The Thames Barrier as a case study in preparedness
Since 1982, partial protection against moderate to high levels of tidal flooding has been provided by the Thames Barrier at Greenwich. Flooding in central London is possible from a range of causes, including rainfall, high tides, and storm surges in the North Sea. These risks are enhanced by the impacts of climate change. The scale and complexity of the risks outlined above makes the Thames Barrier an ideal topic for a case study in preparedness. The NPC held a roundtable discussion with key stakeholders on Thursday 22nd September 2022 resulting in some important observations and conclusions. This case study reflects the discussion, held under the Chatham House Rule, as well as some next steps for follow-up research.
Like many capital cities, London has developed around a major tidal river. Whilst the River Thames provides much that makes it attractive and vital for a vibrant, flourishing city, its very presence places London at risk. Flooding in central London is possible from a diffuse set of causes, including local and regional heavy or prolonged rainfall, high tides, and storm surges in the North Sea. These risks are overlain and enhanced by the impacts of climate change; including shifts in patterns of extreme rainfall events and seasonality, rising sea levels occasioned by warming ocean temperatures, and melting ice caps. The entire system is complex and understanding the full range of possibilities is dependent upon assumptions about future sea level rise and other environmental shifts.
At risk are billions of pounds’ worth of property, over a million residents, and an almost incalculable volume of business. Historic buildings such as the Houses of Parliament and Whitehall, Westminster Abbey and the Tower of London lie in the area that could be inundated, as do power stations and substations, train and underground stations, museums and galleries, hospitals, schools, and universities.
Previous experience of UK flooding also highlights that economic and social damage is normally felt well beyond the area of inundation, since transport, power supplies, water and other elements of critical infrastructure are interrupted, preventing people from working and going about their day-to-day lives. Moreover, the economic impact of a major inundation in London may be expected to spill over to the rest of the UK, and beyond.
The scale and complexity of the risks outlined above makes the Thames Barrier an ideal topic for a case study in preparedness. The NPC held a roundtable discussion with key stakeholders, resulting in some important observations and conclusions. The discussion was held under the Chatham House Rule, and these notes take account of this.
The Thames Barrier as part of a system of controls and protection
Since 1982, partial protection against moderate to high levels of tidal flooding has been provided by the Thames Barrier at Greenwich, and eight smaller barriers on tributary streams, along with 330km of river embankments and sea walls. Currently 125km2 of central London is estimated to be protected by the Thames Barrier, and a much larger area of the eastern suburbs is protected by the sea walls. Current estimates in the Thames Estuary TE2100 Plan are that the Thames Barrier will serve to protect London until 2070, but a decision will need to be made by 2040 on whether, how and where to replace it.
Different scientific forecasts may be used to model flood risk, but in every case the maximum extent of the floodable area is the River Thames’ ‘natural’ floodplain – although this has been heavily modified over last five or more centuries by construction.
As a result of the granularity of the urban landscape, small differences in the extent of flood risk may produce large differences in the cost of damage. Moreover, tidal flooding may be exacerbated by large volumes of fresh water coming down the River Thames from its catchment area west of London; or down the River Lea catchment in east London, which is unable to flow out to sea during high tides. Localised flooding from intense rainfall also occurs in response to specific storms when surface water sewers are filled beyond capacity or overwhelmed. Whilst the Thames Barrier was designed to manage tidal flood risk, it has also been used to manage river flooding, but its ability to do so in worsening scenarios is not unlimited.
Some issues of concern include:
- System resilience: The Thames Barrier is part of a complete system of controls, which is only as strong as its weakest point. Risk owners and stakeholders, including London Boroughs, have varying levels of expertise and capability in managing and/or mitigating the risk. The strength of the entire system is therefore guaranteed only at the lowest level of efforts deployed.
- Stakeholder awareness: All stakeholders need to know what level of protection is in place, know where their own vulnerabilities are, and be prepared to mitigate or respond to residual risk. An Environment Agency surveys suggest that two thirds of people at risk of flooding are not aware of that risk.
- Risk quantification: Some of the most significant stakeholders include utility and transport providers. Their ability to invest in flood protection is shaped by government policy, including HM Treasury Green and Orange Book parameters for quantifying risk. Quantification methods take account only the risk to human life and habitation in the form of residential buildings and fail to consider either the economic risk to London and the wider economy, or the cost of a failure in service provision. These could well include:
- Exit strategy: Inability to transport staff into/out of London, even if businesses adequately protect their own building and data infrastructure.
- Built environment: Inundation and subsequent drying-out and repair of non-residential buildings, including hospitals, schools, heritage sites or government buildings, leading to impacts which may well continue over an extended recovery period.
- Transport disruption: Regular closures of parts of the transport infrastructure (and subsequent economic loss) due to avoidable low-level flooding.
- Organisational impact: Organisations viewing risk in the limited scope of their own operations and not looking more widely at the implications for their suppliers, neighbours, customers, and staff. This will inevitably lead to inadequate investment in preparation and mitigation measures.
- Integrated response: Infrastructure and service providers have a mutual reliance in the face of flooding events, yet there is no mechanism to ensure their plans, and the assumptions within them, are integrated. One example is the reliance on power to enable emergency alerts, sirens, pumps, and mobile communications once phone batteries are depleted.
- Net zero: The effects of the transition to Net Zero energy sources may exacerbate and further complicate this situation as changes are made across the infrastructure system of systems. Awareness needs to be maintained of how that system is changing.
- National economy: The effects of an inundation or failure in flood protection for London could have economic consequences at a national level. This issue demands a much higher political profile. Whilst councils have responsibility for flood protection there is at present no statutory duty to act, compromising their ability to secure funding at the necessary level.
Adaptive pathway approach
The Thames Estuary 2100 Plan takes an innovative ‘adaptive pathway’ approach, involving monitoring of change drivers and forecasts and decision points within the plan period, to ensure that plans remain fit for purpose. The Environment Agency conducts extensive communication activities and scenario planning exercises with the communities most affected. It also undertakes regular data analysis and modelling, the results of which are freely available on the gov.uk website. This raises further issues, largely concerned with the extent to which the assumptions, monitoring and data used to support the approach are disseminated, tested, and used more widely:
- Data use: With data being subject to regular review, the likelihood of all those responsible for risk mitigation or management plans being fully up to date is questionable. Data availability without comprehensive multi-way communications does not guarantee its incorporation into plans.
- Risk modelling: Conversely, it is highly likely that full benefit is being gained from the wealth of high quality information available, both domestically and in evidence from other countries managing similar risks. Scenarios are modelled by the Environment Agency, for example, which do not appear to be routinely reflected in other people’s cost/benefit analyses for flood risk prevention measures.
- Weather events: The interaction between increasingly high likelihood of extraordinary weather events leading to surface water flooding that coincides with tidal surges is of particular concern and demands scrutiny and management at national level.
Residual risk and planned responses
The management plan for the Thames Estuary is designed to be strategic and adaptive in response to future risks. However, like any risk management plan, it is built around a set of models and assumptions and can never provide 100% certainty. There remains a possibility of failure – of the Thames Barrier or other measures – with the areas potentially liable to inundation at risk of compromise by, for example:
- Climate change: leading to increased sea levels, increased storminess in the North Sea, and increased flood flows in the freshwater Thames and tributaries
- Failure of the Thames Barrier: due to of mechanical or electrical failure, lack of maintenance, operational error, damage from shipping or terrorism
- Failure of the embankments: as a result of temporary breaches for maintenance, riverbank erosion and deterioration, or acts of terrorism
Despite this, there appears to be a widely-held assumption that riverine flooding is under control and that surface water flooding constitutes the greatest vulnerability. Response plans need to take account of different sources of risk, which may demand nuanced responses.
Human stories lie behind the economic and infrastructure impacts that are quantified in risk analyses. Whatever the cause, the potential for a major flood remains, despite the mitigating measures in place. Damaging implications of a major flood event could include: deaths and serious injuries, longer-term health and mental health implications, loss of housing, infrastructure and critical services, disruption to central government and security services, disruption to the Stock Exchange and related financial services, as well as indirect and cascade effects, such as reduced tourism, trade, loss of work and unemployment. A final group of concerns in this area include:
- Tidal range: The River Thames has one of the highest tidal ranges in the country. Those most at risk are more likely to recognise sirens (in the places where sirens are in operation) and know what to do in response. However, many areas do not have area-wide alert systems in place, and since much of the population in the catchment is transitory (tourists, or those coming into London for work or leisure) most cannot know what is expected of them.
- Response action: Poorly understood response actions risk adding pressure to emergency services and other response leaders, for example in crowd control or managing evacuation.
- Funding models: National responsibility for this major area of risk requires new considerations of funding models that promote equity of outcome. Local areas can deliver mitigation and response plans but are unlikely to be able to afford them within normal budget envelopes.
The NPC is considering its role in addressing the issues raised in this discussion. It is likely that further research will be commissioned and participants in the roundtable will be invited to remain as part of a ‘reference group’ on the topic. The Environment Agency has forwarded details of a new consultation on the next 10-Year Review to participants and has invited anyone wanting to join a new strategic advisory group that is beginning in Spring 2023 to contact them.