CHANNEL BELT ARCHITECTURE FORMED BY A MEANDERING RIVER

Braided rivers are relatively easily formed in the laboratory, whereas self-formed meandering rivers have proven very difficult to form. Our objective is to create self-formed dynamic braided and meandering rivers in a laboratory, and to quantitatively compare the resulting morphology and deposits. We applied a transverse moving inlet funnel for flow and sediment at the upstream boundary, mimicking meanders migrating into the control section. Conditions in the meandering and braided experiment were exactly equal except that slightly cohesive silt-sized silica flour was added to the feed sediment of the meandering channel. This was to test the hypotheses that i) meandering rivers have relatively narrower and deeper channels due to bank cohesion, and ii) that floodplain-filling sediment fills potential chute channels that would otherwise lead to braiding.

Our experiments were conducted in a flume of 10x6 meter, which was split up into two separate fluvial plains (each 10x3 m). The parallel setups have identical cycled discharge regimes with a longer duration low flow and a shorter duration high flow simulating floods. The bed sediment consisted of a poorly sorted sediment mixture ranging from fine sand to fine gravel. High-resolution measurements of time-dependent surface elevation were used to quantitatively relate the preserved stratification to the river morphology.

In agreement with earlier work, the experimental river without silica flour evolves from alternate bars to a fully braided river. With silica flour added to the feed, a meandering system evolved with frequent chute cut-offs that nevertheless remained mostly single-thread. Furthermore, the mean set thickness agrees well with the theoretical prediction from channel morphology. The mean preserved set thickness is 30% of the mean channel depth. Finally, there is much systematic spatial variation in set thickness related to repetitive point bar growth and chute cutoff. We find undisturbed and thick sets close to channel belt margins and more irregular stratification with stacked thinner sets in the channel belt center.