Here's a smaller version of the same bridge. I placed the posts a little closer to the ends, and the arch is a little steeper. I used a V-shaped router bit to cut a groove near the top under the copper caps to give it some contrast.
Here are a couple bridges I've designed and built over the years. Maybe you can get some ideas from them - they're really not hard to build. Make sure you use bolts (not screws) to attach the posts to the base and don't forget how heavy the bridge can be once you've got it all put together! I usually make the base and post/rails as separate pieces then assemble everything on site.
Build the bridge out of pressure treated lumber or cedar. They’ll be resting on the ground so you want something that is rot and insect resistant. If you can place them on a couple bricks or a cement pier, even better. Your bridge will last a long time.
Here is a 10 ft. lawn bridge that I built a couple years ago for a customer not far from me. He had a swell in his yard that always collected water when it rained, so he asked me to build a wooden, arched walkway so he could get over to his barn without getting wet. Here is the bridge I came up with. The tops of the posts have copper caps on them and the handrails have the same arch as the walkway. I got the idea from viewing pictures of a few Japanese style garden bridges.
To lay out the arch, I simply used a string that I let sag down from each end and carefully traced it onto the wood - gravity can be a useful tool Then I just traced the arc from the base onto some 1" deck boards to make the handrails. This bridge is 10 ft. long, but the same method could be used on longer bridges, you just may need to add a few more posts. Interested in building this bridge? Here are my plans!