Submitted to: American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: 10/15/2000
Publication Date: N/A
Technical Abstract: The interactions between fluid flow and different sediment size fractions in bimodal mixtures control sediment transport rates, grain size sorting patterns, flow resistance, and the characteristics of any deposited sediment. Due to past technical difficulties of recirculating gravel-sized sediment in a laboratory flume, there is currently a limited understanding concerning the morphological and textural characteristics of bedforms generated in sand-gravel mixtures and their associated turbulent flow structure. Here we present preliminary results of flume experiments conducted using a bimodal sediment mixture (grain size ranging from 0.2 to 26.9 mm, D50=1.8 mm) over a range of discharges at two flow depths. Various measurements were taken under fully mobile and fixed-bed conditions to define the sedimentologial and hydraulic characteristics of the bedforms generated. For the range of flow depths and discharges investigated, the following bedforms were observed as bed shear stress increased: ripples (3 5 mm high and 0.03-0.8 m long), bedload sheets (7-12 m high and 0.07-1.06 m long). When the bed is composed of ripples and bedload sheets, total sediment transport rate can be directly related to bedform migration. When low-relief bedwaves are present, additional contributions to sediment transport are made through particle suspension and overpassing. Detailed contour maps illustrate the effect of grain roughness and bed topography on turbulent flow structure over these sand-gravel bedforms.