Archives For January 2014

Recent storms locally in Reigate, around Surrey and across the UK have brought relatively brief periods of squally weather which have got people asking questions about what they have experienced.  In particular, people are wondering about the January 25 squall line and asking… “wow, was that a tornado?!”

In Reigate we recorded our maximum wind gust for at least 4 years of 52mph during the passage of the active “squall line” cold front at 5pm on Saturday 25 Jan. Biggin Hill recorded 68mph and Kenley 60mph peak gusts.  For Reigate 52mph exceeded StJude 2013 and the storms in December, however the duration of the January 25 gusty front was barely 10 minutes.  The weather phenomenon that produced this extreme short-lived gustiness is called a squall line.  Below is the 500hPa chart for temperatures at 5000m and the 850hPa chart (1500m) as the storm passed over Reigate and SE England.  Note the dramatic change in air mass temperature (colour) and pressure at height either side of the front, especially in the SE.

A squall line is a very active cold front.  This one migrated steadily from the NW to the SE during the course of the day (see radar pics above) causing electric storms across the Midlands and some unconfirmed tornadic activity reported there too.  The squall line appeared to break up a little on its passage down towards the SE but still caused significant gusts measured between 50-60mph.

The experience of fierce winds “coming out of nowhere” combined with lashing rain or hail impeding movement, causing tree branches to sway or snap alarmingly and even whole trees or garden walls and sheds to blow over in an instant, followed by power cuts in some areas locally, have all understandably prompted the search for explanations.  “Mini-tornadoes” have been mentioned in the media as a possible cause for this weather phenomenon.  So, was Surrey struck by a “mini-tornado” on Saturday January 25? This post looks at some local evidence and puts forward a round-up of what is “required” before weather phenomenon can usually be labelled a tornado.

It is worth mentioning that there is not really any meteorological phenomenon formally known as a “mini-tornado”.  Whilst spinning vortices of air can vary greatly in size and shape (e.g. vortices of rubbish on a street corner, dust devils, land /water spouts and gustnadoes) and can yield a great range of wind speeds, a “proper” tornado is only formally defined as a violently rotating column of air, in contact with the ground, usually underneath a cumulonimbus cloud and usually (but not always) visible as a funnel cloud that is intense enough to do damage.  So there is no such thing as a “mini-tornado”.  Wind speeds in tornadoes are unknown.  This is because traditional anemometers are destroyed even in weaker rotations.  It is suspected that wind speeds can exceed 200mph and even 300mph in larger Mid West tornadoes.

From evidence seen so far, few local eye-witness accounts from 25 January have revealed any reliable sightings of funnels or convincing sustained surface wind rotation and there are no photos of funnels or vortices, which isn’t surprising given the lashing rain and bad weather and gathering gloom at 5pm!  Any formal tornado identification must therefore rely on gathering evidence of damage and analysis of the prevailing conditions.  To get nearer an answer we must see how close recent events came to satisfying tornado producing conditions and whether any damage evidence points to tornadic activity in and around Reigate / Surrey area.  Tornado formation is a big topic so these are necessarily abbreviated explanations but they are a starting point from which we might be ready to observe and record tornadic activity even better the next time!

1.Were conditions conducive to tornado formation?

Yes, partly! Tornadoes are associated with severe thunderstorms. The biggest tornadoes are produced by storms called super-cells.  These are rare in the UK but relatively common in the Mid-West of the USA where about 1000 tornadoes occur every year usually in spring/early summer and autumn.  Thunderstorms are produced when moist humid air is rapidly lifted from the surface and huge amounts of water vapour condenses to form towering clouds called cumulonimbus. Latent heat is released which drives further lift and draws in further warm moist air in a feedback loop which builds cloud height.  Cb clouds form characteristic shapes like the ones below spotted in Reigate on 28 Jan 2013.  The UK experiences around 30-40 tornadoes per year, so thunderstorms and squall lines like the Jan 25 event can certainly produce tornadoes, albeit weaker than the US Mid-West variety.

An unstable atmosphere is a prerequisite for most thunderstorm formation.  Instability is a condition where warm air rises and keeps rising until it hits an inversion layer or the top of the atmosphere, (itself an inversion layer called the tropopause) whichever is the lower.  Inversions occur where temperatures increase with height, instead of the more usual decrease.  An inversion will prevent thermals rising and any increase to cloud height will cease.

In unstable conditions, warm bubbles of rising air (thermals) will expand with height, cool down and moisture will condense into water droplets creating the bubbly explosive cloud formations that can be seen growing at the top of young thunder clouds (“Turkey towers”).  Skew-T graphs show temperature change with height.  The graphs below show recently unstable conditions in Reigate during a period of thundery activity and the passage of the squall line in Nottingham on 25 Jan.  Note the relatively steep “lapse rates” shown where temperatures fall rapidly with height.  Steep lapse rates are a critical ingredient for thunderstorm development. Steep lapse rates allow convective thermals of rising air to lift rapidly.  US tornadic thunderstorms in the mid-west often have an additional atmospheric feature called a “cap” that “turbo-charges” thunderstorm development.  A “cap” is an extreme temperature inversion created by a warm dry wind aloft that prevents further uplift of thermals until energy at the surface has built sufficiently for thermals to “bust through the cap” and rise unabated, building explosive thunderstorms extremely rapidly. There is no evidence of an inversion cap on the 25 Jan squall line.

The movie below shows how violent super-cells erupted after the cap has been broken last year over El Reno, OK, USA, producing one of the most violent tornadoes ever recorded.

Another essential process in the production of tornadoes is rotation.  Air can start to rotate in a thunderstorm due to changes in wind speed and/or direction with height. This is called wind shear and it is an essential ingredient in creating severe tornadoes.  A twisting motion in rising air masses can be the result of winds from different directions meeting and “rolling” the air into a horizontal sausage. Try rolling a pencil between your hands moving them in different directions… this is essentially what winds from different directions can do to the air.  Note the absence of directional wind shear in the Nottingham Skew-T chart during the passage of the squall line: all the winds at different heights were blowing in much the same direction. There appears to have been some vertical speed shear. Nevertheless, the absence of directional shear seems to reduce the chances that this storm was a tornado producer.

Clouds can show evidence of rotation in the atmosphere: shelf clouds (horizontal rotation, rarely produce tornadoes directly), wall clouds (vertical rotation: often an imminent precursor to a tornado) and funnel clouds (visible tornado condensation funnel in early stages of development that has not yet “touched down”) are all signs of very disturbed rotating air that is a precursor to tornado activity on the ground.  No evidence of distinctive wall clouds or shelf clouds or funnels has emerged from the Reigate storms 25 January.

Horizontally rolling air aloft can be twisted into a vertical spiraling vortex, a tornado, by strong warm updrafts of air in thunderstorms.

Warm updrafts (thermals) can be lifted for various reasons.  During the summer, heating of the surface can trigger thermals  to rise and, in unstable conditions (where rising thermals keep rising as it is warmer than surrounding air), this can trigger thunderstorms.  During the winter, with less sun available to heat the surface, warm air can still be forced off the surface by active fronts or extremely unstable and moist converging air masses. A coastline or range of hills might be sufficient to lift air to unstable levels and create thundery activity.


Once rotation is moved vertically through the storm then a tornado is possible given the continued updraft of warm air.  Thunderstorms will decay if the cold downdraft of air created by heavy rain or hail interrupts or cuts off the rising warm inflow.  This decay occurs in single cell thunderstorms that sit still over the same rising thermal source.  To build a severe storm requires a strong wind shear and motion across the ground to separate warm inflow from cold downdrafts.  Once this separation occurs then the storm engine can keep running and produce long lasting severe weather with a greater chance of tornadic activity. Although the squall line on 25 Jan did not produce super-cells it nevertheless provided lengthy displays of lightning and thunder, especially across the Midlands.

life cycle of a tornado

  1. The early stages with a rotating wall cloud (counter-clockwise in NH)
  2. development of a funnel cloud,
  3. damage at the surface
  4. descent of the funnel cloud to the ground.
  5. width of the funnel increases to some maximum,
  6. then begins to shrink,
  7. finally reaching a rope-like dissipation.
  8. after several seconds or an hour

Mature super-cell with no problems separating inflow and outflow

2. What do tornadoes look like in the UK and were any spotted during the 25 Jan event?

Classic photos of US tornadoes do not help identification in the UK.  Tornadoes in the UK tend to be smaller and short-lived.  However, the appearance of a tornado is not a reliable indicator of its intensity. Some narrow rope tornadoes can contain extremely violent winds whilst wide established rotations might be weaker.  In the UK tornadoes often initially appear as small funnel clouds, sometimes hard to spot, emerging from a lowering in the base of a cumulonimbus cloud ( a wall cloud).  Funnels appear to drop from the cloud, though this is not really the case. The funnel is actually condensing water droplets progressively becoming visible at lower altitudes as pressure falls in rapidly rotating rising updrafts of air. The rotating air beneath a rotating thunderstorm remains invisible until pressure and humidity allows condensation to make it “appear”.  It is possible to have an “invisible” tornado, for a period at least.  The other reason funnels become increasingly visible is when they pick up dust and debris in their rotation. Funnel clouds are quite commonly observed in the UK but few “touch down”.  Here is a selection of UK funnels and tornadoes.  Unfortunately there were no similar sightings on 25 Jan so this reduces confidence in tornadic activity on this occasion.

3. Was any convincing tornado-like damage left behind?

Despite the impressive trees knocked down, the damage type did not suggest a tornado. The type of damage can be conclusive in identifying tornado activity.  The damage left behind by a tornado depends on the speed of vortex rotation, the duration and length of the path across the ground and the nature of the objects along the path. Severe storm meteorologists have a keen understanding of what kind of damage tornadoes do on the ground. The US Enhanced Fujita scale and the UK based TORRO both estimate wind speed from the damage inflicted by tornado events.  Meteorologists investigate possible tornado sites and assess the damage using these scales.

The damage left behind by a tornado usually includes a ‘surgically’ well-defined and narrow damage-path, along which significant damage is seen, but away from which there is little sign of damage.  A damage PATH of sorts is therefore an essential piece of evidence in pinning down tornadic activity.  In addition, it is common for projectiles to be thrown to the left of the direction in which the vortex advanced (i.e. looking down-track)

When visiting WIND DAMAGE a diverging pattern of damage with several throw directions, with no clear path, perhaps moving out from a single point in several directions or otherwise isolated damage limited to single trees or blown objects is a good indication of straight line winds or downbursts (severe, but non-tornadic).

To confirm a tornado the existence of a damaging circulation / evidence of rotation at the surface must be found before the event can legitimately be called a tornado. With tornado damage, destruction is generally along one direction or path, debris along the ground is twisted or has spiral characteristics, and often small arcs where the top-soil has been removed are visible.  If such a confirmation cannot be made, the event must be considered to be either straight line winds, or possible a funnel cloud which did not touch-down or simply a “possible” tornado with no sighting.

Damage in and around Reigate and Surrey from the 25 jan episode is not representative of tornado damage (see below).  It is more likely that the damage below was caused by straight line winds and down drafts during the passage of the cold front.

We have to agree with the Met Office conclusion from January 25th 2013 which was that “we have had a line of very intense thunderstorms with strong, squally winds today and it is possible that these conditions could have produced small tornadoes. We have had reports of small-scale events consistent with tornadoes but at this stage it’s mostly hearsay.”  RGSweather would finally add that it is unlikely any damage around Surrey was caused by a tornado on this occasion. 

For reference: how to stay safe in tornadic conditions: for next time!

  • Do NOT open windows to “equalize pressure”: useless and waste of time and you should stay away from windows (NOAA)
  • Do NOT stop under bridges: debris will be blasted under bridges and impaled into people hiding; chances of survival not good as people ripped out from under bridges and fly-overs; bridge may fail
  • Vehicles are extremely risky in tornadoes

Here is a selection of tornado clips showing the formation, typical examples and scale of damage and frightening experience that can occur in Mid-West tornadoes.

(Got pics of “mini-tornado” damage?! Please post them on twitter or send them to me here for a post on the subject to be posted shortly, thanks.)

QUICK UPDATE Sunday: Reigate mixed cool wintry weather this week with always marginal conditions between sleet/snow but mostly rain midweek as the active LOW sinks south and fills and produces some interesting but uncertain possibilities for unstable bands of heavy rain circulating round the LOW in troughs as the week progresses. Convergence at times on the S coast could cause thunderstorms at times.  The coldest air overlying SE is due Sunday-Monday but, given the subtle nature of wind directions from the continent and the uncertainty of models as to how this scenario ends, anything could happen regarding next weekend… heavy Atlantic rain and then cooler could be possible. Watch UKMO warnings carefully for updates regarding Tuesday and Weds rain. 

Temperatures are set to dip next week below the balmy winter conditions we have been used to recently.  Below average temps will arrive for the first time this winter to Reigate.  It has been mild so far this January with CET temps over 6c average, courtesy of an extremely lively east-bound jetstream that has flooded (literally) the UK with mild and moist westerly winds from the Atlantic.  A cold plunge has been on the model-cards for a while but has come to nothing so far. However, a LOW pressure spinning up in the Atlantic on Friday is set to bring stormy conditions especially to the W and NW of the UK this weekend with fronts sweeping some significant rain right across the country, including SE and Reigate on Sunday.  It is the track of this LOW pressure that is likely to cause the dip in temps next week. Whilst significant snow is not likely for the SE it is possible that we might get some wintry mix of sleety/ snowy /rain precipitation on occasions through next week and, as Atlantic fronts possibly push in across the region at the end of the week / next weekend these might bring more significant snow on the leading edge of fronts, possibly for the end of Jan or start of Feb.  This is a way off so not certain at all but a possible interesting outcome to ponder that might bring at least fleetingly proper wintry weather to our winter-starved region.

The cold air is set to be dragged down across the UK as the LOW pressure over NW Scotland moves south and east across the country eventually sitting, cut-off, over the continent through next week.  A cut-off LOW is a cold low with plenty of cold air wrapped up in the circulation (called “cold uppers”: cold air throughout the depth of the atmosphere) that becomes cut-off from direct polar or tropical feeds of air.  It becomes a closed circulation with the cool air gradually sinking down to the surface, chilling things further.  Convective showers are also possible as these lows “warm through” from any surface heating.

When this “cut-off low” or cold pool settles over the near continent by early next week, the winds on the northern back edge of the LOW will pull in cool easterly and NE winds that could be cold enough to produce wintry precipitation on occasions, certainly to the North of the UK and maybe to the SE as well, but this is by no means certain for us and probably remains unlikely at the present time until maybe later in the week.  Whilst the source region for the wind is not as cold as 2013, the continent is nevertheless pretty chilly at this time of year so expect Tmax temps to crawl to just 5 or 6c in Reigate but not much higher.

So, we are set for a “cold rinse cycle” next week as this cut-off low circulates cold air over the SE with resulting change in wind direction from mild westerlies, through cool showery NW and, eventually easterlies with the potential for incursions of even colder air from the continent at times.  This is not a severe winter episode, merely a cool/cold snap with some wintry precipitation possible.

Here are some charts and figures from our Vantage Pro2 weather station summarising 2013 weather in Reigate.  All data is posted on our data page for you to download freely and use, but please credit RGSweather for any subsequently produced materials.  Excel pivot tables have been used to generate some of the more fun charts and statistics below.

Our overall conclusion from 2013 is that Reigate has some of the best weather in the UK, probably!  Read on to find out why…

Reigate Temperature

Reigate topped the charts for recording the highest temperature in the UK at times during 2013 summer.

Average temperature 9.9C

Highest Tmax 33.5C on 1 August at 4:22pm. (Heathrow recorded 34.1C)

Lowest Tmin -5.9C on 22 January at 3:37am.

Lowest wind chill -11.9C on 12 March at 5:03am.

The chart below shows daily average temperatures in Reigate throughout 2013 as a full circle.  Note the annual average temperature for Reigate (10c) is shown as a red circle.

Reigate Rainfall

Despite recent experiences, it doesn’t usually rain that much in Reigate compared to elsewhere in the UK.  The extraordinary rainfall of 23-24 Dec recorded a rare 70mm for that period on our manual rain gauge. The graph shows that this rainfall total is unsual for Reigate.

Total 654mm (VP2 rain collector; uncalibrated on site; intended calibration v soon; we suspect some under-reading of totals at this stage)

Average daily rainfall 1.8mm per day

The wettest day of the week, overall, in 2013 was… Friday (but not significantly) with Thursday being a significant driest day.

Highest rainfall intensity 182mm/hr (briefly!) on 22 November at 8.38am.

Reigate Wind

The strongest gust recorded in 2013 was 48mph, more exposed hills and places locally recorded 60-70mph (e.g. Redhill aerodrome).  Reigate is sheltered from Northerly winds by Reigate Hill and from more common gales from the South or SW by high ground at Priory Park and woodlands.

Average highest daily wind speed  8.7mph

Max gust 48mph from WSW on 28 October at 6:15am

Dominant wind direction WSW.

The windiest months were October and December in respect of highest maximum wind gusts measured.

Wind run is a measurement of how much wind has passed a given point over a period of time. A wind blowing at three miles per hour for an entire hour would give a wind run of three miles.  Wind run is a measure of the persistence and duration of a particular wind direction.  The longest wind runs for Reigate in 2013 were associated with NE winds in March, which were persistent.  The fateful Channel Blizzard wind run that met warm SW winds and caused such unusual snowfall in the Channel Islands recorded the longest wind run of 267 miles. It’s interesting to note that our most common westerly and SW winds achieve relatively short wind runs, this is probably because our prevailing winds are associated with LOW pressure systems that cause rapid changes in wind direction as they pass through.

Rain and Wind

Most of the heaviest rainfall totals for Reigate in 2013 arrived on South (S) and West-South-West (WSW) winds.  The wind direction bringing highest average rainfall intensity (rainfall rate mm/hr) in 2013 was SSW.  There looks to be a strong correlation between rainfall rate and wind speed.  The chart shows that the heaviest rain tends to occur at lower wind speeds.  The strongest gales it seems often just precede, or arrive soon after, warm or cold fronts, prior to arrival of or just after the heaviest rain has passed.  This was certainly the case in the October StJude storm when the, now infamous, stingjet winds followed after the heaviest rain on the cold front had well and truly passed through Reigate.

Wind and Temperature

The wind direction bringing coldest temperatures to Reigate in 2013 were those bitter Easterlies in our cold 2013 Spring.  An interesting element of SSW winds also brought in cold minimum temperatures to Reigate, possibly ahead of fronts from the Atlantic when chilly continental air can be dragged in as “polar returning air” from a frigid winter continent.  Interestingly, the dominant wind bearing bringing the lowest wind CHILL temperatures was NNE, different from the winds bringing lowest average air temperature.  More on wind chill here 

Reigate Sunshine (*sunshine recorder added April 27)

Sunshine total hours *1058 hours (from April 27)

Longest daily sunshine hours 11.4 hours June 6

Shortest daily sunshine within recording period 0 hours on 21 December (shortest day)

In conclusion: Reigate, Surrey has the best weather in the UK, probably!  It is one of the driest, least windy, least stormy (we get few thunderstorms compared with many places) and sunniest and warmest places in the country.  Reigate was one of the warmest or nearly-warmest places in the UK a number of times in 2013.

Some factors making Reigate weather some of the best in the country are to do with the uniquely well-placed location of the town.  So, thanks to the Normans we are:

  • south facing: nestled at the foot of the North Downs: sheltered from cold Northerly and NE wind and rain
  • sheltered from S and SW gales by Priory hill and woods.
  • low altitude: 100m above sea level: not exposed.
  • inland from exposed coasts which experience higher winds and, sometimes, more rain
  • on the lowland East of the UK: enjoying a regionally sheltered location from cold easterly winds (Kent gets these) and westerly gales and heavy rainfall.
  • in a location that usually avoids extreme convective thundery activity (2013): Reigate missed several thunderstorm events that passed to the north of London from a SW direction; this may or may not be usual.
thunder monday

Reigate missed the big 2013 thunderstorms which often drifted from SW to NE in lines of convective activity, missing the SE

We plan to post full meta-data describing the location of our Vantage Pro and the data required to get the most our of our records in 2014. Other plans for 2014 include:

  • Calibrate the tipping bucket VP2 rain gauge before Easter.
  • Further outreach to other schools and interested parties regarding all-things weather, via this blog and @RGSweather.
  • Expand the school club and further engage students in the wonders of weather.
  • Expand the use of data in school to include more departments.
  • Provide a local service to help forecast and understand extreme weather, when we can, via Twitter.
  • Engage with the Press and media where we can provide useful information
  • Encourage readers to post comments and together contribute to the wider understanding of weather locally.

A sincere thank you to all readers for making this such a success in 2013 and please come back for more in 2014!

December in Reigate 2013 summary stats:

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

Average temp 6c

Tmax 11.4c 16-17Dec

Tmin -2c 5 Dec

Max wind gust 39mph 27 Dec

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

21 days with rain

Windy! Especially high up

Windy! Especially high up

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

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

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

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

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

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

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

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

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

05-01-2014 14-50-47

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

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

Impacts of River Mole flooding

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

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

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

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

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

Types of Flooding in the Mole Catchment

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

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

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

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

Factors affecting flood risk on the River Mole

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Flood management in the Upper Mole Catchment

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


Gatwick Greenspace partnership working to improve River Mole ecology


further reading and references

EXCELLENT video on sustainable flood management



uppermole flood alleviation scheme







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

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

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

update links:

slow the flow

excellent blog updating situation after Cumbria floods 2015

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

+Contains Environment Agency information © Environment Agency and database right

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

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

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

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

02-01-2014 22-10-19

rain totals fri – sat

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

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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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