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Hydraulics Engineering


Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Appendix B


B.1 Overview

In order to facilitate the development of a POA, the FHWA has created a "standard" template for bridges that are scour critical. 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 required to develop a plan of action for bridges that are scour critical or have unknown foundations.

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 may be downloaded from the FHWA website:

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 a POA for scour critical bridges, the FHWA National Highway Institute has prepared an on-line module (NHI Course No. 135085) which includes the suggested template for a POA and illustrates its application to field case studies. This training module can be accessed (see site reference, p. 2.6).

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.

B.2 Executive Summary

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:

  • Section 1: General information
  • Section 2: Who prepared the POA
  • Section 3: The source of the problem
  • Section 4: What actions are recommended and their status

To assist in completing a POA using the template, the remaining sections of this appendix contain general guidance for each section of the template. Note that an abbreviated set of instructions is appended to the template.

B.3 Standard Template Sections
Section 1 - General Information

Section 1 of the POA Standard Template covers general information about the bridge. This information is usually available from the bridge owner's Bridge Management System or bridge-specific files.

The bridge replacement information provides a framework for decisions regarding the need for a structural or hydraulic countermeasure.

The bridge type information provides insight on how the bridge could fail, if significant scour were to occur. For example, a simply-supported span bridge can fail suddenly, so the bridge should be closed as soon as scour becomes close to critical. Bridges with structural redundancy may allow more time to respond to an emergency situation.

Also, identified in this section is whether the bridge provides service to emergency facilities or is part of an evacuation route. This information is important in bridge closure plans, since early communication of a bridge closure to emergency responders could help them effectively detour their normal routes.

Section 2 - Responsibility for POA

Section 2 of the POA Standard Template provides information on who are the people responsible for preparing and maintaining the POA.

  • Author or Authors refers to the individual or company that developed the POA, such as the State Bridge Maintenance Engineer or a consultant.

Concurrences on POA refers to the individuals or organizations which must concur with the contents of the POA. The individuals or organizations may also refer to upper management who approve funding or county officials and law enforcement agencies who must agree with the bridge closure plans and the disruption the closure will have on the public. Gaining concurrence before an emergency occurs helps to minimize inter-agency conflicts during an emergency.

Also, in Section 2 is information on the POA update: who will do the update, when, and what was updated. The POA should be reviewed and updated on a routine basis to ensure that the contents of the plan are still valid.

Section 3 - Scour Vulnerability

Section 3 of the POA Standard Template provides a summary of the scour status of the bridge.

Section 3a shows the current scour coding. If Other is selected, the appropriate code would be provided. For example, if the foundation is unknown U would be entered in the Other field.

In Section 3b indicate how the Item 113 code was determined. If the bridge has a U code, unknown must be written in the Other field.

Section 3c, provides space for a narrative description to summarize the information from the scour evaluation. The template has been developed to expand automatically as information is entered. Some items to include are:

  • Scour critical flood flow and scour depth
  • 100- or 500-year flood flows
  • Overtopping flow

Section 3d provides space to summarize the scour history, including when, where and how much scour or stream instability has been observed at the bridge. Also, information on scour countermeasures previously installed at the bridge and their performance should be included.

Section 4 - Recommended Action(s)

Section 4 of the POA Standard Template contains highlights from the recommended actions from Sections 6 and 7 of the template. Items 4a, 4b, and 4c refer to the main parts of the POA monitoring program. Item 4d refers to the hydraulic or structural countermeasure selected in Section 7. This section cannot be completed until Sections 6 and 7 are completed.

Section 5 - NBIS Coding Information

Section 5 of the POA Standard Template contains previous and current codes for the hydraulic related items of the NBI. This information provides a quick indication of observed or potential long-term problems or adverse trends that may affect the stability of the bridge foundations.

The Inspection Date corresponds to the date of the inspection when the NBI items were coded. If the Items were coded on a date different than the inspection date, this different date is indicated in the Comments block.

For additional details on Items 113 and 60, see the FHWA Policy Memorandum Revision of Coding Guide, Item 113 - Scour Critical Bridges dated April 27, 2001.

Section 6 - Monitoring Program

There are three types of countermeasures which should be considered as part of the countermeasure program:

  1. Monitoring
  2. Hydraulic
  3. Structural

A monitoring countermeasure can be considered a key component of a POA, either alone or in combination with other countermeasures. Monitoring is highlighted in its own section, Section 6, in the POA Standard Template. Section 6 is subdivided into three additional subsections including:

  • Inspection Frequency
  • Fixed Monitoring
  • Flood Monitoring

The first subsection of Section 6 covers information on the frequency of inspection. Bridges are usually inspected biennially. Bridge owners may choose to keep this schedule but may also specify inspectors look at certain items at the bridge to ensure stability with regard to scour. In this case, the Regular Inspection Program box would be selected along with a list of the items to be watched. These items may include countermeasures, channel bed elevations, signs of movement or settlement.

Some bridge owners may choose to increase the frequency of inspection to less than the 2-year cycle. In this case, the Increased Inspection Frequency box would be checked and the number of months between inspections would be indicated. Usually only items pertinent to scour and stream stability would be observed and inspected.

Underwater inspections may also be required at the bridge. If the Underwater Inspection cycle remains on the regular schedule, then the Underwater Inspection Required box would be checked. If an increased cycle is needed, then the Increased Underwater Inspection Frequency box would be selected and the months between inspections indicated. In both cases the items for inspection and observation are indicated on the POA.

The second subsection of Section 6 covers information on Fixed Monitoring Devices. Fixed monitoring devices can provide continuous information about scour at the bridge site (see Chapter 9). This information can lead to early identification of potential scour problems.

The Fixed Monitoring box would be selected if a bridge owner opts to use fixed monitoring devices at the bridge. The type of devices and location of devices would be described in the plan. Details about the devices may be included in Attachment F to the POA.

In most cases, the fixed monitoring device can send information continuously from the bridge site. However, this amount of information can become cumbersome, so most bridge owners obtain or sample the information periodically. The sampling interval should be indicated on the POA and can be modified during flood events. If modified, the rationale for the change would be noted on the POA.

The information received from the fixed monitoring device should be reviewed for developing scour problems. During normal flow, the information may be reviewed daily, weekly, or monthly. During flood events, the review frequency may increase. The POA should detail the frequency of review and identify who is conducting the review.

Scour Critical Criteria should be determined from a scour evaluation study. This criteria should be indicated on the plan. Selecting an elevation higher than the scour critical elevation ensures sufficient time needed to take action in protecting the traveling public and possibly the bridge. This elevation, called the Scour Alert Criteria, should also be presented on the POA.

The third subsection of Section 6 describes monitoring actions that should be implemented during an actual flood event. If the bridge owner inspects the bridge visually during a flood, then the Visual Inspection box is selected. Those individuals visually observing the bridge may look for movement or settlement in the bridge or for a certain elevation of water, which could trigger the actions prescribed in the POA. If some kind of instrumentation is used to measure scour or water elevation, the Instrument box is selected and the applicable instrumentation type indicated. Both the Visual Inspection and the Instrument boxes may be selected.

The POA should document thresholds for the start and end of flood monitoring and note the frequency of the monitoring (see Chapter 2, Section 2.1.4). These thresholds may include:

  • Flood discharge
  • Stage
  • Water surface elevation
  • Rainfall data

The POA should clearly describe the thresholds and how the threshold is determined. For example, the threshold discharge or stage may be tied to a nearby USGS gage. Some bridge owners have opted to mark their bridges with the threshold water surface elevations to ensure inspectors know when action should be taken. The POA should also describe the actions required when threshold values are reached.

The agency, department, position, or person responsible for inspecting or reviewing instrumentation data should be listed at the end of Section 6 in the POA Standard Template. Some bridge owners may have maintenance staff rather than bridge inspectors monitor the bridge during a flood and then have bridge inspection staff conduct the post-flood inspection. All staff with responsibilities in implementing the POA should be listed. More than one person may be listed, especially to provide back up points-of-contact.

In some cases, the person at the bridge site during a flood event must confer with someone of greater authority in order to decide to close the bridge. This decision maker should also be listed in Section 6 of the POA Standard Template.

Finally, if action must be taken, the agency, department, position, or person responsible for taking the action would be listed. For example, if the local law enforcement is to close the bridge and set up the detour route, this agency is listed in Section 6 of the POA Standard Template.

Section 7 - Countermeasure Recommendations

Section 7 of the POA Standard Template summarizes the alternative countermeasures considered for the bridge, as well as the final countermeasures selected and rationale (see Chapter 2, Sections 2.3 - 2.5).

If a monitoring countermeasure were selected for the countermeasure program, the Monitoring box would be selected. If a structural and/or hydraulic countermeasure were selected, then the Structural/Hydraulic Scour Countermeasures Considered box would be selected. Both boxes may be selected, if needed.

Under the Priority Ranking and Estimated Cost columns, the various countermeasures for consideration along with their corresponding estimated costs. The selected countermeasures are indicated and the reasons for selecting the countermeasures should be explained on the Basis for selection line. Supporting information for the considered countermeasures can be included in Attachment F. Sufficient information to help independent reviewers understand the countermeasure selection process and the resulting decisions should be included.

This section also includes boxes for indicating the countermeasure implementation project type.

The last part of Section 7 of the POA Standard Template requires information on agency/department/position/person responsible for designing and implementing the countermeasure program. More than one person may be listed, if appropriate.

Target design and construction dates should are also required.

Section 8 - Bridge Closure Plan

Section 8 of the POA Standard Template provides instructions for closing a bridge. Specifically, this section should include:

  • Specific criteria indicating when to close the bridge
  • Who should close the bridge
  • Contact information, such as management or local law enforcement

Several examples of bridge closing conditions are provided. If the bridge owner has closure instructions specified in another document, these instructions should be referenced on the Emergency repair plans line.

The POA should also detail the process for reopening the bridge. In some cases, the bridge may be reopened when the floodwater has receded sufficiently. In other cases, the bridge will require inspection to ensure it is structurally sound. The reopening criteria should be listed. The agency or person who will inspect or make the decision to reopen the bridge should be identified in the plan.

Section 9 - Detour Route

Section 9 of the POA Standard Template describes potential detour routes, if the bridge is closed. The description should include route numbers, from/to locations, distances from

closed bridge, as well as any other pertinent information. A map of the detour routes should

be provided in Attachment E to the POA.

The bridges on the detour route should be listed in the Bridges on Detour Route table, along

with restrictive factors for each bridge on the detour route, such as:

  • Load restrictions
  • Clearance restrictions
  • Scour vulnerability condition

Additional items to present in Section 9 include:

  • Required traffic control equipment
  • Critical issues, such as flood overtopping vulnerability for bridges and roadways along the detour route, waterway adequacy of the detour bridges, and lane restrictions
  • Authority to communicate with the media and public Concurrence from local law enforcement agencies on the proposed detour and the closing procedures should be obtained and copies of the POA provided to these agencies.

Detour routes are not set in stone, but information should be provided on potential detours. Some factors to be considered in documenting in advance a detour route for a particular bridge include:

  • Detours must be set up, taking into consideration the conditions existing at the time a detour is needed. For some bridges, it may not be possible to foresee what these conditions might be, which roads might be flooded, which bridges may already be closed, what will be the load and clearance restrictions for the proposed detour route, etc.
  • DOT or bridge owner may have a designated department/office which is intimately familiar with their bridges and road systems, and are in the best position to quickly decide upon and coordinate a detour route for the given set of circumstances. This would be noted in the POA.
  • Decisions are made on the basis of the conditions that exist at the time of the closure. Consideration needs to be given to floods that may overtop bridges and approach roadways on the detour route. Actual detour routes should be based on roadway and bridge network status at the time the detour is proposed.
Section 10 - Attachments

Section 10 is the final section of the template and contains the following Attachments.

Attachment A should reference or include all available boring logs, probes, construction inspection and monitoring records, monitoring well readings, test pits, soil laboratory data, and anecdotal information.

Attachment B should reference or include all available channel cross sections for the bridge. Historic cross section comparisons, if available, should also be included.

Attachments C, D, and E are for documenting bridge elevation and plan views, and maps necessary to show detour routes.

Attachment F should include documentation on scour countermeasure alternatives. A comprehensive plan of action should provide enough information that an independent reviewer could arrive at the same conclusion regarding the preferred countermeasure alternative. For proposed scour countermeasures, a conceptual design should be prepared and details including reference to any hydraulic, structural or geotechnical studies that have been completed for the purpose of scour mitigation should be provided. Estimated costs of all proposed scour countermeasures should also be provided.

Details and dates of any recent scour countermeasure that has been implemented to address the current scour critical/unknown foundation status of the bridge should be included. All applicable studies, lead agencies, subcontractors and as-builts should be noted or included in Appendix F.

Attachment G may also reference or include historic and current aerial photographs of the site.

Attachment H may be used for any additional information such as: (1) standard closing and reopening procedures, (2) information on public transit, or (3) special circumstances such as access to emergency facilities, evacuation routes, etc. In some situations, public transportation (e.g., bus routes) may be of importance to the public with respect to detours.

If additional information is included, indicate this in Section 10 of the template.

{the rest of Appendix B can be found at}

Updated: 09/22/2014

United States Department of Transportation - Federal Highway Administration