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
My engineer nephew sent me these sites. I have a pretty good grasp now. Again, thanks for the info. Keep up the good work! Tooger
May 7, 2021,
https://www.springfieldmo.gov/DocumentCenter/View/3325/Chapter-5---Calculation-of-Runoff-PDFhttps://www.lmnoeng.com/Hydrology/rational.phphttps://www.lmnoeng.com/Hydrology/hydrology.phpThe Rational Method looks to be the simplest calculation for Q , runoff. Q=CiA. It appears to be best for under 200 acre watersheds though. Although, from what I read, on a larger watershed the Rational Method will actually over-estimate the runoff. Better than under-estimating I suppose. But it may lead to saying you need a much larger spillway than you actually do. I put a few links above, the last two are actually calculators. First is rational method, second method is more complicated with more variables. There are other formulas for runoff also, many different ways to compute.
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