Not long ago, WBC's planning honchos approved building a supermarket in Winnersh on a site a few hundred yards from the River Loddon, near the Showcase cinema. Now they’ve been told by the Environmental Agency not to approve it, because the long-term risk of flooding there was higher than WBC had previously calculated for.
The Winnersh location is within the flood risk area mapped out in the council’s own Strategic Flood Risk Assessment document, accompanying the 2021 Local Plan draft. It’s also inside the flood risk area shown on the government’s ‘check-for-flooding’ website.
This might look like just typical bungling by council officials, but in fact all over the world public authorities have been waking up to the fact that their previous estimates of liability to flooding need to be corrected. (Perhaps it takes WBC just that little bit longer…)
Council planners have long been advised by government to consider residential development as having a minimum lifetime of a 100 years. Within that time-span, government guidance lays down that flood risk needs to be reckoned with, framed in terms of flood risk zones. Zone 1 has the lowest risk, with a less than 0.1% annual probability of flooding. Zone 2 means medium risk, with a 1% chance of river flooding. Zone 3 has a 1% or higher chance.
This aerial view of the neighbourhood (from Google) shows the rough boundaries of Flood Zone 2 superimposed in blue. Superimposed in red is the rough outline of the site at Hall Farm where the 2021 LPU proposes to build houses, claiming there is no material risk of flood to those properties. However, this may surely need reconsidering, as a good part of the development site is within Zone 2. That zone is where the council must envisage a one-in-100 year flood event. Let’s take a closer look at what that means, and why the Hall Farm housing proposal now looks vulnerable.
What is a one-in-100 flood event?
A one-in 100-flood event doesn’t mean there’s a 1% chance of it happening for 100 years. It means that there’s a 1% chance of it occurring in any and every given year. This isn’t always appreciated by local authorities. Following severe floods in Cumbria in December 2015 the Mayor of Keswick was among local figures saying their flood defences were “designed for a one-in-100-year event - and since it's six years since we had the last one, we are sort of surprised”.
Not so many people would be surprised now, as repeat flooding is undoubtedly becoming more common. In 2007 parts of South West London were under 2 feet of water, and the London Underground was severely disrupted. In July 2021, the Underground flooded again, just 14 years later. Closer to home, a house in Loddon Drive, Wargrave, was built in 1951 with the ground floor almost a metre off the ground, higher than the river had ever risen before. In 2003 the house was flooded when the river burst its banks, then in 2014 it flooded again, as reported in the Henley Standard.
How does this past experience relate to Hall Farm, and WBC’s keenly expected local plan update? Here’s the view of a well-known local figure, as reported in Wokingham Today, 21 February 2022: ‘ “If you add the impact of new development to the climate change risks, we have a flooding catastrophe. There has been flooding in many of the sites considered in the local plan update.” He believes that any building at Hall Farm will have an impact on the River Loddon downstream, in the borough’s northern parishes of Wargrave and Remenham. “If it’s over-full, experts have said that it won’t go into the Thames, but just burst its banks,” he said. “Northern areas in the borough will be more susceptible to flooding as a result of Hall Farm.” '
Who said this? Stephen Conway, now leader of Wokingham Borough Council.
Arborfield Zone 2 floods
Our Council certainly can’t be accused of ignorance of what a 100-year flood event means. Arborfield experienced river flooding in February 1990 and February 1991 just a year later. These flood events ‘both extended into most of Flood Zone 2’, according to WBC’s own Strategic Flood Risk Assessment document of January 2020. That’s the zone overlapping with the Hall Farm estate, where the University and its ‘partner’ the Borough Council want to build houses. So they must know some of those houses would have been underwater in two successive years, had they been built then.
Below: 1970s flooding near Hall Farm (images from Reading Chronicle)
‘100-year’ floods are being reclassified
In any case, to talk of a ‘100 year event’ using historical data assumes there’s been no significant change to the underlying conditions affecting flooding. But rainfall and streamflow are changing in many parts of the world, and that’s driving the risk of flooding higher. Greater rainfall frequency values in parts of the USA have led the authorities there to redefine the amount of rainfall it takes to qualify as a 100-year event. Previously classified 100-year flood events have now been reclassified as 25-year events.
Nor is this a brand-new discovery. A 2011 US study found that average streamflows and flooding in many areas had increased greatly, two- to three-fold per decade since the mid-20th century and suggested the changes resulted primarily from urbanization. More recently, larger cities such as Austin saw ‘100-year’ rainfall amounts for 24 hours went from 3 inches up to 13 inches, and Houston, where they rose from 13 inches to 18 inches. Likewise in Canada: it’s been reported that most of the worst floods over the past century in Manitoba, around Lake Winnipeg, have taken place over the past 25 years.
The Met Office in this country has similarly been warning of heightened flood risk. It points out that from when their observational record began in 1862, ‘six of the ten wettest years across the UK have occurred since 1998’. It forecasts that ‘Rainfall from an event that typically occurs once every 2 years in summer is expected to increase by around 25%. This will impact on the frequency and severity of surface water flooding.’ We have been warned.
Below: Recent flooding in Winnersh and Arborfield (images Wokingham Today)
New guidance on flood risk assessment
No doubt responding to these heightened concerns, the government published new guidance for local authorities on how to take account of more frequent extreme weather events in future. Their May 2022 provisions include percentage allowances for increases in the likelihood of a 1% flood event. They also require any building development to be ‘safe from surface water flooding’ - increased flood risk can result not only from extreme rainfall, but also from land clearing. When the soil is concreted over across a wide area, surface water can much less easily drain away.
How far WBC's local plan, published in November 2021, was able to take account of the government measures introduced in May 2022, I don’t know.
One thing is clear, though, from the Environmental Agency’s recent intervention in WBC’s handling of the Winnersh supermarket planning application: This government agency does not believe WBC is correctly applying flood risk assessment procedures.
In that case, how confident can we be that WBC is correctly applying the EA procedures when it comes to flood risk at Hall Farm?
Pat Phillipps
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