## 4 Initial Conditions and Numerical Results
### Numerical Results  
#### 4.2.1 Sensitivity to Rainfall Intensity of the Model  
Figure 5 shows the water height propagation in the Tongo-Bassa watershed for rainfall intensity of 15 mm/h. After 15 h, a surge in the water converges toward the watercourse located in low-altitude or valley-bottom areas, which eventually comes out of its minor bed; the water height reaches 1.2 m in the deeper section.  
The same simulation has been repeated, taking 85 mm/h as the rainfall intensity, and the results are illustrated in Figure 6. The distribution of water over the watershed appears  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Land** & \(i_{f}\) & \(i_{o}\) & **r (min\({}^{-1}\))** & **n (s/m\({}^{1/3}\))** & **Rainfall Intensity** (mm/h) \\ \hline Vegetated & 0.029 & 0.097 & 1.383 & 0.065 & Case 1: 15 mm/h \\ Bare & 0.005 & 0.017 & 1.383 & 0.020 & Case 2: 50 mm/h \\ Cultivated & 0.021 & 0.069 & 1.383 & 0.050 & Case 2: 85 mm/h \\ Urban & 0.003 & 0.008 & 1.383 & 0.015 & Case 2: 85 mm/h \\ \hline \hline \end{tabular}
\end{table}
Table 2: The parameters applicable for the study of sensitivity to rainfall intensity.  
Figure 5: Water height propagation at different time steps for a constant rainfall intensity (15 mm/h).  
greater than in case study 1 (at 15 mm/h). The floodable surface is more significant and covers the urge space of the watershed. The water heights in the lowest points (watercourse) reach a maximum value of 4 m after 11 h of simulation.  
Overall, the results show that the greater the rainfall intensity, the shorter the time needed for the water in the watershed to reach a significant height. The floodable surface is well-represented in the watershed, with an emphasis on flooded areas with a wide distribution of water height.  
In summary, the results indicate that water depth in the watershed is sensitive to both rainfall intensity and the simulation duration.  
The curves presented in Figure 7a,b show the heights of the water and flow discharge obtained at the outlet for each value of rainfall intensity (15, 50, and 85 mm/h). It transpires that the height at the outlet increases with rainfall intensity and simulation duration. This can be explained by the fact that the study area is located in a sedimentary zone; the water saturation rate of the soil is very important, resulting in the low infiltration of water. The runoffs on the surface are more important and all converge toward the outlet. This explains how the greatest water depth (4.04 m) was obtained with high rainfall intensity (85 mm/h) and the lowest value (0.87 m) was obtained with low rainfall intensity (15 mm/h).  
All the curves start at point 0 because it was assumed that the point located at the outlet was dry at the start of the simulation. The flow rate curves (Figure 7b) for different rainfall intensities show an increase in flow rates that is proportional to the water height. The maximum value of the flow (35 m\({}^{3}\)/s) is reached at 85 mm/h. When the rain intensity is 50 mm/h, the flow rate reaches 13.5 m\({}^{3}\)/s while at 15 mm/h, 2.5 m\({}^{3}\)/s is obtained as the maximum value of the flow at 12.14 m of water height.