Formation mechanisms of transverse ribs: insights from experiments on antidune and keystone processes

Transverse ribs are prominent bedforms in gravel-bed rivers on alluvial fans, playing a crucial role in maintaining riverbed stability and ecological diversity. Their decline, caused by human interventions such as channel excavation and altered sediment supply for flood control, is a growing concern. This study investigates transverse rib formation through two conceptual frameworks: antidune-type and keystone-type. Hydraulic experiments replicated both mechanisms. Antidune-type ribs formed via hydraulic jumps and standing waves, producing upstream-migrating bedforms. Keystone-type ribs resulted from coarse sediment accumulation in low-velocity zones induced by immobile keystone elements. Varying keystone spatial arrangements significantly influenced flow patterns, sediment transport, and rib development, with alternating lateral placements enhancing flow complexity and rib formation under specific spacing conditions. Grain size analyses showed coarse sediment concentrated in ribs for both types, compared to adjacent pools. The coexistence of both rib types suggests complementary roles in channel morphology shaping. These findings highlight the potential of keystone-based designs to promote natural transverse rib reformation in managed rivers. Establishing design principles for keystone arrangements can contribute to sustainable river engineering by enhancing morphological diversity and hydraulic functionality.