Stormwater Runoff in New Development 3

[abusementpark]

Ok, please entertain a basic question from a structural guy.

New commercial/residential developments create more impervious surfaces which increase the stormwater runoff demand on local drainage systems. How do most municipalities determine how much new runoff is acceptable? I understand there are some things that new developments can do to mitigate the effect through things like retention ponds. But who decides what is acceptable?

 

[beej67]

Do storm water engineers generally consider a certain amount of infiltration to occur in retention ponds? Or is usually negligible?

Careful with your language there. I think you mean 'detention ponds.'

Varies widely by region, because different regions have different rainfall amounts, different storm distributions, different water table depths, and different soils. Always count it in Florida, almost never in the rest of the Gulf Coast states, sometimes in Georgia or North Carolina if you're using a water quality BMP that utilizes it. Often times you'll count it for your water quality analysis but skip it for your detention analysis, because worst case for the detention storm is another storm came by a few days earlier and saturated the ground.

A 'retention pond' is a pond that holds water with zero discharge and typically lets the water leak into the ground, so some sort of infiltration analysis is almost always required in a retention scenario.

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

http://www.campbellcivil.com

 

[abusementpark]

beej67, thank you for the excellent post!

A few follow-up questions:

What you need to do for stormwater management varies widely, WIDELY on where you are. East coast / west cost, north / south of the mason dixon line, everybody does it differently. Before accepting any advice here you must must must check your local regulations. They're typically driven either by your local municipality, your state, or some state subdivision such as a "water management district," sometimes broken out by basins.

Can you highlight some of the broad practices in the Gulf South?

In Georgia, you often have to match the 2 year, 5 year, 10, 25, 50, and 100 year storms as a matter of courtesy to your reviewer, even though the regulations don't often state that anymore.

Does someone usually check your calculations to verify this has been done?

Careful with your language there. I think you mean 'detention ponds.'

Varies widely by region, because different regions have different rainfall amounts, different storm distributions, different water table depths, and different soils. Always count it in Florida, almost never in the rest of the Gulf Coast states, sometimes in Georgia or North Carolina if you're using a water quality BMP that utilizes it. Often times you'll count it for your water quality analysis but skip it for your detention analysis, because worst case for the detention storm is another storm came by a few days earlier and saturated the ground.

A 'retention pond' is a pond that holds water with zero discharge and typically lets the water leak into the ground, so some sort of infiltration analysis is almost always required in a retention scenario.

So then does a "detention pond" hold some water, but releases the overflow in a controlled rate?

Which is more common?

 

[beej67]

Florida is it's own animal, and each WMD in Florida is a different breed of that animal. In Florida you're typically on the hook for 25 year detention, and water quality via dry infiltration/retention ponds. The usually don't care about 100 year discharge, because for them that means hurricane, which means they're all watching on TV from a motel in Georgia. They're very protective of their water tables, since they're so close to the surface. Development in the flood plain is often allowed as long as you do something called a glass box analysis, which means pretend your whole site is a pond, turn the outflow off, dump the 100 year storm on it, an make sure that comes below the BFE. There are some other complications as well. SFWMD is requiring a pre/post match on P and N mass loading now too, which is fairly new and befuddling engineers down there. The panhandle regs often look more like Georgia or Alabama. FDOT is it's own animal, and requires not only modeling of certain storm events, but also of different duration storms to find out which unit hydrographic stresses your site the most.

Bama, Mississippi, and Louisiana are usually behind the times, sometimes don't have a wate quality standard at all, and will want to see detention for the 25, 100, or both. Water quality in Mississippi for instance is done through the state and detention is local.

Georgia is by municipality, but they mostly like to claim that they follow the Blue Book (Georgia stormwater management manual) even though the Blue Book is more a list of suggestions than a comprehensive regulation. They also interpret it all differently, and they also latch on to some older practices such as the 2-100 year storms, even thou the Blue Book supposedly abandoned those. Blue Book says 80% TSS reduction, channel protection in a drawdown, 25 year detention, and a 100 year safety check between you and a spot downstream where the basin is equal to ten times your project site.

North Carolina wate quality is run through NCDENR and detention is local, usually just the 2 and 10 year storms unless someone's downstream you might flood out. NCDENR's bmp manual can be frustrating becuse they update it online all the time, sometimes mid project.

South Carolina is famous for being very obstructionist with license by comity, so few engineers outside the state do work there. I have no experience there, but they supposedly do some fascinating stuff with sediment modeling for their erosion control plans.

I'm licensed in Virginia but haven't had a chance to do anything up there yet. I understand discharge to the Chessapeake can be very regulated, but I can't imagine it's any more difficult than SFWMD currently is.

East coast BMPs are typically some version of the same things, stormwater ponds, stomwater wetlands, bioswales, etc. Everyone wants to promote infiltration but they all seem scared to give you credit for doing it. Concepts are the same but design criteria vary by state.

 

[beej67]

Sorry for typeos above, I'm on an iPad this afternoon. To answer your final two questions, designs are always reviewed by somebody, either locals or district/state staff. Detention is far more common down here, but that's probably going to vary by region. I've done some ponds in Florida that were 100% retention if FDOT wouldn't allow us to discharge to their swale.

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

http://www.campbellcivil.com

 

[bank]

You can also reduce stormwater runoff as well as comply with storm water quality requirements by directing your runoff into rain gardens, bioswales, and/or bioretention ponds. I've designed developments where runoff is directed into these type facilities. They are designed so the runoff percolates down through engineered soils before infiltrating into the subsoil. In the case of fat clay soils like they have down here in Houston, perforated pipe underdrains pick up the filtered water and discharge it into the receiving system.

The rate of percolation through the engineered soil is dependent upon the soil mix, and can be documented and used to offset all or some of the detention requirements.

 

[fattdad]

You may find some useful resources at this web page:

http://www.cwp.org/

Regarding earlier posts: I'm not a fan of perc testing for infiltration design. Neither are the jurisdictions around Central or Northern Virginia. Fairfax County, Virginia (metro DC) actually has a protocol for conducting an infiltration test that uses a 4-in diameter PVC pipe that's positioned in the most restrictive layer within 4 ft of the proposed pond subgrade. There are other factors in this method also. It's just a different approach.

There are design standards for pervious pavement and infiltration basins (bio-filtration too). All of which can re-introduce stormwater to the ground of your site. Virginia Department of Conservation and Recreation has information on these methods.

I have a problem with many of the approaches to stormwater infiltration, however. Many of these design methods require a minimum 4 ft separation between the pond bottom and the high ground water table elevation. That's all well and good before the pond is constructed. Fact is when the pond is in service there will be a mounding of the water table below the footprint of the pond. The only way for this ground/pond water mound to attenuate is via horizontal flow. Nobody considers the horizontal permeability or the transmissivity of the pervious layer in these analyses. I mean if you have a 50 ft by 100 ft pond that's 4 ft removed from the pre-construction water table and you have a sand layer that's 2 ft thick, all that water has to move horizontally through the 2-ft thick sand layer. Who cares how long it takes for the water to get to the water table?

Hope the web link helps. Look for the stormwater handbook at the Virginia DCR too.

f-d

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