There is an art thing called Value Degration. Basically you can impart a 3d effect by establishing bright in the foreground, dark in the background or visa versa. Here you scene is of the same lightness regardless of distance. This causes the back to pile onto the front. Once you establish a scheme don't break it. For example if you do dark in front and light in back be careful not to stick something light in the foreground or that will act like an anchor and destory the illusion. Also some artifacts where the bridge meets the dirt would be nice. A rock in the right spot can imaprt depth if its in front of the bridge rails. You can also perform detail degration which works similar to value degration only as far as focus. A sharper, more focused and etail item between the viewer and the bridge and duller items behind the bridge can add depth as well. Right now you have nothing in the foreground to establish depth or scale. The eye gets to the bridge then the scene cascades off into flatness going toward the viewer. Maybe a bunny rabbit or bird on a tree would help. Nice pic. I like the grass and trees. Again the grass is monotone in detail. Having sharper grass in front and bluryer in back could help.