SRH-2D numerical model

The Sedimentation and River Hydraulics-Two-Dimensional model (SRH-2D) was implemented in this study. The SRH-2D is a module integrated into the surface water modeling system package (SMS-2D) which is a comprehensive package of tools for simplifying the development of 2D hydraulic models. (SRH-2D) is a 2D hydraulic numerical model based on 2D hydraulic principles for river hydraulics and sediment transport developed at the U.S. Bureau of Reclamation (USBR) (Aquaveo 2013). (SRH-2D) solves the time and depth-averaged Navier Stokes equations (known as the depth-averaged St.Venant Equations) to govern the flow regime (Lai and Greimann 2008) as follows:
\(\frac{\mathbf{\partial H}}{\mathbf{\partial t}}+\frac{\mathbf{\partial HU}\ }{\mathbf{\partial x}}+\frac{\mathbf{\partial hV}\text{\ \ }}{\mathbf{\partial y}}=0\)(1)
\(\frac{\mathbf{\partial HU}\text{\ \ }}{\mathbf{\partial t}}+\frac{\mathbf{\partial HUU}\ }{\mathbf{\partial x}}+\frac{\mathbf{\partial HVU}\text{\ \ \ }}{\mathbf{\partial y}}=\frac{\mathbf{\partial H}\mathbf{T}_{\mathbf{\text{xx}}}}{\mathbf{\partial x}}+\ \frac{\mathbf{\partial H}\mathbf{T}_{\mathbf{\text{xy}}}}{\mathbf{\partial y}}-\text{gH}\ \frac{\mathbf{\partial z}}{\mathbf{\partial x}}-\frac{\mathbf{\tau}_{\mathbf{\text{bx}}}}{\mathbf{\rho}}\)(2)
\(\frac{\mathbf{\partial HV}}{\mathbf{\partial t}}+\frac{\ \mathbf{\partial HUV}}{\mathbf{\partial x}}+\frac{\mathbf{\partial HVV}}{\mathbf{\partial y}}=\frac{\mathbf{\partial H}\mathbf{T}_{\mathbf{\text{xy}}}}{\mathbf{\partial x}}+\frac{\mathbf{\partial H}\mathbf{T}_{\mathbf{\text{yy}}}}{\mathbf{\partial y}}-\text{gH}\frac{\mathbf{\partial z}}{\mathbf{\partial y}}-\frac{\mathbf{\tau}_{\mathbf{\text{by}}}}{\mathbf{\rho}}\)(3)
where x and y: horizontal cartesian coordinates, t: time, H: water depth, U, V: depth-averaged velocity in x and y directions respectively, g: gravitational acceleration, T𝑥𝑥, T𝑥𝑦, T𝑦𝑦: depth-averaged stresses due to turbulence, 𝜏𝑏𝑥, 𝜏𝑏𝑦: bed shear stresses, 𝜌: water density, Z=Z𝑏 + h, Z: water surface elevation, Z𝑏: bed elevation.

Mesh generation and boundary conditions

The generated mesh which represents the study area using the SRH-2D numerical model consists of a total number of 180,000 triangular elements with a minimum width of 30m. The bed level elevation was assigned to mesh elements at each node. The upstream boundary conditions are the flow discharge from the delta barrage and five drains. The downstream boundary condition is the water level at the Shabrakhet gauge station corresponding to the upstream flow as shown in Figure 3.

Model calibration and verification

The numerical model was calibrated using flow discharge of the year 2021 with the corresponding water level and stream velocity at two cross-sections as shown in Figure 4. The best suitable value for the manning roughness coefficient (n) is 0.015 after several model simulations. The numerical model was also verified using flow discharge of the year 2018 with the corresponding water level and stream velocity at two cross-sections as shown in Table 1. Model performance was verified by