Hopefuly breaking ground on 2 acre pond in about a week. Please someone simplify where and how deep and how large to install my pvc pipe? I have tried to read everything and look at photos posted but get confused easily. How far from the top of the dam should my emergency spillway be? How large of pipe? How far from the top of the dam should my regular drain pipe be? How large of pipe and should I install more than one in various locations? I would love to have a siphon style but I don't think my builder will know how to design it. Where exactly do I install a "drain valve" if I want to manually lower my water level or drain the pond for some reason? Please understand I need simple answers for my simple mind!! Thanks
As Esshup says, there a no set answers because the answers depend on the environmental conditions your pond will see.
But there are some generalities that will at least give you a rough idea of what to expect. The height of the dam above normal pond full pool level is called freeboard. This height is dependent on things like how big your watershed is, how big of rainfall events you could get in extremes, and how much water your emergency overflow can handle. In other words, the freeboard has to contain any amount of water that could ever flow into your pond that could not get out fast enough to where the water would run over the top of the dam. Because if the water ever tops the dam, it is likely going to breech and destroy the dam. Some extra freeboard also allows for normal settling of the dam and loss of height do to erosion. My 3.1 acre pond called for 3' of dam height above full pool, or in other words 3' of freeboard. My emergency overflow area is 1' above full pool. My overflow pipe is 8". So in other words, during a high rainfall event first the overflow pipe handles the volume as the pond rises up to 8" above full pool. Then the overflow pipe goes into siphon mode which flows more water than a standard horizontal overflow pipe. If the pipe still can not handle the flow the water rises to a foot above full pool where the emergency overflow starts to flow water. At this point I still have 2' of freeboard left. Two foot of protection of my dam. As the water flows 6" deep over the emergency overflow it will be about 15' wide at the emergency overflow at that point and I will have 18" of freeboard left to protect the dam from being breeched at that point. That was the original design and I have actually raised my overflow pipe 4 additional inches when I extended it over my bench area, so in reality my overflow pipe will never go into siphon mode as the water will be going over the emergency spillway by the time my pipe is covered, but I'm digressing into a tangent.
Three foot of freeboard above full pool and emergency spillway a couple foot below the top of the dam is very common in this area for a regular size farm pond. But it is by no means absolute. For example a huge lake that gets flood inflows might have 15' of freeboard. Or a pond that is designed for a watershed project to reduce flooding might have ten foot of freeboard above a very small overflow pipe that allows the pond to rise to high levels during high rainfall then meter the water out slowly down to normal level. As the famous PBF saying goes, "it all depends".
