Our work with rivers involves understanding the mechanics of flow and bar evolution in gravel-bed channels.  We are turning now to streams that contain both alluvial and bedrock reaches, focusing on how these elements interact to transport, store and sort sediment as it moves down a river system.  In addition, we are pursuing the exciting topic of coupled physical-biological behavior.  This work involves growing collaborations with aquatic ecologists, and is aimed at clarifying how the trophic-structure "play" unfolds on the "stage" set by flow and transport processes.

River Bars, Meandering and Alluvial-Bedrock Channels

Channels containing both alluvial and bedrock reaches represent a diverse template for aquatic life, owing to marked differences (and variations) in the physical character of the channel substrate.  In some channels the large-scale longitudinal structure consists of alluvial reaches (large sediment "wedges") spanning a few to tens of channel widths separated by similarly sized bedrock reaches.  At a finer scale alluvial reaches contain well-developed bar forms, and bedrock reaches contain sediment patches.  Our initial work on this topic is aimed at understanding the basic conditions that determine where alluvial versus bedrock reaches are located, the extent to which alluvial features persist in their locations versus migrate, and the transfer and sorting of sediment from one alluvial "wedge" to another during flooding.

Heller, P. L., Beland, P. E., Humphrey, N. F., Konrad, S. K., Lynds, R. M., McMillan, M. E., Valentine, K. E., Widman, Y. A. and Furbish, D. J. 2001. Paradox of downstream fining and weathering-rind formation in the lower Hoh River, Olympic Peninsula, Washington. Geology, 29, 971-974.

Byrd, T. C. and Furbish, D. J. 2000. Magnitude of deviatoric terms in vertically-averaged flow equations. Earth Surface Processes and Landforms, 25, 319-328.

Byrd, T. C., Furbish, D. J. and Warburton, J. 2000. Estimating depth-averaged velocities in rough channels. Earth Surface Processes and Landforms, 25, 167-173.

Furbish, D. J. 1998. Irregular bed forms in steep, rough channels: 1. Stability analysis. Water Resources Research, 34, 3635-3648.

Furbish, D. J., Thorne, S. D., Byrd, T. C., Warburton, J., Cudney, J. J. and Handel, R. W. 1998. Irregular bed forms in steep, rough channels: 2. Field observations. Water Resources Research, 34, 3649-3659.

Thorne, S. D. and Furbish, D. J. 1995. Influences of coarse bank roughness on flow within a sharply curved river bend. Geomorphology, 12, 241-257.

Furbish, D. J. 1993. Flow structure in a bouldery mountain stream with complex bed topography. Water Resources Research, 29, 2249-2263.

Furbish, D. J. 1991. Spatial autoregressive structure in meander evolution. Geological Society of America Bulletin, 103, 1576-1589.

Furbish, D. J. 1988. River-bend curvature and migration: How are they related? Geology, 16, 752-755.

Furbish, D. J. 1988. A test of recent convolution-integral models describing the migration of river meanders. International Association for Hydraulic Research, Proceedings, International Conference on Fluvial Hydraulics, Budapest, pp. 298-303.

Furbish, D. J. 1987. Conditions for geometric similarity of coarse stream-bed roughness. Mathematical Geology, 19, 291-307.

Coupled Physical-Biological Behavior in Streams

The physical character and behavior of the channel substrate is but one, albeit important, ingredient influencing where and when aquatic plants and critters reside and undergo their life cycles.  Our work is aimed at describing how transport processes, including exchanges of biotic and abiotic materials between the fluid column and the bed, influence primary production and nutrient spiraling.

Understanding Hydrodynamic Dispersion

We are combining theory and field experiments on Panther Creek, Tennessee, to clarify ingredients of nutrient transport and spiraling.  Below is an AGU abstract summarizing the work of Daniela Stefan and Ben Iobst.

Characterizing hydrodynamic dispersion in open-channel flow is a key element in environmental studies aimed at modeling the transport and cycling of nutrients and pollutants.  We use a simple flow model together with a particle-tracking algorithm to explore first-order influences of bed topography on the hydrodynamic dispersion.  The model is based on linearized versions of the shallow-water equations for flow over an irregular bed topography composed of alternate bars.  Theoretical dispersion curves were generated by simultaneously releasing tracer particles across the channel at a fixed location and keeping track of their positions for various intervals of time and different channel geometries.  Particles were subject to fluctuating motions mimicking effects of turbulence. The shape and length of the tail of the dispersion curve appears to depend primarily on the time elapsed since the particles were released. For short time intervals, the curve is characterized by a steep leading edge which later transforms into a peak with a less steeply sloping front.  This transition occurs more rapidly with increasing bar amplitude, and also with increasing number of alternate bars in the section traveled – thus with shorter bar wavelengths.

Rhodamine WT was used in a field dye test conducted on a 150 m straight reach of Panther Creek, KY. This section of the creek has an average channel width of 6.3m, and exhibits a loose alternate bar structure with wavelength of ~55 m and amplitude of ~0.1 m. The bed of the channel has an average slope of 0.01 and consists of coarse gravel with a D₈₅ of 6 cm. Consistent with the modeling results, the tracer test revealed a relative steep leading front and slowly decaying tail.  In both the simulated and field case, this tail is similar to the behavior predicted by “dead zone” models of dispersion, and is attributable mostly to spatial variations in the local flow (with superimposed fluctuating motions) associated with vertical velocity structure combined with shoaling and deepening over the bed topography.