Gatwick Airport Radar Tower avoids Christmas 2013 River Mole Flood
The floods at Gatwick Airport around Christmas 2013 in relation to the River Mole caused considerable disruption when an electrical substation flooded. Such disruption to key infrastructure is completely unnecessary if competent Civil Engineering Consultants are employed to assess sites and produce appropriate flood mitigation solutions.
At Wilsham Consulting our Engineers have experience of flood mitigation design for a wide range of clients, from domestic sites to key infrastructure. A good example of this is the NATS Gatwick Airport Radar Tower and its technical equipment and backup electrical generator.
When I designed the civil engineering works for the NATs Gatwick Airport Radar it was critical to me to first understand the flood risks presented and the consequences of a flooding event occurring. Only then could an appropriate mitigation scheme be designed. This is why it is critical to employ highly experienced Civil Engineering Consultants with specialist hydrological experience, such as Wilsham Consulting
A channel called Crawer's Brook leads to the River Mole, which goes under the Airport. Detailed flood maps were provided by the Environment Agency. The flood model indicated that between a 1:30 and 1:100 year event, at least part of the site. The Brook takes flows between Crawley and the South side of Gatwick Airport. The brook takes flows from a catchment which is highly urbanised. As such flash flooding can also be an issue, with storms with a high intensity.
Gatwick Airport is in hydrological terms a new facility and we would not rely on the flood history across the last 50 years as having any effective relevance.
Gatwick Airport's Climate change adaption report confirms "Due to the airport being sighted on a flood plain, flood risk is by far the greatest risk to our business as a result of climate change."
It states "Flood risk from the River Mole principally arises on the south side of the airport before it enters the twin box culverts. Here the capacity of the culverts can lead to localised flooding of a small part of the airfield but this is of limited extent.". The report confirms that surface water drainage systems at the airport are on the basis that flooding will occur during a 1:5 year storm return period. This is considerably greater risk than applicable under the building regulations 1:10 year or national minimim standards of 1:30 year.
The EA flood models can often be inaccurate, especially in relation to small feeder channels. These channels, which are often low during normal condition are commonly culveted and have little spare capacity to expand during high intensity storms. Consequently the levels within them can rise several metres within hours.
However, once the banks have been breached the flooding will occurred across the land generally, creating a temporary enlarged flood area. The capacity therefore increases often by a factor of 50+. This means that the scope for the river to rise further is severely limited.
The risk of flooding or other event disrupting the power supply to the Gatwick Airport Radar was considered. I therefore proposed to have a backup generator which, similar to the main radar equipment cabin, I set at a level above the 1:100 + climate change allowance + specific site freeboard.
What is critical is not just to consider the hydraulic statistics, but to consider the implications of flooding.
It was also essential to enable safe access to the site at all times. I adjusted levels and made other changes to try to maintain, so far as reasonably possible, dry access to the site, for use when the main airport roads were flooded.
It was important that such changes did not increase the flood risk elsewhere. I designed a series of source control sustainable drainage techniques to be used across the site, to maximise natural infiltration to the ground.
The radar site was not affected by any flood waters [during the Christmas 2013 floods]. Kevin Briscoe NATS Communications Consultant
This project shows the long term benefit in employing experienced professional civil engineers who can bring together expertise in structural highway foundations, sustainable drainage and flood mitigation to support critical UK infrastructure.
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