ISC,
It is also possible to control your water level by controlling the elevation of the drain tube at the discharge end of the pipe.
It will cost more money for pipe, but it may be much easier to install.
You can install a 45 ell, then a little straight pipe in a horizonal run, a 90 ell, and then a pipe riser that is vertical and extends up to your desired pond elevation level.
In this scenario, the next time a water inflow event raises your pond level, your outflow tube will fill with water. Any excess water above your outlet elevation will run out of the tube, but your pond will stop draining at the new level.
You can even make a variable water level by making a "turn down stand pipe". Basically, your highest water level is when the outlet pipe riser is pointed straight up. As you "turn down" the pipe by rotating it at the 90 ell, you can select a lower water level elevation.
[It would be very difficult to make a turn down stand pipe with spiral metal culvert, but it can work for other piping material for people in a similar situation.]
Good luck on your project!
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