Drainage design and performance standards


A range of stakeholders are responsible not only for different drainage systems but also for different levels of performance of the same system. There are numerous design guides that cover drainage design and recommend appropriate standards. These standards often overlap and this can cause considerable confusion with stakeholders. CIRIA’s C635 publication on Drainage Exceedance gives more detail.


At present there are no guidelines on the return period of event (extreme event) that should be used for designing for exceedance. It is suggested that return periods of one in 30 to one in 100 or one in 200 year events would form a suitable framework for most applications. Where health and safety issues are important it could be argued that the concept of “any conceivable event” inherent in the procedures set out in the Reservoirs Act 1975 might be applicable. For this purpose the 1000 year event may be suitable. Further guidance on design criteria is given in CIRIA's C635 publication.


Existing design guidance


Although designing for exceedance as a defined approach has been around since 2006, it is still a relatively new concept for many and its uptake has so far been limited. Increasing practitioner and decision makers’ (eg between drainage and highways) confidence in applying the approach is important.


Clear guidance on the principles and approaches is already available in CIRIA C635. Many drainage and planning documents reference the guide (although its contents may need to be refreshed).


CIRIA outputs C738 Managing urban flooding from heavy rainfall – encouraging the uptake of designing for exceedance provides an update to CIRIA C635 and further strengthens the approach by providing further case studies (C738c), a summary of lessons and success factors (C738b) and a recommendations and summary report (C738a)


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