Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 11/30/2002
Publication Date: 12/9/2002
Citation: Horton, J.K., Bennett, S.J., Best, J.L., Kuhnle, R.A. 2002. Flow and bedform dynamics of a bimodal sand-gravel mixture. USDA-ARS National Sedimentation Laboratory Research Report. No. 32. 200 pp.
Interpretive Summary: Little information exists regarding the characteristics of bed topography in rivers and streams with sand and gravel beds. This information is important for determining water depth and total sediment load in rivers and streams for a given flow condition. To address these issues, a unique, extensive data set has been compiled from laboratory flume experiments conducted using a streambed composed of a mixture of sand and gravel over a range of flow conditions. For the flow conditions examined, several distinct streambed states were identified, and these can be grouped into ripples, bedload sheets, and low-relief bed waves. Each group had its own flow and sediment characteristics. Moreover, it was observed that the total sediment load in the experimental channel was wholly dependent upon the movement and characteristics of the streambed states. A conceptual model is presented that describes the evolution of streambed state with changing flow conditions. These data and this conceptual model will aid river engineers and practitioners in the understanding and prediction of total sediment load and flow resistance in rivers with sand and gravel beds.
Technical Abstract: Little information exists regarding the morphology, texture, dynamics and turbulent flow characteristics associated with bedforms developed in mixed sized sediments, which are ubiquitous in nature. A unique, extensive data set has been compiled from laboratory flume experiments conducted using a bimodal sediment mixture (grain size ranging from 0.177 to 32 mm) over a range of discharges at two flow depths. Extensive measurements were taken under equilibrium flow conditions to define the characteristics of the bedforms generated. For the range of flow depths and discharges investigated, several distinct bedforms were identified as flow stage increased: ripples, bedload sheets, and low-relief bed waves. Fluctuations in fractional and total sediment transport are linked to bedform migration. A conceptual model of bedform development in bimodal mixtures is proposed, incorporating the effects of bedform amalgamation, selective entrainment, and variations in shear stress and surface roughness. These data and new conceptualizations will aid in the understanding and prediction of total sediment load and flow resistance in rivers with sand and gravel beds.