Scour is among the leading causes of bridge failure and among the hardest to investigate, for a reason peculiar to the mechanism: the evidence tends to erase itself. Flowing water removes streambed material from around a pier or abutment during high flow, undermining the foundation. As the flood recedes and velocity drops, the sediment the water is carrying settles back into the hole it just created. An inspector arriving afterwards may find a streambed that looks entirely normal above a foundation that has already lost its support.
Three mechanisms that 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 structure narrows the flow, raising velocity across the whole section. Local scour is the vortex action around an individual pier or abutment. All three act at once, and the total is what matters to a foundation — which is why an analysis addressing only local scour understates the condition.
Why the hole refills
Scour depth peaks at the height of the flood, when velocity is greatest and the water is carrying the most sediment. As flow subsides, that sediment deposits, and the scour hole fills. The refilled material is loose and recently deposited rather than the consolidated bed it replaced, so it provides far less support than the original streambed. This is why probing and visual inspection after a flood are unreliable, and why the mechanism can remove a foundation's support without leaving a visible hole.
Finding the hole after it has filled
Refilled scour is detectable because the fill differs from the surrounding bed. Borings through the material at the pier show loose, poorly graded recent deposits over the original stratigraphy, and the depth of that transition marks how deep the hole went. Geophysical methods — sub-bottom profiling and ground-penetrating survey from the water surface — can map the boundary across an area more quickly. Sediment sampling and comparison against the native bed material corroborates it.
The foundation type governs the consequence
A spread footing bearing directly on the streambed loses capacity as soon as scour reaches its underside, and undermining it is generally catastrophic. A pile-supported foundation retains capacity as long as sufficient embedment remains below the scour depth, so 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, and it is frequently where the record is weakest on older structures.
Comparing the flood 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. Establishing the discharge that actually occurred — from stream gauge data, high-water marks, and hydrologic analysis of the watershed — allows a direct comparison. An event within the design flood that produced failure points toward the scour analysis, the foundation design or subsequent channel changes. An event well beyond it raises a different set of questions about the design basis itself.
The channel may not be what it was
Scour vulnerability changes over a structure's life. Upstream dams reduce sediment supply and cause downstream degradation. Gravel extraction, channel straightening, bank armouring and development that increases runoff all alter the regime the bridge was designed for. A structure adequate under the conditions at construction can become vulnerable without anything about it changing, and reconstructing the channel history from successive surveys and aerial imagery is often what explains the failure.
Countermeasures and whether they were maintained
Riprap, articulated block and other armouring are common countermeasures and they are not permanent. Riprap can be undersized for the velocity, can be displaced progressively over successive floods, and can settle into the bed until it no longer protects. Finding displaced armour downstream, or finding it in place but undersized against the velocities calculated for the event, distinguishes a countermeasure that failed from one that was never adequate.
The inspection record for underwater elements
Substructure elements below water are inspected on their own cycle, typically by divers, and often less frequently than the superstructure. The relevant file includes those underwater inspection reports, any scour evaluation or scour-critical designation for the structure, the plan of action required for scour-critical bridges, and post-flood inspections following prior events. A structure previously designated scour-critical and left without the required monitoring is a materially different case from one never so identified.
What to preserve and do quickly
Bathymetric survey and sediment sampling as soon as conditions permit, because the record continues degrading with every subsequent flow. High-water marks photographed and surveyed before they weather away. Gauge data and rainfall records for the event. Construction records establishing the foundation type and depth, and the full underwater inspection and scour evaluation history for the structure.
This article 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.