Kennet storm overflow

Storm overflow schemes are being delivered across the industry against fixed regulatory deadlines, which changes the nature of the environmental work: the technical difficulty is moderate and the programme risk is severe. A permit variation that takes twenty weeks instead of fourteen does not delay a report, it delays a construction start that has a statutory date attached to it.

We ran the Water Framework Directive assessment and the permit variation in parallel rather than in sequence, which meant preparing the WFD screening on a design that was still moving. That is uncomfortable and it is the only way the programme worked. Two design changes landed mid-assessment; the second one changed the outfall position by 40 metres and we reissued the screening in nine days.

The modelled outcome: spill frequency at the overflow drops from 19 events a year to 3, against a permit condition of 10. We reported the 3 with the caveat it deserves — it is a modelled figure on a rainfall series, and the first genuinely wet winter will be the test of it.

The scheme is in construction. We are doing the discharge monitoring during commissioning, which we asked for, because a consultant who never sees whether the prediction held is a consultant who never learns anything.