Catch Basins? Why Not Drywells?

[EngWade]

I'd like to open a discussion regarding catch basins and why they are not replaced with drywells with overflows (essentially catch basins with holes in them to promote infiltration). The two theories I have come across are:

1. Way back in the day, many sewer systems were combined sewers and so promoting the infiltration of sanitary waste into the ground (and ultimately groundwate table) is a bad idea. So today, with the separation of the sewer systems, catch basins are installed without allowing infiltration because that is how they were always done.

2. The general strategy has typically been to "collect runoff ASAP and get rid of it ASAP". Thereby, preventing saturated subsurface conditions that could affect current or future structures.

I believe, in many instances, we could benefit from designing our conveyance systems using perforated (for lack of a better term) catch basins, to allow infiltration, ultimately reducing peak runoff rates and volumes.

Thoughts?

 

[cvg]

most catch basins are installed within roadways and are subject to traffic loading. Intentionally introducing water into the catch basin / or roadway subgrade could reduce the bearing capacity of that soil, promoting failure of the road. Additionally, unless the native soil is granular, then little infiltration will occur anyway. Another issue is water quality and water coming directly off pavement is likely to have low water quality, especially the nuisance water which is the most likely water to percolate. In addition, storm runoff generally is very short in duration, leaving little time for any effective percolation. In order to promote more infiltration, you need to store the runoff in a basin for some extended period of time. If you think that percolation of a small amount of water through a catch basin will in any realistic way reduce your peak runoff, you should actually run the numbers and I believe you will the effect is entirely negligble.

 

[psmart]

As cvg suggests, you have to store a significant fraction of the runoff volume for many hours (or days) to allow time for it to infiltrate. Otherwise it doesn't provide much peak reduction.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[msquared48]

Plus, you have to treat the water to remove oils and other material before it enters the drywell.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beej67]

I did a lot of research into this stuff in grad school. The whole pollution thing is a pretty big misnomer. Unless your aquifer is extremely shallow, infiltration is a lot better for the environment than letting pollutants run off.

Some folks in Japan have had huge success with tying porous pavement systems in with catch basins that have perforated pipes. Most runoff ends up in the road subgrade and infiltrates there, and whatever doesn't, ends up in the pipe trench and infiltrates there. I heard stories of springs that'd dried up from urbanization "springing" back to life, etc.

There's other research (French I think) that shows frequent inundation of a road subgrade with stormwater from the road actually turns itself into an aerobic bioreactor, treating the runoff of bio compounds before it leaches into the ground.

Why isn't it used more? Lots of reasons.

It's used like heck in places like Florida that have porous, sandy soils that can support it. It's not used much elsewhere because either the soils don't support it, or nobody checks to see if they are, which goes back to a business model thing.

For example, in Georgia, your civil design is usually done off a topo survey, with no regard for geotechnical investigation other than pulling a county soils map and doing your Phase 1 ESA. If there is any geotech work done, it's done concurrently or after the civil design is done, and they're only looking for bearing capacity and foundation stuff. In order to design a proper infiltration system, you need to know your soil permeability, you need multiple borings in your stormwater management area, and you need to know your seasonal high water table if you're in a coastal area.

Back to business - the developer rarely has the money for this kind of crap up front, because he often doesn't pull his financing together until the development permit is essentially in hand, so he knows for sure what sorts of yield he's going to get on his property. Which in turn means there's no money to find out the information you need to find out to properly design an infiltration system when you're in that phase of engineering.

Which isn't so terrible in Georgia, because if you're in clay, you can't really gain much out of an infiltration system anyway. In Florida, you need the SHWT elevations and the permeability measurements to do *any* design, so developers build it into their budgets, so you have the info you need when you do the design.

Then there's other places like Louisiana, where all your soils are mucky and all your water tables are high and there's simply nowhere for the water to infiltrate. And there's places like Alabama where the regulatory agencies will respond to you by saying, "Son, we don't do that sort of thing around here."

Stormwater management practices are highly regional, because different stuff works in different places, and Civil Engineering is heavily rooted in a "do what works" philosophy.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[EngWade]

Well said Beej67. I suppose the root of my position is, even with all that said, why don't we do it anyway? If you are in an area with clayey soils, you will get very little infiltration.... so it would essentially be acting the same way closed systems do now. But if you were in a sandy soil, you could gain some advantages. While I understand that quantifying these gains may prove to make them minimal over the course of a full storm event, is that reason enough not to do it?

From an environmental standpoint, any opportunity should be taken to return runoff to the groundwater table. Stormwater in general is such a gigantic approximation anyway, you would think an intuitive concept such as this would be taken advantage of (except of course in the obvious situations where the added saturation of the subsoils would cause impairment on other site elements).

 

[ACtrafficengr]

My main concern would be frost-susceptible soils. Adding moisture to a silty soil during cold weather could wreak havoc on the roadbed.

"...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

 

[cvg]

there are many reasons not to do it the way you suggested (using perforated catch basins) and many better ways. Perforated catch basins is largely ineffective at both reducing peak flows or percolating water. Porous pavement is another subject altogether.