Fig.08 shows Layer 1, which has been color corrected to look green in the render.
Fig.09 shows Layer 2, which has also been color corrected to appear dark green and also uses a mask to ensure it doesn't cover the previous texture completely.
Fig.10 shows Layer 3, which again has been color corrected to make it blue and also takes advantage of a mask restricting it to the left hand side. This layer has been set to Screen mode in order to lighten the effect in relation to the ones below it.
Fig.11 shows Layer 4, which constitutes the dirt at the upper and lower edges of the wall and therefore is set to Multiply. In order to get the mask to create a stain along both horizontal edges, it is mirrored in the V axis only and the Tiling has been altered so that it is visible above and below.
The wooden poles and bridge supports were textured using a similar technique. I found a generic wood texture which I then tiled more along the V axis in order to create a suitable scale. Then I used another Layer set to Darken in order create a random array of marks and mask the tiling somewhat (Fig.12). It looks somewhat crude upon close inspection but in the context of the scene it provides just enough detail to work.
The last remaining aspect that is probably worth a mention is the pile of rubbish below the bridge. To save time modeling a few components and then scattering them, I chose to Planar map some geometry that approximated an accumulated pile. I added a few token pieces of geometry that I placed around the edges to create a more uneven edge, but allowed the texture to do most of the work. The template was composed using cropped selections from numerous sources and then pasted together to form a texture.
I found some useful images in 3DTotal's free reference library, which can be found at the following locations:
After cropping and pasting these into a new file I then duplicated and re-positioned various parts and used the Clone Stamp tool to blend everything together.
Fig.13 shows the final image, which is an amalgamation of each of the three photos. The render was taken into Photoshop and a background was added along with some color adjustments and post work.