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Stormwater Runoff in New Development
3

Stormwater Runoff in New Development

Stormwater Runoff in New Development

(OP)
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?

RE: Stormwater Runoff in New Development

2
abp....I'm not a general civil, but have done stormwater management design.

The general answer is that their mitigation methods usually depend on some overall watershed and regional drainage plan, usually controlled by some organization at a state level.

Given that, a common goal of an individual develop is that post-development runoff not exceed pre-development conditions for the same area....this is both in quantity of runoff and flow rate.

This is done by slowing the runoff down (increasing the impervious area speeds it up) through the use of retention and detention ponds, controlling the amount and rate of downstream runoff from the site to its pre-development condition.

There are other mitigation methods, but this is a simple one.

RE: Stormwater Runoff in New Development

Ron provided a good response to your question.  Another component is location of site in regard to the receiving waters.  If the developed site is adjacent to the receiving water the agency may not require detention/retention due to the regional hydrology and times to peak.  Each governing agency may have stormwater guidelines in place, at least at the county/state level.

 

RE: Stormwater Runoff in New Development

"Who decides what is acceptable?"  Most places will have stormwater laws and regulations which define the parameters.  An environmental agency will usually review plans for compliance with the regulations.

A common management approach is that the runoff from the developed site must be metered to a pre-development rate.  These days, there are many places that also have requirements for water quality management, groundwater recharge, as well as stricter runoff rate requirements.

RE: Stormwater Runoff in New Development

Typically the State will develop the basic legislation for storm water control.

The county and/or local municipality will adopt the legislation (and typically make stricter) and administer the stormwater permitting process.

RE: Stormwater Runoff in New Development

In my area, we have to take the stormwater detention design to the pre-1979 site status for vegetation and development, and not exceed 1/2 the 2 year (for fisheries), not exceed either the 10 or 25 year storms, still providing 30% more storage capacity than the 100 year event.  

The problem is that we have getting a lot of 100 year storms here recently.  Go figure.

Infiltration is encouraged where possible, but it is becoming less and less of an option as most of the permeable sites are developed, rendering permeable pavement a moot point here in my opinion.   

Mike McCann
MMC Engineering

 

RE: Stormwater Runoff in New Development

(OP)

Quote:

This is done by slowing the runoff down (increasing the impervious area speeds it up) through the use of retention and detention ponds, controlling the amount and rate of downstream runoff from the site to its pre-development condition.

What other possible measures can be taken?

RE: Stormwater Runoff in New Development

abp...there really are not any.  You can tweak the hydrographs for the runoff from your site, but the bottom line is that you will have to slow the runoff (since time of concentration is critical to rational analysis of large watersheds).  This can be done by onsite storage, infiltration and routing.  Routing is the most difficult, infiltration is the least effective and onsite storage is the more costly....now balance all that!!

RE: Stormwater Runoff in New Development

(OP)
Does a certain amount of infiltration occur when the water sits in a retention pond?

What is routing?

RE: Stormwater Runoff in New Development

abuse... there are other options to surface/pond detention/retention as in under ground vaults and etc... these typically are used for smaller sites.  The underground systems can either infiltrate or discharge to an existing stormsewer if allowed.  Groundwater levels may dictate whether these systems are viable.  

RE: Stormwater Runoff in New Development

The underground vaults are great system and save a lot of space if you dont have it for a retendtion/detention pond. Look up StormTech, they produce a vault system.  

RE: Stormwater Runoff in New Development

abuse.....infiltration depends on what soils you have, you have a higher sands or gravel you will have high infiltration then if you have silts or clays. IF you are curious about how much could infiltrate (absorb) you can run a percolation test.

RE: Stormwater Runoff in New Development

(OP)

Quote:

One option is permeable pavement. You may consider the use of a permeable interlocking concrete pavement (PICP) parking lot.

Has long has this permeable pavement been around?  Has it gained common use?
 

RE: Stormwater Runoff in New Development

(OP)

Quote:

abuse.....infiltration depends on what soils you have, you have a higher sands or gravel you will have high infiltration then if you have silts or clays. IF you are curious about how much could infiltrate (absorb) you can run a percolation test.

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

RE: Stormwater Runoff in New Development

It depends on your subsoils. If you have clay, it likely will be neglegible. If you have coarse sand or gravel, you can take advantage of it and include infiltration.

Vegetation will effect infiltration. Rain gardens planted with long-rooted prairie grass will have infiltrate better than short-rooted turf grasses, even in clay.(http://www.sustainablecitynetwork.com/topic_channels/water/article_b65fe518-3de0-11e0-82aa-00127992bc8b.html)

Many designers omit infiltration to be conservative. Depending on the native soils, use of deicing grit, etc., infiltration systems may plog if not maintained. That's also true of pervious pavements.

     Maybe the tyranny of Murphy is the penalty for hubris.  - http://xkcd.com/319/
 

RE: Stormwater Runoff in New Development

Abuse...In Arizona by regulation the infiltration rate is not allowed to be considered in the detention/retention routing computation.  However, we do include it in the drain computation for retention basins.  With that said you will need to consult with your governing agency on their acceptance of the infiltration rate used in computations.  

RE: Stormwater Runoff in New Development

Abuse permeable pavement has been around for several years, but its use depends on your location. Here in Alaska it is not used at all, in California where I was, used more often. In 2007 when I started to hear about it more and its use on parking lots.  

RE: Stormwater Runoff in New Development

abp....pervious pavement has been around for over 25 years.  It was initially popular in concept, but soon found that the interstitial spaces filled with soil and debris, so the initial permeability values got compromised.  Still drains better than solid pavement, but has issues.

Check with the Florida Concrete and Products Association and the Florida DOT....both have pretty good info on it.

 

RE: Stormwater Runoff in New Development

Sorry I'm late to this thread, been too busy in the last week designing ...

...stormwater management ponds for new development.

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.  

You've typically got several different factors to consider with these things:

1)  Detention of an event storm or event storms to a specified level, which could be to predevelopment, or to some idealized version of predevelopment (as if the site were all trees) or to some specified discharge rate (so many cfs per square mile of drainage area) because of a flooding history in your basin.  Sometimes you have to match more than one event.  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.  

2)  Water Quality treatment of runoff is often required by municipalities as a function of their permit process, so they can turn around and hand that off to the state EPD or DNR as evidence of non-point-source Clean Water Act compliance.  These sorts of regulations are often framed in terms of a TSS reduction, or a maximum TSS load rate, or a mass load rate of other pollutants such as nitrogen or phosphorous.  Sometimes this varies by basin or by which watercourse you're discharging to, if it's got a TMDL.  Water Quality treatment is typically provided with either a proprietary stormwater cleansing device, or a retention/infiltration volume, or incorporating elements such as littoral shelves into your stormwater management pond.  

3)  Other criteria - there always seems to be a third wheel in these things.  "Channel Protection" is a big thing in some areas, and is handled by a drawdown of a specified volume over a specified time.  Water Quality is sometimes handled with a drawdown as well.  Sometimes you'll have land-use specific criteria that kicks in for hot spots, such as the 0.5 inch of required retention volume for commercial development in SFWMD.  I've seen some areas make you do not only a peak discharge match, but a volume match as well, which means you must retain and infiltrate (or reuse) a certain amount of water, which involves geotechnical investigations and water balance math.

4)  LEED credits are there if you want them for this stuff, one for detention and one for water quality, and you might be able to get them for free just by meeting local code, depending on the code.

5)  Reuse (cisterns/etc) also get you LEED credits, and may get you water quality credit too if you can show you're capturing the first flush and running it through your toilets.  

7)  Out West things get weirder, because your water rights usually aren't riparian, so something as simple as a rain barrel could be outright illegal.  You don't own the rain that falls on your head, some user downstream does.

There are a lot of elements to this, and you should speak to someone who knows about them when you're in the land planning phase to get a good, solid understanding of how you're going to handle it.  An engineer should be able to investigate the details, help you choose a stormwater strategy, and give you a ballpark estimate on the size of your system for a pretty small fee.  (one to a few thousand depending on the size of the project and the complexity of the regulations)

 

 

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Stormwater Runoff in New Development

Quote:

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

RE: Stormwater Runoff in New Development

(OP)
beej67, thank you for the excellent post!

A few follow-up questions:

Quote:

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?

Quote:

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?

Quote:

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?


 

RE: Stormwater Runoff in New Development

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.   

RE: Stormwater Runoff in New Development

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

RE: Stormwater Runoff in New Development

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.

RE: Stormwater Runoff in New Development

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

¡papá gordo ain't no madre flaca!

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