Sewage once again flooded into streets and properties from Thames Water's Horley Sewage Treatment Works (STW) on 5 January 2025.

This is the third occasion in recent years when the unacceptable has happened and hazardous raw sewage effluent has poured out of the works inundating Lee Street and local properties. This recurring and increasingly frequent problem, adding to the decades of sewage flooding from storm tanks to the north of the facility, serves as compelling evidence of woeful underinvestment in the plant, rendering Horley STW unable to effectively manage the incoming wastewater flows. Thames Water blames the weather.

We disagree with Thames water and think blaming "heavy rainfall and high river levels" for raw sewage inundating public paths, streets and properties and causing serious pollution incidents is Out of Order! Let's look at the root causes of sewage flooding from the works.

Multiple Design Deficiencies
As far back as 2022 a Hydraulic Report prepared by engineering firm Atkins highlighted "multiple design deficiencies" at Horley sewage treatment works. This means the works is not currently capable of properly treating incoming wastewater flows. The deficiencies are known to Thames Water and some are listed in the table below.

Furthermore, an Ofwat report from last year lists Horley STW as a site of concern, joining numerous other underperforming assets in the River Mole catchment. Sites of Concern indicate the asset is at risk of non-compliance and "incapable of meeting Full Flow to Treatment". Ofwat states the causes of underperformance at Horley STW are "hydraulic and operational", not local weather conditions.
In it's thorough evidenced-based report last year, Ofwat asserted that...
Thames Water has persistently failed to ensure that its wastewater treatment works and networks are designed, constructed, operated and maintained so that, under normal local climate conditions, all wastewater is collected and treated sufficiently in order to minimise adverse effects on the environment.

We will show evidence that the root cause of sewage flooding from Horley Sewage Treatment Works (STW) is not the rainfall or river levels experienced on the dates in question. Instead, the root cause is insufficient investment which should have resolved design deficiencies that have persisted for decades. Unfortunately, as shown in the map above, the woeful performance at Horley is repeated across the catchment at many other Thames Water assets and, whilst upgrade works are now taking place at a number of sites such as Burstow and Dorking and are planned for Horley, these measures are playing catching-up after years of neglect. Given the "staggering challenges"* presented by local growth and climate change the upgrades are also likely to fall short of providing the necessary additional capacity to address rising incidences of pollution unless the real causes of sewage flooding and excessive storm overflows are found and addressed. (*Thames Water quotation from DWMP Drainage and WasteWater Masterplan).
The multiple deficiencies in the Horley plant have been well known and stated in different reports for several years. Technical hydraulic reports and Ofwat analysis are not needed to spot deficiencies at the plant. On a recent site visit to Horley STW we saw perforated sludge tanks pouring raw sewage sludge onto the ground.
Sewage sludge can emit harmful gases like methane, ammonia, and hydrogen sulfide, which are toxic and can cause respiratory issues. Pathogens in the sludge, including bacteria, viruses, and parasites, can contaminate soil, water, and air, posing risks to human health and ecosystems. The poor condition of the works combined with increasingly frequent sewage flooding from both the north and south, is stark evidence of the misappropriation of financial resources that should have been allocated to investment in timely upgrades and maintenance, rather than being diverted to pay dividends, bonuses, and bank obligations. Moving forward, we encourage Thames Water to provide an honest account of the real causes of sewage flooding which are rooted in the inadequate performance of wastewater treatment at local plants rather than local weather conditions which we show are well within normal range and not "extreme".
Sewage to the North, Sewage to the South
Let's examine the two distinct issues of sewage flooding in Horley. Sewage flooding happens from two locations at Horley STW. Over several years, raw sewage has frequently overflowed from the works into public areas both to the north and the south, as indicated on the map below. But the causes of flooding to the north are different from those in the south.

To the North: West Vale Park Sewage Flooding
To the north of the works, sewage floods out of overtopping storm tanks. This occurs frequently during periods of wet weather and when storm tanks are filled which usually occurs as rainfall exceeds around 15mm in 24 hours. Overtopping of storm tanks has been correlated with high river levels (>2.3mASD) in the River Mole.

Once overtopping starts, the untreated sewage flows across the service road and through perimeter fencing and escapes out the plant and flows across footpaths adjacent to West Vale Park housing estate. The raw effluent was often observed residing across the footpath and in the adjacent ditch for weeks after each event. Hazardous sewage fungus was observed on the footpath.
Lab tests confirm effluent from Horley sewage treatment works is hazardous to human and animal health.
In the worst events, a large pool of dilute sewage effluent was created in recreational fields opposite West Vale Park housing estate where children play football and locals walk their dogs. Our water quality tests showed the outpouring effluent had very high phosphate levels indicative of raw sewage and samples sent to the lab for analysis showed E.coli levels over 1,300,000 cfu/100ml, well above "safe limits".
Sewage flooding in West Vale Park has been a recurring issue for many years, occurring at least 10 times during the winter of 2023-2024 alone. The bund constructed near West Vale was likely intended, at least partially, to prevent sewage from flooding into new properties at West Vale Park. However, it was only through the efforts of River Mole River Watch in collaboration with West Vale Park Residents Association and local councillors that eventually brought this issue to wider attention. We spent 18 months collecting evidence of sewage flooding to determine the causes and effects, and to pressure Thames Water for action to address the wholly intolerable and unacceptable situation of sewage flooding. Due to our combined efforts, Thames Water constructed a perimeter concrete wall to contain the most frequent flooding. Although the wall does not tackle the root cause of sewage flooding and was not our preferred solution, it was proposed as a means to quickly address severe sewage pollution posing a real hazard to local people using the footpath and open space, including children who regularly played in the area. Our monitoring so far this winter suggests that the wall is reasonably effective in containing the worst sewage outflows into the park.
What is causing Storm Tanks to Overflow?
Thames Water blames river flooding for backing-up into the storm tanks which they say causes overtopping.
"Tanks do not empty at high levels due to flooding of the site by River Mole" Thames Water
From our own investigations we have established that there is a correlation between river level and storm tank overtopping. When the River Mole reaches ~>2.4mASD the risk of storm tanks starting to overtop is high. We have verified this by ground truthing evidence. We also find that the EDM (Event Duration Monitor) that monitors the duration of storm tank overflows, stops working or becomes unreliable once storm tanks overtop. This is possibly because the EDM itself becomes flooded and unable to report overflows reliably. The chart below shows accumulated rainfall, river level at Horley and EDM reporting during a wet period in winter 2023-24. The patchy nature of EDM reporting can clearly be seen at times of highest river level when storm tanks would be overflowing.

Whilst this data appears to show river levels are linked to EDM reporting, or lack of it, they do not prove a causal link. Correlation is not causation and our research strongly suggests that, while the level of the River Mole is an indicator of storm tank condition, it is unlikely to be the root cause of overtopping. Some other deficiency on the site is causing storm tanks to overflow.
Part of our evidence is based on elevations across the site. The map below shows elevation at key spots. The storm overflow outfall is 52mAOD (Above Ordnance Datum / sea level), while the storm tanks are >55mAOD, indicating a substantial 3-meter height difference between the storm tanks and the outfall.

This 3-meter height difference, along with pumping, should allow for a continuous flow of storm tank effluent into the river even at high levels. The significant height disparity should make it improbable that the river backs up and reverses the flow in a properly functioning storm overflow system set at over 3m above the outfall.

Furthermore, the River Mole never comes close to flooding the main sewage treatment works or storm tanks. At highest levels the river can flood the lowest Tertiary Ponds, pictured above. The highest level of flooding in recent years was 3.44mASD in 2013 which is still at least 1.5 metres below the level of storm tanks. The map below shows the 2013 flood level using LIDAR data. Our sincere thanks to Floodmapper for producing these helpful maps.

As shown on the map, the bulk of Horley Sewage Treatment works sits well above the EA 0.1% Annual Probability of Flooding which means it is well above even a 1000-year return period flood event.

Flooding of the Tertiary Ponds should have no material impact on wastewater processing "upstream" in the rest of the works. Despite this Thames Water commonly refer to the River Mole "flooding the sewage works" to explain sewage flooding. This is a misleading explanation of sewage flooding. The sewage works itself is never flooded by the River Mole even at its highest levels. Additionally, there is no evidence that storm tanks are being overwhelmed by river water backing up through the storm overflow pipe as these are pumped into the river.

The evidence suggests that the root cause of storm tank overtopping is not the river but more than likely due to a flawed design or compromised storm overflow system. Without a survey we cannot know for sure but possible culprits could be the absence of a storm flap, pipe undersizing, or a blockage like tree roots or partial collapse. A single one of these deficiencies or a combination might be the key to understanding storm tank overtopping.
While the new containing wall offers a temporary solution to sewage flooding, we urge Thames Water to conduct engineering surveys to confirm the root cause of sewage flooding before starting AMP8 engineering works. Otherwise, there is a risk that planned upgrades may not resolve the issue.
In conclusion, the frequent overtopping of storm tanks at Horley STW and subsequent sewage flooding at West Vale Park cannot be attributed to river levels or rainfall alone. The River Mole's highest recorded levels remain well below the storm tanks, and the facility's 3-meter height advantage over the storm overflow outfall should prevent river water from backing up. Instead, evidence points to systemic infrastructure deficiencies, such as design flaws, blockages, or the absence of a storm flap, as the likely root causes. Temporary containment measures, like the perimeter wall, address the symptoms but fail to resolve these underlying issues. Urgent engineering surveys are essential to identify and rectify the causes to prevent further public health and environmental risks.
Sewage Flooding South: Lee Street Sewage Flooding
Sewage flooding to the south of the STW into Lee Street is not caused by overtopping of storm tanks. The storm tanks are on a different "slope" where the ground eases away to the north, so flows are very unlikely to extend south. Instead, the source of Lee Street sewage flooding is the Inlet Works which have been seen to overflow from the tower.
The resultant flows of sewage effluent from the Inlet Works have been modelled below.

Sewage flooding into Lee Street is a more recent problem than the long term issue of storm tanks overtopping. The first event of Lee Street flooding that we know about was reported in February 2020. From the evidence we have gathered it is possible that dates of sewage flooding can be linked to new local housing developments which have added to the volume of incoming wastewater flows which Horley sewage treatment works is wholly incapable for managing. The new housing in West Vale, first occupied in 2017, was connected to Horley STW via new pumping stations in West Vale Park. The first recorded sewage flooding was, perhaps coincidentally, reported 3 years later and the frequency of sewage flooding has increased since.

The result of increased volumes of incoming wastewater is sewage flooding out of the inlet works, down the service track and into Lee Street and people's properties.

The sewage effluent on the street poses a health risk as vehicles drive through it, possibly splashing pedestrians. Timely responses by Lanes Teams and Thames Water, following alerts from local residents, have helped reduce the flow by placing sandbags around the source and pumping out drains. However, if these flows are not controlled, there is a risk of more severe and widespread flooding, as illustrated in the flow maps below.
Thames Water once more attributes the sewage flooding on Lee Street to "heavy rain and high river levels." However, river levels are clearly not the reason for this flooding. The Inlet Works labeled "X" on the LIDAR elevation map is situated well above river flood levels and functions independently from the River Mole's level.

It is widely understood that rainfall enters combined sewers and subsequently reaches sewage treatment facilities, occasionally causing a surge of wastewater that overwhelms the system. When incoming effluent surpasses the full flow treatment capacity, it must be redirected to storm tanks. This design helps sewage facilities manage "extreme rainfall," particularly in areas with combined sewers. Sewage flooding on Lee Street occurred on 16 February 2020, 27 November 2024, and 5 January 2025. The rainfall during these incidents, which led to sewage flooding, cannot be classified as "extreme." Statistically, these are relatively moderate precipitation and river flood events that happen quite frequently in the Mole catchment. Other local treatment facilities handle similar events without causing sewage flooding.
The following is a detailed analysis of each sewage flood incident on Lee Street, evaluating each event with the Flood Estimation Handbook's event calculations to determine the rarity of each occurrence and therefore ascertain how "extreme" each event was.

First Reported Sewage Flood: 16 February 2020 - 50 mm in 48 Hours
On 16 February 2020, the river level recorded at the Environment Agency (EA) gauge in Horley peaked at 3.25 meters above ordnance datum (mASD). This measurement represents the highest river level recorded at this gauge in recent years. The peak was attributed to a significant precipitation event, characterized by:
Rainfall accumulation: 50 mm over a 48-hour period.
Preceding wet period: Total rainfall of 73.2 mm over five days.
Although this precipitation can be classified as a wet week, the total rainfall is not statistically rare or unusual for that time frame. According to the Flood Estimation Handbook (FEH22), a 48-hour rainfall accumulation of 50 mm has a return period of approximately 3 years. It is crucial to note that a return period of 3 years does not classify this event as "extreme" in a hydro-meteorological context. Extreme weather events are typically defined as those with return periods exceeding 50 years, and more commonly, over 100 years. Therefore, attributing sewage flooding to a relatively common 3-year rainfall event is not justified. A properly functioning sewage treatment facility should readily be capable of managing such frequent precipitation events without resulting in sewage flooding in local streets and properties. Notably, other local sewage treatment works in the River Mole catchment, such as Burstow and Earlswood, did not report significant sewage flooding during this event.
Second Reported Sewage Flood: 27 November 2024 - 33 mm in 24 Hours
The second sewage flooding incident occurred on 27 November 2024, characterised by:
Rainfall accumulation: 33.4 mm over a 24-hour period.
River level: Just over 3 mASD at the Horley gauge.
A 24-hour rainfall of 33.4 mm has a return period of approximately 1.75 years, while the preceding 5-day rainfall of 54 mm has a return period of about 1.39 years. Again, this level of rainfall cannot be classified as "extreme." A well-designed sewage treatment facility should possess adequate capacity to manage incoming flows and prevent sewage flooding in local properties during such common weather events.

Third Reported Sewage Flood: 5 January 2025 - 36 mm in 24 Hours
The most recent sewage flooding incident occurred on 5 January 2025, with the following characteristics:
Peak river level: 3 mASD at Horley (2.9 mASD just downstream at Kinnersley Manor).
Rainfall accumulation: 36 mm over a 24-hour period, followed by an additional 20 hours of lighter rainfall.
This event has a return period of less than 2 years, thus cannot be categorized as "extreme." Complicating factors included:
Rapid snowmelt at the onset of the event, with 1cm of snow lying across parts of the catchment upstream of Horley STW melting quickly due to a temperature rise.
Estimated snowmelt: Up to 1 cm, equating to approximately 10 mm of rainfall.

The rapid melting of snow, resulting from a temperature increase of 7°C within one hour, may have contributed to increased inflows to Horley STW. This transformed a modest rainfall total into a more substantial inflow, including runoff from the snowmelt. The peak river level of 3 mASD indicates that the two-year return period event is reasonable; even if we assume a generous 10mm of snowmelt occurred within less than one hour this would be a 3-year return period; however, this still does not classify as an "extreme" event.

The chart above shows rainfall intensity (mm/hr) and red bars indicate when the EDM at Horley STW was reporting. The data shows that the storm tanks at Horley STW began to overflow after just 17 mm of rainfall. Consequently, sewage flooding into Lee Street commenced at 16:30 hours following only 36 mm of rain. This is less than a 2 year return period event and underscores the facility's inability to withstand even modest weather events.
Exception: 52 mm in 48 Hours on 17 November 2022
While the river levels exceeding 3 mASD during the aforementioned sewage flooding events represent the highest levels recorded in Horley in recent years, it is important to note the exception of 17 November 2022. On this date:
Rainfall accumulation: 52 mm over 48 hours.
Peak river level: 3.1 mASD.
Despite the significant rainfall and elevated river levels, no sewage flooding was reported. This observation suggests that not every instance of heavy rainfall and high river levels necessarily leads to sewage flooding incidents.
In conclusion, sewage flooding in Lee Street very likely results from systemic deficiencies in the sewage infrastructure unable to cope with additional volumes of wastewater, particularly the Inlet Works at Horley STW, rather than from high rainfall or river levels. All sewage flooding events occurred during moderate rainfall with return periods of 1.75 to 3-years, conditions that a properly functioning facility should manage without issue. Additionally, the elevation of the Inlet Works above river flood levels and the absence of flooding during heavier rainfall in November 2022 further undermines the claim that rainfall or river levels are root causes that would occur if assets were properly designed. The increased wastewater volume from post-2017 housing developments more than likely exacerbate flooding due to the inadequate capacity at the treatment works. Addressing these deficiencies is essential to prevent further public health risks and property damage.
Looking forward
Climate change has already caused a measurable increase in both the amount and intensity of rainfall and flood peaks in the River Mole catchment.

Local weather stations in the area have noted significant rises in rain intensity. The above chart shows a notable increase in the number of days recording more than 30mm as recorded by a keen amateur weather station in Reigate.

Additionally, the Environment Agency's river gauge at Horley has shown a consistent rise in the frequency of larger flood peaks over the past 50 years. Although rain and river levels are not the root cause of sewage flooding, they clearly act as triggers. The trends of heightened rainfall intensity and elevated peak river levels are likely contributing factors to the apparent rise in sewage flooding incidents in recent years, especially in Lee Street.
As there seems little hope of a global decrease in carbon emissions we must plan for more extreme climate driven weather events which means investing in sewage infrastructure to cope with volumes of incoming wastewater that far exceed anything seen before.
Thames Water do have investment plans for Horley, however the upgrades wont be completed until 2027 or even 2028. Whilst waiting for Thames Water to upgrade their assets, reducing sewage overflows can be expedited by diverting storm water runoff before it enters foul sewers installing Sustainable Urban Drainage and Natural Flood Management upstream. Unfortunately, too many SuDS in new developments in the catchment turn out to be poor quality as illustrated below.

Sometimes SuDS are dutifully shown in the plans but are abandoned for cheaper alternatives in the build. Or the quality is so poor that they are not fit for purpose or even pose a water-hazard to residents.

Properly planned Nature Based Solutions to slow the flow through natural flood management and well designed sustainable urban drainage retrofitted upstream in the worst performing sub-catchments are urgently needed especially in first order catchments and retrofitted to older urban catchments with combined sewers.

The diagram above shows elements that can be used to reduce the volumes of flow of into struggling wastewater treatment plants to help reduce sewage flooding while Thames Water upgrade their infrastructure.
River Mole River Watch is currently working with the Catchment Partnership, SERT and Local Councillors to prospect suitable locations for wetlands and other NFM.
Conclusions on the Root Causes of Sewage Flooding at Horley Sewage Treatment Works:
Inadequate Investment: Chronic underinvestment over many years in infrastructure upgrades and maintenance at Horley STW has left it unable to manage current wastewater volumes even during the "normal" range of weather. Reports from Ofwat and Atkins have highlighted design deficiencies and systemic underperformance.
Misattributed Causes: Thames Water’s claims that flooding is caused by "extreme rainfall" or "high river levels" are inconsistent with evidence. Rainfall events linked to flooding had relatively low return periods (<7 years), and the River Mole’s peak levels remain significantly below critical elevations at the STW.
Design Flaws: Evidence points to unresolved infrastructure issues, including storm tank overflow mechanisms and inlet works incapacity, as the primary drivers of flooding, rather than external weather factors.
Initiatives to Resolve Sewage Flooding:
Immediate Actions:
Conduct comprehensive engineering surveys to identify and address design deficiencies (e.g., potential blockages, pipe sizing issues, or missing storm flaps).
Implement interim and emergency containment measures, such as raised road humps in the works to divert overflowing sewage from escaping down the service road, to mitigate immediate risks.
Medium/Long-Term Solutions:
Plan Nature Based Solutions across the catchment to slow the flow, improve water quality and separate storm runoff from entering foul sewers.
Increase investment in upgrades to expand capacity, modernize facilities, and address deficiencies at Horley as part of AMP8 planning.
Ensure Thames Water commits to proactive maintenance and operational improvements to meet future demands driven by climate change and urban growth.
Strengthen regulatory oversight to enforce compliance with performance standards and allocate resources efficiently.
These measures are essential to address the root causes of sewage flooding and to protect public health, local ecosystems, and community well-being.
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Credits: with our grateful thanks to partners FloodMapper who were so helpful in creating the amazing flood maps and height elevation. Please visit their website here https://www.floodmapper.co.uk/
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