Tentative Conclusions and Recommendations
Vertical sorting of sediment in bedforms is preserved in the bed and provides the bed sediment available for entrainment in subsequent discharge waves. The height of the bedforms determine the depth from which the (vertically sorted) bed sediment may be entrained.
Combination of the vertical sorting in bedforms with the lag in bedform height development during discharge waves may result in anti-clockwise hysteresis of bedload transport and fining of bedload transport.
The given hypothesis for the explanation of anti-clockwise hysteresis of bedload transport due to vertical sorting in the bed and the lag of dune development can partly explain the observations in the river Waal and in the flume experiments. However, local storage and erosion of sand in the river Waal probably also plays a role.
Less well developed bedforms than dunes are less effective in vertical sorting. It is therefore necessary to be able to predict the bedform type. A tentative conceptual model is proposed for the prediction of the bedform type in different flow conditions and with different sand content in the bed.
A modelling approach is proposed for the prediction of bedload sediment transport incorporating the vertical sorting of bed sediment due to vertical sorting in dunes during previous discharge waves. As a boundary condition the sorting in bedforms during the highest historical discharge wave (in the last 10-20 years) is taken. The bed sediment available for entrainment in a subsequent discharge waves is determined by the height of (or depth of erosion by) the bedforms in these discharge waves.
Acknowledgements
The present research is part of a research programme on sediment transport in sand-gravel-bed rivers during high discharges at Utrecht University. The investigations were in part supported by the Netherlands Earth and Life sciences Foundation (ALW) with financial aid from the Netherlands Organization for Scientific Research (NWO).
The National Institute for Inland Water Management and waste water Treatment (RIZA) and the Directorate Eastern Netherlands of Rijkswaterstaat in the Netherlands financed and carried out the measurements in the river Waal. The pleasant cooperation with the Rijkswaterstaat - ANIM crew during the measurement campaigns in 1997 and 1998 is thankfully remembered. Antoine Wilbers (Utrecht University) is thanked for allowing me to include one of his beautiful multibeam echosounding pictures of the Waal bed at the top a high discharge wave.
The Sand flume tests were financed by 1) the Transport and Mobility of Researchers programme of the European Commission and 2) the consortium of Twente University, The Institute for Inland Water Management and Waste Water Management (RIZA) and WL|Delft Hydraulics. The pleasant cooperation with Astrid Blom (Twente University), Klaus Basso (Karlsruhe University), Luigi Fraccarollo (Trento University), Erik Mosselman, Freek de Groot and Joop Ouderling (WL|Delft Hydraulics) is thankfully remembered, as well as the many informal lessons of Nico Struiksma (WL|Delft Hydraulics) on river modelling, operating flumes and how to hinder causes of stress.
The comments of Ward Koster, Leo van Rijn, Janrik van den Berg, Antoine Wilbers, Annika Hesselink and Kim Cohen on earlier drafts of this paper were much appreciated.