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Credit language straight from USGBC
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36 Comments
Runoff Volumes
This credit is actually extremely technical.
I can still use the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size. to deturmine the Runoff Rates, but as for the Runoff Quantities, I'm stuck. The US NRCS/SCS charts are obviously made from US data and therefore not suitable for projects outside the US.
I don't find anything helpful on the internet.
PS. Nice site.
Runoff volumes
The way that we have calculated run-off volume is to calculate a weighted average run-off coefficient, as you would for the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size.. You do this for pre and post development conditions, and essentially, the Run-off Coefficient needs to be the same. For instance, if the Run-off Coefficient is 0.35 for an undeveloped site, and the Run-off Coefficent for the developed site is 0.70, then the volume that must be mitigated is the difference in Run-off Coefficient times the rainfall for the one-and two-year 24 hour design storms. The approach to meeting the volume requirement can be with green-roofs, porous pavment, cisterns and reuse, infiltration basins, bioswales, etc.
Working through the rational
Working through the rational methodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size., I've calculated the Time of Concentration (Tc), then itterated using IDF (intensity-duration-frequency) charts until the duration = time of concentration (since the Tc is influenced by the corrisponding intensity at a given duration).
This really means to me that even though you can calculate this for a given statistical return period, i.e. 2 years, the duration of "24hrs" is badly worded and should only apply to run-off VOLUMES using the SCS methode after finding the run-off RATES with the rational method.
The weighted average Run-off Coefficent should then be used with the Intensity given at the corrisponding Tc as calculated using the Rational Method giving the required run-off RATES, but this still leaves me short for calculating the VOLUMES, which as previously stated, is usually done using American Charts based on American rainfall data.
I mean which CN number must I use if I'm building somewhere in Brazil? Does anyone have a concrete example of the complete set of calculations?
I know in America this is done by the Civil Engineer, but it does't alway fall into his camp in all parts of the world...even though maybe it should.
Runoff volumes
The key for volume calculation is knowing the 1 and 2-year, 24 hour storm depth. This is calculated fordifferent locations and it is a long process and it is not the same as calculating runoff rate using the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size. and multiplying by time. For this reason, most jurisdictions publish a series of 24-hour storm depths (1,2,10,50, and 100 year depths). LEED asks that we calculate volumes for the 1 and 2-year 24 hour storm to show that there is no net increase in the volume of runoff from the site after development..
For example, a 2-year 24-hour storm depth for Baltimore, Maryland is 3.2 inches.
When the 2-year 24 hour storm depth is known, it can be multiplied by the run-off coefficient (a dimensionless number) and the area of the site to calculate a total volume. The only variable in that calculation from predevelopment conditions to post development conditions is the runoff coefficient. Therefore, the purpose behind the credit is to develop a stormwater management program that does not change the combined runoff coefficient. Pavement will have a higher coefficient, bioswales will have a lower coefficient, detention ponds may have a zero coefficient for a 2-year, 24 hour storm, depending on your overall design. The runoff coefficients from all the various sub-basins, multiplied by the associated areas for the sub-basins can be combined to develop a combined run-off coefficient for the entire study are of the site..
Jean Marais replied b.i.g. Bechtold INGENIEURGESELLSCHAFT MBH May 05 2010
The Example 2 in SSc6.1 captures rainfall for the 90% average annual rainfall event. LEED gives watershed equivalent amounts to treat in SSc6.2 that can be used for credit compliance of SSc6.2 (see footnote at the bottom of p101 BD&C Userguide).
Example 2 in SSc6.1 goes on to state that
"In this example, the captured rain must be drained within 3 days, or at a minimum rate of 1.4 gpm, for the tank to be emptied before the next storm. If the drainage rate is slower, full capacity cannot be assumed to be available during the 2-year 24-hour design storm."
Does this imply that a 2-year 24-hour design storm rain volume = rainfall for the 90% average annual rainfall event?
I ask because I've been made aware of projects that hit SSc6 with a sledge hammer by simply arguing they have cisturn capture capacity for 1.5 inches of rain per 24hrs.
Background is that in Germany (and some other countries), rainfall calculations are generally made using different statistical information (also based on IDF charts) given as say e.g. 5 min 2 year frequency event rates and volumes.
The only way for our projects to get SSc6 credits is to pay the weather service lots of money for the adapted information.
Jean Marais replied b.i.g. Bechtold INGENIEURGESELLSCHAFT MBH May 07 2010
Is there any correlation between 1 and 2 year 24hr design storm depths and watershed classification? This may dramatically simplify things.
Runoff volumes
Thanks Mr. Hurst. That points me in the right direction. For a long time I was trying to find the correlation between the run-off rates and the rainfall depth with no success. Understandable, as it's a whole different set of data from the local weather service. We had big problems in Lybia trying to find proper rainfall data of any sort what so ever so I was hoping there were correlations to draw at least rough estimates from the data I had available.
It also seems I was wrong about the CN charts for the SCS method. They represent the soil attributes (and their part in holding or restraining the run-off) and would most likely still be usable internationally.
This also clarifies the use of the "24hr" return period used in the lead requirement language. It applies then to the run-off Volume calculation and not (if using the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size.) the run-off Rates.
Keep up the great work and let me know when I can help out with the site.
calculation of Cr 6.1 Stormwater design-Quantity Control
I would like to request help for the calculation of Cr 6.1 Stormwater design-Quantity Control. The details are as follows.
I have a site with an area of 28460 SQ.FT.
Pre-Development: The site was covered with flat vegetation (Surface Type) which has a Runoff Coefficient of 0.10. This will be complying for Option-1, since the existing site is with imperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. less than 50%.
Post-Development: The site is designed with 23274 SQ.FT. of hardscape including the building and pavements with same Runoff Coefficient, 0.95. The rest 5286 SQ.FT. was left for landscape.
Perform the sample calculation using a data for rainfall.
Tristan Roberts replied Editor – LEEDuser, BuildingGreen, LLC Dec 30 2009
JP, it might help if you can give more of a hint at what obstacles you're facing with this calculation?
I *think* there's not enough information here to do the calculation. Do you have your regional rainfall data?
The credit says if existing
The credit says if existing imperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. is less than 50%, then
implement a stormwater management plan that prevents the
post-development 1.5 year, 24-hour peak discharge rate from exceeding
the pre-developemnt 1.5 year, 2 4-hour peak discharge rate.
Assume Q= Runoff Quantity
Then Q pre dev (for 1.5 year, 24-hour peak) <= Q post dev (for
1.5 year, 24-hour peak)
Q pre = Rainwater Runoff = Average rainfall X Surface Area X Runoff Coefficient
Pre development the site area, 28460 sq.ft(2643.9 sq.mt)., was flat vegetation.
Post Development the site was designed to have 23274 sq.ft(2162.1
sq.mt.) of hardscape including building and pavements with runoff
coefficient of 0.95 and the rest of 5286 sq.f.t(491.08 sq.mt.) of flat
landscape.
Calculation for 1.5 year, 24-hour peak discharge:
Predevlopment = 0.115 m X 2643.9 m2 X 0.1 = 30.40485 m3
= 30.40485 X 1000 litres
Whereas, 0.115=average rainfall
2643.9 m2 = Area
0.1= Runoff Coefficient
Post Development = (0.115m X 2612.1 m2 X 0.95) + (0.115m X 491.08 m2 X 0.1)
= 285.37 m3 + 5.6 m3
= 290.97 m3
= 290.97 X 1000 litres
Since the Post development runoff is higher than the predevelopment
run-off, we need to design for reducing the runoff from the site.
Let me know if there is any changes to the calculation
calculation of Cr 6.1 Stormwater design-Quantity Control
Your calculations are correct. You will have to develop strategies to infiltrate, evapotranspire, or reuse 260.57 m3 of stormwater to qualify for the credit..
However, depending on your location, please verify that 0.115 m is your 1.5 year, 24 hour storm. That seems high. You are trying to find the rainfall amount for the 1 and 2-year 24 hour storms.
JP Rout replied Jan 06 2010
Thanks Mr. Hurst, Actually the data was given to me.
Can you help me to verify the calculation of the storm. I only have a years rainfall data. I hope that's enough.
Month Monthly Total mm
Jan 11.9
Feb 17.8
Mar 23.7
Apr 32
May 49.9
Jun 232
Jul 326.5
Aug 346.1
Sep 239.1
Oct 86
Nov 11.2
Dec 3.1
Annual
Total or
Mean 1379.3
Thanks in advance.
Calculations for SSc6.1
Calculation of a 2-year, 24 hour storm is a lengthy process. See my comments above on this issue. Rainfall amounts, especially for one year, will not provide you with a 2-year, 24 hour storm event. This is information that you have to obtain from the local jursidiction. I have seen are some world atlas books that may provide a figure if there is no local jurisdiction, but they are not very accurate and should be used if you cannot obtain the information locally to your project.
I gave an example above for a 2-year, 24 hour storm for Baltimore. I found this value on the internet.
Hope this helps.
Stream Channel Protection? (case 1, option 2)
Has anyone out there had any success using the Stream Channel Protection option (Option 2 for CASE 1) where the existing impervious is less than 50%?
I've got an urban project (in Denver, CO, where cisterns aren't an option due to CO water law), with the existing impervious <50%. We're doing all manner of best practice stromwater management PLDs, rain gardens, you name it. But there's no way we can infiltrate our two year storm with the soils on the site.
We are _certainly_ reducing (if not eliminating) erosion and protecting any receiving stream channels (really, just runoff from an urban site into the storm system). But I haven't seen any projects in the past that are urban in nature meet this one. Has anyone else out there had any luck? If so, what's the secret?
Thanks,
Josh
I've done some asking around internally at YRG and
Brenden McEneaney replied May 02 2010
Josh - can you get around the CO cistern issue if you don't reuse the water, but just detain it in a cistern and percolate it to landscape over a longer period of time? Seems like that wouldn't be an appropriation, and actually, you'd be adding water to the system.
But you raise another point that I've wondered about with the intent of the credit. If the stormwater flows through all man-made structures, where is the real benefit from this credit? Here in tiny Santa Monica, just one of the stormdrain outflow points can release 50 MGD during a storm event. It releases 500,000 gal/day in DRY weather. While infiltration to the aquifer is still preferable, and one might make an argument about reducing storm volumes overall, I'm not sure I see how rate reductions have any impact to an enormous storm river flowing through a concrete channel and into the ocean...
Perhaps another relevant distinction to this credit would involve the nature of the local stormwater system infrastructure?
Joshua Radoff replied Principal, LEEDuser Primary Author, YRG sustainability May 03 2010
Brenden,
Thanks for chiming in. We are indeed looking into the delayed release option and the potential sizing of a cistern to allow draw down. Should have results this week. I do, however, remain in the dark about the application of Option 2. Is this just an unspoken rural option that USGBC doesn't want to see applied to an urban site?
Regarding your issues in Santa Monica, wouldn't any form of rate reduction reduce the peak flow into the rivers, and therefore mitigate the issues associated with erosion, sedimentation, pollution, and the ability of a treatment plant to handle the peak?
Brenden McEneaney replied May 03 2010
I suppose it would if there were any rivers or if there were a treatment plant. Stormwater from buildings (and building sites) only ever flows through concrete structures and then goes into the ocean untreated. The rate at outfall is influenced far more by the gradient of the concrete channel than by any additional flows from any individual building (no matter the rate). There could be no erosion throughout the system except at the beach between the storm drain outfall and the ocean, and that area is hardly a natural system, as public works moves the sand around there all the time, wet or dry.
Beyond that, the mitigation provided by any new building is negligible (and probably will always be negligible) since the vast majority of stormwater is coming from existing buildings and infrastructure. Obviously buildings should still pursue best practices, but it seems that for this credit, rate reductions have been tougher to achieve around here even though volume reductions and infiltration are probably more important.
Looking forward to hearing if Option 2 works for you, as I don't know of anyone who has pursued that yet...
Gregory Hurst replied Principal, AECOM May 20 2010
the intent of Option 2 for Case #1 has never been clear to me. I think the simple response is that you don't have to do anything about peak and volume if you can show that there is no downstream erosion. If there is a pipe all the way to the receiving waters and there is erosion protection at the outfall of the pipe, then it seems that you have met the criteria. That does not seem to meet the intent of the credit. I think the key to this option is the second sentence that states: The stormwater management plan must include a stream channel protection AND (my capital letters) quantity control strategies. The first unstated assumption is that the channel must be protected, but how far? I have used the EPA definition that the channel must be protected to the point where the flow enters The Waters of the U.S. This may be along ways or a short distance.
Second, I have put importance on the word AND in the second sentence in Option 2 to suggest that an acceptable strategy may not have to limit volume, but does limit flow rate. Of course, increased volume as well as increased flow rate will cause greater erosion in a natural stream channel, but I would expect to see documentation that can demonstrate that the channel protection will protect against the flow rate and volume entering the stream and that the flow rate has been reduced to predevelopment conditions.
This is really the only reasonable strategy that works in places where there is little opportunity for infiltration (clay soils). This is also consistent with water law in Colorado
measurement of imperviousness
?
"Impervious surfacesSurfaces that promote runoff of precipitation volumes instead of infiltration into the subsurface. The imperviousness or degree of runoff potential can be estimated for different surface materials. have a perviousness of less than 50% and promote runoff of water instead of infiltration into the subsurface. Examples include parking lots, roads, sidewalks, and plazas."
"Perviousness is the percentage of the surface area of a paving system that is open and allows moisture to soak into the ground below."
Perviousness refers only to paving systems, therefore imperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. refers only to paving systems...rubbish!
I have an existing site. The new building will be built on an underground parking garage which is currently covered by a layer of turf...so what is the pre-construction "perviousness" of this area? What units is it measured in?
I can calculate pre and post developement run-off rates and volumes, but what does that help if I can't determine into which of the 2 cases it falls into.
Does anyone have a quantifiable definition of perviousness?
Amy Rider replied Sustainability Consultant, KEMA Services May 21 2010
Keep in mind that this credit looks at the site as a whole not a single paving material. Your pre-development runoff rate should answer the question of whether case 1 or case 2 is applicable by giving you a ratio of runoff to rainfall; thus the perviousness of your site.
Jean Marais replied b.i.g. Bechtold INGENIEURGESELLSCHAFT MBH May 26 2010
What you're saying is that
Run-off Coeffient RC = ImperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground.
right?
This would be a clear quantifiable definition that is not found in the reference guide. I can work with this.
Amy Rider replied Sustainability Consultant, KEMA Services Jun 02 2010
Just to clarify:
Run-off coefficients are for specific materials under specific circumstances (e.g. turf with 1-2% slope) whereas imperviousnessResistance to penetration by a liquid and is calculated as the percentage of area covered by a paving system that does not allow moisture to soak into the ground. is typically applied to an area, or in this case a site, as a whole.
Yes, in this case where your site seems to be of a uniform nature, they are roughly equivalent.
Jean Marais replied b.i.g. Bechtold INGENIEURGESELLSCHAFT MBH Jun 03 2010
Run-off coefficients are also area weighted when calculating the RC for the whole site. This is standard practice. Thanks for the tips.
Case 2 seems a doddle
Here, by the looks of it, no RATE calculations are required.
Just VOLUME...and only for the 2-year 24hr design storm to boot, meaning:
1) Get the total volume ("Depth") of the 2-year 24hr design storm for your area.
FOR BOTH PRE AND POST DEVELOPED SITE:
2) Get the respective site areas
3) and their run-off coefficients...these are usually quite standard (http://www.lmnoeng.com/Hydrology/rational.htm):
4) Calculate total VOLUME for both pre- and post developed cases "generated" by each area...V = run-off_coeff x Area x "Depth"
5) Sum the Total Volume "generated" by pre-developed case area and compare to the Sum of the Volumes of the post-developed area.
6) You need a 25% improvement.
The sample "Sample Calculations – Cheyenne Botanic Gardens" in the Documentation Tool Kit is a perfect example.
Note: the "Depth" of the 2-year 24hr Design Storm is a statistical value deturmined from Depth-Duration-Frequency (DDF) Charts. These charts show the relationship between the rainfall depth (volume) and the corrisponding rainfall duration (minutes). This data is required for 1 and 2 year statistical return periods.
(Hi Gregory, you may want to move the following info somewhere more suitable)
1 and 2-year 24hr Design Storm Depth and Rate values should be obtainable from your local meteorological office. At least the data required by statisticians for methods as described by Ernest W Tollner (see below), should be obtainable here.
Further reading:
Natural resources engineering By Ernest W. Tollner
Theoretical Background Summary for German Speakers:
http://www.boku.ac.at/iwhw/hydsem1/folien/hydsem2_08.pdf
Further reading on calculations relating to RATE and IDF charts:
http://europeicid2009.org/media/trudy/Sbjeschni_Schmidt_Germany.pdf
Amy Rider replied Sustainability Consultant, KEMA Services Jun 02 2010
Check out the Case 2 definition on page 95 of the 2009 Reference Guide. Rate and quantity are both clearly required there and I expect this is the standard to which your submittal will be held.
Jean Marais replied b.i.g. Bechtold INGENIEURGESELLSCHAFT MBH Jun 03 2010
Thanks Amy. Strange that it's not on the requirements page though. There the rate is clearly ommitted. This should be corrected in the next addenda hopefully.
Jean Marais replied b.i.g. Bechtold INGENIEURGESELLSCHAFT MBH Jun 03 2010
This stems from historical LEED-CS 2.0 submittal template for example...where for option 2 only the quantity is required. This is unfortunately still the case in LEEDv3 LEEDonline submittal form still only requires the volume for case 2...that's why I asked.
Case 1, Option 1
There is a point of contention within my office that I'm hoping can be cleared up by posting here. When considering the "1- and 2-year 24-hour design storms", is it possible to meet the quantity control (volume) requirement by providing extended detention beyond the 24-hour period?
In other words, I can model a pre-development and post-development run-off volume using the SCS method. Lets say I'm able to attenuate the peak post-development flow to pre-development conditions using a detention pond, but the total volume increase leaving the site is 1000 cubic feet modeled over the 24 hours. What if I alter my pond outflow controls to extend the detention of this 1000 cubic feet beyond the 24 hour period? Does this meet the intent of the credit or is it simply finding a loophole in the wording?
To me it seems like providing infiltration/reuse/evapotranspiration practices are the intent of this credit. However, the wording could possibly be interpreted as what I wrote above.
Any thoughts on the topic would be appreciated.
Thanks,
Randy
Gregory Hurst replied Principal, AECOM Jun 15 2010
I interpret the requirements of the credit to be met for a 24 hour period. If you detention is sized to reduce the volume to met the pre-development condition for a 1 or 2 year 24-hour storm, and you have not released all the volume within 24 hours, then the detention pond may not have the capacity to hold another 24-hour storm occurring in the next 24-48 hours. I realize that having two 24 hour storms back to back is rare, however, it can happen. The pond, if it used for flow and volume attenuation must be ready to fulfill it's function in the next 24 hour period.
This same thought process comes up when using cisterns. There is a desire to use a cistern for rainfall collection that can be used a a source for irrigation water supply. However, after a storm when the cistern has filled or partially filled, there is little need for irrigation so the water is stored in the cistern until needed for irrigation. If the cistern is full, it does not provie any storage capacity, and therefore no peak or volume attenuation.
Greg Hurst
Amy Rider replied Sustainability Consultant, KEMA Services Jun 15 2010
While I agree with Greg on the approach he uses for cisterns, stormwater harvesting and in general allowing for future capacity, I disagree with his interpretation that the credit only applies to the site for a 24 hour period.
Based on my interpretation, the credit requires extended detention, infiltration and/or evaporation to reduce total post-development stormwater quantities. Admittedly it is not stated clearly in the reference guide.
The debate continues....
Gregory Hurst replied Principal, AECOM Jun 15 2010
We are in agreement that the credit requires extended detention, infiltration, and/or evaporation. In my interpretation, those items need to be engineered as part of the requirement to meet volume and flow. For instance, If infiltration is the approach and the infiltration rate and volume to be infiltrated takes longer than 24 hours, the capacity of the detention will have to be increased to allow for another 2-year 24-hour storm to be accomodated. However, as the debate continues, what happens if you get three days, or ten days of 2-year 24-hour storms? when do you stop? Should sizing be for two 2-year, 24 hours storms back to back? or just one. I don't know the answer, but have assumed that a maximum of two 2-year 24 hour storms for sizing of detention. Just my interpretation and a case I think I can defend.
This brings us back to the original question, can we allow volume to discharge beyond 24 hours but limit the volume that was released over the initial 24 hours to meet the pre-development volume requirement? This implies that the additional release after 24 hours doesn't count toward the increased volume that is a result of the post-development condition. I think not. I think there has to be supporting documentation that shows that the additional volume is being dealt with on site.
Now, this brings up the entire debate for SSc 6.1, Case 1, the "or" option. what consitutes a stormwater management plan that includes a stream channel protection and quantity control strategies. As you say, the debate continues......
Jean Marais replied b.i.g. Bechtold INGENIEURGESELLSCHAFT MBH Jun 16 2010
I was under the impression that the 2-year 24hr design storm as a statistical recurrance frequency of 2 years. Yes, they can occur back to back. Yes, they can also occur with 24 hrs in between them. But this rainfall volume will be reached over a 24 hr period if you waited 2 years, statistically speaking.
During your 2 year wait every other day (24hrs) will bring less volume than that of the "2-year" event, statistically speaking. Meaning that if you can discharge the "2-year" event capacity within 24 hrs safely (using infiltration on site for example) without increasing the predevelopement discharge rate, you're covered for the entire 2 year wait. More importantly, you've not increased the potential (the potential remains the same and is therefore more predictable for city planners) of flooding in the area.
Yes, in real life you may get both droughts and floods during the 2 year wait, but that is also expected. Over a hundred year period your design will be right, on average.
If the USGBC wanted us to decrease the volume rate burden on the stormwater sewers even more, they would need to decrease the design storm frequency from 2 years to say 20 years.
Stormwater runoff in the roof
Dear all,
Our project design stormwater combine 2 options:
1. For stormwater runoff at the ground surface, we can decrease 25% by installing pervious pavement (like hole brick...)
2. For stormwater from the top roof, we will collect by a piping system, and connect this piping to public drainage system, don't let this source to runoff above ground surface.
So, if we design like that, can we achieve this credit requirements, under CASE 2?
Thanks so much for your helping.
Tristan Roberts replied Editor – LEEDuser, BuildingGreen, LLC Jul 12 2010
Fabio, your first option works for this credit, but the second option does not. The intent of the credit is to mitigate stormwater onsite, not to let it runoff into local streams or to a public drainage system (which probably discharges to local streams).
It took me a couple minutes to find a LEED reference that would clearly support my opinion here. It is very clearly implied on page 92 of the LEED BD&C Reference Guide, under "economic issues."
Trying to locate 2yr, 24hr rainfall info for Central America
I am working on a Federal facility project in Honduras that we are designing to LEED Silver. Trying to qualify for Credit SS6.1, but have been unsuccessful in finding rainfall data. Has anyone located a source for this information for Central America?
Jean Marais replied b.i.g. Bechtold INGENIEURGESELLSCHAFT MBH Aug 02 2010
Have you tried the airports? They will only have raw data though and you may have to get an external organisation to process the data into usable statistical charts (IDF and IFD charts for 1 year, 2 year, x year recurance intervals). The German weather organisation would probably offer this service, but then so could others.
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