DISCHARGE AND SAND TRANSPORTIN THE BRAIDED ZONE OF THE

ZAIRE ESTUARY

J.J.PETERS

Reprinted fromNETHERLANDS JOURNAL OF SEA RESEARCH

12 (3/4): 273-292 (1978)

E. J. BRILL, LEIDEN

Netherlands Journal of Sea Research/2 (3/4): 273-292 (1978)

DISCHARGE AND SAND TRANSPORTIN THE BRAIDED ZONE OF THE

ZAIRE ESTUARY

by

].]. PETERS(Laboratoire de Recherche I{ydraulique, Section de Chiitelet, iWinistere des Travaux Publics

de Belgique, Belgium)

CONTENTS

I. Introduction. . . . . . . . . . . . . .n. Description of the area under investigation

1. Hydrography . . .2. Tides. . . .3. Discharges . . . .4. Sediment transport.

HI. ConclusionIV. Summary .V. Resume

VI. References.

1. INTRODUCTION

273276276276278279290291291292

The Zaire (Congo) river, with a discharge ofabont 1.45 X 10'2 m' peryear, is the second largest river in the world on the basis ofannual flow.With 4700 km between Lake Tanganyika and the river mouth on theAtlantic Ocean it is also the fifth largest river on the basis of length.Its basin covers 3.7 X 10' km' and comprises a dense hydrographicnetwork, 16000 km ofit being navigable during almost the whole year(Fig. 1).

Only a schematic representation of the longitudinal profile of theriver and its tributaries eau yet be given (Fig. 2, after DEVROEY, 1951),showing very small slopes in the navigable reaches, generally less thanla cm·km- l .

The dominant characteristic of the river is the remarkable regularityof its regime due to the position of the basin (Fig. 1), one third of itnorth of the equator where the dry season occurs in January, hvo thirdsofit south ofthe equator having a more composite regime. At K-inshasa,where the river begins to drop rapidly down to Matadi throngh thereach of the cataracts (Fig. 3), the minimum and maximum observeddischarges are respectively 23 000 m 3 ·s· l and 80000 m 3 's-1 with anover-all average ofabout 42 000 m"' ,-1. At Inga,just upstream Matadiat a place where the river drops 100 m over a distance of approxi-

274 J. J. PETERS

u'ig. 1. Hydrographic network of the Zaire river.

Okml ,! RW !

IOQGkm

\500

1000

500

Om

Fig. 2. Longitudinal profile of the Zaire river and its principal tributaries. Navigablereaches are connected by railways (RW).

DISCHARGE AND SAND TRANSPORT 275

mately 25 km, this large minimum discharge makes the installation ofworlds largest hydroelectric power plant without storage basin possible.

Fig. 3. The Zaire river downstream Kinshasa.

The maritime reach of the river (Fig. 4), downstream of Matadi, canbe divided into three parts.

SALT WEDGE ESTUARY BRAIDED AREA

Fig. 4. The maritime reach of the Zaire river.

In the first part, extending from Matadi to Iloma over about 60 km,the river cuts through the Crystal Mountains with high flow velocities(up to 6.5 n1-8-1) and large depths.

The second part between Boma and Nlalcla, is a sedimentation area,60 km long and 19 km wide, where shoals and islands divide numerouschannels. Only a few of these channels arc navigable for sea ships, andthe sometimes very quick evolutions require frequent dredging anddisplacements of navigation buoys.

The third part, between Nlalela and the river mouth near Banana,is the estuarine area characterized by the presence of a deep canyonwhich begins near Malela, dropping abruptly to 100 m depth.

276 J. J. PETERS

The aim of this paper is to present the hydraulic and sedimentologiecharacteristics of the meandering area immediately upstream the sub­marine canyoD. Since 1967, a research project of the Belgian StateHydraulic Laboratory is involved with a study of the improvement ofthe navigation in this area. Physical model studies and field investi­gations have provided information about discharges and sedimenttransport. A method for prediction of the evolution of the meanderswas developed. In this way, dredging operations could be reduced,becoming 1110re efficient. Field measurements were performed in closecooperation between the Belgian State Hydraulic Laboratory and theRegie des Voies Maritimes of the Republic of Zaire.

IT. DESCRIPTION OF THE AREA UNDER INVESTIGATION

1. HYDROGRAPHY

The first hydrographie chart of the meandering zone of the estuary wasmade by Commander Purey-Cust with HMS Rambler in 1899(DEVROEY, 1951). Since 1927 the whole zone is mapped regularly-al­most once a year-by the local hydrographic service.

For navigation or dredging purposes, detailed maps of some areaswere made up to twenty times a year, mostly on scales of I : IQ 000 andI : 50 000. This unique documentation is present at the Regie desVoies Maritimes in Boma. Topographic surveys on either Angolese orZairian territory, performed since 1915 and completed between 1968and 1975, provide accurate topographic data allowing comparison ofthe hydrographic data. A study of the general evolution over the last75 years ofthe area under investigation is almost finished at the BelgianState Hydraulic Laboratory. The evolution between 1932 and 1974 isshown in Fig. 5. For some pools the evolutions were follmycd vvithyearly data over a period of 50 years.

2. TIDES

The first tidal data were collected in 1923-1924 (DEVROEY, 1951). Thereference tidal gauge 'Nas first installed at Banana, and stands since1955 at Bulabemba, 4.8 km upstream of Banana. Even at Banana, onthe coastline, the level is influenced by the river discharge. The maxi­mum and minimum amplitudes were 1.82 m and 0.42 m in 1976. Thepropagation of the tidal 'Nave in the estuary is approximately kno\vn asa function of tidal range and river discharge. Further upstream theinfluence of meander evolution on the tidal propagation is evident, butnot well known. At Boma the tidal range was for example ca. 0.00 m

DISCHARGE AND SAND TRANSPORT 277

Fig. 5. The braided area between Boma and 1vlalc1a in 1932 and 1974 (isobathsof 0, 5, 8 and 10 m).

278 J. J. PETERS

at neap tide with rnaxinlum discharge, and ca. 0.17 ill at spring tidewith miniulum river discharge in 1976. Further information about thetides is given by DEVROEY (1951). Other documentation is available atthe Regie des Voies Maritimes (Boma) and at the Belgian State Hy­draulic Laboratory.

3. DISCHARGES

Building up the relationship between water levels and discharges nearthe mouth of the river needed the gathering of all measurements evermade in the maritime reach or at Kinshasa. Between Kinshasa and themaritime reach, the discharge of the tributaries represents only a fewpereent~mostly 2%-of the total river discharge (VAN GANSE, 1959).

For the maritinlC reach, interrupted series of discharge mcasure­lllcnts arc available [rmu 1927 up to now, and almost continuous timeseries of water level lllcasurements were recorded at :Nlatadi since 1909and at Boma since 1914. Besides, similar data were collected at Kin­shasa since 1955 for the discharges and since 1902 for the water levels.

The number of measurements increased frolll 1967 when morcknowledge of the discharges and discharge distributions in the mean­dering zone was required for the study of the improvement of thenavigability of the maritime reach.

Actually all these data are analysed and an almost continuous time­series of daily discharges of the Zaire will be computed for the last75 years and published by the Belgian State Hydraulic Laboratory.

An approximate relationship between water level and discharge isgiven in Fig. 6 for the gauging stations of Kinshasa and Boma.

Because of the almost stationary flow conditions this relation isquite univocal, particularly for Kinshasa. For Boma an influence ofvarying head losses due to the bank and bed configuration can benoticed, chiefly when discharges exceed 40 000 1113 • S-1. 1tleasurementsof discharges by the velocity-area method with propeller currentlueters provide uscfull information on the degree of obstruction by thesedimcnts in the navigation channeL

The distribution of the discharge among the diflerent channels(Fig. 7) is measured regularly, and is an indicator for the changes inmorphology of the meander system. These discharges, in about 15cross-sections, have to be collected in a very short period to eliminate asmnch as possible errors due to tidal influence. In the past the surfaee­float method was used. Since 1969, the moving-boat method allowedquick determinations of the discharges but although the method issimple, the possible errors are numerous, and the technique had to beimproved for its application to this complex meandering river system.

DISCHARGE AND SAND TRANSPORT

WATER LEVEL

{ml PERIOD1971- 1975

279

O'--_--"-__-'-.fL_--'__J-__'--_--"-__-'-_--'

o 10.000 20.000 30.000 40.000 50.000 60.000 70,000 (m'/s)WATER DISCHARGE AT NTUA NKULU

Fig. 6. River discharge (Qin m3's~1) at the gauging section of Ntua~Nkulu as afunction of water level (It in m) observed in Kinshasa and Boma, based on measure­ments in the period 1971-1975. The discharge at Ntua-Nkulu represents 86% of

the total discharge of the Zaire river.

4. SEDHIENT TRANSPORT

Sediments entering the braided area range from pebbles to clay.Selective sedimentation occurs along the channels dovITllstream and bed

Fig. 7. Discharge distribution in the different branches of the braided area betweenBoma and Malela in 1969-1970. The discharges measured in the main channels

are given in % of the total river discharge.

280 J. J. PETERS

sediment grain sizes drop regularly fi'om I mm to 0,3 mm over adistance of 40 km (Fig. 8). Only the smaller particles reach the zone ofthe submarine canyon.

d50(mm)2.0 rr:=:=:==c=,--------------------,

IIN CHANNEL

SOUTHERN CHANNEL

+ ................. NORTHERN CHANNEL

t 1.0

'"~zw~

0w

'":>;0~

i!-++ + ++~

0 •ro

20 20DISTANCE FROM NTUA.NKUll-

Fig. 8. :Nledian grain size (d.Jo in mm) of the sediment of channels and shoals alongSouthern and Northern Channels of the braided area.

Large differences between the sediment grain sizes in and betweencross-sections could be noticed. They have probably several reasons,but chiefly secondary currents iuduced by the bed morphology. Forexample, fixed parts of the bed-clay or rock-create at some places

DISCHARGE AND SAND TRANSPORT 281

bends or bifurcations vvhere secondary currents such as helical currentstransport bed and suspended load in different directions.

BED AND CHANNEL MORPHOLOGY

Bedforms are generally large scale dunes; their wavelength and ampli­tude average respectively lOO m and 2 m. They move at a velocityranging from 2 to 10 m a day. At high river discharges and for sedimentgrain sizes slnaller than 0.5 mm, these dunes arc flattened and otherbcdforms appear, similar to small scale dunes. Their wavelengths andamplitudes average then respectively 20 m and I m. The characteris­tics and the behaviour of these bedforms are poorly known.

They exist on shoals as well as in deeper channels and the correspond­ing sediment transport is always high. During a high flood in 196B,small scale bedforms were developping in the Northern Channel, wherethe median grain size was smaller than 0.5 mm (Fig. 9), while in theSouthern Channel \vhere the median grain size vvas larger than0.5 mlTI the large scale bedforms remained. ])lotting the mean powerof the How per unit area versus sediment grain size, it can be seen thatthe small scale bcdforms develop in the Northern Channel at con­ditions of upper flow regime, while in the Southern Channel con­ditions oflower flow regime still exist (Fig. 10). The Froude numberof the How averages 0.1, and the accepted classification of bedformswould suggest a plane bed (SIMONS & RICHARDSON, 1966).

1Vleanders move sometimes quickly; concave banks in bends mayerode at a rate of 100 m per year, or even more. Many rocks and rockybars influence strongly the ll1candering of the different channels in thebraided area, and therefore analysis of the ll1eandcr characteristics isnot very meaningfull. Length, meander belt width, meander radiusand channel width average respectively 12 km, 3 km, 3 km and 1.5 km.The interconnection between the different branches of the braidedarea, ,,,,here scdiments have different sizes and move at differentspeeds, complicate the analysis and the prediction of the evolution ofthe meanders.

SEDIMENT TRANSPORT MEASUREMENTS

As the goal of the investigations \vas the improvClnent of the navigationby dredging and, eventually, with the aid of hydraulic structures,measurements ,vere chiefly carried out in relation to the transfor­mations of the bed morphology, i.e. near to the bed.

Sediment moving close to the bed consists almost wholly offine sand,containing small percentages of clay, and sediment transport rates are

c

282 J. J. PETERS

A

___L,_~~-:t:-

5 ----.-----~

~~~~~~~~~:ff6~i~~-~~i~~~\.:: WOrn

5 I 5

, ~.1~+~I.W..-.R!..i.:rr''''''

A

Fig. 9. Upper three recordings: bcdforms in the Northern Channel at r.rfatebaduring the flood of 1968: A. September 1968, 41 500 m 3 's-1 ; B. November 1968,51000 m 3 's-1 ; C.January 1969, 58 000 m 3 ·s-1 • Lower three recordings: bcdformsin the Southern Channel at Nisot (Kindu) during the flood of 1968: A. October 1968,45000 m 3 's-1 ; B. November 1968, 51 000 m 3 . 8-1 ; C. January 1969, 58 000 m 3 • 5-1 •

Depths in m.

DISCHARGE AND SAND TRANSPORT 283

,-' "ppees

low. For these reasons, continuous samplers were preferred to instan­taneous ones.

I .... , :

,~,

I /'b~,c>"%tJj

~ jj,'" 1'iiiij~,~~~~I71r~r~'qL-"~'~:'~8'NO SEDIMENT MOTION+-+

IJ 1 1+IJ flTTn±t0,3 0;; qs 0,6 0,7 0.8 0.9 1,0

d50 Imm!

Fig. 10. Relation between stream pmver (,·u in kg·m-Ls-1) and median grainsize of the sediment; indicated are the positions of the bedforms represented in(A, B, C) and at maximum discharge (D) even as in Fig. 9. Classifications of bed­forms by SIMONS & RICHARDSON (1966) (solid lines) and by GUY, SIMONS &

RICHARDSON (1966) and ALLEN (1968).

Two types of instruments were used, the Delft Bottle (D.F.), and theBedload Transport Meter Arnhem (B.T.M.A.), both developed in theNetherlands. The Delft Bottle was used in two versions. For measure­ments elose to the bed, it was mounted on a sleigh (D.F. 2). The inlet,having a diameter of 0.015 m or 0.022 m depending on the velocity ofthe water, could be positioned at 0.05 m, 0.15 m, 0.25 m, 0.35 m abovethe bottom. For the rest of the water column, i.e. from 0.40 m abovethe bottom to the water surface, an integrated sample was taken witha suspended Delft Bottle (D.F. 1).

The B.T.M.A. sampler, sometimes callcd the Dutch sampler, sam­ples a 0.05 m thick layer on the bottom, the sediment being retainedby a sieve. Only the sand fraction of the sediment is sampled in the

284 J. J. PETERS

instrument. Grain size of the samples \vas detcrn1ined with the visualaccumulation tube (COLEY & CHRISTENSEN, 1956).

Special attention was paid to the positioning of the D.F. 2 andB.T.M.A. samplers on the bedforms using eehosoundings.

At each station the velocity profile was measured and the bottomshear velocity computed. To understand the sediment transport andits distribution the variation in bed load and suspended load dischargeat constant mean velocity (Fig. ll) were investigated, as well as the

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DISCHARGE AND SAND TRANSPORT 285

distribution of transport rates near the bottom and the distribution ofsediment transport in a cross-section (Fig. 12) and the influence of thetides on flow and sand transport (Fig. 13).

Besides the measurements an attempt was made to calculate bedload and suspended load using Bagnold's approach (BAGNOLD, 1966).The method was modified in order to provide results in an isolatedstation, using the power of the flow as the product of the mean velocityand bottom shear stress deduced from the vertical velocity distribution.

Sediment sizes used in the calculations were hourly weighted aver­aged values. In Fig. 11 the results of the measurements and calculationsare shown for a single station, sampled during a 7 hours period nearBoma in November 1973 with high river discharges. The tidal ampli­tude at the sampling station was 0.03 m and the mean velocity (1.3m's-1) did not vary significantly. Bed load measurements withB.T.M.A. on the bottom and D.F. 2 at 0.05 m above the bottom showa very erratic variation for the instantaneous as well as for the hourlyaverage values, but less so for the D.F. 2. Although the order ofmagnitude was the same, suspended load measured with sampler D.F.1 varied regularly, higher values being observed before low water.

The sediment transport rates measured with the samplcrs and thesediment transport capacities computed with the modified Bagnold'sapproach change during the 7 hours observation period in a quitesimilar way. Although these data do not have the same meaning, theratio between them is jnst given as an indicator (Fig. 11). The observed

Fig. 11. rVfeasured and calculated flow and sand transport characteristics at fixedsampling station near Boma from 4~ hours before to 2 hours after low water, 1973;mean current velocity almost constant. a. Tidal level, T.L. in m. b. Sedimentcharacteristics used in calculations with BAGNOLD'S approach: median grain sizeof bed load (d50c in mm) and median fall velocity of suspended load (Ws in m' s-t).c, Sediment transport rates per unit width, q~ (m3 . m-t. d-t ); thc curves allowcomparison o[ sand transport data sampled at 0.05 m from the bottom with twoidentical Delft: Bottles D.F.2 (a and b), i(D.F.2a -1- D.F.2b), and in the whole watercolumn with Delft bottles D.F.2 (a and b) and D.F.l, l(D.F.2a + D.F.2b) +D.F.I, with total load, b, and bed load, b', calculated with modified BAGNOLD'Sapproach; measured data are hourly averaged (rvLH.). d. Ratios between sedimenttransport rates calculated with BAGNOLD'S approach and mcasured with D.F.samplers, R t = b(D.F.2 + D.F.l)-t [or total load and Rc = b'(D.F.2)-t for bedload. e. Ratios between mean and surface current velocity, Ulll!US, and betweenbottom and mean current velocity, ur/urn. f. Variation of shear litress, -; (kg'm-Z),

and mean current velocity, Urn (m· S-1). g. :0,.·Iedian grain size (d50 in mm) of samplestaken by two identical D,F.2 samplers (a and b) and by D.r.1 and B.T.NLA.samplers. h. Variation of suspended load sampled by hourly integration (V.H.)from surface to 0.40 m from the bottom with D.F.l sampler, qs in m 3 ·m-l.s-1.i. j. and k. Instantaneous value and hourly average (rvLH.) of load sampled at0.05 m from the bottom with D.F.2 samplers and betwecn 0 and 0.05 m from the

bottom with B.T.:0,.'LA. sampler, qs in m S ' m-l. s-1,

286 J, J, PETERS

W"r;==SS~----L::[iJm J lm24hi i b

M I:;::\: 1\ Jroc

'

.I I !rVi I.I I If \J-CF1 """I V,i \

II. / " .. DF2 j'/"\\.

) /' ,,/ ,/' \\£:.:.-. __..-.f v \,

Fig. 12. Distribution of measured and calculated flow and sand transport charac­teristics in the gauging section of Ntua-Nkulu, November 1973. a. 1vlcdian grainsize (diJO in mm) of samples taken by D.F.2 and D.F.l samplers. b. rvIeasured sandtransport rates sampled with D.F.2 and D.F.l samplers, qs in m3>m~Ls~1, forD.F.2 integrated from bottom to OAO m off it, n.F.! integrated from surface toOAO ill from bottom, and D.F.1 + D.F.2. c. FIm'\' characteristics: mean currentvelocity, Um (m's-1), and discharge per unit 'Nidth~ q = um'h (m3 'm-1 ·s-1).

d. Cross-section \'vith depths, It (m), and isotaches1 It (m·s~l). e. Sediment charac­teristics used in calculations ,vith R"'GNOLV'S approach: median grain size of bedload (d50C in mm) and median fall velocity of suspended load UVs in m' s-l). f. Com­parison of measured transport rates with samplers D.F.2 at 0.05 m from bottom,D.F.2 from bottom to 0.40 m from bottom and D.F.2 + D.F.l from bottom tosurface with sediment transport rates calculated with R\GNOLD'S approach for sus­pended load, h', and total load, h; q5 in m 3 ·m~l·s~l. g. Ratios behvccn measuredand calculated transport rates (indicated in L); Rt = h(D.F.2 -;- D.F.1)-l for totalload and Rc = b' (D.F.2)-1 (or bed load. h. Cross-section ,vith depths h in m.

DISCHARGE AND SAND TRANSPORT 287

changes seem to be cl.osely Telatcd to the varying bottom shear stTCSS.Distributions of transport Tates measured near the bed generally

indicate an intense sediment transport in a layer ofa few ccntimcters toa fe",\, dccimeters above the bottom. It is cliHicult to make a distinctionbetween bed load and suspended load, but probably most of the solidparticles in this layer that contribute to the displacemcnt of tbe bed­forms, are 1110ving by saltation r,lthcr than in suspension.

The distribution of sediment transport was studied in several cross­sections during diHtTcnt conditions of Ho..,,/" and tide. An exanlplc offield data and calculations is given (Fig. 12) for the control section ofI'\tua-Nkulu \'\'hcre the river discharge represents 860/0 of the total fresh"vater flmv through the estuary (Fig. 4). 'The plume of suspendedparticles observed near to the left bank is due to the presence of a rock,iEducing intense secondary currents. Also the remarkable distributionof sediment sizes in the 1800 m \'1'ide cross-section is due to morpho­logical factors and an analysis ofscdiIllent transport distribution shouldtake this factor into account. The agreement betvveen sand transportrates measured \vith samplers D.F. 2 and D.l;". 1, and calculated sandtransport capacities is satisfactory, except for stations 11, 7 and 6because of the secondary CU1Tents just nlentioned. Using Bagnold'sapproach \\'ith the mean characteristics of flow and sediments, thecalculated total transport capacity amounts to 24·5 000 IUS. d~l insteadof 1it5 000 m 3 . cl-I \vhich is obtained when the data of each station areused separately while sand transport nlcasured \v1th the D.F. smuplersamounts to 82 000 nl:3.d- 1.

In this cross-section the influence of the tides 011 sedilTIcnt transportcan be neglected. The tidal influence increases do\vnstrearn and anexanlp1c of this influence on fIa\v and sediluent transport is given inFig. 13. "rhe measurements \vere performed 011 4- stations distributedin a cross-section located approximately 5 klu upstreanl the head of thesubnlarine canyon. The influence of the tidcs on flow and sedimenttransport distribution is clear: rnaximurll sand transport occurs 3 11 to2 h before lmv watcr or immediately after maximum flmv intensity.

An attempt \vas nlade to quantif"y the sediment inflm..v into the mainpart ofthc lYlcandering arca through thc control section ofNtua-Nkulu.All available data oftbe period 1971-1975 are plotted in Fig. 14 versusthe discharge through this section. Sand transport measurementssampled bet\veen bottom and surface with the Delft Bottles are com­pared \vith the sand transport capacities calculated either by themodilied Bagnold's approach using local 1I0w data or by Bagnold'sapproach using cross-sectional average flO'N and sedin1ent characteris­tics. The scattering of the sand transport measurcnlents for low dis­charges is chiefly dne to variations in suspended load. It corresponds

288 J. J. PETERS

-

OL..-c-c--c-c--c-c--c-+------------'d"I I I I I I (j) I I I I I I I I

1:~~~~~ .__~.~~~~~.~=..'::::~.=_=,..~/o.r=;::,-~..:::: ..~~~01 ciJ dJ) ·_~+ ~e

50 ,.-Iq! "<.loP [mYmsJ I/,,--

,0 ......30[-... --_........

"I­"I-

- @ --- _........ '- =-----o 9

Fig. 13. Influence of tides on flow and sand transport rates in a cross-sectionlocated 5 km upstream of the head of the canyon; the 4 stations (encircled) weresampled on 5 successive days and all data gathered in one figure with time of lowwater (L.'i,J\l,) as reference. a. Envelope of tidalleve1s, T.L. in m. b. and c. Sandtransport rates, qs in m 3 'm-L s-1, measured with D.F.l sampler between surfaceand 0.40 ill from bottom (D.F.l), and with D.F.2 sampler from bottom to 0.40 ill

from bottom (D.F.2 total). d. and e. Variation of ratios between mean and surfacecurrent velocity, urn/us, and between bottom and mean current velocity, uf/um. f.Discharge per unit width, q = Urn' h in m 3 , m-l.s-1• g. Mean current velocity,

Urn in m·s-1 •

DISCHARGE AND SAND TRANSPORT 289

probably to long term eflects in the adaption of the thalweg to newflow conditions.

10'

10'

20 sp 6pQ max

7,0 80TOTAL

DISCHARGE

Fig. 14. Measured and calculated sand transport rates (in m3'd-1) as a function ofriver discharge (m3's-1) in the Ntua-Nkulu gauging section. Total sedimenttransport rates calculated on the basis of data of GUY, SllIIONS & RICHARDSON (1966)for sediment sizes of 0.3 mm, using cross-sectional average flow data (..); sandtransport capacities calculated with BAGNOLD'S approach (1966) using cross-sectionalaverage flow data (D); sand transport capacities calculated with BAGNOLD'Sapproach (1966) using local flow data (11) j sand transport rates measured withDe1ft Bottles samplers (*). Data were collected from 1971 to 1975 for almost thewhole range of discharges between the observed minimum and maximum during

the last 75 years (indicated by shaded bars).

Sand transport capacities calcnlated with the modified Bagnold'sapproach correspond ronghly with the measured transport rates. InJnly 1973 measnrements were performed at very low river dischargesclose to the minimum ever observed and the calculated sand transportrate had an order of magnitude of 104 m' sand per day. In December1975 the river discharge amounting approximately 85% of the maxi­mum ever observed, the calculated sand transport rate reached anorder of magnitude of2 X 10 5 m 3 ·d- I •

Transport capacities calculated with Bagnold's approach using eross-

290 J. J. PETERS

sectional average {low characteristics, give larger values. This is prob­ably due to the heterogeneity of flO\\' and sediment distribution in thecross-section (Fig. 12). Using data of GUY, SIMONS & RWI-IARDSON

(1966) for sediment sizes of 0.3 mm, the calculated values range from3 X 104 to 10' mS'd- I

-Using the river discharge relationship (Fig. 6), the recorded daily,·vater levels since 1902 and the sand transport rates (Fig. , an esti­mate can be made based on the hypothesis that these transport ratesrClnained the saBle during the last 75 years. This vlOrk is not J'et com­pleted, but the order of magnitude of the sand transport at the entranceof the Ineandering area "';,vil1 be 50 X 106 ID:}' a -1 \vhich corresponds toan average concentration of 40 g sand per m 8 ,"vater.

The sediment input in the llleandcring area of the maritillle Teach iscontrolled the magnitude and the intensity of the successi,'c floods.An intense one may introduce in the Ineandering area a large al110untof sand. This 'vi/ill have a strong influence on the change of the lllean­ders, but because of the slovy' movement of the sands on the bed along lasting effect can be the result. So the influence of the varyinghydrological cycles ",vill be less marked at the head of the canyon, andthere, sand input \-vill be more conditioned by channel evolution.

Ill. CONCLUSION

The geomorphological configuration of the 150 kIn long maritimereach determines a sedimentation area in the coastal plain zone dmvn­strean1 the harbour of Boma. In this 60 km long sedimentation areaending at the head of the canyon, 30 km upstream the Tiver Illouth, anintricated channel systen1 evolves continuously when meanders llloveunder influence of varying fresh water and sediment inflmv, deter­D1ined hy the hydrological cycles, Although these arc stable, successivefloods of different intellsities ,,yill annually introduce different an10untsof sand. The slow rate of transport of the sand is responsible for oc­casionnal and local obstructions of channels. The journey of the sandthrough the braided sedimentation area takes about 20 ")'cars, Channelobstructions \vill be determined by the difference behvcen the amountof sand brought to a certain location and the sediment transport ratedetermined at the mon1ent by the river P0\-VCL

'The fresh "'ivater flow of the Zaire river into the estuary is very stableand amounts to 1.45 X 1012 m3'a~'1 The sand transport rate throughthe braided sedimentation area to the cany-ol1 has an average of50 X 1013 m 3 ·a-1• Ratios between observed extreme dayly river dis­charges and sand transport rates have an order of n1agnitucle of re­spectively 3 and 15.

DISCHARGE AND SAND TRANSPORT

IV. SUMMARY

291

The sedimentation zone of the nlaritime reach of the Zaire riverlocated upstream the cauyon is described.

The main characteristics of ""vater and sand lllovements are indi­cated: stable regime and large discharges ranging from 23 000 m 3 's-1

to 80 000 m 3·C'

; low tides at the mouth with an average amplitude of0.80 m, quickly damped upstream; sand of the bed with grain sizesdropping dmvnstream from 1 mm to 0.2 mm, moving generally asdunes with a speed of 2 m to 10 m per day. The apparition duringfloods of different and smaller scale bedforms was observed.

Measuring and calculation techniques arc briefly described. Someresults show the influence of geomorphological factors on sand trans­port phenonlena, the erratic variations in time and space ofsand trans­port rates and the influence of tidal movements on these. The role ofsecondary currents on the distribution of sand transport in and be­t\veen channels is emphasized.

Volumes of water and sand evacuated annually through the braidedarea arc respectively estimated at 1.45 X 10'2 and 50 X 106 m S.

V. RESUME

11 s'agit de la description de la zone sedimentaire de la partie maritimeciu fleuvc ZaIre situee a l'mllont du canon.

Les caracteristiques principales des mouvements des eaux et dessables 5011t indiquees: stabilite du regime et debits Cleves variant entre23000 m 3 's-1 et 80000 m 3 ·s··

'; marees faibles it l'embouchure de

0,80 m d'amplitude moyenne, rapideIllent amorties vel'S l'amont; surle fond sables d'une granulometrie decroissant de l'alnont vel'S l'avalde 1 mnl a 0,2 nlm, en mouveIllent generalenlent sous la forme dedunes avan<;ant a la vitesse de 2 nl a 10 In par jour. L'apparitionde formes topographiquc5 tres particulieres en temps de crue estsignalee.

Les techniques de mesurc et de calcul des transports de sable sontsommairement decrites. Quelques resultats illustrcnt l'influence de fac~

tcurs gcomorphologiques sur lcs phenomenes de transport de sablc, lesvariations souvent erratiques de ces transports dans le temps et l'espace,de mcmc que l'influencc des mouvements de maree. Le role des cou­rants secondaires sur la distribution des transports de sable entre et al'intCrieur des differents chenaux est souligne.

Les volumes d'eau et de sable evacues annucllement par la regiondivagante sont estimes rcspectivement a 1,45'10 12 m 3 et 50.10 6 m 3•

292 J. J. PETERS

VI. REFERENCES

ALLEN,]. R. L., 1968. Current ripples. North Holland Pub!. Camp.: 1-433BAGNOLD, R. A., 1966. An approach to the sediment transport problem from general

physics.-ProL Pap. V,S. geol. Surv. 422-1: 1-37.COLBY, B. C. & R. P. CHRISTENSEN, 1956. Visual accumulation tube for size analysis

ofsands.-J. Hydraul. Div. HY3 Am. Soc. dv. Engers, Paper 1004: 1-17.DEVROEY, E.]., 1951. Notice de la carte des caux superficiclles du Congo BeIge et

du Ruanda-Urundi.-Publs Corn. hydrogr. Bassin congo!. Publ. 2.GANSE, R. VAN, 1959. Les debits du fleuve Congo a Lcopoldville et a Iuga. :~v1emoires

de !'Institut de l'Academic Royale des Sciences Calouiales, Bruxellcs. Classedes Sciences et Techniques, N.S., T.V., Fasc. 3: 737-763.

GUY, H. P., D. B. SIMONS & E. V. RICHARDSON, 1966. Summary of alluvial channeldata from flume experiments, 1956-61.-Prof. Pap. D.S. geol. Surv. 462-1:1-96.

PETERS,J. J. & A. STERLING, 1968-1977. Rapports d'activite :NIateba 1 a lVIateba 14dans le cadre de l'e-tude de l'amclioration de la navigabilite du bief maritimedu flcuve ZaIre. Laboratoire de recherche hydraulique, Borgerhout, Anvers.

SIMONS, D. B. & E. V. RICHARDSON, 1966. Resistance to flow in alluvial channels.-Prof. Pap. D.S. geo!. Surv. 422-J: 1-61.