What triggers contraction scour?

Contraction scour is primarily triggered by accelerated stream velocity that occurs due to natural constrictions in the riverbed or channel. When a river narrows, whether by natural formations such as bends, rocks, or other geographical features, the flow of water is forced to increase in speed to maintain the same volume of water passing through a smaller area. This increased velocity can erode the riverbed and banks more aggressively, resulting in scour around bridge piers, abutments, or other structures.

In contrast, heavy rainfall increasing water levels may cause general flooding and increases in water flow, but these conditions do not specifically lead to the localized increases in water velocity that characterize contraction scour. Additionally, while man-made structures can indeed alter river flow and contribute to various types of scour (including contraction scour), they are not the sole trigger. Normal riverbank erosion processes occur gradually and are a part of the river's natural dynamics, but they do not produce the sudden increases in flow velocity associated with contraction scour.

Thus, the correct answer reflects the specific hydrodynamic conditions that lead to localized scour due to constrictions in the channel.