Water samples were hand-collected from five different sampling points along Sebeya River (NYB, KY, GH, GSH and SO) during rainy and dry seasons. Sebeya river discharges were measured using the area velocity method, commonly known as current meter method. Current meter uses a propeller component to

measure the local velocity of flow (v) and water depth was measured using a tape measure. As described by Le Coz et al. [9], the mean-section method was used to calculate the total discharge (Q) by integration of the velocity (v) over the river cross section area (A).

Due to the fact that main socio-economic development projects (Gihira WTP and 3 hydropower plants namely Keya, Gihira and Gisenyi HPPs) are concentrated downstream within 12 km from Lake Kivu, water and bed load samples were collected at the 5 five selected sites along Sebeya river for sampling purpose and presented in Figure 2. The samples were taken to chemistry laboratory of University of Rwanda, College of Science and Technology where different parameters

were analyzed including total suspended solids, turbidity and NPK nutrients content.

In order to compare the results of this study, sampling period was extended from rainy season to dry season (Table 2). The width of the river at Nyamugari bridge is 8 m and the depth of water was about 0.60 m at that date and time. At Figure 3, a field technician taking water samples in Sebeya river for TSS and turbidity analysis. Successively at (2 m, 4 m and 6 m) along Sebeya river width from the left river bank, a sampling bottle of 500 ml was inserted at 15 cm below the water surface for prevention of disturbance due to floating debris [10]. From a well-mixed collection of the 3 water samples, 500 mL water sample was taken for laboratory analysis.

From each water sample of 500 ml taken from the respective sampling site, a small quantity of 100 ml was placed in an aluminium sheet plate which was then heated on a hot plate for about 30 minutes in order to vaporize water. The humid residues left on the aluminium sheet plate was then dried in the oven at 105˚C for 24 hours and after that time the dried TSS was weighed and the concentration of TSS determined in mg/l.

Turbidity meter is a complex and delicate instrument which needs to be calibrated before its use. In this study, a 50 ml water sample was poured in a turbidity grass which was then placed in the turbidity meter to allow readings of the turbidity values in NTU.

Samples were hand-collected in the catchment at five selected sampling points.

The sampler used for this study (Figure 4) was designed referring to the description given by [11]. The characteristics of the designed sampler are as follows: opening front size of 0.3 m * 0.2 m (large enough to allow sediments to enter), total length of 0.9 m and the mesh size of 3.9 mm (for capturing different sizes of sediments greater than 3.9 mm). For bed load sampling, 3 samplers were immersed at the bottom of the river at 3 equidistant points along the river cross-section (at 2 m, at 4 m and at 6 m from the river bank). Before starting the sampling process, the mass of each empty sampler was recorded and samplers placed in contact with the river bed against the flow, allowing the sediment transported as bed load to accumulate or be trapped inside the sampler for 1 hr (Long duration for the collection of infrequently moving large particle sizes near the threshold of motion). After one hour, the samplers were raised up to the surface and the weight of the content of each sampler was recorded. The bed load from the 3 samplers were well mixed and 1 kg taken from this mixture for further analysis in the laboratory.

Laboratory procedures were performed by sieve analysis as per IS: 2386 (Part I)—1963 to determine the particle size distribution of the coarse and fine aggregates. Different sieves with different sizes (20 mm, 10 mm, 8 mm, 6 mm, 4.75 mm, 3.35 mm, 2.36 mm, 1.18 mm, 600 µm, 300 µm, 150 µm and 75 µm). The bed load sediment composition was determined using MIT (Massachusetts Institute Technology) grain size classification standard.

Sebeya river discharge measurement was performed at Nyamugari bridge situated at Kanama Sector in order to determine the value of the surface outflow generated by Sebeya catchment and its temporal variability. By knowing the discharge Q (m³/s) and TSS (mg/l) we can deduce the mass of TSS (kg) passing through the river per day. The measurement of discharge has also high impact on sediment transport that leads to hydropower and water treatment plants damages [12]. For low water level at about 60 cm, a current meter was used to measure the stream velocity at each selected interval and the corresponding discharges were calculated as presented in Table 1. Considering all sampling dates as shown in the Table 1, the average Sebeya river discharge was 2.245 m³/s. During heavy rainfall event, the discharge was found to be high in Sebeya river and reduced significantly after rainfall event.

Water resources occupy 8% of the total area of Rwanda but due to lack of data, it is difficult to estimate at which extent the climate change has impacted on water resources [13]. The following statements stand for the sediments loading rates and water quality in Sebeya river (Table 2).

In broadest context, soil erosion is responsible for increasing river sediment

Sampling dates: [RS: Rainy season (26/04/2019, 3/5/2019 and 18/5/2019); DS: Dry season (26/07/2019, 8/2/2019 and 8/8/2019)].

loading rates with two major types of pollution related to the physical and chemical composition of the sediment [14] [15].

An important parameter for the analysis of water quality is the concentration of Total Suspended Solids (TSS) because these types of solids are good indicators of physical and aesthetic degradation of surface water quality [16] [17].

Due to soil erosion, the top soil and soil nutrients are carried into rivers, lakes and water reservoirs resulting in high turbidity. In general, water is more turbid during rainy season as compared to dry season (Table 2). The turbidity is an important parameter which helps to judge the water quality and it gives the idea on purification work to be carried out [18].

Nutrients lost from agricultural fields due to soil erosion, particularly nitrogen and phosphorus, become common pollutants in waterbodies and are largely responsible for what is known as “eutrophication”. Eutrophication is a process in which excess fertility in a waterbody leads to excessive plant growth, low dissolved oxygen, bad odors and the production of toxic substances by blue-green algae which may cause human diseases or kill fishes and other aquatic animals [19] [20] [21].

The variation in mass flow rates of bed load was studied during the two seasons. Table 2 and Figure 5 show that higher values of bed load sediments were observed during rainy season and decreased significantly during dry season. Sedimentation affects the stability of dams, reduces the reservoirs storage and the carrying capacity of rivers which consequently reduces the hydropower production and flood attenuation capabilities. Large scour may be developed at bridge

piers and abutments threatening their structural integrity [22].

In Table 3, sieve analysis results showed that almost all bed load samples were mainly composed of high proportions of fine during dry season at more 70%. Other remaining portions such as silt, clay, coarse sand, gravel, and medium sand showed low percentages. Sand materials cause abrasion on the hydraulic turbines and consequently reduce the performance of the hydropower plant in producing electricity [5] [23].

After comparing sediment concentration at the inlet and outlet of the sediment basin, it was found that the basin removed only 22% of sediment from diverted water toward the turbine. This means that 78% of sediment escapes the basin to cause erosion on the turbine components resulting in reduction of the plant capacity from 2.2 MW to 900 KW [5]. It is therefore important to enforce erosion control measures within Sebeya catchment in order to mitigate the impacts of sedimentation by reducing the volume of sediment that is brought to the hydropower intakes by Sebeya river. Alternatively, a sediment basin can be built for trapping suspended sediments before they reach the turbine components. Developments in achieving a more sustainable sediment management strategy are of high interest for HPPs, owners, environmental agencies and local government [24].

From Table 2, the average Total Suspended Solids (TSS) for both seasons are plotted in the same graph as shown in Figure 6. The higher values of suspended solids were observed in the rainy season at all sampling points comparatively to

Sampling dates: [RS: Rainy season (26/04/2019, 3/5/2019 and 18/5/2019); DS: Dry season (26/07/2019, 8/2/2019 and 8/8/2019)].

dry season. Spatially, there is a small increase of TSS from Nyamugari bridge to its downstream Keya hydropower due to a frequent illegal mining of sand and unimproved agriculture practices between these 2 river sections. However, from the inlet of Gisenyi HPP to Sebeya outlet, the river bed is flat which allows water to flow at a small velocity and then facilitates suspended particles to settle down.

Turbidity is proportional to the availability of clay, silty and sand from organic and inorganic matters [25]. Table 2 and Figure 7 illustrate the variation of the measured turbidity in Sebeya River along rainy and dry seasons. During rainy season, both suspended sediments and turbidity increase due to soil erosion and low conservation measures in Sebeya catchment. All turbidity values in two seasons are above permissible limit for drinking water at every sampling point. This is one of the issues faced by Gihira water treatment plant in terms of excessive costs of reagents for treating high turbid raw water (Table 4).

Sebeya river water depths are automatically recorded by the hydrometric station installed at Nyundo Sector by Rwanda Water Forest Authority. Based on the available data, the channeling capacity of Sebeya river was estimated at 100 m³/s [27].

In this study, Figure 5 says that 4 kg/hr of sediment bed load are rejected by Sebeya river into Lake Kivu which seems to be negligible compared to the storage capacity of Lake Kivu of V = 500 km³. However, in flood condition, more sand sediments are seasonally generated and if no preventive measures are taken, the useful storage of Lake Kivu at Gisenyi beach location can be reduced significantly and swimming uses can be negatively affected. High sediment load into the Lake Kivu can also make water less recreational and destroy aquatic habitats (Table 5).

Table 2 and Figure 8 describe the variation on NPK in water samples taken from Sebeya river during the 2 seasons of sampling while the Table 2 and Figure 9 indicate the variation of NPK detected from the bed load samples. No bed load sediments were found in the samplers at Sebeya outfall during dry season.

At Sebeya outlet, nutrients loadings into Lake Kivu are mainly linked to heavy

rainfall which is influenced by the rapid growth of population, deforestation and soil erosion along the Sebeya catchment. At Sebeya outfall, the nutrients load could be prevented.