Archives For rainfall


The Atlantic is showing off some classic visible cloud features of cyclone birth and decay today.  Systems labelled 1-4 on the satellite photo above show different features including stages of cyclone / mid-latitude depression formation and decaying high pressure ridge all on one satellite picture.  The chart below shows the same view with fronts.


Starting with LOW number #2 (why not?!): the spectacular classic cloud spiral of LOW #2 indicates a mature low occluding and filling.


This maturing occluding LOW has a couple of interesting extra vortices near the low core.


Despite their angry look, classic cloud spirals like this on satellite photos are actually decaying and filling lows, losing their strength as pressure rises in the low core.  This particular LOW has a spectacular cold front of over 1500 miles stretching from 60N to the sub-tropics. The red colours on the RGB false colour eumetsat image below shows the cold continental polar air surging in behind the cold front.

Low #1 is a rapidly intensifying LOW off the coast of Labrador.  It looks harmless as a smudge of cloud but this shape … a so called “baroclinic leaf” indicates the birth of an angry developing storm: rapid cyclogenesis.


This will deepen and pressure will fall rapidly in the next 24 hours as frigid continental air collides with humid maritime air under the influence of an active 200mph jetstream.


LOW #1 is expected to form a big storm in the Labrador Sea by Wednesday. The fronts on this storm are then forecast to stretch clean across the Atlantic and bring the SE our first frontal rain for over a week by later Thursday.

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System #3  on the top satellite photo shows the HIGH pressure lingering over the South of the UK but regressing into the Atlantic.  This ridge has dominated mid to late March weather in the UK but brought a lot of anticyclonic gloom to the SE.  The deflating ridge will allow a more unsettled Atlantic westerly regime to dominate late March and early April weather.


Low pressure #4 is an interesting developing depression in the Mediterranean, courtesy of a southward limb of the jetstream. Currently a disturbance dumping snow over the N Atlas in Morocco, this LOW is set to deepen across the Mediterranean through the week.  It will track directly ENE through the Med and bring snow to the Atlas mountains, rain to N Algeria and foul wet, windy and wintry conditions to Italy and then more snow and wintry weather to the Balkans.

Finally, for the UK our weak ridge is deflating to the SW and this will open the door to zonal westerlies and frontal systems bringing rain and wind from later Thursday and into the weekend.  Ensembles below show the dry spell ending this week and some notable rainfall spikes in the days to come, especially over the weekend.




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

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

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

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


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

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


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

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


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

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


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

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


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





analysis chart shows HIGH edging out with moist Atlantic winds ready to pounce

The analysis chart above shows a weakening ridge of HIGH pressure over the UK being edged out north by a slow moving Atlantic LOW to the W/SW.  Reigate is still currently (Saturday am) in cool dull easterly winds generated by the HIGH but a significant switch in wind direction will take place over the next 12 hours into the bank holiday period as a wholly different mild and humid Sub-Tropical Atlantic air mass, with a source region round the Azores, takes hold from the SW.


weather cross-section

A mild moist S/SW wind drives in from the Atlantic as the LOW edges north east tonight. The first mass of rain is edging onto radar from the SW and is expected to arrive in Reigate by around mid-late pm today.  Most rain is likely for places further north and west but the SE is still likely to pick up plenty of wet weather overnight with low cloud and rain into Sunday morning when it could turn heavy and showery for a while in the early morning as the trough passes directly overhead and pressure continues to fall. Things are expected to clear to brighter conditions later in the afternoon as pressure rises and winds turn more westerly. Cloud cover will hopefully break and cloud height will lift during the afternoon becoming more cumuliform.


trough and fronts migrate north, showers follow

If the sun comes out then there could be a low risk of an odd heavy shower Sunday afternoon, possibly thundery, but these are more likely further north of our area where more unstable air makes progress across the Midlands and East Anglia.


During Sunday winds will be occasionally blustery with moderate convective gusts possible, especially on hills and nearer the coast, and make the mild temperatures Tmax 16C feel considerably cooler. Temperatures overnight could hold up to a balmy 12-13C.


Overnight Saturday-Sunday rain could linger as showers through the morning

Winds turn from SW to more southerly through Monday and pressure should up-tick slightly giving a mostly dry and warm day and less windy as things stand currently.  Troughs could progress east during Monday and build cloud and produce some showers.  More importantly there is a looming threat of something special for later Monday-Tuesday night.


As the northern block (high over Greenland) holds on, the Atlantic LOW just west of Ireland will usher in a mild and moist S/SW flow of air from the continent.  An unstable LOW brewing in the topical Atlantic today (Saturday) is forecast to sweep up and intensify from Biscay later Monday and into Tuesday and this might bring heavy rain and winds to the south and SE and a possible thundery episode later Monday but more likely overnight into early Tuesday for SE.  The jetstream is dipping well south and is forecast to perk up and approach the UK from an unusually southerly direction by Tuesday.  If this happens the jetstream could deepen this low considerably, as modelled by some charts (latest UKMet shows 980mb).

Depending on the evolution we could find ourselves in the unstable left exit region of a jet where divergence aloft enhances convective action and creates heavy rain.  Warm air from the south will also contain more moisture.  A dry slot at mid-levels might also enhance instability (rising dry air cools more quickly increasing lapse rates and CAPE, enhancing lift).  High dew points near the surface temperature also encourage condensation and indicate extremely moist warm surface flows.

So all these ingredients stirred up could be a good recipe for some briefly moderate-severe weather in our region especially some briefly torrential rain, though totals are unlikely to amount to more than 10mm.  Gusty winds and gales near coasts could also accompany this system.  Latest metoffice chart shows pressure dipping to 980mb in the North Sea which is significantly LOW pressure for the time of year.


coastal gales and convective gusts inland

However these episodes have a habit of tracking across Holland and merely clip Kent with thundery showers and miss us entirely.  Models also generally exaggerate these early on and then things flatten out nearer the time considerably.  Nevertheless, it is worth watching this develop as our first potential “warm plume” of the year.  If we take a direct hit the SE could have some heavy rain.

The GEFS summary below clearly shows the two main rainfall spikes tonight and Monday night.


GEFS 850hPa temperatures and rainfall London

Later mid-week the LOW is expected to drift east across the UK bringing in a more westerly pattern so unsettled showery weather is likely for a while. Thereafter, a rise in pressure from a developing Euro high pressure may then take place from the south and settle things down for us in the SE, though this might only make faltering progress.

Hope you enjoyed this post. Comments are always welcome. Links to websites used to create our blog posts can be found on our links page.


faltering pressure rise later next week


cold plunge of polar air to end April

The newspapers have this colder than average week billed as a “polar plume”.  Cold air cannot really be said to “plume” like warm air (spanish plume).  Nevertheless, it is certainly cold up North with settling snow over the hills and wintry precipitation elsewhere too.

The GFS ensemble (several model runs combined at once) chart below shows that the colder than average (upper air) temperatures will last until the weekend at least.  Thereafter temperatures rise but note the rainfall spikes indicating unsettled conditions.


upper air temperatures

For us here in the sheltered SE it is just cooler than average with patchy overnight frost and bright days with light showers Tuesday pm. A blustery cold front on Wednesday is likely to bring more purposeful rain for a while, as could further active fronts on Thursday when the trough axis moves through our region accompanied by the jetstream nearby to the south.  Friday is likely to see a transient high pressure ridge bringing settled weather and then a change in wind direction by the weekend.

The overall cool set-up is due to “northern blocking” which is when pressure builds over Greenland and the Pole and the, previously strong, Azores HIGH slackens off and nudges south. At the same time the usual Icelandic low pressure weakens or is dominated entirely by a HIGH pressure (see chart below).


In this situation the jetstream works its way south of the UK.  The result is that cold polar air is able to leak south out of the polar regions and into the mid latitudes.


The index shown above summarizes the overall pressure pattern in the Atlantic. It is called the North Atlantic Oscillation. The NAO measures the pressure difference between Iceland and the Azores.  It has been mostly positive this winter: meaning that pressure is LOW over Iceland and HIGH over the Azores.  Typically a positive NAO indicates a strong zonal jetstream and mild often wet conditions for Europe with relatively fast moving LOWS passing through.  The chart below shows how the NAO has gone negative recently and this indicates that pressure has built over the Pole, creating a blocking situation.

The charts above show theta-e temperature which shows cool airmasses over the UK clearly.  These are 850hPa temperature charts which represent temperatures at 1500m (1.5km) above the “boundary layer”.  This height is used to avoid disruptive temperature changes which occur nearer the surface caused by day and night, mountains and water bodies etc that upset the overall temperature pattern for analysis.  The situation shown below by this weekend is quite different, though still unsettled.  Note the warmer flow from the SW.

After the transient ridge on Friday it looks like pressure will fall into the bank holiday weekend as a low pressure nudges in from the Atlantic.


pressure falls into weekend

The milder humid air brought from the SW by the Atlantic low could potentially cause some significant rain at times around the weekend and into early next week as this meets cold air over the country.  Despite the milder upper air arriving from the SW, it is unlikely the “milder” temperatures will be noticeable in such wetter and windier conditions. In winter this set-up could have brought big snow events but in early May it will simply bring rain. For the detail on timing and amount of weekend rainfall we will have to wait and see, but it certainly looks potentially quite wet, though models suggest pressure building briefly thereafter.


Update for storm prospects for Reigate: an intense low pressure is forming out in the far west of the Atlantic this evening where frigid air from Canada is meeting humid warmer sub-tropical air circulating from the south round the Azores high pressure.  This confluence of winds causes lift to occur at the polar front but a fast jetstream blowing at 180mph directly overhead will cause additional lift of the air lowering pressure extremely rapidly.


rapid cyclogenesis under jetstreak, then further deepening on left exit of jetstream

This process is called rapid cyclogenesis and gives birth to a deep low pressure with tropical air circulating into the storm from the SW and polar air sweeping round the head of the storm to follow in its wake.

The low will deepen and race 1300 miles across the Atlantic to arrive off the NW UK coast by midnight Thursday.  For inland Reigate and SE England the impacts will be lower than in the west and NW but the English Channel will experience significant gales overnight in the warm sector of the depression with gusts possible of 70mph.  NW Britain and especially NW Irish coast might see the biggest gusts in the wrap around winds on the south side of the low core where stingjet winds are possible even as high as 100mph.

arrival on Thurs 00hrs

arrival on Thurs 00hrs

Winds for SE and Reigate will build through Wednesday from late afternoon and through the evening and are likely to peak at possibly 50mph gusts as the squally cold front passes sometime 4-6am.  Exposed places on the Downs could experience stronger gusts. Winds will ease after the cold front moves through by breakfast time but Thursday will stay blustery with showers and feel cooler.  There is a chance that our record wind gust recorded in Reigate of 52mph will be broken in this storm but the town is sheltered from southerly / SW winds which will be the dominant wind throughout the event so this might keep wind gusts lower.

For Reigate the rain is likely to arrive mid evening on Wednesday.  Rainfall will be persistent throughout the event and heavy at times and possibly with isolated thunderstorms as the active cold front moves across early Thursday morning.  Rainfall totals could amount to over 20mm, most of it falling in a short period probably at the cold front.

The weather on Wednesday will be interesting: starting with cold temperatures and scattered snow showers courtesy of frigid air originating from Canada and Greenland on a brisk NW wind and then warming through the day as the storm arrives with a deluge of rain arriving in sub-tropical air from the deep south Atlantic.

Finally, the storm is due to usher in cold polar winds which eventually swing to the north as pressure builds in the Atlantic to bring an Arctic flow across the UK during the weekend.  Various troughs and any low pressures sliding down the edge of the developing high pressure could cause a more significant snow event any time from Sunday and through the week.  This wintry spell is likely to last into next week so dig out the warm woollies! Any snow that we get in Reigate will be the first since March 2013. Lots going on and very changeable so stay tuned!



So far this winter Reigate and the SE has been sheltered from much of the weather action, which has focused mainly on the NW and especially Scotland. This coming week might change that somewhat, albeit modestly in comparison with the battering the NW has received.  Charts below show temperatures and pressure are overall on the slide this week while precipitation makes two noticeable spikes.

A moist warm airflow ahead of a slow moving cold front arriving from the north Monday pm could stall over the SE overnight into Tuesday am and bring significant rainfall totals.  Temperatures Monday night could hold up into double figures as a warm moist SW flow funnels up from the subtropics ahead of the sluggish cold front.

slow cold front clearing south

slow cold front clearing south

The wave on the polar front emerges out of the SW through Monday with a characteristic plume of rain and a dip in pressure.  Such waves tend to bring a lot of rain despite hardly showing up on synoptic charts.  A modest kink in the front and isobars (see charts below) is the only hint of potential heavy rain action.  Some wintry precipitation and maybe snow is possible for places on the north side of the front later into Tuesday, in Wales/Midlands for example,  but not for the SE as temperatures remain too high.  The front slips slowly south Mon/Tues bringing some significant rain and then brisk cooler weather from showery westerly winds to follow, some sparse showers could possibly be wintry Tues-Weds but not amounting to much for us, most wintry showers will fall further west.  Temperatures will be cooler at Tmax 5-6 on Tuesday and even cooler on Weds, so cold but frost unlikely as too breezy.

There is a weather warning out for this period especially for the south coast but Reigate totals could reach 20mm+ and exceed 20-30mm for places nearer the south coast in Sussex.  The front is followed by increasingly cool showery westerly winds through Tuesday into Wednesday with the odd wintry shower possible but nothing significant.

There is the possibility of a storm for the UK Wednesday-Thursday bringing gales to our area as a “bomb depression” emerges in the Atlantic this week and probably tracks across Scotland (which would be less windy as a result, for a change).  Details of the storm track remain wobbly at the moment but the strongest winds will be found on the southern flank of the LOW centre and various models suggest that the SE could have some strong to gale force winds, albeit nothing like the scale of Scotland recently.  Models suggest a steep pressure gradient as the low tracks somewhere across the north of the UK.  The precise track will determine the impact on the SE.  Current models suggest the strongest winds for the SE will be associated the warm front approaching Wednesday night / Thursday morning.  Gusts could exceed 50mph inland and more on the coast with some excitable models suggesting more but these usually calm down nearer the time! Nevertheless, wind and rain are likely to be significant and expect updates on weather warnings this week. Colder air follows the storm so showers following cold fronts could turn wintry so watch for updates.

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Our own amateur “Wight-Wash Oscillation (WWO)” is an attempt to quantify the potential impact / severity of storms on the South East of England.  The WWO is simply a crude measure of pressure difference between the Wash and the Isle of Wight ( a mini NAO!).  As a benchmark the St Jude storm (99mph Needles max gust) had a WWO of approx 10mb (976 Wash-986mb Wight) and the modest storm of 12/12/14 was a 9mb on the WWO.


Current models shown below have a variety of WWO results from 8mb UMET up to 12mb for GFSP.  A difference of 1 or 2mb can make a big difference to the severity of the storm as pressure gradient ultimately drives wind speed. Nevertheless, the WWO is just for fun but interesting to see how it works out in the event. On the basis of our WWO this storm currently looks less severe than St Jude, which is a good thing because that storm caused a reasonable amount of damage.

The unsettled weather this week is being driven by an active jetstream over the Atlantic.  This itself is partly a product of the tremendous temperature gradient over the northern hemisphere and particularly over the North Atlantic with freezing air pouring out of a frigid Canada meeting warm sub-tropical air emerging round the Azores High from the south.  These air masses are set to clash on Monday at the polar front over the Atlantic.

Whatever the storm brings it is looking likely that afterwards a cool northerly wind from the Arctic will bring temperatures further down by the weekend.  Although a long way off, for Reigate and the SE it is unlikely to be snowy and most likely to be the frosty and cool type of cold!  Further ahead some models build a pattern into later January (around 20) that could be conducive to cooler conditions: with possible easterly or NE winds for a time as a low sinks into Europe, though this is not looking either particularly extreme or long-lived.

Meanwhile, the North Atlantic Oscillation is set to remain positive, though decline somewhat to neutral, which means that the LOW over Iceland and HIGH over the Azores synoptic pattern is likely to be maintained, albeit possibly less pronounced and with more anomalous meridional moments of cold incursions possible.  This would mean a continuation of the broadly zonal westerly flow of mild windy and fairly wet unsettled weather for the foreseeable future.  Conflicting signals like this have riddled longer range forecasters this winter: the stratospheric warming was only moderate, much to snow-lovers disappointment, and some supposed good indcators of colder winters (the OPI / early Siberian snow cover in October) have apparently failed to live up to forecasting skill expectations as yet.  A month or so of winter to go and no significant signs of persistent cold … yet!

Update: confirmed tornadoes today (see foot!)

Potentially interesting, albeit tricky, weather tomorrow for UK, Wednesday. A deep surface LOW sits to the SW of Ireland and is dragging through complex series of fronts associated with various airmasses.  Occluded fronts are tightly wrapped around the LOW, which is due to migrate NE away from the UK during the weekend and pressure to rise.  For the SE tomorrow it’s not so much the fronts but an unstable mass of warm southerly surface air that will be the main cause of any heavy showers tomorrow and some potentially thundery weather, especially when this warm air is forced up by anything… coast, hills or fronts.

Here’s likely scenario for us in Reigate, Surrey SE: Rain is likely, possibly exceeding 10mm, which is fairly wet for SE: higher is possible.  Most of this is likely to be convective rainfall due to unstable and moist air moving in from the south overnight.  Showers, possibly heavy and thundery, are most likely in the morning as the warm surface air moves into our area, causing lapse rates to increase moderately and this encourages lift and cumuliform clouds.  If the sun comes out then surface heating could spark heavy showers and thunderstorms as warm air rises freely through the atmosphere, encouraged by the jetstream overhead that effectively drags air off the ground.  One of the ingredients for thunderstorms, LIFT, is therefore partly in place tomorrow, although it will depend on sunshine for greatest effect.  If it stays overcast, which is possible, then little exciting weather action beyond just rain is likely.

The above charts also show that the southerly / SW airstream is humid because, as it converges on the coast, the model shows rainfall increasing significantly. This increased rainfall on coasts is often caused by convergence which is due to air arriving onto the coast quicker than it is leaving (check the lower wind speeds inland) so the wind effectively PILES UP on the coast and is forced to rise as it has nowhere else to go except UP!  This is called convergence. It is clear from the charts above that any HILLS also encourage lift as South Wales and even the South and North Downs appear to be pushing rainfall totals up locally: this is orographic or relief rainfall.  The charts below shows another feature of the weather tomorrow: the winds are shown to be VEERING with height (left diag) which allows WARM air to advect (move into) into our region (right diag).  Read on for more about how veering winds and WARM AIR ADVECTION can encourage stormy weather.


Another ingredient for potentially unstable weather is that winds are VEERING tomorrow, albeit not dramatically, which means they are rotating clockwise directionally with height thus allowing warmer air from the south to move into a location: it is like opening the door to warm air: winds move through a southerly direction and therefore allow warm air to “advect” into our area.  A moderate wind veer is taking place overnight and into tomorrow morning.  Warmer air at the surface is overrun by cooler westerlies aloft that increases lapse rates: steepens the temperature difference between surface and air at altitude.  The air at altitude tends to stay the same temperature and is associated more with direction and origin of airmass than it is with surface heating or advection of warmer air at the surface. An increase in lapse rates adds to instability which encourages air parcels to LIFT off the ground, should surface heating occur if the sun comes out.  The chart above right shows how WARM air is ADVECTED into the South of the UK and migrates NORTH during the day.  This has nothing to do with solar heating… it is an 850hPa chart (1500m) and shows the airmass temperature which is largely independent of surface influences.  It’s a good example of WARM AIR ADVECTION with a moist air stream increasing instability causing showers and possible thunderstorms.

Finally, the warm relatively unstable airmass is being overridden by a NE turning jetstream that will encourages lift and wind shear.  Wind shear is the vertical change of direction and/or speed with height: rotation.  Shear is moderate tomorrow which might also add a twist to rising air that could even produce the odd tornado.  Nice 🙂  After writing this Estofex issued a tornado warning Level 1.

Update 8 Oct: confirmed tornados from today 8 October 2014


Monday 25 Aug

Bank Holiday heavy rainfall wash out!


Nasty LOW

Like much of the rest of England and Wales, Reigate and Surrey and the SE will see an August Bank Holiday wash-out tomorrow with lots of rain through the day, likely to be heaviest in the afternoon.  This has the potential to be a moderately severe wet weather event with 48 hour rainfall accumulation possibly topping 20mm in our region with some models taking this even higher to 30-40mm in places but this is probably over-egging things.  The WRF and NMM models push totals to 40-50mm in places while Hirlam and Euro4 (UKMET) keep a lid on rain at 20-30mm.  Anything approaching 40mm in 48 hours would be highly unusual for us in SE in Atlantic frontal depressions but it’s by no means impossible.  Remember that a “wet day” for us in the SE usually adds up to only around 10mm, so any totals over 20mm tomorrow will seem a significant deluge. Check the rogues gallery of models below for the worst rainfall offenders.  By the way, always Check the UKMET forecast for a professional look at the situation, this site is for a local perspective on weather and to generate enthusiasm and interest and share some more understanding of the air around us, especially in these times of such uncertainty regarding weather and climate.  As we approach another autumn and winter please share any of your weather stories and photos here or follow us @RGSweather on twitter.  Twitter is an amazing portal for sharing, learning and discussing the amazing fast moving world of weather… join it if you can.

Either way, surface water local flooding is likely to be a feature on roads tomorrow and the UKMET has issued a yellow warning for heavy rain.  You can expect a cloudy, dull, blustery, cool and wet day: so maybe snuggle up with the TV guide and check out some bank holiday films 🙂  and watch @RGSweather on twitter for exciting weather updates of course!


The bank holiday soaking is courtesy of a low pressure in the Atlantic set to deepen to 981mb overnight as an unseasonably fast jetstream blasting through the Channel draws very moist air from the surface and lowers the surface pressure throwing all types of fronts our way and tightening isobars cross the SE during the course of the day. 

Today, the system lurked in the Atlantic as a significant band of cloud, as shown here by the VIIRS satellite at 13:00hrs.


The system will bring rain first thing tomorrow to Reigate.  Initially rain looks to remain fairly light but will become progressively heavier through the day with even a low risk of heavy thundery showers as the cold front passes later pm and into Tuesday night (updated) before things gradually calm down Tues am. Whilst breezy through the day, max wind gusts are likely to arrive later in the afternoon but only 20mph.  This is certainly more a wet event than a wind event!

Into Monday morning: update:

Satellite pics show a rapidly developing occluded low pressure with tightly wrapped fronts.  The dark are on the sat pics represents high altitude air that is subsiding (sinking) into the depression.  This dry intrusion can cause increased instability around the cold front where it is forced to rise once more nearer the surface, increasing cloud formation.  Sometimes is can cause thundery doinwpours and some of the models are picking up on this by increasing rainfall totals even further for the SE as the frontal system passes over.  Interestingly this does not count as a “bomb” cyclogenesis which requires a 24mb fall in pressure over 24 hours (1mb per hour).  This has managed about half that pressure fall so far. 

Even in her hey-day Hurricane Bertha was not a powerful or well organised storm and she disappeared as a discrete tropical storm a few days ago. What is left crossing the Atlantic is a significant “blob” of heat and moisture that she dragged into the dog-days of the mid-latitudes.  Unfortunately for meteorologists the energy injected “intravenously” into the mid-latitudes upsets super-computer weather model forecasts. Models cannot handle the excitement!  The result is that various weather forecasts have struggled to agree and have been producing significantly differing outcomes as to the strength of Bertha and to when, where and IF she “makes landfall” in the UK.  Usually, the differences in model forecasts gradually reduce and forecasts increasingly agree, but in volatile atmospheric conditions it often goes down to the wire, like on this occasion!  This is why meteorologists have been unable to confidently pin down the exact track and strength and impact of Bertha on the UK.

Today there remains uncertainty, despite being only 48 hours out from her arrival.  The main cause of uncertainty is how she eventually interacts with an unseasonably strong and southerly jetstream.  Where a low pressure like Bertha arrives and moves under a jetstream makes a big difference to where she goes and whether she strengthens or weakens. The so-called “left-exit” region of a jet exerts most influence on deepening low pressures into significant storms.  Where the jet is slowing down or speeding up or curving can create extra lift, dragging air off the surface causing pressure to drop.

Here are three of the possible forecast outcomes from the GFS (including WRF/NMM which are similar), UKMET and ECMWF models. (Thanks to @BigJoeBastardi for the ECM model output from @WBAnalytics)

#1 Bertha could travel more or less down the eye through the English Channel from the SW.  This would take most of her highest winds and gales through France but would clip the south and SE of England with some heavy rain as the low crosses directly over this area.  The highest winds in these tight LOW pressure systems usually occur to the south of the LOW centre, associated with the warm and cold fronts.  The heaviest rainfall is modelled to fall to the north of the low associated with the occluded fronts and low pressure centre, hence the heavy rain clipping South and SE England. This outcome is currently favoured by the BBC and UKMET office models.

#2 Various other models, the GFS and WRF-NMM amongst them, favour a more northerly route and take the LOW from the SW approaches, through St George’s Channel, across Wales and into the heart of England before exiting into the North Sea.  Broadly speaking, this would be a worst-case scenario because the UK would take the brunt of both heaviest rain (to the north) and the highest winds (to the south).  Nevertheless, for @ridelondon and #Reigate and the SE this might be the preferred route because any fronts and poor weather would pass over our area relatively quickly on Sunday, the worst of it probably from around 9am through to 2pm.  It would also leave only trailing fronts depositing up to 10mm or so of rain across our region and some gusty conditions, wet for a while but nothing too drastic to speak of really.

#3 A third model, the ECMWF has, until recent runs been quite the outlier amongst all competing tracks, sending Bertha further south through France in earlier runs.  More recent ECM runs have played catch-up with GFS and now sends the LOW NE through the UK.  Significantly, ECM drops pressure to 987mb in the Irish Sea.

The outcome could, of course, be somewhere in between these two or something totally unexpected!  It is worth also pointing out that extra-tropical storms arriving in the UK are not uncommon and this one is likely to underwhelm in many places inland where gusts are unlikely to exceed 40mph and, in the SE maybe 30mph gusts will be the widespread maximum.  Coasts, hills and exposed areas are likely to see the worst of it with gusts up to 60mph or more.  Even here it will not be anything like as potent as our storms last winter where winds exceeded 90mph on occasions.  Nevertheless, with leaves on the trees there may be some blow-downs and loose branches and rain in some places might disrupt travel but it is doubtful that wide-spread chaos will ensue as compared with the morning after St Jude last October which was a lot more potent. See below.


The best advice is to watch weather warnings and updates carefully on Sunday for any changes.  RGSweather will be posting NOWCAST updates on twitter for Reigate and environs.  Here is the latest from NWS/NCEP showing storm force winds in the sea around the UK, note the unseasonably LOW central pressure on 985mb. In comparison, St Jude storm had a lowest overland central pressure of 976mb back in October 2013.


Interestingly, the longer term weather impact of Bertha into next week is probably better modeled than her immediate track on Sunday.  She is forecast to move slowly up the North Sea and merge with and deepen her “parent” LOW to the N of Scotland.  This will introduce cooler unstable showery NW winds to the UK for the early part of next week. Showers and some more organised bands of rain are likely to be frequent visitors, especially to the NW and west coast.  It will also feel cooler and more breezy for the whole country.  So, put the balmy warm days of summer on hold next week.

Update: Fri pm

More model agreement now on a northerly track.  Potential for UK worst-case scenario. Wind and rain crossing the country.  Inland 30-40mph, coastal and hills poss 50-60mph.


Google Earth image of rivers through LGW

Google Earth image of rivers through LGW

This post explores flood control engineering, water management and pollution controls currently in place at London Gatwick airport.  The post adds detail to the previous article on flood control in the River Mole basin and should provide a fuller understanding of flood control on the River Mole including management of water flows through Gatwick airport and any impact on downstream discharge, including pollution. The previous post on the causes and management of flooding more widely in the River Mole drainage basin can be found here.

Gatwick Airport is built across the low relief, impermeable clay flood plain of the Upper River Mole drainage basin. The solid geology is mostly impermeable Lower Cretaceous Weald Clay with some sandstones overlain by recent drift deposits of river gravels and alluvium deposited by the four rivers.  This mostly impermeable surface gives the Mole a naturally rapid (“flashy”) response to rainfall events as less precipitation is able to infiltrate into rocks and instead flows as surface runoff into streams and channels across the clay plain delivering water to channels rapidly and raising discharge quickly after storms, an altogether more speedy process than soil throughflow which dominates in more permeable drainage basins.

Three tributaries of the River Mole drain into this basin within or near the boundaries of the airport: Crawters Brook, Mans Brook and Gatwick Stream.  All of these rivers have been extensively modified, diverted and culverted during the history of the airport development and both up and downstream during the wider urbanisation of the area most particularly in Crawley, Manor Royal industrial estate and Horley.  Most significantly, the Gatwick Stream was culverted underneath the South Terminal during original airport construction in 1958.  More recently, the River Mole was diverted in 2000 further north and west to accommodate the North Terminal.  A second runway will require further diversions and channel modifications of streams and rivers and further significant attenuation schemes to manage additional runoff from any new runway.

This article outlines how Gatwick airport manages runoff and water flow to reduce the impacts of flooding and pollution not only within the airport boundary but also upstream and downstream of the airport.  It should be noted that the Upper Mole basin is an increasingly urbanised catchment with over 11% of the drainage basin covered in urban development.  Despite initial perceptions, Gatwick comprises only a small proportion of this total urbanised land cover in the River Mole basin, parts of which are expanding as a key growth area in the south east with the M25, M23 and London to Brighton railway providing excellent infrastructure for growing business and commercial activity and attractive locations for housing.  At less than 8 sq/km, the airport itself is currently around three times smaller than the urbanised area of Crawley (30 sq/km).  So Gatwick is just a part of an urbanising River Mole catchment and therefore shares responsibility for managing the river with other land users and the Environment Agency.  Despite the difficult hydrological siting of the runway on a flood plain and the negative press regarding winter flooding, Gatwick airport is managing water flows responsibly and behaving as a good neighbour to the local area by investing in the Upper Mole Flood Alleviation Scheme that will endeavour to control flooding, not just benefiting the airport but also reducing flood risk for Crawley and downstream to Horley, Reigate, Dorking, Leatherhead, Fetcham and Esher and the M25.


All human development within flood plains from the smallest garden patio, private driveway or house extension to the largest scale road project, shopping centre or major airport, reduces the natural capacity of landscapes to moderate the flow of water into rivers.  Human development usually involves the removal of trees and vegetation and the addition of impermeable hard surfaces such as cambered roads, sloping rooves, guttering and drains that are designed to remove water as quickly as possible to avoid local flooding.  Such development prevents the infiltration of water into the ground which slows the movement of storm runoff into rivers by allowing some of the precipitation to “sink in” to the soil, thereby slowing down flow reaching the river.  Trees and vegetation also intercept rainfall and transpire water from the soil and these processes also slow the movement of water into channels or removes it from river basins altogether by the natural process of evapo-transpiration from vegetation.

The time it takes for water to enter rivers is called “lag time”: the time between peak rainfall intensity and peak discharge.   Lag time tends to be longer in naturally vegetated lowland basins and those with permeable geology.  In developed river basins with extensive urban surfaces the lag time tends to be shorter.  Rivers with short lag times are sometimes called “flashy” because they respond rapidly to rainfall events and flood more easily.  Storm runoff reaches the river quickly across impermeable surfaces and this shortens the lag time and exacerbates flooding.  All development has a tendency to speed up the transfer of runoff into rivers.  Airports are large open areas, the runways alone covering 4km2, with extensive impermeable surfaces.  Such wide open impermeable surfaces could therefore have significant impacts on local river discharge unless runoff was carefully managed.   Like all airports in the UK, London Gatwick works closely with the Environment Agency to ensure runoff and pollution are carefully controlled.  The shortening of lag times through the rapid removal of rainfall from urban areas can cause flood problems downstream as water ingresses into channels more quickly and this is why Gatwick “attenuate” runway runoff by the addition of holding or balancing ponds.

29-12-2013 21-15-54

Each airport is unique in how the site creates challenges for water managers.  Gatwick was built in a low flood plain which clearly presents unique flood and pollution control challenges particularly now due to climate change and increased rainfall intensity.  Nevertheless, all airports present one challenge or another with regard to their site.  Heathrow is located on permeable gravels and these present challenges for controlling pollution entering the water table. Other major UK airports located further away from fluvial flood risk are presented with the challenge of shedding excess surface water from heavy rainfall efficiently into balancing ponds and water courses with sufficient capacity to remove pluvial flood water.  So Gatwick is by no means unusual in having its own unique challenges presented by a flat low lying site.

“Balancing ponds” are the main engineering device for “attenuating” runoff from airports.  Balancing ponds are designed to delay or “attenuate” runoff from runways by holding back discharge from reaching the river directly from the runways and hard surfaces inside the airport boundary.  Balancing ponds are large, sophisticated engineering structures and are one of the most extensive spatial land uses for many airports, including Gatwick.  A glance out the window on take-off or landing from LGW will show keen-eyed observers numerous balancing ponds and channels in amongst trees, tracks and open spaces around the runway perimeter. These ponds are linked into a complex gravity-fed system designed to transfer runoff and separate clean from contaminated water and treat it and store it, before eventually returning the water to local rivers in a controlled manner or recycling it for other purposes within the airport itself.

All eight of the Gatwick balancing ponds have been designed to control the rate of runoff so that flows are equivalent to the runoff from a grassy or “greenfield” surface.  In short, from the point of view of precipitation, Gatwick airport acts hydrologically like a vegetated grassy surface because rain falling over the runways flows into balancing ponds that regulate runoff reaching the River Mole at a discharge equivalent to water flowing off a greenfield.

For every square metre of additional concrete built within the airport boundaries the airport is obliged to construct additional attenuation to meet this “greenfield” attenuation requirement. This will also be the case for any second runway.  For example, new multi-storey car parks have subterranean attenuation ponds built underneath.  It is worth noting that the obligation to strictly control and continuously monitor runoff is not the case for all development outside the airport boundaries, for example additional private driveways, housing and roads.

The control system of water management at Gatwick Airport permits regulated discharge of clean water into the River Mole from each of the eight balancing ponds depending on the rate of flow of the river.  At times of high flow more water is discharged than in periods of low river flow.  Pond D receives the bulk of contaminated and clean water from runway runoff (see below).  Water is automatically tested (e.g. measuring biochemical oxygen demand) and contaminated water is separated from clean and is then pumped through a 3.5km pipe to Crawley Sewage treatment works at Tinsley Green where it is held in large Y shaped ponds before treatment and eventual release into the river.  Clean water from Pond D is separated, aerated and enters the River Mole directly.

However, even in flood conditions, a maximum discharge of separated clean water into the river of just 1680 litres per second (1.6m3/sec 1.6 cumecs) is possible, as each archimedes screw can only lift 860 litres per second and only two are allowed to operate at any one time.  The discharge of the River Mole entering the airport boundary during times of flood often exceeds 10 cumecs and can exceed 20 cumecs at Sidlow, near Reigate.  The addition of 1.6 cumecs, regulated to slightly precede the natural flood peak, is therefore of little significance to the overall flood conditions of the river.  In contrast, at times of low flow some 75% of the river discharge comes from Crawley sewage treatment works which outflows at Tinsley Green.  Much of this is treated water from Pond D and homes and industry across the Crawley area.  Despite some local myths, there are no “flood gates” at Gatwick airport or manual or automated systems which could allow significantly sudden discharges beyond a maximum of 1.6 cumecs to be discharged at times of high flow into the River Mole and therefore exacerbate flooding downstream of the airport.  The only “flood gates”, if they can be called as such, are located at the new Gatwick Stream flood alleviation scheme, operational from 2014, and these are described below and would also never cause a sudden peak discharge beyond the natural storm flow.

There is little evidence from hydrographs that there are any spikes or discharge peaks outside the expected normal hydrograph curves for high intensity rainfall events, particularly those associated with the floods during the past very wet winter 2013-2014.  A sudden discharge of water of any significance would show up on Environment Agency hydrographs and, monitoring these during the course of the winter, there was no clear evidence of unexpected peaks or surges outside the normal response to rainfall along the river downstream of the airport.  The winter 2013-2014 yielded 250% more rainfall than average for the area and the Gatwick holding ponds and water management system remained within the design capacity.

On its course through the airport, the River Mole is owned and managed by London Gatwick.  The airport has the responsibility to manage the riparian zone (flood plain) and the channel to maintain efficient discharge. River levels and pollution levels are constantly monitored by means of gauges and biochemical oxygen demand.  The River Mole enters a culvert built in the 1950’s underneath the west end of the runway.  The river was diverted north of the runway in 2000 to allow for the North Terminal.  The new channel for the River Mole around the airport is entirely artificial but has been carefully designed as an attractive wooded park-like area with public access being maintained along much of the stretch around Povey Cross bridge for recreational enjoyment, even inside parts of the airport boundary.  Gatwick Operational staff  walk the entire airport stretch of the Mole twice a year to monitor the state of banks and spot any disruption to the efficient flow of the river along the artificial flood plain. The channel itself has been designed as an efficient shape to discharge flows effectively and an artificial flood plain has been built to safely allow for discharges that exceed bankfull stage (the natural maximum discharge for the channel beyond which floods across the flood plain occur).

Gatwick Airport, just like other extensively urbanised land uses such towns like Crawley or Horley or industrial estates like Manor Royal business park, cannot be solely capable or responsible for controlling flooding on the River Mole.  These land users might endeavour to limit flooding locally to acceptable regulated levels but complete prevention of flooding is not possible.  A holistic approach using a variety of hard and soft engineering techniques across the whole drainage basin is likely to be most successful in controlling floods and the embryonic Upper Mole Flood Alleviation Scheme (UMFAS) is just such an example. Nevertheless, even with UMFAS, some flooding along the River Mole will continue to occur across the flood plain, an area which includes the airport, during times of exceptionally high rainfall totals and during high rainfall intensities (the latter becoming particularly more common).  These weather events are still “natural” on a river basin scale and, whilst airports can attenuate flow off runways by careful management, they cannot completely prevent floods from occurring that are caused by intense rainfall across an entire drainage basin.  Overwhelming discharges will inevitably overwhelm flood plains should they exceed the design capabilities of the engineered defences.  Nevertheless, it is still the responsibility of major land users to control floods and reduce impacts to acceptable levels and to protect key infrastructure and this is what Gatwick is doing by investing in modern flood control schemes both within the airport boundaries and across the whole Upper Mole catchment.

What follows are some details about Gatwick Airport engineering schemes designed to control water flows in and around the airport.



Pond M – is a relatively new balancing pond.  Its role is to attenuate airfield runoff and complete initial treatment of contaminated runoff.  Especially during wet periods, it transfers a controlled discharge of clean water to the River Mole by allowing outlets in a chamber to overflow clean water into the river after passing through “interceptors” that remove any remaining aviation fuel or silt.  Clean water from Pond M is discharged into the River Mole from a controlled overflow shown below. The volume of water discharged into the river at this location is designed to be small even at times of peak rainfall intensity.  During rainfall there is discharge at greenfield rates into the River Mole from this balancing pond.  Summer runoff from runways is cleaner so more water is discharged into river as there are no anti-icing used.  In winter more of the water from Pond M is contaminated and so enters the transfer system to Pond D for further treatment.

Prior to entering balancing pond M, runoff from the runway passes through a sensor that continuously measures biochemical oxygen demand (BOD) and detects anti-icing chemicals  and other contaminants and diverts contaminated water with BOD >10mg/l to the contaminated holding pond for storage.  This is then transferred to Pond D for tertiary treatment such as removal of oil / aviation fuel and silts and thence contaminated water is sent to pollution lagoons for further treatment prior to entering the Crawley Water Treatment processing plant east of the runway near Tinsley Green.

aircraft washing creates contaminated waste

aircraft washing creates contaminated waste

There are two aircraft stands where aircraft washing is permitted.  These two stands drain to a treatment plant which removes heavy metals released during aircraft washing including cadmium. Within Pond M catchment there are 2 aircraft stands.


Pond A – is a balancing pond located at the outflow of the River Mole from its culvert under west end of the runway. Pollution monitoring also takes place here of all water shed from the west end of the runway. After flooding of the runway in 1967 the culvert was found wanting so a “syphon” was added. A syphon is essentially an additional channel which provides more capacity at times of peak flow to reduce the risk of flooding on the runway.

Pond D – this is the central “hub” of water control and treatment at Gatwick Airport and sits just behind the North Terminal.  Pond D receives all dirty contaminated water from LGW runoff from each of the balancing ponds e.g oil / aviation fuel and anti-icing chemicals. D pond also drains the eastern area of airfield and both terminals.  Pond D is sited approximately 50m above sea level and a levelling gauge indicates the height of the water in the pond.  During the 23-24 Dec winter flood Pond D was at its highest ever level at around 55m asl and was above the level of the culvert feeding it.  A more extreme weather event could cause Pond D to be overwhelmed and over-top, in which case the North Terminal, being lower elevation, could be flooded at ground level.

Three massive archimedes screws lift water from Pond D, aerating it at same time, up to separation ponds for clean and contaminated water.  Each screw can lift 840 litres per second.  Gatwick is permitted only to use two screws at any one time, the third being present in case of mechanical failure of the others.  Two screws are therefore said to be “on duty” whilst a third is always on stand-by.  During peak storm runoff the two screws are able to lift an absolute maximum of 1680 litres of water per second from pond D to flow through two parallel separators. Biochemical Oxygen demand (BOD) is monitored continuously to check the status of water entering the ponds and separate accordingly into clean or contaminated ponds.

Clean water enters a separate pond where it is aerated to reach a required standard before being discharged into the River Mole just downstream of Povey Cross Bridge via a surface spillway built to allow water to flow naturally, under gravity and unimpeded out of the pond once a certain level is reached during flood conditions. There are no gates and there is no impounding of large volumes of water that could then be “released” to cause a sudden flood peak in the Mole downstream.  The additional discharge into the River Mole from Gatwick airport from Pond D cannot exceed 1.6 cumecs because the two screws jointly have a maximum capacity of 1.6 cumecs.

During “normal” river flow and “average” rainfall conditions there is little or no discharge of clean water into the River Mole.  The rate of discharge into the River Mole from Pond D depends on the rate of flow of the river itself.  When the River Mole is at high flow conditions more airport water is permitted to be discharged from the clean pond.  Rates of discharge are set by the Environment Agency and automatically controlled by automated systems at Pond D.

The contaminated water from Pond D is further cleaned of silt by use of interceptors and any oil or aviation fuel is removed by running absorbent filaments through the surface of the water.  This is then extracted by means of mechanical pressure.  The process is continuous.  The water from this pond is then pumped along a 3.5km pipe to pollution lagoons outside the eastern boundary of the airport and thence finally to the Crawley water treatment works where it joins the water arriving Crawley which is is cleaned to a standard non-injurious to fish prior to being released into the natural water course of the Gatwick Stream which joins the River Mole near the Hookwood roundabout.

Gatwick airport contaminated runoff is therefore treated through the Crawley water treatment works and is the same quality.  The flow of contaminated Gatwick runoff permitted through Crawley treatment works is 50 litres per second or 0.05 cumecs. The attenuated discharge of runway runoff in the Crawley treatment works represents no significant “additional” runoff other than what would have reached the Gatwick Stream or Mole in natural conditions.

With the maximum discharge of clean water at Pond D at 1680 litres per second (1.6 cumecs) and the rate of flow through Crawley treatment plant at 0.05 cumecs, the additional flow to the river network is comparatively small.  The Gatwick Stream at flood peak can achieve ten times this flow.

A new facility at Pond D also extracts clean water for use in the airport fire hydrant network around the perimeter of the runway, increasing the capacity for the airport to attenuate runoff by recycling water within the airport boundary.

All pumping stations relating to water management across LGW have back-up facilities in case of power outage, especially critical during times of flood.

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

The whole system of water management is mostly gravity fed and fully automated.  Water quality is checked with 100’s of samples every month.  The Environment Agency attaches strict conditions to the quality of water entering the River Mole so that pollution discharges are minimised and that water entering the river is the highest quality possible and not injurious to fish.  Fines can be applied if the airport is found to cause pollution along the river, which has happened in the past but it is fortunately a rare event due to tighter controls and monitoring.  One incident in the late 1990’s involved a new product being used to de-rubberize the runway, which naturally gets rubber build up from aeroplane tyres.  The product being used was based on orange peel but some accidentally got poured down a gutter causing the death of fish downstream in the River Mole.  The Environment Agency fined LGW something like £50,000.  This kind of incident is fortunately rare because of the sophisticated pollution controls on site and strict monitoring by EA and compliance by LGW of pollution treatment and potential run-off.

1958 Gatwick Stream culvert construction

1958 Gatwick Stream culvert construction

The culvert running underneath the South Terminal and railway station, built in 1958, unfortunately was not designed with sufficient capacity to accommodate the river discharge during times of extraordinary peak flooding along the Gatwick Stream (i.e. exceeding 15 cumecs).  The culvert can accommodate only peak flows during 1:50 year flood events.  Larger, less frequent flood events with higher discharges cannot be accommodated by the present culvert.  It is not possible to rebuild the culvert due to the extensive airport, road and railway developments that have taken place above it, so a modern attenuation scheme upstream of the culvert has been constructed and is due to be finished in August 2014.  The Gatwick Stream scheme is an airport initiative that complements the wider Upper Mole Flood Alleviation Scheme (UMFAS).

  • Details for the wider Upper Mole Flood Alleviation Scheme (UMFAS) have been posted here before, find details here.
  • LGW contributed £4 million to the UMFAS inc Worth Farm, Tilgate Lake (both completed), Clays Lake (due to start in September)
  • Gatwick Stream scheme estimated cost £12 million (completion August 2014)
New flood attenuation scheme

New flood attenuation scheme

The new Gatwick Stream flood alleviation scheme will be open to the public and appear as a rather unusual park-like layout not dissimilar to “Teletubby Land”!  Grassy mounds with numerous oak trees have been retained to conserve local bat populations and increase the park-like appearance of the area.  Footpaths will encourage the public to use the area for recreation.  Due to the proximity to the end of the runway, large flocks of birds will be discouraged from the site to reduce the risk of birds striking aircraft.  This will be achieved by pumping out any water residing in the basin into the stream to maintain a dry environment thus discouraging flocks of wetland bird species, for example.

The Gatwick Stream has been diverted and modified into a new meandering natural-looking channel course.  Fish have been re-stocked.  Flow will be monitored continuously at the South Terminal culvert entrance.  If the Gatwick Stream discharge exceeds the 1958 culvert maximum of 15 cumecs then the gates will inch down and attenuate the river flow which will spill over into the newly created basin.  Flood waters will fill the basin but will be allowed to discharge naturally as soon as the flood peak has passed.

The park will be open to the public.  Interpretative signs will hopefully be in place to explain the scheme and link to the UMFAS and Gatwick flood and water management and wildlife conservation to present flood management in the Upper Mole Basin.   This seems to be an excellent opportunity for Gatwick to be on the front foot in terms of displaying a modern flood alleviation scheme put in place on schedule and with public and wildlife conservation interests included.  This could be a triumph of modern hydrological engineering made necessary by past mistakes (i.e.1958 culvert being built too small)! It combines modern soft approaches to flood management, such as reintroducing meanders, conserving wildlife and allowing natural wetland environments to attenuate discharge, while also including the hard engineering necessary to protect critical national infrastructure.  The scheme should also benefit Horley residents by reducing flood risk downstream.  This demonstrates the importance of Gatwick as an international facility requiring protection and the seriousness with which the airport takes its responsibility to control flooding both within the airport and for residents downstream in times of climate change.

Car parks are self-balancing

Airport parking is another large scale land user and comprises further extensive impermeable hard surfaces that could potentially increase surface run-off and create additional flood risk.  Deep ditches surround these car parks and these were designed to reduce the need for balancing ponds.  The ditches themselves provide self-balancing around each car park.

Long term car parks at the North Terminal discharge water from the ditches into a balancing pond known as Dog Kennel Pond and Pond G. Complex control systems handle pond levels and during intense rainfall some car park runoff is discharged into the River Mole near Povey Cross Bridge. The discharge is cleaned using interceptors and so is considered to be a clean flow as it matches the same quality of water that would runoff from any road surface.


Y shaped and new round pollution lagoons:

The final piece of the Gatwick water management jigsaw is a 4km pipeline from Pond D that transfers polluted water to the pollution lagoons clearly visible on maps and Google Earth.  These lagoons are netted to prevent harming wildlife.  The lagoons further treat by aeration contaminated water prior to transferring it to the Crawley Treatment plant from whence it is discharged into the Gatwick Stream along with waste treatment from Crawley.

The final destination for most contaminated airport runoff water is Crawley treatment works which receives airport water from the pollution lagoons nearby which, in turn, received their water from the 4km pipeline from Pond D. The maximum rate of treatment for airport water through the treatment works is 50 litres per second (0.05 cumecs) which, again, is of little significance to the Gatwick Stream at times of high discharge.



Gatwick airport is unfortunately sited in an impermeable flood plain vulnerable to flooding.  In response, the airport has progressively built a sophisticated hard engineering water management system that collects, separates, cleans and discharges a controlled flow of water back into rivers via complex transfer through a series of balancing ponds and cleaning processes.  The system has also had to address past mistakes in engineering that proved wanting in times of more recent intense rainfall events.  The majority of contaminated water is discharged into rivers through Crawley Water Treatment works.  During periods of heavy or prolonged rainfall some clean water can be discharged from Pond M and Pond D directly into the River Mole.  The LGW water management system controls runoff into local streams and this effectively “attenuates” runoff to the equivalent of a grassy greenfield site from each of the eight ponds around the site.

Water management and flood control at Gatwick airport is more strictly regulated and more carefully monitored than many expanding urban areas in the same catchment, for example, in Crawley and Horley.  These urban areas, creeping inexorably onto flood plains can exacerbate flooding to a greater degree than the airport alone, though this would need more detailed investigation to “prove”. The causes of flooding in the Mole catchment are a combination of factors which were explored in previous posts on this blog found here.

Importantly, the management of water flow within Gatwick is sophisticated, automated and ongoing. There are no “flood gates” that can be opened to cause a sudden increase in discharge on the River Mole.

Inside the airport, the major flood event on 23-24 Dec 2013 was related to intense rainfall causing rain water to migrate through the maze of 1980’s airport piping, drains and ducting and, unfortunately, discharge through a wall directly into an electrical facility servicing the North Terminal.  The complex organic historical growth of the airport has created unique problems of management that require continuous investment and engineering to solve.

Finally, the airport has impressive “epic” engineering structures that control, separate, transfer and treat runoff from the runways continuously and discharge an attenuated flow of clean water into the River Mole and contaminated water via the Crawley treatment works.  Discharge is usually low but, it is worth remembering that all engineering structures are built to a design limit.  New water engineering schemes at Gatwick, such as Pond M, are being designed to protect against rare events of exceptionally high rainfall with return periods of at least 1:120 years.  Events yielding rainfall in excess of these design capabilities will still cause flooding both within the airport and downstream.  Plans to add another runway should include awareness of flood attenuation both within the airport boundary and for communities downstream.

The River Mole could become a “beacon” example of modern holistic river management!  It could also become a vital green lung for this part of the South East alongside a major international airport.  As the airport hopefully continues to invest responsibly in both soft and hard management along the river course, as much of the river as possible should be made available for recreation and public access.  The River Mole VISION should be for a continuous green network of footpaths and attractive open spaces to exist along an ecologically rich riparian zone for local residents to enjoy: an unbroken network of paths from Rusper, through Crawley and Gatwick and onward to Dorking and through the Mole Gap into Leatherhead and Fetcham to Hampton Court!  Such a “Mole link” could enhance the natural diversity and varied landscape of the area as well as celebrating the vibrant economy, history and even enhancing local tourism.  This vision will require continued work with local authorities, land owners and the public to encourage a wider understanding of the key importance and benefits of our river network to the whole community and create a network of access agreements to join up this linear park.  Involvement of the public in how they might wish to use the river, particularly with any impact of a new runway,  could be a key to successful management of the River Mole into the future.

The Nutfield Marsh nature reserve, a wetland habitat designed, in part, to attenuate flood runoff on Redhill Brook by designing a natural wetland, could be a design blue-print for the Upper Mole.  This nature reserve has partly been managed by school groups from Reigate Grammar working with Reigate Area Conservation Volunteers.  The nature reserve has a role to play in reducing flood risk downstream on the Redhill Brook. Public access and enjoyment of these interesting locations will be both educational, healthy and yield a greater understanding of the need for careful management of rivers and flood plains as we likely face climate extremes and changing flood risks in the future.

The information in this post is based on a lengthy tour and investigation of the water management engineering in Gatwick airport especially designed for RGSweather and kindly hosted by John Barber (LGW Water Quality Manager) and Tom McShane (Project Manager).  All sites pertaining to water management around LGW were visited at the end of March 2014.  John and Tom imparted expert knowledge and this post represents a collaborative approach to publish, as far as possible, a clear and factually correct outline of flood control in LGW.  Comments / further information /updates always welcome.