The flow in the basin of Mono is very variable. The north of Togo is subject to a tropical transition climate characterized by dry season from October-November to March-April and a wet season from March-April to October-November. The south region of Togo is subject to an equatorial climate characterized by a dry season from mid-November to March and a wet season from March to mid-November, interrupted by a small dry season in August and September. The floods of Mono begin therefore generally in March and extend on five or six months in average. Figure 3(a) highlights the classes of seasons in the basin of Mono, divided into three types of domain (subequatorial domain and humid tropical domain) according to the latitude and thus Figure 3(b) shows the rainfall variability class by class from january to december in the basin of Mono (Figure 3).

The most important rainfalls on the basin, are registered from April to October. During this period, the precipitations are unevenly distributed on the basin; the south of the basin is very watered in June, comparatively to the north which is wet with the most abundant rains from July to September. That could be explained by the cumulated effect of monsoon and the relief. So, the rainfall started earlier in the south than in the north of the basin. The Figure 4 shows the longitudinal profile of the river [6].

According to Le Barbe et al. (1993), in average year, the annual average flow of the Mono River is in order of 100 m³/s at the station of Athiémé; but since 1988, with the production of electric power, the hydrological regime of Mono had been modified with the low water period sustained by an average flow rate of 40 m³/s [3].

The problem of outflow in the river can be summarized as follows: there is on one side the gravity which tends to flow the water and on other side the bed friction (bed and bank) which tends to slow down the flow. The influence of the gravity on the flow depends on the slope, while that of the friction depends on the flow characteristics and the solid interface characteristics (granulometry and topography). For certain flow rates, the flow conditions depend therefore on the slope and also on the fluid flow rate and granulometric and topographical properties of the bed. When the flow velocity is such that the variations of water height induced by topographical perturbations, it cannot move upstream; it is the case of torrential flow. This case is only found in mountain torrents or in special cases (outlet of the limon); otherwise the flow is supposed to be fluvial. It exists between those two cases, a very marked difference in behavior. The cases encountered will be always in fluvial regime. Concerning the applying friction, only on the solid banks of flow (bed and bank), the flow velocity is not uniform in the section. In rectilinear section, the maximum value of velocities of the area is located at the middle of the section, and near the free surface, and therefore, the regime of flow in the river is generally highly turbulent. Turbulent flow features will therefore arise with origin, all areas where strong gradient of velocities are generated; i.e. bed and banks and also all topographic discontinuities of the bed. The maximum size of those turbulent flow features is limited by the maximum spatial scales according to the considered direction (height of water in the vertical plan and width of transversal section in the horizontal plan). Those flow features play an important role in the deformation of bed by the outflow.

Different parameters defined from the particles size distribution can be used to characterize the mixture. Many characteristic diameters of the mixture can be defined, as the median diameter D₅₀ or the average diameter. The explanation of the effects of mixture is the following; the big grains in a mixture are more exposed to the flow than if they were the only ones represented in the bed, and conversely for fine grains. In consequence, the big grains are easier to move and their transport will be more increased than if they were the only ones, conversely for the small grains. This phenomenon is known as “masking et exposure”. This effect tends to counteract the difference of intrinsic mobility of grains. We can imagine that all of grains of the mixture are mobile in same time regardless of their size: so there is no selective transport. This case is not general (it can be considered as approximation, and only in certain cases). One consequence of the transport preferential of finer sediments is that the granulometry of transported sediments is usually finer than that of sediments of the bed [7].

One general characteristic of rivers is that the sediments which compose the bed are increasingly fine in downstream direction. This phenomenon provoked by the preferential transport of most of fine sediments as well as by abrasion, and is associated to a decrease of the slope of the bed to downstream. The diameters of the grains of bed can go of many tens of centimeters in the torrents of mount, to some tens of millimeters at the outlet of the rivers [7].

The sampling is carried out in three ways: by the Bertois’s cone, by coring and diving. With the Berthois cone, we operate on a canoe or a motorized boat, during coring on the banks and finally with the divers. The realized sample have been analyzed in the laboratory CGILE in Togo and in the laboratory of Sciences of Earth Department of University of Abomey-Calavi, Benin [5]. The experience is realized according to the AFNOR series (French Association of Normalization). The screen column is constituted by 5 sieves with decreasing geometrical dimensions of meshes allowing to distinguish soil materials ie. gravels (>2 mm), very coarse sands (>1 mm), coarse sands (>0.5 mm), medium sand (>0.2 mm) et fine and very fine sand (>0.05 mm). The Figure 5 presents the granulometric curve of sediments in Mono river.

From the particle size distribution curve of sediments of the bed bottom [8], we got D_m = 0.75 mm and D₅₀ = 0.375 and the uniformity coefficient can be calculated with the following formula:

C u = D 50 D 10 (1)

After the granulometric analysis, the results obtained for the different sands are as follows:

For clays and other sediments derived from clays, diameters are between 0.002 mm and 0.05 mm; for fine sands, the diameters are between 0.05 mm and 2 mm; from medium sand to rough sand, grains diameters are higher than 2 mm [8].

The uniformity coefficient (Cu) of grains indicates the type of granularity of grains. In the case of Cu < 3, the granularity is uniform. Thus consequently, in the case of Mono river, Cu = 0.33, (Cu < 3), thus the particle size distribution is uniform. Through the granulometric curve, different diameters are determinate (Figure 6).

Table 2 shows the different values of calculated diameters.

The coefficient of Strickler β, is essential for the determination of wet perimeters in the Mono river.

So

β = ( K f / K g ) 3 / 2 (2)

where

K_f: roughness of the bottom

K_g: Roughness of grains

K g = 21 / ( D 50 ) 1 / 6 (3)

Table 3 shows the different values of K_f, K_g and β [8] [9].

The wet perimeter can be calculated with the following expression [10] [11]:

P m / K 3 / 2 = P f / K f 3 / 2 + P b / K b 3 / 2 (4)

where

P_m: wet perimeter

P_f: width to top

P_b: length of bank and the different values used are: P_b1 = 6.33 m, P_b2 = 29.53 m

The bathymetric sampling techniques include depth and sediment samples. This bathymetric measurements are operated on 10 km with an equidistance of 500 m, so we got 20 measuring stations. The measurements of bed of watercourses have been realized in dry season (low water period) between Athiémé and Agbanankin, thanks to two artisanal techniques which take into account the section to be measured.

The first technique consists to measure on sections with an equidistance of five hundred meters by moving the boat from one bank of the river (point 0) to the second and so on. The graduated cord ballasted with 5 kg of lead to stop the waterflow from moving the cord, is submerged for measuring the depth of the lagoon (Figure 7). Each depth measurement is accompanied by the taking of coordinates at the measurement point of the starting river bank at the arrival river bank. The determination of the depth being difficult, especially in time of frequent violent wind (5 m/s or even 10 m/s on a watercourse of 2.5 at 3 km wide), the rating technique consists to quantify two orders of magnitude which are the width and the height of water in one point. The lead comprises a cavity at its end making it possible at the same time to take sample of sediment [3] [4].

The second technique consists to extend a graduated rope with the use of a pirogue, supported on both sides by stakes. This technique concerns the gauging of the bed and the bathymetry to low water period (Figure 8) [5].

Up to now, data of Mono river remained incomplete. Thanks to bathymetry and to the calibrated measurement of depth and of the survey on a number of sites, the transversal profile of the river (down-Mono), is reconstituted, to identify the evolution of the filling, comparing to the previous works. The movement of sediments, which shapes the bottom of the river, is a complex phenomenon. To study the geometry of this watercourse, we assimilated the bathymetric curve of the river to a regular form of figure. So, the following figure present on one hand, the measured bathymetry of a section of the upstream at the station of Athiémé (Figure 9) and on other hand, we present the simulated transversal section of the river at Athiémé (Figure 10).

All section of the Mono river at Athiémé, may be calculated according to the simulated form, where

Y: the water draught

AB: the width of glassy water

DC: the width on top

AD: the left of bank

BC: the right of bank

Considering S_k as the area of the river section located at K point on the average line of longitudinal profile [12], we have:

S k = { 0         if   x i − x 0 = L k y       if   x i − x i − t = l k z k = k W n (5)

where 0 ≤ k < L b , 0 ≤ i < L k , 0 ≤ j < y and 0 ≤ t < i .

L k = ( X j − X i ) 2 + ( Y j − Y i ) 2 and (6)

l k = L k − y ( 1 tg ( α ) + 1 tg ( β ) ) (7)

L_k is the width of glassywater, l_k the width on the top, z_k position of the section on the average line, n the step in the space and L_b the length of the reach. X_j, X_i, Y_j et Y_i are the topographic coordinates of the bank [12].

The study of different geometric parameters of the river on the reach is based on the analysis of the bathymetry in twenty (20) measuring sections of the river from Athiémé to Agbanankin. Considering the sinusoidal character of the river, its geomorphology is handmade by the hydraulic turbulences of meanders. With the geometric model, we evaluate the width to glassy water and to top of the river in the reach on 10 km with a step of 500 m. The knowledge of dimensions of the sections of the river allowed to evaluate the dimensionless stress (Shields parameter) in order to determine the transport mode of grains. Table 4 recaps the dimensions of the sections to each point.

Figure 11 shows the longitudinal profile of the river (left bank and right bank) in the reach upstream of the station of Athiémé, showing four (4) meanders on just 10 km. The river is therefore very sinuous; and its sinuosity had highly influenced the dynamic of watercourse.