Do you use discharge multiplier if storage has mutliple items?

[Michael R. Martin]

I am modeling a subdivision with rain gardens to treat roof runoff. I combined the roof surface area from multiple lots (let us say five). So in my pond (rain garden), I enter five separate rain gardens for storage, each which will infiltrate and have a weir overflow. On the outlet tab, I have exfiltration (wetted perimeter) and sharp-crested rectangular weir. If I do not use the discharge multiplier for the rectangular weir I get run errors that things have WAY overtopped. So, in this situation with five identical storage lines (rain gardens) do I use the discharge multiplier and set it to 5? It fixes the run error but not sure if this is correct modeling.

Also of note, the infiltration seems to be fine but maybe that should be set to Discharge Multiplier of 5, too?

 

[psmart]

If you want to simulate 5 weirs then you need to set the weir discharge multiplier to 5. The exfiltration is a bit more complicated. If you're using the constant flow option, then you do need to set the multiplier to 5. But if you're using a constant velocity, the exfiltration is automatically applied to the TOTAL storage area, which you've already increased 5x, so the exfiltration multiplier should stay at the default of 1, otherwise you'll be making a double adjustment.


Peter Smart
HydroCAD Software

http://www.hydrocad.net