Slakkar,

Thanks. Glad you like the image. :) Caustics are envelopes of rays (light rays) eminating from a point (a light source) and reflected or refracted by a curved surface (water surface riples in this case). So, the caustic effect I'm after is caused by light refracting unevenly when it passes through the surface of the water in my image. Takke a look at the little image above as an example of the caustic effect I'm talking about (image borrowed from Linda Ewing's tutorial "Deep Sea Diving with Bryce"). See how there are squigly areas of greater light intensity cast on the sea floor? That's the caustic effect I'm talking about which is caused by light defracting through the rippled surface of the water.

rickymaveety,

Are you saying the light would not reach the water because it wouldn't be passing through the ground even though you can't see the ground because this is a cutawy view? :) Well... I'm taking the view that the ground really is cutaway and not there or invisible, and that the light can reach the water. In anycase, the light is reaching the water in the storm drain pipe in the image, or at least the part of the water not in shadow. Does that make sense. I hope I understood what you were trying to say correctly.

Kemal & rickmaveety,

Thank you for confirming the notion that it would be best to create two separate renders, one with and one without the caustics, and to edit them together.

 
If anyone else has any comments or ideas please don't hesitate to post them.  
 
Thanks!