Flooding on the River Mole: causes, impacts and management

January 4, 2014 — 31 Comments

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

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

05-01-2014 14-50-47

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

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

Impacts of River Mole flooding

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

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

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

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

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

Types of Flooding in the Mole Catchment

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

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

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

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

Factors affecting flood risk on the River Mole

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Flood management in the Upper Mole Catchment

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

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

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

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

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

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

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

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

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

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

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

28-12-2013 08-18-43

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

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

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

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

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

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

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


Gatwick Greenspace partnership working to improve River Mole ecology


further reading and references

EXCELLENT video on sustainable flood management https://vimeo.com/137996880





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 https://adriancolston.wordpress.com/2015/12/09/storm-desmond-climate-change-land-use-and-what-it-means-for-dartmoor/



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

31 responses to Flooding on the River Mole: causes, impacts and management


    A very interesting read.
    We were flooded out on Christmas eve in Charlwood so we know the mole all too well.


    A very enjoyable post, thank you! Your students are very lucky to have such a knowledgeable member of staff teaching them!


    Very interesting article with a lot of background information as to the flash effect of the river. We too were flooded on Christmas Eve. However, it would appear from reports inthe local press that may have been due to a floodgate being raised at Molesley thus preventing the rising water level in the River Mole from being discharged into the Thames, rather than solely due to the rainwater flowing off the catchment area.


      Aha, many thanks for the info and reading the post… I had heard rumours of this happening but had no verification, so this is most helpful. Info on the Lower Mole flood defences seem to be rather harder to find than the Upper Mole so any user-info gratefully received, thank you.


    An extremely interesting and informative post. Thank you


    The environment agency seem a little disingenuous here : http://www.dorkingandleatherheadadvertiser.co.uk/River-Mole-flood-scheme-designed-protect/story-20418003-detail/story.html
    – in not quite spelling out clearly, that the protection afforded to Hersham, was probably at the expense of Leatherhead. There is no transparency over how decisions are taken. The environment agency is reported to have said: “Leatherhead is 10km upstream of the Lower Mole Flood Alleviation Scheme and [it] is not designed to protect Leatherhead, but it does help reduce flood risk along the River Mole by improving the flow of water into the River Thames. The scheme was operated successfully and protected 10,000 properties in the Hersham and Molesey area.” In other words, I believe she means that the scheme successfully protected 10,000 properties in the Hersham and Molesey area AT THE EXPENSE OF LEATHERHEAD. There might very well, I do accept, have been a positive cost-benefit on that decision (depending on what property damage could have ensued there, near the Thames), or it may have been taken on a rather arbitrary basis ….. there’s no transparency and she’s not even being straight about what sounds like a trade-off, so that it cannot be properly explored or discussed. Just defensive bureaucratic language – the art of public relations. But it does at least hint at what sounds like it may have been the truth. Nor can I find transparent statements about whether or not a reservoir at Gatwick was “emptied” just before or at the start of the floods, presumably so that it could then protect Gatwick by filling up again later. However, if it did later fill up as the rain came down, one can see that its effect could well still have been an ameliorant, or dampener, on peak Leatherhead flooding – but if they won’t be transparent, how can be know ??


      Many thanks for your feedback, this is very interesting.. I have requested detailed information from the EA regarding the events on the Leatherhead – Esher reach of the Mole during the flooding especially 23-24 Dec. The Freedom of Information Act and Environmental Information Regulations state that a public authority must respond to requests for information within 20 working days, so I’ll post their response in due course. many thanks for reading the post. Simon


    It would be interesting to find flow data pre 1972 to see what significance of the run-off from Gatwick has contributed to the flooding. Living at Kinnersley Manor and looking at the old photographs of the river when it used to be kept clear by the river authority, comparing then and now the river if full of fallen trees and these have created what look like beaver dams. Spending millions on new defences may help, but clearing the river should be a priority. It’s like installing bigger gutters because the down pipe is blocked. Contac me if you want some pics of the fallen trees.


      Hi Matthew, Thank you for this, yes hard-management is exactly like installing new gutters really: it speeds water through to the next jam! It’s probably better to address issues like blockages but also of course the nature of land use in the wider catchment. SUDs are probably a good way forward. Any photos very gratefully received and acknowledged.
      best regards


    This is a very interesting web site,so congratulations on producing it.

    I found the site as I’m interested in trying to assess the usefulness of the Green Belt as a “reservoir” helping to protect London from floods, which would be a very complex project to undertake, but one that would be useful to the many groups trying to resist development on the Green Belt. Because of this I was attempting to assess the relative importance of land absorbing water and run off, so looked in some detail at your plots of lag time as they should give some indication of the proportion of rain that creates effectively “rapid” run off and the volume absorbed into the soil. I was horrified to see that about 73% appeared in the river within a few days, as this would mean the Green Belt was not terribly useful. Then I thought some more about it – although the catchment area above Horley includes Crawley and Gatwick, most of the 89 km^2 catchment area should be woods/grass/agricultural land. So I looked at rainfall records on the Sutton and East Surrey web site to see if that had been a wet period. Their figures suggest not, with September and October being exceptionally dry; November rainfall a little above the long term average and December below the long term average. So no evidence that the ground would be saturated. The actual rainfall event, 11.5 mm over 10 hours is not much of a rainfall event, and reflects light rain falling steadily rather than a torrential downpour, so ideal conditions for the rain to be absorbed into soil. So, intuitively, I found it hard to believe that almost 75% ran off in quite a short period of time.
    So then I looked at the maths in the figures – and I cannot get the same figures. For example, in the effective rainfall calculation, 7.6/89 = 0.0853m, or 85.3 mm, not the 8.4 mm reported. By way of a check – rainfall was recorded as 11.5 mm so assuming this was typical of the catchment area (possibly a major flaw in this) the total volume of rain was 89 x 10^6 m^2 x 0.0115 m = 1.024 x 10^6 m^3. But the volume under the curve was given as 7.6 x 10^6 m^3. Something not right here. Can you help? Is there an error in my calculations? Or an error in the area under the curve?

    Ultimately, what I’m interested in is the capacity of Green Belt land to absorb water and know this will vary according to the underlying geology. But I hope in this example that it was rather more than just 25%, especially given that the catchment area above Horley is largely a mix of moderate and low permeability bedrock.

    Hope you enjoyed the holiday period.


      Thank you for feedback Roland, your assessment of Green Belt use as reservoir / flood attenuation sounds like the sort of holistic far-sighted approach to flood management that could be adopted more widely. Your calculations of green-belt lag time also sound interesting, though tbh I need more detail on how you did this to be of much help. You are right about previous months in the lead up to this winter being rather dry too, certainly drier than equivalent months in 2012. Estimating rainfall across a drainage basin is, I think, fraught with error, as from rainfall events we have experienced this winter totals can vary widely. Averaging rain gauge totals from several stations is probably the way forward before accurate lag time for a particular point on the river can be sought. My rough guide above is just that, only a rough estimate for Sidlow. Oddly enough, I cannot find any lag time data from EA sources: if you dig anything up then let me know! Here are some pages that might help you further with this http://www.wou.edu/las/physci/taylor/hydro/prcpanal.pdf


    Thanks for the info – i have aways been intrigued as to why the Mole flooded so often in my home town of Cobham but never take the trouble to look too far. I work as a landscape architect and am very interested in the natural / SUDS approaches to flooding since i feel that the area could definitely benefit from improved wildlife habitat and recreation (notably the continuous Mole path you mention). Growing up in cobham i have always harboured a wish to convert the fields adjacent to the mole into a natural detention pond and wetland area since they appear to have little value as agricultural space and in addition to flood attenuation they could provide a much valued walking / cycle link between the town centre and the station. at present this area is fenced off and residents must walk along the road (or as is more common get in their cars and drive). I am not sure what benefit the current landowner gets from this land but i wonder if there are any EA or similar grants that you know of to purchase and convert strategically valuable areas of land into natural flood attenuation schemes – do you know of any such grants?

    Furthermore with the governments new focus on larger Natural Improvement Areas (in recent White Paper) for promoting landscape level conservation (and providing ecosystem services such as flood reduction / water quality) i wonder if the Mole catchment area could form such an area in future with the necessary stakeholders onboard and promotion etc…. if this is an area you have ever considered pursuing i would certainly be interested in getting involved or discussing further.
    best regards,


      Hi Steve, many thanks for the information regarding the Cobham stretch of Mole, the bit of the river I know least well, so very welcome. I must take a visit to the lower Mole flood scheme (check the update from the Env Agency in the blog above for some details about the flood relief channel: I suspect with this channel in place there would be less possibility of further investment in soft defences, but you never know, worth investigating) You might be interested in news that the Wildlife Trust is recruiting advocates for the River Mole prepared to do some regular measuring and monitoring of the river through a new initiative called Riversearch (@riversearch). If you are interested then please email personally sac@reigategrammar.org and I can forward details of how to get involved directly. Also, you might be interested in the updates on the Gatwick flood alleviation schemes that I have written about more recently: my trip round the airport with the water engineers and project managers was fascinating and filled in plenty of details about the Mole through the airport. They were most helpful.
      best regards,


        HI Simon,
        Many thanks for the quick reply and info on the Wildlife Trust advocates, i will certainly look into this later in the summer when i have less work commitments. As for the lower mole flood alleviation scheme there seems little info available on the web other than that news article. it certainly seems to lack effectiveness for cobham (and leatherhead as Paulo noted in his earlier post) since the mole has flooded onto the adjacent Stoke Road (in cobham centre) for as long as i can remember, even after an expensive upgrading of the weir adjacent to the old mill. However there is also a large flood plain on the other side of the river which takes up much of the water although it is my feeling that these fields could be better managed as a wetland / wet meadow area with improved flood capacity as well as the obvious wildlife and leisure benefits. i am not sure how much difference such an intervention would make in the wider scheme of things (if there is always much more water backed up further upstream) but it would be interesting to raise with the EA. One of these days i hope to take a kayak out and complete the section of river between dorking and cobham and take a closer look at the wider area but again this is something for later summer. thanks also for the new info on the Gatwick scheme, i found the article very interesting and am pleased to see that the drainage engineers are finally starting to consider some softer approaches to flood alleviation (despite the obvious bird strike issues). In my experience most drainage engineering schemes seem to end up with a very bland detention ponds with little wildlife, leisure or aesthetic interest so the new Moors project looks like a positive step forward. And if a further runway does appear in future it may provide some much needed impetus for the wider improvements to the Mole that you mention in your article – we will see!
        thanks again,


        Thanks Steve, agree about using meadows for further flood attenuation wherever possible. I’ll join you in that kayak if you get the chance… is it 2 seater?! What a super way to see the river!


    NB, the Mole used to have mink on it to my knowledge, as up to 2002, the Wealden Mink Hounds used to have a meet on it at Cobham. It then became too hard to have a meet there, as it had very slowly become less rural over time. I don’t know if it has many mink on it these days ? (if it does, there certainly won’t be any water voles)


      Thanks for info Paulo, I think the new Riversearch initiative is tasked with monitoring for mink amongst other key species, so it’ll be interesting to find out.


    Hi Simon,
    yeah, the kayak would be a great way to see the river from a different perspective, especially in this fine weather! i don’t actually own a kayak or canoe but was hoping to rent one from Kingston depending on how easily available transport can be organised. I have made contact with the Riversearch co-ordinator (Glen Skelton) about some future surveying so thanks again for that contact. Glen also advised that i should speak to the Canoe England rep about the kayaking as he said it can be a little contentious with some landowners and fishing clubs where you pass through ‘their’ section of river, certainly not a deal breaker but one to tread carefully around. Will keep you posted.

    Stephen Spark May 21, 2014 at 10:43 pm

    A very thought-provoking and informative post with some very interesting contributions. I have lived (on and off) in Stoke D’Abernon all my life and remember well the destructive effects of 1968 floods, which destroyed Downside Bridge and a railway flood arch south of Cobham station.

    Perhaps with this in mind, the EA’s flood maps show my property, between Stoke and Oxshott, as at high risk of flooding, yet all but one house in the road remained dry in 1968 – because excess water used the railway cutting as a flood channel to exit to the Mole. Until very recently, the EA maps failed to take this manmade feature into account. To give the agency its due, when I raised this point earlier this year, two EA officers came to meet me and we had a very interesting and productive site visit – full marks for responsiveness.

    The neighbours who suffered flooding in 1968 did so because a stream ran in culvert under their garden and this culvert became blocked by a fertiliser sack. In the 1970s the stream (called by the EA, but no one else, Fairmile Ditch) was diverted into a large concrete pipe – fine for conveying excess outflow from the Cook’s Brickworks Pond and the marshy area of Watercut Copse, but useless for dealing with runoff from the fields etc. Areas that used to be well drained have become semi-permanent bogs. The stream bed ought to act as essential storm drainage for the Stoke/Oxshott area but, along with roadside ditches and culverts, has become so clogged by debris and vegetation that it is ineffective, leading to the sort of flooding noted by Simon Rusby.

    So in addition to the ‘macro’ level issue of the overall Mole catchment, which causes the wider flood problem, there is also a ‘micro’ level one of inadequate/poorly maintained drains, ditches and culverts causing highly localised flooding. Confusion between the two has, I think, led to some of the anomalies in the EA’s flood maps, given that the level of risk is based on past flooding incidents as advised by local councils. Whether councils have the knowledge or resources to distinguish between different causes of flooding in the aftermath of a major storm event is open to question. Failure to understand the micro as well as macro aspects could lead to scarce resources being spent in the wrong places and on the wrong solutions. And as your post points out, the wrong solutions can exacerbate rather than reduce flooding risk.

    Finally, I’d add that the fields opposite Cobham Mill (which has a useful measuring post showing the height of past flood events) are known as ‘water meadows’ for good reason, yet still there we hear intermittently of plans to build houses or a road across them. Memories are short… I am fully supportive of all attempts to provide ‘soft’ flood alleviation solutions such as SUDS and, provocatively, would suggest a tax on impermeable surfaces!


      There have been a few comments above on SUDS, and so I think a few observations are in order. Firstly, Gatwick has SUDS systems in operation – but these did not prevent the airport from flooding, on Christmas Eve 2013, nor earlier in that year. SUDS are like any other drainage system – if they are not maintained, they will create problems. Consider a semi permeable surface – over a period of time, with leaf debris, grit, etc etc accumulating – the surface becomes clogged and essentially becomes impermeable, unless cleaned and maintained. Who will do this? – most systems are installed and forgotten about. Then consider the storage element – these are designed to cope with a specific rainfall event. But this takes no account of prolonged periods of rain, such as we had last winter. So storage systems can simply be overwhelmed, either by a single rainfall event greater than the design, or by prolonged rainfall. The overflow from storage is then likely to contribute to urban run off, as is the run off from semi permeable surfaces that have become close to impermeable – just at the time there is the greatest need for the SUDS system to work as designed.
      If you care to check on u-tube, you will find many clips of SUDS that have created problems. They are definitely not a long term trouble free answer to urban run off and should not be considered as such. What would help would be an extensive maintenance programme put in place, with long term funding. But as Gatwick illustrated twice last year, even this may not be enough, systems can simply be overwhelmed.

      I think the answer to the Mole flooding is more temporary storage, and there is a need to fully assess where this can be done. Along Bookham Brook, for example, there is an area (of agricultural land) that could be flooded on an intermittent basis, and the topography of the land is ideal for this. It borders Bookham Commons, and perhaps more temporary storage could be provided here too. I’m sure a careful survey would reveal other areas that are suitable, all along the river. Temporary storage can be provided in rural areas too. In the Netherlands, temporary flood storage is sometimes provided by underground car parks, generally associated with buildings that are close to flood plains. These car parks fill with water and are then pumped out after the flood has subsided.
      I don’t believe there is an easy answer to prevent river floods from causing damage to homes – but am sure much more can be done to alleviate it. The best place to start is keeping river courses and associated drainage ditches clear, combined with using temporary storage, such as natural flood plains, as well as man made temporary storage, in both rural and urban areas. I’m equally clear that the more we build, the greater the flood problems will become, even with the use of SUDS.


        Thanks for interesting update on SUDS, which is very helpful in giving a fuller picture of their “cons”, something which is clearly overlooked in trying to find solutions. Keeping ditches and culverts is a recurring theme in recent replies from contributors which seems crucial to stop acute flooding. Also thanks for the reminder that all systems will have a design capability, something which stood out well when I met with the LGW hydro-engineers who were very helpful in passing on info for the updated post on flood control within the airport (on this blog). The other clearly emerging theme seems to be that the rather unglamorous “hands-dirty” approach to flood control is just as important as big flag-ship attenuation schemes.. e.g. continuous maintenance of drainage ditches, clearing of gutter and culverts. Many thanks for adding your contribution, I am delighted that the post has encouraged people to comment and add their thoughts and hope that this growing wealth of combined community knowledge on the Mole might assist decision makers in due course.


      Thanks for posting the note Stephen, very interesting to hear of specific floods caused by problems as local as a fertiliser sack stuck in a culvert… similar thing happened in Smallfield this year. Macro/micro level management also seems spot on. Many thanks for adding such useful points to the original post. Can I also point you towards my more recent post detailing the hydro-engineering in Gatwick itself.


    Pretty awesome report and observations. Thanks for that! I’ll keep following your posts.


    Hi there
    Glad I found this article.
    I’m wondering if there is any kind of EA support for residents with riparian rights struggling with river bank erosion?
    Many thanks –

Trackbacks and Pingbacks:

  1. Flood-Gate? Gatwick Airport flood and water management in the Mole drainage basin « Reigate Grammar School Weather Station - April 22, 2014

    […] With options for a second runway at Gatwick airport now firmly in the public eye again, this post explores flood control engineering and water management currently in place at the 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. The previous post on the causes and management of flooding more widely in the River Mole drainage basin can be found here. […]

  2. Welcome! « Reigate Grammar School Weather Station - October 31, 2014

    […] weather, observations and educational posts. SkyWarnUK storm spotting. RiverSearch monitor for River Mole. RMetS education committee. Consult professional websites  for decision making purposes. Town […]

  3. #Reigate 2014 annual weather report « Reigate Grammar School Weather Station - January 6, 2015

    […] Atlantic storms battering the west coast and delivering high rainfall totals and significant flooding events across the SE, including our own River Mole and impacting Gatwick airport.  This blog […]

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