Saving velocity values in the HDF file, I realised that only two velocities are saved for each node: velocity in X direction and Y direction. In reality, I think water can flow in all four directions (up, down, right, left), but as the HEC-RAS Node velocity results suggests, water can only flow in two directions? I’m not sure what I am missing in my interpretation of HEC-RAS results.
I think what I was looking for was Face velocity, not the node x and y velocities. Each cell can have four faces (if the cell is not located in the boundary of the 2D area) and the velocity and flow rate is calculated for each face in HEC-RAS.