Calculating Runoff Rates

Calculating runoff rates should be completed using the most conservative rates in order to prevent an increase in flooding offsite.

What the LLFA expect to see 
For greenfield runoff rates we would expect to see the calculations used, including the inputs and outputs. For brownfield runoff rates, within outline applications the modified rational method or the urbanisation method found in the ReFH2 software are acceptable. The calculations themselves along with their inputs should be provided. However, at a more detailed design stage more site-specific calculations should be provided and modelled to show what influence factors such as the current pipe sizes have.

Runoff rates should be calculated based on the full development site area subtracting any areas of large open space that will not be draining via the proposed SuDS. Unrestricted rates will only be allowed where the outfall is to a tidal estuary.

The LLFA preference is that greenfield runoff rates should be calculated using the following rainfall data: FSR and FEH, and should be calculated using the IH124 method. Alternatively, the ICP SuDS method can be used which is the IH124 with sites below 50ha having a pro rata applied. These methods should be adjusted as appropriate to site conditions. Calculations of greenfield rates can use a range of calculation methods, as rates and storage requirements will vary depending on different storm criteria it should be demonstrated during the later stages (reserved matters or discharge of conditions) of the planning process that the method selected is the most conservative of the options available. For brownfield sites, the Modified Rational Method can be used to calculate the peak brownfield rate for the 1 in 1-year storm event, 1 in 30-year storm event, and the 1 in 100-year storm event. The inputs for this calculation should be based upon the inputs recommended by The Wallingford Procedure - for design and analysis of urban storm drainage and BS8005 - Sewerage. Guide to new sewerage construction. Alternatively, runoff from a brownfield site can be estimated using the urbanisation methods within the ReFH2 software. When calculating the brownfield runoff rate, surveying and modelling should be undertaken to confirm how the site currently drains. For example, if the brownfield site is currently drained by a 225mm pipe the brownfield runoff rates should take account of the limits this poses.

Page updated: 17/02/2020

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