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?

 

[Ron]

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.

 

[gbam]

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.

 

[TerryScan]

"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.

 

[bimr]

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.

 

[msquared48]

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

 

[abusementpark]

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?

 

[Ron]

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!!

 

[abusementpark]

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

What is routing?

 

[gbam]

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.

 

[Phidelt218]

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.

 

[bimr]

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

http://www.icpi.org/sites/default/files/casestudy1_mortonarboretumforweb.pdf

 

[Phidelt218]

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.

 

[abusementpark]

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?

 

[abusementpark]

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?

 

[ACtrafficengr]

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.c...le_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/

 

[gbam]

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.

 

[Phidelt218]

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.

 

[Ron]

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.

 

[beej67]

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