Bridge scour

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  2. Bridge maintenance
  3. Hydrology
  4. Hydraulic engineering
  5. Environmental engineering
  6. Physical geography
  7. Fluid dynamics
  8. Fluid mechanics
  9. Erosion


File:Abutment scour2.jpg
Abutment scour
Bridge scour is the removal of sediment such as sand and rocks from around bridge abutments or piers. Scour, caused by swiftly moving water, can scoop out scour holes, compromising the integrity of a structure.[1]

Bridge scour is one of the three main causes of bridge failure. It has been estimated that 60% of all bridge failures result from scour and other hydraulic related causes.[2] It is the most common cause of highway bridge failure in the United States,[3] where 46 of 86 major bridge failures resulted from scour near piers from 1961 to 1976.[4]

Areas affected by scour

File:HomochittoRiver1974.jpg
Mississippi Highway 33 bridge over the Homochitto River failed due to flood induced erosion

Water normally flows faster around piers and abutments making them susceptible to local scour. At bridge openings, contraction scour can occur when water accelerates as it flows through an opening that is narrower than the channel upstream from the bridge. Degradation scour occurs both upstream and downstream from a bridge over large areas. Over long periods of time, this can result in lowering of the stream bed.[2]

Causes

Stream channel instability resulting in river erosion and changing angles-of-attack can contribute to bridge scour. Debris can also have a substantial impact on bridge scour in several ways. A build-up of material can reduce the size of the waterway under a bridge causing contraction scour in the channel. A build-up of debris on the abutment can increase the obstruction area and increase local scour. Debris can deflect the water flow, changing the angle of attack, increasing local scour. Debris might also shift the entire channel around the bridge causing increased water flow and scour in another location.[3]

During flooding, although the foundations of a bridge might not suffer damage, the fill behind abutments may scour. This type of damage typically occurs with single-span bridges with vertical wall abutments.

Bridge examination and scour evaluation

The examination process is normally conducted by hydrologists and hydrologic technicians, and involves a review of historical engineering information about the bridge, followed by a visual inspection. Information is recorded about the type of rock or sediment carried by the river, and the angle at which the river flows toward and away from the bridge. The area under the bridge is also inspected for holes and other evidence of scour.

Bridge examination begins by office investigation. The history of the bridge and any previous scour related problems should be notified. Once a bridge is recognized as a potential scour bridge, it will proceed to further evaluation includes field review, scour vulnerability analysis and prioritize. Bridges will also be rated in different categories and prioritize for scour risk. Future implementation of scour countermeasures includes monitoring plans, inspections after flood events, and procedures for closing bridges if necessary.

Alternatively, sensing technologies are also being put in place for scour assessment. The scour-sensing level can be classified into three levels: general bridge inspection, collecting limited data and collecting detailed data [5]. There are three different types of scour-monitoring systems: fixed, portable and geophysical positioning. Each system can help to detect scour damage in an effort to avoid bridge failure, thus increase public safety.

Introduction to scour countermeasure and prevention

Hydraulic Engineering Circular No. 23 Manual (HEC-23) contains general design guidelines as scour countermeasures that are applicable to piers and abutments. The design guideline numbering in the following table indicates the HEC-23 design guideline chapter.

Table 1.
Description of types of countermeasure in HEC-23
Description Design Guidelines
Bend way weirs/stream barbs 1
Soil cement 2
Wire enclosed riprap mattress 3
Articulated concrete block system 4
Grout filled mattresses 5
Concrete armor units 6
Grout/cement filled bags 7
Rock riprap at piers and abutments 8
Spurs 9
Guide banks 10
Check dams/drop structures 11
Revetments 12

Bend way weirs, spurs and guide banks can help to align the upstream flow while riprap, gabions, articulated concrete blocks and grout filled mattresses can mechanically stabilize the pier and abutment slopes [6]. Riprap remains the most common countermeasure used to prevent scour at bridge abutments. A number of physical additions to the abutments of bridges can help prevent scour, such as the installation of gabions and stone pitching upstream from the foundation. The addition of sheet piles or interlocking prefabricated concrete blocks can also offer protection. These countermeasures do not change the scouring flow and are temporary since the components are known to move or be washed away in a flood.[7] FHWA recommends design criteria in HEC-18 and 23, such as avoiding unfavourable flow patterns, streamlining the abutments, and designing pier foundations resistant to scour without depending upon the use of riprap or other countermeasures, as is available.

Trapezoidal-shaped channels through a bridge can significantly decrease local scour depths compared to vertical wall abutments, as they provide a smoother transition through a bridge opening. This eliminates abrupt corners that cause turbulent areas. Spur dikes, barbs, groynes, and vanes are river training structures that change stream hydraulics to mitigate undesirable erosion or deposits. They are usually used on unstable stream channels to help redirect stream flow to more desirable locations through the bridge. The insertion of piles or deeper footings is also used to help strengthen bridges.

Estimating Scour Depth

Hydraulic Engineering Circular No. 18 Manual (HEC-18) was published by the Federal Highway Administration (FHWA). This manual includes several techniques of estimating scour depth. The empirical scour equations for live bed scour, clear water scour, and local scour at piers and abutment are shown in Chapter 5-General Scour section. The total scour depth is determined by adding three scour components which includes the long-term aggradation and degradation of the river bed, general scour at the bridge and local scour at the piers or abutment [8]. However, research had shown that the standard equations in HEC-18 over-predict scour depth for a number of hydraulic and geologic conditions. Most of the HEC-18 relationships are based on laboratory flume studies conducted with sand-sized sediments increased with factors of safety that are not easily recognizable or adjustable [9]. Sand and fine gravel are the most easily eroded bed materials, but streams frequently contain much more scour resistant materials such as compact till, stiff clay, and shale. The consequences of using design methods based on a single soil type are especially significant for many major physiographic provinces with distinctly different geologic conditions and foundation materials. This can lead to overly conservative design values for scour in low risk or non-critical hydrologic conditions. Thus, equation improvements are continued to be made in an effort to minimize the underestimation and overestimation of scour.

Bridge disasters caused by scour

See also

References

  1. ↑ http://ma.water.usgs.gov/publications/ofr/scour.htm
  2. ↑ 2.0 2.1 [http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1140&context=usgsstaffpub&sei-redir=1&referer=http%3A%2F%2Fscholar.google.com%2Fscholar%3Fhl%3Den%26q%3DShirhole%2Band%2BHolt%2B1991%26as_sdt%3D0%252C22%26as_ylo%3D%26as_vis%3D0#search=%22Shirhole%20Holt%201991%22
  3. ↑ 3.0 3.1 http://www.fs.fed.us/eng/structures/98771207.pdf Bridge Scour Evaluation: Screening, Analysis, & Countermeasures, United States Department of Agriculture Forest Service Technology & Development Program
  4. ↑ "USGS OGW, BG: Using Surface Geophysics for Bridge Scour Detection". Water.usgs.gov. http://water.usgs.gov/ogw/bgas/scour/. Retrieved 2010-07-30.
  5. ↑ Ettouney, Mohammed M.; Alampalli, Sreenivas (2011). Infrastructure Health in Civil Engineering : Applications and Management. CRC Press. Retrieved April 04, 2012, from Ebook Library.
  6. ↑ Lagasse, P. F., Zevenbergen, L. W., Schall, J. D., & Clopper, P. E. U.S Department of Transportation, Federal Highway Administration. (2001). Bridge scour and stream instability countermeasures (NHI 01-003). Retrieved from website: http://isddc.dot.gov/OLPFiles/FHWA/010592.pdf
  7. ↑ "Publications - Hydraulics Engineering - FHWA". Fhwa.dot.gov. 2006-04-26. http://www.fhwa.dot.gov/engineering/hydraulics/library_listing.cfm. Retrieved 2010-07-30.
  8. ↑ Richardson, E. V., & Davis, S. R. U.S. Department of Transportation, Federal Highway Administration. (2001). Hydraulics engineering publications title: Evaluating scour at bridges, fourth edition description (NHI-01-001). Retrieved from website: http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=17&id=37 )
  9. ↑ Chase, K. J., Holnbeck, S. R., Montana., & Geological Survey (U.S.). (2004). Evaluation of pier-scour equations for coarse-bed streams. Reston, Va: U.S. Dept. of the Interior, U.S. Geological Survey.

http://ascelibrary.org/sco/resource/1/ppscfx/v15/i2/p125_s1?isAuthorized=no

Further reading

  • Boorstin, Robert O. (1987). Bridge Collapses on the Thruway, Trapping Vehicles, Volume CXXXVI, No. 47,101, The New York Times, April 6, 1987.

http://ascelibrary.org/sco/resource/1/ppscfx/v15/i2/p125_s1?isAuthorized=no

  • Huber, Frank. (1991). “Update: Bridge Scour.” Civil Engineering, ASCE, Vol. 61, No. 9, pp. 62–63, September 1991.
  • Levy, Matthys and Salvadori, Mario (1992). Why Buildings Fall Down. W.W. Norton and Company, New York, New York.
  • National Transportation Safety Board (NTSB). (1988). “Collapse of New York Thruway (1-90) Bridge over the Schoharie Creek, near Amsterdam, New York, April 5, 1987.” Highway Accident Report: NTSB/HAR-88/02, Washington, D.C.
  • Springer Netherlands. International Journal of Fracture, Volume 51, Number 1 September 1991. "The collapse of the Schoharie Creek Bridge: a case study in concrete fracture mechanics"
  • Palmer, R., and Turkiyyah, G. (1999). “CAESAR: An Expert System for Evaluation of Scour and Stream Stability.” National Cooperative Highway Research Program (NCHRP) Report 426, Washington D. C.
  • Shepherd, Robin and Frost, J. David (1995). Failures in Civil Engineering: Structural, Foundation and Geoenvironmental Case Studies. American Society of Civil Engineers, New York, New York.
  • Thornton, C. H., Tomasetti, R. L., and Joseph, L. M. (1988). “Lessons From Schoharie Creek,” Civil Engineering, Vol. 58, No.5, pp. 46–49, May 1988.
  • Thornton-Tomasetti, P. C. (1987) “Overview Report Investigation of the New York State Thruway Schoharie Creek Bridge Collapse.” Prepared for: New York State Disaster Preparedness Commission, December 1987.
  • Wiss, Janney, Elstner Associates, Inc., and Mueser Rutledge Consulting Engineers (1987) “Collapse of Thruway Bridge at Schoharie Creek,” Final Report, Prepared for: New York State Thruway Authority, November 1987.
  • Richardson, E.V., and S.R. Davis. 1995. "Evaluating Scour at Bridges, Third Edition.", US Department of Transportation, Publication No FHWA-IP-90-017.

External links


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