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Calculating Stormwater Run-Off From Hardstanding Areas

Home » Calculating Stormwater Run-Off From Hardstanding Areas

Surface water pumping stations are designed to take storm water run-off from impermeable areas such as car parks, loading bay, drive ways and roof tops. In its simplest form, the discharge flow rate from the pump must exceed the inflow rate in storm conditions in order to prevent flooding.
To work this out the most common method is the Wallingford Procedure which is detailed below:
QP = 2.76 x Ap x i
Where:
QP = Peak discharge (litres/sec)
Ap = Impermeable Area (hectares)
I = Rainfall intensity (mm/hr)
For example, a surface water pumping station draining stormwater from an impermeable area of 1.89 hectares would require the following calculation:
QP = 2.78 x 0.4 x 50
QP = 55.6 litres / sec
50mm/hr is used as a standard rainfall intensity as part of building regulation’s part H and is sometimes referred to as a 1-in-100-year event. This is a particularly severe storm and as a rule of thumb is used as a worse case scenario for designing storm water drainage systems.
At this point the QP can be reduced by 20% to account for infiltration and loss of water by other means prior to the pumping station giving us a result of 44.5 litres/sec as our final discharge flow rate.
With an inflow rate of 44.5 litres/sec the pumping station needs to be capable of discharging a minimum of 48 litres/sec to reliably prevent the surcharging of the system and potential flooding.

If you’re looking for more information or even some assistance with your pumping station requirements, please feel free to contact our engineering team on 0113 2860813 for free advice.