Abstract
During discharge waves in large sand gravel bed rivers, phenomena associated with both sand bed rivers and gravel bed rivers influence the bedload sediment transport. For example dunes and armour layers may occur simultaneously. The effect of vertical sorting in the bed sediment due to armour layers and bedforms on bedload transport is explored, using both field observations and flume experiments. This history effect of sorting is a relict of a previous discharge wave but may have strong effects on bedload transport and bedforms in subsequent discharge waves.
Prediction of bedload transport is usually based on average bed sediment grain size distributions and bed shear stress. In the presence of bedforms the concept of static average bed sediment is not valid. During rising flow the bed level (defined here as the level of bedform troughs) is lowered while sediment is entrained into the migrating and growing bedforms. During waning flow the bed level rises again while the bedforms become smaller.
The sand gravel bedload sediment is vertically sorted in two ways: first, in the avalanche of grains along the lee side of bedforms and second, due to selective deposition of the coarsest grains in waning flow. If both sand and gravel are transported, the sand fractions are mainly deposited in the top half of bedforms and the gravel fractions in the trough half. The trough region of bedforms may be armoured. In waning flow, the vertical sorting of the largest bedforms is preserved. This vertically sorted sediment is the bed sediment available for entrainment in the next discharge wave.
A conceptual model is proposed in which the vertical sorting due to bedforms is incorporated in the prediction of bedload sediment transport. The bedform height determines from which depth below the initial bed level the bed sediment is entrained. With the vertical sorting of the largest previous discharge wave (in the last 10-20 years) as a boundary condition or history effect, the entrained bed sediment is known. However, a predictor of vertical sorting by avalanches of grains along the lee side of dunes is not yet available.
Observations in the river Waal suggest that a combination of the history effect related to vertical sorting in the bed and bedforms and related to the growth and decay of dunes during a discharge wave may (partly) explain the anti-clockwise hysteresis and fining of bedload transport observed in transport rates during the discharge wave of 1998.
The presence of an armour layer may protect the bed against entrainment by the flow. This may lead to a depletion of the available transportable sediment. Bedforms are then not able to develop as they would in uniform sediment but instead obtain barchanoid forms. This is likely to happen more frequently in rivers with stronger armour layers or less sand in the bed. Barchans probably are less effective in vertically sorting of sediment than dunes because of their smaller dimensions and lesser extent on the river bed. Prediction of the bedform type is therefore necessary to be able to assess the effectivity of vertical sorting by bedforms. A tentative stability concept for bedform types is proposed, based on the flume experiments.
The flume experiments presented here show a number of bedform types and their transitions in different flow conditions. These bedform types and their transitions were also observed in some field studies. Below a certain threshold the bedforms are flow-parallel sand ribbons (unless there are relict bedforms from a previous stage with higher flow). With increasing flow, ripples and small barchans form in the sand ribbons, which develop into barchan fields with further increasing flow. Given enough time or even higher flow conditions and enough sand in the bed sediment, these barchan(oids) develop into more dunelike bedforms. The turbulence due to flow separation at the lee sides of the bedforms contributes to the winnowing process which leads to the transition from barchans to dunes.