What are the three scour mechanisms at a bridge, and how do they combine?
The three scour mechanisms at a bridge are long-term degradation, contraction scour and local scour, and they add together. Long-term degradation is the general lowering of a streambed over years from changes in sediment supply, channel modification or upstream impoundment. Contraction scour occurs where the bridge structure narrows the flow, raising velocity across the whole section. Local scour is the vortex action around an individual pier or abutment. All three scour mechanisms act at once, and the total is what matters to a bridge foundation, which is why a scour analysis addressing only local scour understates the condition.
Why does a bridge scour hole refill after a flood?
A bridge scour hole refills because scour depth peaks at the height of the flood, when velocity is greatest and the water is carrying the most sediment, and as flow subsides that sediment deposits and the scour hole fills. The refilled material is loose and recently deposited rather than the consolidated streambed it replaced, so it provides far less support than the original streambed. This is why probing and visual inspection of the streambed after a flood are unreliable, and why scour can remove a bridge foundation’s support without leaving a visible hole.
How can a scour hole at a bridge be found after it has refilled?
A refilled scour hole at a bridge can be found by borings through the fill at the bridge pier and by geophysical mapping, with sediment sampling corroborating the finding, because the refilled material differs from the surrounding streambed. Borings through the material at the bridge pier show loose, poorly graded recent deposits over the original stratigraphy, and the depth of that transition marks how deep the scour hole went. Geophysical methods — sub-bottom profiling and ground-penetrating survey from the water surface — can map the boundary between the refilled material and the original streambed across an area more quickly. Sediment sampling and comparison against the native bed material corroborates the finding.
How does a bridge’s foundation type determine what scour does to it?
A bridge’s foundation type governs the consequence of scour: a spread footing bearing directly on the streambed loses capacity as soon as scour reaches its underside, and undermining a spread footing is generally catastrophic, while a pile-supported foundation retains capacity as long as sufficient embedment remains below the scour depth. For a pile-supported bridge foundation, the question becomes how much of the pile length was still engaged. Establishing the as-built foundation — from construction records, drawings, or direct investigation — is therefore the first step in judging what the scour did to the foundation, and it is frequently where the record is weakest on older bridges.
How is the actual flood compared with a bridge’s design flood after a scour failure?
The flood that actually occurred is compared with a bridge’s design flood by establishing the discharge of the event — from stream gauge data, high-water marks, and hydrologic analysis of the watershed — and setting it directly against the design assumption. Bridges over water are designed against a design flood, with scour assessed for that event and typically for a larger check event. A flood within the design flood that produced a bridge failure points toward the scour analysis, the foundation design or subsequent channel changes. A flood well beyond the design flood raises a different set of questions about the design basis itself.
Can changes to a river channel make a bridge vulnerable to scour?
Yes: changes to a river channel can make a bridge vulnerable to scour, because scour vulnerability changes over a bridge’s life. Upstream dams reduce sediment supply and cause downstream degradation. Gravel extraction, channel straightening, bank armoring and development that increases runoff all alter the regime the bridge was designed for. A bridge adequate under the conditions at construction can become vulnerable to scour without anything about the bridge itself changing, and reconstructing the channel history from successive surveys and aerial imagery is often what explains a bridge scour failure.
Are bridge scour countermeasures such as riprap permanent, and does maintenance matter?
No: riprap, articulated block and other armoring are common bridge scour countermeasures, and they are not permanent, which is why a bridge scour investigation asks whether they were maintained. Riprap can be undersized for the velocity, can be displaced progressively over successive floods, and can settle into the streambed until it no longer protects the bridge foundation. Finding displaced armor downstream, or finding the armor in place but undersized against the velocities calculated for the flood event, distinguishes a scour countermeasure that failed from one that was never adequate.
What records matter for a bridge’s underwater elements in a scour investigation?
In a scour investigation, the relevant file for a bridge’s underwater elements includes the underwater inspection reports, any scour evaluation or scour-critical designation for the bridge, the plan of action required for scour-critical bridges, and post-flood inspections following prior events. Bridge substructure elements below water are inspected on their own cycle, typically by divers, and often less frequently than the superstructure. A bridge previously designated scour-critical and left without the required monitoring is a materially different case from a bridge never so identified.
What evidence should be preserved quickly after a bridge scour failure?
After a bridge scour failure, bathymetric survey and sediment sampling should be carried out as soon as conditions permit, because the scour record continues degrading with every subsequent flow. High-water marks should be photographed and surveyed before they weather away. Gauge data and rainfall records for the flood event should be gathered, along with construction records establishing the bridge’s foundation type and depth, and the full underwater inspection and scour evaluation history for the bridge.
This article on bridge scour is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.