I am making an RTS game and want to add the ability to dig a trench without redrawing the whole terrain as that is causing lag. The solution would need to allow a user to build a trench on the map and have a unit able to stand in the trench. The game is using 3d models at the moment.
My first thought was to have the trench on a layer that is rendered on top of the layer with the terrain but that would give the problem that a trench would be seen even if it was behind a hill.
Is there an alternative or a solution?