Uh, now that I read your post more closely and saw that you want the caustic pattern to be on the bottom of the sewer. Edit my previous post and remove that part "point it towards the wall". I thought you wanted a reflection on the wall. Other stuff still stands, just put a light above the water, point it down. Try with gel and without it. You might want to increase TIR in Render options. Also, range, intensity and colour will also play a role in how the caustics look.

-- erlik