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After a pleasant dry and sunny day in the SE, the satellite photo from this evening spells trouble ahead for mid-week with a deep depression over Iceland and an increasingly active Atlantic with a long frontal boundary trailing across the ocean into thick bands of cloud.

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Reigate and the south of England are set for a potentially very wet day on Wednesday as shown on the Euro4 chart below showing just 12 hours of rainfall during the morning.

Unusually high rainfall totals could mount up, possibly to around 30mm for the day on some models. However, models usually exaggerate rainfall totals but it is likely to be soggy!

A strengthening Atlantic jetstream is causing the convergence of moist sub-tropical and polar air at the Polar Front over the Atlantic Ocean.  The winds can be seen converging on the Atlantic chart below.


The convergence of Polar and Tropical airmasses can also be seen on this chart showing the trajectory of winds arriving in the UK on Wednesday.  Note the surface tropical airmass circulates round the Azores high and meets the incoming Polar air from Canada.  It’s the less dense moisture laden maritime tropical air which is lifted over the cold, enhancing rain on frontal boundaries.

The 850hPa (15oom) temperature chart below shows the steep temperature gradient between contrasting airmasses across the Atlantic.  The water vapour Meteosat satellite picture shows a broad sweep of moisture laden air crossing the Atlantic from the Sargasso Sea.


The boundary of the contrasting moist Polar and humid Tropical airmasses causes lift and this is set to rapidly form a depression over the UK later on Tuesday into Wednesday courtesy of a jetstreak to the west of the UK.

Large amounts of Atlantic moisture are set to converge in this low pressure as airmasses meet at frontal boundaries.


The result over SE England is an unsually steep rise in dew point to 6-7C, indicative of increasingly moist air.

The atmospheric column looks to become exceptionally moist on Wednesday and saturated through to a height of 25,000 feet.  Cloud depth will make it a very dull day.


Rainfall charts look impressive and, at the moment, show the rain arriving on Wednesday morning. Here is a medley of rainfall charts from 3 different models showing the potential for a deluge, though do note that models tend to exaggerate these totals this far out.

The 6 hourly total chart from GFS shows an extraordinary 26mm over parts of SE on Wednesday morning.  This would lead to local surface flooding on roads.


Note this is not a convective event so no thunderstorms are likely, which makes such high rainfall even more unusual. It could be the biggest daily rainfall total for quite a while, over 36.6mm of rain in a day was recorded on 24 August 2015. Keep posted on twitter and check MetOffice forecasts for updates if travelling.  Some disruption could occur if this comes off as models suggest.  Milder and settled conditions are expected into the weekend after our mini-monsoon!




Reigate experienced an impressive deluge today when over 60mm* (tbc) of rain fell in a few hours causing flooding in parts of the town.  This was a “surface water flood” caused by drains being overwhelmed by intense heavy rainfall, rather than a river flood, our local River Mole will react more slowly to this event and is unlikely to cause any problems. (*by 4.30pm manual rain gauge 64mm; radar netweather est 50mm; total 24 hour rainfall 74mm manual rain gauge; RGS AWS 36.6mm; Reigate AWS 50.8mm)

The entrance to Morrisons car park and the lower section of Bell Street were overwhelmed with water bursting from drain covers adding to the water running down the roads.  It wasn’t just the roads that were flooded… such was the intensity of rain that Priory Park was also left with surface water pooling up several inches deep on the pitches, and even more in the sunken garden and around the cafe.

This rain event was accurately forecast by the MetOffice and MeteoGroup and several specialist severe and convective weather met agencies who all issued warnings of heavy thundery rain, some totals suggested over 60-90mm.

Two models, the MetOffice Euro4 and WRF NMM model output, turned out to be especially accurate with both putting exceptionally heavy rain precisely over our area at least 24 hours in advance and this was reported by RGSweather on twitter.  The GFS model was less convinced about such heavy rain reaching beyond the south coast but, as always, these convective summer events are especially hit and miss.  Today was a “hit” for Reigate but unfortunately would have probably caused a nuisance for businesses and commuters.


The maximum intensity of the rainfall in Reigate was measured at 145mm/hr during the storm between 2-3.00pm.  This intensity of rain is unusual in Surrey (highest rainfall intensity in recent years was 183mm/hr 20 Nov 2013) but we are told to expect more of our rain to fall in intense events like this with global warming.  The last time flooding of this scale took place in town was 24 Dec 2014 but, having seen both events, I think this time was slightly “worse”, although given summer conditions the surface water may have subsided quicker.  There are many areas for further investigation worthwhile here: how much rain, for how long and with what antecedent conditions (soil moisture / season / evaporation etc) and what duration of rainfall intensity causes flooding in Reigate?


rainfall intesity Reigate

The synoptic weather situation that caused this event was a trough over the Atlantic and a blocking HIGH over Scandinavia causing a warm unstable humid airmass to meet cooler Atlantic air along active fronts.  The resulting LOW transported a lot of moisture over the SE and caused the rain in heavy thundery downpours triggered near the fronts.  This situation is not likely to change much until later this week, so expect more rain but hopefully not as intense.

August SE Reigate flood

August SE Reigate flood

An unstable airmass with backing winds over the SE indicated by a skew t chart from the day.

Technically speaking this was part of a “trough disruption” event (see charts above) that started over the Atlantic during the weekend. A “trough disruption” is when an upper trough “breaks” and a southerly section of the trough proceeds purposefully east or NE leaving the centre of the trough behind as a semi-static feature.  The isolated part of the trough can then behave erratically. In this case an unusually active and southerly dipping jetstream for the time of year also played a part in deepening the LOW at the base of the disrupted trough.  Weather events can be “severe” and unexpected with models sometimes struggling to cope when trough disruptions occur.

The LOW pressure that brought heavy showers over the SE and Reigate formed in Biscay yesterday and travelled NE up the English Channel during the day.  SE England therefore experienced SE / easterly winds in the morning, backing northerly and finally swinging westerly during the course of the day, fairly unusual for our part of the world. The anticlockwise change of direction in wind is called “backing”.  Veering is the clockwise movement of wind over time or with height.


glowering clouds menace Reigate in the morning

Here are some more pictures from this event on my google pics page: please use them but do credit rgsweather.

by evening it wasn’t any better:


RGSweather photos featured on the BBC London news that evening (sorry, no sound)

Finally, we can expect more rain this week as the synoptic situation stays static with the Atlantic trough blocked by the high out East. This situation will keep us in a flow of humid moist air that, when it interacts with cooler Atlantic airmasses along fronts, is likely to cause rain.  Add an unusually active jetstream and we have a decent recipe for a wet week.


wet week ahead

links to other reports

the system went on to cause significant tornados in Netherlands

Google Earth image of rivers through LGW

Google Earth image of rivers through LGW

This post explores flood control engineering, water management and pollution controls currently in place at London Gatwick 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, including pollution. 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.

29-12-2013 21-15-54

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.  For example, new multi-storey car parks have subterranean attenuation ponds built underneath.  It is worth noting that the 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.  Pond D receives the bulk of contaminated and clean water from runway runoff (see below).  Water is automatically tested (e.g. measuring biochemical oxygen demand) and contaminated water is separated from clean and is then pumped through a 3.5km pipe to Crawley Sewage treatment works at Tinsley Green where it is held in large Y shaped ponds before treatment and eventual release into the river.  Clean water from Pond D is separated, aerated and enters the River Mole directly.

However, even in flood conditions, a maximum discharge of separated clean water into the river of just 1680 litres per second (1.6m3/sec 1.6 cumecs) is possible, as each archimedes screw can only lift 860 litres per second and only two are allowed to operate at any one time.  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.  In contrast, at times of low flow some 75% of the river discharge comes from Crawley sewage treatment works which outflows at Tinsley Green.  Much of this is treated water from Pond D and homes and industry across the Crawley area.  Despite some local myths, there are no “flood gates” at Gatwick airport or manual or automated systems which could 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.  The only “flood gates”, if they can be called as such, are located at the new Gatwick Stream flood alleviation scheme, operational from 2014, and these are described below and would also never cause a sudden peak discharge beyond the natural storm flow.

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.

aircraft washing creates contaminated waste

aircraft washing creates contaminated waste

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.  A more extreme weather event could cause Pond D to be overwhelmed and over-top, in which case the North Terminal, being lower elevation, could be flooded at ground level.

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 3.5km 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.

Some sources of contamination specific to the airport include fuel spillage, rubber build-up on the runway, cadmium run-off from plane washing and de-icer (ethylene glycol) anti-freeze in winter.

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.  One incident in the late 1990’s involved a new product being used to de-rubberize the runway, which naturally gets rubber build up from aeroplane tyres.  The product being used was based on orange peel but some accidentally got poured down a gutter causing the death of fish downstream in the River Mole.  The Environment Agency fined LGW something like £50,000.  This kind of incident is fortunately rare because of the sophisticated pollution controls on site and strict monitoring by EA and compliance by LGW of pollution treatment and potential run-off.

1958 Gatwick Stream culvert construction

1958 Gatwick Stream culvert construction

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)
New flood attenuation scheme

New flood attenuation scheme

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 run-off and create additional 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 and Pond G. 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.


High pressure persists this week over the UK as shown by the chart and the amazingly clear satellite pic showing great swathes of clear skies over a remarkably snow-free Europe today 11 March. It’s worth remembering that on this day last year we saw the lowest wind chill temps of the year in Reigate at -11.9c early on 12 March with -10c at 9:30am on 11 March during an episode of extremely cold NE winds which brought a blizzard to the Channel Islands.

Back to this week… the high pressure has brought delightfully sunny days and Tmax temps to Reigate over 19c last weekend , especially warm and spring like on Sunday. Since then a cooler NE/E breeze around the edge of the high as it slipped north and east has kept us cooler and occasionally more gloomy with anticyclone gloom and stratocumulus blanket on some days.

The outlook this week remains settled but with a distinct drop-off of temps over the weekend and especially into next week.  This is due to the HIGH slipping west and the jetstream dragging down cooler NW winds fresh from Greenland.  A LOW moving across N Scotland and over to Scandinavia this weekend will start the process of dragging down NW winds that will make next week cooler, especially further east where it looks like the N Sea will get a full blast of chilly air at times.

Nevertheless, it is unlikely to become terribly unsettled down here in the SE as the longer term picture looks like high pressure hangs on in the south keeping us mostly dry.  The north of the UK might see more frontal action as LOWS bring fronts across more northerly parts.  Models seem to be keeping the south relatively dry for another week to 10 days.

After the wettest winter on record, groundwater levels in the chalk aquifers are the highest EVER in the SE and the Mole Valley particularly.  There are still 50 flood warnings in the SE and locally flood water continues to pour across the A25 between Reigate and Dorking near Brockham.  This groundwater flooding, though of course nothing like as damaging as the fluvial river flooding during the winter along the R Mole and the Thames, could last for some months yet as the winter rain continues to percolate into the chalk water table and flows out along geological boundaries at inconvenient locations such as normally dry river beds and bournes and across impermeable surfaces such as roads. The map showing soil moisture below shows how wet the soil remains after this extremely wet winter.

IMPORTANT UPDATE 11AM Fri: UKMET extended AMBER warning to include all of SE inland including Reigate, Surrey, London: gusts of 60mph possible overnight. 

Latest update: (pm Thurs): one HiRes model just HALVED rainfall totals for storm tomorrow: if others follow that’ll be good news for flooding.  This storm is not resolved perfectly yet, some models flip and flop before the event.  Nevertheless, one to watch for signs that, perhaps, the rain is being over-done. In fact, rainfall totals have rarely matched model runs in recent events… fast moving fronts have simply zipped by so fast that rain totals have been rather low.  example: 8.8mm on 12 Feb front yesterday.


Now you see it…?! Rain totals just halved on latest HiRes model run (left). GOOD NEWS for floods!

Update for Reigate and SE England on UKstorm Fri 14 – Sat 15 Feb:  the storm tomorrow is yet another serious weather event, this time focusing most intensely on SW, Southern coasts and SE England and Wales overnight Fri-Sat.  Below are some satellite pictures from today showing the rapidly intensifying storm in the Atlantic to the SW of the UK.


The wind and rain forecast on NWP models for inland SE areas are severe: often the forecast winds and rain do not transpire, coming in at a good deal less than the modeled projections especially for sheltered towns where most people live.  However, the magnitude of figures consistently being predicted for gusts and total rain associated with this storm in our area are consistent enough to be of some concern, especially for places exposed to southerly winds (on hills like North Downs, for example) and at risk of flooding, either surface water flooding or in the flood plain for the River Mole or tributaries.

Rainfall will be heavy throughout Friday and could total over 20-30mm in 24 hours in places.  If this exceptional rainfall does occur then extensive local flooding is likely to result rapidly, especially along the River Mole catchment which has a flashy response to rain within 10hours.  Winds are due to possibly exceed those experienced this winter so far, forecast to peak at over 60mph gusts inland early Sat am.  Heavy rain is also likely to add to the problems of flooding.

UKMO warnings stand at Yellow alerts for Reigate and Surrey from Fri through to Sat 15 Feb and AMBER alerts for all places nearer the coast.  The storm is rapidly intensifying and will bring heavy rain all day to Reigate and Surrey and the SE as a whole through Friday, totals could be over 30mm, which will cause the River Mole to flood in places and plenty of surface water on the roads. Strong winds build gradually through the day and will peak overnight in the small hours of Saturday morning.  The winds, even inland, could even gust at 60-70mph and for southern coasts 75-80mph.

These are exceptionally strong gales for inland and, if these NWP modeled magnitudes transpire, it would be exceptionally bad news for the region already suffering from flooding. Remember our strongest gust for YEARS in Reigate was 52mph in January.  So any wind speeds over 60 mph will be exceptional. The UKMO also forecast wind gusts over 60mph around midnight.  ESTOFEX have issued a Level 1 tornado warning and for isolated gusts from afternoon onwards due to warm air advection and presence of sub-tropical air wrapped in the system.  This will add lift and energy to frontal rain bands which might cause thunderstorm activity, the odd clap of thunder embedded in fronts might be a sign of tornadic potential activity so keep an eye our for funnel clouds or drops in cloud base or tornados.  If you see any of this please tweet or email a picture!

The saving grace is that it passes over relatively quickly and winds will abate on Saturday leaving a bright possibly showery and blustery day. The remainder of half term week looks less dramatic and with a ridge of high pressure flirting with the South it should be more settled, albeit with another LOW early in the week, but this is much less intense.

RGS weather is on holiday for the remaining week, inspecting the other side of the jetstream in the Atlas Mountains of Morocco. Intending to fly out from LGW at 4am on Sat morning! Close up and personal with this storm but sadly no tweet updates are likely.

By the way, press details can be hopelessly inaccurate, especially certain papers!  The pic below is wrong: indicating several storms that were not a special problem for the UK… handle with care!



Four days of UKMO weather warnings and this hasn’t been unusual this winter!

Unsettled is an understatement, the weather is still on over-drive! Here’s a quick update on rain and storm events upcoming this week for Reigate, as the weekend summary didn’t quite do it justice!  The jetstream is still blasting across the Atlantic and giving birth to storm after storm.  Causes of this extraordinary jetstream are complex, but most “local” to the UK is the extreme contrast in temperature across North America with a buckling jetstream dragging frigid conditions across much of that continent.  This creates a steep temperature and pressure gradient that kick-starts a powerful jet across the Atlantic.  Storm force lows are being created amazingly quickly and most of them are being directed at the UK.

So, here’s a summary of storms on offer this week for Reigate and Surrey…

Tuesday: active front with cold air behind sweeps across the UK and arrives in Reigate am for heavy rain at lunchtime with gusty winds for a time. 10mm is possible in a short space of time which could lead to local FLOODING.  The front will whizz through quickly by the afternoon and leave a brighter but cooler feel pm even with possible wintry showers on a brisk cool W/NW wind.

Wednesday: looks grim… another tight storm approaches from the SW.  This one brings more gales to the SW, Wales but also pushes them further inland across the Midlands.  The south coast and the NE coast (on departure) will also see gales.  Wind for the SE will be pushing in off the Channel at possibly 50-60mph with 40mph gusts inland for inland places Reigate, peaking pm.  Rain is due to be heavy, especially pm and early evening and it will be cloudy all day.

Thursday… looks like a brighter showery day with possible heavy thundery showers at times. Friday / Saturday brings in the next storm with high rainfall totals adding to already saturated ground.  The likelihood of more flooding seems high at this stage which is terrible news for those places along the Thames that have been so badly hit today.

After another big “50 year storm” battered the SW and south coast today, the weather next week unfortunately looks to continue very unsettled with the possibility of stormy conditions at times, especially for the South and west.  Reigate, though more sheltered than most, will not escape the action. Mid-week Tues/Weds sees a likely cold snap with a chance of snow mainly falling over high ground to the NW but also snow showers possibly falling anywhere, albeit briefly and probably not reaching the SE. Before all that, let’s do a quick review of the interesting weather today…

In Reigate there was some interesting weather to report today as an unstable brisk SW airflow picked up moisture from the Channel and built cumulonimbus cells that rolled up to Reigate in the morning… see below:

In Reigate gusts peaked at 39mph, more widely gusts reached a measured 43.5mph on N Downs and exceeded that elsewhere to reach 50mph in places. The SW and coastal areas had gusts well over 70mph. In the morning some well developed cumulonimbus thunderstorms rolled across the area and produced hail and some lightning around Reigate, Dorking and Guildford. During these storms downdrafts caused temperatures to drop sharply but also the pressure to rise locally.  Now, let’s look at some charts for next week… 

It’s worth noting that the Northern Hemisphere is exceptionally cold and snowy this year… except for UK where it is exceptionally stormy and wet!  There are signs that things will calm down after this coming week as pressure creeps up.  This week, however, looks a little rough, windy, wet and unsettled.


Models are still battling with the precise intensity and track of storms next week. ECM wants to bring in a southerly tracking rather intense low with some big gales through the South and the Channel prior to opening the door to a cold plunge of NW winds fresh from Greenland on Tuesday / Weds. The NW is not the usual direction for country-wide snow, at least not in the sense of prolonged bitter temperatures like 2013 but this particularly unstable polar maritime airmass is being delivered direct from Greenland/Canada and has some chilly upper air wrapped up in a low pressure behind an active front that will cross the UK through Tuesday and drop plenty of rain / sleet /snow. It’s the air behind this front that may bring snow.  Whether it reaches SE is not certain, but the fridge door will not stay open for long as a bigger Atlantic storm sets to arrive at the end of the week.

Finally, at the end of the week there is a sting in the tale… another almost identical storm to this weekend appears on the scene on Thursday and runs through Fri/Sat.  Some models (GFS) show extraordinary wind speeds for inland at the moment but often they over-do things at this early stage and they usually calm down nearer the time. Lots of interest this week so watch this space for developments.  As weather is so fast moving our twitter feed is the place to get best up-to-date info. @RGSweather

December in Reigate 2013 summary stats:

A quick summary of an historically stormy and wet December in the SE.

Average temp 6c

Tmax 11.4c 16-17Dec

Tmin -2c 5 Dec

Max wind gust 39mph 27 Dec

Total rainfall 110mm of which 47mm fell 23-24 Dec (or 70mm from manual rain gauge)*

21 days with rain

Windy! Especially high up

Windy! Especially high up

For the RGS weather station there were some interesting developments regarding rainfall measurements which are worth sharing.  During the early storms in December, when wind speeds gust 35mph in Reigate, the VP2 rain gauge and ISS (integrated sensor suite), which houses all the thermometer instruments and the tipping bucket rain gauge, was blown over!  The standard tripod had been pegged down but the wind had pulled out the pegs on one side and rocked it over.  This was noticed remotely almost straight away by the lack of rainfall rate turning up whilst it was clearly raining outside! The unit was rescued within an hour, checked thoroughly and secured with guys with no adverse effects.

This prompted a comparison of rainfall between our VP2 and our standard rain gauge and some comparison checks with other neighbouring weather stations (including Kenley and Charlwood, two Metoffice “official” stations). Through studying the rainfall totals of several subsequent storms, It appears that the VP2 rain gauge is sometimes recording significantly less in comparison to our standard rain gauge, which is located a few feet away at ground level.  The most extreme example of this disparity was the 23-24 Dec storm when 70mm was recorded on the standard rain gauge at ground level, while the VP2 recorded 47mm.  This initially looks like the VP2 must be incorrectly calibrated because the standard rain gauge (collecting an “absolute” total rainfall amount in a graduated collecting jar rather than a 0.2 mm resolution of counted tipped buckets on the VP2) must be the more accurate… but hang-on! Our VP2 measurements match other local weather station and the rainfall rate correlates well with other local stations such as Kenley, Smallfield and Charlwood on a daily record resolution.  Such comparisons would never be exactly the same of course because rainfall varies a lot even across a small local area (see pics below), so relying on comparisons with other stations would certainly be incorrect in establishing the accuracy of our own figures. Nevertheless, it does at least appear we are all in agreement to a close degree regarding rain-rate trends as known storms progress across our region.

So rainfall is notoriously tricky to measure and all sorts of differences can arise between neighbouring gauges due to tiny differences including height above ground.  Height of rainfall instrumentation above the ground is especially important in turbulent gusty weather when rain can appear from all angles and get swept in all directions.  The VP2 rain collector is set at the standard thermometer height of 1.2m as it is attached as a single unit.  This means it might have been more susceptible to collecting less rainfall during the very stormy December weather due to being more exposed to the wind at over 1m.  In less gusty conditions the two gauges might agree far more, so a re-calibration at this stage would make our measurements of our more commonly occurring vertically descending rainfall less accurate!

Rainfall comparisons are on-going but evidence remains insufficient as yet to attempt a rushed re-calibration of the VP2 rain gauge immediately as it appears to be working well or, indeed, to go through any awkward resetting of the rain gauge to a location at ground level.  The conclusion is that …

  • Our rainfall measurements and trends in rainfall rate match our sister station in Reigate and rainfall rate follows the nearest official weather station at Kenley precisely enough to show broadly accurate rainfall measurements.
  • The VP2 rain gauge was calibrated last year and currently measures rainfall at 1.2m above the ground.
  • As more automatic rain gauges become the norm the height issue will be standardized.
  • Our standard manual rain gauge measures rainfall at ground level.
  • The two measure rainfall correctly but illustrate the vagaries of data recording and the caution we must use when quoting and comparing figures about the weather from different weather stations, even ones that are close together.
  • We will continue to use un-adjusted VP2 tipping bucket rain gauge readings for the time being but quote our standard rain gauge readings to use as comparison when appropriate and available.
  • A full calibration will be conducted in due course 🙂

Please let me know if you have had similar experience of rainfall data collection.

December will, of course be remembered for some extraordinary stormy weather, but with some spectacularly crystal sunny days in between and few frosts and no snow at all.  Previous posts cover flooding and some individuals storm events but here is a round-up of December pics from around Reigate as they happened.  Comparisons with last December will be analysed with the 2013 summary in due course.

Please check out our new “sister” article on Gatwick flood and pollution control here 

The River Mole is a modest lowland river in SE England that rises near Rusper in the wooded hilly heartland of the Surrey / Sussex Weald, near Crawley and Horsham. The Mole and its tributaries flow broadly north, through Crawley, around and under Gatwick airport, before meandering through mostly rural countryside to Dorking. The Mole Gap at Dorking is one of the defining geomorphological features of the river as it flows through a gap in the North Downs chalk ridge to reach the London Basin where it finally meets the Thames at Hampton Court.  The length of the river is approx 80km, excluding tributaries. The Mole has a catchment of 512km 2 and a mean discharge in its middle course at Sidlow of 2.4 cumecs and a mean discharge at its confluence with the Thames at Esher of 6 cumecs (cubic metres per second).  The long term flow graphs below show how these mean flows can be dwarfed by occasional discharges exceeding 50 cumecs.  The worst flooding in recent years was in Autumn 2000 when a 1:300 year storm caused the Mole to flood 500+ houses, leading to the evacuation of over 200 residents and the closure of the M25. The worst recorded flooding in the River Mole was in 1968 when a rare 1:200 year storm caused rainfall to exceed 100mm in 24 hours and a staggering discharge over 240 cumecs in the Lower Mole (see extract below).  For comparison, the average discharge of the River Thames in London is 65 cumecs and the average September rainfall is 25mm.

05-01-2014 14-50-47

A kmz Google Earth file showing the locations discussed can be downloaded here

So, the River Mole is usually well behaved but, like most rivers, it has rare moments of significant flooding.  Planning, developers and residents should take these rare, but arguably increasingly frequent, extreme events into consideration.  Unfortunately, like many SE rivers, the River Mole is largely ignored and out of the public imagination until it behaves “badly”.  Flooding is, of course, entirely natural river behaviour. This post outlines the causes of flooding in the River Mole catchment, some of the impacts from the recent December/January 2013 storms (worst in 45 years) and the management schemes designed to reduce the flood risk.  It also argues that living with, valuing and understanding our rivers, allowing them to behave naturally and getting residents involved with their restoration is the way forward for managing the River Mole.

Impacts of River Mole flooding

Despite being a small river, flooding along the River Mole has the capacity to cause significant damage and disruption on a local and even national scale due to location of key infrastructure and communications located within the catchment, most notably, Gatwick airport, East Surrey Hospital, the M25 and M23 and the London-Brighton railway. Specific impacts of the winter 2013-14 storms and floods on the River Mole included:

  • Gatwick airport: power failure from flooding causing delays with luggage handling (see below for more on airport flood vulnerability) on Christmas Eve; 100 flights delayed or cancelled; thousands of travellers were left stranded or abandoned as rail connections were disrupted as well.
  • Power cuts across the county e.g.100 homes in Merstham and Sidlow left without power for 3 days. UK Power Networks raised compensation from £27 to £75 for customers without power for 48-60 hrs.
  • Leatherhead crematorium: closed due to flooding
  • Burford Bridge hotel, Dorking and Ye Olde Six Bells in Horley : amongst other commercial properties were submerged by flooding and closed for extended periods.
  • Damage to planes in Redhill aerodrome from wind and floods (71 mph winds measured on N Downs, Kenley)
  • Numerous roads and rail links including two closures of the A24 at Mickleham, A217, A23 around Horley and Salfords and downstream in Leatehrhead
  • Flanchford Bridge, Reigate, damaged by flood waters
  • Flooding of hundreds of residential properties: 40 homes in Fetcham (esp Cannon Grove) under water throughout Christmas
  • Morrisons car park in Reigate flooded
  • Damage to telephone land-line communications in Brockham
  • Landslide caused embankment to collapse Dorking to Horsham railway line: limited service and month to repair
  • River Mole rose to highest level in a generation: cars swept away (e.g. taxi in photo above was swept away, just after 3 people safely rescued) and people stranded as cars stalled in deep flood waters; dramatic rescues throughout the area including 27 guests at Burford Bridge Hotel

A severe flood warning was issued by the Environment Agency for one stretch of the Mole during this period, meaning there was a threat to life.  It should not be forgotten that people who have their homes flooded can suffer seriously from stress and a feeling of dislocation from their property which can be permanent.  This excludes consideration of the significant financial cost of flooding.

The 2012-2016 Surrey Local Flood Risk Management Plan states that “Surrey is a county with a high risk of flooding” and that “we cannot stop flooding in Surrey”. So… why does the Mole occasionally flood so badly and what is being done about it?

Before we investigate the causes and management of floods it is important to understand that there are 4 types of flooding, though the main concern for this post is fluvial (river) flooding. Not all floods you see are to do with rivers… so here are the 4 main “causes of flooding” in outline:

Types of Flooding in the Mole Catchment

1. Fluvial floods: rivers naturally burst their banks and water naturally spills across flood plains when intensive rainfall (acute fluvial flooding) or prolonged periods of rainfall (chronic flood episodes) cause river levels to rise beyond bankfull stage.  Rivers respond differently to rainfall events.  The River Mole responds rapidly to rainfall and is a “flashy” river.  This means that river levels rise quickly after rainfall.  River flooding along the Mole is usually across rural and agricultural flood plains. However, urban development around Crawley, Gatwick, Horley, Reigate, Dorking, Leatherhead and Cobham have substantial numbers of properties at risk from river floods associated with the Mole, as have tributaries such as Burstow Brook and Gatwick Stream.

2. Surface water flooding / pluvial flooding: this type of flooding occurs when the drainage or sewer capacity is overwhelmed by the volume of rainfall.  This happened in Morrisons car park, Reigate after the heavy rain 23-24 Dec 2013.  Some 46,500 properties are at risk from this type of flooding in Surrey during a 1:200 year storm event. Reigate and Redhill are both identified in the list of 5 highest risk areas for surface water flooding in Surrey.

3. Groundwater flooding: Low lying areas, usually in more rural locations with permeable bedrock, are prone to the water table rising in prolonged wet weather causing surface flooding.  This can continue to occur long after rainfall has ceased, sometimes flooding roads and railways.  It is most common on permeable chalk strata, often where it meets clay geology, along the North Downs. The A25 between Reigate and Dorking is prone to such groundwater flooding as the water table rises on the scarp slope of the Downs and flows across the road, often at the boundary between chalk and gault clay.

4. Reservoir / dam breach flooding: The record of UK reservoir safety is excellent but the potential failure of any dam wall, even ones that appear modest in scale, would cause significant acute flash flooding down river valleys.  There are 8 dams in Surrey measured as High Risk category that would cause significant floods if they breached e.g. Mytchett Lake embankment. There are several dams in the Mole catchment which are low risk and smaller in scale, for example Tilgate Lake in Crawley. Legislation requires any such dams to be regularly monitored for safety, so the risk of breaching is very low in the Mole Valley, but nevertheless, part of flood risk assessment in this area.

Factors affecting flood risk on the River Mole

There are several factors that make the River Mole more or less prone to flooding.  Some are static factors that are relatively fixed, such as rock type.  Other factors are more changeable such as human developments and even rainfall intensity and amount.  The causes of floods are physical (rainfall, geology, relief etc) but human activities have a great deal of influence on the nature of flooding and the vulnerability of communities to flood hazards.

Geology: the solid bedrock of the River Mole drainage basin is predominantly impermeable Wealden Clays and Greensand. More than 60% of the solid geology is “low permeability” which means that rainfall is unable to sink into the rocks and quickly runs-off into streams causing the river to respond rapidly to rainfall events. This rapid response to rainfall is known as “flashy” and makes the Upper Mole rise within just a few hours after rainfall.

A key measurement on hydrographs is “lag time”: the time between peak rainfall and peak discharge.  In the Upper Mole the lag time can be measured in hours.  The flood peak then takes some 24 hours to reach the Lower Mole, time to get flood warnings and protection in place.

The chalk ridge of the North Downs forms approximately 30% of the catchment area and allows water to percolate into the chalk and create an aquifer.  The River Mole passes over chalk through the Mole Gap.  At this point, discharge is usually lost into the permeable chalk through 25 swallow holes . The gradient of the river increases through the Mole Gap as it fall some 15m between Brockham and Leatherhead.  (As an aside: the chalk aquifer supplies much of the water supply for the area.  The level of the aquifer is monitored by abstracting water companies at various boreholes, one recently drilled on Reigate Heath. The quality of groundwater abstracted locally from the chalk aquifer is poor and requires treatment before entering the water supply.)

A comparison of the long term Mole hydrograph with the Kennet shows how flashy the River Mole is in comparison with this similarly sized river basin, due partly to it’s geology.

Shape of catchment: the shape of the Mole drainage basin is like a teardrop: with the Upper Mole (sources and tributaries) occupying a larger and more circular basin shape which is then “squeezed” through the Mole Gap into a more elongated basin shape in the Lower Mole.  A circular lowland river basin can be said to collect more rainfall and deliver it more rapidly to rivers than a more elongated shape (though elongated catchments are common in mountainous regions and these respond rapidly to rainfall due to steep relief).  The average distance “raindrops” have to travel to reach a stream is less in a circular catchment, especially one with numerous streams or, in the case of the Mole catchment, a high density of culverts, ditches and drains.  This reduced distance encourages a rapid flood response.  The result is that the larger area of the Upper Mole catchment south of the Downs ridge, is where much of the input from precipitation is derived. It also means that rainfall, flood protection and urban development in the Upper Mole catchment to a great extent controls discharge and flooding downstream into the Lower Mole, which is where the majority of properties at risk from flooding are located on the Mole flood plain. The secret of flood control in the whole Mole catchment arguably therefore largely lies in managing hydrology and development in the Upper Mole basin.

Relief: Whilst the SE is associated with low relief, the Mole catchment has a significant maximum elevation of 265m at Leith Hill, the highest hill in the SE. The North Downs from Ranmore and Box Hill to Reigate Hill are elevated by over 100m above the valleys below.  These elevations are sufficient to cause some local enhancement of precipitation through orographic uplift.  Local rainfall levels can vary considerably as measured by weather stations found here.

Precipitation: The annual average rainfall for the River Mole is comparatively modest at around 700-800mm/pa. Excluding evapotranspiration, much of this precipitation will, eventually, find it’s way into the river due to the impermeable geology (discounting a proportion of abstracted groundwater that might be removed from the river basin).  The rainfall during the 24 hours 23-24 Dec was the wettest for 35 years, yielding over 71mm in parts of the Mole catchment.  Each millimetre of rainfall equals 1litre of rainfall per square metre.  In this rainfall event an average of approx 50mm of rainfall fell over the catchment in 24 hours, locally some places experienced 70mm in 24 hours, which is a high rainfall intensity.  This would be equivalent to some 260 million buckets of water falling onto the River Mole drainage basin!  Extremely intense rainfall events cause water to flow overland into rivers very rapidly, especially if the ground is saturated.  Such rainfall events are likely to increase with climate change and our sheltered area of Surrey is not immune from these.

Effluent discharge: a growing urban population produces more effluent discharge which is treated in numerous water treatment plants and then passed into the River Mole.  There is some additional input of effluent from other catchments which has added to the discharge of the River Mole. Interestingly, Gatwick airport uses about 1 million m3 of water every year (dcreasing with efficiencies made) and this water, presumably makes its way into the river as treated discharge during the course of the year.  The source of water for LGW is not known.

Urban growth: the growth of towns like Crawley, Horsham and Horley and the expansion of Gatwick airport terminals has increased surface runoff by expanding impermeable surfaces that allows precipitation to transfer to the river more rapidly.  The removal of vegetation / deforestation during development also reduces interception storage and reduces the amount of water abstracted naturally by trees.

The effect of urbanisation is to enhance the already naturally flashy nature of the River Mole due to impermeable geology and make it more prone to flooding.  Gatwick airport, Manor Royal and Crawley located in the upper reaches of the River Mole catchment have had an impact on flooding downstream by encouraging the runoff of water into concrete lined channels and culverts designed to remove water efficiently.  This has the effect of transferring flood peaks downstream more quickly and thus potentially increasing the steep rising limb of hydrographs and flood peaks further downstream. LGW has an obligation and an ongoing policy “to incorporate flood risk management in all new developments and to avoid inappropriate development in areas of flood risk “(2011 water quality flood management action plan).  This would include any development of a second runway which would, incidentally, involve the placing of the River Mole in a longer tunnel.

Changing land use: Land use goes beyond the distinction of simply urban or rural.  In common with other SE catchments, the Mole basin has expanding land uses such as golf courses, industrial parks like Manor Royal and numerous airport car parks.  These land uses encourage rapid runoff into culverts and concrete lined channels decreasing the lag time of rainfall into streams.  These channels transfer water to the lower Mole more quickly and can have the effect of raising flood peaks downstream.

Blocked drains: blocked drains and culverts are a local cause of flooding, albeit one requiring the rainfall to make it evident.  Anecdotal evidence suggests that blocked drains caused much of the flooding in Smallfield, near Horley, during the 2013 December/Jan storms. Photos courtesy of Simon Rushby, who commented that “The Wheeler’s Lane flood was caused mainly by a blocked drain, which was eventually cleared last night after 19 days of continuous flood, which was passable in the area in the photo, but impassable further up where it was a foot deep in places. The water disappeared overnight.”

Saturated ground:  though not strictly a “cause” beyond that of precipitation, previously saturated or frozen ground or snowmelt can greatly exacerbate later precipitation episodes to respond more rapidly, flowing overland directly into drains, streams and rivers, and therefore cause more rapid increase in levels on the Mole than would normally be expected.  This was experienced in relatively modest rain events on already saturated ground on 17-18 Jan 2013. Modest rainfall totals across the catchment of 4-10mm characterised the Wednesday and Thursday leading up to the heavier rainfall on Fri17 – Sat18 Jan that totalled 30-40mm in some locations.  The River Mole levels responded unusually rapidly to this rain because the ground was saturated and unable to infiltrate the additional rainfall, rendering more surfaces impermeable.  72 hour rainfall totals measured at Charlwood for this period amounted to 59mm but this is comparatively modest compared to the 70mm in <12 hours overnight that caused the biggest floods back in 23-24 Dec.  Nevertheless, it is the response of the Mole that was rapid: just 10 hours from peak rainfall to peak discharge at Horley.  Put this another way: raindrops falling during that storm anywhere in the catchment over Rusper, Horsham, Crawley, Gatiwkc and Horley took about 10 hours to hit the ground, run into the river and thence flow to the gauging station at Horley where levels rose to an all-time record high on the Environment Agency website (pictured).  This episode of flooding was short lived, causing evening traffic chaos on the Friday as bridges closed and surface flooding slowed commuters.  Rainfalls similar to that of 23-24 Dec would have been a lot more serious on this occasion due to saturation.

Flood management in the Upper Mole Catchment

The River Mole catchment is not, at first glance, heavily protected from floods.  Nevertheless, Environment Agency Flood Alerts and Warnings cover most stretches of the river and there are examples of almost every type of flood management in this modest drainage basin.

Hard flood engineering defences: The Upper Mole Flood Alleviation Scheme is a £15 million Environment Agency project designed to reduce flooding along the River Mole, in particular in urban areas in the upper reaches prone to flooding such as Crawley, Horley and Gatwick airport.  Various schemes are being built to attenuate / delay the flood peak in the upper reaches including upgrading of the flood retention reservoir at Clay Lake and the construction of a higher dam wall at Tilgate Lake (concrete wall construction completed now at Tilagte: with an additional 2.5 metres of dam wall to cope with a 1:1000 year flood).  The Worth Farm flood retention pond on the side of the M23 on the outskirts of Crawley is also designed to delay flood peaks entering the town by filling up a detention reservoir.

Compared to other airports in the UK, Gatwick is at risk from relatively frequent 1:50 year flood events.  This is because it has been built in a low lying area in the flood plain of three streams: Crawters Brook, River Mole and Gatwick Stream.  There is almost no extensive modern flood protection in place for LGW, yet.  Gatwick Stream was culverted in a large drain underneath the South Terminal during construction in 1957.  The River Mole was similarly placed in a big pipe under the runway.  On EA flood risk maps (below) parts of both North and South Terminals are shown to be at risk from medium and high annual flood risk (>1% chance of annual flooding). Compared with other major airports, Gatwick has been built in a relatively soggy flood plain!

Numerous flood ponds (balancing ponds) around the perimeter of the taxi and runways are designed to attenuate the flow of water off the hard surfaces so that flood peaks downstream of Gatwick are delayed and reduced to acceptable levels in high rainfall events.  The water in the ponds is sometimes contaminated with pollutants (such a cadmium, used in plane de-icing, and aviation fuel) and must therefore be treated to a required level that is “not injurious to fish” before disharge is permitted into the Mole river itself.  However, these ponds do not prevent LGW itself from floods from “upstream” sources. During the Dec23-24 flooding the luggage system failed due to power cuts caused by flooding of the Gatwick Stream and thousands of passengers were stranded by more than 100 cancelled or delayed flights.  The airport remains vulnerable to 1:50 year flood events when compared to the 1:1000* year level of protection afforded by river flood defenses for London from the Thames Barrier, for example.

FULL detailed and updated post on FLOOD MANAGEMENT WITHIN GATWICK AIRPORT here

So Gatwick is investing in an £8 million Gatwick Stream Flood Attenuation Scheme and working closely with the Environment Agency to reduce flood risk. This latest scheme is a detention pond with iron flood gates built between Manor Royal and the A23. The pond is designed to store over 180,000 cumecs of water from the Gatwick Stream at high flow periods. During normal flow the pond will be empty and the trees and habitats and footpaths built into the design will encourage both human and wildlife visitors on completion.  The project is designed to “double the level of flood protection” for Gatwick Airport from 1:50 year flood risk to defences able to protect against a 1:100 year flood risk event.  It is under way. Gatwick is also investing £4million as a contribution to the Upper Mole Flood Alleviation scheme mentioned previously.  LGW is therefore taking serious steps to protect the catchment from flooding and, from ongoing policy documents it is clear that LGW are endeavouring to be good neighbours with the local community, stating clearly that they will incorporate flood risk management in all new developments and avoid inappropriate development in areas of flood risk (PPS25; water quality management plan 2011).  LGW is also involved with sustainable ongoing flood management through groups such as Gatwick Greenspace, for example.

Further downstream in the Lower Mole there are hard engineering structures designed to reduce flooding such as weirs and bank defences at Molesey, built after the 1:200 year floods of 1968 and upgraded since. There are some unverified accounts that the recent flooding in December 2013 in the middle parts of the Mole near Leatherhead may have been exacerbated by these flood defences being put into operation downstream at Molesey, specifically a barrage or gate preventing discharge flowing into the Thames. (see comments from contributors below).  Here is a reply from the Environment Agency to questions I posed to the recent flooding and rumours about whether the Lower Mole flood scheme “caused” flooding in Leatherhead.  The reply supplies compelling evidence that it did not, due to the steep gradient between the two locations making any “backing up” extremely unlikely. (see footer)

The Lower Mole Flood Alleviation Scheme (FAS) extends from Hersham through Esher and Molesey to the confluence with the River Thames near Hampton Court. Operation of the FAS has no adverse impact on flood levels through Leatherhead and Fetcham. The Lower Mole FAS starts approximately 10 kilometres downstream of Leatherhead, and between Leatherhead and the FAS the Mole follows a series of meanders under the M25 and past Cobham. Over the course of this length, the bed level of the River Mole drops by over 20 metres. The Lower Mole FAS is predominantly comprised of a man-made diversion channel, along the course of the old river Ember, which has capacity to convey large flood flows through Molesey without causing property flooding. Along the course of the diversion channel are a series of tilting and radial gates, which are in place to regulate water levels during times of normal river flow. These gates are lowered during times of flooding to allow flow to pass safely along the diversion channel without causing river levels to rise upstream in areas of South Hersham, where raised walls and banks provide protection to local properties.

Development of the FAS was instigated after significant flooding in Esher and Molesey in 1968. During the recent flood event, when flows in many parts of the Mole catchment were comparable to those seen in 1968, the FAS was operated in accordance with established procedures and was able to pass the flows through Esher and Molesey, preventing flooding to many thousands of properties in these areas. The operation of the scheme is based on water levels upstream of the main gates and there are no circumstances where any decision would have been taken to operate the scheme at the expense of other communities further afield. Upstream of the first sluice, the River Mole occupies a wide floodplain, with raised banks and defences completing the FAS at the very edges of the floodplain. This means that changes in flow through the FAS result in only minor changes in level across this large area upstream during a flood. The 20 metre drop in elevation along the River Mole between Leatherhead and the FAS, occurring over a distance of 10 kilometres, means that it is not possible for minor variations in water level in the location of the scheme, to influence flood levels in Leatherhead and Fetcham. Flooding in these locations was an unfortunate result of the unprecedented flow in the River Mole, which, as your website posting points out, was comparable only to the flooding in 1968.

Since the Lower Mole FAS has been operating successfully for many years and pre-dates many of the newer schemes you have researched in your posting, we do not currently have information available on it on our website. Given the interest in the scheme over the course of the recent flood event, this is something that we shall consider, should resources allow it, in the future. Later in February, subject to weather conditions, we plan on operating structures to temporarily lower levels through the length of the Lower Model FAS. Lowering the ‘normal’ water levels temporarily will allow us to inspect all elements of the FAS following its operation during the recent flood events. We will use the information obtained during this inspection to plan and prioritise our maintenance works, to ensure the FAS continues in future to provide the high level of flood protection it provided to local communities over recent weeks.

We are developing a project for potential flood risk management options along the Middle River Mole, taking into consideration many of those local areas flooded this winter.  As these plans develop, there will be opportunity for the public to contribute to their development through formal and informal consultation. We would encourage you to get involved as these opportunities arise.

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Sustainable approaches to flood management: hard engineering is expensive and **arguably creates more problems downstream** and is expensive to maintain.  More sustainable modern approaches to flood management involve restoring natural wetland areas to absorb flood peaks naturally in habitat and wildlife rich ecosystems along the course of rivers.  Such sustainable approaches to flood management exist in the Upper Mole catchment and include Grattons Park in Crawley and the Moors Project in Redhill / Nutfield Marshes.  Although different in character the schemes share the common purpose of putting in place more natural, meandering water courses with vegetated banks and wetlands and flood plains which can be occupied during times of flood.  This delays the surge of water downstream and therefore reduces the risk of more damaging floods in urban areas.  The Moors Project is managed by Surrey Wildlife Trust with regular key conservation work by Reigate Area Conservation Volunteers and Reigate Grammar School students.  The Mole Gap, near Westhumble, has also had some restorative work reinstating an old meander, though this has since silted up.  There are numerous other possibilities along the river to restore more wetland habitats, reinstate lost meanders, increase natural sinuosity, naturalize banks and recreate water meadows along the length of the Mole flood plain.

**though note that flooding on RMole at Leatherhead and Fetcham 2013-2014 was not caused by any of the Lower Mole Flood Alleviation Schemes (FAS) at Molesey or downstream to Cobham or Esher.  The fall of the river between Leatherhead and the FAS at Molesey is over 20 metres in 10km and this gradient makes it impossible for any impact of flood management downstream to “back-up” to the Mole reach at Leatherhead and cause any flooding there.

Flood warnings and alerts: Warning and educating the public is a key to successful flood management.  The Environment Agency have identified areas at risk from all 4 types of flooding in the Mole catchment.  Local Authorities have detailed plans in place to deal with floods of different scales.  EA flood zones can be viewed on detailed maps here. The EA also issue warnings and alerts.

Finally, councils and planners have adopted the policy of keeping development off the flood plain along the entire length of the River Mole.  

In conclusion, the River Mole can cause significant impacts from flooding on a local scale and these are likely to get worse with predicted climate change and urban development and population growth in the area.  There is also key infrastructure across the catchment which can be impacted severely by relatively high frequency flooding of 1:50 year probability, such as Gatwick airport and rail routes like the Gatwick Express.

New “hard” engineering schemes being built in the catchment, such as at M23 Worth Farm and the Gatwick Stream flood attenuation project which address some of the increased flood risk and double protection to defend against 1:100 year flood events.  However, the future, in the view of this writer, is to adopt a more holistic approach to managing the River Mole catchment which involves all stake holders and local residents. This might include greater use of sustainable urban designs and approaches to flood management that use the restoration of wetlands and ecologically sensitive management of the whole catchment including careful woodland management, like coppicing, and the restoration of naturally meandering river courses to absorb any increased runoff.  A continued policy to restrict development on flood plains is also essential.  The resultant areas of beautiful open space, rich with interesting wildlife and diverse habitats can then be open to the public for recreation and exercise during dry periods.  A continuous footpath along the entire length of the River Mole from source to “mouth” could be an achievable target with which to measure the progress of such a holistic approach.

Update: 20/10/2015: Gatwick Greenspace and Gatwick airport manage the river through the NW corner of the airport. This interpretative signage is being designed to go into the stretch of artificial channel.  This shows how partnerships such as this can benefit rivers and communities alike.


Gatwick Greenspace partnership working to improve River Mole ecology


further reading and references

EXCELLENT video on sustainable flood management



uppermole flood alleviation scheme







RBBC Flood Plan 2013 v0.8 – UNRESTRICTED – Public Version_tcm9-52048

note: this post is for educational purposes; all figures and data are posted in good faith.  Feedback and updates welcome to keep things as accurate as possible! 

News! This post formed the basis of an interview that contributed to a programme on River Mole flooding for Radio4 File on Four to be broadcast in February 2014.

update links:

slow the flow

excellent blog updating situation after Cumbria floods 2015

notes: Why does flooding along the Thames at Hampton Court not back-up the R Mole and cause flooding in Cobham or Esher?  Environment Agency: +”The downstream-most part of the Lower Mole scheme includes a series of water level control structures. These have sufficient drop in level across them to prevent impacts on the Thames ‘backing-up’ upstream. Furthermore, the timing of the flooding on the River Thames was such that the River Mole was back to relatively normal flow conditions when the worst of the flooding was occurring along the Thames valley.”

+Contains Environment Agency information © Environment Agency and database right

No, not a record low pressure this one but the stormy weather continues for the UK with another deep area of low pressure sweeping up past the NW of Scotland tomorrow, Friday.  This one, called Anne, has violent storm force westerly winds out in the Atlantic building high waves and matching high tides, causing unusual storm surge conditions for the west coast. It is the orientation and track of this storm that appears to be causing most trouble: spot those isobars directed straight at the UK and building high seas with those high tides.  The distance which wind travels over the sea is called FETCH and the longer the fetch the greater the possible wave height.  Note also the waves and wind that build in the Channel.

There are currently 17 severe flood warnings from the Environment Agency for the west coast of Wales and SW England.  The River Severn estuary is also at risk as as it faces the SW winds, funnels tides and has high river flows all to contend with.

Envt Agency are even warning people to evacuate if possible from vulnerable locations on West coast.  N Ireland also adopting this too.

For Reigate and the SE (inland) heavy overnight rain will pause for a while in the morning only to resume as winds increase to gusts possibly 30-40mph, gusting higher on hills, and with possible heavy showery rain.  Rain totals for Fri-Sat 48 hours could amount to 30-40mm, but more likely 10-20mm for Reigate and Mole Valley.

02-01-2014 22-10-19

rain totals fri – sat

Is there any sign of improvement? Met experts watch the high atmosphere for some long range forecasting. In particular, the stratospheric Polar Vortex is a a possible indicator of how the tropospheric jetstream might be acting in weeks to come.  The stormy weather we have been experiencing has been “caused” by an extremely powerful polar vortex: a great contrast in temperature difference between high and low latitudes sets up high winds in the stratosphere which act like a belt to hold in cold air to the Pole.  The North Pole stratosphere is extremely cold this winter and this has encouraged a powerful polar vortex.

02-01-2014 23-32-17


You can see from the chart below that the vortex is forecast to change shape: split or squeeze, meteorologists are watching for this to tell them if the jetstream will weaken, and there are signs that it will in mid-Jan.  This may also have the effect of allowing polar leaks of cool air to reach the UK: so maybe less stormy but a tad cooler. Check these guys for further details:

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Finally, the Express is unusually conservative with this article: the wind speeds are too low!

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