Jan H. van den Berg

Prediction of alluvial channel pattern of perennial rivers

Abstract Purely braided, meandering and straight channels can be considered as end-members of a continuum of alluvial channel patterns. Several researchers have succeeded in separating channel patterns in fields defined by flow related parameters. However, the discriminators of the principal channel patterns derived from these diagrams all require some a priori knowledge of the channel geometry. In this paper a method is presented which enables prediction of the equilibrium conditions for the occurrence of braided and high sinuosity meandering rivers in unconfined alluvial floodplains. The method is based on two, almost channel pattern independent, boundary conditions: median grain size of the river bed material, and a potential specific stream power parameter related to bankfull discharge or mean annual flood and valley gradient. This can be regarded as a potential maximum of the available flow energy corresponding to the minimum sinuosity condition, P = 1. Based on an analysis of 228 datasets of measurement sites along rivers from many parts of the world an independent discriminating function was found that separates the occurrence of braided rivers and meandering rivers with P > 1.5. The function applies to equilibrium conditions of rivers that neither incise nor show rapid aggradation, with a bankfull or mean annual flood discharge above 10 m3/s and a median bed material grain size between 0.1 and 100 mm.

Overbank and channelfill deposits of the modern Yellow River delta

Abstract The Huanghe is noted for its high transport rate of silt and clay, which may reach depth-averaged values of 200 kg m−3 during peak discharge. The sediment load transported through the river on entering the delta plain, amounts to 1012 kg per year. In contrast to most other large deltas only one distributary channel is active at any one time. The high sediment load causes the rivermouth to prograde at a yearly rate of 1–4 km into the shallow (less than 20 m deep) Bohai gulf. The vertical aggradation of the channel belt and mouth bar complex is also rapid (decimetres per year on average), so that after a normal average of twelve years increasing channel instability and avulsion create the start of a new delta lobe. A series of satellite images covering the last fifteen years has provided insight in the evolution of the river pattern as well as the progradation of the delta front. A newly developed distributary passes from a multichannel to a single, straight channel system, and ends with the formation of meanders. The protruding mature delta lobe shows a radiating pattern of crevasse channels. Overbank/ crevasse deposits are made of vertically stacked dm-scale waning flow sequences, structurally characterized by (from bottom to top) small scour-and-fills, even (parallel) lamination, and climbing-ripple crosslamination. Accumulation rates on crevasse splays can be predicted on the basis of estimated river sediment discharge. It can be concluded that each sequence has been deposited within a few hours, and that tidal waterlevel fluctuations may have played a role in the generation of a single sequence.

Dynamics and sequential analysis of a mesotidal shoal and intershoal channel complex in the Eastern Scheldt (southwestern Netherlands)

Abstract A detailed survey was carried out on a mesotidal shoal and intershoal channel complex near the mouth of the Eastern Scheldt in the southwestern part of The Netherlands during the summers of 1976 and 1977. The objectives were to establish a relationship between the morphological changes through time and its influence on the development of the preserved sedimentary sequence. The investigated area is divided into four zones, each one characterized by specific depositional processes and their related bedforms: (a) the outer channel bend and margin, which is mainly covered by flood-oriented large-scale catenary ripples and sand waves; (b) the channel thalweg and inner bend, which is dominated by ebb-oriented mega-ripples; (c) the inner channel margin, which is characterized by a levee wall; (d) the adjacent shoal areas, which generally are flat but may be covered by patches of flood- or ebb-oriented large-scale ripples. The preserved vertical sequence of sedimentary structures in each zone is more or less in accordance with the presence of the dominant bedforms within the respective zones. Sequential analysis shows that the net sedimentation within the outer channel margin is much less when compared to the sediment accumulation within the inner channel margin. This means that the lateral accretional units of the inner channel margin are a dominant feature within the preserved sequence of such a shoal and intershoal channel complex, and also that the preservation potential of sedimentary structures in the inner margin sequence is considerable higher as that compared to the outer margin sequence. Consequently, preservation of the megaripples, a major bedform in a large part of the investigated area, will be limited and will form only a minor part within an ancient analoguous sequence.

Prediction of suspended bed material transport in flows over silt and very fine sand

Three equilibrium sand transport formulae were tested for prediction of the suspended bed material transport rate in the silt-laden Huanghe (Yellow River): Ackers and White (1973), Engelund and Hansen (1967), and Van Rijn (1984b). The best results were obtained with the Van Rijn function. However, at low flow stages better results were produced by the Engelund-Hansen equation. The Ackers-White formula seriously overpredicted the measured values. Some modifications of the Van Rijn formula were proposed for application in flows over very fine sand and silt. In a verification analysis it was demonstrated that these modifications slightly improve its predictive strength. Also, attention was paid to the effect of turbulence damping by sediment particles at high concentrations.

Comment on Lewin and Brewer (2001): ¿Predicting channel patterns¿, Geomorphology 40, 329¿339

With the aim of assessing basic alluvial channel planforms as a function of the main determining parameters, a number of stability diagrams have been published during the past several decades, starting with the well-known plots of channel slope versus bankfull discharge of Leopold and Wolman (1957) and Lane (1957). In more recent versions of these stability diagrams, a parameter representing flow energy is plotted against some geometric or grainsize parameter. In one way or another, the diagrams indicate that straight, meandering and braided patterns represent a trend of increasing flow energy (sensu Ferguson, 1987, and Knighton and Nanson, 1993). However, the discriminators could not be used in a truly predictive way, as the value of one or both of the ‘‘independent’’ variables was predicated upon a priori knowledge of one or more geometric properties of the pattern that was to be predicted, such as the bankfull width, depth or slope of the channel (Parker, 1976; Fredsøe, 1978; Struiksma and Klaassen, 1988). Therefore, a diagram was proposed by the first author using the parameters potential stream power (based on valley gradient as opposed to channel gradient) and median grainsize, variables that can be considered almost independent of channel pattern (Van den Berg, 1995). Potential specific stream power, x, was defined as: