Hydrology of Braided River Systems

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

0207556 Foufoula Braided rivers are complex dynamical systems comprising multiple unstable alluvial channels that divide and rejoin around bars and islands and migrate frequently across the river's braidplain. Although not common in lowland areas, braided rivers are dominant at high latitudes and in areas with rapid sedimentation. They represent a major management challenge in areas such as Alaska and the Pacific Northwest where bridges and pipelines must be built across them. In addition, the alluvial deposits of braided rivers are important reservoirs of water, oil, gas, coal, sand, gravel and heavy minerals. Their instability and opportunistic flow patterns make prediction of braided rivers extremely difficult. Recent advances have suggested several new ways of analyzing these complex dynamical systems. It is now clear that braided rivers exhibit both spatial and dynamic (time-space) scaling; i.e. a small part of the river network is statistically similar to a larger part, in terms of planform geometry and evolution, under appropriate resealing. This observation opens up the possibility of statistical prediction of rare, high-magnitude channel shifts from readily obtainable records of lower magnitude but more frequent events occurring at smaller scales. It is also clear that at least some of these system-scaling or "emergent" properties of braided rivers can be predicted on the basis of simplified models of the stream dynamics. Finally, recent experimental work has illustrated that the planform pattern and dynamics of braided rivers can be dramatically affected by modest amounts of bank vegetation; it may even be possible to convert a braided to meandering river by vegetation effects alone. We propose to unite and build on these three lines of research, focusing on the following basic issues: (1) extension of scaling analysis in braided rivers to the third (vertical) dimension, i.e., scaling of flows, velocities, bed elevation and sediment flux, with implications for statistical prediction of floods, scour and mobility in modern rivers, and stratigraphic architecture in ancient deposits; (2) testing and refinement of existing braided-river models using hydrologic scaling results as a model metric; and (3) study of the effects of vegetation on braided rivers, focusing on how repeated cycles of flow and vegetation can lead to the 'regularization' of braided-river flows into a small number of well defined channels, breaking the scaling and perhaps leading to the development of the first truly self-regenerating meanders.