DISCHARGE AND SAND TRANSPORT
IN THE BRAIDED ZONE OF THE
ZAIRE ESTUARY
J.J.PETERS
Reprinted from
NETHERLANDS 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 TRANSPORT
IN 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. . . . . . . . . . . . . .
Description of the area under investigation
1. Hydrography . . .
2. Tides. . . .
3. Discharges . . . .
4. Sediment transport.
HI. Conclusion
IV. Summary .
V. Resume
VI. References.
n.
273
276
276
276
278
279
290
291
291
292
1. INTRODUCTION
The Zaire (Congo) river, with a discharge ofabont 1.45 X 10 '2 m' per
year, is the second largest river in the world on the basis of annual flow.
With 4700 km between Lake Tanganyika and the river mouth on the
Atlantic 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 hydrographic
network, 16000 km ofit being navigable during almost the whole year
(Fig. 1).
Only a schematic representation of the longitudinal profile of the
river and its tributaries eau yet be given (Fig. 2, after DEVROEY, 1951),
showing very small slopes in the navigable reaches, generally less than
la cm·km- l .
The dominant characteristic of the river is the remarkable regularity
of its regime due to the position of the basin (Fig. 1), one third of it
north of the equator where the dry season occurs in January, hvo thirds
ofit south ofthe equator having a more composite regime. At K-inshasa,
where the river begins to drop rapidly down to Matadi throngh the
reach of the cataracts (Fig. 3), the minimum and maximum observed
discharges are respectively 23 000 m 3 ·s· l and 80000 m 3 's- 1 with an
over-all average of about 42 000 m"' ,-1. At Inga,just upstream Matadi
at 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.
\500
1000
500
Om
Okml
!
RW
,
!
IOQGkm
Fig. 2. Longitudinal profile of the Zaire river and its principal tributaries. Navigable
reaches are connected by railways (RW).
DISCHARGE AND SAND TRANSPORT
275
mately 25 km, this large minimum discharge makes the installation of
worlds 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, can
be 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 numerous
channels. Only a few of these channels arc navigable for sea ships, and
the sometimes very quick evolutions require frequent dredging and
displacements 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 canyon
which begins near Malela, dropping abruptly to 100 m depth.
276
J. J.
PETERS
The aim of this paper is to present the hydraulic and sedimentologie
characteristics of the meandering area immediately upstream the submarine canyoD. Since 1967, a research project of the Belgian State
Hydraulic Laboratory is involved with a study of the improvement of
the navigation in this area. Physical model studies and field investigations have provided information about discharges and sediment
transport. A method for prediction of the evolution of the meanders
was developed. In this way, dredging operations could be reduced,
becoming 1110re efficient. Field measurements were performed in close
cooperation between the Belgian State Hydraulic Laboratory and the
Regie 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 was
made 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 areas
were made up to twenty times a year, mostly on scales of I : IQ 000 and
I : 50 000. This unique documentation is present at the Regie des
Voies Maritimes in Boma. Topographic surveys on either Angolese or
Zairian territory, performed since 1915 and completed between 1968
and 1975, provide accurate topographic data allowing comparison of
the hydrographic data. A study of the general evolution over the last
75 years ofthe area under investigation is almost finished at the Belgian
State Hydraulic Laboratory. The evolution between 1932 and 1974 is
shown in Fig. 5. For some pools the evolutions were follmycd vvith
yearly data over a period of 50 years.
2.
TIDES
The first tidal data were collected in 1923-1924 (DEVROEY, 1951). The
reference tidal gauge 'Nas first installed at Banana, and stands since
1955 at Bulabemba, 4.8 km upstream of Banana. Even at Banana, on
the coastline, the level is influenced by the river discharge. The maximum and minimum amplitudes were 1.82 m and 0.42 m in 1976. The
propagation of the tidal 'Nave in the estuary is approximately kno\vn as
a function of tidal range and river discharge. Further upstream the
influence of meander evolution on the tidal propagation is evident, but
not 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 (isobaths
of 0, 5, 8 and 10 m).
278
J. J.
PETERS
at neap tide with rnaxinlum discharge, and ca. 0.17 ill at spring tide
with miniulum river discharge in 1976. Further information about the
tides is given by DEVROEY (1951). Other documentation is available at
the Regie des Voies Maritimes (Boma) and at the Belgian State Hydraulic Laboratory.
3.
DISCHARGES
Building up the relationship between water levels and discharges near
the mouth of the river needed the gathering of all measurements ever
made in the maritime reach or at Kinshasa. Between Kinshasa and the
maritime reach, the discharge of the tributaries represents only a few
pereent~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 time
series of water level lllcasurements were recorded at :Nlatadi since 1909
and 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 morc
knowledge of the discharges and discharge distributions in the meandering zone was required for the study of the improvement of the
navigability 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 last
75 years and published by the Belgian State Hydraulic Laboratory.
An approximate relationship between water level and discharge is
given in Fig. 6 for the gauging stations of Kinshasa and Boma.
Because of the almost stationary flow conditions this relation is
quite univocal, particularly for Kinshasa. For Boma an influence of
varying head losses due to the bank and bed configuration can be
noticed, chiefly when discharges exceed 40 000 111 3 • S -1. 1tleasurements
of discharges by the velocity-area method with propeller current
lueters provide uscfull information on the degree of obstruction by the
sedimcnts 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 in
morphology of the meander system. These discharges, in about 15
cross-sections, have to be collected in a very short period to eliminate as
mnch as possible errors due to tidal influence. In the past the surfaeefloat method was used. Since 1969, the moving-boat method allowed
quick determinations of the discharges but although the method is
simple, the possible errors are numerous, and the technique had to be
improved for its application to this complex meandering river system.
279
DISCHARGE AND SAND TRANSPORT
WATER
LEVEL
PERIOD
1971- 1975
{ml
O'--_--"-_ _-'-.fL_--'_ _J -_ _'--_--"-_ _- ' - _ - - '
o
10.000
20.000
30.000
40.000
50.000
60.000
WATER DISCHARGE
70,000
(m'/s)
AT NTUA NKULU
Fig. 6. River discharge (Qin m3's~1) at the gauging section of Ntua~Nkulu as a
function 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 between
Boma 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 a
distance of 40 km (Fig. 8). Only the smaller particles reach the zone of
the submarine canyon.
rr:=:=:==c=,--------------------,
d50(mm)
2.0
IIN CHANNEL
SOUTHERN CHANNEL
+ .................
t
NORTHERN
CHANNEL
1.0
'"z
~
w
~
0
w
'"
:>;
0
~
~
0
ro
i!-++ + ++
•
20
-
20
DISTANCE FROM NTUA.NKUll
Fig. 8. :Nledian grain size (d.Jo in mm) of the sediment of channels and shoals along
Southern and Northern Channels of the braided area.
Large differences between the sediment grain sizes in and between
cross-sections could be noticed. They have probably several reasons,
but chiefly secondary currents iuduced by the bed morphology. For
example, 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 currents
transport 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 velocity
ranging from 2 to 10 m a day. At high river discharges and for sediment
grain sizes slnaller than 0.5 mm, these dunes arc flattened and other
bcdforms appear, similar to small scale dunes. Their wavelengths and
amplitudes 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, where
the median grain size was smaller than 0.5 mm (Fig. 9), while in the
Southern Channel \vhere the median grain size vvas larger than
0.5 mlTI the large scale bedforms remained. ])lotting the mean power
of the How per unit area versus sediment grain size, it can be seen that
the 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 number
of the How averages 0.1, and the accepted classification of bedforms
would suggest a plane bed (SIMONS & RICHARDSON, 1966).
1Vleanders move sometimes quickly; concave banks in bends may
erode at a rate of 100 m per year, or even more. Many rocks and rocky
bars influence strongly the ll1candering of the different channels in the
braided area, and therefore analysis of the ll1eandcr characteristics is
not very meaningfull. Length, meander belt width, meander radius
and channel width average respectively 12 km, 3 km, 3 km and 1.5 km.
The interconnection between the different branches of the braided
area, ,,,,here scdiments have different sizes and move at different
speeds, complicate the analysis and the prediction of the evolution of
the meanders.
SEDIMENT TRANSPORT MEASUREMENTS
As the goal of the investigations \vas the improvClnent of the navigation
by 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
282
J. J.
PETERS
A
,
___L
_~~-:t:-
----.-----~
5
c
~~~~~~~~~:ff6~i~~-~~i~~~\.
:: WOrn
5
I
,i.:rr''''''
~.1~+~I.W.
A
5
- .R!..
Fig. 9. Upper three recordings: bcdforms in the Northern Channel at r.rfateba
during 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: bcdforms
in 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
low. For these reasons, continuous samplers were preferred to instantaneous ones.
I
.... ,
,~,
/'b~
I
,-' "ppees
:
,c>"%tJj
jj,
~
'" 1'iiiij~,~~~~I71r~r~'qL-"~'~:'~8'
+IJ flTTn±t
NO SEDIMENT MOTION+-+
IJ 0;;1 1qs
0,3
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 grain
size 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 the
Bedload Transport Meter Arnhem (B.T.M.A.), both developed in the
Netherlands. 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 of
the water, could be positioned at 0.05 m, 0.15 m, 0.25 m, 0.35 m above
the bottom. For the rest of the water column, i.e. from 0.40 m above
the bottom to the water surface, an integrated sample was taken with
a 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 retained
by a sieve. Only the sand fraction of the sediment is sampled in the
J. J.
284
PETERS
instrument. Grain size of the samples \vas detcrn1ined with the visual
accumulation tube (COLEY & CHRISTENSEN, 1956).
Special attention was paid to the positioning of the D.F. 2 and
B.T.M.A. samplers on the bedforms using eehosoundings.
At each station the velocity profile was measured and the bottom
shear velocity computed. To understand the sediment transport and
its distribution the variation in bed load and suspended load discharge
at constant mean velocity (Fig. ll) were investigated, as well as the
2,52
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"
om
id50!c
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------i§ ,
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-"
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V----.......-----------.,
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i
r;:::c
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RI
!
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-- ...... /
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. Ih
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(m/si
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- I
-
1h
2h
-,"
(m o/m2 I,hl!
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lie
liu::iJ1LtJtJJI1Jij;mu:Ltlhl
-
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ro
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c [kg/m")
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285
DISCHARGE AND SAND TRANSPORT
distribution of transport rates near the bottom and the distribution of
sediment transport in a cross-section (Fig. 12) and the influence of the
tides on flow and sand transport (Fig. 13).
Besides the measurements an attempt was made to calculate bed
load and suspended load using Bagnold's approach (BAGNOLD, 1966).
The method was modified in order to provide results in an isolated
station, using the power of the flow as the product of the mean velocity
and 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 calculations
are shown for a single station, sampled during a 7 hours period near
Boma in November 1973 with high river discharges. The tidal amplitude at the sampling station was 0.03 m and the mean velocity (1.3
m's- 1 ) did not vary significantly. Bed load measurements with
B.T.M.A. on the bottom and D.F. 2 at 0.05 m above the bottom show
a very erratic variation for the instantaneous as well as for the hourly
average values, but less so for the D.F. 2. Although the order of
magnitude 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 the
sediment transport capacities computed with the modified Bagnold's
approach change during the 7 hours observation period in a quite
similar way. Although these data do not have the same meaning, the
ratio between them is jnst given as an indicator (Fig. 11). The observed
Fig. 11. rVfeasured and calculated flow and sand transport characteristics at fixed
sampling 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. Sediment
characteristics used in calculations with BAGNOLD'S approach: median grain size
of 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~ (m 3 . m-t. d- t ); thc curves allow
comparison o[ sand transport data sampled at 0.05 m from the bottom with two
identical Delft: Bottles D.F.2 (a and b), i(D.F.2a -1- D.F.2b), and in the whole water
column 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'S
approach; measured data are hourly averaged (rvLH.). d. Ratios between sediment
transport 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 bed
load. e. Ratios between mean and surface current velocity, Ulll!US, and between
bottom 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 samples
taken 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 at
0.05 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::
J
b
[iJm lm24hi i
M
I:;::\
Jro
.I
: 1\
c
'
I !rVi
I.
I If \J-CF1 ""
V,i
\
I
"I
)
I.
/
"
..
DF2
/' ,,/
,I£:.:.-. __ ..-.f
v
,/'
j'/"\\.
\\
\
Fig. 12. Distribution of measured and calculated flow and sand transport characteristics in the gauging section of Ntua-Nkulu, November 1973. a. 1vlcdian grain
size (diJO in mm) of samples taken by D.F.2 and D.F.l samplers. b. rvIeasured sand
transport rates sampled with D.F.2 and D.F.l samplers, qs in m3>m~Ls~1, for
D.F.2 integrated from bottom to OAO m off it, n.F.! integrated from surface to
OAO ill from bottom, and D.F.1
D.F.2. c. FIm'\' characteristics: mean current
velocity, Um (m's- 1 ), and discharge per unit 'Nidth~ q = um'h (m 3 'm- 1 ·s- 1 ).
d. Cross-section \'vith depths, It (m), and isotaches 1 It (m·s~l). e. Sediment characteristics used in calculations ,vith R"'GNOLV'S approach: median grain size of bed
load (d 5 0 C in mm) and median fall velocity of suspended load UV s 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 to
surface 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 measured
and calculated transport rates (indicated in L); R t = h(D.F.2 -;- D.F.1)-l for total
load 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 of a few ccntimcters to
a fe",\, dccimeters above the bottom. It is cliHicult to make a distinction
between bed load and suspended load, but probably most of the solid
particles 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 of
field data and calculations is given (Fig. 12) for the control section of
I'\tua-Nkulu \'\'hcre the river discharge represents 860/0 of the total fresh
"vater flmv through the estuary (Fig. 4). 'The plume of suspended
particles observed near to the left bank is due to the presence of a rock,
iEducing intense secondary currents. Also the remarkable distribution
of sediment sizes in the 1800 m \'1'ide cross-section is due to morphological factors and an analysis ofscdiIllent transport distribution should
take this factor into account. The agreement betvveen sand transport
rates measured \vith samplers D.F. 2 and D.l;". 1, and calculated sand
transport capacities is satisfactory, except for stations 11, 7 and 6
because of the secondary CU1Tents just nlentioned. Using Bagnold's
approach \\'ith the mean characteristics of flow and sediments, the
calculated total transport capacity amounts to 24·5 000 IUS. d~l instead
of 1it5 000 m 3 . cl-I \vhich is obtained when the data of each station are
used separately while sand transport nlcasured \v1th the D.F. smuplers
amounts to 82 000 nl:3.d- 1 .
In this cross-section the influence of the tides 011 sedilTIcnt transport
can be neglected. The tidal influence increases do\vnstrearn and an
exanlp1c of this influence on fIa\v and sediluent transport is given in
Fig. 13. "rhe measurements \vere performed 011 4- stations distributed
in a cross-section located approximately 5 klu upstreanl the head of the
subnlarine canyon. The influence of the tidcs on flow and sediment
transport distribution is clear: rnaximurll sand transport occurs 3 11 to
2 h before lmv watcr or immediately after maximum flmv intensity.
An attempt \vas nlade to quantif"y the sediment inflm..v into the main
part ofthc lYlcandering arca through thc control section ofNtua-Nkulu.
All available data oftbe period 1971-1975 are plotted in Fig. 14 versus
the discharge through this section. Sand transport measurements
sampled bet\veen bottom and surface with the Delft Bottles are compared \vith the sand transport capacities calculated either by the
modilied Bagnold's approach using local 1I0w data or by Bagnold's
approach 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:~~~~~
ciJ
01
50
,0
.__~.~~~~~.~=. ': : ~=._=,..~/o.r=;::,-~. ::::..~~~
·_~+
dJ)
Iq! "<.loP [mYmsJ I/,,--
~e
,.-
......
30[-... - - _........
"I"I-
o
@
---
_.. . . . '-
=-----
9
Fig. 13. Influence of tides on flow and sand transport rates in a cross-section
located 5 km upstream of the head of the canyon; the 4 stations (encircled) were
sampled on 5 successive days and all data gathered in one figure with time of low
water (L.'i,J\l,) as reference. a. Envelope of tidalleve1s, T.L. in m. b. and c. Sand
transport rates, qs in m 3 'm- L s-1, measured with D.F.l sampler between surface
and 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 surface
current 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 •
289
DISCHARGE AND SAND TRANSPORT
probably to long term eflects in the adaption of the thalweg to new
flow conditions.
10'
10'
20
sp
6p
Q max
7,0
80
TOTAL
DISCHARGE
Fig. 14. Measured and calculated sand transport rates (in m 3 'd- 1 ) as a function of
river discharge (m3 's- 1 ) in the Ntua-Nkulu gauging section. Total sediment
transport 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 (..); sand
transport capacities calculated with BAGNOLD'S approach (1966) using cross-sectional
average flow data (D); sand transport capacities calculated with BAGNOLD'S
approach (1966) using local flow data (11) j sand transport rates measured with
De1ft Bottles samplers (*). Data were collected from 1971 to 1975 for almost the
whole 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's
approach correspond ronghly with the measured transport rates. In
Jnly 1973 measnrements were performed at very low river discharges
close to the minimum ever observed and the calculated sand transport
rate had an order of magnitude of 10 4 m' sand per day. In December
1975 the river discharge amounting approximately 85% of the maximum ever observed, the calculated sand transport rate reached an
order 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 the
cross-section (Fig. 12). Using data of GUY, SIMONS & RWI-IARDSON
(1966) for sediment sizes of 0.3 mm, the calculated values range from
3 X 10 4 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 rates
rClnained 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 entrance
of the Ineandering area "';,vil1 be 50 X 10 6 ID:}' a -1 \vhich corresponds to
an average concentration of 40 g sand per m 8 ,"vater.
The sediment input in the llleandcring area of the maritillle Teach is
controlled
the magnitude and the intensity of the successi,'c floods.
An intense one may introduce in the Ineandering area a large al110unt
of 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 a
long lasting effect can be the result. So the influence of the varying
hydrological cycles ",vill be less marked at the head of the canyon, and
there, sand input \-vill be more conditioned by channel evolution.
Ill. CONCLUSION
The geomorphological configuration of the 150 kIn long maritime
reach determines a sedimentation area in the coastal plain zone dmvnstrean1 the harbour of Boma. In this 60 km long sedimentation area
ending at the head of the canyon, 30 km upstream the Tiver Illouth, an
intricated channel systen1 evolves continuously when meanders lllove
under influence of varying fresh water and sediment inflmv, deterD1ined hy the hydrological cycles, Although these arc stable, successive
floods of different intellsities ,,yill annually introduce different an10unts
of sand. The slow rate of transport of the sand is responsible for occasionnal and local obstructions of channels. The journey of the sand
through the braided sedimentation area takes about 20 ")'cars, Channel
obstructions \vill be determined by the difference behvcen the amount
of sand brought to a certain location and the sediment transport rate
determined at the mon1ent by the river P0\-VCL
'The fresh "'ivater flow of the Zaire river into the estuary is very stable
and amounts to 1.45 X 10 12 m3'a~'1 The sand transport rate through
the braided sedimentation area to the cany-ol1 has an average of
50 X 10 13 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
291
IV. SUMMARY
The sedimentation zone of the nlaritime reach of the Zaire river
located 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 of
'
0.80 m, quickly damped upstream; sand of the bed with grain sizes
dropping dmvnstream from 1 mm to 0.2 mm, moving generally as
dunes with a speed of 2 m to 10 m per day. The apparition during
floods of different and smaller scale bedforms was observed.
Measuring and calculation techniques arc briefly described. Some
results show the influence of geomorphological factors on sand transport phenonlena, the erratic variations in time and space of sand transport rates and the influence of tidal movements on these. The role of
secondary currents on the distribution of sand transport in and bet\veen channels is emphasized.
Volumes of water and sand evacuated annually through the braided
area arc respectively estimated at 1.45 X 10 '2 and 50 X 10 6 m S.
V. RESUME
11 s'agit de la description de la zone sedimentaire de la partie maritime
ciu fleuvc ZaIre situee a l'mllont du canon.
Les caracteristiques principales des mouvements des eaux et des
sables 5011t indiquees: stabilite du regime et debits Cleves variant entre
23000 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; sur
le fond sables d'une granulometrie decroissant de l'alnont vel'S l'aval
de 1 mnl a 0,2 nlm, en mouveIllent generalenlent sous la forme de
dunes avan<;ant a la vitesse de 2 nl a 10 In par jour. L'apparition
de formes topographiquc5 tres particulieres en temps de crue est
signalee.
Les techniques de mesurc et de calcul des transports de sable sont
sommairement decrites. Quelques resultats illustrcnt l'influence de fac~
tcurs gcomorphologiques sur lcs phenomenes de transport de sablc, les
variations 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 a
l'intCrieur des differents chenaux est souligne.
Les volumes d'eau et de sable evacues annucllement par la region
divagante 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.: 1-433
BAGNOLD, 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. Classe
des Sciences et Techniques, N.S., T.V., Fasc. 3: 737-763.
GUY, H. P., D. B. SIMONS & E. V. RICHARDSON, 1966. Summary of alluvial channel
data 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 14
dans le cadre de l'e-tude de l'amclioration de la navigabilite du bief maritime
du 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.