Your question is too general to be answered here. You need to provide more detail and ask specific questions. Constructing a road for (presumably) 15 miles along a river requires many, many considerations. It is not a simple task and requires planning, engineering, environmental and construction expertise.
You said "along" which means parallel to. You're actually talking about "across" and it's more properly called a "bridge." Surely, there's a word for "bridge" in the Philippines?
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Yeah, may be a bridge-like structure designed to allow vehicle to cross over the waterway while allowing adequate passage for the water. Because I'm refering to a culvert type structure.
There are many variables which need to be considered, including width and depth of river at present, flow rate / velocity, what the range of flow is ( storm, rainy season / flood), ground type, height and width of road, type of vehicles / weight/ quantity, environmental restrictions etc etc. A few more details or a picture would help....
At its most basic this is simply a matter of lowering in your culvert pipe or pipes and backfilling around it quicker than the water washes it away.
We have even cut the ends off of shipping containers, set them side by side, and set them in concrete to make multiple box culverts. Top them with a reinforced slab and you are good to go with a site-built, impromptu bridge crossing. Or use smaller culverts, and let the high water levels during floods wash over the top of the roadway, as in a low-water crossing.
Dave
I think the general theme here is: there are lots of structural solutions to a culvert type bridge structure. The most important and critical part of the process is understanding the hydraulics.
This is on a larger scale than your project, but a recent reminder of what can go wrong if the hydraulics aren't understood:
