Swiftness of biomorphodynamics in Lilliput- to Giant-sized rivers and deltas

Physical experiments of self-formed river channels and floodplains with live vegetation are pathways for understanding that complement numerical modelling. Recent experiments succeeded in creating braided rivers and dynamic meandering systems with clastic and vegetated floodplains. However, application of the insights gained from such experiments to natural systems depends on understanding potential scale effects, temporal, and spatial. Here we combine review, analysis, and experiments to identify fundamental problems of biomorphological river pattern formation that are open for further research in experiments. We first show by review and analysis that physics-based, linear bar theory predicts negligible spatial scale effects in bar and bend wavelength relative to channel width. Time scaling, on the other hand, remains problematic because it integrates multiple processes of sediment transport, floodplain formation, and bank failure affected by bank stratigraphy and riparian vegetation. As a tentative solution, we secondly present experimental methods to assess bank strength effects that can be used in the design of river pattern experiments. The third issue is that riparian vegetation has often been represented in experiments by uniformly seeded sprouts of a single plant species, whilst spectacularly different patterns are obtained with contrasting seeding protocols, showing the need for other experimental procedures, and alternative riparian species. The main challenge for future experiments is better understanding of temporal scaling of biomorphodynamics.