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Beyond standard practice, but cost-effective
Development usually comes with increased stormwater runoff due to 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. like roofs and parking lots. To earn this credit with previously undeveloped sites, you’ll need to avoid any increase in runoff, while on mostly impervious developed sites, you’ll need to reduce runoff. You will probably need to go beyond standard practice to achieve this credit, and you might see increased costs, although an integrated approach can make this credit cost-effective....
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127 Comments
Calculating Runoff to a Rain Garden
Hello,
Our project is an adaptive reuseAdapted reuse is the renovation of a space for a purpose different from the original. on an urban site that was previously about 95% impervious. We are increasing the area of vegetated space (the old parking lot is being demolished), collecting rainwater from about 6700 SF of the roof areaRoof area is the area of the uppermost surface of the building which covers enclosed Gross Floor Area, as measured when projected onto a flat, horizontal surface (i.e. as seen in Roof Plan view). ‘Roofs’, or portions of roofs, covering unenclosed areas (e.g. roofs over porches and open covered parking structures) are not included in the areas used to evaluate compliance with SSc7.2, though they may be applicable to SSc7.1., and creating a rain garden out of a vegetated parking island that is about 1800 SF. We had already put together the stormwater volume reduction calculations using the NRCS method before the team decided to add the rain garden (we had previously been considering pervious pavement for a section of the front driveway), and now we are having trouble determining the proper way to quantify the runoff that will now be directed to the rain garden in the calculations. We plan to slope a section of the paving which surrounds the rain garden towards the rain garden, but how should this be quantified? Should the volume of water which will be draining to the rain garden be subtracted from the Runoff Volume Total in the design case (assuming that we have properly accounted for the rain garden being able to handle that 2-year 24-hour design storm volume based on the inches/hour infiltration rate for the soil), or should the CN value for that section of the paving be changed?
Stormwater Master Plan
Our project site is located on property owned by an International Airport. The project site is located within a much larger basin for which the Airport created a stormwater master plan. The master plan stormwater system consists of a series of pipe and ponds on the Airport property. Our project site fits within the parameters of the master plan by not exceeding the allowable impervious coverage and discharging into the designated offsite pond. There is no onsite stormwater management for this project. Calculations that demonstrate the system as a whole meets stormwater quantity and quality requirements were performed by an engineering firm that acted as a consultant to the Airport. Could LEED Credit for Stormwater Design – Quantity Control and Stormwater Design – Quality Control be awarded to this project by referencing the calculations that were provided by the Airport, as prepared by another engineer, for their master stormwater management system? Since we did not perform the extensive analysis for the system, we do not feel that it is correct to imply that we have prepared or validated their calculations.
In my opinion, this is an acceptable approach to achieve the credit. See my response to a similar inquiry to Kim Sosalla-Bahr below. The key point is that the treatment area has to be set aside in perpetuity and your project has to conform to the assumptions set forth in the drainage master plan (run-off coefficient, conveyance, etc.), and the drainage master plan must have been implemented to account for your property, and must have been designed to meet the LEED reuirements. The information developed by the master drainage engineer will have to be used as supporting documentation, and the narrative that you provide will have to demonstrate that you meet the criteria set forth in their plan, and your supporting calculations will have to demonstrate that your project's drainage will conform to the master plan, as implemented. I do not expect that you have to validate their calculations. The assumption is that their work as a professional engineer was accepted by the local stormwater jurisdiction.
Stormwater Run Off to Tidal Waters
Are there any cases where an alternative compliance path has been taken for this point. Project is dockside straight on to tidal waters - while the stormwater quality issue can be tackled there is little point in attenuation and the site would be given the appropriate discharge consents from the local authority. I can see potential impacts on a 1 in a 100 year situation but this would far exceed the requirements within LEED. The site is an existing area of hardstanding.
thanks
Are you proposing an alternative compliance approach? Based on your description, it seems you are not taking action that would make you eligible for this credit.
SS credit 6.1 and LEED boundary
In order to satisfy this credit (and SS 6.2) can a facility on a very urban site, with greater than 50% perviousness, use an area on the facility's campus, but is outside the project's LEED boundary for stormwater management related to quantity and quality. The project is looking to slow down and hold water on the campus, so it can infiltrate.
Yes, in my judgement it can. However, you will have to demonstrate that the area in question that is outside the boundary of your project is being set aside in perpetuity for water quality, and that it is sized aequately to provide the same level of treatment for any other properties that discharge stormwater to this area. It is the same argument that is made for a combined stormwater detention or retention pond, in that it is sized to meet the volume requirements of all the properties discharging into the shared pond. In your case, it has to be sized or engineered to meet the water quality requirements of all of the properties discharging to the area.
Alternative Compliance path for international projects
The USGBC has published a draft for additional guidance for international projects. That includes alternative compliance paths and even additional LEED online forms for international projects. This credit is included in the guideline. Find more here: http://www.leeduser.com/topic/international-projects-alternative-complia...
peak flow calculations for 24-hr duration
Hello -
My question is in regards to the 2-yr, 24-hr requirement for calculating peak runoff rate. As far as I understand, by requiring the 24-hr duration to be used with 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. for both pre- and post-development, any design that increases the site's time of concentration (thus reducing the peak runoff rate) is not accounted for. It seems to me that the rainfall duration used in the Rational Method calcs should be based on the site Tc, just as they are with traditional stormwater design. Using a 24 hr duration for peak runoff doesn't make much sense to me....even the local stormwater criteria in my area does not provide rainfall IDF curves for such long durations.
Perhaps I am misinterpreting the language. Either way, I would appreciate any insights as to how this has been used and/or interpreted by others in the past.
Thanks,
Chris
In my experience you are not limited in your calculation methodology, but do not need to account for time of concentration per the reference guide. I suggest uploading your calculations with an accompanying explanation for whichever approach you use and your results.
SSc6.1 Pond with Flow Restrictor and Total Stormwater Volume
I am having the same issue as Lucy (October 18, 2010) on one of our current projects, and the comments by Gregory and Jennifer were helpful but I still have some questions.
Scenario:
-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%
-new buildng footprint will increase imperviousness
-we will build a new detention pond/swale with an outlet to city storm system
-there will be a restrictor to control the rate of outflow from the pond/swale
-we do not plan to use the water for reuse in the building or landscaping, etc.
1.) I thought that showing the restrictor outflow rate would be enough for the calculations but we got the following comment from the GBCI: "peak rate from a detention pond is typically a function of the outlet structure configuration, outliet pipe size, and the depth of water in the pond. Peak rate does not necessarily decrease at a constant rate with a decrease in runoff volume. Please revise and resubmit calculations for the peak rate leaving the pond..." So, can someone please illustrate how to calculate the peak rate if we have a restrictor (at the outlet from the pond) which allows a maximum flow of 0.5cfs/acre?
2.) The second issue is as follows: if we are catching storm water in the pond, the restrictor will solve the issue of the "rate", but not the problem of the "volume". Does someone know how to overcome this issue?
Thank you in advance!
Even though I haven't done such calculations, I might be able to shed some light on this.
1.The reviewer is arguing that your outlet will allow for more water to be discharge depending on the height of the water table in your pond. So lots of water in the pond --> higher pressure at the outlet --> higher water flow.
You will have to account for this or explain, why your outlet will only allow for a certain maximum
2. Besides reduction of the flow rate, you will have to reduce your volume. So the question is how much of that water, which is sitting in the pond, will infiltrate into the ground. Your civil engineer should be able to answer that.
I hope that helped at all.
Thank you, this is helpful. But I think we might be losing this credit, since the soil in our property has proven uncapable of absorbing as much water as we need. It seems like the only way to get this credit--at least in terms of quantity--are to use the water for something else, or to have the project on a sandy beach where the water can be absorbed by the site. :(
Is it possible to get post development run off in negative?
Our project is located in Kathmandu, Nepal and I have a few questions regarding the following points.
1. As per the Table 1 page # 104 of 'LEED Reference Guide for Green Building Design & Construction', the Annual Rainfall in Kathmandu is more than 40", which is under Humid WatershedWatershed that receives at least 40 inches of rainfall each year. area. To design the system, to capture the runoff, the average rainfall event can be taken as 1" (one inch) of rainfall per 24 hour.
2. But in 'SS Credit 6.1: Stormwater Design - Quantity Control Form' it is mentioned that 'Site Runoff: Two-Years, 24-Hours Design Storm.
Question -1:
"Which value should I take to calculated the site Stormwater Runoff?"
Question - 2:
"While calculating, if the Stormwater Runoff reduction is in negative values or less than 25. What procedure should we follow?"
In response to question #1: The credits are different and require a different set of calculations. Credit 6.2 is addressing water quality and the requirment is to treat the first 1-inch (in this case) of a rainstorm. The intent is that most pollution is contained in the small frequent storms, and that developing a pollution prevention strategy to treat stormwater runoff in the smaller storms will provide an adequate and cost effective level of protection to receiving steams and lakes to minimize the effect of sediment. The requirement is actually to treat all the stormwater from 90% of the annual runoff volume. This is a cumbersome calculation so LEED has provided a guideline based on conditions in the watershed (humid, semi-arid, and arid) to help make the calculations easier.
Credit 6.1 has been developed for a different reason and is more concerned with flood control and downstream flooding. For this reason, a two year-24 hour storm (which can have a rainfall depth greater than the 1-inch suggested for evaluation of credi 6.2) is the chosen basis for design, and, depending on the location, is intended to reduce the peak flows leaving a site that can cause downstream soil erosion, sediment transport, and a negative impact on drainage infrastructure.
Depending on your solution, usually detention and infiltration for volume reduction for 6.1 or 6.2, the solution may serve for both credits, but the requirements for both credits must be checked independently.
Question #2: I am not entirely clear on your question, if the reduction is negative, that means that there is an increase in runoff. If there is an increase in runoff, you will have to develop strategies to reduce the volume and/or flow rate to meet the LEED criteria for the credit. Again, the reduction may be different, depending on which credit you are evaluating. If you want to clarify the question, I may be able to provide a better response.
Option 2: Stream Protection Plan
I apologize if the requested information is included on this site but I have not seen it to this point. Can anyone give me a definition of what constitutes a stream channel? We are looking at a 5.0 acres site that is unimproved and the proposed development will create an 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. area of approximately 4.0 - 4.5 acres of the total 5.0 acres. With this in mind there does not appear a way that Option 1: Quantity can be met without creating a large wet pond on the site to accommodate the tremendous increase in storm water that will be generated from this site. Detaining for the peak discharge is not a problem but the total volume is the issue with Option 1 for this site. The proposed site has a concrete lined drainage ditch running the entire length of the property that the site currently sheet drains to. This ditch eventually routes to an existing detention pond before discharging to the outfall stream. We will maintain all existing drainage patterns on the proposed site. If we are wanting to achieve Option 2 are we to implement the stream protection plan on our site even though technically the stream it will be discharged to is not on our site? If we slow the water down with some design techniques, swales, rain gardens, rip rap, etc. will this be sufficient to achieve the point for this credit?
Robert, I don't think the credit language intends to restrict your efforts to streams on your site. I think you would want to be as comprehensive as possible in looking at both the drainage on your site and ther receiving stream.
Calculation of drawdown rate Qr
I have a question for a Czech colleague of mine. The have a project and want to install a stormwater storage tank. The water shall be used for toilet flushing in the building. The problem is, that he does not know how to determine the design storm interval. I looked into the LEED BD+C Reference Guide on page 97 in the example (last calculation) at the top of the page. From where do the 3 days / 72 hrs come. I understand that the tank must be emptied, but I cannot help him identifying the design storm interval. Can anybody please help?
Christian, if I understand the question properly, proper sizing of this tank should be based not on LEED compliance data but on meteoroligical data from the area and calculations of how much water will be required for flushing.
existing imperviousness
Is this the # I should be using to determine 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.: "Pre construction coefficient of imperviousness: C = .25"? Therefore, our project falls into Case 1?
Kash, if I understand your question, yes, you are correect.
The data I have to work with
The data I have to work with for filling this credit out are pre- and post-development calculations for a 100 year storm with a duration of 1 hour. How does this relate/convert to 1-yr, 24 hr and 2-yr, 24 hour design storms?
Thank you!
It doesn't. You will have to obtain the 1yr, 24hr and 2 yr 24 hour storm data form your local weather service. It's a totally different value. In general if the systems were designed to handle the 100 year storm than they are very much capable to handle the others. I did a project in German with the exact same question.
Case 2 Site with little decrease in Impervious Ratios
I am in the processing of accessing the feasibility of attaining this credit with a site that is currently 88% impervious and the proposed site is roughly 81% impervious. While i will be able to show a slight reduction in site runoff via a retention swale (+/- 4%), i am wondering if an exfiltrationExfiltration is air leakage through cracks and interstices and through the ceilings, floors, and walls. system would be a means of achieving the additional runoff decrease. I know this goes against the standard runoff reduction calculations, but has this credit been achieved via structured stormwater system?
We had an infiltration system to reduce the storm water runoff to zero for two projects with very limited site area. Here is a link to a similar system. http://www.intewa.com/drain_max_tunnel_infiltration_ditch_system_1258504... not sure if that's what you are planning on using, but you can install these system under neath the surface parking. It's pretty much common practice in Europe.
Project Boundary
Our site is served by a large, adjacent regional retention basin (permanent large pond) that includes the runoff from our site. The basin is directly adjacent to our project boundary and is covered by cross-easements between all the owners served, including our project. Our comment came back on this credit that we need to prove that we meet the requirements specific to our project boundary, We have provided the calculations for the pond which exceed the LEED requirements. What am I missing? The calculations submitted are quite extensive.
If you are using a shared "facility" in this instance the pond, you will have to to account for all the other buildings connected to the pond also and make sure that they too achieve compliance with the credit. I actually had this questions in my LEED AP exam. The campus guidelines give you some backup information on the topic.
Hi,
I have quite simmilar situation. Our building is located in offices business park. In the center there is a lake which we want to use as a source of water for irrygation system.
If our pipes are attached to the lake do we have to take whole lake to account for Project Boundary or just a part where we have a pipes?
As indicted in the response above your inquiry, you must be able to show that the lake has adequate storage volume to meet the LEED requirements for your project, and all the other properties draining into the lake must have adequate volume also. many times there is a master drainage plan for a business park that can provide you with the anticipated runoff for all properties draining into the lake, and the calculations for lake storage volume that can handle all of the properties. Essentially, your property has a share of the storage volume in the lake and it must be adequate to meet the LEED requirements.
Green roof
Green roofs can reduce peak runoff rates on developed sites. However, the volume reduction potential of any green roof will depend on its moisture-retention capacity, which depends on the soil profile. One storm may saturate the soil, leading to a conventional amount of runoff resulting from a second storm in close succession.
So does this mean that a green roof can not be used? Or do we need to install additional equipment to capture rainwater ?
Emmanuel,
A green roof can satisfy the credit if it has sufficient soil depth and type to detain the 2-year, 24-hour design stormA 2-year, 24-hour design storm is a nationally accepted rate that represents the largest amount of rainfall expected over a 24-hour period during a 2-year interval. The rate is the basis for planning and designing stormwater management facilities and features.. Many projects have used a green roof to achieve this credit, but not all green roofs will work - as you mentioned, you do need to have sufficient soil depth to provide enough detention for LEED criteria. So if your soil profile is deep enough and calculations show you can detain the necessary quantity of water, you should be okay.
Whether you need to have additional capacity to detain more storm water immediately following that storm event is a little unclear but has been discussed in several different threads below for cisterns and detention ponds, which are functionally similar to a green roof. See Greg and Amy's comments in particular. You can always submit a CIRCredit Interpretation Ruling. Used by design team members experiencing difficulties in the application of a LEED prerequisite or credit to a project. Typically, difficulties arise when specific issues are not directly addressed by LEED information/guide to get more clarity if you are still unsure.
SSc6.1 Option 2 - Pre-Project site > 50% Impervious
The requirement for this credit states, "Implement a stormwater management plan that results in a 25% decrease in the volume of stormwater runoff from the two-year, 24-hour design storm," but within the text it says, "The post-development rate AND quantity must be at least 25% less than the pre-development values to earn this credit." Further, the template online only calls for the 2-year, 24-hr pre- and post-project Volume values. Does anyone know if the true intent of this credit is only for the Volume of the 2-yr storm? Has anyone ever been requested to furnish more information for the flow or for the 1-yr data? Thanks.
Evan, you seem to be quoting text from Case 2 for "volume" and Case 1 for "ratea and quantity." As far as I can tell, that is the source of the differences in what you are seeing.
Which case are you?
possible or not?
Hello everyone,
we have a site 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%, and I can't understand can we archieve this point or not. We have rainwater drain system with watertreatment equipment, which capture runoff. After cleaning water is used on-site: for irrigating, flushing.
How can we calculate peak discharge rate?Is it 0 for postdevelopment??
I can't understand the intent of credit and ways we can archieve it in our case.
Armen, are the systems you mention ones that are already in place prior to your LEED construction, or are they planned? Either way, it sounds like you have a good shot at this credit.
To get oriented to it, I would recommend carefully reviewing the information under Bird's Eye View, Checklists, and Resources tabs above.
Thank you
They are planned.
I've read comments here (Bird's Eye View, Checklists, and Resources tabs I've seen too - i had the access in the past and will pay for it this days)
BUT I still don't understand what is 1 and 2-year 24 hour design storm? Our wheather service has only annual and monthly dates. Can I calculate 1 and 2-year 24 hour design storm??
Armen, the 1- and 2-year storms are storms that have a 100% and 50% chance, respectively, of happening in a given year. These are statistically measured storms that the national weather service should be able to provide. You cannot just derive them from a year's worth of data, unfortunately.
ok, thank you! try to get the data from weather service.
How to quantify a evaporation rate
I was hoping I could get some advice on obtaining the quantity requirement of SS CR 6.1. We are trying to obtain this credit on an approximate 19 acre site. The site was originally a multi-office complex that will be removed and replaced with a much larger building. The impervious area of the site will be increased slightly (69% to 71%) but we are increasing the quality of the landscaping so the runoff curve number is actually lower than the existing conditions (CN 89 to 87). To achieve this credit the volume of runoff must be 25% lower than the existing conditions for the 2-yr 24hr storm event, which we determined will be 155,347 gal of runoff to remain on-site. There will be 2 underground rainwater cisterns on-site; a 20,000 gal tank to be re-used as graywater1. Defined by the Uniform Plumbing Code (UPC) in its Appendix G, titled "Gray water Systems for Single-Family Dwellings," as "untreated household wastewater which has not come into contact with toilet waste. Grey water includes used water from bathtubs, showers, bathroom wash basins, and water from clothes-washer and laundry tubs. It shall not include wastewater from kitchen sinks or dishwashers." 2. The International Plumbing Code (IPC) defines graywater in its Appendix C, titled "Graywater Recycling Systems," as "wastewater discharged from lavatories, bathtubs, showers, clothes washers, and laundry sinks." Some states and local authorities allow kitchen sink wastewater to be included in graywater. Other differences with the UPC and IPC definitions can probably be found in state and local codes. Project teams should comply with the graywater definitions as established by the authority having jurisdiction in their areas. within the building and a 105,000 gal tank to be re-used for irrigation purposes. My first question is when calculating the volume for the underground tank, can I only use the total (125,000 gal) or can I determine a draw down and get some additional volume. Secondly, due to the volume shortage we are proposing to create a constructed wetland with enough storage volume to achieve this credit. Unfortunately the soil conditions are not great and we can only expect an infiltration rate around 0.04 in/hr. Although, with the wetland we will see some evaporation and evapotranspiration rates that would increase the volume of removed from the wetland. Will this rate need to be quantified for this credit, if so is there some documentation out there to help determine this. Also, in general do you think what we are proposing will satisfy the intent of the credit or do you think we are out of luck, other than adding more pervious surface? Thanks
Question 1: Tank Volume.
Some tank drawdown assumptions seem reasonable in this case given the greywater reuse, but the effort to prove a minimal amount of added volume may prove to be more trouble than it is worth given the addition of the constructed wetland.
Question 2: Constructed Wetland Evaporation Rate.
In general I think that any surface water retention system that is well designed and therefore adequately sized can help achieve this credit without the need to account for evaporation rates insofar as LEED is concerned. That being said it isn't entirely clear from your description that the constructed wetland will actually discharge water. If water will leave the constructed wetland the rate of infiltration and evaporation will be key to showing your project's net reduction of stormwater quantity.
Thanks for your response Amy. The wetland has been designed to be anywhere from 2' to 6" deep. An overflow wier on a riser pipe will be installed to take the storrmwater out once it reaches an elevation over the 2' depth. Also, could I include some assumption of the infiltration rate of the approximately 5.5 acreas of landscaping (sod / native plantings, etc.) and green roof on site. I would have plenlty of volume by doing this...
Thanks again.
How to show 100% Infiltration
Our project is the only development on a small, flat, island. The surface is sand or sandy topsoil and is extremely porous. After development, the only 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. will be roofs, some of which will harvest rainwater. Rainfall from the other roofs will infiltrate naturally into the sand. There is no runoff into the surrounding lagoon as all rainfall infiltrates into the water table. Given that no specific stormwater design is needed, will it be sufficient for documentation to show that 100% infiltration remains post development? Thanks
Yes, 100% on site infiltration is an acceptable means to obtaining this credit. The reviewers will likely want to see some calculations or supporting documentation showing that the soil types and grading can adequately infiltrate the additional rainwater from the 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. that aren't being captured, but this shouldn't have to be very extensive. Be sure to describe your rainwater harvesting system as well to further support your claim.
runoff coefficient
what the value of runoff coefficient for gravel area (2% gradient)?
While this is somewhat dependent on the design (material thickness, subgrade permeability, etc), 0.5 is a good rule of thumb for low slope gravel areas.
1-yr 24hr storm event
why does the credit require for both 1-yr and 2yr storm events 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%? and who knows the 1-yr 24hr storm event (intensity) for Idaho area?
I cannot speak as to why both 1- and 2-year events are required only that they are, and are often requested for projects with more than 50% 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. as well.
As for where the information can be found (since it doesn't appear to be reported by the USGS) I recommend speaking with the City Engineer or Public Works Officer in the jurisdiction where your project is located.
Good luck!
Thank you for suggestion!
I did that way and is still unable to find out 1-year storm 24 hr data for this area. Further question is if just fill leed's docu. with only 2-year 24 hr results and give an explanation that we don't have 1-yr data, would that be ok?
Eric,
This post has been up for awhile, but I thought I would throw in my two cents. The 1-year storm event is usually not documented, however, thde two-year and the 100-year 24-hour storm events can usually be found in your regional hydrology manual (which should be posted on-line). The 2000 Maryland Stormwater Design Manual (also online) Table 2.2 has 1-yr, 24-hr rainfall depths for regions within Maryland. Find the County that matches ratio's of 2-yr and 100-yr rainfall with your data, and then use that ratio on the 1-yr. This gives a pretty good value for your 1-yr, 24-hr storm event, which will help you document your LEED SSc6.1 point.
This is an old post, but if anyone comes looking for information here like I did, I found a national isopluvial map for the 1-year, 24-hour storm. Go to page 50: http://www.nws.noaa.gov/oh/hdsc/PF_documents/TechnicalPaper_No40.pdf
Multiple Drainage Patterns
Can you tell me how to calculate for multiple draingage sites, i have Three on this particular project. The TC is based on the length and travel and the slope towards the discharge point . Do i take a seperate calc for each then average?
There are two scenarios that depend on the basin layout and interaction and it is not clear from your question if you have multiple discharge points, one from each basin, or if all three basins combine to a single discharge point..
The most simple scenario is that all three basins are separate, with no connection or flow through from basin to basin. In this case, you would analyze each basin separately. If one basin does not quite meet the requirements for LEED, but another basin can more than meet the requirements and make up for the deficient basin, then you may still be able to meet the LEED requirements as you add the results together.
The more complicated analysis is if the basins flow through each other (they are essentially sub-basins) in which case, you may want to analyze the sub-basins and determine if there is the opportunity to reduce the peak flow rate based on timing of the discharge from the separate sub-basins. Of course, the volume will be additive from basin to basin.
In any case, it is not appropriate to average the separate calculations, unless you developed a weighted average.. It is more probable that you will analyze each basin and add the reults together to get the combined result.
Thanks for the feed back, on my scenario. i have a pre development (2yr-24 hour) CFS of .14 and post development i have one CFS of .02 and another of .13. how would i get the average?
Joseph, I thought you had three drainage basins, not two? In any case, I would follow Greg's advice for adding or averaging the numbers.
Which point in time do I take for ascerting imperviousness?
Hello,
following "problem": we have a project which will be built on a site which was an abandoned industrial site up to 2009. Then all buildings and surfaces were demolished and the site lied idle. So which situation do I take for the calculation of the predevelopmentPredevelopment refers to before the LEED project was initiated, but not necessarily before any development or disturbance took place. Predevelopment conditions describe conditions on the date the developer acquired rights to a majority of the buildable land on the project site through purchase or option to purchase. discharge rate? The long-time situation when it was used as an industrial site with a high 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. until around 2009 or the situation we had for the last year, when there were nearly no impervious areas on the site?
Thanks for your advice!
I would take the condition at the start of your LEED project, with no impervious areas.
GIB8 stormwater management credit
Hey there,
I was stuck in understanding a technical phrase -"over a 20- to 40+- period" from LEED ND GIB Credit 8 stormwater management, especially the meaning of minus and plus symbols noted behind 20 and 40.
Original requirement of GIB Credit 8 is: the percentile rainfall envent is the total rainfall on a given day in the record that is greater than or equal to X percent of all rainfall events over a 20- to 40+- period.
Does anyone help me out and give a specific example of -"over a 20- to 40+- period"?
Thanks a million
Is Imperviousness = Runoff coefficient ?
Hello,
Can I presume that the 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. of a site is less than 50% if the overall runoff coefficient is less than 0.5 ?
Apologies if the answer is obvious but I am not an export in this field.
Thanks,
Yes.
It is generally the opinion that site "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." (fraction) = area weighted Run-off coefficient (also fraction) for the site.
Hi.
I have a little doubt of comment from Jean.
See the page 95, on BD+C Reference guide.
Rv(volumetric Runoff Coef.) = 0.05 + 0.009xI
(where, I = percentage of 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.)
Following the equation above,
when "I" is 100, Rv will be 0.95.
So, Rv and I are not equal, but propotional.
I think Jean is right. Taking into acount the equation, you are actually considering the "I" value and doing a weighted average based on surface area. Hope my explanation is clear and correct.
this view is correct for the Credit SS6.1?
hello greetings from mexico, I have a question, in our project, we have an impervious area, less than 50%, according to studies of soil mechanics, water dislodge channels THROUGH made us into a cistern that far exceeds the capacity more severe rainstorm, and then send this into a well the rain that reaches a river, this view is correct for the Credit SS6.1?
Milton, I think you are on the right track, but—and I apologize—the translation of your Spanish is a little hard to follow. Can you try explaining again your approach?
NC 2009- SSc6.1 Storwater Quantity Acceptable Methods
Has anyone ever utilized a bio-retention basin with an underdrain that eventually slowly releases the filtered runoff and discharges to a stormdrain system instead of infiltrating naturally to the subgrade to achieve the requirements of this credit? Our site is a greenfield, so we fall within the less than 50% impervious category. It is located in the southwest and the soil conditions do not allow for adequate infiltration on site; mostly limestone. There are no stream channels that the storm water will directly drain to, so option 2 does not seem feasible. Does anyone have an opinion on if this credit is able to be achieved if all stormwater is not being retained, infiltrated, evapotranspired, or reused on site?
You describe a scenario that is common in the west. We have the same issue in Colorado with our clay soils. Look at my comments, posted on October 11, 2010.
Also, our most successful approaches have been to use the filtered water as a source for irrigation. Of course, this only works if you are using a raw water source for irrigation. If you are using a potable waterPotable water meets or exceeds EPA's drinking water quality standards and is approved for human consumption by the state or local authorities having jurisdiction; it may be supplied from wells or municipal water systems. source, it is difficult to use two sources for irrigation. However, if you want to look hard at this option, it does, potentially, provide you with some credit on the water conservation issue (WEc1). This seems to work best if you are proposing a native landscape pallete that can survive with less water, and the source of the water becomes your stormwater that has been filtered. You may have to supplement the filtered stormwater during longer dry periods, and you may have to over apply water if you have stormwater that you need to dispose of.
The other option for this water is to direct it to a cistern that may also collect gray water from your building and rooftop (depending on your water laws). This gray water/stormwater can be used for toilet flushing or irrigation and the combined sources will allow for a smaller cistern.
Lastly, the filtered stormwater can be applied to raw land and allowed to evaporate or supplement natural rainfall to grow native grasses. This is similar to a land application approach for treated wastewater disposal.
Bottom line, this is not easy with the soil conditions we have, and the best option is to look for a good use for the filtered stormwater, and not just view this as an isolated problem of disposal. I don't have a suggestion for an alternative if you cannot use the water on site or infiltrate or evaporate it.
I have used bioretention facilites with underdrains several times for storm water quantity peak reduction. These projects were in Virginia where there is a lot of clay in the soil which makes infiltration not feasible. Your civil engineer can route the bioretention facility similar to a dry pond with a "grate" (some routing programs have underdrains, but a grate will also work). This will show that the water is exiting at a lower peak flow rate over a longer period of time which is the requirement for <50% impervious area. We typically use a 40% void ratio for the media within the facility when calculating the volume within the facility. It shouldn't be a problem to send this water directly into an existing storm sewer system and is common in urban areas.
If your site had greater than 50% 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., then you would have to find someplace else to use 25% percent of the water.
I am having the same issue as Lucy on one of our current projects, and the comments above were helpful but I still have some questions.
Scenario:
-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%
-new buildng footprint will increase imperviousness
-we will build a new detention pond/swale with an outlet to city storm system
-there will be a restrictor to control the rate of outflow from the pond/swale
-we do not plan to use the water for reuse in the building or landscaping, etc.
1.) I thought that showing the restrictor outflow rate would be enough for the calculations but we got the following comment from the GBCI: "peak rate from a detention pond is typically a function of the outlet structure configuration, outliet pipe size, and the depth of water in the pond. Peak rate does not necessarily decrease at a constant rate with a decrease in runoff volume. Please revise and resubmit calculations for the peak rate leaving the pond..." So, can someone please illustrate how to calculate the peak rate if we have a restrictor (at the outlet from the pond) which allows a maximum flow of 0.5cfs/acre?
2.) The second issue is as follows: if we are catching storm water in the pond, the restrictor will solve the issue of the "rate", but not the problem of the "volume". Does someone know how to overcome this issue?
Thank you in advance!
SSc6.1 Post-development & Pre-development rate
We're trying to show compliance with SSc6.1. On the form they ask for Pre-development and Post-development runoff rates and quantities. The credit requires "Case 2. 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 greater than 50% and a stormwater management plan will be implemented that results in a 25% decrease in the volume of stormwater runoff from the two-year, 24-hour design storm." With"25% decrease in VOLUME" being the key phrase for us.
Our Pre-development Rate is 0.234 CFS and Quantity is 2,022 CF
Our Post-development Rate is 0.2754 CFS and Quantity is 0 CF
Notice that the Rate is higher but we will now have ZERO runoff (Quantity is lower) because all runoff will be/is infiltrated on-site. Since the form is not recognizing this and allowing us the point, should we try and go with:
"Special circumstances preclude documentation of credit compliance with the submittal requirements outlined in this form."
OR
"The project team is using an alternative compliance approach in lieu of standard submittal paths."
Thoughts? Recommendations?
My inital thought is that you are infiltrating the entire volume within your LEED boundary (I assume) and if this is the case, then your run-off flow rate is also zero. You only need to measure the rate and volume as it leaves your site.
Gregory,
Thank you for the response. That was what we decided to do until we heard opinions otherwise. Our Post-development Rate and Quantity are now set to zero. The form allowed this and allowed us the point.
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