No problem. What I’d do is to set up one scene that contains things (such as cameras, significant lights, and reference geometry) that are to be common to all other scenes. Then, create additional scenes, linked to the first, that represent things that you actually want to produce to make the shot. (Or, you can use entirely separate files and still link them, but I digress.)
“First, do the roadway,” and render that roadway. (If the surface of the road and the interior of the hole are significantly different e.g. in lighting, you can also render these separately, for later compositing. Likewise the dirt field. Anything. “Break it down. Break it down …”)
“Next, do the water.” Let the water flow within a box or guide of some kind that will be “seen” by the water simulation but that will not appear on-screen (i.e. it’s in a hidden layer). Render the water. (Don’t worry about the pot-hole: you can mask that later. But also don’t bother to render stuff that will be forever blocked by the hole.
In both cases, render to MultiLayer OpenEXR files and in those files capture whatever RenderLayers you think might be useful in the next step(s).
Remember that you can link to actual geometry, such as the interior of the hole, if only to use it as a visual reference in placing the water-guides.
“Finally, compositing.” This step uses the outputs generated by the other two steps, and it is at this point purely a two-dimensional process. The water is superimposed over the road, but it is made partially transparent. You can use characteristics of the water to modulate exactly how the two (or more …) channels of information (“road,” “water” …) are combined. There are a wide variety of things that you can do at this step, and “they’re all fast and cheap,” because it’s all two-dimensional processing at this point … although it can make use of any of the data that you captured in the MultiLayer OpenEXR file format. (It can, in other words, be “3D aware.”) You can color things, for example, or tweak the colors. You can vary the opacity of things, like water. You can dynamically adjust the hue and saturation of things, like the road, to further the illusion that you are “looking through water.” You have lots of opportunity to be very creative here.
A good first step, after rendering the road in this way, is to create nothing more than a rough piece of geometry, with appropriate materials, that will represent the water more-or-less everywhere it’s eventually going to be, so that you can immediately shift ahead to the compositing step to get that “block of water” looking sufficiently “like water” in the compositor output. Use this to guide you as you go back and construct the water simulation. (It might then become your bounding-box …)
This is a multi-step workflow that will take you to a result that looks convincing [enough], and that minimizes the expensive process of “3D rendering.” The use of “MultiLayer OpenEXR” is critical because this format captures all of the digital data in high-resolution numeric form. You have “all of the numbers” that those renders produced, in addition to the pixels.
