I'm relying on an overflow for my primary drain. It will be concrete on virgin soil and drain off over bermuda grass on a 2 to 3 degree slope over an area 30 feet wide. The overflow will be 20 feet wide and two feet tall. After that, I'm in trouble.
To help manage my pond I put in a bottom drain. I used six inch PVC with a butterfly valve. (Thanks Brettski).
The drain will be six inches lower than the overflow to handle light duty rains, but be pretty much useless on heavy rains.
The biggest reason for putting it in is that I can adjust my water level by opening the valve. The siphon system seemed like more effort than I wanted to deal with.
Two reasons really influenced my desire for a drain.
One is to lower the water level to kill shorline vegitation. I'll be able to drop the water level several feet and let the plants die, then mow them down.
Second reason is that if I'm expecting a severe storm, tropical storm or maybe a hurricane, I can drop the water level before it gets here. Hurricane Rita was supposed to drop over 20 inches of rain in my area. It didn't happen, but I realize that one day it could.
This is also why I'm using concrete for my spillway. For a few hundred bucks, I'll never have to worry about erossion. This will also support a bridge over the spillway that I'll build sometime down the road.
Eddie
