So, I back tracked and read down 35 out of 66 posts, and kinda regret it....
I did a very quick Google search, and found 18" flood gates for anywhere from $200 to $1500 for proper gate value types. Some have a weir style, where when they over top the gate, they are free to flow, some look like a exhaust flap on a peice of equipment. Now, there is a hazard to letting 5 or 6 ft of head build up.
So, if we walk through this; we add a good base of 18" of clay/gravel, and compact that we'll, we then put a 8 ft long section of 18" HDPE pipe on top and back fill, and compact. We then install the flood gate; we have an gate open depth of 18", very manageable. We close the gate, we build upto 36". If you are not there daily, I would look for a weir style, just incase you get heavy flooding while not Around. As a safety feature, we can install a 8" pipe at about 4 ft elevation, or better yet, we build our 'dam' 5 ft high, Except in the a spillway area, we leave it down at 4ft, and lay soil cement bags over the spill way (quikcrete from Home Depot is fine, leave it in bag and lay over the spillway). This acts as an armor for our spillway to prevent erosion, But also acts to so degree as disapator blocks, slowing water to some degree, also slowing erosion.