Use 5 Year Rational Method or 5 Year Modelled Design Storm for Subdivision SWM Pond Forebay Dispersion Length Calculation?

Hey all,

Looking for a sanity check here on calculating SWM Pond forebay dispersion length. I'm based in Ontario and per our provincial guidelines the dispersion length is calculated as follows:

Dist = (8*Q)/(d*Vf)

Where

Dist = length of dispersion

Q = inlet flowrate (pipe design capacity - in this case the municipality requires pipes to convey the 5 year storm)

d = depth of permanent pool in forebay

Vf = desired velocity in the forebay (recommended 0.5 m/s)

My question is - how are you determining your Q (inlet flow rate), using Rational Method from a storm sewer design sheet, or from modelling output (e.g., a modelled 5 year Chicago storm)?. Assuming this forebay is servicing a large 30 ha subdivision.