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.
-existing imperviousness 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!