Detention vs Retention Ponds: Which Does Your Site Need

Your engineer's plans show a pond in the corner of the site, and somebody has already called it a retention pond, a detention pond, and just the pond, all in the same meeting. You want to know what the difference is, why your site got the one it got, and what building it actually involves. The short version is that both are holes that catch stormwater, and the difference is what happens to the water after it lands in the hole.

The plain difference

A retention pond holds the water and keeps it. Runoff comes in during a storm, sits in the basin, and leaves by soaking down into the ground and evaporating off the top. There is no outlet doing the work. On the right dirt a retention basin is a shallow, grassed depression that is dry most of the time and full for a while after a hard rain.

A detention pond holds the water and lets it go, slowly. Runoff comes in fast, the basin fills, and an outlet the engineer sized meters it back out at a rate the ditch or the bayou downstream can take. The point is to stop a site that used to be woods from dumping its whole storm on the neighbors in one rush. Some detention ponds hold a permanent pool with the storm volume stacked on top, which is the wet pond with the fountain in front of shopping centers around Destin and Fort Walton Beach.

People use the two words interchangeably. Engineers do not, and neither should you.

Why the soil decides more than the name does

Which one your site gets is mostly a question of what the ground under it will do with water. The sugar sand along the coast, from Navarre and Gulf Breeze across to Destin and down 30A, drinks water. Dig a shallow basin in it and rainfall disappears into the sand on its own, which is why retention basins show up so often on coastal sites. The catch on the coast is the water table. Near the bays and bayous it can sit close to the surface, and a basin dug down into it holds water year round whether the plans meant it to or not.

Head inland toward Milton, Pace, Crestview and DeFuniak Springs and the ground turns to clay and hardpan that does not soak up much of anything. A retention basin dug in clay is a mud hole that breeds mosquitoes and never empties, so inland sites tend to get detention with a controlled release, because the water has to go somewhere and the ground is not going to take it. Your engineer figures this out from soil borings and the seasonal water level on your parcel, which is why two sites a few miles apart can get different ponds.

What the earthwork actually looks like

From our side a pond is grading with a hole in it. We stake the pond off the plans, strip the topsoil and anything organic, and dig with the excavator to the bottom elevation the drawings call for. The bottom gets shaped the way the plans show, and the banks get cut to the slope the engineer drew, which is usually gentler than people expect so a mower can run it and the sides do not slump into the water.

What comes out of the hole is the interesting part. On a coastal site it is often clean sand, and clean sand is fill. A pond dug early can supply the building pad and the parking areas on the same site, which cuts trucking both ways. On a clay site the spoil is usually not pad material, and it either shapes the site or gets hauled off. Where the pond sits in the sequence, and where the spoil goes, is part of the overall grading plan for the site, and it is worth settling before the first machine shows up.

How the water leaves the pond is the engineer's design, and the plans spell it out. Our part is the hole, the bottom, the slopes and the ground around it, shaped so runoff from the site actually reaches the pond instead of finding its own way to the neighbor's lot. Then the banks get stabilized with sod or seed before the first big rain, because a bare sand bank in a summer storm ends up in the bottom of the pond.

Who decides which one you get

Not us, and not you. Your engineer designs the pond from the site, the soil and what the finished site will shed, and the people who review the plans sign off on it. We build what the sealed plans show, to the elevations and slopes on the sheet. If the pond looks too big to you, or you would rather have it in a different corner, that is an engineer conversation before the plans are final, not a dozer conversation.

Keeping it working after we leave

A pond that gets ignored stops being a pond. Sediment is the enemy of both kinds. During construction the pond catches everything that washes off the bare site, and by the end of the job the bottom can be a layer of silt that has to come out. On a retention basin that silt seals the sand and the basin stops soaking, so the last thing we do on a lot of sites is clean the bottom back to clean sand. After that, mow the banks, keep the sod alive, do not let trees or brush take the slopes, and watch the places where water enters the pond, because that is where a gully starts.

NOBL Sitework is a licensed general contractor (CGC1537340) building ponds and grading sites across Santa Rosa, Okaloosa, Walton, Escambia and Bay counties. If your plans show a pond and you want to know what it means for the dirt work, call us at 850-238-3307 and we will walk the site with you and go through the sheet together.