Meander chute cutoffs are a common and geomorphically important feature of meandering rivers. They exhibit complex dynamics and distinctive morphologic features. To date, however, the geomorphic processes governing the evolution and formation of these features are poorly understood due to limited knowledge of cutoff hydrodynamics. This paper investigates three-dimensional mean flow structure, turbulent flow structure, and bed shear stress distribution from high-resolution flow velocity data in a fixed-bed, sediment-free physical model. The results show that 1) the chute channel conveys around 1.4 times the unit-width flow discharge as the cutoff bend; 2) mean flow structure is highly three-dimensional, with strong convective acceleration throughout the bends and pronounced flow separation zones in both the chute channel and the cutoff bend; 3) turbulent kinetic energy is intense at shear layers bounding the flow separation zones at several locations in the channel; and 4) bed shear stress is elevated due to strong turbulence in the chute channel and is low in the cutoff bend. The unique hydrodynamics of meander chute cutoffs explain their distinctive morphologic behaviors, including the rapid widening and deepening of chute channels and locations of bars and pools. Moreover, this paper quantitatively compares the secondary flow structure before and after the cutoff, showing that cross-sectional redistribution of streamwise momentum by secondary flow remains largely unchanged in the presence of the chute cutoff. This provides support to the use of current 2D depth-averaged hydrodynamic models for chute cutoffs with secondary flow parameters calibrated in single-channeled meanders.

A document by Chao Gao. Click on the document to view its contents.

The problem of meandering in mixed bedrock-alluvial rivers is more challenging than that of purely alluvial streams, in that alluvial, bed incisional and bank incisional morphodynamics must be accounted for. Here we present a numerical formulation that addresses heretofore unanswered questions. Bed incision is based on abrasion due to saltating grains. The model satisfies mass conservation of alluvium over a partially-covered bedrock surface. Bank incision is treated in terms of a measure of incipient collision of bedload particles with the bank. It is assumed that land accretes along the inside of point bars when the water depth falls below a specified shallow value. All but one of the runs are performed with repetitive two-step hydrographs. Runs starting from a low-amplitude sine-generated curve indicate that sinuosities at least as high as 2.5 can be achieved. The rate of increase of sinuosity declines in time, but does not vanish. For the same hydrograph, increasing the initial thickness of alluvium on the bed causes the rate of vertical bedrock incision to decline, and bend sinuosity to increase at a faster rate. At a sufficiently high thickness, the channel migrates laterally without bed lowering. Bend shape can change dramatically with increasing alluvial thickness, with high thickness favoring more regular bend trains. For the same initial alluvial thickness, increasing the peak flow of the hydrograph causes the vertical incision rate and the rate of sinuosity growth to increase. The model thus captures a wide range of behavior associated with bedrock meandering.