I have a few questions on this credit that are buggering me off. Since I'm working on a site with with existing imperviousness higher than 50%, the main objective of this credit is to create appropriate infrastructure in order to reduce the total runoff volume of a 2-yr 24-hr design storm in 25%. The project site is largely dominated by a clay pan, so even though I'm increasing perviousness through the use of permeable pavements and rain gardens, infiltration is not feasible, so I'm retaining filtered water (from pavers and bioswales) and excess water in a wet pond, including excess from cisterns. I have two questions:
1. The cisterns are capable of holding approximately 25% of the design storm, however, they won't be empty the whole time, and therefore won't guarantee its retention throughout the year. Should I apply a given measure (say, on average), and compensate for the excess in the wet pond?
2. How do I account for water retained in planters, gardens and bioswales?
Greetings from Brazil
Michael DeVuono
Regional Stormwater LeaderArcadis North America
LEEDuser Expert
186 thumbs up
September 7, 2012 - 7:32 am
Question 1:We deal with this all the time on capture/reuse projects. Basically you can not store more than you have the ability to use. You need to have sufficient volume in the cistern to capture back to back lower level storms.
The problem I always have with capture and reuse, is not the capture part, it's finding a way to use all the water!
Question 2: Ideally I would have a geotech get infiltration rates for the underlying soil, then convert the flow rate to a constant flow (cfs) applied across the surface area of the BMP. This will allow you to "net out" some of the runoff volume, thus reducing peak rate as well. Additionally, you can take credit for the storage volume in the BMP. The ammended soils and mulch in your BMP should have (on average) a void ratio of about 17% So take 17% of the total depth of the BMP, and net out that volume as well.
Lucie Cervena
Ekowatt CZ13 thumbs up
October 17, 2012 - 8:22 am
Could you please give me an advice regarding question 1?
Our cistern is designed to store the volume that is required in this credit – the predevelopmet runoff is 90 m3, the post development runoff is 130 m3 and the difference 40 m3 we would like to store in the cistern (60 m3). The captured rainwater is reused for flushing – the need of water for flushing is 4 m3 per day that means, we could empty the cistern in 10 days after design storm. I think this assumption is not sufficient in order to meet the credit requirements.
We must declare that we reuse all the captured rainwater as states upwards. Is it sufficient to calculate annual balance of rainwater that is captured by the cistern and the amount of water that is reused for flushing in monthly steps? Which time step do you recommend?
Another question: which part of the monthly rainwater volume I must reuse for flushing to meet the requirement of SS6.1? I think it is not necessary to reuse all the rainwater because the credit requires retaining the predevelopment conditions (90 m3 during design storm). Is it for example possible to reuse 30% of the monthly rainwater volume to meet the content of this credit (30 % results from the design storm – we must capture 30 % of rainwater = 40m3/130 m3 to meet the requirement).
Could you describe briefly how it is possible to declare sufficient volume of the cistern and water for reuse?
Thanks a lot!