I did a tr-55 analysis for my pond. It was a bit tedious, but it gave me some real confidence in the design. I used a free computer program, wintr-55, made by the nrcs to do the nitty gritty calcs. You just have to measure your watershed using county mapping with soil types. Then input some other parameters, and wala, it spits out flows for 10, 25, 100 year storms. You can even input spillway parameters for testing.
It told me that during a 25 year storm my water level would rise 1.5 feet to right at the emergency spillway. For a 100 year storm it would flow through emergency. This was enough info for the county lady who gave me the driveway permit to cross the dam.
There are older nrcs techniques using watershed, soil types, slope, etc, that are simpler than the full tr-55.
