Fluvial flooding
| Impact | 5 |
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| 3 |
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| 2 |
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5 |
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| ID | 51b |
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| Risk theme | Natural and environmental hazards |
Impact & Likelihood
| 5 | Catastrophic |
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| 4 | Significant |
| 3 | Moderate |
| 2 | Limited |
| 1 | Minor |
| 5 | >25% |
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| 4 | 5-25% |
| 3 | 1-5% |
| 2 | 0.2-1% |
| 1 | <0.2% |
Background
Fluvial flooding occurs when waterways such as rivers, streams or brooks overflow their banks into surrounding areas. This risk is most likely to occur following periods of intense rainfall and will become more frequent as a result of climate change. Impacts are widespread and may include damage to the local environment, properties and essential services, with disproportionate effects on vulnerable groups. The government has well- established arrangements for minimising the risk from flooding including, the deployment of fixed and temporary defences, public warning and informing alert systems, and local and national response mechanisms.
Scenario
The reasonable worst-case scenario is based on a significant river flood event, resulting from cumulative local events or a series of concurrent events across multiple geographic regions following a sustained period of heavy rainfall. This could possibly be combined with snow melt and surface water flooding.
Flood defences would become overtopped by river levels and breaches may occur in river banks and hard defences as they are put under pressure. Across urban and rural areas there would be flooding of homes and businesses. There will be casualties and fatalities. A large number of people would require evacuation, with a significant proportion of these being vulnerable and requiring assistance. There would be medium-term (days to weeks) loss of essential services (electricity and telecoms) to up to a substantial number of homes and businesses, with disruption to water supplies.
Key assumptions
The scenario assumes the event would occur at night time after an extended period of rainfall lasting 2 weeks in a large urban area. There would be a loss of essential services to homes and businesses possibly lasting several weeks.
Variations
A lower-impact scenario could involve fewer breaches and an overtopping of flood defences, with more localised areas impacted. A higher-impact scenario would involve the additional risk of severe surface water flooding over already-saturated catchment areas.
Response capability requirements
An advanced flood forecasting capability is available via the Met Office and the Flood Forecasting Centre (FFC). The FFC produces products that identify potential flood impacts from weather scenarios. These products help inform long-term emergency planning and in the lead up to a potential flooding event. The FFC target is to consistently provide at least a 3-day lead time for fluvial events.
The Environment Agency (EA) leads operational preparedness and responseto flood impacts and, during local-level operation response, would workas part of a multi-agency team, coordinated through the Local Resilience Forum (LRF) to support flood preparedness, warning and informing the public, operating defences and systems and coordinating any evacuation including accommodation requirements. The local response will have access to operational resources including temporary flood barriers, mobile pumps and the necessary logistical support to transport and deploy these resources. Flood rescue teams (which consist of over 100 specialist flood rescue teams on standby to be deployed across the country), national mutual aid and military assistance can also support a local response. Any England-wide coordination would be led by the EA and the Department for Environment, Food and Rural Affairs.
Recovery
Major recovery impacts and long-term economic, environmental, infrastructure and humanitarian implications would go beyond 2 years especially if significant repairs are required to infrastructure, homes and business. Mental and physical impacts on affected citizens will last for years. Businesses would experience significant impacts, and the long- term contamination of agricultural and other land would also be likely.