Water Quality - Stormwater

[martin888888]

We have a crowned 24' wide local road that discharges water down a approx. 10:1 roadside slope and about 1000' to a waterway. What kind of water quality treatment does this overland flow give us normally? Would it be better to go through the cost and create a roadside ditch and direct it through a swale or design the ditch for treatment?

The same goes for any discharge through an existing wetland. What kind of treatment does that give you. Probably depends on how degraded and what type of wetland it is?

 

[cvg]

hard to say. is there grass? does the flow concentrate in a swale or draw? what is the slope on the 1,000 feet? how many feet of roadway drains to this area? what is the longitudinal slope of the road?

 

[francesca]

Your water quality credit for 1000' at 10% grade is either zero or negative. To get credit for water quality (reduction in TSS), you need a ditch or swale at 2% or less. A 10% slope gives extremely short sheet flow conditions and high velocities. The result is erosion, especially if you are concentrating runoff.

To get credit for discharge through a wetland, it would have to be a new wetland (actually a wet pond). You have to discharge only pristine water into the existing wetland.

 

[beej67]

I agree with Francesca conceptually. If the whole run of 1000 feet is 10:1 there's not much you can claim there. However, it really varies by what state you're in as far as what you can claim.

In Georgia for instance, you can get credits for sheet flow through a stream buffer, which can counterbalance the water quality requirement, so it's effectively treated as a TSS reduction in the calculation. How much you can claim is set rather arbitrarily in the Georgia Stormwater Management Manual. You can also claim "overland flow filtration" credits if the overland flow is flat enough. From memory I think the limit is down around 2%, not 10%, and I think you have to be discharging as sheet flow.

I don't know that those credits are based on any real science, they're just set out there as an encouragement for "better site design practice," and that's just Georgia. In North Carolina, for instance, you could put a level spreader and filter strip somewhere between you and the creek and gain credits, and you could utilize some of the stream buffer as credits as well if you design it properly. They're all different.

Of course, the truth of the matter is that TSS doesn't come from pavement anyway, it comes from stream morphology changes due to the urbanization of the watershed, but state EPDs jumped the shark a long time ago on that one. It's much easier to plug your fingers in your ears and pretend correlation = causality when the EPA is breathing down your neck, than it is to try and model increases in sediment transport due to stream morphology in your TMDLs.



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

http://www.campbellcivil.com

 

[3feethighandrising]

You will have to treat the stormwater before the natural wetland, and cannot use a natural wetland for treatment. You can design a treatment wetland but it would have to outlet to the buffer of the native wetland.

Over here (Washington) you could get full dispersion and WQ treatment credit for sheet flow dispersion through native vegetation for flow paths greater than 150' at native slopes less than 15%. This applies to native vegetation over native soils only.

An Ecology Embankment retrofits easy into a road shoulder and provides basic and enhanced (metals) WQ treatment.

Your flow velocity in a grass lined ditch will probably be above scouring velocity @ 10% slope, unless you have a lot of check dams