Pond Boss Forums POND CONSTRUCTION Building a dam Dam question

Here's how to decide.
Figure out the size of your watershed that feeds the pond. Determine the size of pond you want to fit the site.
Remember this number...1 acre, 1 inch deep, is 27,000 gallons.
Next, learn what your average annual rainfall is and get the best guess as to what the average percentage of that rainfall is runoff. (That's based on surrounding terrain...forest, farmland, rocky soils, etc.)
Then, figure out your "100 year flood" event. From that, do the math and you'll see how much water you'll need to manage.
Remember this...your dam is a water management structure. It's designed to impound water and release excess water safely.
Many ponds have a primary spillway at pool level and then a secondary, emergency spillway above that. The primary spillway is usually a pipe, of diameter to accomodate a "normal" runoff event. The secondary spillway is designed to handle that 100 year flood event.
Here's an example of what I mean. Say there's a site for a 4 acre pond in a swale/creek that is fed by a 50 acre watershed. That part of the planet, over a 30 year average, gets 32 inches of rain/precip per year and 80% of that is runoff. That means, over a year-span, that pond will receive 25-26 inches of runoff. Over 50 acres, that's 1,200-1,300 acre inches of rain...or close to 100 acre feet. That means the pond is too small for that watershed...so the emergency spillway needs to be wide enough to dispose of excess water. In that case, freeboard (the elevation between spillway and dam) needs to be more than if that pond was 8 acres.

Here's the bottom line...most ponds are designed with 6-10" of freeboard between the primary spillway and secondary spillway. Most ponds have 3 feet of freeboard between the emergency spillway and top of dam. Those ponds/lakes with bigger watersheds have wider emergency spillways...so the water rarely flows out any deeper than 6-8".