Sorting out meandering and braiding: discriminating formative conditions Seminar

For various river channel patterns, the necessary formative conditions differ, but how is not entirely understood. Our objective is to identify the necessary and sufficient conditions for forming dynamic meandering and braided rivers. We reproduced both patterns experimentally and with a physics-based numerical model.

Experimental meandering was produced using a mixture of poorly sorted sediment and silt-sized silica flour and a transversely moving inflow boundary. Braiding was produced in exactly the same conditions but without the silica flour. These experiments represent gravel-bed rivers in nature, where both experimental meandering and braiding channels were close to the transition between the types. Onset meandering was also produced in the numerical model (Delft3D) with a similar transversely moving inflow boundary, whilst braided rivers formed with fixed inflow or some noise on the transverse discharge distribution at the inflow boundary.

The silica flour deposited on crevasse splays and in chute channels, forming new floodplain. This caused much less chute cutoffs and stronger banks. The resulting meandering river formed multiple sets of scroll bars forming pointbars, overlain by splays and floodplain. The braided river, in contrast, showed mid-channel bars and multiple active channels, faster and more haphazard bar and channel migration, and frequent chute cutoffs. We conclude that some floodplain-filling sediment or vegetation is required for meandering to prevent chute cutoffs to lead to weak braiding, but, more importantly, that some dynamics are required at the upstream boundary, where periodic perturbation causes meandering whilst stochastic perturbation causes braiding.