Computer simulation of sedimentation in meandering streams

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ABSTRACT

A dynamic mathematical model for simulation of sedimentation in meandering streams is briefly described. This is composed of component mathematical models which are formulated to predict the following aspects of the system for a given physical situation and a single time increment. (1) The characteristics of the plan form of the meander; (2) the movement of the meander in plan, and definition of cross-sections across the meander in which erosion and deposition are considered in detail; (3) the hydraulic properties of the channel in the bend and the erosional and depositional activity within the channel as defined in specific cross-sections; (4) the nature and occurrence of cut-off; (5) a relative measure of the discharge during a seasonal high water period, which is used in (3) and (4); (6) aggradation.

The model, in the form of a FORTRAN IV computer program, has been used to simulate various aspects of sedimentation in meandering streams by performing a set of experiments with the program under different input conditions.

The geometry of simulated point bar sediments, as controlled by channel migration over floodplains with variable sediment type, agrees broadly with the natural situation, however extensive sheets of point bar sediment cannot be simulated because large scale meander-belt movements are not accounted for.

In the simulated sediments, successive surfaces of the point bar before falling stage deposition (lateral and vertical) may be picked out, and these delineate the epsilon cross-stratification of Allen (1963b). The epsilon unit thickness is that measured from bankfull stage down to the lowest channel position existing prior to deposition.

The model records the characteristic fining upwards of grain sizes in the point bar, and the systematic distribution of sedimentary structures. Channel migration combined with seasonal scouring and filling across the channel section produces a characteristic relief in the basal scoured surfaces and facies boundaries (as defined by variation in grain size and sedimentary structure). A related lensing and inter-fingering of the facies may also be present. The model also records large-scale lateral changes in grain size and sedimentary structure associated with changes in the shape of developing meanders.

When channel migration is combined with a constant aggradation rate the model predicts a general slope (relative to the land surface) of facies boundaries and scoured basal surfaces upward in the direction of channel movement. If aggradation sufficiently increases the thickness of fine-grained overbank material, there is a channel stabilization effect. It is shown that a complete sequence of point bar sediments capped by overbank sediments would rarely be preserved in the moving-phase situation. Such preservation only becomes likely when an aggrading section lies out of range of an eroding channel for a considerably longer time span than it takes a meander to move one half-wavelength downvalley. Deep channel scours have a higher preservation potential than contemporary shallower ones.

Where appropriate field data exist the model can be used in the more accurate recognition of ancient fluviatile sediments. Inferences may be made about the erosion-deposition processes operating in the ancient channel system, and the geometry and hydraulics of the system can be alluded to. A representative application of the model to the quantitative interpretation of an ancient point bar deposit is illustrated. There is reasonable agreement between the natural and the simulated deposits, and a broad quantitative picture of the palaeoenvironment of sedimentation is obtained.

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