The equipment used for this study includes a Garmin GPS for georeferencing, a “Rio Grande” Acoustic Doppler Current Profilers (ADCP), equipped with four acoustic transducers emitting at a frequency of 1200 kHz, for flow measurements by gaging and a Garmin 521s dual-frequency Doppler echosounderbathymeter for bathymetric measurements (Plate 1).

· Selection of hydrological data

Flow data for the years 2010 to 2022 from the Bonou station located on the Ouémé River and upstream of the study area were collected from the DGEau. Water level data were from Bonou station located at the North of the study area as well as from stations of Hêtin-sota, Sô-Ava and Fisheries Directorate located in our study area.

(a) (b) (c)

Plate 1. Views of the ADCP (a) et (b) and Garmin 521s echosounder (c) deployed on the Ouémé River and Nokoué lake (July 2022).

· Field work

The seasonal field campaigns took place in July, September, November 2022 and March 2023 for flow measurements and bathymetry.

○ Flow measurements and specific power estimation

Stream flow can be calculated in a number of ways. According to [2] , flow can be determined by:

Q = ∑ ​ ( V × A ) (1)

where: Q—flow (in cubic meters per second); V—current velocity measured at each cell by the ADCP (in meters per second); A—cross-sectional area of the flow corresponding to each cell (in square meters).

Flow data acquired during the 2022 and 2023 seasonal gaging campaigns, have been processed to obtain campaign and river averages values. Additional flow and water level data from stations close to the study area were used. Monthly and multi-year averages from 2010 to 2022 were calculated and some hydrographs were produced. This enabled a comparative analysis of flows in the various tributaries. In addition, flow data were used to determine total and specific power values along the studied watercourses [24] .

A river power is used to measure a river’s efficiency in sediment transport and bed modification. It can be determined by:

Ω = γ Q J (2)

where: Ω—power in W/m; γ—water density in N/m³ (9810 N/m³): J—slope of the energy line in m/m and Q—flow rate considered in m³/s.

The specific power ω is defined as the quantity of energy transported by the water flow per unit width of the river. It thus characterizes the dynamic potential of erosive processes per river section (energy) for a given flood and cross-section [24] .

ω = Ω / L (3)

where: ω—specific power in W/m²; Ω—power in W/m; L—bed width for flow Q.

○ Bathymetric measurements

Bathymetric measurements were carried out in October 2022 and March 2023.

These measurements concerned Nokoué lake, Sô and Ouémé Rivers, Cotonou channel and Totchè canal.

The bathymetric data collected were processed with various software programs to produce bathymetric maps and profiles [25] [26] [27] .

○ Spatial analysis

Satellite images (SPOT5 from 2010 and Sentinel 2A and 2B from 2022) were downloaded from the ESA website (https://scihub.copernicus.eu/dhus/#/home)

and used to analyze land and water distributions around Nokoué lake, Ouémé and Sô Rivers from 2010 to 2022 (Figure 2). An object-oriented classification approach based on pseudo-spectral bands was adopted [28] [29] (Table 2).

The characteristics of the satellites are summarized in Table 3 below.

Object-oriented classification with the hierarchical classification algorithm is an effective method for classifying areas where land use is fragmented and complex [29] . The implementation steps led to change detection, using transition matrices to represent changes from one category to another [4] [19] . The methodological approach made it possible to present the spatio-temporal variations of the different classes in 2010 and in 2022-2023.

· Water levels and flows

Water level measurements have shown that the hydrological regime of the various tributaries is linked to seasonal variations. The monthly average of water levels on the tributaries from 2010 to 2022 is shown on Figure 3. During the year, the peaks are reached in September but the highest water levels (over 300 cm), is noted in October at the Fisheries Directorate station which is located at the southern part of the study area. For the Bonou station located upstream of our zone, the peak recorded is 440 cm.

Comparative analysis of water level and flow data clearly shows a unimodal regime, with perfect correlation between these parameters (Figure 4). The Ouémé River, in accordance with his morphological and hydrological characteristics, exhibits higher flows and water levels than the Sô River, indicating greater water transport capacity.

Average of flows measured during the 2022-2023 campaigns vary from −185.19 m³/s in Totchè canal to 1404.15 m³/s in Cotonou channel (Figure 5). The negative flow values on the Totchè canal indicate the inversion of the usual water flow direction, resulting from the daily influences of the tides. This observation underlines the importance of the tides in the hydrological dynamics of the region, and their effects on flows near coastal areas. The average of flood flow obtained on the studied Ouémé River is 415.21 m³/s, (compared with 773.35 m³/s recorded at Bonou during the exceptional flood of 2010), making it the most important tributary in terms of flow. This underlines the importance of the Ouémé River in hydro-sedimentary transfer processes toward Nokoué lake, particularly during flood periods.

· Specific power and bank erosion

Specific powers and bank erosion have been estimated for Ouémé and So tributaries. Bank erosion processes are a function of specific power (Table 2) and soil type. The coordinates of the measurement points and corresponding values are shown in (Table 4). The highest specific power values are noted upstream and decreasing downstream as the river approaches its mouth. On the Ouémé River, the highest specific power (12.92 W/m²) comes from a segment with a section of 92.39 m wide. This value corresponds to a relatively high flow rate of 608.57 m³/s and a bed topography, that favours increased conversion of potential

energy into kinetic energy, is capable to induce more significant erosion and geomorphological modification processes. The lowest specific power, 2.69 W/m², comes from a segment with a section of 81.72 m wide, and reflects a lower level of energy per unit width. This value corresponds to a low flow rate (112.10 m³/s) and a bed topography that restricts the transformation of potential energy into kinetic energy. On the Sô River, the hightest specific power is 10.99 W/m² for a segment with a section of 63.73 m wide, while the minimum specific power is 2.46 W/m² is related to a segment with a section of 34.54 m wide.

Segments colored in red along our studied sections of tributaries represent areas of high erosion, mainly due to high flows and soil characteristics. Segments colored in salmon-pink correspond to areas of moderate erosion, while segments colored in blue represent areas of low-bank erosion (Figure 6).

Different types of water erosion were observed in the study area. Areolar type of erosion (wash load) is observed in large sections, notably on the Cotonou channel and on certain tributary stretches (segment). Linear type of erosion is dominant and mainly observed in the northern part of the study area where removal of solid particles from the banks and the bottoms of watercourses and collapse of banks are noticeable (Plate 2). Near the lake, depending of the power of the flows, erosion is of gully or claw types. While runoff causes significant soil loss and watershed degradation, it also defines areas of deposition and erosion. Along the studied sectors, the sections with low specific powers are determined by a low erosion capacity, a zone of deposition and accumulation of fine sediments and a relatively flat bottom.

Plate 2. Views of eroded sections bank on the Sô and Ouémé rivers.

· Bathymetry

At flood stage (October 2022), the bathymetry of Nokoué lake and its channels shows significant depths of up to 10 m in some sectors (Figure 7). At lake level, the edges have shallow depths, generally between −1 m and −2 m, while the center of the lake has average depths between −2 m and −4 m.

During the low-water period (March 2023), the bathymetry of Nokoué lake

and its channels shows depths of up to −6 m and −7 m on the Ouémé and Cotonou channels studied (Figure 8). At lake level, the edges have depths of between −0.5 m and −1.2 m, and the center of the lake has average depths of between −1.6 m and −2.4 m.

The bathymetric results for October 2022 and March 2023 highlight significant seasonal variations in depths in Nokoué lake and its channels. These variations reflect the influence of the hydrological regime, essentially controlled by rainfall, dredging activities and the clogging of the lake with plant debris.

· Bottom morphology

To present the bottom morphology of Nokoué lake, bathymetric data from seven (07) transects (four oriented North-South and three West-East) were acquired during low-water conditions. This made it possible to present the corresponding bathymetric profiles at non-standardized scales.

○ Lake bottom morphology

For depths not exceeding 3 m in the lowest central zone, the profiles show, overall, three types of morphology at Nokoué lake level: “V”, “U” and “Intermediate” (Figure 9 and Figure 10). The north-south profiles (Figure 9) show a relatively gentle slope from north to center, and a steeper slope from the center

of the lake to the south. The eastern sector (profiles AA’ and BB’) is less indented than the western sector (profiles CC’ and DD’), which has a “V”-shaped bottom with clearer variations in depth due to significant hydro-sedimentary inputs from tributaries. The different bottom shapes correspond to flow zones.

The lake bottom in a west-east direction has an intermediate, almost flat morphology (Figure 10). In the eastern and western sectors, the erosive effect of the tributaries becomes less pronounced from north to south (profiles GG’ and FF’), leading to a significant accumulation of sediment on either side of the central zone in the southern part (profile EE’). The central zone, which is less notched in the northern sector of the lake, is heavily notched in the transition zone with the Cotonou channel. Notched areas are associated with sectors close to tributary mouths and flow corridors. “U” shapes are associated with weaker currents, favouring sedimentation, while “V” shapes can be observed at mouths and along flow corridors.

○ Longitudinal profiles of tributaries

Cross transects on tributaries essentially yielded asymmetrical “U” and “V” profiles. Longitudinal profiles were produced to observe morphological variations along the studied river courses.

1) The longitudinal profile of the main tributary, the Ouémé River, shows a variation in depth from −6 m to −4 m upstream and from −4 m to −2 m downstream (Figure 11). There is a gradual decrease in depth towards the lake. The numerous gradients observed mainly reflect the connection zones with the multiple branches observed along the linear route. Also, flow corridors and dredging activities should not be overlooked.

2) The longitudinal profile of the Sô River shows a relatively flat bottom of around -4 m in depth over the more or less straight northern half, with almost no branches. Over the last six (6) kilometers of the river’s length, there is a gradual decrease in depth, with two major drops of almost 3 m each marking the junction with the Ganvié and Vêkky branches (Figure 11).

The Ganvié and Vêkky arms, shown in green and blue respectively (Figure 12), have almost flat bottoms with depths of less than −4 m. These depths gradually decrease towards the lake, reaching −2 m. Only the connection zones with the Sô River are marked by deep notches ranging from −6 to −4 m.

3) The longitudinal profile of the Cotonou channel can be divided into two topographically homogeneous sectors with varying gradients (Figure 12). The upstream section has a relatively steep, non-rectilinear gradient over the first half of its length. The second half is characterized by a gradual rise of around 1 m over the remaining 2 km of the linear section. The bottom profile is highly variable, with gradients that punctuate the profile. This unevenness in the central zone may be the result of development work carried out to install structures (bridges) on the linear section.

4) The longitudinal profile of the Totchè canal shows a gentle slope from west to east, resulting in an increase in the depth of the lake towards the Porto-Novo lagoon. Two notches at 500 m and 2500 m from the lake are probably linked to junction points between the canal and river branches of unequal size (Figure 13).

Flow facies are generally linked to the hydrodynamic conditions and morphological characteristics of the watercourse. The simplified key to determining flow facies is based on four criteria: depth, flow velocity, cross-sectional profile and longitudinal profile of the stretches of the studied segment [30] . Based on these criteria, the Ouémé river, the Totchè canal and the Cotonou channel can be characterized by wetland-type facies which, depending on the section, can be subdivided into lentic channels, concave wetlands and lotic channels. The Sô River as a whole displays flow facies of the lotic channel type upstream and flat lentic to running downstream. These flow facies, generated by the geodynamic processes of erosion and solid transport, are one of the main tools for linking the hydromorphological and ecological functioning of rivers.

For the spatial analysis of 2010 and 2022 satellite images of the study area, the visible spectral bands (blue, green, red) were used to visualize the presence of water, the near-infrared bands to map vegetation, and the mid-infrared bands to study soil composition (Figure 14). Classification was used to present the different classes of features or areas present in the images. The spatial analysis made it possible to identify nine (09) thematic classes. The results were then compared, class by class, in order to evaluate the changes. The information quality evaluation criteria calculated for each image gave values close to 1, validating the methodology used in image processing and indicating high information reliability. For the 2010 SPOT image, the kappa is 0.88 and the overall accuracy is 0.90. For the Sentinel 2B image from 2022, the kappa is 0.90 and the overall accuracy is 0.92. The classifications made it possible to obtain quantitative information on land occupation during these periods and on the temporal and spatial evolution of this occupation in the study area.

For the year 2010, variations in water distribution within the study area are highlighted by comparing low-water and flood periods. These variations are shown in blue. The surface area of all the water bodies is 270 km² at low water (Figure 15(a)), and 277 km² at high water, with a floodplain (wetland) of around 421 km² (Figure 15(b)).

For the year 2022, the surface area of the water bodies is 274 km² at low water and 280 km² at high water, with a floodplain (wetland) of 419 km². The results of processing SPOT5 and Sentinel 2B satellite images show that morphological variation in the study area over a 12-year period is not very significant, with only minor changes to the shape of the lake (Figure 15(a) and Figure 16(a)). At the scale of the maps presented (Figure 15 and Figure 16), the channels show virtually no morphological variation.

Changes in land use over the period 2010-2022 were assessed. Among the factors influencing land cover change, human factors are more important than physical ones. Firstly, there is a significant and gradual expansion of the urban environment associated with a densification of existing areas then, a spatial expansion of this environment. The results show a trend towards an increase in anthropogenic formations at the expense of natural formations. Barren land and built-up areas have not undergone any conversion, but have increased at a rate of 3.6% and 69.15% respectively. Forest formations as a whole regressed by 4.29% and converted by 4.82%. Much of the vegetation has been destroyed for agricultural purposes, handicrafts (acadjas) or even domestic use (firewood).