I am doing a job about flood simulation. First Step I add a terrain into the RAS mapper and then edit the geometry data in which I add an 2D area and upstream and downstream BC lines and generate the computation cells. The cells showed in RAS mapper are like this:

After the computation, some results showed in RAS mapper are like this:

The pink color represents the Water Surface Elevation. At first I thought every cell would be filled to full if the cell had a value and then the interface would be rendered. But the image showed above means that's wrong. So what's the true strategy or algorithm to render the interface? How does the data or value be used to achieve the image above?

Could anyone help me to solve the problem? I'm glad for any help.