This case is a simulation of the water velocities in a reach of the river Sokna, located 1 hour drive south of Trondheim in Norway. The river has large roughness elements, rocks and stones. A bridge spans the river at the upstream part of the section. The picture below is taken from the bridge in the downstream direction.
The roughness elements are smaller than the grid size, so a porosity model is used. The model is based on the x,y and z coordinates of a large number of measured points at the bed of the river. The points are so dense that individual rocks are resolved. The bed of the grid is located in a plane cutting through the points, closer to the lower coordinates than the average bed level. The points above the grid bed are used to determine the porosity in the grid cells. The formula is given by Olsen and Stokseth (1995).
The velocity field is compared with measurements (Olsen and Stokseth, 1995) for a high and a low water discharge. The figures below show the velocities for the higher discharge of 188 qubic m/s.
Velocity vector and color map, seen from above, close to the surface
Velocity vector and color map, seen from above, close to the bed
Three-dimensional color map, where a slice has been cut in a longitudinal section of the river. The colors show the magnitude of the velocity
Olsen, N. R. B. and Stokseth, S. (1995) "Three-dimensional numerical modeling of water flow in a river with large bed roughness", Journal of Hydraulic Research, Vol. 33, No. 4., pp. 571-581.