BED FORMS IN SAND GRAVEL MIXTURES

Knowledge of the characteristics of bed forms are important in alluvial streams for determining channel resistance, flow depth, and for use in calculating sediment transport. Bed forms may also be preserved as primary sedimentary structures which potentially contain a large amount of information on the nature of the generating flow and on the type of depositional environment present when the sediment was deposited. Bed forms that occur in mixtures of sand and gravel are poorly understood. In this study, bed forms were examined in a 15.2 m long by 0.36 m wide laboratory channel and in a 24 m wide field channel. The sediment in both cases consisted of bimodal mixtures of sand and gravel (median size 1.8 mm and 8.3 mm in the laboratory and field, respectively). All flows were subcritical. Using cluster analysis on the heights and periods, three sizes of bed forms were recognized in the laboratory channel. These three groups had heights of 3 to 5 mm, 7 to 12 mm, and 8 to 15 mm, with lengths ranging from 0.03 to 4.85 m, resulting in length to height ratios from 60 to 397. The smaller bed forms in the laboratory channel were mainly formed of the sand sizes of the sediment. The larger bed forms in the laboratory channel were composed of all sediment sizes in the bed material. Bed forms in the field channel had mean heights from 60 to 260 mm, lengths from 3 to 28 m, and length to height ratios from 31 to 136. During most flows in which bed forms were measured in the field, the larger sizes of the bed material were under represented in the bed load. The height of the bed forms was positively related to the shear stress in the field channel. Flow resistance decreased with increasing shear stress and bed form height for both the laboratory and field cases. Classifications need to be modified to account for the bed forms that form under flow conditions for which not all of the sizes of the bed material are in motion. In addition to the flow and sediment size, parameters describing the nature of the sediment size distribution and the degree of mobility of the bed material should be included in classification schemes.