Zbigniew Zwoliński

Quaternary Research Institute, Adam Mickiewicz University, Poznań, Poland**)
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  1. In the last decade a few examples of non-typical distribution of depositional processes have been found at meander bends. Generally speaking, they involve inversion of geomorphic functions at sharply curved bends. The most important symptom of this inversion is the deposition of alluvial material at the concave bank of the river channel, i.e. in places where the processes of bank erosion, as a rule, develop. Carey (1969) was the first to describe such events. Up to now they have been particularly reliably reported from Australia where forms built up nowadays, as well as ancient ones were subjected to study. The development of the latter forms has been reconstructed.

    The literature provides an exchange of proposals for the nomenclatural problem concerning this deposition of sediments. Up to now this sedimentation has been termed eddy accretion (Carey 1969), concave-bank bench (Hickin 1979, Nanson Page 1985, Page, Nanson 1981, Taylor, Woodyer 1978, Woodyer 1975, Woodyer, Taylor, Crook 1979), barbed channel planform and irregular floodplain depression (Lewin 1978, Thorne, Lewin 1979), channel-side bar (Witt 1979), concave bench (Page, Nanson 1982), counterpoint sedimentation (Lewin 1985), counterpoint bar (Smith 1985). The latter two proposals seem most suitable since they refer to the location and origin of sedimentation of this type. Besides, note should be made of the fact that erosional modelling of the inner bank, i.e. a point bar, is the counterpart of sedimentation occurring at the outer bank of the bend. Hence, the use of the term COUNTERPOINT SEDIMENTATION/BAR is sufficiently justifiable and appropriate. The Polish nomenclature offers the term sedymentacja/łacha wsteczna (=backward sedimentation/bar).

    There is no information in the Polish literature concerning the deposition of sediments along the outer bank of the bend. Merely Witt's publication (1979) provides certain data on this subject. However, the above author was not fully aware of the conditions and mechanism of such sedimentation. The study and observations of the Parsęta River channel, carried out since 1979, have provided a deeper insight into the counterpoint sedimentation.

  2. More or less distinct imprints of the counterpoint sedimentation have been found at many sites along nearly the entire length of the Parsęta River. Thus, three observation reaches of the Parsęta River have been chosen in the vicinity of Krosino, Dębczyno and Bardy and subjected to detailed study in respect of counterpoint bar characteristics. Three cross-sections located in different settings of channel planform, namely in the straight reach and at bends with short and long curvature radius, have been chosen along each reach. Such a selection of channel cross-sections has allowed hydraulic, sedimentary and morphologic differences to be discovered for each type of the cross-section.

  3. It has been established that the counterpoint sedimentation takes place at sharply curved bends, i.e. at those whose curvature ratio is less than 2. The main planform characteristics of sharply curved bends are an angle of meander opening which is less than 90° and channel width (W) approaching the length of curvature radius (Rc). The curvature ratios (CW) for a bankfull stage are 0.48, 1.24 and 1.12 for Krosino, Dębczyno and Bardy, respectively. The fullest sequence of geomorphic events is registered at the Krosino bend. Events observed and/or recorded at the bends of the other two Parsęta reaches either support or complete the evidence of counterpoint sedimentation detected at Krosino.

  4. The provenance of bends with a low curvature ratio remains an open question. Are they one of the evolutionary stages or types of a meander bend? Are they hereditary forms resulting from one of the earlier developmental stages of the river, perhaps climatic ones? Are they determined by lithology of the valley floor? Are they conditioned by water flow hydraulics? Mention must be made of the fact that the literature does not provide evidence of the occurrence of these bends as palaeomeanders.

  5. The most important characteristic of channels at sharply curved bends is specific water flow within them. It is associated with the existence of a zone of separation of the current that impinges on the outer bank. This zone occupies the deepest channel sections and is characterized by vertical vortices. On impinging on the outer bank, the current is separated into two water limbs. One limb is the river current proper which has, however, been shifted towards the inner bank. The other limb is back flow along the outer bank. The occurrence of the two water limbs is indicative of a number of hydraulic, sedimentary and morphologic implications in the river channel behaviour.

  6. The shifting of the river current proper towards the inner bank of the bend results in the displacement of the zone of maximum water flow velocity in the channel cross-section in the same direction. This zone occupies more or less one third of the channel width, moving from the inner bank. Asymmetrical development of bank ice covers during the winter is a spectacular expression of this displacements.
    1 - back flow, 2 - counterpoint bar deposits

  7. A wide range of cross-section. changes in the position of the channel bottom (up to 2.18 m) along the inner bank is one of the results of displacement of the zone of maximum water flow velocities towards this bank. High dynamics of a channel morphologic cross-section together with high velocities do not facilitate the deposition of sediments on the convex bank in the form of a point bar. In case such deposition occurs, sedimentary material is eroded from point bars, especially at low water stages. Mention must be made of the fact that secondary currents, the operation of which becomes reflected in a concentric pattern of ripple marks in phase in the submerged part of point bars, contribute a lot to the erosion of these bars. Shifting transverse bars may sometimes be encountered instead of point bar sediments being laid down.

  8. With regard to the shifting of maximum velocities in the channel cross-section, there is a non-typical relationship between the channel depth at a point (Dx) and the median of grain diameter of bed material (M) collected at a given point. For comparative purposes, this relationship is presented in a graphic form on three diagrams for channel cross-section at a bend with high curvature ratio (A), in a straight river reach (B) and at a bend with low curvature ratio (C). From the latter diagram, it can be inferred that the size of transported grains decreases with increasing channel depth. Thus, the finest grains that are largely carried in suspension, as can be inferred from field measurements, remain in the zone of maximum depths of bends with low curvature ratio and thus, in the zone of separation.

  9. The second water limb due to flow separation comprises back flow moving upstream along the outer bank of the channel. Back flows at sharply curved bends have minimum potential energy which decreases with higher water stages. This is manifested by an extremely slow rate of bank erosion (B), compared with the rate of bank erosion occurring at the edge of a bend with high curvature ratio (A). Apart from minimum energy levels of back flows, the lithology of concave banks which are built up largely of alluvial silty sediments or cohesive fine sands also has an effect on the intensity of bank erosion. A slow rate of bank erosion affecting the edges under consideration or sometimes the absence of this process results in the presence of aquatic and bank plants. The vegetation begets the loss of energy of back flows. There is concurrent deposition of fine particles deposited by these flows.

  10. The operation of counterpoint sedimentation in the river channel is clearly defined against hydraulic, sedimentary and morphologic events described in brief. Besides the above events, two more geomorphic causes which reflect the events that take place in the drainage basin assume principal significance for the sedimentation of counterpoint bar deposits. Firstly, flash and high-magnitude floods, bankfull or overbank ones at the optimum, occur, resulting in large quantities of suspended load; such floods may be generated by heavy rainfalls or due to the melting of a snow cover in the spring. Secondly, these floods are preceded by a period of rather stable hydrodynamic conditions of the river channel and of geomorphic "standstill" in the drainage basin; such a preceding period provides indications of intensive delivery of solids derived from both the river channel and drainage basin.

  11. The back flow from the zone of separation carries large quantities of suspended material associated with the vertical vortices. As the potential energy of the back flow is lost, rapid and chaotic primary deposition of counterpoint bar sediments takes place due to great roughness of the bed and bank. The first stage of sedimentation which is correlated with a rising phase of flood in the channel leads to smooth basal surface of the bar. A change in its roughness precedes the total or partial cessation of suspended material deposition, which leads to a change in the type of material transport from that in suspension to transport by rolling and saltation. The transported material is gradually deposited along the concave bank, depending on the rate of decay of back flow transporting capacity. In this stage the bar surface may remain smooth (flat) or covered with ripple marks in accordance with the existing hydrodynamic conditions.

  12. Further stages of deposition of the counterpoint bar sediments depend on the nature and duration of floods. The intensest moment of sedimentation can be identified with a period prior to the flood peak or with the very peak, Although the previous stages of sedimentation are chiefly associated with fine sands, the last stage may be identified with the deposition of a thin silt cover under tranquil hydrodynamic conditions of the back flow or even under those of nearly stagnant water. During the falling phase of floods, the counterpoint bar sediments may become partially eroded.

  13. The two-dimensional geometry of counterpoint sedimentation corresponds with the distance of back flow along the concave bank and with the width of a back flow limb. In case the limb of back flow is narrow, the counterpoint bar surface is horizontal; otherwise, the surface remains gently inclined towards the channel axis. The height of sediment accretion in the counterpoint bar is associated with either the moment of suspended material exhaustion in the river channel, or maximum water stage in the channel, or floodplain level.

  14. The counterpoint bars are built up of fine sands and silts. The spectra of cumulative size frequency curves for one of the counterpoint bars indicate fine texture of sediments (average Mz=2,09 phi) and good and extremely good sorting (average dI=0.45 phi, average g=1.01 phi). They also suggest that the sediments are due to transport by suspension in the river channel. In most cases they display symmetrical (average SkI=0.10 phi) and mesokurtic (average KG=1.12 phi) distribution of grains in samples. Thus, they are mostly sediments displaying logarithmonormal distributions with distinct maxima.

  15. Horizontal parallel and lenticular lamination as well as small-scale cross-bedding which register differing conditions of water flow regime are regarded as typical sedimentary structures of counterpoint bars. Structures corresponding to climbing ripple marks of type B (after Reineck, Singh 1973) are relatively frequently observed. The presence of sedimentary structures in the counterpoint bar sediments is indicative of the influence of transport by rolling on the growth of these forms.

  16. After a flood wave falls, a newly produced form at the concave bank becomes a new segment of the floodplain. The recognition of counterpoint deposits as sediments deposited by lateral / vertical accretion is of importance in terms of the functioning of the fluvial sedimentary environment. It appears that the above problem should be dealt with in respect of two aspects. One aspect is concerned with a single counterpoint bar as an independent form. The bar sediments should be then regarded as vertical accretion deposits in spite of the fact that the sedimentation takes place within a river channel. In addition to hydraulic and morphogenetic evidence, attention should be given to a fast rate of sedimentation which was up to 1.5 m on the Parsęta River during a flood. The other aspect tackles a series of a few segments of the counterpoint bar, formed during successive floods. The floodplain can be thought then to expend laterally. However, this cannot be identified with lateral accretion of deposits. Witt (1979) has provided evidence on such a mechanism for the growth of a floodplain. Note should be made of the fact that such a sequence of counterpoint bar segments has not been found on the Parsęta River.

*) This electronic publication was presented as poster during International Association of Sedimentologists 7th European Regional Meeting in Krakow-Poland, May 1986.
**) Present address: Institute of Paleogeography and Geoecology, Adam Mickiewicz University, Dziegielowa 27, 61-680 Poznan, Poland