Just signed up but I have been reading for a while trying to decide if I can build a pond on a piece of property I am looking at (40 acres total). There are 2 drainage ditches/wet weather creeks that converge and empty into a fairly large wet weather creek. I would like to dam them before they enter this larger creek. Issues I see
1. Larger creek could flood and wash out dam from back side.
2. Pond could back up to road depending on height of dam and desired size.
3. Smaller drainage ditches carry a lot of water sometimes (8ft sides while standing down in bottom of ditch) can I handle the inflow/outflow?

Dam would be about 200ft long and I figure around 25ft high. Red is dam, blue is my estimate of pond and orange is where I plan to build my home.

I'm not sure picture has enough detail. I tried to overlay topo on google earth.

I am no expert, but a carefully designed and large emergency spillway may take care of some of your concerns (at an elevation based on the size of your watershed and the height of the raod). Another thing to consider is the size and drop on your tube through the dam. More drop equals more capacity once the tube fills and a siphon starts (gravity pulling on the water through the pipe). Plenty of free board on the dam is also a great benefit. My 1.25 acre pond filled in 12 hours just 2 weeks after being built (in a deluge), and all of these features prevented any damage. However, there is no substitute for having a good contractor. I also had all of my excess soil placed behind the dam and sprung for a long pipe to maximize drop. The extra money was well worth it for me.