Guest Post by David Walton
Previous posts have discussed the River Westbourne and its tributary the Malvern, which together form the borough boundary between City of Westminster and Brent in Kilburn and which have a tendency to flood impact lives and homes both sides of this administrative, social and political divide.
City of Westminster's Finance, Smart City and City Management Policy Scrutiny Committee 30th September 2021 published its Lead Local Flood Authority Section 19 Interim Flood Investigation Report findings. LINK This is based on what Westminster knows and disclaims any error in or omission from this report.
The nearest rainfall gauge is at Putney Heath 8.5 km away and so it proved impossible to establish exactly how heavy the rainfall was on 12th July, but it is estimated by Thames Water that more than the entire month's average rainfall fell in 1 hour*. Local sewers/rivers in this area were unable to cope. In Westminster approximately 230 properties were internal flood impacted with Kilburn Park Road (shared with Brent) needing to be evacuated. Underground lines closed due to flooding as was one primary school, three libraries and three community centres.
Thames Water reported that the rainfall on 12th July was equivalent to a 1 in 300 year event. The second flooding on 25th July in this same cross borough boundary area (this time a month's rainfall in 2 hours and another 1 in 300 year's event) is now subject to a second separate Section 19 Flood Investigation by the City of Westminster. The report states that the Environment Agency is not the responsible risk management authority for the 12th July flood event.
The River Westbourne is report named as a combined sewer and it is described how in heavy rain it sewage discharges into the River Thames. The Kilburn sewer rivers were likely already at full capacity when heavy rains* started and tried to enter the network. The £22 million Maida Vale Flood Alleviation Defence is meant to deal only with a 1 in 30 year storm event/ not a 1 in 300 year one? The report states that the entire cross borough boundary flood area is Flood Zone 1 having an annual flooding probability of 0.1% from fluvial and tidal sources. There is however High Surface Water Flood Risk throughout this floods prone area.
Westminster is in 2021 trialling gully sensors to give real time information on silt level build-up in gullies, the aim being to clean gullies before surface water events. In the flood area these gullies were cleaned within 3 months of the event and were in effective condition for July 12th event. (For the 25th event many would have likely had blockages?)
Westminster Lead local Flood Authority interim recommendations are for
a) an increase in reliable rain gauges on site b) The LLFA to keep better records of past flood events for future reference c) to further optimise drainage maintenance d) wider catchment considerations (South Kilburn destroying all of its natural parkland flood defences) will be investigated by flood responsible agencies where the Regional Flood and Coastal Committee may be involved. e) Thames Water is organising meetings with LLFA's and its own Independent Review and these findings will inform the final S19 report. This process could take 6 months!
Thames Water reported that the July 12th rainfall was more than the drainage "network was ever designed to cope with. To the best of our knowledge, our sewers and pumping stations performed as designed….." No system blockages were found either. The Northwest storm relief sewer was overloaded and a River Thames high tide prevented overflow flaps from opening.
"Where there is uncertainty over the cause of the incident and therefore responsibility for it, Thames Water needs to take action to support customers without concerns that in taking action we are wrongly accepting liability."
An Independent Flood Review has been commissioned by Thames Water, in a time scale which can meaningfully inform Thames Waters and other parties' plans (such as South Kilburns natural park flood defences all being destroyed upstream). Thames Water also now supports the Mayors 'Grow Back Greener' scheme to make sure that every Londoner lives within 10 minutes of a green space. So, extra funding for South Kilburn Public Open Space and Chippenham Gardens natural flood defence parks major upgrade works?
Westminster LLFA is attempting to be open about its struggles with its rivers and vales environment and climate change in this London boundary area and I would now expect Brent to follow this lead and to also seriously engage the issues to proactively protect Kilburn major Town lives and properties from escalating flood risk.
David Walton, FLASK (Flood Local Action South Kilburn
Thames Water Interim Report on July 12th events LINK
London Fire Brigade Briefing on 'Major Incident' July 12th LINK