What bad thing can happen to a bridge support if 'scour' happens around its base?

Scour is the erosion and removal of streambed material, such as sand, gravel, or soil, from around a bridge support's foundation by the force of flowing water, creating a deepened hole. The bad thing that happens when scour occurs is the critical loss of foundation support. The foundation, which is the substructure component (like piles, caissons, or footings) designed to transfer the bridge's weight and other loads into the ground, becomes undermined or exposed. Undermining means the material directly beneath the foundation is washed away, while exposure means the foundation components, which are designed to be embedded to a specific depth, are no longer surrounded by the necessary amount of soil or rock. This loss of surrounding and underlying material significantly reduces the foundation's bearing capacity, which is its ability to safely support the vertical weight of the bridge without sinking. Concurrently, it drastically diminishes the foundation's lateral stability, which is its resistance to horizontal forces such as those from water currents, wind, or seismic activity. If the bridge support relies on piles, scour exposes a greater length of the piles, effectively increasing their unsupported length and making them more susceptible to buckling under the bridge's load. Ultimately, this reduction in support can lead to excessive settlement (sinking), tilting, or catastrophic structural failure of the bridge support, potentially causing the entire bridge to collapse.