Case 1, Option 1

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

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....

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......

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.