Geomorphology and hydrography

Fig. 3.5: Comparison of historical precipitation measurements with observations in 2006 for the two gauging stations (ASECNA) at Parakou and Djougou. The measured daily precipitation is accumulated over the year. Data ranges from 1950 to 2006 (with permission of M. Gosset, IRD 2007).

Fig. 3.6: Perspective view on the DEM of the HVO. Z-level is 20 times exaggerated (horizontal scale 1:10.000; vertical scale 1:500). The morphologic valleys contain seasonal rivers.

(Projection: UTM, Zone 31P, WGS 84). Cross sections A-A’ and B-B’ are traced by black lines are represented in Fig. 3.7.

Fig. 3.7: Cross sections A-A’ and B-B,’ as shown in Fig. 3.6. The z-level is 50 times exaggerated.

Along the trace of the Kandi fault quartzitic crests (Qz) occur. Especially in the northern half of the study area they form a characteristic N-S striking elevation of 30-50 m above surface level. The Kandi fault is as shear fault accompanied at both sides by zones of increased stress due to shear movements. This zone of weakness has promoted erosion

but also intrusions of plutonic rock bodies (Wari-Maro massive). Originally of granite or monzodiorite origin, these plutonites were strongly metamorphosed and show gneissic and migmatitic structures. In the southern half of the HVO the trace of the Kandi fault is accompanied by a number of inselbergs.

Fig. 3.8: Inselberg of Wari-Maro (620 m asl) at UTM 407864/1013209.

Characteristic morphological elements of the Upper Ouémé catchment are inland-valleys, also known as bas-fonds in Francophone countries or dambos in English respectively (FASS 2004). In this text the term bas-fond is preferred, as it is commonly used in Benin. Bas-fonds constitute the main drainage system in the African surfaces (WRIGHT 1992).

Bas-fonds are seasonally waterlogged bottomlands (MCFARLANE 1987a). They appear in the study area as seasonally swampy, mainly grass-covered depressions of either linear or circular shape. Bas-fonds accumulate thick clay deposits in their centre. These deposits are mainly made up by smectite, which indicates stagnant high water levels (Fig. 3.9, MCFARLANE 1992a). Most of the runoff water accumulated in the bas-fonds is evapotranspired while a minor part may run off downstream. The evapotranspiration of already concentrated solutions favours the precipitation of smectite which is enriched in bas-fonds clays (MCFARLANE 1987a).

Fig. 3.9: Schematic transect of a bas-fond to show the distribution of dominant clay minerals.

B.H. = Borehole (taken from MCFARLANE 1987a).

Bas-fonds in the HVO are of varying size but often less than 100 m in diameter. More than 700 bas-fonds, or similar structures, were counted in the HVO (oral comm. S.

GIERTZ, IMPETUS 2006). Many different models for the evolution of bas-fonds are discussed (VON DER HEYDEN 2003; MCFARLANE 1987a and 1992a). However, it seems

that the occurrence of bas-fonds is often connected to river systems above densely fractured systems (MCFARLANE 1987a). The groundwater in the bas-fonds clays is connected to the regolith aquifer.

Due to the strong precipitation and the inundation of the bas-fonds during the rainy season most of the settlements are grounded on crests and hilltops. Perennial surface water and groundwater resources are predominantly found downhill.

The biggest rivers in Benin are the Ouémé, the Pendjari, the Niger and the Mono whereby the Ouémé is the most important river. Its complete catchment covers almost half of the countries area.

The source of the Ouémé is within the Monts Taneka in the Northwest of the study area at an altitude of 550 m above sea level. The river’s total course is 415 km when it enters the Lake Nokoué close to Cotonou. The slope of its thalweg on the first 12 km is 12.5 m/km. Then the slope progressively declines to attain in average 1.5 to 0.38 m/km (LE

BARBÉ et al. 1993). The mean annual discharge of the Ouémé for the period 1951-1995 (SOGREAH and SCET 1997) at Bétérou was 51.14 m³/s, 100 m³/s at Savé and 200 m³/s at Bonou station.

The basin has a total surface of approximately 49,540 km² until it joins the river Zou. In the South of this joining point the limits of the catchment are less easy detectable due to a flat surface (slope = 0.13 m/km) and regular inundations during the rainy season. The Ouémé river bed is in the HVO rocky and shows no terraces or alluvium. Larger tributaries have sandy beds and are accompanied by riverine forests. Smaller tributaries show hydromorphic clay deposits with herbal plant cover and can be associated with bas-fonds (ZOUMARO 1998). The water divide between the Niger and the Ouémé basin roughly follows the 10th parallel, north of the Nikki-Djougou dorsal line (FAURE and VOLKOFF 1997). The northern part of the Ouémé catchment is known as the Upper Ouémé catchment (Fig. 3.1). The main tributaries of the Upper Ouémé are the Térou in the Southwest and the Donga in the Northwest. In the northern part of the HVO people often refer to the Ouémé as the Affon. In this text the term Ouémé is exclusively used.

Fig. 3.10: Riverbed of the Ouémé in the northern half of the HVO during the dry season (UTM 407864/1013209).

Fig. 3.11: Hydrographic net of the rivers in the HVO catchment. The striking of the Kandi fault has a strong impact on the course of rivers in the East of the HVO. (Projection: UTM, Zone 31P, WGS 84).

Within the catchment the hydrographical net (Fig. 3.11) is quite dense, but during the dry season almost all rivers dry up leaving stagnant puddles and swampy area in the riverbed (Fig. 3.10). Remaining water holes are called “marigot” in Francophone West Africa. For local people without access to wells or other water points the marigots are the only water source during the dry season.

The course of rivers is in general alleged by the direction of fractures and fault zones.

The flow net of the Upper Ouémé catchment has been derived from the DEM. The results were compared to general flow patterns on crystalline basement in West Africa given by CEFIGRE (1990).

Most of the rivers show the change between angular and sinuous flow patterns.

However, it seems that on this rather rough scale of investigation the angular type prevails.

The angular thalweg generally appears where the riverbed follows fractures or fault zones whose course abruptly changes in sharp angles (see Fig. 3.12). The sinuous river courses are controlled by the topography and the weathering surface. Hills with duricrust cover and inselbergs force the river to turn around them (CEFIGRE 1990). This could be helpful for local analysis of subcatchment characteristics.

a) b)

Fig. 3.12: a) The water course is controlled by fractures (“en baionette”). b) Sinuosity depends on morphological features and not on fractures (from CEFIGRE 1990).

It is clearly visible that the Kandi fault has a major impact on the river net. The outcrop of the dense and impermeable Kandi quartzite is accompanied to the west and to the east by the almost parallel flowing Alpbuoro River and Yérou-Maro River. Both rivers join the Ouémé in an area where the Ouémé starts to make sharp angular turns until the Térou River from the West joins it. The course of the river is clearly controlled by the trace of major fractures.

Tab. 3.1: Typical flow net pattern in West Africa taken from CEFIGRE (1990).

Type Flow net Observations Regolith thickness

[m]

1 rectangular or askew net

with eroded interfluves;

temporary, straight marigots; riverine forests;

inselbergs

15-20

2 Gneiss terrain; weak slopes;

duricrusts; riverine forests

15-25

3 Schist and granite terrain;

all indicators as above may occur to different degree

variable

Tab. 3.1 shows three types of morphological observations which fit to the general flow net and can be identified from satellite images as seen in Fig. 3.13. The descriptions given by CEFIGRE (1990) are in accordance with field observations in the HVO. Type 1 is typically seen along the major rivers in the central part of the study area. Type 2 and 3 share in changing portions the rest of the area in dependence to the geology.