I am renovating a pond that I had built about 30 years ago. The pond is about 0.6 acre surface area and is 18' deep at its deepest spot, but is generally pretty deep throughout most of the pond. The pond had filled with sediment which I have removed. There was 6' of sediment at the bottom of the pond in it's deepest spot.
The pond was built with a 6" galvanized pipe as a surface withdrawal spillway. I would like to install a bottom withdrawal spillway to improve the water quality and to prolong the life of the pond. The original galvanized spillway is probably nearing the end of it's life, so I thought I might use 4" PVC to build the new bottom withdrawal spillway and feed it thru the existing galvanized pipe. The galvanized pipe would be sealed to prevent water from entering. This would allow me to not have to cut thru the dam to install the spillway outflow pipe. I am in east central Iowa. My question is if the 4" pipe is large enough for what I am trying to do. Any opinions would be appreciated.