Development threats to runoff in the Upper Mole Basin

Let’s look at three large scale, contrasting development sites in the Upper Mole basin and try to assess their potential impact on river flow and their plans to manage drainage and flood protection. (I hope to look at other, even bigger, housing developments eg West of Ifield, separately in due course).

  1. West of Bewbush: Housing estate for 2500 homes on the far SW edge of Crawley.

  2. Center Parcs: proposed new holiday village on the forested watershed in an ancient woodland on hills south of Crawley: 900 lodges, parking for >4000 people.

  3. Hookwood housing development: 563 houses in flat, poorly drained, low lying land north west of Gatwick airport.

First, some context.

The streams in the Upper Mole catchment descend steeply down wooded slopes from hills surrounding Crawley and Gatwick. The hilly and wooded landscapes of Buchan Park, Tilgate and Worth Forests and Oldhouse Warren are the source of numerous streams flowing from the watershed into the highly urbanised, largely flat, Upper Mole basin. Forested watersheds like this slow down runoff into river basins and act to reduce flooding.

However, the steep slopes, high density drainage (lots of streams) and impermeable Wealden Clay geology still combine to create naturally flashy streams in spite of the forested hills above Crawley. This means that streams in the Upper Mole rise quickly and threaten flooding in urban areas. The area suffered from significant flooding in 1947, 1960, 1968, 1974, 1990, 2000 and the winter of 2013 to 2014.

Flood risk is not restricted to housing but has also impacted national infrastructure and business, for example the M23 and other major roads (where tributaries are culverted), the London to Brighton railway (e.g. Three Bridges), Manor Royal industrial estate and Gatwick Airport.

It is the behaviour of tributaries in the Upper Mole that sets the scene for flood risk downstream. Flood peaks, as shown above, travel all the way from Gatwick to Horley, Dorking, Leatherhead and Cobham and to Esher, reaching the Lower Mole in around 48 hours, sometimes much less. Careful management of the River Mole headwaters in the wooded hills to the south of Crawley is therefore vital to sustainable flood management and to maintain the quality of water.

An holistic catchment approach is required to manage potential downstream impacts from development in the Upper Mole. This means stakeholders, land owners and residents working together to manage the whole Mole catchment. To a great extent this is happening already with incredible work from several groups and charities like South East Rivers Trust, Friends of the River Mole (FoRM), Lower Mole Partnership, Gatwick Greenspace and Angling groups, working with Local Authorities, Thames Water, Wildlife Trusts, Gatwick airport and the Environment Agency and some farmers towards a better Mole. From my personal experience meeting with several of these groups, there is a passion and a determination with plenty of hard work happening on all sides to improve the Mole.

Recognising the risks and after further floods in 2000, large sums of money were jointly spent by the Environment Agency, Gatwick Airport and local authorities on the Upper Mole Flood Alleviation Scheme. The UMFAS improved flood protection for the Upper Mole by raising the dam wall at Tilgate Lake, upgrading Clays Lake attenuation pond, completing a flood detention reservoir at Worth Farm next to the M23, reinstating meanders and natural flow management in Grattons Park and building a £12 million flood alleviation scheme on the Gatwick Stream. These schemes were designed to delay the flood peak by retaining it behind dams or in attenuation ponds that fill up in times of peak discharge and can then be released slowly to avoid flooding downstream.

Tilgate and Clays Lake were designed to manage storms of up to 1:1000 year return period at the time of building, which seems impressive (and I must confirm this is correct again!). However, stated flood protection capacity does not take into account development or land use change upstream or the increased intensity of rainfall likely due to climate change. Unmanaged, these could mean a reduction in the effectiveness of flood protection schemes as discharge increases and lag times reduce beyond their engineered capacity. This has already happened at the Gatwick Stream Flood Alleviation Scheme where protection has reduced from the original 1:100 flood protection to around 1:75 year return period due to a combination of climate change and building upstream increasing discharge. The same magnitude flood will now return more frequently.

In any case, despite the UMFAS, the streams themselves naturally remain flashy, discharging water quickly as shown above by the recent hydrographs for a storm in November 2022 which caused some over-bank flooding. The hydrographs show that the stream which flows through Maidenbower from the hills in the Worth Forest, is more responsive to rainfall than the River Mole itself as measured at Gatwick. The stream running through Crawley (which is confusingly called the Gatwick Stream!) rose to a higher peak more quickly in the same rainfall event than the River Mole as measured at Gatwick (apologies for the confusing names here). I would suggest this is because of the hilly terrain from the sources to the south of Crawley.

So, the streams running through Crawley and under Gatwick and down into Horley, are highly sensitive to rainfall. The predominantly forested watershed increases interception and soil storage and helps to reduce flood peaks. Any development in the Upper Mole basin must therefore avoid increasing discharge or reducing lag times in an already flashy basin by inappropriate land use change and poorly planned development in watershed catchment areas. I would argue that the forests in the hills above Crawley are vital to retain, manage and conserve. This is for the benefit of flood control as well as the incalculable value of the biodiversity, wildlife and water quality provided by extensive woodland.

The Three development sites

So, let’s look at how three contrasting development plans perform purely from their potential impact on rivers and what their proposals look like to manage runoff. I have previously written about water management and flood protection schemes at Gatwick… in a nutshell the airport is highly regulated and adheres to strict guidelines working with the EA and Thames Water to mitigate flooding and pollution. Whilst no one would now choose to have an international airport built across the floodplain of a highly responsive, vulnerable river, Gatwick is the best neighbour at this scale we could ask for. The other obvious point is that the airport is already built! So the question is whether adding further pressure to the catchment through more development is desirable or manageable.

Gatwick Airport has invested in sophisticated and extensive water engineering to attenuate and process water. The airport has strict regulations on water use and how runoff gets into the River Mole.

The three new developments are at different stages: building at the West of Bewbush housing development is well underway; I understand that plans for the housing development at Hookwood are at an advanced consultation and planning stage with many of the objections I’ve read being somehow categorised as “unjustified”. Plans for the Center Parc development at Oldhouse Warren is at an early stage with no formal planning application lodged just yet.

I am not an activist or have a political agenda and I am not a NIMBY (“not in my back yard”) or indeed a BANANA (“Build Absolutely Nothing Anywhere Near Anything”)! The Upper Mole catchment is a densely populated and busy economic area where some carefully planned development will need to take place. Nevertheless, the location and management of any development is crucial to get right for the health of our river and the protection of the population in the future.

1. Housing development West of Bewbush

Location of housing development West of Bewbush

The site “West of Bewbush” at Kilnwood Vale, is an extension to housing already built along the north side of the Crawley Road between Crawley and Horsham. It is an extensive area of 132 hectares planned for 2500 homes to meet local needs. Part of the site was a former inert landfill (mostly construction waste). Parts of the estate are at medium risk from flooding shown below.

Land clearance is well under way with diggers and trucks working on the site moving earth around a huge series of knolls descending down to Bewbush Brook. I came across it for the first time on a walk around the Upper Mole which included Ifield and Bewbush Brooks.

A glimpse of the building site in November 2022 at Bewbush

The plans available online appear to take some account of the risk of developments in the Upper Mole basin increasing runoff and flooding downstream. The Joint Area Action Plan briefly mentions attenuation ponds and the retention of some meandering sections of Bewbush Brook in woodland upstream of the site.

No development is proposed in the Flood Zone. It is also likely that this area will accommodate any flood or surface water attenuation measures that the neighbourhood Flood Risk Assessment identifies.

Flood attenuation measures and implementation of SuDS will ensure that runoff levels are not increased.

JAAP Flood risk management p26-27

In addition, sustainable urban drainage systems (SUDS) are mentioned including rainwater harvesting from green rooves.

At the development level, this should include appropriate sustainable drainage systems (SUDS) that are capable of reducing peak storm run-off and increasing local infiltration that can reduce the likelihood of flooding and protect aquatic ecosystems.

JAAP p.37

SUDS involve the collection of water from rooves or hard surfaces from where it is filtered, sometimes naturally in reed beds, and stored in tanks and then used for laundry, toilets and outside use. The housing at the Olympic Village uses green rooves and reed beds to filter the water.

SUDS at Olympic Park

In addition to greywater recycling these techniques would manage immediate runoff into the brook especially at times of intense rainfall and reduce burden on water treatment works downstream. From reading the JAAP the need to provide additional sewage treatment capacity and a requirement for Thames Water to expand Crawley Sewage Works is also in the text. From my understanding, Thames Water are working to improve many of the SWT plants in the Mole catchment, for example, investing in screening out phosphorus to a tight new consent level of 0.25 mg/l before discharging into water courses.

Bewbush Brook, while diverted in a straight line on the edge of the site for now, seems to be planned as an integrated part of the housing and greenspaces. The straight line channel shown in the sketch below would not be ideal for a natural river either from wildlife or flow point of view. I hope this is adjusted to a more sinuous channel providing riffles and pools and meanders for aquatic wildlife, insects and birds.

So there is some evidence from plans and ongoing works of attenuation on site using swales (shallow, flat bottomed, vegetated open channels designed to convey and attenuate surface water runoff), vegetated naturalised channels and attenuation ponds.

Downstream of the new development, Bewbush Brook flows immediately into Bewbush Water Gardens. This is an attractive linear urban park that follows the river into Ifield Mill ponds, an area of historic interest and a natural beauty spot with board walks extending across the mill ponds.

The park is well integrated into the urban landscape and offers residents a lovely green space with a connection to rivers and streams, important if we are to appreciate them. The green space, ponds, channels and wetland area of riparian woodland and channels also act to attenuate runoff and flow into Ifield. So the new estate will hopefully add to this attractive green lung corridor.

A glance at Bewbush Water Garden

During construction of Kilnwood Vale there is likely to be an impact on water courses, for example Bewbush Brook has been diverted and will be again with an obvious impact on aquatic biodiversity and likely silty runoff during the building stage. However, notwithstanding the impact of building work going on now, the use of this brownfield location for much needed housing appears, at least from the plans available, unlikely to cause a significant increase in flood risk or deterioration of water quality downstream IF all the sustainable and flood attenuation designs are implemented fully. For example, there is no extensive loss of trees and there appears to be enhancement of vegetation in the plans along with efforts to attenuate flow downstream. If the measures in the JAAP are fully implemented then this addition to the housing corridor between Crawley and Horsham should be “hydrologically neutral” and add to the green corridor in place through Bewbush and Ifield, connecting these streams to the River Mole.

Center Parcs development in Oldhouse Warren

Below is an outline of what is proposed by Center Parcs at Oldhouse Warren from the CPRE Sussex website. Many objections have been raised about the tragic prospect of losing ancient woodland as well as taking chunks out of an AONB and a sensitive SSSI.

These reasons alone should be enough to persuade Center Parcs to abandon this location as wholly inappropriate as a holiday village. However, I want to look at the additional potential impact that a Center Parcs development might have on runoff into the River Mole at this location.

In July 2021 Center Parcs announced that they intend to seek planning permission to develop a new large leisure complex at Oldhouse Warren near M23 junction 10A between Worth and Balcombe. Whilst we have not yet seen their detailed plans; we expect that it will involve major swimming and multi-franchise restaurant and retail complexes, 900 lodges and treehouses, forest trails and a spa that will be open to daily visitors as well as residents. The park will have a capacity of around 4,500 visitors. The proposed site is in ancient woodland within the High Weald Area of Outstanding Natural Beauty (AONB), and adjacent to a Site of Special Scientific Interest. Mid Sussex District Council is required by law to give the highest status of protection from development to such irreplaceable, specially designated areas.

—CPRE Sussex

I don’t think formal plans have been made available yet (Nov 2022) but the general location of the proposed Center Parcs is shown on the map above. The map below shows that Oldhouse Warren is an extensive area of forest on hills over 150m high, some 90m elevation above Crawley and Gatwick. It connects with Worth and St Leonard’s Forest to the south west forming a woodland footprint larger than the Ashdown Forest. From these hills flow numerous streams that descend quickly into the lower lying Upper Mole basin.

Location of Oldhouse Warren

The Stanford Brook and Cowdray Brook flow from Tilgate Forest and Oldhouse Warren respectively meeting the Gatwick Stream in Maidenbower housing estate. The Gatwick Stream then flows under Three Bridges railway station, through Grattons Park, into the Gatwick Stream Flood Alleviation Scheme, through Riverside Park in Horley and into the Mole north of Gatwick at Longbridge roundabout (near Tesco). Like most of the Upper Mole tributaries, the nature of this catchment promotes a responsive regime and rivers which respond quickly to rainfall and can rise to significant levels.

The Stanford Brook and Gatwick Stream catchment have relatively steep slopes, descending some 90 metres to the river gauge at Maidenbower. The geology is 98% very low to moderate permeability and the forest extent is a significant 43% cover while urban area covers 30% of the catchment and is growing.

Despite extensive forest cover the regime is highly responsive to rainfall and numerous channels discharge water into Crawley quickly from Oldhouse Warren.

Flood risk map around Oldhouse Warren and Maidenbower, Crawley

An example of the rapid response was the September 1968 flood. Over 140mm of rain fell in a few days, with 100mm in 24 hours. This caused an estimated peak river level of 2.32m in Stanford Brook in Crawley, corresponding to a flow of 15.3m/s according to an extrapolation of the rating curve. In 2008 and 2014 the river level reached over 3m at Three Bridges. The lag times calculated in recent high rainfall events in November 2022 show a lag time of only 4-5 hours for peak discharge to reach Crawley after heavy rain. This is 2-4 hours quicker than the River Mole at Gatwick airport, for the same rainfall event. This is a highly responsive stream network!

Without conserving complete forest cover the flashy nature of these rivers will only increase causing enhanced flood risk downstream and, in all likelihood, reducing the effectiveness of parts of the UMFAS including Grattons Park and the Gatwick Flood Alleviation Scheme .

Steep slopes deliver flood risk to the Upper Mole Basin

Typical land uses of a Center Parcs holiday village turns forest into car parks, areas for lodges and tracks and large swimming pools and restaurant complexes. They claim Center Parcs “nestle into forests” and can even improve them. However, developments in a sensitive watershed like Oldhouse Warren could add considerably to the speed of runoff off due mainly to the building of impermeable rooves, surfaces, guttering, drainage and the necessity to remove water away from amenities. Parking for 4000 people alone will involve extensive felling of tree cover with an inevitable reduction in interception provided by mature trees. In addition, ancient forest soils tend to be thicker after years of leaf fall and this promotes infiltration and water storage further delaying runoff. Removal of any part of this native woodland would likely, without extensive engineering to attenuate enhanced runoff, lead to shorter lag times, increased discharge downstream and possible silting of streams due to erosion. A result of this could be more flooding in urban areas downstream and reduced effectiveness of Gatwick Stream Flood Alleviation Scheme 10km downstream. Any development of Oldhouse Warren should include rigorous analysis of this risk.

Oldhouse Warren and a Center Parcs car park

A quick overview of other Center Parcs suggests Longleat is the closest equivalent to Oldhouse Warren in terms of relief and stream network. It is similarly steep with a number of streams descending to a large river which then flows near an urban area. However, unlike OldHouse Warren, chalk and sands and gravels are the predominant geologies around Longleat and some 90% of the catchment is high or moderate permeability, so attenuating overland flow maybe a less pressing matter as more overland flow should infiltrate into permeable soil and percolate into rocks.

With numerous ponds, aqua parcs and other man made water features visible within the Longleat holiday village it is likely Center Parcs would have been obliged to undertake reservoir risk assessments and constant monitoring of strict rules around reservoirs.

The recreational Aqua Parc lakes and attractive water features typically seen at Center Parcs are not the same as attenuation ponds which are designed to lie empty until intense rainfall fills them up, delaying the flood peak reaching streams. The other possibility for attenuating flow is the use of SUDS (Sustainable Urban Drainage Systems) in the design of buildings and roads. This is perhaps what Center Parcs is most likely to present as mitigation, if any, to the loss of interception storage and addition of impermeable surfaces in their holiday village plans.

However, the location of a holiday village in this ancient forest on the watershed of an already highly responsive river basin is undesirable. The deforestation and loss of forest interception and natural storage, with associated impact on water quality, would likely lead to more flooding downstream into densely populated urban areas parts of which are at risk from Zone 3 flooding. In addition, it could risk reducing the effectiveness of costly alleviation schemes already in place to manage flooding at Gatwick and downstream such as Gatwick Stream Flood Alleviation Scheme (£12 million) and Grattons Park.

Hookwood Housing development

Location of housing planned in Hookwood

Plans for new housing to the west of Hookwood are broken into several areas. The largest site of 22 ha will have a capacity for 446 dwellings while the three other small plots will have capacity for another 117 dwellings. (Update: total housing for whole site now 563)

Deciduous woodland priority habitat

The plot is described as “former greenbelt”, and is a designated rural area as well as containing woodland priority habitat in the hedgerows and fields with stands of mature oak. Soils are described as seasonally wet with slow permeability.

Seasonally wet soils with slow permeability

On a visit a week after a rainy November the soil was still saturated with considerable expanses of standing water in the fields. The numerous drainage ditches were full of water.

This location is extremely flat and low lying. The River Mole bounds the area to the south and east as it meanders around Gatwick and then turns NW at the confluence with the Gatwick Stream. Two brooks drain sluggishly west to east across the A217.

Lidar digital terrain model shows the elevation of locations at a resolution of 1m

The LIDAR high resolution digital terrain model shows the shallow gradient descending less than 1m elevation in 600m across the site in some places. The main A217 running NW/SE bounds the development to the east. At 59m at Hookwood roundabout, the road is actually higher in elevation than most of the development site. The shallow or, in places, negative slope towards the Mole means brooks have a tendency to back-up and not drain effectively when the Mole is at high discharge. For example, flood water might back-up into these brooks if the Mole is running at more than 3m asd.

Other evidence for the poor drainage at this site is shown by the existing network of drains around many of the fields. On our visit these were still full of water with surface flooding across parts of fields despite the Mole having returned to normal flow for several days.

Flood risk map Hookwood

In line with the LIDAR elevation model the same areas near the Hookwood Common Brook and (un-named) brook to the south are designated Flood Risk Zones 2 and 3. The whole area is covered by a Flood Alert zone.

Of course, developers are likely to be aware of the flood and drainage risks, but what is required in the plans? The following is taken from the flood risk policy outlined in the plans for land west of Reigate Road:

Avoid built development within Flood Zones 2 and 3Incorporate SUDS (sustainable drainage) measures to address and mitigate the risk of fluvial floodingin accordance with Policy INF3 and site-specific guidance in the Level 2 Strategic Flood Risk Assessment.
Liaise with Thames Water to ensure that necessary upgrades to wastewater infrastructure can be delivered and engage in pre-application discussions on phasing.
Provide water efficiency measures such as water butts, rainwater harvesting, water-saving appliances and fittings, with the aim of exceeding the requirements of policy H10, to reduce increased pressure on water supply infrastructure.
Any developer of this site should, where possible: Work in partnership with Flood Risk Management bodies including Surrey County Council and the Environment Agency to design flood risk mitigation which co-ordinates with flood risk alleviation measures in the wider area around Hookwood, contributing to the reduction of flood peaks downstream.

Policy DS41

Avoiding building in Zones 2 and 3 restricts the area open to housing. It is good to see SUDS included and water efficiency measures. However, for most of the year, retaining and harvesting water and slowing the flow will not necessarily address surface water flooding due to the extremely shallow gradient and low permeability, often saturated, soils. The extensive drainage ditches are testament to the efforts of land owners to deal with saturated ground and surface water over the years and the area is still flood prone.

The wording for addressing flood mitigation downstream also seems rather weak: stating that “any developer should..where possible” work with risk managers to ..reducing flood peaks downstream. What happens upstream has real impacts downstream and developers need to work closely with partners from the outset.

The new housing estate at West Vale which is built on the Mole flood plain just 2 km north of Hookwood is an example of what could be appropriate to tackle local drainage and flood control. The housing is built on a raised terrace 1-2m above the flood plain. In addition, the edge of the estate contains a number of flood retention ponds with swales channeling water to the Mole.

In flood conditions in November 2022 the need for the raised terrace was self-evident. Over night on the 17 November the river rose to 3m above stage datum. The photos above were taken from the bridge at 12:30pm the next day when levels had fallen to about 2.7m asd. Data from the NRFA suggests 3m asd level corresponds to a discharge of 40m³/s. The maximum flow recorded just upstream at the Horley gauging station was 63m³/s in the huge 1:200 year flood of 1968 when 100m of rain fell in one day and up to 145mm over 3 days. The river level peaked in excess of 3.5m asd, so a full half a metre above the current level. The Mole is a flashy river and floods like this, and worse, will happen again so planning for the whole catchment is essential to avoid worse scenes than those shown below.

Landlord raised this cottage above 100 year flood risk level: this is a ROAD, not the river!
R Mole
Brockham Bridge
Brockham Bridge Dec 24 2013

Building on the flood plain does two things: the first is to put residents at risk of high magnitude but low likelihood flooding, for example the 1968 1:200 year flood. However, it’s worth remembering that such stated return periods do not take into account the impact of climate change, particularly increased rainfall intensity. This means that a 1968 scale flood will become more likely at shorter recurrence intervals and could render flood protection less effective in the future.

Gatwick Stream Flood Alleviation Scheme, funded by LGW

The second impact of building on flood plains is to increase discharge and reduce lag times risking more flooding downstream. Even with SUDS and attenuation ponds, a 1968 style flood could overwhelm the design capacity of flood protection schemes. For example, the Gatwick Stream Flood Alleviation scheme excavated >100,000 m3 of soil to form a huge retention basin to remove the flood peak before it enters the culvert under the South Terminal. When built the scheme was designed to cope with a 1:100 year flood. With house building upstream and climate change intensifying rainfall the design capacity has been downgraded to a 1:70 or 1:75 year storm event, more frequently reaching design capacity. It cost £12 million! The modest use of SUDS and attenuation ponds in housing estates may not be sufficient in the most intense rainfall events of the future. For example, studies show the cataclysmic flooding that occurred in Germany in July 2021 is possible in the UK and is worth planning for now.


A more resilient plan would be to avoid building estates on low lying flood plains and find an alternative brownfield location at a higher elevation or find ways to increase the density of housing in existing urban areas. However, I understand that Mole Valley is short of brownfield locations so there are clearly no easy “off-the-shelf” solutions.

My summary

The River Mole flowing through a beautiful stretch near Flanchford, Reigate

There is an aspiration for a catchment-wide approach to river management on the Mole. This holistic approach needs to be pursued with vigour and closer coordination between all parties especially regarding the numerous development projects going on. I think the will is there amongst all groups and stakeholders interested in the River Mole but it’s challenging to achieve! From what I have seen, which is obviously just a snapshot so far, charities and local groups with live and local knowledge of the river, can be helpful catalysts that bring the big players, like Local Authorities, the Environment Agency and Thames Water, closer together. From what I have also experienced there are no “goodies and baddies” in this story.. raising the River Mole safely back to a more healthy life seems to be a passionately shared aim across everyone who sees the river as an invaluable service delivering huge benefits to wildlife and human well being.

Thank you for reading this post. I hope you enjoyed it and found it interesting. Please like and share my post with whoever you think might be interested and I’d be delighted if you leave a comment. The aim is to be as informative and objective as possible using what I learn from numerous sources, without too much opinion. So any corrections or ideas for improvements are of course welcome. Thank you!

My thanks to Lisa Scott, Councillor MVDC, for the informative tour of the Hookwood area.