Gatwick new runway would increase likelihood of more local flooding along River Mole
A paper has been prepared for GACC by the distinguished naturalist and author Jeremy Early. It shows why Gatwick – and towns downstream – are liable to flood. And that the situation would be made worse by the construction of a new runway and associated infrastructure. Jeremy points out that Gatwick has areas of higher land in its vicinity, which increase the amount of rainfall that has to be drained away. In addition the huge amount of development locally consisting of impermeable surfaces, makes the flooding in several parts of Crawley, other local villages, and at Gatwick Airport understandable. Jeremy points out that Crawley is built on a floodplain and the Environment Agency has said: ‘The decision to site Gatwick Airport across 3 watercourses means that it is vulnerable to flooding from all 3 watercourses as well as local drainage. Run-off from main airfield paving flows by gravity to a storage pond and is then discharged by pumps directly to the River Mole.” They consider the chance of the North Terminal flooding again to be high (about 8% chance). The report considers it misguided to plan to use 900 hectares of greenfield site to create a 2nd runway involving a vast quantity of impermeable surfaces, not to mention associated infrastructure, roads, homes etc.
A paper has been prepared for GACC by the distinguished naturalist and author Jeremy Early. It shows why Gatwick – and towns such as Dorking and Leatherhead which lie downstream – are liable to flood. And that the situation would be made worse by the construction of a new runway and associated infrastructure.
Read the paper (4 pages) here.
A few extracts from Jeremy Early’s paper:
[As Crawley (100,000+ residents) and Horley (20,000+ residents) have developed, the watercourses have been diverted into a total of 75 culverts with an overall length of 5,430
metres.] ‘Once water enters a drainage network, it flows faster than either overland or subsurface flow. With less storage capacity for water in urban basins and more rapid run-off, urban streams rise more quickly during storms and have higher peak discharge rates than do rural streams. In addition, the total volume of water discharged during a flood tends to be larger for urban streams than for rural streams.’
In addition, the Mole has ridges and hills in close proximity and elevated ground always tends to increase the intensity of rainfall through what is known to meteorologists as the ‘seeder-feeder mechanism’. The December  rainfall totals for Dorking and Leatherhead were 210mm and 215mm respectively, whereas the figure for Heathrow Airport, which has little in the way of elevated ground close by, was less than 100mm. Varied elevation explains why, in 24 hours on 23-24 December 2013, Pease Pottage [village] and Reigate had 70mm of rain and Charlwood had a lower total, 58mm. The Pease Pottage figure, combined with the huge amount of development consisting of impermeable surfaces that has occurred locally, makes the flooding in several parts of Crawley (including Bewbush, Ifield, Langley Green, Maidenbower, Three Bridges and Tinsley Green) and at Gatwick Airport including the North Terminal understandable. (Flooding is either pluvial, from surface water, or fluvial, from watercourses; pluvial flooding usually disperses into watercourses, adding to their flows.) One unnamed resident of Tinsley Green, quoted in the local press, said that the flooding in Crawley was worse than in any other area she saw driving to Brighton and back on the morning of 24 December.
Crawley is built on a floodplain and as the Environment Agency points out: “The decision to site Gatwick Airport across three watercourses means that it is vulnerable to flooding from all three watercourses as well as local drainage. Run-off from main airfield paving flows by gravity to a storage pond and is then discharged by pumps directly to the River Mole. As the 1 in 100 chance flood level in the Mole is at the same level as the ground level at the North Terminal, the system is totally dependent on the pumps and on-site storage, with the latter likely to be inadequate at times of prolonged high rainfall due to its modest volume.
“It is estimated that there is currently a 1 in 20 (5%) chance of Main River flooding closing Gatwick Airport, and with 10% increase in flows due to climate change, this increases to a 1 in 12 (8%) chance. The probability of flooding of the North Terminal area due to backing up from local drainage depends on the storm duration and intensity and it is understood that the on-site drainage capacity was designed for a 1 in 5 (20%) probability event.’ The map below (Environment Agency) shows the Flood Zones around Gatwick Airport and Horley. Flood Zone 2 (pale blue) comprises land having between a 1 in 100 and 1 in 1000 annual probability of flooding. Flood Zone 3 (mid-blue) comprises land having a 1 in 100 or greater annual probability of flooding. The Flood Zones ‘show the extent of the natural floodplain if there were no flood defences or certain other manmade structures and channel improvements.”
On the admission of a spokeswoman for Gatwick Airport quoted in thisislocallondon.co.uk [see below] on 8 January 2014, an unspecified quantity of water was pumped into the River Mole from at least one flood storage reservoir/ balancing pond at the airport on 24 December 2013. She said: ‘The reservoir will have been opened. The river flooded in a way which was unexpected in terms of the modelling we have conducted with the Environment Agency. The problems we had were because it was unexpected.’ This action – effected under the terms of an Environment Agency permit – did not cause the damaging flooding at Leatherhead and other areas, but it can hardly have helped. Perhaps the permit and its validity require reassessment, since to some observers there may be a suspicion that the rule ‘I’m all right, Jack’ applies with Gatwick Airport. The Environment Agency’s Upper Mole Flood Alleviation Scheme (UMFAS) was estimated to cost £15 million, of which the British taxpayer would contribute £11 million, GAL £4 million and Crawley Borough Council £100,000. The fairness of this division could make for an intriguing discussion. Be that as it may, the Scheme involved raising Tilgate Dam and river restoration and environmental mitigation works in Grattons Park (both already effected); creating a flood detention reservoir at Worth Farm (expected to be operational early in 2014); replacing the existing dam at Clay’s Lake with a new and higher dam (scheduled for summer 2014); and installing a flood detention reservoir at Ifield. The last-named has been put on hold, seemingly because of major cutbacks in funding for the Environment Agency by the Department for Environment.
Gatwick Airport has spent £20 million on a flood resilience scheme for the South Terminal and is spending £8 million on an additional on-site flood resilience scheme. Despite the substantial expenditure, on 16 October 2013 the South Terminal experienced flooding in an electrical substation, forcing a switch of passengers to the North Terminal. Even assuming that these works and those included in UMFAS are all completed within two or three years, can either the Environment Agency or GAL guarantee that, given an extreme weather event, there will be no flooding in Crawley, Horley or at Gatwick Airport? That alleviation in the upper reaches of the Mole will automatically be beneficial for all those who live close to the river downstream? This is to be doubted if, say, 50mm or more of precipitation were to fall rapidly on saturated ground, as it may well in the short-term let alone the long-term given the local topography and recent rainfall records. On 17 January 2014, after 36.8mm of rain fell in 24 hours at Charlwood, six flood warnings were issued for the River Mole and its tributaries, including at Gatwick Airport.
Given that it was constructed on a floodplain with a number of watercourses running through the site, the fact that Gatwick Airport is located where it is must be seen as somewhat eccentric. So how eccentric, not to say misguided, would it to be to use 900 hectares of greenfield site to create a second runway involving a vast quantity of impermeable surfaces? To build the associated infrastructure, led by tens of thousands of new homes and numerous roads, in an area vulnerable to flooding? Even with the consistent use of Sustainable Drainage Systems (SuDS), which in the most effective examples can involve significant land take, making them unpopular with developers, could there ever be certainty that the local watercourses would not sometimes behave ‘in a way which was unexpected in terms of the modelling’? Could there ever be certainty that there would not be a significant cost on occasions to homeowners living close to the Mole and its tributaries, to the environment and to wildlife? No guarantees made on this particular subject would be likely to convince an observer possessing the capacity to make reasoned judgements based on evidence.
See the full paper at
More about Jeremy Early on his website
25th January 2014 (Local Guardian)
Mole Valley District Council is offering to help some residents affected by the recent floods
Mole Valley District Council is offering council tax support to certain residents who have had to vacate their homes due to the recent floods.
The authority has announced it will provide financial support through a discretionary hardship provision fund for some residents whose household insurance does not cover flooding.
The council will consider granting a discount equivalent to its element of the council tax bill, approximately 10 per cent, for the period the property is empty.
The deal is subject to remedial works taking place to make it habitable again up to a maximum of twelve months.
Council leader Chris Townsend said: “Mole Valley District Council is sympathetic to the circumstances that those residents affected by the floods are experiencing.
“We are committed to doing as much as we can to support our communities through what has been a challenging period for the district.”
Flooded properties in Mole Valley that are substantially emptied of furniture are usually given a council tax holiday of 28 days.
If the property remains furnished but the resident moves to alternative accommodation, they are still liable to pay council tax at the full amount – and could potentially be liable for council tax at both addresses.
Coun Townsend said: “This is where insurance companies should step in, but if not, MVDC can help.”
Each case will be considered on its own merits.
For more information, call Mole Valley District Council’s revenue section on 01306 879293.
ie. taxpayer subsidy if it turns out that Gatwick airport releasing water did not help the flooding situation for people downstream. The details of what actually happened are still being ascertained.
Local Surrey Guardian newspaper asks: “Was Leatherhead sacrificed in the floods to save Gatwick?”
January 17, 2014
After exceptionally heavy rain and wind on 23rd December, Gatwick airport had serious problems with unexpected flooding, with many flights cancelled or delayed. It is still unclear to what extent actions taken at the airport to divert water from its holding ponds and prevent the airport from flooding meant more water surged down the River Mole, making flooding worse downstream in areas such as Dorking, Leatherhead and Cobham. It is understood that investigations are under way, and councillors for Leatherhead are seeking clarifications from the airport. The local press reported that an Environment Agency spokesman had said that Gatwick airport are constructing a further water storage reservoir directly on the Gatwick stream. The Gatwick Stream, where river levels rose rapidly, meets the River Mole south of Horley. Flooded residents feared that the contents of Gatwick airport’s balancing ponds may have been dumped into the River Mole and sluice gates further down were not opened in time. Click here to view full story…
Passengers stranded at Gatwick Airport as flooding causes power outages
by SIMON CALDER (Independent)
Tuesday 24 December 2013
Thousands of people remain stranded at Gatwick airport after a power outage caused by flooding has closed the airport’s north terminal.
A statement from Gatwick airport read: “Gatwick has 250 departing flights in total today. Out of 133 departing flights from North Terminal, 15 have been cancelled while 58 have departed. South Terminal is operating as normal.
“The cause of the power outages – flooding from the River Mole into airfield substations and North Terminal – is related to the heavy rain overnight and fixes for the issues are being progressed as quickly as possible.
“Gatwick would like to apologise for the inconvenience caused to some passengers today and we are working hard to keep disruption to a minimum.”
The main airline at Gatwick, easyJet, had cancelled a total of 54 flights by 2pm. It was offering any passengers due to depart from Gatwick the chance to re-book to another date free of charge.
The airline has reserved 450 rooms in hotels around the airport, and will secure more if needed. Six flights will now depart tomorrow.
British Airways has cancelled 22 flights in and out of Gatwick, to destinations including Rome, Venice and Naples. Three round-trips to Edinburgh have also been cancelled.
From 1pm today, all the airport’s departing flights will now be leaving from the south terminal.
A BA spokesman said: “We are experiencing long flight delays at Gatwick. Gatwick Airport is investigating the fault and trying to restore power as soon as possible. We are sorry for the delays and our customer service teams are doing all they can to help customers. We strongly urge customers to check their flight details on ba.com before leaving for the airport.”
Ryanair uses the South Terminal, but decided to delay its services by an hour to Cork, Shannon and Dublin by an hour “To ensure all those affected by rail delays at Gatwick get home”.
The airport’s IT systems were also affected. One passenger, “Nievey C”, tweeted: “Jesus Christ @Gatwick_Airport if your computers aren’t working & there’s no source of visible info, at least make your announcements audible.”