Bedrock river lateral erosion plays a crucial role in landscape
evolution, sediment transport and deposition, and the occurrence of
geohazards. Lateral erosion is driven by the impacts of bedload
particles (BPs). However, BPs generally move parallel to the channel
walls and thus need to be deflected sidewards to cause wall erosion.
Sideward deflection of BPs occurs when they interact with roughness
elements (REs) fixed on the riverbed. We set up 21 sets of flume
experiments to systematically investigate how spacing (5, 10, 20, 30,
40, 50, and 60 mm) and size (5, 10, and 20 mm) of REs influence sideward
deflection of BPs. The deflection length and speed peaks at intermediate
values of the spacing of REs. The likelihood for a BP to leave the
roughness zone decays with the BP’s distance to its edge. Our results
suggest that lateral erosion rates in bedrock channels are dominantly
controlled by the position of the roughness zone within the channel and
its relation to the particle path.