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May 22, 2016 — 3 Comments

Saharan dust reaching the UK gets in the news quite regularly, usually unfavourably in connection with pollution events. Desert dust is one of several types of minute particles, called aerosols, that are emitted into the atmosphere including salt, carbon and volcanic ash. Human made aerosols, such as CFCs and sulfate aerosols from the burning of fossil fuels, are infamous for destroying the ozone layer and causing climate change but 90% of atmospheric aerosols have natural origins and they all contribute in major ways to the global weather machine. The aerosols constantly floating around in the air that we breathe are made up of a complex mix of particles clumping together in an invisible soup that we are unaware of most of the time.  On occasions dust concentration becomes “thick” enough to become visible and reveals itself as haze. Here is some information about desert dust to help get to grips with this impressive weather phenomenon and hopefully clear the air of those mysterious arid particles!  This post concentrates mostly on Saharan desert dust but volcanic dust is mentioned as a comparison and is worthy of a separate post at a later date.

Where does it come from?


Saharan dust blown across the Atlantic reaches South America

Dust is naturally lifted into the atmosphere from deserts and is an important component of global weather and climate processes and nutrient transport to ecosystems.  Global mineral / desert dust emissions into the atmosphere are estimated to be up to 1500-1800 Tg/year (teragrams) per year and emerge from numerous arid and semi-arid source regions.  For comparison, the average global annual volcanic output of ash from average scale small eruptions has been estimated as an average of only 20 Tg/yr (20 million tonnes per year).  Less frequent, larger eruptions, inject much more ash into the atmosphere. The Icelandic volcano, Eyjafjallajökull, erupted 250 million tons of volcanic ash during the eruption in 2011. This was still small compared with the largest mega eruptions which blast huge volumes of volcanic dust higher into the atmosphere. Desert dust is usually swept up by winds only as high as the planetary boundary layer 2-4km (PBL), whereas volcanic ash can be injected into the high troposphere or even the stratosphere where it encircles the Earth quickly. Nevertheless, in average conditions desert dust usually dwarfs volcanic ash in the atmosphere, unless there is a colossal eruption. (1 Tg = 1 million tonnes ) and albeit desert dust resides at lower altitudes.

global dust source regions

global dust source regions

The biggest global source of atmospheric dust is the Sahara Desert, a huge area of sand dunes, stone and gravel plateaus, dry valleys and salt plains creating nearly 5 million sq km of potential dust producing terrain.  Within the Sahara Desert the Bodele Depression in Chad is thought to contribute half of all Saharan dust.

How does dust get into the air?

Dust is lifted by strong surface winds produced at different scales, from small local convective processes such as dust devils to meso-scale convective systems such as large thunderstorms through to regional scale frontal depressions. Importantly, rainfall in arid areas contributes to available dust by causing flash floods that wash fine debris into river and lake beds. These rivers and lakes then dry out and provide an important source of desert dust when the wind blows. A good example is the Bodélé Depression in Chad, which is part of the dried out Lake Chad.  This area has dust storms on average of 100 days per year and can loft 700,000 tonnes of dust into the atmosphere every day.

Meso-scale convective weather systems in deserts can cause strong cold downdrafts of out-flowing evaporatively cooled air descending from cumulonimbus storm clouds that can entrain particles and lift them vertically into powerful upward thermals. Sandstorms known locally as haboobs are created in this way and appear as frightening “Hollywood”style dust fronts in Africa, Australia, China, the USA and recently in the film Interstellar.

haboob dust storm formation

haboob dust storm formation

Dust can also be lifted from the surface by powerful winds covering a large area associated with troughs and fronts sweeping across, or near to, desert regions.  One such wind is called the Sirocco which occurs in eastward tracking Mediterranean lows where the warm sector produces strong southerly winds which can bring dusty conditions into Europe especially in Spring and Autumn.


A significant sirocco event occurred 23 March 2016.  The event shows up well on a synoptic chart and satellite photo.

Various other synoptic scale meteorological scenarios bringing European / UK dust events are discussed below.  Once elevated, coarse dust (sand) falls out nearest to the origin but fine dust (clay), less than 0.002mm in diameter, can be lifted high into the troposphere, up to 10km, where it can remain aloft for weeks and be driven thousands of miles across oceans by jetstreams.  Saharan dust routinely travels to the Caribbean in the summer on an easterly jetstream.  Dust is eventually deposited in light winds, usually in anticyclonic high pressure systems, or is washed out in rainfall.  In this way some 40 million tons of dust is transported from the Sahara and deposited in the Amazon rainforest every year.


desert dust entrainment and transport

There are broadly two types of dust storm.

  1. Dust plumes have a streaky linear point pattern of dust emerging from a point source and spreading into a cone.
  2. Dust fronts are walls of dust rising on an extensive, frequently curved path.


Desertification of environments in China and Africa seem likely to be increasing the area of global dust producing regions and potentially making the planet more dusty. However, it is not certain whether global atmospheric dustiness will increase or decrease due to expected climate change in source regions like North Africa.  The world has certainly been more dusty in the past. It is understood that during past glacial periods (last glacial maximum 18,000 years before present) water was locked up in glaciers creating drier conditions particularly in periglacial mid-latitudes.  In Europe, China (Yellow River) and the US (Idaho, Washington, Iowa and Mississippi), huge areas of wind-born dust deposited thick aeolian sediments, one of which extends across the North European plain which now forms very fertile soil called loess.  Loess has become some of the most productive agricultural terrain in the world.  “Dust to dust” seems more apposite than ever considering our reliance on natural dust transport for our food.


loess hills, fertile farmland courtesy of dust!

What are the impacts of dust on climate and environment?

Over long time scales dustiness increases during cold climatic periods (glacials or ice ages). Evidence for this comes from ice cores in Antarctica and Greenland shown below.


Reasons for increased dustiness during cold glacial periods includes:

  • Increase desertification (less rainfall generally in cold periods as more water locked up as ice)
  • Increased land area (due to falling sea levels, so more dust sources available)
  • Increased winds

Over shorter time scales, dust plays various complex and sometimes contradictory roles in atmospheric processes, including modification of solar energy receipt, temperature, cloud formation and influencing rainfall.  Dust also has impacts on ecosystems and human activities which can be beneficial or detrimental and even hazardous. So, what can atmospheric dust do exactly?


  • absorbs and scatters incoming sunshine causing surface cooling
  • increases cloud condensation nuclei enhancing rainfall or…
  • increases cloud condensation nuclei enhancing condensation of small droplets which stay aloft so reducing rainfall
  • causes “blood / red / mud rain” events creating dirty cars and windows
  • neutralizes acid rain: dominant minerals in dust are usually >pH7 and include acid neutralizing carbonates
  • imports important beneficial minerals and nutrients such as nitrates, phosphates, iron, calcium, silicates etc to ecosystems like the Amazon rainforest. 200 million tons of fertilizing dust is transported from Africa to the Amazon each year of which about 40 million tons is deposited directly into the forest ecosystem: this is possibly the main nutrient source for the forest. Marine ecosystems also benefit from dust inputs e.g. stimulating growth of phytoplankton and subsequent food chain.
  • imports pernicious alien spores and soil fungus to coral reefs potentially causing coral death events
  • reduces Atlantic hurricane formation: enhanced dust from the Saharan Air Layer (SAL) over the Atlantic during the hurricane season has been correlated with reduced numbers of hurricanes, possibly due to the dust reducing sunshine which suppresses Atlantic sea surface temperatures in the hurricane development zone.  The EUMETSAT satellite image below shows a dust veil (pink) killing off convection cells (brown and green) as it moves across the Atlantic towards the Caribbean.  The Saharan Air Layer is a hot, dry and dusty stream of upper air emanating from West Africa, especially during summer.  The SAL could also inhibit convection, and hurricane formation, by creating an inversion preventing updrafts necessary to kick-start tropical storms.


  • Dust also impacts human activities and health.  Severe dust storms impact activities requiring good visibility such as air travel and some sports. It can also carry organisms such as spores, fungus, bacteria and viruses which could introduce disease far away from the origin of the dust.  Serious cardiovascular and respiratory problems might also be aggravated by fine airborne dust.

So, dust is clearly a critical part of the weather machine and can bring both benefits and problems.  The next section attempts to explain how desert dust can get all the way to Britain from the Sahara.

What weather patterns typically bring dust to the UK and Europe?


In Winter, the subtropical Saharan HIGH pressure is strong with winds wanting to spill away in all directions, potentially carrying dust.  However, with a more southerly jetstream and visits by low pressure systems, the Mediterranean is often unsettled and wet during winter. Despite the Sahara being dusty in winter, dust events extending to Europe in winter tend to be restricted because particles are washed-out by winter rainfall before it gets very far north.


The transitional seasons of Spring and Autumn can produce the most significant dust episodes in Europe. The desert heats up and dries out creating ideal conditions for dust to be elevated by strong winds.  Low pressure can still dip south on meridional jetstreams and create Genoa low pressure which typically increases wind speeds across North Africa.  A cooler Mediterranean Sea surface temperature means that less convection occurs and creates less wash out opportunities as any dust travels north. Therefore, springtime is potentially more dusty for Europe given the right conditions.


In Summer the Mediterranean  HIGH pressure develops as a semi-permanent feature.  This inhibits transport of dust from the Sahara.  Nevertheless, occasional heat lows over Iberia or cut-off lows can create the right southerly wind on a Spanish Plume to bring dust as far as the UK.


Saharan dust moves north associated with Spanish Plume

Dust events in Europe vary in scale and can occur at any time of year but it seems usually and most effectively in transition seasons, especially Spring. In 1901 an historic dust event created the first recorded “blood rain” across Europe.  In this single dust event, well documented, some 50,000 tonnes of dust was deposited across Europe (this would have required a 250km long convoy of 2500 20-tonnes lorries to transport). It has been estimated that dust build up across Europe is 4-5mm per century.

Some case studies of European dust events

Here are some examples of past European dust events showing the synoptic evolution of how dust gets to Europe. Note that meso-scale convective systems (MCS) typically producing dust storms in the Sahara are sub-synoptic and sometimes the dust lofting event barely shows up on these charts.  Nevertheless, the synoptic patterns transporting the dust into populated parts of Europe are well illustrated in these examples.

European Dust March 2014

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European Dust April 2011

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So what is the future for dust?

There is no certainty on the impact of climate change on the future of dustiness in the atmosphere.  There have been press articles suggest there is increasing Saharan dust emission due to population increase, intensive farming and land degradation in North Africa.

“There has been a dramatic increase in some aspects of dust flux [emissions], which have doubled over the last 50 years. Population pressure alone is likely to exacerbate the problem and if current trends continue the amount could double again over the next 50 years,” said Dr Bryant, a Reader in Dryland Processes at the University of Sheffield.

Nevertheless, the impact of these activities is not certain and others suggest dust emissions are not increasing. For example, despite human desertification and degradation of semi-arid environments causing increased potential source areas of dust, it appears that the most significant dust source globally, the Sahara  desert, has not in fact been perturbed by human activities since the major dust sources are mostly in uninhabited areas and in true-deserts.

The IPCC predict that North Africa will get drier and therefore presumably more dusty.  However, models suggest that specific dust source regions could become wetter.  There are significant uncertainties over African dust and climate change and there seems to be no clear correlation over recent decades between measurable climate change and dust load in the atmosphere. Models cannot agree on rainfall changes in North Africa.

Here are some links for further information on dust…


excellent detail:

Greek forecasting dust:

Barcelona dust forecast centre

satellite dust over Western Europe:

cross-sections of dust across Europe:

ecosystem impacts

more info:


Imogen is the ninth named MetOffice storm this winter.  She formed in the Atlantic in an area of steep temperature gradients under control from an active jetstream.


Storm Imogen is deepening rapidly today to 953mb, though on arrival in the UK she will be occluding and filling gradually to above 960mb on her track over N Scotland into the North Sea on Monday. The exact track makes a big difference to where the strongest winds are.  Current trends are for the storm to pull wind fields further north so impacts could be less than expected. Keep an eye on the MetOffice forecast as things are likely to change. Below is an outline of Imogen’s likely activity:

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Unlike the previous eight named storms, Imogen has a more southerly track, guided by a more southerly tracking jetstream, and the field of strongest winds and heavy rain are possibly set to impact the densely populated southern part of the UK, including the SE. High waves are also expected on the Channel coast.


Strong winds on Sunday night will be associated with Imogen’s fronts running ahead of the depression.  The cold front is an active kata-front, associated with descending cold dry air from the stratosphere running ahead of the surface front and enhancing lift and potentially generating heavier rain and gusty conditions (image and info courtesy UKweatherworld).

On Monday gusts up to 80mph on the Channel coast are possible, while inland the MetOffice consider 60mph possible in exposed places.  Around Reigate and sheltered parts of Surrey, 40-50mph gusts are more likely.  The North Downs could see gusts approaching 60mph. The strongest winds for the SE are likely to be through midday and in the afternoon.

Yellow warnings apply to inland parts of Surrey and SE England while the entire Channel coast has an Amber MetOffice warning. The first impact will be frontal rain tonight.  Fronts passing through overnight into Monday could drop over 20mm of rain in places particularly linked to the occluding “triple point” forecast to cross the SE overnight.


Monday is likely to see showers, some heavy, appearing through the day.  Warm sea surface temps in the Channel are likely to cause more on the coast but the brisk winds could bring them inland as the day progresses.

The cause of the strong winds behind the cold front on Monday is a steep pressure gradient.  On Monday tightening isobars show the steep pressure gradient bringing gusty showery conditions in unstable Polar Maritime air behind the cold front later on Monday.


The Wight-Wash Oscillation (WWO) measures the pressure difference between The Wash and the Isle of Wight and is designed as a guide to wind speed in the SE corner of the UK. The WWO on Monday shows a significant 16mb gradient between The Wash and the Isle of Wight on the WRF model.  The Euro4 model has a more modest 12mb WWO.  16mb would be the largest WWO pressure gradient recorded and greater than St Jude, which was 12mb.

On Tuesday models show a wave depression bringing more rain to the SE, some even show fleeting wintry precipitation on the back end of this low as colder air ingresses from the north.  This is unlikely to be significant, at least on Tuesday, as upper air temps remain mostly too high for snow in the SE.


ECM colder flow mid-week

Colder conditions are preferred by the ECM as northerly winds bring cool polar air further into the country through mid-week.  The Arctic Oscillation is again going negative which shows pressure rising over the Poles trying to push Arctic air south into mid-latitudes.  However, the NAO remains positive so Atlantic depressions will continue to bring frontal depressions for this week.

The 8-10 day mean shows a deep trough over the UK meaning low pressure and unsettled conditions remain likely into half term.


The ECM builds heights over southern Greenland which links with higher pressure over the Atlantic, a more northerly feed of cold polar air is likely in this scenario into half term . The ECM has been outperforming the GFS so the more Atlantic driven GFS chart would be the less favoured option.

The Sudden Stratospheric Warming going on over the Pole is another astonishing feature of weather at the moment.  Today (Sunday) temperatures in the stratosphere over Siberia has got up to an amazing +12C from a more usual -70C.  SSW events often build pressure over the Polar troposphere a few weeks later which can cause cold incursions into mid-latitudes.  This is by no means certain but is perhaps our last chance of any sustained cold this winter… if it were to happen it would be late Feb/March. One to watch!

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

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.

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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.

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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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Severe weather warnings out: Gales and heavy rain across the South…

UKMO warnings page:

Flooding map:

Brief update on storm “Caspar” (our own name, middle one of Three Kings ): possibly worst in 130 years for UK (BBC source).

WIND: Reigate can expect possible gusts of 50-60mph, though the town itself is sheltered from southerly and SW winds by Priory Park hill and woodland so that we can shave off 20% from max wind gusts, usually.  The max quoted gust from UKMO is 66mph in small hours of Tues am as cold front goes through. StJude, the last big storm in October, had a max gust in town of 48mph during high winds that lasted only a few hours.  Caspar is set to be a longer duration, across a wider part of the country and is a bigger beast, the storm being centred off the NW of Scotland, rather than running through the Midlands like StJude.

Strongest ever jetstream? Chris Fawkes BBC weather

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Winds will spring up early tomorrow and by 11am could be gusting at 50mph.  They remain strong for the next 12 hours, peaking as the cold front arrives overnight/early morning Tuesday.

The emerging storm is now visible in the Atlantic as a familiar hook shape develops with cold and warm fronts.  The storm is set to explode over the next 24 hours in a process called cyclogenesis, caused by a very powerful 270mph jetstream “sucking” air off the ground as air diverges aloft.  This lowering of pressure by some 50mb in a few hours will cause air to “rush” into the “void” (converge) and this, in a simple way, is the cause of the maelstrom.  The spin of the Earth causes the winds to rotate into the centre of the Low pressure in a process called coriolis effect.



The surface pressure is expected to fall to somewhere around 926-928mb, one of the lowest pressures recorded near mainland UK (lowest land surface pressure 925.6mb, 1884, Perthshire). The BBC explains the development here:

RAIN: Reigate can expect almost continuous rain for 12 hours, starting early Monday morning, around 6am, through to around the same time on Tuesday. Heavy rain is caused as air rises, cools and condenses especially where warm air is forced to rise over cooler polar air.  This will happen especially at two moments as the storm passes…  first the warm front and then a cold front (overnight, probably Tuesday early am).  They have different characteristics.  Of most concern to us and the Environment Agency tomorrow, is that this storm will be causing more or less non-stop rain from 9am Monday through to about the same time Tuesday.  Rainfall totals 20-30mm can be expected during the course of the storm and, on saturated ground, this will cause flooding and plenty of surface water on roads.

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The “good thing” is that cold fronts pass pretty quickly and moderated wind conditions arrive soon thereafter, this means Tuesday is likely to see brightening ameliorating conditions arrive during the morning for any clear up. Flooding will be a problem by then and it’s likely that our River Mole will flood key back-routes and surface water on more major roads could be a problem.  Winds of more than 50mph can also causes trees to be knocked down and other damage is possible.

Finally, waves in the Atlantic are expected to reach near “phenomenal” heights with significant wave heights of 13m by Christmas Eve (which means waves double this height could occur once an hour).

After a quieter rest over Christmas things sadly don’t get a lot better… another storm is heading our way for Friday and the weekend.

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The 2013 December 5-7 North Sea storm caused “the biggest UK storm surge for 60 years” (UK Environment Agency).  With associated gales across Scotland, coastal flooding in North Wales, Merseyside and the UK East coast, tidal river flooding in Hamburg, the closure of all major North Sea coastal surge barriers and disruptive snow further south in Europe, this storm system was arguably more powerful than StJude back in October.  Thankfully, this storm only killed 7 people across Northern Europe (

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Essentially a storm surge is a higher-than-normal sea surface caused by low air pressure coinciding with high tides which, when thrown into shallow coastlines by winds, can produce exceptional coastal flooding.  A surge can also include associated lower-than-normal water levels with off-shore winds pushing water away from the coast at low tide.

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This post outlines the factors that makes the North Sea so vulnerable to storm surges and, further down, there is a summary of some impacts and a quick resume of the successful responses to this hazard event with some useful links.  Finally, before we get too smug and chill out entirely about future storm surge hazard…will development land lapped-up on exposed coasts, for example in the Thames Gateway, increase our future vulnerability in the face of sea level rise and climate change?  Is it sensible to build in these locations?

Animation shows storm surge rolling round the coast and into the North Sea.


The North Sea is particularly vulnerable to storm surges because of an unlucky combination of factors that come together to occasionally make the “perfect storm”.  Fortunately, not every North Sea storm produces a surge!  Remember that Tacloban in the Philippines was hit by an even bigger storm surge generated by Typhoon Haiyan due to similar forces and a funnel shaped bay.  Compare videos on this blog to see the difference between Tacloban and North Sea surges. So what comes together to produce the most significant storm surge hazards in the North Sea? There are at least 6 factors that combine to produce the biggest storm surges: here they are:

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1. Sea shape and low lying coastlines: The North Sea is particularly prone to dramatic storm surges because it is open to the North Atlantic and then tapers towards the south in a funnel shape. This funnel shape has the effect of allowing strong northerly winds to direct storm surges towards cities like London, Amsterdam and Hamburg and surrounding vulnerable low lying coastal areas including Norfolk, Lincolnshire, Essex, Kent and the Netherlands. Some of these areas are at or below sea level and require sea walls and dykes and barriers to protect them during storm surge events otherwise they will be flooded. The 1953 storm surge broke the rather primitive sea walls of the time and flooded large areas of Essex and even more of the Netherlands causing the worst European peace-time disaster since the war and killing 307 people in the UK and thousands in the Netherlands (see you tube documentaries below)


2. Sea depth/ bathymetry: the North Sea gets shallower towards the bays and wetlands towards the south.  These shallows have the effect of increasing the height of tides and surges as they are forced up over submerged shelves into narrowing bays.  This is possibly why Boston, Lincs and Hamburg suffered some of the worst flooding because surges were forced up bays and rivers.

3. Intense low air pressure: A 1 millibar reduction in air pressure allows sea level to rise by 10mm.  This effect can be replicated by sucking water up through a straw. The storm that crossed to the north of Scotland on 5 December had a central pressure of 976mb that deepened to 968mb over the North Sea. This is a similar central pressure to the storm that caused the 1953 storm surge that killed 307 people in the UK and 1800 people in the Netherlands.

4. Storm track: the LOW pressure has to track east over north of Scotland, which will drive a surge of water into the North Sea that is then pushed south by vigorous onshore Northerly winds into the low lying east coast of UK.  Ideally, the storm should deepen on its’ track across the North Sea, thus allowing northerly winds to gain in strength driving the surge and associated wind waves south.

LOW track

LOW track

5. High tides: high spring tides are the final requirement for the biggest surges.  Tides migrate as a bulge of water around the coast and, for the worst impacts, any surge travelling south down the North Sea must match the dome of the highest tide to produce the highest water levels in any one place. Since high tides occur twice a day it is quite likely that high elements of the surge will match a high tide level somewhere down the east coast.

6. Wind driven waves: Finally, surge and tide heights can be increased yet further by strong on-shore winds producing locally high wind driven waves that can over-top sea walls.

Warnings and impacts

The impacts of the 2013 storm surge included flooding in coastal towns on the east coast of the UK with perhaps worst hit being Boston in Lincolnshire. Houses on some vulnerable stretches of coasts such as Hemsby were washed into the sea as waves eroded sand dunes.  There was also significant flooding in Rhyll, North Wales and along the Merseyside coast at New Brighton (note: not Brighton!) where a Morrisons supermarket was flooded. The worst impacts on major populations and cities were avoided by the raising of the Thames Barrier to defend London and the closure of the flood gates on the Delta Scheme in the Netherlands.

The storm was modeled over a week prior to impact.  Initially GFS and UKMO models were seeing a cold surge as the main factor bringing possible snow across the UK but from about 6-7 days out it became increasingly obvious that the exact track and orientation of the LOW meant that powerful northerly winds and a possible storm surge were the greatest risk.  The UK Met Office, with Environment Agency, then started preparations for warning those at risk from flooding.  Most news channels were airing significant coverage from 24 hours out.


Significant flooding did occur along the East coast, notably in Scarborough in Yorks, Boston in Lincs and Hemsby in Norfolk. In Hemsby some vulnerable houses located on the sand dunes were washed into the sea. Bridges near the sea were shut for a time, like the Humber Bridge; and rail services in some eatern counties were disrupted for a time.  Power was cut to homes in Scotland due to high winds.  Hundreds of residents were evacuated prior to the floods in various locations but some claimed to have little warning.

The worst impacts were successfully controlled by the massively impressive engineering schemes built since the devastating 1953 floods.London has nearly 200 miles of flood walls and 8 barriers holding back the tidal Thames. The Thames Barrier was opened by the Queen in 1982.

The Eastern Scheldt storm barrier was closed for the first time since the 1970’s.  The Netherlands barriers are built to withstand a 1 in 10,000 year storm surge event so it is perhaps unsurprising that they easily saw off this event.  It is also noteworthy that the Dutch have great faith in their storm surge protection barriers.

These measures, along with warnings and on the ground assistance for places that were flooded, proved extremely effective.

Further useful links on 1953 and 2013 storm surges:

1953 storm surge: original newsreel and timewatch documentary

The sting in the tale?

London is sinking into clay and, along with the rest of the SE, it is tilting into the sea partly due to an epeirogenic / isostatic adjustment taking place since the glaciation released the north of the country from the burden of millions of tonnes of glacial ice causing positive isostatic rebound in the north and related subsidence in the south.

Flood plains and reclaimed land exposed to storm surges are still being lapped up by hungry developers as places ripe for building, like the Thames Gateway in London.  But is it sensible to concentrate massive new urban development in low lying areas vulnerable to coastal flooding when we have sea level rise and climate change?

STORM SURGE WARNING FOR NORTH SEA COASTS WIND and North Sea storm surge update later today: check twitter @RGSweather for updates!

Check environment agency website for flood warnings

Point to note: the original weather forecast (below) for Thurs-Sat dec5-7 emphasised a plunge of cold Arctic air on the back of the low passing across N Sea.  However, in the event, as things quickly transpired nearer the time the strong northerly winds became the focus and the storm surge down the east coast on the back of it. 528 dam (coldest air) stayed well to the east of the SE and so we had squally showers on the cold front with no wintry ppt.  

Express meaning rapid / short-lived icy blast for later this week is firming up in most model runs now, though with differences in arrival, departure and intensity (especially the GFS which is least chilly, but I understand this is not deemed as performing well on this event).  Anyhow, it would be sensible to assume that it will be chilly or cold to very cold for early December across much of the UK from Thursday onward and through much of next weekend with the North and coastal areas, especially North Sea coasts, getting some really icy gales at times, particularly Thurs-Fri/Sat.  

30-11-2013 22-29-39Some snow is likely for north facing coasts both East and West and Scotland and parts of Northern England.  Reigate and the more sheltered SE will, as usual, be protected from the worst but we can still expect cold blustery conditions at times with such a cold upper Arctic airmass incursion. Air mass temps are usually measured at 5000ft (850hPa) (to avoid surface interference and changes day/night, urban/rural etc above the so-called boundary layer). Currently our 850hPa temp is around 0c, by the end of this week they will drop to -8c or even -10c.  

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Include the wind expected, especially Thursday – Friday and this will feel cold if you are out and about, watching football matches on the touchline for example: much colder than this weekend!  Air temps may struggle to reach 2-3c in Reigate on Friday and the 15-20mph wind will make it feel well below freezing.  An interesting additional feature is a possible LOW forming in the southern part of the North Sea on those icy northerly gales.  If this occurs it might bring snow to the far SE of the UK and certainly into Netherlands and, later, the rest of N Europe. 

However, it is likely that this episode will be a brief SHOT of cold as a high pressure stirs in some warmer air later next weekend and the winds also die down making it feel more tolerable, though an anticyclonic gloom is more likely than sparkling sunshine. Latest Aus model shows an easterly picking up sustaining some cool conditions in SE in the week after this cold shot…check below…

01-12-2013 08-56-03

but a tad warmer later

but a tad warmer later

The culprit is the jetstream which is looping wildly around the Northern hemisphere at present.  The jetstream acts as a kind of “belt” around the Poles holding the cold Arctic air in.  If the jet starts weakening and wiggling from North-South it allows cold air to leak out from the Poles and plunge south in huge swoops of icy air.  The Arctic swoop expected later next week is going to inject icy weather to much of Europe from Greenland and the North Pole  The UK, being on the western edge of this swoop, will miss the most intense cold (that’ll hit Netherlands and Germany and, eventually, get as far as E Europe and the Alps).   The lead-up to this event is pretty benign with a high pressure giving calm cool cloudy conditions early in the week, turning into a more westerly zonal flow for a time before the northerly plunge hits from Thursday. More on this later in the week as more details emerge.

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A current picture of Greenland shows conditions this weekend.  Think of air leaving here this week and reaching the UK by Thursday: thankfully, it’ll warm up from around -40c on the ice sheet to about +4c by the time it reaches Reigate high street.

30-11-2013 12-09-50

Haiyan officially deadliest storm in Philippines history with 5209 people killed.

Some 30 storms of varying intensity have formed in the Western Pacific region in 2013 and nearly 10 of these have crossed the Philippines or come near. People in the Philippines and this part of the Pacific are entirely used to tropical cyclones which is perhaps why so many so sadly did not heed warnings this time, choosing instead to sit it out in their homes. It is also dreadfully ironic that perhaps the welcome recent expansion of a more populous middle class in LIC’s such as the Philippines meant that people may possibly have felt safer in their new concrete homes than they may previously would have done in simpler shacks or self-built dwellings.  The storm surge would have meant they were trapped in a deadly and terrifying “washing machine” inside their own home rather than making the journey to shelters on higher ground given the warnings to evacuate low lying coastal districts. This, of course, is unsupported conjecture but was mentioned today by a previous resident of Tacloban interviewed on BBC Radio 4.

Haiyan was spotted before the 3 November when a cluster of intense thunderstorms began to rotate near the central Pacific Micronesia islands. At this point it became a Tropical depression, the weakest status for a cyclone. Rotation is the precursor for hurricane formation which is why hurricanes / typhoons never occur on the Equator due to the zero Coriolis (spinning effect) near the low latitudes.
During the next 3 days the storm traveled 2000 miles to the west, varying in intensity but staying below supertyphoon status. Nevertheless, the Joint Typhoon Warning Centre based in Pearl Harbour was monitoring it and several storm chasers and camera crews chose to fly direct to the line of fire in Tacloban. 

So, the Philippines may have been unlucky this time because the Haiyan / Yolanda reached maximum intensity as a Cat 5 supertyphoon just before it made landfall and, due to the very warm sea surface temperatures to the east of the Philippines at the end of the summer, it did not weaken at all.
This led to the ferocious winds exceeding gusts of 200 mph and massive storm surge of some 6m that hit Tacloban and regions around there.

On the other hand, if Haiyan had been a terrorist attack: we knew his precise location, direction and potential threat several DAYS beforehand and we knew that Tacloban was staring down the barrel of this most unprecedented attack at least 48 hours before Haiyan unleashed with such ferocity.

With masses of technology already invested in weather prediction, the challenge for this century is to get appropriate, meaningful and timely warnings to people on the ground so they can react accordingly.  This will mean education and responsible leadership.

Tacloban and the Philippines more widely, is known to be exposed to the potential for typhoon damage, they experience storms frequently. Nevetherless, it is the lack of human planning and preparation that made her so vulnerable this time.

The picture below of the father carrying his child is so distressing that it should perhaps not be on this blog, but it might help us to understand the immeasurable impact this massive storm has had on real people.  Social media has made the world SO small now, allowing us to communicate with and get to know more about individuals who live across the planet than sometimes live across the street. It means that the people in these photos should, more than ever, be seen as our neighbours.

Here is an excellent explanation from the BBC

More on hurricane / typhoon formation here: this is old but a good outline of hurricane formation:

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Notes and links on RESPONSES and more long term impacts

6 days later:

PH Govt now facing criticism as “despair and chaos” descend over Tacloban and effected areas.

Tonnes of aid remains undelivered due to Govt inefficiency? No large scale food distribution until now.

US Military / US Marine Corps arrive in force with aircraft carrier George Washington and other vessels with helicopters to rapidly deliver aid.  US have taken control. There is a dramatic change on the streets with dead being collected now, 6 Days later.

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$5m Canada

$10m UK

15-11-2013 06-58-54 15-11-2013 07-00-06

A superb article here : are events like Haiyan just “acts of God” or partly “acts of Man”?

16-11-2013 10-24-05Fear of disease: mass vaccination program of 33000 children started in Tacloban amongst fears of cholera epidemic threat in devastated city.

Psychological damage

Good summary on management of aftermath and aid and reconstruction:

storm surge science


STORM STATS TO DATE…(10/11/2013) President declares “state of national calamity”

Latest 15/11/2013 PH Gvt website and BBC  5000 dead; 3853 injured; 921,000 displaced; 243,000 homes destroyed

At least 2 million families (11.8 million people) in 51 cities; 1444 evacuation centres. Estimated $4billion damages to PH


This page has essentially become a chronological scrap-blog of emerging information from the pre-storm warnings to the immediate post-disaster phase. Scroll down for maps, sat pics, videos and photos and links as they were forthcoming during the storm.  Live information from web sources including twitter and facebook direct from chase teams and embedded weather journalists as well as local Philippino sources traversed by the eye wall are included. This video was shot for CNN by storm chaser James Reynolds @typhoonfury who went to Tacloban knowing the eye wall would cross directly over.

This is an historic tropical cyclone: a truly huge storm and the strongest in recorded history with devastating impacts emerging from the areas hit by the hurricane eye, such as Tacloban City, Leyte and S Samar.  (Typhoon Tip was the previous record breaking strongest storm). Several extreme storm chasers and journalists embedded themselves into the path of the EYE WALL including James Reynolds @typhoonfury and Jim Edds @ExtremeStorms bringing up-close and personal stories of the true horrifying nature of the storm and the terrible aftermath now unfolding.

(LIVE now a recording offline COVERAGE OF LANDFALL


A major super typhoon with winds gusting more than 200mph has emerged out of the NW Pacific and is on track to cross the Philippines at the end of this week. With 175 mph sustained maximum winds this is the most powerful storm this year and, were it an Atlantic Hurricane, it would be about the most powerful on record. Although it will weaken slightly when it crosses land, it is still an exceptionally dangerous storm and a great threat because the Philippines is a densely populated country and, while the capital Manila will not receive a direct hit, some 10 million people in Luzon are in the area predicted for Haiyan to make first landfall.  The storm is then predicted to track rapidly across to Vietnam where it will arrive on Sunday, possibly still as a strong typhoon.

Update: poss most powerful landfall typhoon ever: off the NOAA Dvorak instensity scale

It was a FAST moving storm and the eye passed over Guiuan district and Tacloban City in just a few hours but the impact was immense.

“The typhoon moved fast and didn’t last long– only a few hours– but it struck the city with absolutely terrifying ferocity.”

The last tweet from storm journalist Jim Edds in Tacloban was sent just before the eye wall hit and then..nothing. He and other extreme film makers were reported safe and evacuated later

IF the typhoon had passed over Europe… how big would it be?

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Instagram videos of destruction in Tacloban

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Morgerman posted this harrowing report on Facebook:

First off, Tacloban City is devastated. The city is a horrid landscape of smashed buildings and completely defoliated trees, with widespread looting and unclaimed bodies decaying in the open air. The typhoon moved fast and didn’t last long– only a few hours– but it struck the city with absolutely terrifying ferocity. At the height of the storm, as the wind rose to a scream, as windows exploded and as our solid-concrete downtown hotel trembled from the impact of flying debris, as pictures blew off the walls and as children became hysterical, a tremendous storm surge swept the entire downtown. Waterfront blocks were reduced to heaps of rubble. In our hotel, trapped first-floor guests smashed the windows of their rooms to keep from drowning and screamed for help, and we had to drop our cameras and pull them out on mattresses and physically carry the elderly and disabled to the second floor. Mark’s leg was ripped open by a piece of debris and he’ll require surgery. The city has no communication with the outside world. The hospitals are overflowing with the critically injured. The surrounding communities are mowed down. After a bleak night in a hot, pitch-black, trashed hotel, James, Mark, and I managed to get out of the city on a military chopper and get to Cebu via a C-130– sitting next to corpses in body bags. Meteorologically, Super Typhoon HAIYAN was fascinating; from a human-interest standpoint, it was utterly ghastly. It’s been difficult to process.
09-11-2013 19-16-12

09-11-2013 16-19-09

Damage far worse than people thought:

Massive online efforts from PH govt to help aid and search and rescue efforts:
600,000 people evacuated from Vietnam and Hainan as Yolanda Haiyan arppaches Sunday 10/11/2013
2 months to fully restore power as 90% poles lost in storm surge


11/11/2013 Tacloban now officially placed under a state of emergency

Red Cross report “absolute bedlam”

11-11-2013 19-16-15UK Govt promises £10 million–governments-pledge-millions-but-tacloban-is-still-waiting-8933515.html