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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.

28-12-2013 08-18-43

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

The popularly named St Jude storm (officially named Christian) of 27-28 October 2013 was the most severe to hit southern Britain for over a decade. Whilst it was less powerful than the 1987 Autumn storm, St Jude lived up to its forecasted strength and caused an estimated £1 billion worth of damage and losses across the southern half of the UK.  Here is a round up of  causes and some impacts of this severe mid-latitude storm.  Locally, @RGSweather covered the storm continuously overnight, providing updates and advisories on twitter as things developed minute by minute.  This is a summary of causes, what happened and what we have learned from this storm…


The “ingredients” for the birth of St Jude include…

1. A big warm soup:

A warm Atlantic Ocean, some 3-5c warmer than 30-year average, acted as a perfect birthing pool and nursery for StJude. The warm sea surface temperatures provided plenty of extra water-vapour, heat energy and lift ready for stirring up a potentially big storm.

Warmer than usual Atlantic

Warmer than usual Atlantic

2. Add some extreme pressure!

The North Atlantic Oscillation is a measure of the difference in pressure between Iceland and the Azores and it was in increasingly positive phase during the storm build-up.  Whilst this is more of a measure than a cause, a positive phase nevertheless indicates LOWER than average pressure over Iceland and higher pressure over the Azores, which usually indicates a strong zonal west to east flowing jetstream across the Atlantic and fast moving weather with the potential for plenty of low pressure systems from the west.  This rapid west-east flowing weather was a necessary ingredient in the set-up for St Jude.

29-10-2013 09-44-40

bubble bubble toil and trouble

bubble bubble toil and trouble

3. Throw in Mr Muscle

A very strong jetstream: blowing at 240mph across the Atlantic towards the UK acted as the main ingredient in the birth of storm St Jude. The jetstream directs weather on the ground.  The jet over the Atlantic in the days leading up to St Jude was extremely strong and blowing directly across UK latitudes. The jetstream is a product of the temperature and pressure contrast between cool Polar air to the north and warm Tropical air to the south.  The temperature difference between polar air and tropical air is particularly marked at this time of year: with the tropics still very warm, while the Polar ice sheets seeing a marked fall-off in temperatures with their attendant air masses.  This builds steep pressure gradients and a strong jet. The jet is also a key factor in creating and guiding LOW and HIGH pressure systems on the surface.  Like a dog on a lead, St Jude was dragged across the Atlantic by it’s angry owner, the jetstream. At times during the passage of the storm wind speeds above the Channel exceeded 180mph.

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4. Whisk up some bad parents!

So all the ingredients are ready, but no storm yet!? The mother of St Jude was a deep robust LOW south of Greenland.  This formidable storm produced hurricane force polar winds directed from the NW in the days before St Jude was even a twinkle in her eye.  The father was a weakening and slow moving tropical storm called Lorenzo.  He had spent the week meandering slowly in the Mid-Atlantic but Lorenzo, despite his old age. still arrived with plenty of hot air from the Tropics.  Their respective air masses collided in the mid-Atlantic some 1000 miles off the SW coast of the UK and, encouraged upwards by the jetstream, they produced their only child, St Jude!

baby bomb is born

baby bomb is born

5. Give it a stir!

Cyclogensis is the process of rapid growth of a baby storm in the mid-latitudes: due to converging warm and tropical air and, encouraged by the jetstream, air rapidly lifted off the surface and Jude’s central pressure, as predicted by the UKMO fell steeply.  This so-called meteorological BOMB exploded (or perhaps imploded, as air was dragged into the low pressure causing all that wind) formed a deep wave depression LOW that charged across S Wales and England in less than 12 hours.  It made a rapid exit from the UK via the Wash and then continued to deepen across the North Sea before smashing into Scandinavia. Arguably the storm did not deepen over the UK quite as spectacularly as some models forecast, but nevertheless, the track and winds were much as predicted and the storm went on to cause significant damage. The chart below shows the pressure falling at a NOAA weather buoy in the development zone of St Jude some 300 miles SW of Cornwall. Note the INCREASE in pressure before the sudden drop-off.  This is entirely in line with cyclogenesis: pressure builds ahead of rapidly developing warm fronts as isobars are buckled up ahead of the storm.  This is popularly known as the “calm before the storm” where winds die down before the maelstrom hits.  This was marked across the country on Sunday evening. At that stage people wondered “what storm?”.

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calm before storm

The satellite picture below of St Jude still in development phase shows the characteristic wave form kink of a rapidly developing storm.

6. Watch out for that sting in the tale!

More immediate “causes” of storm damage from StJude, making it extra-powerful, include the relatively newly discovered weather phenomenon called a stingjet wind.  These are isolated fierce gusts of wind experienced behind a departing deep area of low pressure, often behind a cold front. Oddly, they tend to occur as conditions more widely are improving. In very tight depressions descending air from the upper troposphere pushes gusts to the surface and, like a giant invisible hand, these can, in a careless whim, push down whole swathes of mature forest, take rooves off houses, rip down scaffolding, push over cranes, roll over double decker buses and blow trampolines clear out of your garden!  The sting in the tale is an appropriate analogy, as the curl of winds descending round from the NW of the departing LOW are frequently the last hurrah for these storms. **NOTE: Stingjet NOW confirmed!**

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The storm was perfectly forecast by the UKMO up to a week before the event.  It was always going to be hit-and-miss up to the last minute, not least with inevitable media-hype; but the consequences of playing this down would have been potentially disastrous.  Overall, it was well predicted and people were warned effectively days beforehand.  Whether they prepared effectively or took warnings seriously is another matter.

Below is a slide-show of synoptic charts showing the progress of the storm.  Note that the central region of LOW pressure experienced light winds like a hurricane “eye” (but not as extreme in contrast!). Below this is a brief list of impacts.

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The highest winds were largely restricted to places south of the M4, excluding Wales which had 80mph+ winds in the run-up to the storm on Sunday but calmed down for a time overnight Monday as the centre of the LOW passed over the Severn Estuary and S Wales. Across S and SE England wind speeds were widely 40-60mph and 70-80mph+ on the south coast.  Remember that average wind speed across an area can seem surprisingly low during a storm.  The average wind speed across Reigate from 5-8am during the height of the storm was only 16mph! It is, of course, the random gusts that cause the most damage. The highest gust in Reigate was 48mph at 6:20am on Monday morning.  The highest official max gust was 99mph on the Needles, Isle of Wight, other notable wind speeds were Heathrow 70mph and 62mph at Redhill aerodrome.  Reigate, as predicted by @RGSweather, was spared the worst as our max wind gust was 48mph. Our location in Surrey is away from the coast and locally the town is low down in a vale with low wooded hills to the south, Priory Park, that shelters the town from S or SW gales such as the ones StJude produced during the worst of the storm.  More exposed parts of our local area certainly experienced higher wind speeds.  Rainfall was intense for a period of time and caused localised flooding. In Reigate 25mm of rain fell overnight, which is more than for the whole month of July or, August, in less than 10 hours!  St Jude crossed at night and only keen meteorologists were awake to see it go through.  If this had been a daytime storm, impacts listed below are likely to have been worse with more people getting out and about, or attempting to.

Here are some of the impacts from St Jude in the UK:

  • 3 people were killed by falling trees, 1 boy was very sadly swept out in rough seas on s coast in the lead-up to the storm
  • 147 flood alerts, 17 flood warnings issued by Environment Agency, including our own River Mole
  • In the English Channel and approaches there were 20-30 foot waves and storm force winds.
  • power cuts in SE across 270,000 homes, some for 2 days
  • 5 train companies cancelled all their trains in SE
  • 130 flights from Heathrow cancelled, delays at Gatwick
  • Port of Dover closed, horrifying stories of Channel ferry crossings
  • crane collapsed onto Cabinet office
  • Major bridges were shut in high winds including Severn Bridge and QE2.
  • Dungeness B nuclear powerstation had a power cut in 90mph winds and had to shut down both reactors
  • In Suffolk a double decker bus was rolled over by a gust of wind
  • Clacton pier helter skelter was blown down
  • 1000’s of trees blocked roads and caused travel delays and closures
  • other impacts across the SE here
  • here
  • and costs here (sorry, only one I could find 🙂

Emergency service response was predictably very effective in dealing with thousands of calls.  Public were advised not to call 999 for tree falls, and only call in real emergencies.

The storm went on to cause significant damage and some 15 people in total died across the UK, N France, Netherlands, Belgium, Germany and Scandinavia.  It deepened across the North Sea and became more intense with stronger winds, with 120mph reputedly being recorded in Denmark (

So… hundreds of trees down, some scaffolding torn apart, helter-skelters blown away and some very unfortunate people killed out in the storm… plus £1billion lost through days off and travel chaos.  Inevitably, forecasters are stuck between over-blowing storms and under-playing them so as not to cause panic.  Personally, I think they got this spot on from the start, so congratulations UKMO! The fine balancing act between under-playing and exaggerating potentially serious events is not an enviable task for forecasters.  Despite being very powerful, computer forecast models were still flip-flopping 24 hours ahead with the exact track and severity of this storm.  It was an on-then-off affair right down to the line!  For the future it is worth raising awareness in the public that, despite computers producing forecasts (and who trusts them!?), predicting the weather is still based on the judgement of experts at the UKMO and elsewhere. (photo of clouds over Channel below left was taken by an airbus pilot on his way over Channel during storm). other resources for this storm:

Which was bigger, 87 or 2013?

Finally, the old chestnut “was it a hurricane?”… Despite getting winds exceeding hurricane force ((74mph+) UK storms cannot be classified as hurricanes. Hurricanes are tropical weather phenomenon and do not form in the Mid-Atlantic at our latitude, neither do they ever get to the UK.  We may experience hurricane force winds in extreme low pressure systems which are confusingly also called cyclones, although they are NOT tropical cyclones!  At our location, on this side of the Atlantic and this far north, we have never experienced a true-hurricane.  Even the ’87 storm was not technically a hurricane despite having even stronger winds. We sometimes get “old” hurricanes impacting the UK but this is not the same, and neither St Jude nor 1987 were one of these characters. Handy pic below illustrates this nicely. Below this are a selection of photos posted on twitter mainly from our local area in E Surrey.

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Lead-up to storm in Reigate on Sunday pm