Hydraulic Modeling of Channel-Control Structures in Self-forming Braided Channels.

Robert Ettema, Christopher Thornton, Steven Abt, Caroline Ubing

Tuesday 30 june 2015

16:30 - 16:45h at Mississippi (level 1)

Themes: (T) Sediment management and morphodynamics, (ST) Sediment transport mechanisms and modelling

Parallel session: 7A. Sediment - Erosion

There are scant few references describing moveable-bed hydraulic modeling of channel-control structures located in braided channels. Such modeling involves several challenging considerations, because braided channels characteristically are relatively wide and shallow; have relatively large bed-sediment loads; readily shift; and (importantly) are self-forming – they are shaped by the magnitude and frequency of their flows, the amount and properties of the sediment they transport, and the slope of the valley they drain. These characteristics of braided channels pose substantial complications for designing and interpreting hydraulic models of channel-control structures intended to modify or constrain the morphology of braided channels. Our paper describes the similitude guideline and model-design approach we adopted for modeling a series of large channel-control structures intended to trap bed sediment, and elevate a broad sediment plain, along a two-kilometer reach of the North Fork of the Toutle River, in Washington. The Toutle River conveys large amounts of sediment originating from a huge debris avalanche deposited at the upper end of the river’s watershed during the 1980 eruption of Mount St. Helens. Our approach departed from the normal similitude considerations associated with moveable-bed models, and expressly involved considerations of channel-morphology similitude. Furthermore, various geometrical distortions (vertical, slope, sediment diameter) were needed to achieve morphologic similitude, and ensure reasonable similitude of the local flow field at the channel-control structures. The only prior study somewhat comparable to ours was that of a braided reach of the Jamuna River, Bangladesh (Klaassen 1990). Reference Klaassen, G. J., 1990. On the Scaling of Braided Sand-Bed Rivers. In: Moveable Bed Physical Models, H. W. Shen (Ed.), Kluwer Academic Publishers, Dordrecht, Netherlands, pp. 59–71.