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19
Sept
Upgrade of Knaresborough drains after devastating flood ‘not realistic’, investigation finds
An upgrade to Knaresborough’s drainage network following a devastating flood in the town last year would “not be realistic”, a review has found.
More than 50 homes and businesses were affected by flooding on May 6 last year, which led to some people being forced to find emergency accommodation.
An intense storm hit the town which saw 54mm of rain fall within 35 minutes.
The sheer volume of rainfall and its impact on the community prompted questions about whether better maintained drains would have helped. But the council said the flooding was "not related to condition of drains".
Now, a North Yorkshire Council investigation has made a series of recommendations following the floods.
‘Struggled to cope’
The council report, which was authored by Finlay Kell, flood risk engineer at the council, and Meirion Jones, lead local flood authority team leader, found that more than 50 properties and business were affected by the flood.
Homes on Park Avenue, Orchard Close and Halfpenny Lane were among the areas affected by the storm. St Margaret’s Gardens, Queen’s Road, Dentdale Drive Lundale Avenue and Cotterdale Close were also hit by the flooding.
The report, which is due to go before a Harrogate and Knaresborough area committee next week, highlighted “key areas at high risk of surface water flooding”.
The view over High Bridge outside Mother Shipton's during last year's flooding in Knaresborough.
In its findings, the investigation found that 54mm of rainfall fell within a 35 minute period — exceeding the average May monthly rainfall total of 43mm in half an hour.
As a result, the drainage network in Knaresborough could not cope with the volume of rainfall.
The report said:
In the context of current drainage design standards, drainage is designed for no above ground flooding in a 1 in 30 year event and no flooding to properties in the 1 in 100 year event. The rainfall intensity therefore far exceeded current design standards for drainage systems.
The event exceeded all current reasonable worst case scenarios and was not something that could be effectively forecasted.
Blocked drains not to blame
The report added that the highways gullies “struggled to cope” with the significant amount of surface water as they were not “designed to take this volume of water”.
It was “not an indication” that additional cleansing was required, the review said.
The investigation found that a “small number” of gullies were found to have been blocked for a significant period of time — but these were “isolated cases”.
The council’s highways team responded to 422 gullies in the immediate aftermath of the storm and found 43 required follow up investigation.
Some were found to be “blinded” — meaning debris was blocking surface water entering the system — but these were cleared, the report said. These were found in flooded areas on Blind Lane, near Orchard Close.
The review found that concerns over “non-operational” gullies in the town "cannot be ruled as a clear reason for the flooding or that the flooding was amplified”.
It said:
Given the drainage networks were already beyond capacity and were not allowing any further surface water to enter their systems, the apportionment of blame on the non-operational gullies is likely to be less than expected and cannot be ruled as a clear reason for the flooding or that the flooding was amplified due to these non-functional gullies, in such an extreme event.
Had the event been of a lesser magnitude and closer to the design standard of the local drainage networks then a more in-depth investigation as to the impact of the non-functional gullies would be entirely justified.
‘Not realistic’
Mr Kell and Ms Jones said in their report that any changes to Knaresborough’s drainage system so that it has sufficient capacity to cope with storms such as that in May 2024 would require changes in national policy and legislation.
They added that it would also require “huge levels of investment” and “major on-going disruption” while the work was undertaken.
The report said:
It is not realistic at this time to expect drainage infrastructure to be upgraded to a capacity sufficient to cope with this level of event.
Instead, Mr Kell and Ms Jones made six recommendations following their investigation.
These include:
- Subject to funding eligibility, the LLFA to assess the potential for property level resilience in areas where multiple properties (including Businesses) are at risk from repeated events.
- All risk management authorities to continue responsive service within the Knaresborough area.
- Yorkshire Water to consider and implement appropriate options to manage the frequency and impact of surcharging of foul drainage on Park Avenue.
- Yorkshire Water to continue the development of their Development and Wastewater Management Plans (DWMP)
- Where opportunities are identified either through the DWMP, or Combined Sewer Overflow Reduction plans and where funding sources are identified, YW and NYC to work collaboratively to maximise opportunities to separate surface water and foul drainage by promoting the use of Sustainable Drainage Systems (SuDS) to reduce the impacts on flooding in the High risk areas.
- All risk management authorities to work with the communities to encourage and promote improved level of resilience. With the impacts of climate change becoming ever more clear, it is critical that communities play an active role in helping themselves to be resilient to the increasingly prevalent risk of flooding.
Councillors will discuss the findings of the investigation at a Harrogate and Knaresborough area constituency committee meeting on September 25.
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