ABSTRACT: Using the Depth-Velocity-Size Diagram to Interpret Equilibrium Bed Configurations in River Flows

John B. Southard

Data from flume studies that report equilibrium bed configuration as well as water temperature, flow depth, flow velocity, and sediment size were used to develop the best approximation to the relationships among the various bed phases (ripples, dunes, lower regime plane bed, upper regime plane bed, and antidunes) in a three-axis graph (depth-velocity-size diagram) with dimensionless measures of mean flow depth, mean flow velocity, and sediment size along the axis.

Relationships are shown in a series of depth-velocity and velocity-size sections through the diagram. Boundaries between bed-phase stability fields are drawn as surfaces that minimize misplacement of data points. A large subset of the data, for which reliable values of bed shear stress are reported, was also used to represent the stability relationships in a graph of dimensionless boundary shear stress against dimensionless sediment size, but with results less useful for fluvial flow interpretation. The diagram covers about one order of magnitude in flow depth. To be useful for river flows, the diagram must be extrapolated in flow depth by about one more order of magnitude, but this is not a serious problem for approximate work.

The depth-velocity-size diagram permits prediction of equilibrium bed configuration in river flows when the approximate flow depth and mean flow velocity are known. Because the diagram is essentially dimensionless, the effect of water temperature (via the fluid viscosity) on the bed configuration is easily accounted for by use of the diagram.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990