hi
i think that you could do a fake ( i mean with this :: easy, fast, not accurate but realistic with a bit of job)...
you could use 2 or 3 textures moving down mixed to achieve the look of the fast falling of water ( separated from the wall) and the slow falling down of water that is sliding over the wall
i repeat it's a fake but if you want one advice, in the day after tomorrow ( i think) they used realflow to simulate the sea entering in NY and when they tried to modify the simulation and they weren't getting the desired effect, they decided to fake the water with particles following the behaviour showed in the simulation fo realflow... i want to say that sometimes it's better a fake that a simulation that you cannot modify to be realistic or at least seem to be
luis