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Sizing the emergency spillway
#534697 05/02/21 03:55 PM
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Greetings. I am sure someone has discussed this, but I can not locate it.
I am planning a 7 to 8 acre lake. The watershed is about 250 acres. Location is west central part of Indiana.
I am trying to locate the formula for determining the emergency spillway size when the above is given. The watershed is about one third tillable and the rest in woods and pasture land. I would like during the worse rain the water would not be more than 2-3 inches deep on the spillway.
Would someone direct me to the thread or to the formula?
Thank you. Blessings


Tooger Smith
Re: Sizing the emergency spillway
Tooger Smith #534700 05/02/21 07:04 PM
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Thirty to one watershed to lake ratio likely means you're gonna need a big emergency spillway. Especially if you want to have only 2 or 3 inches deep during largest rain events. I have about seven to one ratio and have had several major flood events in just six years. Of course, I do have a constant low flow creek, which you may not.

What is your annual rainfall? How about soil types? Is the watershed mostly flat, or is there a pronounced slope? Do you have a flowing creek? If so, what is the flow rate? All will play a role in the answer.

Last edited by anthropic; 05/02/21 07:05 PM.

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Re: Sizing the emergency spillway
Tooger Smith #534708 05/03/21 10:47 AM
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I am west of you in South Central Illinois. Anthropic provides good questions. Based upon my lake and experience, I would encourage you to have a larger dedicated spillway than you might first consider too.

My lake is in the 17 - 21 acre range (depends if count sediment basins, etc.) and its watershed is one hundred and some acres (maybe 170?) of mostly farm fields and CRP. I used the emergency spillway as the main overflow for some time - until a major rain event occurred, it cut a trench big enough to bury a bulldozer, and made me fear major repair expenses. There was apparently some non-clay dirt in the undisturbed hillside over which my spillway ran. I now have an 8" siphon installed and drain it down some whenever it appears there will be a big rain event. I still want to put in at least one or two dedicated overflows a little lower than the emergency spillway just in case. Play it safe in advance so you don't risk a big rain event taking out your dam. That is a relatively huge watershed ratio for this general area so a much smaller rainfall will have an even bigger impact on your lake.


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Re: Sizing the emergency spillway
Tooger Smith #534710 05/03/21 12:00 PM
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What they said, that's a lot of acres of drainage per acre foot of pond, during an extra heavy spring rain event you will have a lot of water looking for a place to go and water is a powerful force, it will create its own exit strategy, I would not want to error on the side of having a barely adequate overflow system.


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Re: Sizing the emergency spillway
anthropic #534717 05/03/21 03:13 PM
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Thanks for the info.
The annual rainfall is 47 inches. Bloomington IN, close city.
Elevation of the watershed at the highest point is about 80 feet above the lake and three fourths of a mile away.
There is no flowing creek.
About half of the water shed is on the other side of a highway and the other side of a black topped county road. This provides some restriction since there is only three culverts under those roads.
I built another lake about 20 years ago and I know I found a "formula" for the width of the emergency spillway. The various things you have mentioned were a part of the data needed.
The lake I now have is about 30 acres and the emergency spillway is about thirty feet wide. I had no real choice as to the size. This one gets a full foot or more deep on a major rainfall event. It should be at least 60 feet wide or more.

Blessings


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Re: Sizing the emergency spillway
Tooger Smith #534774 05/05/21 05:38 AM
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I am in west central Indiana, but considerably north of Bloomington (but both of my kids live down there). It might take a while, but the NRCS can help size a spillway, at least my local office can. Nice topography in that part of Indiana. We are a lot flatter up here.


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