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Field Manual - Scour Critical Bridges: High-Flow Monitoring and Emergency Procedures Idaho Transportation Department
[Idaho Transportation Department Logo]
by Ayres Associates
PDF Version (2.3 mb)Table of Contents
1. IntroductionList of Figures
Figure 1. Schematic illustrating local scour processes at a cylindrical pier
Approximately 198 of Idaho's highway bridges are rated scour-critical, meaning they are at risk of failure due to scour. Some of the scour-critical bridges require monitoring during periods of high flow in order to protect the traveling public. This manual provides guidance on the procedures for high-flow monitoring. It is intended as a field reference for use by monitoring crews.
The monitoring crew for a scour-critical bridge has a very important job with significant responsibilities. As the field personnel assigned to the bridge, you are the eyes and ears of the department. To properly perform your job, you must understand the general nature of scour, know what to look for in the field, understand how to make detailed scour measurements, and properly execute notification processes and bridge closure should a serious problem be discovered. The responsibility you carry is great and lives may depend on how you perform your job.
Section 2 explains basic scour concepts and definitions. This is necessary background information for anyone attempting to measure scour in the field. Section 3 describes the content of Plans of Action for scour-critical bridges. A Plan of Action has not yet been developed for every scour-critical bridge. Where available, however, a Plan of Action provides useful information for the field monitoring crew.
Section 4 describes commonly used scour monitoring equipment. The important concept of scour-critical elevations is discussed in Section 5. Monitoring procedures are explained in Section 6. Finally, Section 7 describes emergency action protocols.
2. Basic Scour Concepts and Definitions
Crews responsible for monitoring scour-critical bridges should have a basic understanding of scour concepts. This section explains what scour is, different types and components of scour, and the factors that influence scour potential.
Scour is the result of the erosive action of flowing water, removing sediment from the streambed and banks of streams and from around the piers (intermediate supports) and abutments (end supports) of bridges. Different types of sediment scour at different rates. Loose granular soils such as coarse sands, gravels and larger materials, are rapidly eroded by flowing water, while cohesive or cemented soils, such as clays and silts, are more resistant to erosion. However, ultimate scour (maximum scour) in cohesive or cemented soils can be as deep as scour in sand-bed streams. Under constant flow conditions, scour will reach a maximum depth in sand-bed and gravel-bed material in hours; cohesive streambed material in days; glacial till, sandstones, and shale in months; limestone in years, and dense granite in centuries. Under flow conditions typical of actual bridge crossings, attainment of the ultimate scour depth may take several floods.
2.1 Types of Scour
Scour can be classified into two types, clear-water and live-bed, defined below:
A scour hole formed by clear-water scour may be visible during a post-flood inspection. The scour hole at a pier formed by live-bed scour may not be visible during a post-flood inspection because the scour hole is deepest during the peak of the flood and then partially or fully refills as the flood recedes.
2.2 Components of Scour
Total scour at a highway crossing is comprised of three components:
Long-term aggradation raises the streambed and degradation lowers the streambed over a long distance along the stream. Aggradation and degradation are typically not caused by the bridge, but rather by other factors related to a longer segment of the stream.
General scour at the bridge can be clear-water or live-bed, and it usually occurs as contraction scour. Contraction scour is caused by the width constriction of the waterway or floodplain imposed by the road embankments and the bridge structure. It occurs when the waterway opening through the bridge is smaller than the natural flow area. Contraction scour typically extends across the entire width of the bridge opening. The contraction scour depth may or may not be uniform across the bridge opening. The scour that occurs at the outside of bends is another example of general scour.
Local scour is a result of the obstruction of flow caused by the piers and abutments. The obstruction caused by a pier creates a horseshoe-shaped vortex around the front and sides of the pier, as illustrated in Figure 1. A similar process causes scour at abutments that form an obstruction to the flow, as shown in Figure 2. The horizontal extent of local scour is relatively small compared to the other components of scour, but the depth of local scour is often greater than the other components. Local scour can be either clear-water or live-bed. Under live-bed conditions, the local scour is deepest near the peak of the flood, and refills at least partially as the flood recedes.
In scour prediction calculations, the computed values of the three scour components are added to obtain the total predicted scour at a pier or abutment (Figure 3).
2.3 Lateral Migration
In addition to the three components described above, potential lateral migration (side-to-side movement) of the stream must be assessed when evaluating total scour at bridge piers and abutments. Lateral migration of the main channel occurs naturally, but can also be induced or magnified by human activities. A channel moving laterally may affect the stability of piers along the channel bank, erode abutments, damage the approach roadway embankments, or increase the total scour by changing the flow direction at piers and abutments. Figure 4 shows an abutment undermined by lateral migration. Factors that affect lateral stream movement are the geomorphology of the stream (the ongoing trends of change in the stream alignment), the location of the crossing on or near a channel bend, flood characteristics, and the characteristics of the streambed and bank sediments.
2.4 Tips to Remember When Monitoring Contraction Scour
2.5 Tips to Remember When Monitoring Pier Scour
2.6 Tips to Remember When Monitoring Abutment Scour
3. Plans of Action for Scour-Critical Bridges
3.1 What is a Plan of Action?
Each scour-critical bridge should ideally have an associated Plan of Action and an information packet developed for the monitoring crews. The full Plan of Action includes:
The elements contained in a Plan of Action for a given bridge will vary depending on the economic risk of bridge failure and the annual probability of failure. Idaho's scour-critical bridges have been subdivided into four scour categories. These scour categories are described below.
Scour Category A - Vital Bridges. These bridges have an economic risk of failure in excess of $5,000,000. Consequently, these bridges should not be allowed to fail due to scour processes. The minimum response to scour-critical conditions for Category A scour-critical bridges focuses on the design and installation of effective scour countermeasures. However, prior to countermeasure installation, Category A bridges are treated as Category B, C, or D bridges, depending on the predicted annual probability of failure. Therefore, the Plan of Action for these bridges may include interim monitoring and closure plans. Once the scour countermeasures are installed, no high flow monitoring will typically be required. However, if the channel, abutments, or piers are armored to resist scour, then post-flood scour inspection will be required to ensure the long-term performance of the countermeasures.
Category B - Extreme Scour-critical Bridges. These bridges have an economic risk of failure of less than $5,000,000, but have a predicted annual probability of failure in excess of 10 percent. The minimum scour response for this category will provide instructions to close the bridge for any flood event. The Plan of Action will also describe inspection requirements for the bridge. It will not typically include any high-flow monitoring beyond the minimal monitoring necessary to determine that a flood is occurring.
Category C - Severe Scour-Critical Bridges. These bridges have an economic risk of failure of less than $5,000,000 and a predicted annual probability of scour failure between 1 and 10 percent. A bridge is also in this category if it has a predicted annual probability of failure less than 1 percent, but has shallow (spread footing) foundations subject to sudden, catastrophic failure. A bridge monitoring plan should be developed by the bridge owner, in consultation with ITD personnel, for each bridge in this category. Category C bridges should be treated as Category B bridges until a monitoring plan has been developed and implemented.
The typical response plan for a Category C bridge will focus on a monitoring and closure strategy. Once the monitoring plan is implemented, however, structural, monitoring and hydraulic countermeasures may be developed for each bridge as funding allows. Successful installation of structural or hydraulic countermeasures at these structures may allow their reclassification from scour critical status to low-risk status in the NBI (Code 7 under Item 113).
Category D - Moderate Scour-Critical Bridges. These bridges have an economic risk of failure less than $5,000,000, deep foundations, such as pile or drilled-shaft foundations and a predicted annual probability of failure of less than 1 percent. The scour response strategy for these bridges typically focuses on inspection, qualitative monitoring, and closure protocols.
3.2 What do You Need from the Plan of Action?
The scour monitoring crew members should familiarize themselves with the contents of the Plan of Action for a given bridge prior to commencing monitoring activities. At a minimum, the crew should review the cover page and executive summary and determine whether all the necessary data, monitoring equipment, and signage are available for the crew.
Certain items from the Plan of Action should be reproduced and prepared for field use (i.e., laminated) in advance of monitoring activities. These items include the cover page and executive summary, the Scour-Critical Elevations Table (or the Monitoring, Closure, and Scour-Critical Water Surface Elevations Table if a WSEL-based scour monitoring strategy has been implemented at the bridge), the detour map, closure protocol, and signing layout plan, the current bridge inspection cross section plotted with historical soundings and bridge foundation elements, contact information, and any special bridge-specific monitoring and closure instructions. An equipment list should be developed for monitoring activities at each bridge. Sufficient stocks of signage and other supplies should be kept to allow high-flow monitoring at several scour-critical bridges simultaneously within the jurisdiction.
The crew performing high-flow monitoring should be focused on looking for indicators that the bridge is at imminent risk of failure. When it is necessary to monitor a bridge during high flows, the monitoring crew should have the necessary guidance information readily at hand. If a Plan of Action has been prepared for the bridge to be monitored, it should provide the following useful information to the monitoring crew.
The following sections describe the typical steps required within each scour category to meet the expected monitoring requirements.
4. monitoring activities at scour-critical bridges
4.1 Safety Precautions During Monitoring Activities
The monitoring crew should look for signs of bridge distress before getting on the bridge and throughout the high-flow monitoring effort. Such signs would include, but not necessarily be limited to:
If any of these or other signs of structural distress are apparent at any time, the crew should call for a bridge closure and should avoid getting on the bridge if at all possible. A bridge under extreme distress, near Henry's Fork, Idaho is shown in Figure 6. Note the abrupt sag in the profile (due to a failed pier), the debris buildup (which shows that the bridge deck was overtopped), and the water surface still very near the low chord.
4.2 Step-By-Step Procedure for Scour Category A Bridges
Category A bridges are of such importance that an individual Plan of Action should be developed for each bridge, with specific monitoring instructions, rather than a general plan of action applicable to all bridges in the category. The guidelines given below for other scour categories will assist in understanding the monitoring activities required by each Plan of Action for Category A bridges.
4.3 Step-By-Step Procedure for Scour Category B
The appropriate response for Category B bridges is bridge closure for any flood event. The Plan of Action will describe what flood event triggers bridge closure. The critical flood level may be described on the basis of water surface elevation, discharge, bankfull flow, official flood warnings (e.g., National Weather Service) or rainfall conditions. See Appendix A for a description of flood event triggers that could be used to close a Category B bridge.
4.4 Step-By-Step Procedure for Scour Category C
The monitoring requirements for Category C are the most demanding and involve detailed on-site inspection activities, including both qualitative and quantitative monitoring procedures. For bridges with established scour monitoring and scour closure water surface elevations, no direct streambed elevation monitoring is required. The water surface elevation at the upstream face of the bridge will be monitored visually using signs posted on the abutments, piers or other elements visible from the bridge deck or road shoulders, or measured using a weighted tape, and compared to the closure water surface elevation reported in the Plan of Action.
For direct streambed elevation monitoring, the type of instrument used depends on the site conditions. In many cases simple physical probing will be most effective. Some conditions, such as water flowing fast and deep can make the use of a physical probe impractical. In these cases sonar type instrumentation may be better suited. Appendix C provides guidance on portable monitoring instrument selection. Fixed scour monitoring instrumentation will rarely be available. When available, however, it can provide valuable information to supplement portable monitoring.
The following sections describe the step-by-step instructions for either water surface elevation or streambed elevation monitoring protocol. For additional information on water surface elevation monitoring techniques, see Appendix B, and for streambed elevation monitoring, Appendix C.
Step-by-step instructions for Water Surface Based Monitoring
Step-by-step Instructions for Streambed Elevation Monitoring
4.5 Step-By-Step Procedure for Scour Category D
The appropriate response for Category D involves on-site inspection activities and qualitative monitoring procedures. Qualitative monitoring methods are based primarily on visual observation of conditions at the bridge.
5. Emergency Procedures
5.1 Bridge Closures
During high-flow monitoring, the crew may decide to call for a bridge closure if:
If the crew chief decides that a closure should be called for, he or she must contact the office of the District Engineer (for ITD-owned bridges), or in the case of local-government-owned bridges, whoever has the authority to order the closure. The contact protocol for the closure should be delineated as part of the Plan of Action. At a minimum, the Bridge Inspection Engineer, the Bridge Engineer, the Maintenance Supervisor, the State Highway Patrol or local law enforcement agency, and the IDT or local authority public affairs coordinator should be contacted regarding the closure.
The Plan of Action should include a detour map and signing layout plan in case of a bridge closure. The barriers for the closure and the detour signage are to be installed by ITD or local government maintenance forces. After the closure has been called for the monitoring crew should remain at the bridge site until the bridge closure crew or law enforcement arrives at the scene. If the bridge becomes unsafe for traffic while the crew is waiting for a formal bridge closure, the crew should perform an emergency closure of the bridge. The monitoring crew should be equipped with the necessary signage and temporary traffic barriers and authorized to perform an emergency closure, if necessary.
Once a bridge has been closed, it should remain closed until the flood has passed. As soon as possible after the flood, a full inspection of the bridge foundations, substructure, and superstructure should be conducted. For ITD-owned bridges the District Engineer has the authority to reopen the bridge to traffic once it has been assessed as structurally sound.
5.2 Emergency Protection
The high-flow monitoring crew may find it necessary to call for emergency protection. Emergency protection efforts usually involve placing rock riprap to slow or arrest the scour at a pier or abutment, or to stabilize an eroding bankline. Depending on the type of problem being addressed, the riprap may be placed:
The methods employed for placing the emergency protection will depend to some extent upon the size of the bridge, its load rating and its condition. Other factors are the extent of the flooding. For instance, placing riprap at a pier in the middle of a long bridge during high flow must be done from the bridge deck, probably using the bucket of a hydraulic excavator. The width or load rating of the bridge may not accommodate the necessary equipment. Placing large rock from a bridge deck may itself cause damage to the structure. Placing riprap along a bankline is possible only if the equipment can reach the bankline. If significant overbank flooding is occurring, there will typically be no way to quickly and safely reach the bankline.
Contraction scour is a general lowering of the entire streambed beneath the bridge. If contraction scour is occurring during the emergency situation, it may render the emergency protection measures futile. Riprap placed locally at a pier experiencing contraction scour will simply be lowered with the rest of the streambed.
As with a bridge closure, the decision authority to place emergency protection rests with the District Engineer for ITD-owned bridges. The District Engineer may consult with the Hydraulics Engineer, the Bridge Engineer and the Maintenance Supervisor. As with any roadside activity, all appropriate traffic control measures must be taken.
Another important form of protection is the removal of debris from the bridge. Removing debris from the nose of a pier, for instance, can prevent significant additional scour from that flood event. Debris removal during high flow is usually accomplished from the bridge deck using a bridge inspection truck, crane, or excavator arm. It may not be feasible at bridges with a large bridge deck overhang because of the difficulty in reaching the pier nose under the deck and in pulling the debris away from the pier against the flow.
5.3 Monitoring Countermeasure Performance
If it is known that scour protection countermeasures (such as riprap at piers or abutments) have previously been installed at the bridge, but the bridge is still considered scour-critical, the monitoring effort should include probing (if possible) or visual inspection to assess the condition of the countermeasures. If the countermeasures are judged to be failing or non-functional during the high flows, the crew should call for a bridge closure or emergency repair work.
[ Appendices ]
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