Quantity Control
Intention:
- Limit the disruption of natural hydrology
- Increase on-site filtration
- Manage storm water runoff
- Eliminate sources of contaminants
- Reduce impervious cover
- Reduce/eliminate pollution
- Remove pollutants from storm water runoff
- Stormwater Design (for reduction of impervious surface)
- alternative surfaces: rain gardens, vegetated swales, rainwater recycling
- non-structural surfaces: vegetated roofs, pervious pavement, grid pavers
Implementation:
- note:
- humid watershed: 1″ rainfall / 40″ per year
- semi-arid watershed: 0.75″ rainfall / 20-40″ per year
- arid watershed: 0.50″ rainfall / less than 20″ per year
- case 1: impervious surface less than or equal to 50% of total site area
- post-development peak discharge rate and quantity equal to or less than pre-development (for 1 and 2 year, 24-hour design storms) OR
- implement a stormwater management plan that protect receiving stream channels from excessive erosion by implementing a stream channel protection strategy and quantity control strategies.
- case 2: impervious surface more than 50% of total site area
- post-development peak discharge rate and quantity 25% less than pre-development (from the two-year 24-hour design storm)
Codes/Standards Applied:
- none
Extra Credit:
- none
Submittal Phase:
- Design
Links from Reference Guide:
Other Sustainable Sites Credits
- SS P1 – Construction Activity Pollution Prevention (prerequisite)
- SS 1 – Site Selection
- SS 2 – Development Density & Community Connectivity
- SS 3 – Brownfield Redevelopment
- SS 4.1 – Alternative Transportation – Public Transportation Access
- SS 4.2 – Alternative Transportation -Bicycle Storage & Changing Rooms
- SS 4.3 – Alternative Transportation – Low Emission & Fuel Efficient Vehicles
- SS 4.4 – Alternative Transportation – Parking Capacity
- SS 5.1 – Site Development – Protect or Restore Habitat
- SS 5.2 – Site Development – Maximize Open Space
- SS 6.1 – Stormwater Design – Quantity Control
- SS 6.2 – Stormwater Design – Quality Control
- SS 7.1 – Heat Island Effect – Non-Roof
- SS 7.2 – Heat Island Effect – Roof
- SS 8 – Light Pollution Reduction
51 Comments On This Post
For “case 1: impervious surface less than or equal to 50% of total site area
pre-development peak discharge rate and quantity equal to or less than post-development” should not pre- and post- be switched with each other?
Thanks, Good luck, I ike your blog, Jerry-testing March 24.
Thanks Jerry! Fixed that mistake. Much appreciated, and good luck!!
I know it’s not necessary to know this to be able to get the questions about this credit right, but can anyone tell me what exactly does
1-yr and 2-yr 24 design storm stands for? To my knowledge the reference guide doesn’t explain.
Is it the worst storm in a 24hr period within 1 year? Then that would make the 2-year one a “bigger” storm than the 1-year… or maybe the same one if a …. ok, ok… whatever with the assumptions… anyone care to illuminate me?
Thanxs!
A 1-yr 24hr design storm refers to the the worst storm that you can expect to happen every year. For example, a L3 hurricane for the Miami area.
A 2-yr 24hr design storm is the worst storm you can expect to happen every other year. For example, a L4 hurricane in the same Miami area.
The concept continues. Therefore, the longer the period the worst the storm. For example, a tsunami that devastates and entire city should have a return period of (let’s say – I’m assuming these values) 100 years. Meaning that every hundred years a tsunami, strong enough to devastate a city, should happen. That would be a 24hr 100-yr design storm.
In these credit you should evaluate both design storm, both histograms (since the two are different), and assure that the post-development peak would not be higher than the pre-development peak.
nice…
thank you!
and I suppose then…
this 24hr 100yr storm should not be confused with the FEMA 100-yr storm since they say it’s not necessarily the worst storm in a hundred years but the storm that has a 1% probability of happening each year…
right?
Right. However, FEMA does not define a 100yr “storm”, but rather the 100yr “flood line”. According to FEMA:
“The term “100-year flood” is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1- percent chance of being equaled or exceeded each year.”
So, in that sense, you are right.
However, if you find another standard that defines a 100yr storm, you should check both histograms, because they might not we exactly the same (therefore the peak might be different).
Thanxs AA!
I’m taking the test next Tuesday May 20.
Have you taken it already?
No problem Sofia, you’re welcome.
Yes, I took it last Friday and passed it.
It’s not that difficult, but very tricky. They just try their best to confuse you, but if you know the credit’s concepts well you should not have any problem. Good luck
Hi,
Here is a question regarding runoff coefficient hopefully any of you here can help me out.
Which has lower runoff coefficient, 6″ green roof or landscaped area(assuming they have same plants and slope)?
Many thanks!
Rachel
hi,
I’ve gotten some sample test questions that are doing a good job of confusing me with impelementation switch and bait.. I am needing to clarify that 6.1 and 6.2 use same technologies and measure them differently..
I cannot find in 6.1 any mention of ‘structural’ impelementation…right? they lump it all into ‘alternative’
smaller bldg footprint, PERVIOUS PAVING, storm water reuse, green roofs, BIOSWALES, retention ponds,… of these only the Bioswales were mentioned in 6.2 as ‘non structural’ along with ‘disconnection of impervious areas, and pervious pavement(!)
6.1 only interested in how much h20 – submittal is in cubic ft per second
6.2 interested in the particles in the water and where they go, so the the same technologies are listed in terms of structural and nonstructural in terms of their infiltration capacity
It is not intuitive to me..so must memorize..is this the final list? (why are pervious paving area non structural and ponds structural…and the green roofs?)
Rachel, that is a tricky question, especially without knowing the soil type of the landscape area. With all things being equal, I am going to have to go with the landscape area, because there will likely be a greater volume of infiltration.
valeria…can you rephrase your question just a bit. I think i know what you are getting at, just want to make sure.
FWIW…I’m a PE, and do stormwater management daily. The LEED Ref guide does a horrible job of trying to get their points across.
I’ll give your question a shot…6.1 reduce stormwater QUANTITY and discharge RATE. Typically as engineers, we do this via detention basins. As impervious surface increases, so does the runoff rate and runoff volume. We route this into a basin, and let it come out slowly, thus reducing the peak rate and volume.
6.2 deals with removing TSS from stormwater. You can do this several ways, the most common ways I do this are by using infiltration basins, which store the entire volume of the water quality storm (what they call 90% average annual rainfall) then slowly let it perc back into the ground. We also use structural means, stormtech, vortechnics, etc…structures designed to do just this, remove 80% TSS.
ok, thanks for the “how it is written” comment.. I fell this whole green building thing is coming down so fast and furious, they have no time to proof read anything..
In your expertise, what is the criteria for a non structural technology?(inLEED land) I get bioswales, form land and put vegetation on it. no structure there, but they list in 6.2 also pervious pavement- pervious says structure to me..and disconnection of impervious areas- i imagined drains or some STRUCTURE to make the water go somewhere on purpose, instead of randomly eroding or gushing into a creek…
In the construction industry “structures” is a term that is synonymous with mahnoles, inlets, etc. not parking lots.
I think that is what they are getting at here.
ahaaa! thanks!!!!
Can anyone tell me if Option 1 and Opiton 2 both refer to rate AND quantity?
I assume that they do, but I have found a contradiction in the book:
Summary page (77 in my 3rd Ed) says rate and quant for Option 1, and quant “volume” for Option 2.
Over on page 80, calculations, it says rate and quant for both???
I am trying to understand the required calc’s for SS 6.1 under Option 1. I understand the pre/post for the peak discharge. However, I don’t understand the pre/post for the volume.
Is it peak volume or total (cumlative) volume for the site discharging through the weir/drawdown orifice?
If someone could enlighten me it would be greatly appreciated.
I have a Project Site about 0.6 Ha and this land has not been developed. and it has 0.75(75%) imperviousness. Now I have Post development site runoff quantity and rate, since We are going to install X amount of Stormwater storage tank. However It is not understanble to find PRE-Development Runoff rate(I do have result in quantity but not the rate). Do I have to assume the amount of Pre site water storage or something else?
Can someone clarify Eq.1 (p.79 of V2.2, third edition) for calculating the amount of runoff captured?
I do not understand the Vr calculation lister as:
Vr = (P)(R)(A) / 12′
I am unsure what the P is in the equation. Also, is the 12′ a constant or project specific?
I also do not understand the Rv calculation listed as:
Rv= 0.05 + (0.009)(I) = 0.05 + (0.009)(100) = 0.95
If I= % impervious, are the other numbers constants or were they derived from something project specific?
please see the follwoing web site :
http://www.mde.state.md.us/assets/document/chapter2.pdf
page no 2.2
Maryland Stormwater Design Manual , Volumes I & II states that the A in the equation listed above is acre-feet, not square feet. However, the equation’s example uses a 10,000 sq. ft. roo.
There is no errata issued for this. Which A value should I be using?
here it states that
“alternative surfaces: rain gardens, vegetated swales, rainwater recycling
non-structural surfaces: vegetated roofs, pervious pavement, grid pavers ”
However, in the greenexamprep it asks:
What are nonstructural techniques to reduce water pollution from stormwater for LEED certification?
the answer was: rain gardens, vegetated swales (which are listed here as the alternative not the nonstructural).
which is correct?
Thanks alot!
Mina,
They do seem to make a distinction between non-structural surfaces and alternative surfaces. I think they also make a distinction between “alternative approaches” and “alternative surfaces.”
The gist of “quantity control” is to either make precipitation sink into the ground or collect it for re-use, cutting down on the amount of water that is flowing off the site. The best way to do this is to reduce the amount of impervious surface (i.e. solid pavement.)
“Alternative approaches” can include rain gardens, vegetated swales, vegetated rooftops, grid paving, pervious paving, and rainwater recycling.
I believe “nonstructural” means vegetated swales and rain gardens – those measures that are plantscaping related, where no one exactly built anything, but just moved a bunch of dirt around and planted things there.
Green roofs, to me, sound structural (although alternative because they’re unusual) because of the weight and hassle of supporting a vegetated roof.
“Alternative surfaces” would be the grid pavers and pervious paving – those measures that are added constructed surfaces different from the typical choices – but are still not part of the building structure – the key word here perhaps being “surfaces,” as in active surfacing or paving.
I hope that helps – maybe someone else can jump in here, but that’s the way I understood it when I took the test.
It’s probably more important to understand the pre- and post-development runoff rules. That if the site was largely undeveloped, there shouldn’t be any more runoff when the project is complete than there was when it started. But if the site was already half paved over or covered, the runoff needs to be 25% less when you’re done than when you started.
Anyone?
Does anyone have a good sample test question that demonstrates changes in stormwater quantity? Using percentages before and after and trying to figure out how many gallons it will be reduced by? Sorry I am so vague. I read the question somewhere online and now I cannot locate it.
Can anyone answer this for me?
Decreased permeability increases which of the following aspects of stormwater on a site? (Choose 1.)
a. volume
b. frequency
c. volume, frequency
d. volume, velocity
e. volume, temperature, frequency, velocity
Q =(0.28) x Cx i x A
Q – run-on flow (m3 /sec).
C – run-off coefficient for drainage area (%).
i – rainfall intensity (mm/hour).
A – area draining onto the site (km2 ).
I would choose a. since in this formula to calculate the run-on there is nothing regarding frecuency, velocity or temperature!
Thanks Omar, actually the place I got this question from says its e. But I think there is something wrong.
You may be right..
anyone any comments??
Mark,
I also looked the definition of STORMWATER RUNOFF (ON A SITE) I have the second edition, this definition shows up on page 81, it is the very last paragraph before SSc6.2 starts. This definition DOES NOT say anything either about frequency, velocity or temperature. I continue thinking A) is the right answer. What do you think?
Can anybody will xplain me terms post-development peak discharge rate and pre-development?
Mark
I am a Civil and Environmental Engineer. The right answer is indeed E. With decrease in permeability the volume of the runoff increases. Furthermore these hard surfaces also increase the temperature of the flowing water. Due to lesser resistance to flow the velocity of the flow also increases. More volume also increases the frequency of flows.
I’ve had a question where the pre-construction runoff was 35% and the post-construction runoff was 88%. Which implementation option should be used for decreasing post construcion runoff – SS-6.1 opt 1 or 2?
Patrick:
I really need to know the imperviousness of the site but most likely the answer is option 1 because because although the post development runoff rate (CFS) and quantity (CF) must remain the same or less than pre development rate and quantity . . .Option 1 does allow post development rate and quantity to exceed predevelopment rate and quantity given appropriate steps are taken to prevent excessive stream velocities and associated erosion.
Anyways the breakdown is as follows:
Option 1: For sites that have an imperviousness less than or equal to 50%, the post development discharge rate and quantity must remain the same or less than the pre development peak discharge rate and quantity.
OR
You can implement a waterway protection strategy that reduces receiving waterway velocities and associated erosion.
OPtion 2:
For sites with imperviousness greater than 50%, the post development peak discharge rate and quantity must be 25% less than the pre-development peak discharge rate and quantity.
On a second pass of the Reference guide, im a bit unsure of option 1b. Perhaps someone could clarify if my interpretation is correct?
Here is as a formula (just memorize this for option 2) :
(3/4) PreDev ? PostDev
?=bigger or equal
If you are designing for a 2 year 24 hour design storm, why would you need to also design for a 1 year 24 hour design storm. It seems to me that a 2 year 24 hour design storms peak discharge rate and quantity would exceed the 1 year 24 hour design storm (because it would be a more intense storm).
Can someone clarify why for option 1, we design for both 1 and 2 year 24 hour design storms?
I had the exact same question… would there ever be an instance where the 1-year, 24-hr storm exceed the rate or quantity of a 2-year, 24-hour storm?
This is to Mark’s question on March 4th:
I found this little section in the Reference Guide (although it is the first addition I don’t think it has changed)It’s small but I think it addresses your question;
“The geometry and health of streams is closely linked to stormwater runoff velocities and volumes. Increases in the frequency and magnitude of stromwater runoff due to development can cause increased bankfull events. As a result, the stream bed and banks are exposed to highly erosive flows and more frequently and for longer peroids.”
It’s not an equasion but it does explain about Velocities, Volume, and Frequency.
~Amy
I think there is a typo under
“Stormwater Design (for reduction of impervious surface):
alternative surfaces: rain gardens, vegetated swales, rainwater recycling
non-structural surfaces: vegetated roofs, pervious pavement, grid pavers”
In the v2.2 Reference Guide (under SSc6.2), it says that:
-Alternative Surfaces inlcude vegetated roofs, pervious pavement or grid pavers.
-Nonstructural Techniques include rain gardens, vegetated swales, disconnection of imperviousness and rainwater recycling
Hello,
I’m just trying to fullfill the Pre-Development Site Runoff Quantity in my SSc6.1 Template. I just determined my Pre-Development Site Runoff Rate (cfs), but I can’t figure out how I find the run off quantity (cf). How do I relate this two information.
Thank you
If I read the manual right, if I picked the option 1b (how excessive stream velocities were prevented) I need not calculate the pre/post development rates. However when I enter just the narrative on option 1, I dont get the point in ‘points documented’ section. Is this an error or do I still have to calculate the rates?
Thanks
Vid
I am not clear on why we must check BOTH 1-year and 2-year qtys for 50% imperv, why are we checking 2-yr only, not 1-yr?
thanks
So, I am not clear on why we must check BOTH 1-year and 2-year qtys for 50% imperv, why are we checking 2-yr only, not 1-yr?
Is there any suggested guidance or standard on determining the acceptable interval for drawing down a stormwater storage cistern? The 2.2 Example uses 3 days, but I would believe there has to be some value based on local rainfall patterns.
If nothing else exists I would default to the typical annual rainfall file that Wisconsin references. This file shows the rainfall record for what has been designated as a typical year for that part of the state.
However, the LEED Option 2 is tied to the 2 yr rainfall event, and the typical rainfall year files referenced by the Wisconsin regulatory authority do not necessarily include the 2 yr event.
Could someone tell me what the difference is between pervious paving and open grid paving systems? One of the LEED prep courses I took mentioned that they were not the same but I’m finding it a bit subtle. I believe one is a subset of the other, but the terminology is not clear to me yet. Thanks.
alternative surfaces: rain gardens, vegetated swales, rainwater recycling
non-structural surfaces **TECHIQUES**: vegetated roofs, pervious pavement, grid pavers
Pat this is other way around!
Hi,
Would anyone be able to tell me what is the unit for the 1-yr and 2-yr 24 design storm? Is it a probability value, with no unit? Or is it in mm/inches of rainfall per minute or second?
Thanks,
Maya
Does anyone know if you achieve SS 6.1 (quanity control) where you held stormwater in a cistern and then slowly released (say 72 hours after the storm) it into the storm drain or combined SS drain, or does LEED require the captured runoff to be reused onsite (retained permanently)?
Amy, did you ever get an answer, I have a project that I am looking for the same information for.
Thanks!
Could someone tell me which rainfall data that one has to consider for designing his harvesting system(storage or recharge)for peak ranfail or for average rainfall? and that could be for what duration,either for 15min? or for 8 hr or for even 24hr or for _hr?
Hey CP, i thought it was the 1 year and 2 year 24 hour design storms.