A detailed Flood Risk Assessment and Surface Water Drainage/SUDS Strategy was prepared for my site at Outline Planning stage. As it was done before I purchased the site and poor drainage/flooding is the number 1 bugbear of the neighbours, I have been through it and The SUDS Manual (968 pages !!) with a fine tooth comb. As you say, the jargon and the methodology is very off-putting for the lay person.
A couple of extracts from my report summarise the first steps (as well as the jargon) required:
"In order to quantify the existing runoff rate from the site, the methodology outlined within the Institute of Hydrology Report Number 124 (IoH 124) entitled Flood Estimation for Small Catchments, has been adopted. This document together with the guidance stipulated in the Interim Code of Practice for Sustainable Drainage Systems, compiled by the National SUDS Working Group in July 2004, suggests that an estimation of peak runoff rates from areas below 50 ha, and up to 200 ha, can be derived from the calculated mean annual flood flow, QBAR."
"The ICPSUDS function within the Microdrainage software Version 2016.1 can be used which implements IoH 124 method with a pro-rata below 50 ha. The SAAR value of 623mm has been determined from the catchment descriptors taken from the FEH CDROM Version 3. The soil value has been determined using the information from the Winter Rain Acceptance Potential (WRAP) map within the Flood Studies Report, 1975, together with Table 6 and equation 12 of the ADAS document entitled Pipe Size Design for Field Drainage, 1980."
Thankfully Microdrainage is not required and the online tools at Online tools for sustainable drainage systems (SuDS) (uksuds.com) will allow you to calculate mean annual flood flow (QBAR) for the existing site. The methodology is based on the premise that the flow rate discharge constraints for storm water runoff from the site are defined by the greenfield runoff rates for the 1 year, 30 year and 100 year return periods. The tool is flexible in allowing the use of three different methods for estimating greenfield runoff rates along with various options for assessment of storage volumes using variations in the design criteria (e.g. minimum design flow rates from the site, requirements for control of runoff volumes for the 1:100yr, 6 hour event).
Note that in Microdrainage a range of storm durations and intensities are modelled (i.e. 15 - 10080 minute for both summer and winter). For my site, the 1440 minute summer storm was the worst case used for the permeable paving design.
Note also that although SUDS attenuation/infiltration devices are usually designed for 1 in 100 year events, the design is checked against an exceedance case which is usually the 1 in 1000 year storm events.
Let us know how you get on ?
