2. HYDROLOGIC AND CLIMATIC DATA ON THE CONGO RIVER

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Hydrologic studies spanning over the period 1902-1956 have been used, inter alia, for the feasibility studies and the design of the Inga dams. The following TABLE I summarizes the characteristic discharges of the Congo River at the Inga site. It can be soundly advocated that these figures provide a good representation of the discharges that one may expect at Moanda, given the short distance and the lack of major input or output of water to the river.

Table 1: Characteristic Discharges of the Congo River at Inga (1902-1956)

Water Level	Exceeding Frequency		Discharge in m3/s	Method
	in %	in days/year
Min. Low-Water Mark observed	—	—	23000	INGA's limnigraphic mark
Very Low-Water	98	358	26300	Idem
Low-Water	90	328	29000	Idem
Median-Water	50	182	35400	Idem
High-Water	10	37	49900	Idem
Max. Flood observed	—	—	67000	Idem
100 year Low-Water Mark computed	99.9973	—	22300	Gumbel's Law
50 year Low-Water Mark computed	99.9450	—	25600	Gumbel's Law
100 Year Flood	0.0027	—	72000	Gumbel's Law
Catastrophic Flood	1/36500	—	9000	Gumbel's Law

It stems from the data in Table 1 above that the Congo River has a very regular discharge as compared to European rivers. Indeed, absolute values of the maximum flood and the minimum low-water mark are not sufficient per se in the evaluation of the safety of hydraulic constructions. The rate of change and the ratio between the maximum flood and the minimum low-water mark are central to that. In Table 2 that regularity is confirmed at three different sites in comparison with European figures.

Table 2: Ratios of Extreme Discharges

River	Site	Maximum Discharge in m3/s	Minimum Discharge in m3/s	Ratio
Loire	Roanne	9000	35	260
Garonne	Toulouse	6000	36	167
Rhone	Lyon	7000	150	47
Seine	Paris	2500	48	52
Rhine	Strasbourg	5690	150	38
Meuse	Sedan	700	13	54
Congo	Kinshasa	90000	23000	3.91
	Kinsangani	19800	5050	3.92
	Ubundu	18100	4850	3.73

That draws attention to the study of the hydrologic regime and the climatic conditions of the Congo River Basin. In general, the hydrologic regime and the discharge of streams depend on the influence of stable and variable parameters:

• Stable parameters include: the relief, the nature of the soil, and the forested area.
• Variable parameters are the climate, the rainfall, and the temperature.

An hydrologic regime can be simple or complex according to the particular manner in which the above-mentioned factors combine in given river basin. In inter-tropical regions of Africa, two typical climatic conditions have been identified, namely:

The tropical regime and its variants essentially characterized by one dry season and one rainy season.

The equatorial regime and its variants typified by the occurrence of two rainy seasons and two dry seasons of unequal duration.

The climatic conditions of the DRC are quite complex and are called the inter-tropical regime. The region enjoys generally high temperatures and is under the influence of both the Indian and Atlantic Oceans. The North is an equatorial zone, whereas the East is a mountainous region. Therefore, the hydrological regime of the Congo River is characterized by two annual swellings:

• A big swelling in December thanks to the rainy season in the Southern Hemisphere.
• A second flooding of a lesser importance takes place in May because of the rainy season in the Northern Hemisphere.

As a consequence there are also two dry seasons, the driest happening in August.

Other important information on the Congo River:

The Quality of Water

Table 3: Chemical Analysis of Water of the Congo River
(Location and period not specified in the source document)

Element	Unit	Range
PH	—	6.1-5.8
Hardness	Millivols	0.3-0.4
	German degrees	0.8-1.1
	French degrees	1.5-2.0
CO2	mg/l	6
C1	—	—
S	mg/l	3
Suspended load after drying	mg/l	30-70
Suspended load after calcination	mg/l	20-36

Accordingly the Congo River's effluent characteristics are:

• Very fresh water
• Slightly salted water
• Corrosive water

The Solid Load
Measurements near the estuary provided the following data (1941):

• The solid-load comprises 90% of suspended-load and 10% of bed-load.
• The suspended-load is composed of fine sand with 0.2mm mean diameter.
• The median diameter for the bed-load sands is 0.5mm.
• The solid load discharge varies with the Congo River's regime: as a first
• approximation the mean value of 0.05 is prudently brought forward.
• During flooding seasons, the Congo River drifts various objects and mainly freshwater hyacinths.

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