Flow4 Storm

Event Monitoring


In response to the Environment Agency’s (EA) Flow4 driver, CFS worked with three of Severn Trent’s (ST) ‘tier one’ contractors to scope 40 sites identified by the EA and ST as ‘at risk sites’.

Scoping required our specialist knowledge to: confirm the site process, identify the correct storm spill point and calculate the accuracy of the storm control gauging structure (i.e. the inlet flume). In addition, we needed to: confirm duct routes, power supply and spare capacity on the telemetry outstation.


Working closely with the EA and the stakeholders, we were able to develop a number of innovative solutions where the storm control was not straight forward.

At Stoke Bardolph ST (Nottingham) the combined uncertainty of eight inlet flumes was above the 8% uncertainty target figure required and alternative solutions were investigated.

Area velocity flow meters were installed upstream from the storm weirs, this accurately represented the flow to treatment up to the storm spill point. Beyond that point, an element of flow passed to the storm tanks. However, the criteria of the Flow4 driver is only to record the flow rate at the point of spill.


This solution prevented a potentially awkward non-compliance issue or the cost of a possible re-build of the inlet to one of the largest sewage works in the country.

Additionally CFS were able to help develop the arrangements for ongoing compliance and highlight areas which had not been previously considered.

All of the Event Duration Monitors that we installed were fitted with a datum reference system, so the meters can be calibrated by Severn Trent Instrument Technicians.