Last week I picked up some 2mm & 5mm thick brown craft foam to get started with the formation of the creek.
After a little bit of doodling I cut some to form the banks of the creek and test fitted the bridge to make sure three of the bents were going to sit in the water. Then glued it all in place.
The area in front of and under the bridge will be at this level: 7mm above the creek surface. Most of this area will be covered by tall static grass pretty much right up to the water's edge as per the prototype. Before the grass goes down I'll add some sculptamold to the make the surface a little more uneven in front of the bridge.
I started cutting the bottom of the bents to match the landform by removing 7mm (thickness of the craft foam) off the bottom of all the bents (except the three that would be sitting in the water). I sliced up some tape to 7mm and applied it to the bents as a guide. Using more cut tape, I removed another ~1/8" from all of the bents so the bridge would match the height of the existing tracks.
Next I mixed up some Ground Goop and added it the edges of the craft foam to form the creek banks just under the bridge, for now. I'll add the rest when I get the rest of the thicker foam set in place.
It may seem like I'm doing this the hard way, but if I had planned for the bridge before laying the track and getting it ballasted, I'm sure it would have been a much easier process?