morphological studies and data needs b russets,...

MORPHOLOGICAL STUDIES AND DATA NEEDS Jean J. Peters

Consultant B russets, Belgium

Introduction

Fluvio-morphological or river behaviour studies are needed for predicting the future evolutions of the Bangladesh river network: the natural, partly man-induced development of the delta with its long term trends and the impact on these trends of the Flood Action Plan. There appears to be a general consensus about the urgency to start with studies, but the tools required for conducting such studies are not yet well defined and experts do not always agree which tools to use or to develop for answering specific questions.

River behaviour or morphology is rightly considered as an extremely difficult and complicated matter. River behaviour problems are addressed by experts from mainly two disciplines: the engineering and geographic sciences. However, its broad multi-disciplinary character is not so often well recognized, neither by decision makers nor in the scientific community.

These river behaviour problems can be subdivided in two main categories: ( 1) the more general problems like overall bed levels and planform changes and (2) the more local changes in river course as e.g. those occurring near hydraulic structures.

The first category relates usually to the assessment of medium-and/or long-term trends when the parameters that determine the river's overall morphology are affected: changes in river flow (e.g. under i~uence of dam construction, during the year, or annual, or inter-annual), in sediment yield (e.g. under influence of land use changes), in geology/tectonics (e.g. under influence of earthquakes), etc ..

The second category is usually resulting from the human impact on the river system. On the one hand, there is a direct or indirect influence of man on the development of the river morphology (e.g. by construction of groynes, bank revetments or levees)~ on the other hand, the effect of the morphological changes on human settlements has also to be considered (e.g. bank erosion close to villages, cities, transport facilities).

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The distinction between these two categories remains however often unclear, especially when dealing with deltaic systems, as the evolution of the rivers can be progressive during long periods and suddenly become erratic, e.g. when 'avulsion' occurs. Little is known about the triggering of sudden changes by small causes, among which are the human interferences. This implies that the two categories of problems must be handled together. Furthermore, the multi-disciplinary character of fluvio-morphology should be thoroughly recognized, especially when dealing with large tropical deltas.

Tools for River Behaviour Studies

Today, the most common tool for solving general morphological problems in large rivers is the numerical or mathematical modelling, and this is for mainly two reasons. The first is that the size of the problem makes scale models inappropriate, the second is that morphological field data are extremely complex, difficult to collect and to express in mathematical relations. Mathematical models are said to allow a thorough investigation of the sensitivity of the various parameters involved in the morphological processes. The model concepts are founded on basic theoretical or on purely empirical approaches. It is not the place in this paper to consider this matter in detail, but a controversial discussion was supposed to take place during the present International Workshop on Morphological Behaviour of the Major Rivers in Bangladesh.

Scale models have been. widely used for more detailed studies of constructions, their design or possible local impact on the river behaviour. Attempts to model the general morphological trends in large deltas have failed because it is not possible to model thoroughly the complex phenomena, especially those related to sediment transport (Peters, 1990).

Field investigations as such (not the collection and processing of data to be used as input in models) receive generally little attention. This can be explained by the difficulty (some prefer to say the 'impossibility') to perform reliable or meaningful measurements of some phenomena. Bed material transport is one example for which exists only very limited experience. But another reason is that not so many experts are interested in performing field work considered solely as a supporting (to modelling) activity: They leave it to technicians. The development of new, modem tools may help in the breakthrough, as more detailed data become available, increasing in this way the power of· field investigation techniques. The river becomes the model on

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scale 1/1 for the analysis of some phenomena. Inherent is again the danger for too theoretical approaches if investigators loose contact with the field when data acquisition becomes automated.

One argument presented by some experts against field investigation is its high cost. This is only partly true, because the cost of field work can be reduced significantly by optimization of survey programmes. This can be achieved with experience, with good concepts and with the understanding of the phenomena. Data acquisition programmes for river behaviour studies have too often been based on the usual hydrological procedures. Although the instruments, equipment and methods may be similar, the data acquisition in fluvio-morphology should differ from what is done in the routine surveys performed by hydrological services.

The ideas presented in this paper about the tools and data for the studies on river behaviour in large tropical deltas are derived from the experience gained in the Zaire (or Congo) delta. Although the main objective was restricted to the maintenance of a navigation channel, the tools for assessing and predicting morphological changes deal with the overall changes (not only those of the channels), as everything is linked.

The study project started in 1968 and is still going on, presently without any technical assistance. It was successful because it could make profit of the experience gained (since 1900) with analysis of the morphological changes, of the natural evolutions and of the impact of dredging (Peters & Wens, 1991).

From 1968 on, the studies of the Zaire delta were making use - and taking profit - of all three methods: scale modelling, numerical modelling and field investigations. This combination appeared to be the most effective way to achieve the final goal: the development of a simple, practical, easy-to-use morphological assessment/ prediction method. Easy-to-use does not mean that the interpretation of the morphological changes is easy - because the phenomena are extremely complex - but the method was set in such a way as to avoid sophisticated data acquisition programmes.

Field investigations (surveys and data interpretation) permitted to reach a fairly good insight in the process governing the morphological changes (Peters, 1988). Scale models helped understanding complex hydraulic and sediment transport processes, e.g. in which the 3-dimensionality of the flow determined the morphology (Peters, 1990). In a comparison of the modelling

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results with the field data, it appeared that the numerical modelling was a less effective tool for prediction and this because the limitations of the presently available models appeared to be important (Lukanda, Peters, Cornet & Swartenbroeckx, 1992).

Data Needs for Fluvio-Morphological Studies in Large Tropical Deltas

A comprehensive discussion of the data needed for river behaviour studies is presented in the report 'The River Environment' (Simons et al., 1975) prepared for the US Department of the Interior, Fish and Wildlife Service, Twin Cities, Minnesota. This reference document emphasizes the need for a multidisciplinary 'potamological' approach (from the Greek language: potamos =

river and logy = science).

The data needed for a morphological study in a specific delta depend obviously on the objectives and on the characteristics of the catchment and/or of the delta. When initiating a new study, the difficulty resides in the often limited knowledge of the system under investigation. A 'geographical' approach - in which the geographic features are analyzed with a good hydraulic background -is therefore useful to restrict the amount of information to be collected during a first phase of the study. Otherwise, the cost of the field surveys could indeed become prohibitive. This is one reason to choose labour-intensive, low-tech equipment and methods for investigations in countries under development (although the same remark could be made for many developed countries).

In order to make the above clear, the data needs are presented hereafter, with reference to the Zaire delta, illustrated with examples.

Maps, Charts, Aerial Photographs and Satellite Images

Maps and charts are not always available, or fitting the minimum requirements for morphological studies. Different kind of maps are needed:

topographic maps hydrographic charts geological maps (possibly geotechnic or soil maps)

They can be complemented by aerial photographs and satellite images, possibly processed for specific uses.

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The hydrographic charts should be established together with the bathymetry by depth contours, as currently in use for navigable waterways. These charts differ from a conunon representation with elevation contours used in many engineering applications. There is a growing tendency to apply the latter (elevation), also because of their use in numerical models. Interpretation of iso-depth contours yields very useful information on the evolution of bars, chars and channels, for which the iso-level contour would give a deformed piCture (Figure 1 ).

Hydrographic charts must be complemented with hydraulic and possibly geological information:

surface flow lines at the moment of the survey readings at the key water level gauges at the moment of the smveys, and possibly at the secondary gauges, with indication of the gauge lecture used for depth reduction (depth on chart = measured depth -correction for the zero depth at the flow line) alignment of the sounding profiles with their sense and preferably indication of sounding depths at regular intervals indication of specific points such as rock outcrops or bars, etc ..

The intervals between iso-depth contours must be chosen carefully, depending on the type of morphological studies.

The choice of the scales of the hydrographic charts depends on .the kind of morphological studies and the size of the most characteristic morphological features. In the case of the Zaire river, where charts are used for morphological predictions and for the organization of the dredging activities, the basic scale is 1/10,000 (Figure 4); for detailed studies, such as the effect of a rock outcrop on the shift of a bar/char, scales of 1/5,000 (Figure 5) and even 111,000 (Figure 6)~ for analysing the more general evolutions, partial charts are assembled on scale of 1/20,000, 1/50,000 (Figure 3) and 1/100,000 (Figure 2).

A conunon error is to neglect the depth contours (possibly 'negative' depths) of the dry parts as chars and bars, or of the minor channels. The Zaire studies showed clearly the importance of these zones, some to be considered as areas of overland flow during extreme events (Figure 7).

The importance of the interpretation 'by eye' of the iso-depth contours in morphological studies was another element revealed by the Zaire studies. This requires to have the charts easy to read, thus coloured, using a standardized procedure to be set up for each chart scale. If the charts are

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produced by computerized processing, attention should be paid to the possible errors inherent to some mapping programmes.

River cross-section analysis is very often used in river behaviour studies. This technique is quite useful for smaller or meandering rivers, but is difficult to apply and to interpret in the kind of large, braiqed and wandering rivers such as the Zaire or the Jamuna. Indeed, the large discontinuities observed along the channels e.g. because of the moving bars and chars, as well as the quite erratic development of many channels in one single reach may give misleading conclusions (the transverse geometry does not necessarily yield a good image of the flow conveyance pattern).

Topographic and geological (or geotechnical) maps provide useful information about the possiblt! . 'controls' that govern the morphological changes. A similar information can be rendered by aerial photographs, aerophotogrammetry and satellite images. More attention should be given to the ways of interpreting these remote sensing techniques, more with eye and brain and less with computers. This does not mean that automatic satellite image processing is to be avoided, well on the contrary, but each river has its specific features that require the identification of the important or relevant elements to be looked at before the automatic processing. One example is the importance of vegetation as an indicator of sedimentation processes when the fiver carries sediments with variable mineralogical composition: due to the difference in particle size and shape, the various particles will deposit at different settling velocities~ this results in river bars formed by the depositions of the larger particles (sand), in the local depressions, the smaller particles (clay) on top. This difference in composition is reflected when vegetation starts to grow: grass on sand, bushes and trees on the more fertile clay soils. The formation of clay plugs in dead arms of calm zones can be explained by the same mechanism as given earlier: the reduction in velocity at these places causes the deposition of very fine material~ these depositions, called clay plugs, may affect significantly the evolution of minor and major channels due to their higher resistance to erosion.

FlowDatd

As a general rule, flow data are collected in key stations for establishing flow rating curves. Flow lines take profit of the water level gauges established along the river, but at inter-distances that are often very large.

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The flow data are processed in order to yield the input for hydraulic models: water levels and discharges at model boundaries, roughness coefficients etc .. Usually, these coefficients are calibrated with the traditional concepts, used for flow resistance in canals, that are not applicable in straightforward manner for large alluvial rivers.

The Zaire studies revealed that stage-discharge relationships in a large wandering multi-channel delta are influenced by other elements than the 'roughness'. The stage-discharge relationship may remain stable for longer periods, but suddenly deviate from the average due to morphological changes in themselves related to hydrological or sedimentological 'incidents': extreme flood events or sediment 'overloading' of particular reaches. This kind of deviation is one of the elements used in the method for assessment and prediction of the morphological changes (Figure 8).

Also the idea of a regular, smooth water surface slope was abandoned for the Zaire study. Slopes are also good indicators of incidents, like channel loop cut-offs or the dying of a river branch. In the Zaire delta, the number of gauges has been increased significantly. Its water level gauging network is composed of fixed gauges (almost fixed as they have to be moved occasionally in the same area), established on morphologically meaningful and stable locations, besides other gauges that are placed at locations where they can work as indicator for changes. The latter are moved regularly, when the channel system evolves.

Besides flow discharges and water levels, surface flow mapping at various stages of the flood, preferably together with the establishment of bathymetric charts appeared to be efficient tools for monitoring morphological changes. Charts with sufficient float tracks produce very useful lines of equal surface flow intensity, one of the elements utilized in the method for assessment and prediction of morphological changes in the Zaire delta (Figure 9).

Sediment Data

Different kind of sediment data are needed for different categories of studies: soil characteristics when dealing with bank erosion (geotechnical aspect), settling velocities of suspended sediments, sediment sizes for bed form and bed material transport. The sizes of the bed sediments and of the transported material vary with space and with time. The complexity of the sediment processes makes that investigations have been mainly based on

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fundamental research and laboratory experiments instead of on field surveys. The standards and guidelines for sediment measurements are not well suited for studies on large alluvial deltaic systems.

Bed Forms:

A comprehensive data set of bedforms and flow measurements was used by Delft Hydraulics in the frame of the study for flood protection of the river Rhine. Compared to the laboratory data, very little data of large alluvial rivers were found. In 1985, a set of Zaire data from 1968 were made available to Delft Hydraulics by the Belgian Hydraulic Research Laboratory for analysis. The data processing showed that the bed forms did not fit the current theories. The Belgian team came already to this conclusion at the end of the sixties and this had been one of the reasons to abandon the theoretical work on bed forms· , they were used rather for visual and semi-quantitative interpretation in relation with sediment transport phenomena (Figure I 0).

Two types of bed form observations and processing are suggested, on the basis of the Zaire experience: ·

longitudinal sounding profiles in one test reach during the year or at least during the entire flood period, over a length that is sufficient as to encompass the deeper and shallower parts of the thalweg (Figure 11). These would allow (1) a qualitative or semi-quantitative analysis of bed form characteristics and (2) to assess the usability of bedform data for 'roughness' determination. detailed bathymetry of a restricted area (± 10 Km2

), by transverse soundings and possibly complemented by side-scan sonar data. detailed surveys over typical bed form types identified in the pervious steps, together with hydraulic measurements (mainly current velocities, · shear stresses and slopes), to understand the relation between bed form changes, sediment movement (both bed material and suspended transport) and flow (Figure 12).

For the Zaire, bed form appeared to be of little use for roughness and bed material load transport studies (Anonymous, 1986~ Lukanda et al., 1992).

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Sediment Characteristics:

Sampling of bed or transported sediments can yield very interesting results such as the decrease in size in downstream direction, the selective transport and sedimentation phenomena during the different flood phases, peculiar processes e.g. at off-takes, bifurcations and confluences, etc ..

The analysis of sediment characteristics requires also simple but well thought methods, especially when the sediment sizes cover the sand, silt and clay ranges, and when the particle shapes vary. This is clearly the case for the Jam una river, carrying fine sands but also large, flat mica particles, besides other silt and small sized clay elements. The use of different size determination methods ·and visuatanalysis by microscope are needed in a first phase to set up the optimum laboratory procedures. The mineralogical composition of a limited number of samples can help in most sedimentological studies.

Sediment Transport:

Sediment transport data are one of the most difficult to acquire. For most large alluvial rivers, only suspettded sediment transport is measured directly, and bed load is computed (Milliman and Meade, 1983). Several investigators (Meade and Stevens, 1990~ Meade, 1985) have shown the importance of differentiating transport rates for the various sediment size fractions. Despite these experiences, little attention has been paid to the contribution of bed material load to the morphological changes in many large deltas, as in the Bangladesh rivers.

In their paper on 'Worldwide Sediment Delivery to Oceans', Milliman and Meade (1983) mention that the Zaire is quite an exceptional cas~; as bed load represents on average about half of the total load of the river. This statement is· based on the results provided by the Zaire river studies conducted by the Belgian Hydraulic Research Laboratory. The contribution of wash load to the total sediment transport is indeed very low due to the Zaire catchment characteristics: suspended and bed load are mainly composed of fine sand.

The fact that the contribution ofbed material load to the morphological processes would be underestimated may be explained by the very limited and often unreliable data on bed load transport rates in other large alluvial rivers, especially those with mild slopes. Zaire delta field studies revealed that the traditional distinction between bed and suspended load does not apply to this

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river. Processes close to the bed are much more complex than presented in theories, e.g. with the siltation concept. It was only by well thought measuring campaigns that the relevant processes could be identified and included in the method for assessment and prediction of the morphological changes (Peters and Goldberg, 1989; Peters & Wens, 1991).

The sediment sampling under intense flow conditions is not an easy task and most samplers were not designed for this purpose. Simple and efficient sampling equipment and methods were adopted to the Zaire conditions (Peters, 1981; Peters and Goldberg, 1989) and providing information on sand transport distribution in the cross-sections; they appeared to be quite reliable even for flow velocities upto 3 rnls in large depths (Figure 13). These sediment transport measurements organized in particular reaches may help understanding 'abnormal' morphological changes. Figure 14 shows a reach of the Zaire delta where a 'sediment plume' produced by a large sand bar triggered a channel shift.

Sediment transport surveys should thus be organized mainly for two purposes:

to serve as input of models, mathematical or scale models; to be included in field studies in order to help understanding the processes that govern the morphological changes;

the latter can be as well in an investigation phase as in the routine river and flood plain management work.

For river studies, surveys should concentrate on the collection of sediment data over the entire depth: it should include transport data as well as the characteristics of the sediment. This requires obviously the development of gauging procedures that permit the collection of samples for size analysis; probably it is best to combine modern sophisticated equipment - that provides a good global and qualitative picture of the suspended sediment transport rates in the river cross-section - with more traditional instruments of sampling (collapsible bag sampler (Richey et al., 1986), point sampler, Delft Bottle, etc.). At the same moment, bed load measurements should be done bearing in mind the transition at the vicinity of the. bed. Surveys and studies on sediment transport should also include bed forms, as mentioned abOve.

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Conclusions and Recommendation for Surveys and Morphological Studies in Bangladesh

Many questions have been raised about the future development of the delta of the Ganges, Brahmaputra-Jamuna and Meghna. The opinions about the impact of the Flood Action Plan are at least divergent, as are the proposed actions, ranging from 'doing nothing' to 'taming the rivers' solutions. An interesting approach was presented in the F AP-22 Project: the 'Active Flood Plain Management'.

In line with a presently quite common idea, some propose to develop tools for predicting the general morphological trend. It would be possible to anticipate with fair confidence the river(s) responses to training or management works. This is not the place to argue about the validity of the theories, most of them derived from experiments on canals.

It is useful to consider the present concern about the response of the Bangladesh rivers to the human impact - both in the entire catchment and in the delta, within the country's borders - in two categories:

the overall, general morphological changes in response to natural or man-induced changes (deforestation and flow abstraction m neighbouring countries, Global Change, etc.). the more site specific problems (e.g. the effect of the Jam una Bridge, of the flood protection schemes, of protection works close to economically important but endangered areas, of the dying off-take rivers, etc.).

For the studies related to the first category, mathematical modelling with overall, average parameters are said to suffice. For the second, more detailed data are required.

Although the tools proposed for the studies of the first category proved to be useful in other rivers, future morphological studies of the Bangladesh rivers should look at the need of introducing more elaborated tools. One example is the use of a representative discharge responsible for the river morphology. About the so-called 'channel-forming' (different values can be used, such as dominant or bankful discharge), the concept of a single discharge conditioning the river's morphology should be questioned (Thorne et al., 1993). In the Zaire delta, the overall river's morphology appeared to be very sensitive to the magnitude of the flood and controlling, with other factors, the evolution of the many rivers branches. Modelling can help understanding the relative

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importance, of these factors, but they have first to be identified on the basis of thorough field studies.

For the second category, FAP-24 approach can help understanding sufficiently the mechanisms on the basis of a sound interpretation of the field investigations.

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References

ANONYMOUS - 1986, Dimensies van beddingvormen onder perrnanente stromingsomstandigheden bij hoog sediment transport (Dimensions· of bedforms under steady flow conditions with high sediment transport rates), report M2130, Delft Hydraulics.

M. LUKANDA - J.J. PETERS - P. CORNET - P. SW ARTENBROECKX -1992, Applicability of sediment transport theories to large sand-bed rivers, Proceedings of the Fifth International Symposium on River Management, Karlsruhe, FRG, 6.- 10.04.1992.

R.H. MEADE - 1985, Suspended sediment in the Amazon River and its tributaries in Brazil during 1982-84, U.S. Geological Survey, Open-File Report 85-492, 39 p.

R.H. MEADE - OTHERS - 1979, Suspended sediment and velocity data, Amazon River and its major tributaries, June-July 1976 and May-June 1977, U.S. Geological Survey Open-File Report 79-515, 42 p.

R.H. MEADE - H.H. STEVENS JR. - 1990, Strategies and equipment for sampling suspended sediment and associated toxic chemicals in large rivers with emphasis on the Mississippi River, The Science and the Total Environment, no. 97/98, 125-135.

R.H. MEADE- T.R. YUZKY- T.J. DAY- 1990, Movement and storage of · sediment in rivers of the United States and Canada. In: Wolman, M.G. and

Riggs, H.C., eds., Surface Water Hydrology: Boulder, Colorado, Geological Society of America, The Geology of North America, Vol. 0-1, pages 225-280.

J.D. MILLIMAN- RH. MEADE- 1983, World-wide delivery of river sediment to the oceans, Journal of Geology 1.

C.F. NORDIN - C. C. CRANSTON - M. B. ABEL - 1983, New technology for measuring water and suspended-sediment discharge of large rivers, Proceedings of the Second International Symposium on River Sedimentation, Nanjing, China, Water Resources and Electric Power Press, Beijing, 1145-1158.

C.F. NORDIN- J.V. SKINNER- 1977, Sediment-sampling equipment for deep fast currents, Proceedings 17th IAHR Congress, Vol. 6, 606-609.

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J.J.PETERS- 1971, La dynamique de la sedimentation de la region divagaute du bief maritime du fleuve Congo, (Dynamics of sedimentation in the braided zone of the Congo river maritime reach), Award 1971 of the Royal Belgian Academy for Overseas Sciences, Published by the Hydraulic Research Laboratory, Ministry of Public Works, Report "Mateba 7": 1-120~ 109 figs.

J.J. PETERS - 1977, Sediment transport phenomena in the Zaire river - in "Bottom Turbulence". Proceedings of the 8th International Liege Colloquium on Ocean Hydrodynamics, Elsevier Oceanography Series, 19: 221-236.

J.J. PETERS - 1978, Discharge and sand transport in the braided zone of the Zaire estuary, Netherlands Journal of Sea Research, 12: 273-292.

J.J. PETERS - 1981, Water and sediment gauging of the Zaire (Congo), 19th Congress of the International Association for Hydraulic Research, New Delhi: 2: 173-182.

J.J. PETERS - 1988, Etudes recentes de la navigabilte: la methode des dragages diriges, (Recent studies about navigability: a method to improve dredging by prediction of morphologic evolutions), Workshop "Maritime access to Zaire", Belgian Royal Academy of Overseas Sciences (ARSOM), Brussels, 5 December 1986, 89-110.

J.J.PETERS - 1990, Scaling of sediment transport phenomena in large alluvial rivers with very low slopes - Proceedings NATO Workshop on Movable Bed Physical Models, Delft (De Voorst) - The Netherlands, August 18-21, 1987, edited by Hsieh Wen Shen, Kluwer Academic Publishers, NATO ASI Series, Series C: Mathematical and Physical Sciences- Vol. 312: 149-158.

J.J. PETERS - A. GOLDBERG - 1989, Flow data in large alluvial channels, International Conference on Interaction of Computational Methods and Measurements in Hydraulics and Hydrology- HYDROCOMP '89, Dubrovnik. Yugoslavia, 13-16 June 1989, Proceedings published by Elsevier's Applied Sciences, 1989: 77-86.

J.J. PETERS - A. STERLING - 1978, Observations hydrologiques dans le bief maritime du fleuve Zaire (Hydrologic observations in the Maritime reach of the Zaire river), Communication at the mini-symposium "Experience of the members concerning projects of technical cooperation in hydrology", Commission Hydrology ofW.M.O., Sixth session, Madrid: 8 pages and 5 fig.

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J.J. PETERS - F. WENS - 1991, Maintenance dredging in the navigation channels in the Zaire inner delta, Proceedings COPEDEC Ill Conference, Mombassa, 16-20/09/1991 .

J.E. RICHEY- R.H. MEADE- E. SALATI- A.H. DEVOL- C.F. NORDIN JR. U. DOS SANTOS - 1986, Water Discharge and Suspended Sediment Concentrations in the Amazon River: 1982-1984, Water Resources Research, Vol. 22, No. 5, pages 756-764.

D.B. SIMONS- P.F. LAGASSE- Y.H. CHEN- S.A. SCHUMM- 1975, The River Environment, A Reference Document prepared for the US Department of the Interior, Fish and Wildlife Service, Twin Cities, Minnesota, December 1975.

C.R. THORNE- A.P.G. RUSSELL- M.K. ALAM- 1993, Planform pattern and channel evolution of the Brahmaputra River, Bangladesh. From: Best, J.L. & Bristow, C.S., eels., Braided Rivers, Geological Society, Special Publication No. 75, pages 257-276.

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Zaire River - Mateba Amont Detail - Region "Oiseaux" -1973.. Rock formation river bed. Original Scale: 1/5000 (Reduded)

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Zaire River - Mateba Amont Detail - Region "Oiseaux" -1973. Rock formation in river bed. Original Scale: 1/1000 (Reduced)

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Syledis Automatic Sounding - Navigation Channel Mateba Amont - Region "Oiseaux" - 1992. Original Scale: 1110,000 (Reduced).

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:ba 00


NORMAL Gauge 2

Datum

L 6X

WITH "OVERLOADING"

Figure Sa:

h1 ~~ <Y-~

&~ ~,o .s .Stit. ~ vo

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Influence of an "obstruction" on the local slope of the water surface line. The "obstruction" may be due to various causes e.g. local sediment overloading, lateral channel squeezing.

355

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Technical Session 7

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Figure 8b: Use of flow discharge distribution among channels to identify morphological evolutions. In this example, a cut-off channel developed in several phases between 1981 and 1987. The · scatter in flow data is not only due to gauging uncertainty but also to morphological variability.

356

tify tnel rhe hut

Figure 8c:


UPPER STRETCH h 2-.-------------------------

1 (m.)

15/96

0 -l----+----1--- -+-----+-----+-----+-----l

0 2 4 6 8 10 12 14

LOWER STRETCH 2~-----------------~----------,

15/66

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2

ENTIRE CHANNEL (UPPER + LOWER STRETCH)

N' Period 1/65 1/1 - 15/1

5/65 2/3 . 16/3 10/65 16/5 - 30/5 15/65 3Cl/7 - 13/6 7#)/65 15/10 - 29/1 ?~165 17/12 - 31/12 1/66 1/1 - 30/1

5/66 213 - 16/3 10/66 16/5 - 30/5 15/66 30/7 - 13/6

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---•.. "SLOPE" .(cm/km)

Variation of the slopes (in cm/km or 10-5) with stage in a cut

off channel. Three water level gauges were installed in the channel by which the "lower" and '\lpper" stretches are defined.

357

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Hydrographic chart with surface float tracks (chart usually coloured for better revealing the bed topography).

358

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Hydrographic chart with the lines of equal surface velocities, deduced from the float tracks and usually in different colours, "lso-vel' lines interpreted on their shape~ used m morphological prediction maps.

359

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Figure 10: Bedforms evolving during the flood of 1968-1969 in two main channels of the braided area (Peters, 1977).

360

(J) (!) (J)

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nain

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,...,....,....,...,_., .... _V **

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Evolution of the bedforms in the reach ''Seuil Mpuasi" from November 1972 till May 1973 (Mateba 14, 1977).

361

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Technical Session 7

Figure 12:

EeEEE .0 (I) 10....- 0

I ,

(Cl

E 1-------~ U)

~ E]. ;~

----· ~-

Cl.> (Cl 0 (/) (/) <1>

c :=J rou

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u <1>

...0

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(Cl

Q_

Localization of sampling stations and the bedforms occurring at the different places on which flow and sediment transport were measured extensively for studies (Peters and Goldberg, 1989)

362

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ing XJrt ~rg,

Figure 13:

0 0 0 ClJ >

c CO

ClJ 0.0 .... CO


•oo I

100

<O

I• 11 11 ' ,, '• t"> :>O 11 "

BANC NTUA NKULU

10- 16/12/1975

/s)

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>--+-

u 0 CV > c ..c

..c 0 E 0..

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E ..c 0'" ::::>

WIDTH: 2000 m DEPTH: 29 m

0 500 IOOOm

Results of flow and sediment measurements in the Zaire main gauging section, left channel: discharge of 58560 m3/s. Sediment load by Delft Bottle (Mateba 13, 1975).

363

,,

Technical Session 7

Figure 14:

!' 'Delft Bottle • DF2' on a oleigh

-.•· ·----~ I-

Results of sediment transport measurements in 54 stations (5 cross sections). The survey was conducted to explain an "abnormal' morphological evolution, triggered by a sand plume originating from a sand bar).

364

I

morphological studies and data needs b russets,...

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