Storm overflow register · Thames Water
Gerrards Cross sewage works: storm overflow spills 2025
Gerrards Cross sewage treatment works, run by Thames Water, has 1 monitored storm overflow discharging to Misbourne. In 2025 it recorded 88 spills totalling 1,605 hours, against 144 spills and 2,838 hours in 2024. By hours spilling, it ranks 10 of 233 Thames Water works, where 1 spilled longest.
Works size: 8,204 population equivalent (GERRARDS CROSS, GERRARDS CROSS, STW", UWWTD 2018). Used for the spill volume estimate.
Every spill in 2025
Each blue mark is one spill, placed by when it started and sized by how long it lasted. Hover over a mark for the date and duration.
Estimated spill volume by month, 2025
Hours spilling each month × the estimated spill rate for a works of this size. The bars are central estimates and the whiskers show the low–high range. Method.
Longest single spills
| Started (GMT) | Overflow | Duration | Estimated volume |
|---|---|---|---|
| 23 Feb 2025, 08:45 | Gerrards Cross | 14.0 days | 18,855 – 118,842 m³ |
| 26 Jan 2025, 15:45 | Gerrards Cross | 11.9 days | 16,028 – 101,020 m³ |
| 5 Jan 2025, 02:30 | Gerrards Cross | 10.3 days | 13,830 – 87,168 m³ |
| 9 Feb 2025, 19:30 | Gerrards Cross | 4.3 days | 5,711 – 35,997 m³ |
| 14 Nov 2025, 07:30 | Gerrards Cross | 2.0 days | 2,702 – 17,028 m³ |
Estimated 2025 spill volume and pollution
Estimate, not a measurement. Monitors record how long an overflow spills, not how much. This calculator back-calculates a spill rate from the size of the works, using the Environment Agency's permit formulae, and multiplies it by published storm-sewage concentrations. How it works.
Assumed spill rate while spilling: 15.6 – 98.0 L/s (central 39.0 L/s), from a dry-weather flow of 15.6 L/s.
| Pollutant released | Low | Central | High | Concentration used (low / typical / high) |
|---|---|---|---|---|
| BOD₅ (organic load) | 3,595 kg | 20,308 kg | 113,298 kg | 40 / 90 / 200 mg/L Low/high: Europe range, Ellis & Jenkins 2005; typical: UK, Ellis & Jenkins 2005 (both via botturi) |
| Suspended solids | 9,437 kg | 95,899 kg | 408,439 kg | 105 / 425 / 721 mg/L Low/high: Europe range; typical: UK, Ellis & Jenkins 2005 (via botturi) |
| Ammonium-N | 171 kg | 1,422 kg | 5,268 kg | 1.9 / 6.3 / 9.3 mg/L Low: Germany, Brombach 2005; typical/high: Paris range 3.3–9.3 midpoint and top, Gasperi 2012 (via botturi); no UK value found |
| Total phosphorus | 108 kg | 587 kg | 3,059 kg | 1.2 / 2.6 / 5.4 mg/L Low/high: Paris, Gasperi 2012; typical: Slovakia (via botturi). UK figure of 10 mg/L treated as an outlier |
| E. coli | 9.0 trillion organisms | 225.6 trillion organisms | 5.7 × 10¹⁵ organisms | 10,000 / 100,000 / 1.0 million per 100 mL Median range at CSO and retention-tank outlets, Stott et al. 2018 (via botturi); typical = log midpoint |
Low combines the low spill rate with low concentrations, and high combines high with high, so the true figure is very likely inside the range. Long spills become more diluted as they go on, so the central figure tends to overstate the load of long events. The calculator does not estimate the effect on the receiving water, which depends on river flow or tide at the time of the spill.
Gerrards Cross
Storm tank at sewage works · discharges to Misbourne
- Spills 2025
- 88 (1,605 hours) · down 39% on 2024
- Spills 2024
- 144 (2,838 hours)
- Long-term average
- 92.5 spills a year (monitored since 2020)
- Monitor uptime
- 100% of the year
- High-spill reason (company)
- Performance - Asset configuration (e.g. PS/rising main/storm tanks)
- Investigation
- Env Act (SODRP) investigation ongoing
- Improvement
- No improvement action in reporting period
- WFD catchment
- Misbourne (GB106039029830)
- Outlet location
- TQ0190087700 (OS grid reference) · View on Apple Maps
- EA ID / permit
- TWL00196 · TEMP.2613 · company name: Gerrards Cross STW
How the estimate works
- Dry-weather flow (DWF) = population × water use per person × (1 + infiltration). Water use is 136.5 L per person per day, England's per-capita consumption for 2024–25 [EA]. Infiltration of groundwater into sewers is taken as 0–40% of that flow [Escritt, via HBF]. The Environment Agency defines DWF as PG + I + E [EA].
- Population. The size of each works comes from the load entering it as reported under the Urban Waste Water Treatment Directive, in population equivalents (1 p.e. = 60 g of BOD a day) [UWWTD]. We treat population equivalent as population. That overstates flow where trade effluent adds load, and no official source links the two. Works serving fewer than 2,000 p.e. are not reported, so their size has to be entered by hand.
- Low spill rate = DWF. This is the method used by Giakoumis & Voulvoulis (2026), which they note likely underestimates storm loads [G&V].
- High spill rate = Formula A − flow to full treatment = (DWF + 1360P) − 3DWF. This is the largest flow the permit expects the storm tanks to handle before it passes on to treatment or overflows [EA]. The central rate is the geometric mean of the low and high rates.
- Volume = spill rate × hours spilling, with hours taken from the event duration monitor.
- Pollution = volume × published storm-sewage concentrations [Botturi et al.] [US EPA]. Organic load is also shown as person-days of untreated sewage at 60 g of BOD per person per day [UWWTD].
What this cannot tell you. It gives no real flow for any single spill, and no effect on the river or sea, which depends on the flow, tide and the other pressures on the water at the time. It gives no figure for spills from overflows on the sewer network, which serve catchments of unknown size. It is a way to put hours into rough physical terms, not a substitute for flow monitoring.
Sources
- EA: Water companies – environmental permits for storm overflows and emergency overflows
- EA: Water resources 2024 to 2025 – per capita consumption
- HBF: Foul sewer design – historic conventions (citing Escritt 1984 on infiltration)
- Giakoumis & Voulvoulis (2026), Environ. Sci.: Water Res. Technol., doi:10.1039/D5EW00860C
- Botturi et al. (2021), Crit. Rev. Environ. Sci. Technol. 51:1585 – CSO quality review, Table 2
- US EPA (2004) Report to Congress on CSOs and SSOs, chapter 4
- Urban Waste Water Treatment Directive, Article 2(6): 1 p.e. = 60 g BOD5/day
- Olympic-size swimming pool: 2,500 m³ at the nominal 2 m depth
← All Thames Water works · Contains Environment Agency data licensed under the Open Government Licence v3.0. Spill start and stop times: © Environment Agency copyright and/or database right 2026. All rights reserved. They are water-company data and have not been verified by the Environment Agency. Works size: EEA Waterbase UWWTD. How to read the data.