London Flash Flooding - 5 lessons learned, 5 years on

In July 2021, London was hit by some of the heaviest downpours on record for the capital. On 12 and 25 July, intense summer thunderstorms dumped up to 3 inches of rain in just a few hours, more than a month’s worth in some boroughs. The sheer volume overwhelmed drains and sewers, triggering flash floods across streets, tube stations and homes. 

Areas such as Kensington, Camden, Westminster, Hammersmith, and Wandsworth saw roads turn to rivers, basement flats and businesses inundated, and critical transport links shut down. Insurance claims ran into tens of millions of dollars, while councils and Thames Water faced urgent calls to strengthen flood resilience. 

The events became a wake-up call: climate change was no longer a distant threat for Londoners, it was surging up from the drains beneath their feet. Five years on, boroughs have begun to translate that shock into action. 

1) Make the streets act like sponges (retrofit SuDS, not just bigger pipes) 

Following the summer flash flooding, Hammersmith & Fulham (H&F) council created new street-level rain gardens on King Street, plus additional sites planned on Blythe Road, Westville Road and Manbre Road. 

Camden council delivered major SuDS retrofits and reports over 1,850 m² of highway de-paving to create rain gardens/pocket parks. 

In Westminster, a formal SuDS Delivery Programme (2025/26) is targeting North Paddington catchments and embedding SuDS in wider public-realm schemes. 

2) Maintain and upgrade highway drainage as a first line of defence 

Westminster council reviewed its gully strategy after 2021; $814k was then allocated to replace/repair gullies and optimise function; cleans ~14,000 drains at least annually. 

H&F council launched a formal flood-mitigation programme including stepped-up gully cleansing on strategic roads (twice yearly) and residential roads on a cycle. 

3) Tighten planning policies so new development manages its own water 

Camden council now require SuDS in all major applications, aiming for greenfield runoff rates (1 in 100 year, 6-hour event) where practicable.  

Westminster council adopted a Local Flood Risk Management Strategy 2024–2030 and public SuDS guidance to prioritize rain gardens, permeable paving, tree pits and green roofs. 

The City of London updated its Strategic Flood Risk Assessment (2023/24) with new modelling and emergency plan guidance, used to steer planning decisions. 

4) Investigate every major event and turn findings into borough-level action plans 

Section 19 Flood Investigations are reports conducted by councils after flooding events occur. 

Westminster and Wandsworth published detailed S.19 reports after the 2021 floods; Wandsworth’s 2023 report sets actions like increasing permeable surfacing and installing rain-garden SuDS on specific streets.  

Royal Borough of Kensington and Chelsea published their investigations and updates; as well as refreshing its Surface Water Management Plan identifying critical drainage areas. 

Wandsworth Borough Council followed with a new Local Flood Risk Management Strategy (2023–2029) containing a monitored action plan. 

5) Coordinate city-wide—with utilities—and invest in property-level protection 

The London Surface Water Strategy (2025–30) was developed: A new, multi-agency blueprint (GLA, boroughs, Thames Water, London Councils) to scale SuDS and nature-based solutions across highways and public realm.  

The London Resilience Group created a Strategic Flood Response Framework after 2021 and Thames Water revised procedures per the London Flood Review.  

In the Royal Borough of Kensington and Chelsea, Thames Water surveyed properties and installed protection at the highest-risk homes following the 2021 event. 

Five years after the shock of July 2021, London’s councils are proving that lessons learned can be turned into lasting change, though with heavier downpours on the horizon, the real test of resilience may still be to come.