With options for a second runway at Gatwick airport now firmly in the public eye again, this post explores flood control engineering and water management currently in place at the airport. The post adds detail to the previous article on flood control in the River Mole basin and should provide a fuller understanding of flood control on the River Mole including management of water flows through Gatwick airport and any impact on downstream discharge. The previous post on the causes and management of flooding more widely in the River Mole drainage basin can be found here.
Gatwick Airport is built across the low relief, impermeable clay flood plain of the Upper River Mole drainage basin. The solid geology is mostly impermeable Lower Cretaceous Weald Clay with some sandstones overlain by recent drift deposits of river gravels and alluvium deposited by the four rivers. This mostly impermeable surface gives the Mole a naturally rapid (“flashy”) response to rainfall events as less precipitation is able to infiltrate into rocks and instead flows as surface runoff into streams and channels across the clay plain delivering water to channels rapidly and raising discharge quickly after storms, an altogether more speedy process than soil throughflow which dominates in more permeable drainage basins.
Three tributaries of the River Mole drain into this basin within or near the boundaries of the airport: Crawters Brook, Mans Brook and Gatwick Stream. All of these rivers have been extensively modified, diverted and culverted during the history of the airport development and both up and downstream during the wider urbanisation of the area most particularly in Crawley, Manor Royal industrial estate and Horley. Most significantly, the Gatwick Stream was culverted underneath the South Terminal during original airport construction in 1958. More recently, the River Mole was diverted in 2000 further north and west to accommodate the North Terminal. A second runway will require further diversions and channel modifications of streams and rivers and further significant attenuation schemes to manage additional runoff from any new runway.
This article outlines how Gatwick airport manages runoff and water flow to reduce the impacts of flooding and pollution not only within the airport boundary but also upstream and downstream of the airport. It should be noted that the Upper Mole basin is an increasingly urbanised catchment with over 11% of the drainage basin covered in urban development. Despite initial perceptions, Gatwick comprises only a small proportion of this total urbanised land cover in the River Mole basin, parts of which are expanding as a key growth area in the south east with the M25, M23 and London to Brighton railway providing excellent infrastructure for growing business and commercial activity and attractive locations for housing. At less than 8 sq/km, the airport itself is currently around three times smaller than the urbanised area of Crawley (30 sq/km). So Gatwick is just a part of an urbanising River Mole catchment and therefore shares responsibility for managing the river with other land users and the Environment Agency. Despite the difficult hydrological siting of the runway on a flood plain and the negative press regarding winter flooding, Gatwick airport is managing water flows responsibly and behaving as a good neighbour to the local area by investing in the Upper Mole Flood Alleviation Scheme that will endeavour to control flooding, not just benefiting the airport but also reducing flood risk for Crawley and downstream to Horley, Reigate, Dorking, Leatherhead, Fetcham and Esher and the M25.
All human development within flood plains from the smallest garden patio, private driveway or house extension to the largest scale road project, shopping centre or major airport, reduces the natural capacity of landscapes to moderate the flow of water into rivers. Human development usually involves the removal of trees and vegetation and the addition of impermeable hard surfaces such as cambered roads, sloping rooves, guttering and drains that are designed to remove water as quickly as possible to avoid local flooding. Such development prevents the infiltration of water into the ground which slows the movement of storm runoff into rivers by allowing some of the precipitation to “sink in” to the soil, thereby slowing down flow reaching the river. Trees and vegetation also intercept rainfall and transpire water from the soil and these processes also slow the movement of water into channels or removes it from river basins altogether by the natural process of evapo-transpiration from vegetation.
The time it takes for water to enter rivers is called “lag time”: the time between peak rainfall intensity and peak discharge. Lag time tends to be longer in naturally vegetated lowland basins and those with permeable geology. In developed river basins with extensive urban surfaces the lag time tends to be shorter. Rivers with short lag times are sometimes called “flashy” because they respond rapidly to rainfall events and flood more easily. Storm runoff reaches the river quickly across impermeable surfaces and this shortens the lag time and exacerbates flooding. All development has a tendency to speed up the transfer of runoff into rivers. Airports are large open areas, the runways alone covering 4km2, with extensive impermeable surfaces. Such wide open impermeable surfaces could therefore have significant impacts on local river discharge unless runoff was carefully managed. Like all airports in the UK, London Gatwick works closely with the Environment Agency to ensure runoff and pollution are carefully controlled. The shortening of lag times through the rapid removal of rainfall from urban areas can cause flood problems downstream as water ingresses into channels more quickly and this is why Gatwick “attenuate” runway runoff by the addition of holding or balancing ponds.
Each airport is unique in how the site creates challenges for water managers. Gatwick was built in a low flood plain which clearly presents unique flood and pollution control challenges particularly now due to climate change and increased rainfall intensity. Nevertheless, all airports present one challenge or another with regard to their site. Heathrow is located on permeable gravels and these present challenges for controlling pollution entering the water table. Other major UK airports located further away from fluvial flood risk are presented with the challenge of shedding excess surface water from heavy rainfall efficiently into balancing ponds and water courses with sufficient capacity to remove pluvial flood water. So Gatwick is by no means unusual in having its own unique challenges presented by a flat low lying site.
“Balancing ponds” are the main engineering device for “attenuating” runoff from airports. Balancing ponds are designed to delay or “attenuate” runoff from runways by holding back discharge from reaching the river directly from the runways and hard surfaces inside the airport boundary. Balancing ponds are large, sophisticated engineering structures and are one of the most extensive spatial land uses for many airports, including Gatwick. A glance out the window on take-off or landing from LGW will show keen-eyed observers numerous balancing ponds and channels in amongst trees, tracks and open spaces around the runway perimeter. These ponds are linked into a complex gravity-fed system designed to transfer runoff and separate clean from contaminated water and treat it and store it, before eventually returning the water to local rivers in a controlled manner or recycling it for other purposes within the airport itself.
All eight of the Gatwick balancing ponds have been designed to control the rate of runoff so that flows are equivalent to the runoff from a grassy or “greenfield” surface. In short, from the point of view of precipitation, Gatwick airport acts hydrologically like a vegetated grassy surface because rain falling over the runways flows into balancing ponds that regulate runoff reaching the River Mole at a discharge equivalent to water flowing off a greenfield.
For every square metre of additional concrete built within the airport boundaries the airport is obliged to construct additional attenuation to meet this “greenfield” attenuation requirement. This will also be the case for any second runway. It is worth considering that this obligation to strictly control and continuously monitor runoff is not the case for all development outside the airport boundaries, for example additional private driveways, housing and roads.
The control system of water management at Gatwick Airport permits regulated discharge of clean water into the River Mole from each of the eight balancing ponds depending on the rate of flow of the river. At times of high flow more water is discharged than in periods of low river flow. At Pond D, which receives the bulk of cleaned water from runway runoff (see below), a maximum discharge into the river of 1680 litres per second (1.6m3/sec 1.6 cumecs) is possible. The discharge of the River Mole entering the airport boundary during times of flood often exceeds 10 cumecs and can exceed 20 cumecs at Sidlow, near Reigate. The addition of 1.6 cumecs, regulated to slightly precede the natural flood peak, is therefore of little significance to the overall flood conditions of the river. Despite some local myths, there are no “flood gates” at Gatwick airport or manual or automated systems which could ever allow significantly sudden discharges beyond a maximum of 1.6 cumecs to be discharged at times of high flow into the River Mole and therefore exacerbate flooding downstream of the airport.
There is little evidence from hydrographs that there are any spikes or discharge peaks outside the expected normal hydrograph curves for high intensity rainfall events, particularly those associated with the floods during the past very wet winter 2013-2014. A sudden discharge of water of any significance would show up on Environment Agency hydrographs and, monitoring these during the course of the winter, there was no clear evidence of unexpected peaks or surges outside the normal response to rainfall along the river downstream of the airport. The winter 2013-2014 yielded 250% more rainfall than average for the area and the Gatwick holding ponds and water management system remained within the design capacity.
On its course through the airport, the River Mole is owned and managed by London Gatwick. The airport has the responsibility to manage the riparian zone (flood plain) and the channel to maintain efficient discharge. River levels and pollution levels are constantly monitored by means of gauges and biochemical oxygen demand. The River Mole enters a culvert built in the 1950’s underneath the west end of the runway. The river was diverted north of the runway in 2000 to allow for the North Terminal. The new channel for the River Mole around the airport is entirely artificial but has been carefully designed as an attractive wooded park-like area with public access being maintained along much of the stretch around Povey Cross bridge for recreational enjoyment, even inside parts of the airport boundary. Gatwick Operational staff walk the entire airport stretch of the Mole twice a year to monitor the state of banks and spot any disruption to the efficient flow of the river along the artificial flood plain. The channel itself has been designed as an efficient shape to discharge flows effectively and an artificial flood plain has been built to safely allow for discharges that exceed bankfull stage (the natural maximum discharge for the channel beyond which floods across the flood plain occur).
Gatwick Airport, just like other extensively urbanised land uses such towns like Crawley or Horley or industrial estates like Manor Royal business park, cannot be solely capable or responsible for controlling flooding on the River Mole. These land users might endeavour to limit flooding locally to acceptable regulated levels but complete prevention of flooding is not possible. A holistic approach using a variety of hard and soft engineering techniques across the whole drainage basin is likely to be most successful in controlling floods and the embryonic Upper Mole Flood Alleviation Scheme (UMFAS) is just such an example. Nevertheless, even with UMFAS, some flooding along the River Mole will continue to occur across the flood plain, an area which includes the airport, during times of exceptionally high rainfall totals and during high rainfall intensities (the latter becoming particularly more common). These weather events are still “natural” on a river basin scale and, whilst airports can attenuate flow off runways by careful management, they cannot completely prevent floods from occurring that are caused by intense rainfall across an entire drainage basin. Overwhelming discharges will inevitably overwhelm flood plains should they exceed the design capabilities of the engineered defences. Nevertheless, it is still the responsibility of major land users to control floods and reduce impacts to acceptable levels and to protect key infrastructure and this is what Gatwick is doing by investing in modern flood control schemes both within the airport boundaries and across the whole Upper Mole catchment.
What follows are some details about Gatwick Airport engineering schemes designed to control water flows in and around the airport.
Pond M – is a relatively new balancing pond. Its role is to attenuate airfield runoff and complete initial treatment of contaminated runoff. Especially during wet periods, it transfers a controlled discharge of clean water to the River Mole by allowing outlets in a chamber to overflow clean water into the river after passing through “interceptors” that remove any remaining aviation fuel or silt. Clean water from Pond M is discharged into the River Mole from a controlled overflow shown below. The volume of water discharged into the river at this location is designed to be small even at times of peak rainfall intensity. During rainfall there is discharge at greenfield rates into the River Mole from this balancing pond. Summer runoff from runways is cleaner so more water is discharged into river as there are no anti-icing used. In winter more of the water from Pond M is contaminated and so enters the transfer system to Pond D for further treatment.
Prior to entering balancing pond M, runoff from the runway passes through a sensor that continuously measures biochemical oxygen demand (BOD) and detects anti-icing chemicals and other contaminants and diverts contaminated water with BOD >10mg/l to the contaminated holding pond for storage. This is then transferred to Pond D for tertiary treatment such as removal of oil / aviation fuel and silts and thence contaminated water is sent to pollution lagoons for further treatment prior to entering the Crawley Water Treatment processing plant east of the runway near Tinsley Green.
There are two aircraft stands where aircraft washing is permitted. These two stands drain to a treatment plant which removes heavy metals released during aircraft washing including cadmium. Within Pond M catchment there are 2 aircraft stands.
Pond A – is a balancing pond located at the outflow of the River Mole from its culvert under west end of the runway. Pollution monitoring also takes place here of all water shed from the west end of the runway. After flooding of the runway in 1967 the culvert was found wanting so a “syphon” was added. A syphon is essentially an additional channel which provides more capacity at times of peak flow to reduce the risk of flooding on the runway.
Pond D – this is the central “hub” of water control and treatment at Gatwick Airport and sits just behind the North Terminal. Pond D receives all dirty contaminated water from LGW runoff from each of the balancing ponds e.g oil / aviation fuel and anti-icing chemicals. D pond also drains the eastern area of airfield and both terminals. Pond D is sited approximately 50m above sea level and a levelling gauge indicates the height of the water in the pond. During the 23-24 Dec winter flood Pond D was at its highest ever level at around 55m asl and was above the level of the culvert feeding it.
Three massive archimedes screws lift water from Pond D, aerating it at same time, up to separation ponds for clean and contaminated water. Each screw can lift 840 litres per second. Gatwick is permitted only to use two screws at any one time, the third being present in case of mechanical failure of the others. Two screws are therefore said to be “on duty” whilst a third is always on stand-by. During peak storm runoff the two screws are able to lift an absolute maximum of 1680 litres of water per second from pond D to flow through two parallel separators. Biochemical Oxygen demand (BOD) is monitored continuously to check the status of water entering the ponds and separate accordingly into clean or contaminated ponds.
Clean water enters a separate pond where it is aerated to reach a required standard before being discharged into the River Mole just downstream of Povey Cross Bridge via a surface spillway built to allow water to flow naturally, under gravity and unimpeded out of the pond once a certain level is reached during flood conditions. There are no gates and there is no impounding of large volumes of water that could then be “released” to cause a sudden flood peak in the Mole downstream. The additional discharge into the River Mole from Gatwick airport from Pond D cannot exceed 1.6 cumecs because the two screws jointly have a maximum capacity of 1.6 cumecs.
During “normal” river flow and “average” rainfall conditions there is little or no discharge of clean water into the River Mole. The rate of discharge into the River Mole from Pond D depends on the rate of flow of the river itself. When the River Mole is at high flow conditions more airport water is permitted to be discharged from the clean pond. Rates of discharge are set by the Environment Agency and automatically controlled by automated systems at Pond D.
The contaminated water from Pond D is further cleaned of silt by use of interceptors and any oil or aviation fuel is removed by running absorbent filaments through the surface of the water. This is then extracted by means of mechanical pressure. The process is continuous. The water from this pond is then pumped along a 4km pipe to pollution lagoons outside the eastern boundary of the airport and thence finally to the Crawley water treatment works where it joins the water arriving Crawley which is is cleaned to a standard non-injurious to fish prior to being released into the natural water course of the Gatwick Stream which joins the River Mole near the Hookwood roundabout.
Gatwick airport contaminated runoff is therefore treated through the Crawley water treatment works and is the same quality. The flow of contaminated Gatwick runoff permitted through Crawley treatment works is 50 litres per second or 0.05 cumecs. The attenuated discharge of runway runoff in the Crawley treatment works represents no significant “additional” runoff other than what would have reached the Gatwick Stream or Mole in natural conditions.
With the maximum discharge of clean water at Pond D at 1680 litres per second (1.6 cumecs) and the rate of flow through Crawley treatment plant at 0.05 cumecs, the additional flow to the river network is comparatively small. The Gatwick Stream at flood peak can achieve ten times this flow.
A new facility at Pond D also extracts clean water for use in the airport fire hydrant network around the perimeter of the runway, increasing the capacity for the airport to attenuate runoff by recycling water within the airport boundary.
All pumping stations relating to water management across LGW have back-up facilities in case of power outage, especially critical during times of flood.
The whole system of water management is mostly gravity fed and fully automated. Water quality is checked with 100’s of samples every month. The Environment Agency attaches strict conditions to the quality of water entering the River Mole so that pollution discharges are minimised and that water entering the river is the highest quality possible and not injurious to fish. Fines can be applied if the airport is found to cause pollution along the river, which has happened in the past but it is fortunately a rare event due to tighter controls and monitoring.
The culvert running underneath the South Terminal and railway station, built in 1958, unfortunately was not designed with sufficient capacity to accommodate the river discharge during times of extraordinary peak flooding along the Gatwick Stream (i.e. exceeding 15 cumecs). The culvert can accommodate only peak flows during 1:50 year flood events. Larger, less frequent flood events with higher discharges cannot be accommodated by the present culvert. It is not possible to rebuild the culvert due to the extensive airport, road and railway developments that have taken place above it, so a modern attenuation scheme upstream of the culvert has been constructed and is due to be finished in August 2014. The Gatwick Stream scheme is an airport initiative that complements the wider Upper Mole Flood Alleviation Scheme (UMFAS).
- Details for the wider Upper Mole Flood Alleviation Scheme (UMFAS) have been posted here before, find details here.
- LGW contributed £4 million to the UMFAS inc Worth Farm, Tilgate Lake (both completed), Clays Lake (due to start in September)
- Gatwick Stream scheme estimated cost £12 million (completion August 2014)
The new Gatwick Stream flood alleviation scheme will be open to the public and appear as a rather unusual park-like layout not dissimilar to “Teletubby Land”! Grassy mounds with numerous oak trees have been retained to conserve local bat populations and increase the park-like appearance of the area. Footpaths will encourage the public to use the area for recreation. Due to the proximity to the end of the runway, large flocks of birds will be discouraged from the site to reduce the risk of birds striking aircraft. This will be achieved by pumping out any water residing in the basin into the stream to maintain a dry environment thus discouraging flocks of wetland bird species, for example.
The Gatwick Stream has been diverted and modified into a new meandering natural-looking channel course. Fish have been re-stocked. Flow will be monitored continuously at the South Terminal culvert entrance. If the Gatwick Stream discharge exceeds the 1958 culvert maximum of 15 cumecs then the gates will inch down and attenuate the river flow which will spill over into the newly created basin. Flood waters will fill the basin but will be allowed to discharge naturally as soon as the flood peak has passed.
The park will be open to the public. Interpretative signs will hopefully be in place to explain the scheme and link to the UMFAS and Gatwick flood and water management and wildlife conservation to present flood management in the Upper Mole Basin. This seems to be an excellent opportunity for Gatwick to be on the front foot in terms of displaying a modern flood alleviation scheme put in place on schedule and with public and wildlife conservation interests included. This could be a triumph of modern hydrological engineering made necessary by past mistakes (i.e.1958 culvert being built too small)! It combines modern soft approaches to flood management, such as reintroducing meanders, conserving wildlife and allowing natural wetland environments to attenuate discharge, while also including the hard engineering necessary to protect critical national infrastructure. The scheme should also benefit Horley residents by reducing flood risk downstream. This demonstrates the importance of Gatwick as an international facility requiring protection and the seriousness with which the airport takes its responsibility to control flooding both within the airport and for residents downstream in times of climate change.
Car parks are self-balancing
Airport parking is another large scale land user and comprises further extensive impermeable hard surfaces that could potentially increase surface runoff and create flood risk. Deep ditches surround these car parks and these were designed to reduce the need for balancing ponds. The ditches themselves provide self-balancing around each car park.
Long term car parks at the North Terminal discharge water from the ditches into a balancing pond known as Dog Kennel Pond. Complex control systems handle pond levels and during intense rainfall some car park runoff is discharged into the River Mole near Povey Cross Bridge. The discharge is cleaned using interceptors and so is considered to be a clean flow as it matches the same quality of water that would runoff from any road surface.
Y shaped and new round pollution lagoons:
The final piece of the Gatwick water management jigsaw is a 4km pipeline from Pond D that transfers polluted water to the pollution lagoons clearly visible on maps and Google Earth. These lagoons are netted to prevent harming wildlife. The lagoons further treat by aeration contaminated water prior to transferring it to the Crawley Treatment plant from whence it is discharged into the Gatwick Stream along with waste treatment from Crawley.
The final destination for most contaminated airport runoff water is Crawley treatment works which receives airport water from the pollution lagoons nearby which, in turn, received their water from the 4km pipeline from Pond D. The maximum rate of treatment for airport water through the treatment works is 50 litres per second (0.05 cumecs) which, again, is of little significance to the Gatwick Stream at times of high discharge.
Gatwick airport is unfortunately sited in an impermeable flood plain vulnerable to flooding. In response, the airport has progressively built a sophisticated hard engineering water management system that collects, separates, cleans and discharges a controlled flow of water back into rivers via complex transfer through a series of balancing ponds and cleaning processes. The system has also had to address past mistakes in engineering that proved wanting in times of more recent intense rainfall events. The majority of contaminated water is discharged into rivers through Crawley Water Treatment works. During periods of heavy or prolonged rainfall some clean water can be discharged from Pond M and Pond D directly into the River Mole. The LGW water management system controls runoff into local streams and this effectively “attenuates” runoff to the equivalent of a grassy greenfield site from each of the eight ponds around the site.
Water management and flood control at Gatwick airport is more strictly regulated and more carefully monitored than many expanding urban areas in the same catchment, for example, in Crawley and Horley. These urban areas, creeping inexorably onto flood plains can exacerbate flooding to a greater degree than the airport alone, though this would need more detailed investigation to “prove”. The causes of flooding in the Mole catchment are a combination of factors which were explored in previous posts on this blog found here.
Importantly, the management of water flow within Gatwick is sophisticated, automated and ongoing. There are no “flood gates” that can be opened to cause a sudden increase in discharge on the River Mole.
Inside the airport, the major flood event on 23-24 Dec 2013 was related to intense rainfall causing rain water to migrate through the maze of 1980’s airport piping, drains and ducting and, unfortunately, discharge through a wall directly into an electrical facility servicing the North Terminal. The complex organic historical growth of the airport has created unique problems of management that require continuous investment and engineering to solve.
Finally, the airport has impressive “epic” engineering structures that control, separate, transfer and treat runoff from the runways continuously and discharge an attenuated flow of clean water into the River Mole and contaminated water via the Crawley treatment works. Discharge is usually low but, it is worth remembering that all engineering structures are built to a design limit. New water engineering schemes at Gatwick, such as Pond M, are being designed to protect against rare events of exceptionally high rainfall with return periods of at least 1:120 years. Events yielding rainfall in excess of these design capabilities will still cause flooding both within the airport and downstream. Plans to add another runway should include awareness of flood attenuation both within the airport boundary and for communities downstream.
The River Mole could become a “beacon” example of modern holistic river management! It could also become a vital green lung for this part of the South East alongside a major international airport. As the airport hopefully continues to invest responsibly in both soft and hard management along the river course, as much of the river as possible should be made available for recreation and public access. The River Mole VISION should be for a continuous green network of footpaths and attractive open spaces to exist along an ecologically rich riparian zone for local residents to enjoy: an unbroken network of paths from Rusper, through Crawley and Gatwick and onward to Dorking and through the Mole Gap into Leatherhead and Fetcham to Hampton Court! Such a “Mole link” could enhance the natural diversity and varied landscape of the area as well as celebrating the vibrant economy, history and even enhancing local tourism. This vision will require continued work with local authorities, land owners and the public to encourage a wider understanding of the key importance and benefits of our river network to the whole community and create a network of access agreements to join up this linear park. Involvement of the public in how they might wish to use the river, particularly with any impact of a new runway, could be a key to successful management of the River Mole into the future.
The Nutfield Marsh nature reserve, a wetland habitat designed, in part, to attenuate flood runoff on Redhill Brook by designing a natural wetland, could be a design blue-print for the Upper Mole. This nature reserve has partly been managed by school groups from Reigate Grammar working with Reigate Area Conservation Volunteers. The nature reserve has a role to play in reducing flood risk downstream on the Redhill Brook. Public access and enjoyment of these interesting locations will be both educational, healthy and yield a greater understanding of the need for careful management of rivers and flood plains as we likely face climate extremes and changing flood risks in the future.
The information in this post is based on a lengthy tour and investigation of the water management engineering in Gatwick airport especially designed for RGSweather and kindly hosted by John Barber (LGW Water Quality Manager) and Tom McShane (Project Manager). All sites pertaining to water management around LGW were visited at the end of March 2014. John and Tom imparted expert knowledge and this post represents a collaborative approach to publish, as far as possible, a clear and factually correct outline of flood control in LGW. Comments / further information /updates always welcome.