Thanks for the quick feedback! A couple of things to help w/ response:
The original waterway is pictured below. The dam goes exactly where the trail is shown, with the pond to the right of it. That was part of the reason we built the pond, as in the winter the trail was too muddy for effective travel, cutting off 90% of the property from the front 10%. The dam was used in lieu of a very large bridge / very large culvert.
Downstream of the dam, originally, about 500 yards northwest, the gulley is about 400' wide, with a trench as deep as 25' cut into it. We've had such significant rain on mother's day, that the trail that crosses the drainage intersecting the main gulley northwest of the pond was also cut for the first time in 30 years, washing out the trail entirely and turning it into a new 12' deep creek.
As for acreage, I manually calculated 80 acres going into it, Plan My Land Operation is all over the map, calculating from 0.5 acres to 300 depending on where in the pond you click. The county roads all mess with the normal drainage, forcing water down into those three creeks, so I roughly estimated the 180. I could be very high in that calculation.
The pond filled by mid October, after having been down 6' from the summer drought (3.5 months w/ no rain and > 100 degree temps), and then we had a big storm dropping 5" over 12 hours, a camera on the spillway I installed just before the storm showed 12" of freeboard during the worst of the flow. We haven't had a topping event since we widened the spillway. Given that the pond is always full by the time the big rains start, for most of the wet season 100% of the water that comes out of the drainages goes through the spillway, there is no holding capacity in the pond its self.
With respect to a syphon, a concern I have is that it is complicated and may not always work as expected until tuned properly, based on what I'm reading. That would be a concern for us, as we're usually only out there every two weeks, and most of the big storms hit when we're not there. Are they reliable enough to be built and forgotten for several years?
We're also willing to raise the dam another few feet and move the spillway uphill a bit, to install a traditional drain. Heck, we could add 8-10' to the pond (and likely about 5 acres) without encroaching on neighboring properties.
