1School of Geography, University of Leeds, West Yorkshire, LS2 9JT, UK
2School of Earth Sciences, University of Leeds, West Yorkshire, LS2 9JT, UK
Email:
p.ashworth@geog.leeds.ac.uk,
URL:
http://www.geog.leeds.ac.uk/staff/p.ashworth/
http://earth.leeds.ac.uk/research/seddies/best/jim_best.htm
Relationships between flow structure, sediment transport and bed morphology were investigated over bedforms generated in a bimodal mixture of coarse sand and fine gravel (combined median grain size = 1.07 mm) in an experimental flume. Six experimental runs covering a range of flow conditions (mean bed shear stress = 1.34 to 2.44 N/m2), allowed two classes of contemporaneous bedforms to be observed: (1) low-relief bedforms (0.4 to 1.7 m in length and 5 to 25 mm in height), and (2) bedload sheets (up to 0.5 m in length and 7 mm in height), and (2) bedload sheets (up to 0.5 m in length and 7 mm in height). Bedload sheets form on the back and crestal regions of low-relief bedforms and increase in amplitude towards the bedform fronts. The variability of flow structure over low-relief bedforms is quantified through simultaneous collection of bed height and flow velocity data using a high resolution, ultrasonic bed profiler and a laser Doppler anemometer. Boundary shear stress is low over bedform troughs, high over bedform backs and intermediate over bedform crests, with low-relief bedform crests being finer grained and better sorted than bedform troughs. The spatial pattern of total and fractional bedload transport rates over low- relief bedforms is predicted using the Bridge and Bennett (1992) surface-based bedload transport model. Excellent agreement is achieved between model simulations and observed bedload transport rates. Comparison of the geometry and dynamics of low-relief bedforms with other work suggests they share some characteristics of both two-dimensional dunes and barforms. Low-relief bedforms may represent an incipient dune phase, intermediate between a nominally flat lower-stage plane bed and fully developed dunes. An explanation is presented for bedload sheet formation in response to the interaction of grain roughness, flow structure and fractional sediment transport.