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Genesis of Gravel Bars

At normal water levels a river flows in its river bed. The river bed is an elongated hollow shape through which the water of a river flows. It consists of a river bed and banks, which limit the river bed on both sides. In a watercourse material is transported, including mineral and organic, solid and dissolved substances. The carried material is also referred to as a flow load.

The relationship between flow rate and transport behavior of the entrained flow load that comprises various particle sizes (clay, silt, sand, grus) is represented by the Hjulström diagram (fig. 2.1). The upper section of the diagram show the velocities at which the respective grain sizes can be eroded. The border between transport and deposition is limited by a narrow transition region (a), since in this recion not only the flow rate, but also the form of deposition is an important factor.

Hjulström Diagram. Shown is the relationship between grain size and critical flow velocities for erosion and transport. a) critical erosion velocity, b) critical velocity for the onset of deposition (Ahnert 2003, 200). On the y-axis are the flow rates [cm/s], the grain size [mm] is plotted on the x-axis respectively on a logarithmic division.

Although the grain sizes fine sand and silt have larger equivalent diameters, lower flow velocities are neccessary for erosion. Clay particles are harder to mobilize, as the single grains are additionally held together by electrostatic forces. The lower graph of the diagram (b) represents the critical flow velocity at which the particles can not be transported and thus are sedimeted at the reiverbed. Obviously, fine silt and clay remain in suspension for long time periods. Between erosion and deposition, the occuring velocities result in transportaiton of the sediment. The rule is that with increasing distance between the two graphs, the transportation of sediment becomes more continuous.

Clay- (< 2 µm) and silt particles (2 - 63 µm) are suspended load due to their long residence time in the water column. The main source of suspended load is surface runoff and thus the amount of clay and silt particles in the river decreases with increasing drainage.

Schematic outline of a meandering watercourse with associated morphometric characteristics and examples of river bed cross-sections (Ahnert 2003, 223)

The debris load of a river consists of gravel (63-200 mm), which is transported along the bottom of the riverbed. In a broader sense, the sand fraction (63-2000 µm) can be counted to debris load as well. Nevertheless it rather represents a transitional form between suspended particles and debris load. The movement of large grain sizes such as gravel is mainly caused by the drag force (shear force) at the riverbed. The drag force is approximately proportional to the flow velocity squared. However the flow velocity within the flow cross-section varies exceedingly: The line of maximum flow rate in a watercourse is also referred to as line drift. With more or less straight river sections the drift line is approximately in the middle of the watercourse, meanwhile in river bends near the outer bank. In this context, it is in the river bends, the so-called meander bends (fig. 2.2) (Ahnert 2003, 2007, Schmidt 2007).

In meander bends the drift line is closer to the outer curve. Hence lateral erosion is predominant and leads to the emergence of undercut slopes and steepened banks. At the inside curve of the meander accumulation processes are dominant and slip-off slopes develop. Here prevails a lower flow velocity, so that entrained flow load is deposited on the bank slopes by progressive accumulation. Gravel bars in the river bed arise when transported debris reaches areas where the drag force is less than the critical value for the corresponding grain size. The debris is deposited, forming an additional barrier which reduces the flow rate and hence the drag force. A positive feedback occurs and also smaller grain sizes are deposited. Additionally a certain sorting in grain size occurs because only those gravels remain, that can not be moved by the existing drag force at the gravel bar (Ahnert 2003, 2007, Schmidt 2007).

Often gravel bars form on both, the left and on the right bank. This is an indication of the current drift line swings from side to side. In meanders gravel bars are always deposited on the inside of the river bend. There the flow velocity is very low, because this side is farthest away from the drift line. Gravel bars typically have a streamlined contour which is elongated in the direction of flow. In river beds with sandy subsoil sand bars can form in a similar manner (Ahnert 2003, 223).