Scouring in bridge
Scouring is defined as the lowering or loss of top bed soil (erosion) around the bridge piers, abutments, spurs etc. due to the water flow currents. Scouring is always a potential threat to bridge stability eventually it can lead to foundation failure.
As per the research data 60% of bridge failure in the US are because of scouring. The bridge scouring is a dynamic act which depends on various factors like flow depth, flow angles, water currents around the piers or abutments, the shape of pier and abutments, sediments properties etc. Scouring in the bridge is very difficult to monitor and also the foundation failure due to scour is sudden without any warnings.
Scour Mechanism in Bridge substructures
When a bridge is constructed across a river a unique flow pattern will arise across the pier/abutments, i.e. the flow velocities got change due to the presence of piers or abutments which reduces the flow area and it make changes in flow patterns. The figure below shows the working mechanism of scour. When the flow layer in the channel got separated by the pier it converges to the downstream of the pier. A portion of separated flow layer later rolls up to form vortex during the flow moves down. The vortex which formed has a characteristic shape of a horseshoe and thus it is named as horseshoe vortex.
The movement of horseshoe vortex to the downward side of the pier along with downflow results in increased shear stress and when it reaches the channel bed, sediments began to transfer from around the pier base. Later it turns to the formation of a hole around the pier/abutment and in this way scouring takes place. The strength of horseshoe vortex got reduced when the depth of the scour hole increases.
The separation of flow at the bridge pier also generates wake vortex too, which is the rotation of vortex along the vertical axis. The wake vortex takes up the sediments like a vacuum pump and carries away from the pier surroundings. The wake vortex effects got decrease when the distance increases and so the sediments can usually found on the downstream of the pier. The river channels with high flow velocities can seriously damage the foundation (by scouring) if the foundation is not deep enough. In other words, scouring will expose the foundation of the bridge.
Types of Bridge Scour
The bridge scour generally classified into four different categories;
General scour is the normal decrease in the elevation of the river bed. In most cases, the general scour is happen for only a small time period.
Long term scour
This scour refers to the change in bed elevation over time, over the entire reach of the water body. This scour type also involves deposition sediments carried from the upstream channel and carry away or lowering the bed level.
This scour occurs when the channel’s cross-section got reduced due to the presence of natural or manmade structure. The reduction in cross-area initiates high velocity which creates unusual flow patterns which removes bed sediments.
Local scour can be defined as the removal of bed sediments or degradation of river banks which is localized to a specific area. As mentioned earlier the Localized scour is the most common scour happening to bridges. The localized scour is happening because of the changes in flow patterns due to the presence of a structure (pier/abutments). Local scour can be either clear water scour and live bed scour. Clearwater scour means there is no deposition of sediments to the scouring area from the upstream side of the river. On the other hand live in bed scour, the sediments are continuously supplied from the upstream to the scour zone.
Scour Depth calculation
There are different types of formulas with respect to different parameters are used to find out the scour depth. In US the most commonly used equation is the following one;
- ds= scour depth
- y= flow depth at upstream of the pier
- K1,K2,K3 are pier nose shape correction factor, angle of flow attack, and bed condition respectively
- b= pier width
- F= Froude number
In India method used for estimating the scour depth is Lacey’s method. Lacey and Inglis developed this method mainly by observing the canals of India and Pakistan. This method is commonly used to find out the scour depth around the piers in alluvial soil.
DLQ= 0.47(Q/f )1/3
- DLQ is the scour depth in meters below the design flood level.
- Q is the discharge in m3
- f is the Lacey’s silt factor ( f= 1.76 were d (median size of bed material) in mm)
- P is the perimeter
Factors Affecting Scour Depth
- Whether the flow is clear water flow or flows with sediments. The clear water scour carries 10% more sediments compared to live bed scour.
- Effect of change in depth of flow. As per the experiments, if D/b is greater than 2.6 the scour depth does not depend on the flow depth. For smaller depth, the depth of the flow depends on the flow depth.
- The shape of the pier nose which will highly influence the strength of vortex flow and as well as the separation flow around the pier. Hence it affects the scour depth. The table below shows the effect various pier shape on scour depth. Here in the cylindrical pier, the maximum scour depth is taken as unity.
- In the case of non-cohesive materials, the characteristics like density, particle size, standard deviation, stratification of sediments will influence the scour depth. For a given flow condition, if the sediment size d is small the clear water scour will be higher. Also, other factors like flow parameters, flood level, opening ratio influences the scour depth.
Scour Monitoring Techniques – Scouring in bridge
Sonars: Sonar devices also known as acoustics transducer which uses sound waves for underwater navigation or communication or depth analysis. In this case, the sound waves are sent to the potential scour region and sound wave will reflect back when it reaches the surface of the river bed.
The reflected sound wave is captured and the time taken from transmission to reflection is analyzed and converted into a range by knowing the speed of the sound. Most of the sonar devices mounted directly on the pier or substructure of the bridge. Sonar instrument can track both scour and fill process.
Radar is an effective technology that can use in scour analysis. In radar electromagnetic waves are transmitted to the medium or ground and reflect back when it reaches an interface. By analyzing the reflected signal we could able find out the surface characteristics test zone.
Fibre optic sensors
Fibre optic sensors have many advantages over other traditional sensors such as durability, reliability, resistance to corrosive environment etc. Fibre optic sensors are installed in a cantilever beam which mounted on a bridge pier. These sensors are designed based on the concept that it picks up strain acts on the beam and generate electric signals.
The fibre optic sensors exposed to water flow only pickup strain and those buried under sediments won’t pick up the signal and this because the buried sensors won’t be bent due to absence of hydrodynamic force. The scouring can be analyzed verifying the exposure condition of buried Fibre optic sensors and variations in signals.
Time Domain Reflectometry
The Time Domain Reflectometry (TDR) is another technique used in recent years for scour analysis. In TDR system an electromagnetic pulse is transmitted through a transmission line with a fixed velocity. The pulse propagates down through the transmission line until the end reached or discontinuity (interface) found. Then the pulse reflects back and by analyzing changes in sent and received electromagnetic pulse the scour variation can be tracked.
Scour & Scouring in bridge Prevention & Mitigation
The scouring counter measures can be broadly divided into two;
- Armoring countermeasures
- Flow altering countermeasures.
Armouring countermeasure is providing an additional protective layer to the base or on the top of potential scour zone to prevent the hydraulic shear stress from the vortex. Commonly used armour material is riprap (arrangement of stones to a particular thickness). Also, other types of armours are used such as tetraprons, grout filled bags, concrete blocks etc. one of the main disadvantage of this system is this armours consumes more flow area and it triggers additional contraction scour.
The principle behind flow altering countermeasures is to alter the flow alignment or break up the vortices. Different proposed flow altering measures include submerged vane, collar and slots, parallel wall etc. The figure below shows the working of collar. As you can see the collar divides the downflow into two, which also obstructs the vortices from downward movement. Hence the strength of horseshoe vortex got reduced. The efficiency of the collar depends upon the size of collar, location of pier, flow rate etc.
The selection of countermeasures depends upon various factors like location, nature of the problem, means the type of scouring, channel properties, flow characteristics etc. Also, other important factors like cost, efficiency, maintenance required are to be considered while selecting scour protection. Sometimes different scour protection techniques need to be work together to get good output in scour prevention.