Sod at the bottom of a detention pond?

[Stonehenge]

I've seen a few detention ponds in my area, built for warehouse-type establishments (Not Home Depot, but think that as far as size and type). They've had sod laid at the bottom of the detention pond.... Is it me or does this seem bone-headed?

In those cases where the pond drains enough to see the bottom, the sod is of course totally dead.

Wouldn't rip-rap be a better choice?

[Paul]

Most times they call for a vegetated bottom, we all know that seed won't work. plus with the problem with erosion sod is ideal. The problem with Rip/rap is the cost have you priced RR4 or RR5 lately? Here it's over $40 per ton and a ton doesn't go very far.

[Stonehenge]

But the root structure of that sod will break down after the sod dies - how long before it's just mud and mess at the bottom?

I used to be able to get assorted rip-rap (no specific specifications) for $4-5/ton, with the clause 'you get what you get' when it's ordered. Could be anything from 2# pieces to 300 pounders.

[Paul]

City planners and others won't let you get away with that any more 2# to 300 lbs. Besides how long does water last in those ponds? Most are geared towards 24 hours after the storm.

[agla]

A detention pond does just that, detains. It, by definition, should only hold water for a short period of time as not to over burden the runoff swale, stream, or whatever form of surface drainage it is protecting. In other words, it should usually be empty and does not have the primary function of percolating water into the soil.

A retention pond retains drainage to percolate into the soil. It is designed to trap water and not let it continue as surface runoff. These often hold water for very extended periods of time and should have a bottom designed to let water into the soil.

Detention delays surface runoff.
Retention traps surface runoff.

[Stonehenge]

So if the sod is dead at the bottom of one of these, it might have to do with a poorly graded bottom, and not from being underwater during heavy storms.

Agla, I have to chuckle - I appreciate the lesson or detention vs retention - funny thing is, I was accidentally using them interchangeably, but was using it correctly here, for the pond I was describing. Dumb luck, I guess.

[agla]

I just through that out not because I thought anyone in particular did not know the difference, but because most people do use the terms interchangably.

I would expect a detention pond designed with a sod bottom to only be underwater for very limited times during big storms. In that case, the sod should not be permanently injured. It is good design if that is the case and it is constructed according to the design (grading).