and illustrates its application to field case studies. This training module which is referenced as NHI Course No. 135085 is offered by NHI and can be accessed at:
A state's Bridge Management System is a useful source of data for developing a POA. Many DOT's are now using information technology (IT) systems that provide immediate access via the bridge engineer's desktop computer to an integrated system of bridge management information and data bases. Much of the information outlined in the template may be obtained from these systems.
The standard template contains ten sections. Sections 1 through 4 are intended as an executive summary for the busy reviewer/manager who may not need the details of Sections 5 through 10, and show:
To assist in completing a POA using the template, Appendix B contains general guidance for each section of the template. An abbreviated set of instructions is also appended to the template.
Selecting the countermeasures to be included in the plan of action requires evaluating a number of alternatives. These alternatives could include hydraulic countermeasures, structural countermeasures or monitoring, either individually or in some combination. To facilitate selection of alternatives to be considered in the plan of action, a matrix describing the various countermeasures and their attributes has been developed. This countermeasure matrix is introduced and described in this section .
A wide variety of countermeasures have been used to control channel instability and scour at bridge foundations. The countermeasure matrix, presented in Table 2.1, is organized to highlight the various groups of countermeasures and to identify their individual characteristics. The left column of the matrix lists types of countermeasures in groups. In each row of the matrix, distinctive characteristics of a particular countermeasure are identified. The matrix identifies most countermeasures used by DOTs and lists information on their functional applicability to a particular problem, their suitability to specific river environments, the general level of maintenance resources required, and which states have experience with specific countermeasures. Finally, a reference source for design guidelines is noted, where available.
Countermeasures have been organized into groups based on their functionality with respect to scour and stream instability. The four main groups of countermeasures are: hydraulic countermeasures , structural countermeasures, biotechnical countermeasures, and monitoring . The following outline identifies the countermeasure groups in the matrix:
Group 1. Hydraulic Countermeasures
Group 2. Structural Countermeasures
Group 3. Biotechnical Countermeasures
Group 4. Monitoring
Hydraulic countermeasures are those which are primarily designed either to modify the flow or resist erosive forces caused by the flow. Hydraulic countermeasures are organized into two groups: river training structures and armoring countermeasures . The performance of hydraulic countermeasures is dependent on design considerations such as edge treatment and filter requirements, which are discussed in Sections 5.2 and 5.4, respectively.
Group 1.A River Training Structures. River training structures are those which modify the flow. River training structures are distinctive in that they alter hydraulics to mitigate undesirable erosional and/or depositional conditions at a particular location or in a river reach. River training structures can be constructed of various material types and are not distinguished by their construction material, but rather, by their orientation to flow. River training structures are described as transverse , longitudinal or areal depending on their orientation to the stream flow.
Transverse river training structures are countermeasures which project into the flow field at an angle or perpendicular to the direction of flow.
Longitudinal river training structures are countermeasures which are oriented parallel to the flow field or along a bankline.
Areal river training structures are countermeasures which cannot be described as transverse or longitudinal when acting as a system. This group also includes countermeasure "treatments" which have areal characteristics such as channelization, flow relief, and sediment detention.
Group 1.B Armoring Countermeasures. Armoring countermeasures are distinctive because they resist the erosive forces caused by a hydraulic condition. Armoring countermeasures do not necessarily alter the hydraulics of a reach, but act as a resistant layer to hydraulic shear stresses providing protection to the more erodible materials underneath. Armoring countermeasures generally do not vary by function, but vary more in material type. Armoring countermeasures are classified by two functional groups: revetments and bed armoring or local scour armoring.
Structural countermeasures involve modification of the bridge structure (foundation) to prevent failure from scour. Typically, the substructure is modified to increase bridge stability after scour has occurred or when a bridge is assessed as scour critical. These modifications are classified as either foundation strengthening or pier geometry modifications .
Vegetation has been used increasingly over the past few decades to control streambank erosion or as a bank stabilizer. It has been used primarily in stream restoration and rehabilitation projects and can be applied independently or in combination with structural countermeasures. There are several terms that describe vegetative streambank stabilization and countermeasures. The use of 'soft' revetments (consisting solely of living plant materials or plant products) is often referred to as bioengineering. The techniques that combine the use of vegetation with structural (hard) elements include biotechnical engineering and biotechnical slope protection. Where riprap constitutes the "hard" component of biotechnical slope protection, the term vegetated riprap is also used (see Chapter 6).
The matrix considers representative categories for biotechnically engineered counter-measures (which incorporate rock) including:
Biotechnical engineering can be a useful and cost-effective tool in controlling bank or channel erosion, while increasing the aesthetics and habitat diversity of the site. However, where failure of the countermeasure could lead to failure of a bridge or highway structure, the only acceptable solution in the immediate vicinity of a structure is a traditional, "hard" engineering approach.
Monitoring describes activities used to facilitate early identification of potential scour problems. Monitoring could also serve as a continuous survey of the scour progress around the bridge foundations. While monitoring does not fix the scour problem at a scour critical bridge, it allows for action to be taken before the safety of the public is threatened by the potential failure of the bridge. Monitoring can be accomplished with instrumentation or visual inspection. A well designed monitoring program can be a very cost-effective countermeasure. Two types of instrumentation are used to monitor bridge scour: fixed instruments and portable instruments (see Chapter 9).
The countermeasure matrix (Table 2.1) was developed to identify distinctive characteristics for each type of countermeasure. Five categories of countermeasure characteristics were defined to aid in the selection and implementation of countermeasures:
These categories were used to answer the following questions:
The functional applications category describes the type of scour or stream instability problem for which the countermeasure is prescribed. The six main categories of functional applications are local scour at abutments and piers, contraction scour, vertical and lateral instability, and overtopping flow on approach embankments. Vertical instability implies the long-term processes of aggradation or degradation over relatively long river reaches, and lateral instability involves a long-term process of channel migration and bankline erosion problems. While overtopping flow considers, primarily, roadway approach embankments, the functional application could also include overtopping of countermeasures such as spurs or guide banks. To associate the appropriate countermeasure type with a particular problem, filled circles, half circles and open circle are used in the matrix as described below:
well suited/primary use - the countermeasure is well suited for the application; the countermeasure has a good record of success for the application; the countermeasure was implemented primarily for this application.
possible application/secondary use - the countermeasure can be used for the application; the countermeasure has been used with limited success for the application; the countermeasure was implemented primarily for another application but also can be designed to function for this application.
In addition, this symbol can identify an application for which the countermeasure has performed successfully and was implemented primarily for that application, but there is only a limited amount of data on its performance and therefore the application cannot be rated as well suited.
unsuitable/rarely used - the countermeasure is not well suited for the application; the countermeasure has a poor record of success for the application; the countermeasure was not intended for this application.
N/A not applicable - the countermeasure is not applicable to this functional application.
This category describes the characteristics of the river environment for which a given countermeasure is best suited or under which there would be a reasonable expectation of success. Conversely, this category could indicate conditions under which experience has shown a countermeasure may not perform well. The river environment characteristics that can have a significant effect on countermeasure selection or performance are:
For each environmental characteristic, a qualitative range is established (e.g., stream size: W ide, M oderate, or S mall) to serve as a suitability discriminator. While most characteristics are self explanatory, both HEC-20 ("Stream Stability at Highway Structures") and HDS 6 ("River Engineering for Highway Encroachments") provide guidance on the range and definitions of these characteristics of the river environment (Lagasse et al. 2001a; Richardson et al. 2001). In the context of this matrix, the bank condition (slope) characteristic ( V ery Steep, S teep or M ild) considers the effectiveness of a given countermeasure to protect a bank with that configuration, or in some cases, the suitability for the countermeasure to armor a bank with that configuration.
The checked block means that the characteristic does not influence the selection of the countermeasure, i.e., the countermeasure is suitable for the full range of that characteristic. For example, guide banks have been applied successfully in braided, meandering, and straight streams; however, bendway weirs/stream barbs are most suitable for installation on meandering streams.
For further discussion of these and other river environment characteristics, see Chapter 3.
The maintenance category identifies the estimated level of maintenance that may need to be allocated to service the countermeasure. The ratings in this category range from "Low" to "High" and are subjective. The ratings represent the relative amount of resources required for maintenance with respect to other countermeasures within the matrix shown in Table 2.1. A low rating indicates that the countermeasure is relatively maintenance free, a moderate rating indicates that some maintenance is required, and a high rating indicates that the countermeasure requires more maintenance than most of the countermeasures in the matrix.
This category identifies DOTs for which information on the use of a particular countermeasures was available. These listings may not include all of the states which have used a particular countermeasure. Information on state use was obtained from three sources: a National Cooperative Highway Program questionnaire (University of Minnesota survey for NCHRP Project 24-07(1); Brice and Blodgett, "Countermeasures for Hydraulic Problems at Bridges, Volumes 1 and 2," (1978); and correspondence with DOT staff. Certain countermeasures are used by many states. These countermeasures have a listing of "Widely Used" in this category. Both successful, and unsuccessful experiences are reflected by the listing.
Reference manuals which provide guidance in countermeasure design have been developed by government agencies through research programs. The FHWA has produced a wealth of information through the federally coordinated program of highway research and development (see Chapter 10, References). In addition, each Design Guideline in Volume 2 identifies reference manuals where additional guidance on design can be obtained. Countermeasures for which design guidelines are provided within this document are referenced using DG# , where # represents the number assigned to the design guideline (see Chapter 7, Countermeasure Design Guidelines and Volume 2). Where additional design guidance can be found in Volume 1, the appropriate Chapter (CH) is referenced in the matrix.
The countermeasures matrix is a convenient reference guide on a wide range of countermeasures applicable to scour and stream stability problems. A comprehensive plan of action should provide conceptual design and cost information on several alternative countermeasures, with a recommended alternative based on a variety of engineering, environmental and cost factors. The countermeasures matrix is a good way to begin identifying and prioritizing possible alternatives. The information provided in the matrix related to functional applications, suitable river environment, and maintenance issues should facilitate preliminary selection of feasible alternatives prior to more detailed investigation.