Tuesday 23 November 2021

The River Westbourne flood defences, the tale of two boroughs

 An update post by David Walton of FLASK


Brent used to have more River Westbourne flood defences but still has some, publicly owned natural parkland flood defences throughout South Kilburn Vale, that were built in the 1950's and 1960's. These flood defences have been incrementally built on since 2000 and the impacts are already being felt.  The new intention is to establish this as a tall building zone as set out in the Brent Local Plan to 2041 which awaits final approval.  Population growth is planned to rise from 6,000 in 2000 to 36,000 by 2041. Brent has no plan to mitigate growing flood risk which is exacerbated yet further by excavating giant underground car parks. A mainline electrified railway luckily severs South Kilburn Vale from the rest of Brent.


For its River Westbourne flood defences, the City of Westminster uses complex and expensively engineered solutions built inside its borough boundary, but it also ( cf July 2021 major Incident) clearly relies on Brent playing its full part in the  flood defence of the City of Westminster upstream of the River Westbourne.


Westminster has the Carlton Hill natural hill (pending new developments area) which drains down onto the Brent floodplain vale, with Kilburn Park Road on the east bank of the River Westbourne (Westminster) relying on Brent's depleting natural parkland flood defences for safety. Then at the main borough boundary at Shirland Road, Westminster engineered flood defences start and which though of considerable scale failed in July 2021 and will with certainty fail again unless Westminster and agencies look at the bigger River Westbourne flood attenuation cross borough boundary picture. (See key Kilburn Park Road flood defences already removed like the 40 veteran trees roundabout flood defence or the Granville Road park flood defence three-quarters removed).


New map fragments recently obtained from Thames Water show how the culvert straightened high speed River Westbourne takes a dramatic giant sweeping curve from Kilburn Park Road into Shirland Road, and at this point (underneath the zebra crossing) also connects to the North West Storm Relief Sewer which heads west down to the River Thames at Hammersmith, while the Mid Level 2 Interceptor Sewer which heads east to Beckon Sewage Works connects to the River Westbourne nearby at the south east end of Shirland Road. Flood protection support is also supplied by two new large flood storage reservoirs underneath Tiverton Gardens and Westbourne Green. Both are rivers connected and were built in 2016 at a cost of £22 million. To quote from this new project’s 2012 description:


"The Sewer Flooding History Database (SFHD) lists 105 properties that have a flooding category of either AI or BI; however, it is known that the flooding issues affect many more properties in the area. Optimise (the contractor) are targeted with removing 177 properties from the SFHD flooding register and contracted to remove a minimum of 147 properties.


Primarily, the identified flooding areas are located around Formosa Street and Shirland Road. Prior to 2005 the problem was much smaller with far fewer properties affected; however there have been severe flooding events in 2005, 2007, 2008 and 2009. In both areas the flooding occurs incommercial and residential property basements.


Having considered a number of options, the preferred option proposed by Optimise is to construct a 20m dia, 20m deep storage shaft in Westbourne Green. From this a 3m dia tunnel will be driven to a 7.5m dia reception shaft in Formosa Street. In conjunction with managing flows at the Kings Scholar’s Pond and at a number of bifurcations in the Formosa Street area this will effectively resolve the flooding issues at Formosa Street. Flows from the shaft in Westbourne Green will be returned to the Ranelagh sewer (River Westbourne) by means of a pumping station with a return pump rate of 400 l/s. The Shirland Road flooding will be resolved by diverting more flow to the Mid-Level 2 sewer and constructing a 20m dia, 20m deep storage shaft in Chippenham Gardens.


In order to remove properties from the SFHD it has also to be proven thatthe properties flood due sewer surcharge / local incapacity. This information was collated through existing databases already connected to the properties, and via interviews with current residents in the area. There was an initial reluctance to complete the survey by residents and this was for a number of reasons, including many residents were not living at the properties at the time of the flooding events and property owners do not want their property on a flooding register.  As such, the verified model has and will continue to be used to validate the number of properties that suffer from flooding".


The sheer scale of the City of Westminster's engineered flood defences that are place and   being rapidly extended  indicate that the wild River Westbourne is a major environmental risk to lives and property for this entire area of London. Yet this river is deregulated from Environment Agency responsibility and often commercially driven boroughs so Thames Water must work out what to do in an ad hoc and uncoordinated way instead. 


The City of Westminster does seem at least to be trying seriously to take mitigating actions to protect its own residents and businesses on a borough boundary frontline siege basis, but these actions have clearly failed to accept this area’s wider geography and factor in the housing infrastructure in Brent’s urban growth zone.  Brent seems to think that leaseholders and tenants in Brent and City of Westminster should 'learn to live with' traumatic flood risk escalation  and then pay the costs created by its tall buildings growth area, built on a flood plain.


Liability is being cleverly being passed entirely to leaseholders and tenants for the moment, as this area’s big freeholder housing block owners will just make sure that flood repairs are actioned in a timely manner and that costs are then fully recovered from block leaseholders and tenants. They will be  paying literally forever for the extreme over development of this floodplain. This, when natural parkland flood defences (that Brent is destroying) had proved excellent in protecting South Kilburn and North Westminster for decades.



David Walton

FLASK (Flood Local Action South Kilburn)

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