Unlike traditional piped systems which collect and convey polluted runoff indiscriminately through a piped drainage system, with SuDS the first part of any rainfall event (or the whole volume in the case of permeable paving) is isolated ‘at source’ to be cleaned and released slowly to the management train. The longer this ‘first flush’ volume can be retained and treated, the better the overall quality of runoff.

This first flush volume (normally considered to be the first 10-15mm of runoff from a discreet sub-catchment) carries the most surface pollution and so the longer it’s retained the better. A flow control is required to hold back this volume to enable primary cleaning and encourage silt collection in dedicated ‘source control’ features. But some flow controls do not allow this first flush process to occur, quickly discharging polluted and silt-laden water down the system.

Controflow flow controls are all designed with small orifices to retain this first flush volume. Then, with heavier rainfall events, clean runoff can be accommodated by integral overflows within the chamber. Cleaned water from ‘source control’ features and larger volumes from larger rainfall events will travel along the management train to an appropriate outfall. Each of these SuDS features requires a flow control to maximise storage along the management train to encourage infiltration, continue the cleaning process and reduce the size and depth of storage structures towards the end of the system.