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Idaho Transportation Department Office Manual Plans of Action for Scour Critical Bridges
Ayres Project No. 32-0629.00
PDF Version (1.5 mb)
Table of Contents
1. Introduction1.1 Purpose2. Basic Concepts and Definitions of Scour
This manual is intended for use by those responsible for the safety of Idaho bridges that may be threatened by scour. The purpose of this document is to enable engineers, bridge inspectors and maintenance personnel to prepare appropriate and cost-effective responses to scour conditions at each scour critical bridge in Idaho. To that end, this manual provides guidance on:
Additional technical background useful for the implementation of the recommended minimum responses are included in the appendices to this manual.
The most common cause of bridge failure is from floods scouring bed material from around bridge foundations. Scour is the engineering term for the water-induced erosion of the soil surrounding bridge foundations (piers and abutments). During the spring floods of 1987, 17 bridges in New York and New England were damaged or destroyed by scour. In 1985, 73 bridges were destroyed by floods in Pennsylvania, Virginia, and West Virginia. A 1973 national study for the Federal Highway Administration (FHWA) of 383 bridge failures caused by catastrophic floods showed that 25 percent involved pier damage and 75 percent involved abutment damage. A second, more extensive study in 1978 indicated local scour at bridge piers to be a problem about equal to abutment scour problems.
Minimizing future flood damage to Idaho’s bridges requires attention to developing and implementing improved procedures for protecting bridges against scour. Because it is not economically feasible to construct all bridges to ensure absolute invulnerability from scour damage, some risks of failure from future floods may have to be accepted. However, the added cost of making a bridge less vulnerable to scour is small when compared to the total cost of a failure, which can easily be 2 to 10 times the cost of the bridge itself.
Each bridge identified as scour critical in National Bridge Inventory (NBI) Item 113 (designated by Code 3, 2, 1, or 0) should have a monitoring and closure plan, addressing the inherent public safety hazard. For scour critical bridges whose failure would cause severe economic disruption, a hydraulic or structural countermeasure plan addressing the underlying scour conditions should be developed. Any unknown-foundation bridge (Item 113 code U) should be programmed for further action in accordance with the prioritization for unknown-foundation bridges presented in this manual. The bridge owner should make every attempt to determine the foundation type and depth. Once the foundation has been determined, a scour evaluation should be performed to determine whether the bridge is actually scour critical. Until the foundation is determined and the scour depths are known, a monitoring plan with closure protocols should be implemented.
This manual subdivides the ITD scour critical bridges into four categories based on lifetime risk of failure and annual probability of failure. It describes the minimum monitoring and/or countermeasure response required for each category. The required responses vary from simple high-flow monitoring plans and bridge closure protocols to a full Plan of Action covering both monitoring and countermeasure development.
1.3 Manual Organization
This manual is organized as follows:
2. Basic Concepts and Definitions of Scour
Scour is the result of the erosive action of flowing water, excavating and carrying away material from the bed and banks of streams and from around the piers and abutments of bridges. Different materials scour at different rates. Loose granular soils are rapidly eroded by flowing water, while cohesive or cemented soils are more scour-resistant. However, ultimate scour in cohesive or cemented soils can be as deep as scour in sand-bed streams.Under constant flow conditions, scour will reach maximum depth in sand- and gravel-bed material in hours; cohesive bed 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, several floods may be needed to attain maximum scour.
Measuring the magnitude of scour is complicated by the cyclic nature of the scour process. Scour can be deepest near the peak of a flood, but hardly visible as floodwaters recede and scour holes refill with sediment.
Designers and inspectors need to carefully study site-specific subsurface information in evaluating scour potential at bridges, giving particular attention to foundations on rock.Massive rock formations with few discontinuities are highly resistant to scour during the lifetime of a typical bridge.
The type of scour at highway crossings and encroachments is an important factor. Clearwater scour occurs where there is no transport of bed material upstream of the crossing or the material being transported from the upstream reach is transported through the downstream reach at less than the capacity of the flow. Live-bed scour occurs where there is transport of bed material from the upstream reach into the crossing.
This chapter provides a brief overview of scour concepts. For more detailed information, refer to the following documents published by the Federal Highway Administration (FHWA):
2.2 Total Scour
Total scour at a highway crossing is comprised of three components:
These three scour components are added to obtain the total scour at a pier or abutment. This assumes that each component occurs independently of the other, which may not always be true, but provides for a conservative design. In addition, lateral migration of the stream channel must be assessed when evaluating total scour at bridge piers and abutments.
2.2.1 Aggradation and Degradation
Aggradation and degradation are long-term streambed elevation changes due to natural or man-induced causes. Aggradation involves the deposition of material eroded from the channel or watershed upstream of the bridge. Degradation involves the lowering or scouring of the streambed due to a deficit in sediment supply from upstream.
The streambed may be aggrading, degrading, or in relative equilibrium in the vicinity of a bridge crossing. Long-term aggradation and degradation do not include the cutting and filling of the streambed in the vicinity of the bridge that might occur during a runoff event (see general and local scour below). A long-term trend may change during the life of the bridge. Such changes may be the result of natural processes or human activities. The engineer must assess the present state of the stream and watershed and then evaluate potential future changes in the river system. From this assessment, the long-term streambed elevation changes can be estimated.
2.2.2 Clear-Water and Live-Bed Scour
There are two classifications for scour: clear-water and live-bed scour. Clear-water scour occurs when there is no movement of the bed material in the flow upstream of the crossing or the bed material being transported in the upstream reach is transported in suspension through the scour hole at the pier or abutment at less than the sediment transport capacity of the flow. At the pier or abutment, the acceleration of the flow and vortices created by these obstructions cause the bed material around them to move. Live-bed scour occurs when there is transport of bed material from the upstream reach into the crossing. Live-bed local scour is cyclic in nature; that is, the scour hole that develops during the rising stage of a flood refills during the falling stage.
Typical clear-water scour situations include: (1) coarse-bed material streams, (2) flat gradient streams during low flow, (3) local deposits of larger bed materials that are larger than the biggest fraction being transported by the flow (rock riprap is a special case of this situation), (4) armored streambeds where the only locations that tractive forces are adequate to penetrate the armor layer are at piers and/or abutments, and (5) vegetated channels or overbank areas.
During a flood event, bridges over streams with coarse-bed material are often subjected to clear-water scour at low discharges, live-bed scour at the higher discharges and then clearwater scour at the lower discharges on the falling stages. Clear-water scour requires more time to reach its maximum than does live-bed scour (Figure 2.1). This is because clearwater scour occurs mainly in coarse-bed material streams. In fact, local clear-water scour may not reach a maximum until after several floods. Maximum clear-water pier scour is about 10 percent greater than the equilibrium live-bed pier scour.
2.2.3 General Scour
General scour is a lowering of the streambed across the stream or waterway bed at the bridge. This lowering may be uniform across the bed or non-uniform, that is, the depth of scour may be deeper in some parts of the cross section. General scour may result from contraction of the flow, which results in removal of material from the bed across all or most of the channel width, or from other general scour conditions such as flow around a bend where the scour may be concentrated near the outside of the bend. General scour is different from long-term degradation in that general scour may be cyclic and/or related to the passing of a flood. Also, long-term degradation is usually associated with a longer segment of the river whereas general scour is more of a local issue due to a scour-inducing feature of the channel geometry (flow constriction, a bend, etc).
2.2.4 Local Scour
Local scour is the removal of material from around piers, abutments, spurs, and embankments. It is caused by an acceleration of flow and resulting vortices induced by obstructions to the flow. Local scour can be either clear-water or live-bed scour.
2.2.5 Lateral Stream Migration
In addition to the types of scour mentioned above, naturally occurring lateral migration of the main channel of a stream within a floodplain may affect the stability of piers in a floodplain,erode abutments or the approach roadway, or change the total scour by changing the flow angle of attack at piers and abutments. Factors that affect lateral stream movement also affect the stability of a bridge foundation. These factors are the geomorphology of the stream, location of the crossing on the stream, flood characteristics, and the characteristics of the bed and bank materials. HEC-20 and HDS 6 provide detailed information on lateral stream migration.
The following sections provide a more detailed discussion of general scour, local scour, and lateral stream migration.
2.3 General Scour
2.3.1 Contraction Scour
Contraction scour occurs when the flow area of a stream at flood stage is reduced, either by a natural contraction of the stream channel or by a bridge. It also occurs when overbank flow is forced back to the channel by roadway embankments at the approaches to a bridge. In order for continuity of flow to be satisfied, a decrease in flow area results in an increase in average velocity and bed shear stress through the contraction. Hence, there is an increase in erosive forces in the contraction and more bed material is removed from the contracted reach than is transported into the reach. This increase in transport of bed material from the reach lowers the bed elevation. As the bed elevation is lowered, the flow area increases and, in the riverine situation, the velocity and shear stress decrease until relative equilibrium is reached; i.e., the quantity of bed material that is transported into the reach is equal to that removed from the reach, or the bed shear stress is decreased to a value such that no sediment is transported out of the reach. Contraction scour causes removal of material from the bed across all or most of the channel width.
Live-bed contraction scour is typically cyclic; for example, the bed scours during the rising stage of a runoff event and fills on the falling stage. The cyclic nature of contraction scour causes difficulties in measuring contraction scour depths after a flood.
Typical conditions that can cause contraction scour are (1) natural stream constrictions, (2)long highway approaches to the bridge over the floodplain, (3) ice formations or jams, (4)natural berms along the banks due to sediment deposits, (5) debris, (6) vegetative growth in the channel or floodplain, and (7) pressure flow (the condition in which the bridge low chord is submerged).
Figure 2.2 shows the conditions after live-bed contraction scour at the I-5 bridges near Coalinga, California. The failure occurred March 10, 1995, and seven people were killed.The photo is looking downstream.
2.3.2 Other General Scour
Other general scour conditions can result from erosion related to the plan form characteristics of the stream (meandering, braided or straight), variable downstream control, flow around a bend, or other changes that decrease the bed elevation. General scour conditions can occur at bridges located upstream or downstream of a confluence.
2.3.3 Tips for Assessing General Scour Potential
2.4 Local Scour
The basic mechanism causing local scour at piers or abutments is the formation of vortices (known as the horseshoe vortex) at their base (Figure 2.3). The horseshoe vortex results from the pileup of water on the upstream surface of the obstruction and subsequent acceleration of the flow around the nose of the pier or abutment. The action of the vortex removes bed material from around the base of the obstruction. The transport rate of sediment away from the base region is greater than the transport rate into the region, and, consequently, a scour hole develops. As the depth of scour increases, the strength of the horseshoe vortex is reduced, thereby reducing the transport rate from the base region. Eventually, for live-bed local scour, equilibrium is reestablished between bed material inflow and outflow and scouring ceases. For clear-water scour, scouring ceases when the shear stress caused by the horseshoe vortex equals the critical shear stress of the sediment particles at the bottom of the scour hole.
In addition to the horseshoe vortex around the base of a pier, there are vertical vortices downstream of the pier called the wake vortex (Figure 2.2). Both the horseshoe and wake vortices remove material from the pier base region. However, the intensity of wake vortices diminishes rapidly as the distance downstream of the pier increases. Therefore, immediately downstream of a long pier there is often deposition of material.
2.4.1 Tips for Assessing Pier Scour Potential
2.4.2 Tips for Assessing Abutment Scour Potential
Figure 2.4 presents the various types of abutments including spill-through, vertical wall, and vertical wall with flared wingwalls.
2.5 Lateral Shifting of a Stream
Streams are dynamic. Areas of flow concentration continually shift banklines, and in meandering streams having an "S-shaped" planform, the channel moves both laterally and down valley. A braided stream has numerous channels that are continually changing. In a braided stream, the deepest natural scour occurs when two channels come together or when the flow comes together downstream of an island or bar. This scour depth has been observed to be 1 to 2 times the average flow depth.
A bridge is static. It fixes the stream at one place in space. A meandering stream whose channel moves laterally and down valley into the bridge reach can erode the approach embankment and can affect contraction and local scour because of changes in flow direction.A braided stream can shift under a bridge and have two channels come together at a pier or abutment, increasing scour. Descriptions of stream morphology are given in HDS 6 and HEC-20.
Factors that affect lateral shifting of a stream and the stability of a bridge are the geomorphology of the stream, location of the crossing on the stream, flood characteristics, and the characteristics of the bed and bank material. It is difficult to anticipate when a change in planform may occur. It may be gradual or the result of a single major flood event.Also, the direction and magnitude of the movement of the stream are not easily predicted.While it is difficult to evaluate the vulnerability of a bridge due to changes in planform, it is important to incorporate potential planform changes into the design of new bridges and design of countermeasures for existing bridges. These factors are discussed and analysis techniques are presented in HEC-20.
Countermeasures for lateral shifting and instability of the stream may include changes in the bridge design, construction of river control works, protection of abutments with riprap, or careful monitoring of the river in a bridge inspection program. Serious consideration should be given to embedding foundations located on floodplains to the same elevations as those in the main channel. Control of lateral shifting requires river training works or bank stabilizing by riprap, or other revetment material. The design of these works is beyond the scope of this manual. Design methods are given by FHWA in HEC-23, HDS 6, HEC-11, and similar publications. The USACE and AASHTO provide additional guidance.
3. Prioritization Strategy and Category Definition
An effective response program for scour critical bridges and those with unknown foundations requires a systematic method of prioritizing the bridges. This chapter presents the prioritization strategy for the 198 scour critical bridges and 536 unknown foundation bridges in Idaho. The method used to prioritize these bridges is based on lifetime risk, defined as the lifetime cost of failure multiplied by the lifetime probability of failure (Equation 3.1) and for scour critical bridges, the estimated annual probability of failure.
Risk = TotalCostFailure × PFailure (3.1)
The probability of failure and failure cost values are based on an expanded HYRISK methodology (Pearson et al. 2000). The methodology was modified and expanded to incorporate information developed by ITD as part of the scour assessment program along with information from the NBI database. The assumptions and methods used to develop the cost and probability of failure over the lifetime of a bridge are discussed in the remainder of this chapter.
The lifetime risk is the expected cost of bridge failure due to scour, delivered by combining the cost of failure with the probability of failure. A bridge may have a very high cost of failure (a long bridge with a very high average daily traffic (ADT)) but a very low probability of failure. The lifetime risk for this bridge may be quite low. Conversely, a bridge with a lower cost of failure but with a high probability of failure may have a higher lifetime risk.
3.2 Total Cost of Bridge Failure
The total cost of bridge failure due to scour is the sum of the cost of reconstruction, time and mileage costs due to the detour around the failed bridge, and the assumed cost of any fatalities (Equation 3.2).
TotalCost = ReconstructionCost + DetourTimeCost + DetourMileageCost + FatalityCost (3.2)
Each of the four costs is estimated based on information in the NBI, information from ITD, and judgment. The reconstruction cost equals the total deck area multiplied by the cost per unit area. The total deck area is extracted from the National Bridge Inventory (NBI) for the bridge. The unit cost of the bridge replacement, $90/ft2, was determined using cost estimation documents provided to Ayres Associates by ITD personnel. The reconstruction cost is adjusted upward to account for additional costs due to unexpected, early replacement. This cost factor varies with the ADT and was determined using information from National Highway Institute (NHI) Course 135048, "Countermeasure Design for Bridge Scour and Stream Instability." It is assumed that high-ADT bridges would be replaced on an accelerated schedule. The reconstruction cost is presented in Equation 3.3:
ReconstructionCost = CostFactor × Cost/Area × W × L × EF (3.3)
where W is the width of the bridge deck (NBI Item 52), L is the bridge length (NBI Item 49), EF is a factor equal to 1.2 (based on communication with ITD personnel) that accounts for engineering and traffic control costs. Table 3.1 gives the Cost Factor as a function of ADT.
The time cost due to the detour around the failed bridge equals the cost per hour of traffic passing through the detour multiplied by the time required to traverse the detour, multiplied by the ADT and the duration of the detour. The duration of the detour varies with ADT and was determined using materials found in NHI Course 135048. ADT and detour length are extracted from the NBI record for the bridge. It is assumed that high-ADT bridges would be replaced on an accelerated schedule. The time cost is shown in Equation 3.4:
Time Cost = Duration × Length × ADT × ((TCP × 0 × (1 – ADTT)) + (ADTT × TCT))/DS (3.4)
where Duration is the assumed time it takes to complete the reconstruction (Table 3.2), Length is the total length of the detour (NBI Item 19), TCP is the time cost per person (assumed $7.05/hour), O is the occupancy (assumed to be the default HYRISK value of 1.56), ADTT is the fraction of the ADT that are trucks (NBI Item 109), TCT is the time cost per truck (assumed $20.56/hour), and DS is the speed of the detour (assumed 64 km/hour).
The mileage cost due to the detour around the failed bridge equals the duration of the detour multiplied by the cost per kilometer driven, the ADT, and the detour length. The mileage cost of the detour is presented in Equation 3.5:
Mileage Cost = Duration × Length × ADT × Cost/Length (3.5)
where the cost per length is the HYRISK default value of $0.16/kilometer.
The assumed cost per fatality is $500,000. This value assignment is obviously subjective and could vary more considerably based on both economic and sociological factors. The number of lives lost is assumed to vary depending on the ADT and functional classification as shown in Table 3.3. High-ADT crossings, interstates and principal arterials are assumed to have more potential for fatalities.
3.3 Annual and Lifetime Probability of Failure
The annual and lifetime probability of bridge failure due to scour is determined using an extended form of the HYRISK approach to estimating the annual probability of failure. A 50-year bridge design life is assumed, based on ITD bridge design standards. This design life is adjusted to account for superstructure and substructure condition (coded in the NBI),recognizing that bridges in good condition often remain in service past their design life. The remaining service life of the bridge is then determined based on the year of last major reconstruction, coded in the NBI.
If the bridge has failed due to scour (NBI Item 113 equals 0), then the annual probability of failure is assumed equal to 1.0.
If the bridge is considered scour critical for low-flow conditions, with an observed severe scour or substructure condition, the annual probability of failure is 0.5, corresponding to a 2-year event. The bridge is considered scour critical by inspection if so indicated in the scour report text or if the Item 113 code is 1 or 2. The annual probability of failure is 0.1 if the bridge is scour critical for the 10-year event. Failure probability information is not given explicitly in the ITD scour reports and is qualitatively determined using the hydrology, hydraulic, and scour information presented in the scour reports.
The HYRISK approach is used to determine the annual probability of failure for bridges considered scour critical in the 500-year event without a determination for smaller floods, and for all unknown foundation bridges. HYRISK determines the annual probability of failure based on the frequency of roadway overtopping and scour vulnerability. Frequency of roadway overtopping is determined using the Waterway Adequacy NBI Item 71 and the bridge functional classification. Scour vulnerability is determined as a function of the reported Channel Protection(Item 60) and Substructure Condition (Item 61) NBI ratings. A reality check of the computed annual probability of failure is made by converting the annual failure probability to the expected life of the bridge, and checking it against the bridge age. If the 90th percentile expected lifetime of the bridge is less than the actual bridge age, the annual probability of failure is adjusted downward such that the 90th percentile expected bridge life equals the actual bridge age.
The resulting annual probability of failure is further modified to account for foundation and bridge structural designs that reduce the probability of bridge failure under high scour conditions(pile foundations, redundant load paths, etc.). The lifetime probability of failure is determined from the annual probability of failure and the remaining bridge service life. The total risk is equal to the lifetime probability of failure due to scour multiplied by the total cost of bridge failure(Equation 3.1). Appendix A details the ranking of the scour critical bridges, using the approach outlined above, as well as the ranking of the unknown foundation bridges. Appendix B details the full methodology and tables for this prioritization strategy.
3.4 Limitations of the HYRISK Method
For the scour critical and unknown-foundation bridges in Idaho, long detour lengths tend to dominate the total cost of failure. Bridges with long detour lengths are predominately located in rural areas with relatively low ADT. The assumed duration of the detour increases with decreased ADT. The combination of a long detour length with long detour duration tends to increase the total failure cost of these relatively small rural bridges, thus increasing their total risk under this prioritization strategy.
It appears that the HYRISK methodology predicts higher annual probabilities of bridge failure than might be expected. While this method may overestimate risk, it is internally consistent and appropriate for ranking purposes.
3.5 Application of Priority Rankings to Scour Critical Bridges
Scour critical bridges are subdivided into four categories based on lifetime risk and annual probability of failure. Each category corresponds to a recommended minimum response level described in this document.
Category A: Vital Scour Critical Bridges. Lifetime risk of failure for these 37 bridges exceeds $5,000,000. This lifetime risk cutoff value was set in consultation with the ITD Scour Committee.
Category B: Extreme Scour Critical Bridges. The lifetime risk is less than $5,000,000, but the calculated annual probability of failure equals or exceeds 10 percent. There are 12 extreme scour critical bridges in Idaho.
Category C: Severe Scour Critical Bridges. The lifetime risk is less than $5,000,000. The annual probability of failure is between 1 and 10 percent, or less than 1 percent, but the bridge is founded on spread footings. There are 109 severe scour critical bridges in Idaho.
Category D: Moderate Scour Critical Bridges. The annual probability of failure is less than 1percent and driven pile foundation. There are 37 moderate scour critical bridges in Idaho.
The recommended minimum response level for each category is presented in Chapter 4 of this manual. Appendix C presents a template for a full Plan of Action for scour critical bridges in Idaho.
The categorization was partially validated in a qualitative sense by conducting field visits to five bridge sites: State Route 75 over the Big Wood River (Category A); County Road 61over South Fork Boise River (Category B); County Road 21 over Mores Creek at Dunnigan Creek Road (Category A); and State Route 55 over South Fork Payette River (Category A).The field visit observations supported the categories that had been assigned to these bridges by the prioritization/categorization scheme.
3.6 Unknown Foundation Bridges
Unknown foundation bridges should be prioritized for further action based on lifetime risk,determined as described above. The bridge owner should make every attempt to determine the foundation type and depth. Once the foundation has been determined, a scour evaluation should be performed to determine whether the bridge is actually scour critical.Until the foundation is determined and the scour depths are known, a monitoring plan with closure protocols should be implemented.
4. Scour Critical Response Categorization
Idaho has 198 scour critical bridges, based on 2002 NBI records and available scour reports.These bridges have a significant probability of failure due to scour processes under high-flow conditions. Some action is necessary at each of these bridges to prevent scour-related bridge failure and/or to protect public safety in the event of such a failure. This section subdivides Idaho's scour critical bridges into four categories and recommends a minimum response level for each category. It should be noted that scour critical bridges represent a significant hazard to the traveling public compared to bridges that are not classified as scour critical, and that the monitoring, inspection, and countermeasure activities at these bridges should reflect that status.
The four categories of scour critical bridges are Category A: Vital Bridges, Category B: Extreme Scour Critical Bridges, Category C: Severe Scour Critical Bridges, and Category D: Moderate Scour Critical Bridges. The classification criteria and minimum recommended response level for each category are described below. Appendix C presents the ITD Plan of Action template, which provides a framework for developing response plans for individual bridges.
4.1 Category A: Vital Scour Critical Bridges
This category represents bridges whose lifetime economic risk of failure exceeds $5,000,000, as computed by the extended HYRISK methodology discussed in Chapter 3 and described in Appendix B. See Appendix A for a listing of the 37 Category A scour critical bridges in Idaho. These bridges, due to the economic risk of failure, should not be allowed to fail. Consequently, a full Plan of Action including both monitoring and countermeasures should be developed and implemented in a timely manner to prevent scour-related failure of the bridge, (see Appendix C for the Plan of Action template). In the interim period before scour countermeasures are installed, each Category A bridge should be treated as a Category B, C, or D bridge, depending on the annual probability of failure and the structural features of the bridge. Successful installation of structural or hydraulic scour countermeasures at these bridges may allow their reclassification from scour critical status to low-risk status in the NBI.
It is impracticable to develop a generic Plan of Action to fit all of the bridges in Category A,because the characteristics of these bridges vary widely. For example:
Some of the crossings are over streams with small, steep watersheds, where extreme floods are likely to be flashy, requiring a very rapid monitoring and/or closure response. By contrast, some are over the Snake River, with a watershed of over 69,000 square miles and regulated flood flows. Floods on the Snake River will rise more slowly than on the smaller, steeper streams. Much more warning will be provided, and the flood duration will be much longer due to the flow regulation and large volume of upstream storage.
4.2 Category B: Extreme Scour Critical Bridges
These bridges have a predicted annual probability failure in excess of 10 percent, estimated by the extended HYRISK method. In other words, they are considered extremely vulnerable to scour (see Appendix A for a listing of the 12 Category B bridges in Idaho). These bridges should be closed under high-flow conditions due to the substantial probability of bridge failure. A closure plan should be developed and implemented by the bridge owner detailing closure trigger events, closure methods, a recommended detour route, and contact information for the District Engineer and traffic enforcement personnel. Field crews sent to close the bridge should be provided with adequate signage and materials for closing the bridge and marking the detour route. Once a bridge is closed due to high flow, it should be inspected for stability prior to reopening the bridge to traffic.
The typical response plan for a Category B bridge will not initially call for hydraulic or structural countermeasures. At some of the bridges in this category, however, the economic cost of frequent bridge closures may lead the owner to consider hydraulic and structural countermeasures as a cost-effective alternative. An example of such a case would be where the bridge conveys a vital access route. Successful installation of structural or hydraulic scour countermeasures at these bridges may allow their reclassification from scour critical status to low-risk status in the NBI (Code 7 under Item 113). Appendix C shows the Plan of Action template.
4.3 Category C: Severe Scour Critical Bridges
Bridges in this category typically have a predicted annual probability of failure between 10 percent and 1 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. Appendix A presents a listing of the 109 Category C bridges in Idaho. 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. The cost-effectiveness of scour countermeasures at these bridges may be determined using the methods outlined in NHI course 135048 and the economic risk rating developed as part of this effort. Successful installation of structural or hydraulic scour countermeasures at these bridges may allow their reclassification from scour critical status to low-risk status in the NBI (Code 7 under Item 113). Appendix C shows the sample Plan of Action template.
4.3.1 Water Surface Elevation-Based Scour Monitoring
Direct monitoring of the streambed elevations during flood flow conditions is the most accurate method for determining whether a bridge foundation element is approaching a scour critical bed elevation. Appendix D of this manual describes fixed and mobile scour monitoring equipment in more detail. Appendix C of this manual presents a sample table of scour critical elevations useful for direct streambed monitoring and for the analysis required to develop water surface elevations for monitoring purposes.
However, if direct streambed sounding is not feasible, then a scour critical water surface elevation may be estimated and used as part of the scour monitoring plan. However, additional hydraulic, scour, and structural analysis beyond what has been performed for the scour assessment program may be necessary to adequately develop a water surface elevation-based monitoring plan for most Category C bridges. In addition, the decision to close a scour critical bridge must be made more conservatively (closure trigger set for less severe conditions) when using a water surface elevation bridge closure trigger. This added conservatism is required because monitoring scour indirectly based on the water surface elevation incorporates the assumptions and uncertainty of the scour prediction method. Consequently, bridges may be closed more frequently than necessary, resulting in economic losses and inconvenience to the traveling public.
Additional structural and geotechnical analysis should be undertaken as necessary to determine the scour critical bed elevation for each bridge foundation element. This analysis may be very simple, as with spread-footing foundations, or more complex, as with pile foundations. Hydraulic and scour analyses should then be performed to determine the 10-, 25-, 50-, 100-, and 500-year scour elevation and upstream water surface elevation at each bridge foundation element. The critical foundation element and the approximate return interval for the flood event that produces scour critical conditions at the bridge, Ti, may be estimated from this information.
The predicted water surface elevations (WSELs) and predicted post-scour ground elevations (scour GRELs) at all scour critical foundation elements for the 10-, 25-, 50-, 100-, 500-year, and scour critical elevation should be plotted against return interval, T. Using this plot, the approximate scour critical recurrence interval Ti and the standard recurrence intervals Ti-1 and Ti-2 may be determined for each bridge (see Figure 4.1 for an example plot).
For example, the maximum scour critical GREL of any foundation element for the bridge represented in Figure 4.1 is 4403.6 ft-NGVD. This GREL corresponds (approximately) to the 70-year event (Ti). The first standard recurrence interval below the scour critical recurrence interval corresponds to the 50-year event and is labeled Ti-1. The second standard recurrence interval below the scour critical recurrence interval corresponds to the 25-year event and is labeled Ti-2.
If the scour critical recurrence interval is within 5 years of the first standard recurrence interval below the scour critical recurrence interval, or if scour depths are expected to develop suddenly (flashy stream conditions), then serious consideration should be given to adjusting the trigger recurrence intervals downward. For the hypothetical bridge conditions plotted on Figure 4.1, if the scour critical recurrence interval were the 55-year event, then the first standard recurrence interval would be the 25-year event, and the second standard recurrence interval would be the 10-year event, depending on how quickly scour conditions may develop at the site.
If the return interval of the scour critical event determined by this analysis exceeds the 100year event, then the bridge should be reclassified as a Category D bridge, unless the bridge has shallow foundations subject to sudden, catastrophic failure.
Continuous bridge monitoring for Category C bridges should commence when WSEL (Ti-2) is equaled or exceeded at the upstream bridge face, or if other severe flow conditions are predicted at the bridge. Severe flow conditions triggering monitoring for Category C bridges are defined as the rainfall or stream discharge event corresponding to the Ti-2 return interval, or if debris buildup, stream migration, or other conditions at the bridge are observed and have been identified as contributing to the bridge scour status by the scour assessment.
The elevations corresponding to the monitoring WSEL (typically WSEL (Ti-2)) and the bridge closure WSEL (typically WSEL(Ti-1)) should be clearly marked at the upstream bridge abutment wingwalls or other area using reflective signage legible from the roadway or from an accessible viewing point within the right-of-way. Example sign layouts are given in Figure 4.2. If practicable, the measure down distance from the top of bridge rail to the closure WSEL should be marked with reflective signage on the bridge rail at the location where measurements would be made.
A single, mobile monitoring crew may be able to adequately monitor several Category C bridges during the initial phases of a flood event. Once the scour monitoring WSEL has been exceeded at a bridge, however, a monitoring crew should be immediately dispatched to continuously monitor scour conditions at that bridge and close the bridge to traffic if necessary.
4.3.3 Bridge Closure Criteria
The bridge should be closed if the water surface elevation exceeds the marked closure WSEL (Ti-1). The bridge should also be closed if any other evidence of bridge distress is noted. Evidence of bridge distress includes, but is not limited to:
Furthermore, if, at any time, monitoring personnel do not feel the bridge is safe or if they are uncomfortable working on the bridge due to flood conditions at the bridge, they should close the bridge to traffic and stay off of the structure until it has been inspected for stability.
The bridge monitoring team should be given sufficient information, training, and equipment to perform the scour monitoring, observe the WSEL marks, take measuredown readings to the WSEL with a weighted tape measure, and perform an emergency closure of the bridge, if necessary. See the Appendix of the field manual for a partial list of materials required for these tasks. In addition, an information card with necessary bridge data, a detour route, emergency contact information for traffic enforcement and the district engineer, and closure directions should be provided to the monitoring team for each bridge (see the Appendix of the field manual for a sample information card).
4.4 Category D: Moderate Scour Critical Bridges
These bridges have predicted 500-year scour depths that would be expected to cause bridge instability, but the predicted annual probability of failure is less than 1 percent and the bridges have structural features (driven pile foundations) that tend to reduce the probability of catastrophic failure due to scour (see Appendix A for a listing of the 37 Category D bridges in Idaho).
The typical response plan for a Category D bridge will not initially call for hydraulic or structural countermeasures. It will focus on a monitoring and closure strategy. These bridges should be observed under high-flow conditions and closed if any signs of bridge distress are observed. Furthermore, if, at any time, monitoring personnel do not feel the bridge is safe or they are uncomfortable working on the bridge due to flood flow conditions, they should close the bridge to traffic and stay off of the structure until it has been inspected for stability.
The monitoring team should be provided with sufficient training to monitor the bridge, equipment and signage to effect an emergency bridge closure, detour route information, and contact information for the bridge engineer and traffic enforcement personnel (see Appendix C for the Plan of Action template).
4.5 Scour Critical Bridge Inspection Frequency and Procedures
The inspection procedure and frequency for every scour critical bridge should be assessed for adequacy given the increased risk of bridge failure due to scour processes. All scour critical bridges should be inspected on the regular biennial (24-month) schedule and after severe flood events.
Post-flood scour inspection should be performed after any event exceeding the 2-year flood for Category A and Category B scour critical bridges. Post-flood scour inspection should be performed for Category C scour critical bridges after any event exceeding the Ti-2 event, once a predicted scour depth/WSEL/frequency curve has been developed for the bridge. In the interim period, Category C bridges should be inspected after any flood exceeding the 10-year discharge and otherwise treated as Category B bridges. Post-flood scour inspection should be performed for Category D bridges for any flood exceeding the 50-year discharge.
The regular inspection frequency, normally biennial, may also be increased for scour critical bridges depending on the site and crossing characteristics. For example, if a bridge is scour critical due to channel lateral migration or ongoing channel degradation, more frequent regular inspections may be necessary to adequately monitor the scour conditions and channel bank positions at the bridge. Alternatively, a fixed monitoring program may substitute for an increased inspection frequency. The parties preparing the response to scour critical conditions at a given bridge should recommend an adequate inspection frequency or proposed fixed scour monitoring program. See Appendix E for a review of the HEC-18 recommended bridge scour inspection procedures. Appendix D presents an overview of scour monitoring systems and methods.
ITD staff should implement a scour coordination system to enable bridge owners, bridge inspectors, maintenance personnel, and the scour committee to adequately monitor and react appropriately to bridge scour issues as they develop. Several states have implemented "flag" systems, which formalize communication channels, notification procedures, and immediate responses when bridge inspectors or maintenance personnel identify scour conditions affecting the safety of the traveling public at a bridge.
5. Scour Critical Plans of Action by Category
There are 198 scour critical bridges in Idaho, according to the current National Bridge Inventory (NBI). The bridges have been prioritized in terms of risk and annual probability of failure according to the scheme described in Chapter 3. A database file has been developed that contains a summary of information taken from the scour reports for most of the scour critical bridges. The compact disc at the back of this manual contains the database file. This database file (created in Microsoft ACCESS) and the NBI are useful resources for future use in preparing Plans of Action.
5.1 Prioritizing Plan of Action Development
The Scour Critical Plan of Action should be immediately developed and implemented on all Category B bridges and on Category A bridges that have a predicted annual probability of failure in excess of ten percent. The remaining Category A, C, and D bridges should then be addressed in order of scour category and, within each category, economic risk. The prioritization should be revisited on a regular basis, say every five years, or as new inspection information becomes available, traffic rates change, or new scour assessments are completed. Occasionally, situations may arise to justify implementing a response out of priority. For example, if an extremely high spring runoff is expected on a certain river for a particular year, scour critical bridges over that river may be addressed out of priority in order to minimize the risk of damage during the spring runoff.
5.2 Basic Elements of the Idaho Scour Critical Bridge Plan of Action
The full Idaho Transportation Department Scour Critical Bridge Plan of Action consists of four sections. An outline of the full Plan of Action is given below.
III. Bridge Closure
IV. Scour Countermeasures: Design Plan and Implementation Schedule
The bridge plan and elevation drawings; the closure detour map and signing plan; and monitoring equipment checklist should be attached, along with the scour summary table and countermeasure design alternative information, if applicable.
Each scour critical bridge in Idaho has been categorized into four scour critical categories depending on economic risk and annual probability of failure, determined using the extended HYRISK method described in the report narrative and in Appendix B. The minimum response level for each category of scour critical bridge will determine which elements of the full Plan of Action apply to a given bridge. The individual implementation of the Plan of Action for a given bridge will vary from this minimum response level depending on its current scour categorization, current economic risk of failure, and other factors not directly examined as part of this document, such as whether the bridge is part of an emergency evacuation route.
The minimum Plan of Action for each Category A bridge contains Sections I, II, III, and IV of the ITD Scour Critical Bridge Plan of Action.
The minimum Plan of Action for each Category B bridge consists of Section I and Section III of the ITD Scour Critical Bridge Plan of Action.
The minimum Plan of Action for each Category C bridge contains Sections I, II (A) and II (B), and III of the ITD Scour Critical Bridge Plan of Action.
The minimum Plan of Action for each Category D bridge contains Sections I, II (A), II (B.1.a), and III of the ITD Scour Critical Bridge Plan of Action.
Please note that the response levels for each scour category represent minimum responses and that additional steps should be taken as necessary and as resources allow to further safeguard the traveling public and Idaho’s transportation infrastructure. The full Plan of Action Template is presented in Appendix C. The scour critical response for each scour critical bridge in Idaho should be implemented by the bridge owner in a timely manner, in coordination with ITD personnel.
The remaining sections of this chapter provide guidance on the implementation of a Plan of Action for Idaho bridges, following the sequence shown in the Plan of Action Template in Appendix C.
5.3 General Bridge Information
The first section on the cover page includes the bridge number, its district and county, the route it carries and the stream it crosses. It also provides a brief description of the location (such as distance and direction from the nearest town) along with the NBIS code under item 113 and the foundation status.
5.4 Summary of Recommended Actions
This section is also on the cover page and provides a complete at-a-glance summary of the Plan of Action for a given bridge. Without looking past the cover, bridge managers can determine:
More detailed descriptions of the recommendations are provided within the body of the Plan of Action.
5.5 Authority and Acknowledgment
As a practical matter, each Plan of Action will be prepared by an individual or a small team of engineers. It should incorporate, however, the contributions and review of an interdisciplinary team. Engineers with expertise in inspection, maintenance, structural engineering, hydraulics, geotechnical, and traffic engineering all have a role in creating an effective response to scour conditions at the bridge. The individual responsible for preparing the Plan of Action is most likely to be a hydraulic engineer, but could come from any of these disciplines, provided he or she is conversant in scour issues and that input is sought and obtained from the others. That individual should sign in the "Prepared by" block on the cover page. The approval signature should come from the District Engineer or higher-level official. The District Engineer is named as responsible for certain key decisions in the Plan of Action, and therefore must be familiar with the Plans of Action for the bridges in the district.
The cover page of the Plan of Action provides convenient access to the essence of the planned response. It is intended that the information on the cover alone will suffice for many bridge management activities, such as database development, scheduling inspections, allocating emergency response resources during widespread flooding, etc.
After the cover page, the Plan of Action contains a brief narrative report that documents the recommendations. The narrative should comprise about 2 to 3 pages of text. It provides the reader with an explanation of the logic behind the recommendations.
This text is expected to be very similar for most or all bridges. It simply explains the needs for and benefits of a Plan of Action.
5.6.2 Bridge History and Description
This section tells when the bridge was constructed, when any rehabilitations occurred, and the scour history of the bridge. It also describes the number of spans and types of substructure foundations. All of this information is readily obtained from the NBI, from archived bridge inspection reports, and from scour reports for the bridge.
5.6.3 Summary of Scour Assessment
The Plan of Action is typically implemented only for bridges rated scour critical. It is assumed, therefore, that a scour assessment of some type has been prepared for the bridge of interest. Typically, the scour assessment will estimate the scour depth at some or all of the substructure elements for floods of certain recurrence intervals. This section of the Plan of Action provides a summary of that information. Of particular interest are the predicted scour depths and post-scour elevations at any abutment or pier for which a calculation has been made. It will report the amount of each type of scour: long-term degradation or aggradation, contraction or other general scour, and local scour. This information is useful in determining what types of countermeasures would be most effective and which substructure elements need the closest monitoring during high flows.
5.6.4 Inspection and Monitoring Requirements
The text in this section of the narrative provides support for and detailed explanation of the recommended inspection and monitoring program. Section 5.7 describes the content of a good inspection and monitoring program.
5.6.5 Countermeasure Selection
The narrative summarizes the process of eliminating countermeasure types from consideration and developing designs for the types that were considered. Section 5.9 describes the countermeasure selection process in detail.
Any document that was significant in implementing the Plan of Action at a bridge, along with other useful resources, should be cited here. Examples include:
5.7 Recommendations for Scour Inspection and Monitoring
Immediately after the narrative sections, the template provides a fill-in table that allows for thorough description of the inspection and monitoring plan. This plan should be tailored to each individual bridge depending on its unique scour critical situation.
All scour critical bridges should be inspected on the regular biennial (24-month) schedule and after severe flood events.
Post-flood scour inspection should be performed after any event exceeding the 2-year flood for Category A and Category B bridges. Post-flood scour inspection should be performed for Category C bridges after any event exceeding the Ti-2 event, once a predicted scour depth/WSEL/frequency curve has been developed for the bridge. In the interim period, Category C bridges should be inspected after any event exceeding the 10-year flood event and otherwise treated as a Category B bridge. Post-flood scour inspection should be performed for Category D bridges for any event exceeding the 50-year flood event.
The regular, 24-month inspection frequency may also be increased for scour critical bridges depending on the site and crossing characteristics. For example, if a bridge is scour critical due to channel lateral migration or ongoing channel degradation, more frequent regular inspections may be necessary to adequately monitor the scour conditions at the bridge. Alternatively, a fixed monitoring program may substitute for an increased inspection frequency. The parties preparing the response to scour critical conditions at a given bridge should recommend an adequate inspection frequency or proposed fixed scour monitoring program. See Appendix E for a review of the HEC-18 recommended bridge scour inspection procedures. Appendix D includes an overview of scour monitoring systems and methods.
Both routine biennial inspections and post-flood inspections should include stream cross sections along the bridge faces and local scour depth measurements at the ends of piers, and at the four corners of each abutment (two wingwall ends plus the two inside corners). Figure 5.1 is a plan-view schematic showing where measurements are needed. These measurements will be taken using portable monitoring instruments such as probes, portable sonar, etc. The cross sections at each bridge face and the local scour depth measurements should later be plotted to scale on a common vertical datum on grid paper and compared to cross section plots from previous inspections. Note the vertical datum tie on all plots and measuredowns. Any shifting of the thalweg (channel minimum invert elevation) or any vertical trends noted from the plots should be evaluated for potential impacts to bridge stability and the bridge scour rating.
In addition to the elevation measurements, the inspections should look for visible scour problems such as undercut banks, undermined abutments, dislodged protection riprap, exposed footings at piers or abutments, road washouts, etc. Look for signs of stream channel migration, such as freshly scoured banks on one side of the channel and new gravel bars on the other side. Long-term degradation can progress in the form of upstream-migrating headcuts. The inspector should examine the downstream reach for any signs of headcuts. Debris can aggravate scour problems. The inspector must note any significant debris buildup on the bridge and maintenance forces should be arranged to clear the buildup. The inspector should also look upstream to identify any potential debris sources, such as fallen or falling trees in or near the channel.
The inspector should examine previous inspection reports, and if they noted any visible scour problem, the current inspection should look for improvement or worsening of that problem. It is helpful to include photographs of visible problems in the inspection report.
ITD staff should implement a scour coordination system to enable bridge owners, bridge inspectors, maintenance personnel, and the scour committee to adequately monitor and to react appropriately to bridge scour issues as they develop. Several states have implemented "flag" systems, which formalize communication channels, notification procedures, and immediate responses when bridge inspectors identify scour conditions or structural problems affecting the safety of the traveling public at a bridge.
5.7.2 High-Flow Monitoring
Monitoring during high flows is a critical activity for bridges that could be destroyed or substantially damaged by a single flood. 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. The Plan of Action includes a table (Section II under Recommendations for Scour Inspection and Monitoring) that indicates:
All scour critical bridges should be evaluated for signs of bridge distress prior to and during WSEL or bed elevation monitoring activities. Such signs would include:
If any of these or other qualitative signs of structural distress are apparent at any time, the crew should implement an emergency bridge closure, call for a formal or full bridge closure, and should avoid getting on the bridge if at all possible.
The type of scour monitoring will depend on the availability of resources. If necessary or desirable, fixed bed-level monitoring devices may be installed at any scour critical bridge.
For Category A bridges, high-flow monitoring may not be necessary once scour countermeasures are installed. Prior to scour countermeasure installation, Category A bridges should be treated as Category B, C, or D bridges, depending on the predicted annual probability of failure. Category B bridges are closed during flood conditions and consequently require no high-flow monitoring. Teams monitoring Category C bridges ideally will be equipped with portable bed elevation monitoring equipment suitable for flood flow conditions at the bridge. However, when direct bed elevation monitoring is not feasible, Category C bridges should have two water surface elevations marked at the upstream face of the bridge: Ti-2, the monitoring trigger WSEL, and Ti-1, the scour closure trigger WSEL. The procedure to develop and mark these WSELs is outlined in Section 4. Category D bridges should be qualitatively monitored for signs of bridge distress as described above.
For bridges where direct bed monitoring is to be used, the type of bed monitoring instrument will depend on the site conditions. In many cases a simple probe will be most effective. Some conditions, such as water flowing fast and deep, may make the use of a probe impractical. See Appendix D and HEC-23 for guidance on portable monitoring instrument selection. In the case of a bridge where some fixed-instrument monitoring has already been installed, it can provide information to supplement the portable monitoring. In rare cases, all of the high-flow monitoring can be accomplished by previously installed fixed instruments, as long as the instruments cover all of the threatened substructure elements.
High-flow monitoring ideally should be triggered at a particular bridge by the water surface elevation reaching some pre-determined elevation associated with a certain recurrence interval, such as the 25-year flood. The Idaho scour reports typically do not report a computed water surface elevation for floods smaller than the 500-year event. The original hydraulic design documentation for the bridge, or a FEMA Flood Insurance Study for the stream may provide water surface elevations for more moderate floods. If a computed water surface elevation for a moderate flood cannot be determined, then other types of events can be used as the trigger. Category C bridges and some Category A bridges will have a monitoring WSEL set as part of the Plan of Action.
For bridges where scour monitoring and scour closure trigger WSELs have been established, the bridge monitoring crew may initiate continuous WSEL monitoring when the scour monitoring WSEL has been exceeded. The bridge should be closed immediately if the WSEL exceeds the marked scour closure WSEL. A simple weighted tape may be employed to check measuredowns to the WSEL from a fixed bridge element, typically the bridge guardrail, at scour critical bridge elements.
For Category D bridges, practical definitions of high-flow events as triggers for high-flow monitoring are listed below in descending order of preference. The operative definition should be chosen based on availability of information.
Once the predetermined monitoring trigger event has occurred for Category A and C bridges, assuming that immediate closure is not warranted and the bridge is safe to occupy, the monitoring crew will begin the measurement process. They will periodically measure the bed elevations or water surface elevations at critical substructure elements and compare the measured WSELs or bed elevations to the scour closure elevations for those elements. The field crew should be provided with the scour-closure WSEL or bed elevations for all bridge substructural elements and which elements are most critical. The crew may also judge that another pier or abutment should be monitored closely if high velocity flow is impinging on the element at a high angle of attack.
If scour is causing the bed elevation to fall rapidly toward the scour critical elevation, but significant embedment remains, the crew should call for emergency protection. If the critical elevation has been reached, or will be reached soon, the crew should perform an emergency closure to traffic and contact the District Engineer, ITD maintenance personnel, and traffic control authorities to implement a full bridge closure. If scour countermeasures were previously installed and are found to be failing or non-functional, the crew should call for abridge closure.
5.7.3 Fixed-Instrumentation Monitoring
The ITD has not historically made extensive use of fixed scour monitors. They may be very valuable, however, for some situations. Under the right circumstances, a fixed monitor can be considered a scour countermeasure. Appendix D and HEC-23 provide extensive guidance on the application of fixed scour monitoring. If they are to be incorporated in a bridge’s Plan of Action, the template provides a fill-in table to describe the instrument to be used, where to install it, and the sampling time interval. The Plan of Action should also indicate how often to download the data and whether to use telemetry to allow remote data downloads.
5.7.4 Tables of Scour Critical Bed and Water Surface Elevations
For Category A and C bridges, the scour critical elevations must be known, or reasonably estimated, if the inspection and monitoring plan is to be effective. Ideally, engineers with structural and foundations expertise should determine the scour critical elevation for each substructure element analytically. For certain foundation types, a scour critical elevation can be reasonably estimated without extensive analysis. A spread-footing foundation, for instance, can be assumed scour critical at the base of the footing. A buried-footing foundation with piles extending below it may be also be considered scour critical at the base of the footing, as further scour would expose the piles, which may not be designed to tolerate any exposure to the flow. The scour critical elevation of driven-pile or drilled-shaft foundation cannot be accurately estimated without analysis, since these foundation types depend upon some amount of friction and lateral support from the soil to function properly. For in-field analysis or emergency purposes, if less than 50 percent of the original pile design embedment remains, then the bridge element may be assumed scour critical.
The inspection and monitoring crews need to know the critical bed elevation or WSEL triggering closure. They also need a convenient way to measure the bed elevation or WSEL in relation to the closure trigger elevation. One example of a measurement reference is a known-elevation benchmark set near the bridge structure. The crew would need a level and tripod to use this method. An independent benchmark allows the monitoring crew to precisely check for vertical displacement of the bridge deck. A method requiring fewer instruments and quicker setup is to establish a known elevation at the top of the rail or the top of curb above every substructure element to be monitored. As an alternative, a tight,level string line of known elevation could be stretched across the waterway immediately adjacent to the bridge deck. This taut wire placement could be facilitated by setting posts off of the corners of the bridge with hooks or eye bolts at equal elevation. The string or wire would be stretched between the posts through the hooks by the monitoring crew when they arrived at the site.
The Plan of Action includes a table for filling in the monitoring and closure trigger bed elevations or WSELs for each monitored abutment and pier. The table also indicates what reference line is to be used and what the elevation of the reference line is above each element.
5.8 Bridge Closure and Emergency Protection Installation
The District Engineer, in urgent consultation with the Hydraulics Engineer, the Bridge Engineer, the Bridge Inspection Engineer and the Maintenance Supervisor, should approve the placement of emergency protection. Emergency scour protection should only be considered for bridges with a direct streambed scour monitoring approach, due to the inherent uncertainties of a WSEL-based scour monitoring strategy. Maintenance crews will install the protection. They should have a ready stockpile of adequately sized riprap available for such emergencies. It should be noted that in flashy streams, emergency protection approaches are likely to be futile, because of the short duration and high intensity of scour-causing conditions. Furthermore, attempting to implement emergency countermeasures may place workers and equipment at significant risk during extreme flooding.
The District Engineer must also approve a full bridge closure and initiate the bridge closure plan, which should be part of the Plan of Action. Bridge monitoring crews should be authorized to perform an emergency bridge closure if signs of bridge distress are noted, or if measured WSELs or scour depths exceed the relevant closure criteria. Most important to the bridge closure plan is to identify the key contacts. These are the Bridge Inspection Engineer, the Bridge Engineer, the Traffic Engineer, the State Highway Patrol and local law enforcement agency, and the ITD or local authority public affairs office. The full closure of the bridge, including additional physical barriers and detour signs, will be placed by maintenance or traffic crews according to the maps and plans attached to the Plan of Action.
5.9 Countermeasure Recommendations and Schedule
Category A scour critical bridges include countermeasure recommendations as part of the minimum response to scour conditions due to the high economic risk of scour failure of these bridges. Category B, C, and D bridges should be scheduled for countermeasure design and installation by category and, within each category, in decreasing economic risk order, as resource availability allows. Appendix A presents the Scour Categorization and Ranking for scour critical and unknown foundation bridges in Idaho.
Inspection and monitoring can safeguard public safety and alert bridge managers to developing problems. However, inspection and monitoring cannot prevent a bridge from failing. The prioritization results in Appendix A show that significant costs are associated with bridge failure. If there is a reasonably high probability of failure at a particular bridge, and that failure is to be avoided, the inspection and monitoring plan must be connected to a plan and schedule for implementing scour countermeasures.
The Plan of Action template includes a check-box table indicating the types of countermeasures considered and the approximate cost of each, along with the preference ranking of each type. The selection of countermeasures should consider the scour critical conditions of the bridge, such as:
In addition to the scour critical conditions, site-specific constructability issues may govern the type of countermeasure selected. Examples of such constraints may be:
With scour critical conditions and constructability issues known, the engineer can develop conceptual countermeasure designs that are suitable and effective. The countermeasure matrix in Appendix C and HEC-23 should be consulted as design resources. The conceptual design of each countermeasure type should go far enough to enable a reasonable estimate of the cost. For instance, the conceptual design of riprap-based countermeasures should include the median riprap size and the thickness of the riprap mat to be placed, along with appropriate toe-down depths. For each countermeasure type considered and found suitable, an approximate cost estimate should be developed.
Some of Idaho’s scour critical bridges are small single spans where contraction scour alone, without considering abutment scour, is expected to threaten the abutments. This situation typically calls for a full-channel armor across the bridge opening, using riprap, articulating concrete block, or grout-filled mats. Alternatively, the contraction scour could be addressed by lengthening the bridge (widening the opening) or providing a relief culvert. If the contraction scour is eliminated, then only the abutments of the bridge require revetment.
Other Idaho bridges are scour critical because of predicted pier scour at a few of the piers. In this case, protection of the individual piers with well-designed pier armoring is indicated. If some scour can be tolerated, then fixed-instrument monitoring of the bed level at the critical piers may be more cost effective.
Another possible case is where the bridge is barely rated scour critical for a 500-year flood and is rated stable for moderate floods. This condition describes many Category D bridges. Under this scenario, if the foundations have significant embedment, then a countermeasure may not be necessary, and diligent inspection on the biennial schedule and after floods may be adequate.
The Plan of Action should indicate the schedule for countermeasure implementation. This schedule should be developed in consultation with agency management for realistic programming into capital or maintenance budgets.
The Plan of Action should include several attachments. At a minimum, the bridge plan and elevation drawings and the detour map and signing layout should be attached. The scour summary table and backup information documenting the design of countermeasure alternatives are also valuable attachments.
Barksdale, G. 1994, "Bridge Scour During the 1993 Floods in the Upper Mississippi River Basin," proceeding of the 1994 ASCE Hydraulic Engineering Conference.
Bath, W.R., 1999, "Remote Methods of Underwater Inspection of Bridge Structures," FHWA Report RD-99-100, Turner-Fairbank Highway Research Center, McLean, VA.
Bertoldi, D.A., J.S. Jones, S.M. Stein, R.T. Kilgore, and A.T. Atayee, 1996, "An Experimental Study of Scour Protection Alternatives at Bridge Piers," FHWA-RD-95-187, Office of Engineering and Highway Operations R&D, McLean, VA.
Bradley, J.N., 1978, "Hydraulics of Bridge Waterways," Hydraulic Design Series No. 1, U.S. Department of Transportation, FHWA, Washington, D.C.
Brice, J.C. and J.C. Blodgett, 1978, "Countermeasure for Hydraulic Problems at Bridges, Volumes 1 and 2," FHWA-RD-78-162 and 163, USGS, Menlo Park, CA.
Brown, S.A., 1985, "Streambank Stabilization Measures for Highway Stream Crossings, Executive Summary," Federal Highway Administration, Report No. FHWA-RD-80-160, U.S. Department of Transportation, Washington, D.C.
Brown, S.A., 1985, "Design of Spur-type Streambank Stabilization Structures," FHWA/RD-84/101, FHWA, Washington, D.C.
Brown, S.A., 1985, "Streambank Stabilization Measures for Highway Engineers," FHWA/RD-84/100, FHWA, Washington, D.C.
Brown, S.A. and E.S. Clyde, 1989, "Design of Riprap Revetment," Hydraulic Engineering Circular No. 11, FHWA-IP-89-016, Washington, D.C.
Chang, F. and M. Karim, 1972, "An Experimental Study of Reducing Scour Around Bridge Piers Using Piles," South Dakota Department of Highways, Report.
Clopper, P.E., 1989, "Hydraulic Stability of Articulated Concrete Block Revetment Systems During Overtopping Flow," FHWA-RD-89-199, Office of Engineering and Highway Operations R&D, McLean, VA.
Clopper, P.E., 1992, "Protecting Embankment Dams with Concrete Block Systems," Hydro Review, Vol. X, No. 2, April.
Clopper, P.E. and Y. Chen, 1988, "Minimizing Embankment Damage During Overtopping Flow," FHWA-RD-88-181, Office of Engineering and Highway Operations R&D, McLean, VA.
Federal Highway Administration, 1989, "Design of Riprap Revetment," Hydraulic Engineering Circular No. 11 Report No. FHWA-IP-89-016, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C.
Federal Highway Administration, 1995, "Recording and Coding Guide for the Structure Inventory and Appraisal of the Nation's Bridges, Report No. FHWA-PD-96-001, U.S. Department of Transportation, Washington, D.C. Also see errata sheet, October 2001.
Federal Highway Administration, 1998, "Scour Monitoring and Instrumentation," Demonstration Project 97 Participants Workbook, FHWA-SA-96-036, Office of Technology Applications, Washington, D.C.
Fotherby, L.M. and J.F. Ruff, 1995, "Bridge Scour Protection System Using Toskanes -Phase 1," Pennsylvania Department of Transportation, Report 91-02.
Karim, M., 1975, "Concrete Fabric Mat, " Highway Focus, Vol. 7.
Keown, M.P., 1983, "Streambank Protection Guidelines for Landowners and Local Governments," U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.
Lagasse, P.F., P.L. Thompson, and S.A. Sabol, 1995, "Guarding Against Scour," Civil Engineering, American Society of Civil Engineers, June, pp. 56-59.
Lagasse, P.F., E.V. Richardson, J.D. Schall, and G.R. Price, 1997, "Instrumentation for Measuring Scour at Bridge Piers and Abutments," NCHRP Report 396, Transportation Research Board, National Research Council, National Academy Press, Washington, D.C.
Lagasse, P.F., J.D. Schall, and E.V. Richardson, 2001, "Stream Stability at Highway Structures," Hydraulic Engineering Circular 20, Third Edition, FHWA NHI 01-002, Federal Highway Administration, U.S. Department of Transportation, Washington, D.C.
Lagasse, P.F., L.W. Zevenbergen, J.D. Schall, and P.E. Clopper, 2001, "Bridge Scour and Stream Instability Countermeasures – Experience, Selection, and Design Guidelines, Hydraulic Engineering Circular No. 23, Second Edition, FHWA NHI 01-003, Federal Highway Administration, Washington, D.C.
McCorquodale, J.A., 1991, "Guide for the Design and Placement of Cable Concrete Mats," Report Prepared for the Manufacturers of Cable Concrete.
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