Winchester flooding 2020: Major report reveals causes

Around 70 properties, both residential and commercial, were impacted when surface water flooding hit parts of the city centre on August 27, last year.

Hampshire County Council, as the lead local flood authority, have released the findings of a significant investigation to determine the causes of the flooding and how a similar incident can be prevented in the future. The authority has to carry out a formal investigation under Section 19 of the Flood and Water Management Act 2010.

The report reveals that some of the drains in the affected areas were not cleared and, in some cases, the county council, as the highways authority, did not know where some of its flood assets were. Recommendations have now been put forward to mitigate the causes or impact of any future flooding.

Flooding in Winchester. Picture: Isabella Evans

Summarising the cause, the report said: “The source of this flooding was surface water run-off during an intense rainfall event, as a result the highway drainage systems could not reasonably be expected to manage surface water flows associated with this event. Furthermore, the rapid drain-down following the flooding suggests that these drainage systems were functioning and that these systems suffered a hydraulic overload due to the volume of water rather than from a blockage.

“This run-off was rapidly conveyed as high velocity sheet flow on the public highway, which was exacerbated due to the steep topography and high degree of urbanisation and impermeable surfaces in the city. Surface water flowed through and pooled in low spots such as the Broadway and Cranworth Road, where the depth of water was sufficient to cause internal property flooding.”

JBA Consulting, environmental consultants, which carried out the investigation, found months after the flooding some of the drains in the affected areas “appeared to be blocked by silt”, while during the flooding gullies may have become blocked from falling debris or materials being washed into them, including soil, waste and leaves.

It said: “More frequent cleansing in specific areas such as the High Street could improve their performance and reduce residual flood risk. A recommendation for increased gully maintenance and clearance in priority areas could help mitigate surface water flood risk to properties, by reducing the amount of water pooling on roads.”

It also discovered that HCC was not aware of the presence of some of its flood assets, which “could lead to potential issues with lack of maintenance exacerbating or causing flooding issues if left unaddressed”.

Flooding in Cranworth Road, Winchester

The council has been recommended to undertake additional mapping of highway drainage assets to ensure that all locations are known; in places install sustainable urban drainage systems (SUDS) or tree pits, rain gardens or permeable paving to tackle the problem.

The report also recommends adding extra protection directly to some houses and businesses and, potentially, a surface water forecasting system for Winchester.

But the report has drawn criticism from the Liberal Democrat spokesperson for economy, transport and the environment Cllr Martin Tod.

Cllr Tod, who represents Winchester Westgate which includes part of the affected area, said: “Climate change means we’re going to have more and more of these floods and this report shows the county council just isn’t ready.

“The floods last year were devastating for local residents and businesses. But, at the time of the flood, the county hadn’t been keeping drains and gullies clear in high-risk areas. In some cases, it didn’t even know where their soakaways were.

“And it’s still the case. I went round and checked last week – and many of the drains highlighted in the report were completely blocked. Even after reporting them and highlighting that they’re in a high-risk area, they’ve still not been cleared.

“There’s got to be a change. The report sets out measures that can be taken to better protect homes and businesses. We need to see a plan to see them funded and implemented.

“The county council must also stop being so complacent. The report says that the storm event was between a 1 in 29 and a 1 in 42-year event – but almost exactly 11 months later, on July 24 – almost all the houses and shops referred to in the report were flooded again.

“This wouldn’t matter if that assumption didn’t have consequences, but it uses that figure to make the case that the sewer network is adequate.

“It’s time for the council to raise its game. Its lack of urgency in tackling the climate emergency is already a huge problem. This report makes clear that its lack of urgency in responding to the consequences of climate change is a huge problem too.”

Councillor Rob Humby, deputy leader of Hampshire County Council and executive lead member for economy, transport and environment, said: “The rainfall event at the end of August in Winchester last year was exceptional with 35mm falling, half of the average August rainfall, in 15 minutes. As we saw then and more recently last month, with rainfall of this intensity, traditional highway drainage systems and household rainwater systems are quickly overwhelmed and the topography of the centre of Winchester meant that large volumes of water ran off the paved surfaces at tremendous rates.

“The County Council is committed to doing all it can to reduce the risk of flooding for communities across Hampshire. For example, following the flooding in 2014, we worked with the City Council and the Environment Agency to implement measures to reduce the risk of flooding to homes and businesses in the centre of Winchester. And in 2010, we worked with Southern Water to reline and replace drainage pipes in the High Street before repaving the pedestrianised area.

“After last summer’s flooding event in Winchester, as promised, we have undertaken a full investigation. The report that has now been published sets out the causes of the event and identifies a range of measures which may help in reducing the impact of the flooding in future. The report confirms that the highways drainage systems in Winchester were functioning properly but were overloaded due to the sheer volume of rainfall in a very short space of time.

“We will continue to do all that we can to ensure existing drainage systems remain as effective as possible, however, there may be further occasions where these will not be able to cope with severe rainfall intensities on this scale. Sadly, we are likely to see more of this with the impacts of climate change and, while we will bring forward improvements where we can, providing further drainage capacity across all of our drainage assets in all of our urban areas is not practical either from a feasibility or cost perspective. As some have already done, individual property owners are advised to take appropriate steps to understand the risk of flooding and protect themselves. We have a list of resources available for residents seeking advice on our website and we will work with them, the Environment Agency, City Council and the water company to identify improvements wherever possible both in terms of new drainage infrastructure and policy.”