What’s really interesting about this hydrograph is that it has two separate and unique peaks. One at the expected 5 minute point (because the design storm had a time of concentration of 5 minutes) and another at the 15 minutes mark! This is largely in part because the hydrograph represents the inflow to the detention vault, for which there are two separate inflow points. Now, if you’re wondering why two separate points coming from the same site would cause this, i can explain. You see, one inflow source is from sheet flow of the proposed site, which drain directly into the storm sewer system. But the second is from a bioretention filter, and this is where it gets really interesting. If there was a hydrograph for in the inflow into the bioretention filter, it would have a single peak, but the outflow mechanism is what causes the dual peak of the hydrograph shown. The bioretention filter retains the first flush (first half inch of runoff) and therefore has no discharge until the first half inch of runoff occupies the storage of the filter, but once this storage is occupied, the inflow is discharged immediately, creating the second peak. And for those of you keen enough (Micah) to realize the constant flow of 0.1 cfs, I’m sure you’re wondering where that’s coming from. Well, while the bioretention filter “retains the water” in actuality it only detains, because the percolation through the soil media allows for a constant 0.1 cfs to discharge into the detention vault!

Good stuff. Enyoy.