GEOMORPHOLOGICAL APPROACH TO THE RIVER RUNOFF EVALUATION IN THE GEOLOGICAL PAST (Paper 2. Estimation of river palaeo-discharges based on channel hydraulics)

The methods of open channel hydraulics, designed to calculate the capacity of canals and river channels, can be used to determine the hydraulic and hydrological characteristics of ancient river channels. Large available empirical data allows calculations of channel capacity with an accuracy of 10–15%. The same accuracy is possible for the calculation of bankfull paleochannel discharge in the conditions of correct choice of the cross-section and the reconstruction of its morphometry: altitude of floodplain edges, paleochannel width at bankfull stage, average depth and surface slope of the flow. Morphometric characteristics of ancient channels and particle size distribution of alluvial deposits are obtained by detailed coring at channel cross-sections. In modern rivers, straight channel reaches without backwater effects, flow divergence or convergence are usually selected for the calculation of discharges with Chezy formula. Given that, coring of meandering paleochannels is rational to be organized at straight insertions between two adjacent bends. These reaches correspond to shallow crosses with low longitudinal and lateral flow non-uniformity and negligible backwater effects at high water levels.
The most reliable results can be obtained for the non-deformable (slightly deformable) paleochannels. The uncertainty in the estimation of ancient discharges increases significantly for deformable paleochannels. Changes in the paleochannel cross-section shape during its conservation, the uncertainty in the determination of textural contacts in alluvium and filling deposits, errors in determining of paleochannel slope lead to an increase in the total error of a palaeo-discharge estimate. The conversion from the bankfull discharge to a discharge of another frequency (i.e. to annual or mean maximum discharge) is processed with the help of empirical equations. This reduces significantly the accuracy of the calculations, since the empirical relationships are characterized by implicit errors, which are difficult to identify and reduce. Therefore, the accuracy of the hydraulic methods for estimating the annual discharge is about the same as that of the morphometric methods (regime equations), i.e. about 30–40%.

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