Flooding –mitigating the risk to buildings in areas prone to flooding

A report issued by accountancy company KPMG following the December 2015 floods in the UK estimated the cost to business to be over £100m, with many companies having insufficient insurance to cover the damage and losses incurred.

Prevention is better than cure, so what should companies do to mitigate their risk from flooding?  The EA recommends that all organisations find out if they’re at risk, that they make a flood plan and that they consult a professional to find out if there is anything they can do to improve their property's flood protection.  https://www.gov.uk/prepare-for-a-flood/get-insurance

Consulting a professional would have saved occupants of Unit C, an indoor play centre for juniors, on the Greatham Rd Estate in Watford, a great deal of hardship.  Unfortunately Clarkebond’s flood risk experts were contacted too late to prevent damage when the estate suffered flooding to the car park and to Unit C.

Investigations by Clarkebond’s flood risk team revealed that the flooding was caused by both fluvial flooding from the River Colne as well as flooding from both foul and surface water drainage.  The area is prone to flooding because it is very low lying and often flooded by heavy rainfall, but this is made worse by the flooding of the Colne. This is caused by rapid runoff from relatively impermeable clay areas and from hard paved urban areas.

Heavy rainfall causes the river level to rise and stops the local drainage from discharging causing back up into the drains and flooding of roads and properties. As the event becomes more severe, overtopping of the river banks and flooding from the river occurs.

“Had these investigations been carried out sooner, measures could have been put in place to prevent flood damage to the building,” comments Seymour D'Oyley, Clarkebond’s Principal Hydrology & Flood Risk Engineer.

The River Colne is contained in a culvert running along the western boundary of the site and, this being the case, there was no viable way of protecting the whole of the estate.  Flood protection is the only feasible option to prevent water from entering Unit C in the event of another flood such that the car park could be flooded but at least no damage inside the warehouses could occur.

Clarkebond liaised with the local authority flood engineer and the EA to ensure that we had all the relevant information and then used that information, together with a topographic survey, to make sure that the protection to the building would be to the correct level and the defences would not be overtopped.  The team then carried out a detailed building and external survey to establish the actual condition and ability of the buildings for options to be assessed with a costed option feasibility report provided to the building owners.

Once the owners had given the go ahead for the mitigation measures suggested, Clarkebond undertook detailed design and scoping of the works required which included:

  • Replacement of escape and access doors with flood proof versions
  • Construction of flood walls and doors across roller shutter doors
  • Installation of non return flood valves to foul drainage runs
  • Sealing of penetrations
  • Re-provision of air vents and cavity vents with flood proof versions
  • Sealing of cracks in walls
  • New flood proof gullies and sealed manhole covers
  • Sealing of porous brickwork

The detailed repairs and works required were tendered and the contract for the installation and site works administered by Clarkebond, including payment scheduling, programmes and full contract administration role under a JCT minor works contract.

Subsequent flooding events have proven the measures taken to be effective as the unit remained unaffected.