How to Calculate the Proper Spillway Size for a Pond

The point of a spillway is to passively divert water runoff from your pond safely under or around your dam wall. It is extremely important that the size of your spillway is large enough to accommodate the 100 year storms in your area. If it cannot, you will have water spilling over your dam wall causing erosion which can ruin the integrity of the wall, which can lead to failure of the wall.

Spillway Location


We all know that spillways are important, but how do you know how large it should be? There is actually a calculation that will help you determine the size of your spillway. You will need two numbers to start with. Your area of catchment and your areas 100 year 24 hour rain event. I covered how to figure out your area of catchment in my last article entitled “How to Calculate Catchment for a Pond”. 100 year rain event data can be found in this PDF document from NOAA.


Spillway to run into swale


My catchment area is 90,000 square feet and my 100 year 24 hour rain event is 8 inches. I am rounding up to 1 foot for my rain event to allow for slightly larger rain events. The calculation is as follows:


Catchment in Square Feet x 24 hour rain event in feet x 7.48 (Gallons converter) / 24 hours / 60 minutes / 60 seconds= Gallons of water moving across your spillway per second in an 100 year storm


90,000 x 1ft x 7.48 / 24 / 60 / 60= 7.8 Gallons of water moving across my spillway per second


Based my results, I can visualize 8 gallons of water moving across my spillway every second. I like to think of it in terms of milk jugs. Can I fit 8 milk jugs across my spillway easily? Or does it look a little tight? I have a 3 foot wide spillway that is 1 foot tall, so my spillway is adequate for the job. However, if I had more space for my spillway, I would widen it to 5 or 6 feet to be safe. When in doubt, a wider spillway is safer, they are inexpensive and you never know when that 100 year storm will hit.

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