Bridge scour
Encyclopedia
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
such as sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
and rocks from around bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
abutments or piers. Scour, caused by swiftly moving water, can scoop out scour holes, compromising the integrity of a structure.
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. It is the most common cause of highway
Highway
A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...
bridge failure in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, where 46 of 86 major bridge failures resulted from scour near piers from 1961 to 1976.
Areas affected by scour
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.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
The examination process is normally conducted by hydrologists and hydrologicHydrology
Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability...
technicians, and involves a review of historical engineering
Engineering
Engineering is the discipline, art, skill and profession of acquiring and applying scientific, mathematical, economic, social, and practical knowledge, in order to design and build structures, machines, devices, systems, materials and processes that safely realize improvements to the lives of...
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.
Prevention
RiprapRiprap
Riprap — also known as rip rap, rubble, shot rock or rock armour or "Rip-rap" — is rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures against scour, water or ice erosion.It is made from a variety of rock types, commonly granite or...
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 gabion
Gabion
Gabions are cages, cylinders, or boxes filled with soil or sand that are used in civil engineering, road building, and military applications. For erosion control caged riprap is used. For dams or foundation construction, cylindrical metal structures are used...
s 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. 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.
Mathematical equations
It is possible to determine the effects of scour using mathematical formulas.Lacey's formula
According to Lacey's formula, the width of a natural channel at bank-full flow is proportional to the root of the discharge.To determine scouring depth:
where,
- d = normal depth of scouring below HFL
- Q = discharge (in m3/s)
- f = Lacey's silt factor, which is a function of bed material
Other equations
Various formulas exist related to other kinds of scour, including:- live bed contraction scour
- clear water contraction scour equation
- local scour
- unconstricted waterway assessment procedure
- procedural adjustments for constricted waterways
(See external link at the bottom of this article.)
See also
- Bridge maintenanceBridge maintenanceMaintenance of today's bridge infrastructure presents many challenges. Transportation engineering and maintenance personnel must maintain around the clock service to millions of people each year while maintaining millions of cubic meters of concrete distributed throughout their facilities. This...
- Baer's lawBaer's lawIn geology, Baer's law, named after Karl Ernst von Baer, says that, because of the rotation of the earth, in the Northern Hemisphere, erosion occurs mostly on the right banks of rivers and in the Southern Hemisphere on the left banks...
- Fluid dynamicsFluid dynamicsIn physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
- List of bridge disasters
- Schoharie Creek Bridge collapseSchoharie Creek Bridge collapseThe Schoharie Creek Bridge was a New York State Thruway bridge over the Schoharie Creek near Fort Hunter, in New York State. On April 5, 1987 it collapsed due to erosion at the foundations after a record rainfall...
- Glanrhyd Bridge collapse
- Hintze Ribeiro disasterHintze Ribeiro disasterOn the night of March 4, 2001, the Hintze Ribeiro disaster occurred when the Hintze Ribeiro Bridge, made of steel and concrete, collapsed in Entre-os-Rios, Castelo de Paiva, Portugal, killing 59 people, including those in a bus and three cars that were attempting to get to the other side of the...
- Custer Creek train wreckCuster Creek train wreckThe Custer Creek train wreck is the worst rail disaster in Montana history. It occurred on June 19, 1938 when a bridge, its foundations washed away by a flash flood, collapsed beneath Milwaukee Road's Olympian as it crossed Custer Creek, near Saugus, Montana, south-west of Terry, killing at least...
- Homochitto River
- Breakwater (structure)Breakwater (structure)Breakwaters are structures constructed on coasts as part of coastal defence or to protect an anchorage from the effects of weather and longshore drift.-Purposes of breakwaters:...
- MIKE 21CMIKE 21CMIKE 21C is a computer program that simulates the development in the river bed and channel plan form in two dimensions. MIKE 21C was developed by DHI Water.Environment.Health...
- Armor (hydrology)Armor (hydrology)Armor, in hydrology and geography is the association of surface pebbles, rocks or boulders with stream beds or beaches. Most commonly hydrological armor occurs naturally; however, a man-made form is usually called riprap, when shorelines or stream banks are fortified for erosion protection with...
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.