We are looking at repairing a pond. It has a 72” pipe in it now. The beavers dammed up the pipe and the rain from hurricane Michael blew out the run around on one end. The pipe is plenty big enough to drain the flow it has but it needs a new run around. Is there a rule of thumb for the height of the run around in relation to the normal level of the pond or the max water height when pipe is full? Normally it has about a foot of water in it. When they want to hunt ducks they put in a couple 8” y’all slats to raise the level. But the beavers and heavy rain broke it lol. Thanks
What is a "run around"? Not familiar with that term in pond construction.
Emergency spillway I guess is another term. Where the overflow water goes when the pipe can’t handle it
OK that's what I thought it was. I am not educated on the hydraulics of pond design but most spillways I've installed are 1-2' taller that the overflow pipe/structure in the pond. Is the 72" pipe the overflow pipe/structure? That's a big pipe for overflow - how many acres is the pond?
Yes that’s what I was thinking a foot above the top of the pipe. Yes it’s the overflow pipe. I know it big lol. Really not sure the acreage. Pretty sure they used that pipe because they already had it. It has a strong flow though. But that pipe should easily handle the amount of water is dumps. Problem is the beavers dames it up and there was no where for the water to go
Understand using what you got. If beavers will be a continuing problem I would get some rip-rap for the emergency overflow or pour it in concrete if the budget allows.
Not a pro. From what I have read about pond dams is the spillway you are talking about is an emergency spillway. They are at an elevation above the normal water level but below the top of the damn to prevent water from running over the top and eroding the damn. Spillway seems to be located the on either end of damn.
Tenfo that’s right. That’s what the one dude said that looked at it today with me. About a foot above the normal water level. I was thinking a foot above the top of the pipe. But above the normal water level makes more sense to me. I can’t ever see a pipe running at full capacity without some kind of damage. But idk. He’s a pond man so he would know better than me fosho!
As long as the spillway is at an elevation sufficiently lower than the main pond bank to prevent erosion you will be alright. "Sufficiently" is kind of based on factors determined in field. If you have a lot of inflow or the pond catches a lot of stormwater, you might want to leave the spillway a little lower than a pond which doesn't see many high water instances. You want to be able to drain water as fast as it is flowing in, given a situation where the normal drain is failed or is operating at max capacity. The deeper the spillway in relation to the top of the bank, the more protection room you have before the water would overrun the entire bank. If the pond is fed by a stream that has a tendency to flood during heavy rain, you might want a different spillway height than say a pond that is simply spring fed. You can also vary the width of your spillway to provide more or less flow capacity. As mentioned in another response, some reinforcement on the spillway to fight erosion would be ideal. If nothing else, make sure you get some grass or something growing in there to hold the soil.
It sounds like someone needs to run a quick and dirty storm water model to estimate the flow that this spillway needs. Otherwise we are going to be back where we are at now. This overflow needs to have pretty substantial erosion protection on it because it's the EMERGENCY overflow. It has to handle everything that the normal outflow cannot without failing. Just look at that dam in California that had the EMERGENCY spillway fail..... the wider the spillway the more flow can go over it with less energy at the top, the path that the water takes down over the dam needs to be looked at as well. If you get the water moving to fast. It goes super critical and can create a hydraulic jump at the bottom. That will eat away the toe of the dam (if that is where it occurs) very quickly. There is nothing wrong with a pipe flowing 100% full. There is however a problem with that flow hitting a natural channel with such energy that it starts causing erosion. I don't do pipe hydraulics for a living but im going to guess a 72" pipe is going to need solid concrete and Wingwalls with some serious shot rock to help the channel withstand the energy.
You aren't giving us much to go on. The size of the pond doesn't matter. Its the size of the drainage area that feeds the pond that matters. As far as the elevation of the emergency spillway, I'd shoot for about half way between the tube and the dike. If its washed out, it wasn't wide enough. And the beavers? They will be back, so size the spillway to be adequate without the tube helping.