What Impact Does the Revised Breach Model Have on London Flood Risk Assessments?
The 2017 update to the Thames Breach Model expanded from 113 to 5,679 breach locations, significantly changing residual flood risk assessments across central London.
Central London is protected by flood defences maintaining a 1 in 1,000-year standard, primarily through the Thames Barrier. Despite these protections, developments in Flood Zones 2 and 3 still require Flood Risk Assessments per National Policy.
The Challenge of Residual Risk
Because of existing defences, assessments must evaluate “residual risk” — essentially, what happens if defences fail. This breach scenario analysis has become increasingly important for planning applications.
Updates to the Thames Breach Model
In June 2017, the Environment Agency significantly revised its Thames Breach Model:
Previous approach: 113 tidal models at predetermined breach locations from Teddington to the Mar Dyke and River Darent.
New approach: “5,679 breach locations along the entire extent of the Thames from Teddington to the Thames Barrier.”
The shift from predetermined locations to comprehensive modelling covers substantially more potential failure points.
Real-World Impact
A Deptford site example demonstrates the change: the pre-2017 model showed no breach risk, while the updated 2017 model indicated vulnerability to flooding from modelled 2100 events due to the expanded breach simulation locations.
Planning Implications
Different London Boroughs have varying mitigation requirements. Developers must implement measures like appropriate sleeping accommodations and finished floor levels to secure planning approval. Understanding the breach model outputs and how they interact with borough-specific policies is essential for successful planning applications in central London.