What Is Tidal (Sea) Flooding? Causes, Risks and Planning Implications

Tidal flooding occurs when sea water moves inland and inundates coastal land. This can happen when sea levels rise above coastal defences or natural barriers, allowing water to spread across low-lying coastal areas.

Tidal flooding is an important source of flood risk in the UK, particularly for communities located along the coastline or within tidal estuaries. Understanding how tidal flooding occurs is important for property owners, developers and planning professionals working in coastal areas.

How Tidal Flooding Happens

Tidal flooding occurs when sea levels rise high enough to overtop coastal defences or natural barriers.

Under normal conditions, the movement of the tide causes sea levels to rise and fall predictably throughout the day. However, several factors can combine to produce unusually high sea levels that lead to flooding.

When these conditions occur together, sea levels may rise significantly above normal tidal levels, allowing water to overtop sea walls, coastal defences or natural coastal features.

Unlike surface water flooding, tidal flooding events are often associated with major weather systems and can affect large areas of coastline.

Where Tidal Flooding Occurs in the UK

Tidal flooding occurs in coastal locations and areas influenced by tidal water levels.

Typical locations affected include:

Low-lying coastal areas

Flat coastal landscapes are particularly vulnerable to tidal flooding because water can spread inland once defences are overtopped.

Estuaries

Many rivers in the UK flow into tidal estuaries, where river levels and tidal levels interact. During extreme events, high sea levels can prevent rivers from draining effectively, increasing flood risk.

Coastal towns and cities

Many settlements in the UK developed along the coast for trade and maritime access. Some of these communities are located close to sea level and may be exposed to tidal flooding.

Areas protected by coastal defences

Sea walls, embankments and other coastal defences protect many coastal areas. However, flooding may still occur if defences are overtopped or fail during extreme events.

Because of these factors, tidal flood risk is typically concentrated in defined coastal zones and estuarine floodplains.

How Tidal Flood Risk Is Mapped

In England, flood risk from the sea is mapped nationally by the Environment Agency.

The government’s Flood Map for Planning service identifies areas that may be affected by flooding from rivers and the sea and is used within the planning system to determine whether development proposals require a Flood Risk Assessment (FRA).

Tidal flood risk is represented through the Flood Zones, which indicate the probability of flooding:

These zones are based on modelling of coastal water levels and floodplain behaviour.

National flood risk datasets, including the updated National Flood Risk Assessment (NaFRA2), provide a broader picture of flood risk across England from rivers, the sea and surface water.

As with other national flood mapping, these datasets are intended as a screening tool rather than a precise prediction of flooding at individual properties. Site-specific flood risk is usually assessed through more detailed analysis and modelling.

Local authorities may also provide additional information on coastal flood risk through Strategic Flood Risk Assessments (SFRAs).

Tidal Flooding and Planning Applications

Tidal flood risk is an important consideration in coastal planning and development.

Planning policy requires development proposals to consider flood risk and apply the Sequential Test, which aims to steer development away from areas with the highest probability of flooding.

Where development is proposed in areas affected by tidal flooding, a Flood Risk Assessment is usually required to demonstrate that the development will be safe and will not increase flood risk elsewhere.

Typical mitigation measures for tidal flood risk may include:

• setting finished floor levels above predicted tidal flood levels
• incorporating flood resilience and resistance measures into building design
• maintaining safe access and egress routes during flood events
• designing development layouts that avoid the most vulnerable areas of a site

Coastal flood risk assessments may also need to consider future sea level rise, which can influence flood risk over the lifetime of a development.

Key Things Property Owners Should Know About Coastal Flooding

Several important points are worth understanding about tidal flooding.

Tidal flooding can affect large coastal areas

Extreme coastal flood events can impact extensive stretches of coastline, particularly where land is low-lying.

Flood risk is closely linked to coastal elevation

Areas located close to sea level or within tidal floodplains are typically more exposed to coastal flooding.

Sea level rise may influence future risk

Long-term sea level rise and changing storm patterns may increase tidal flood risk in some locations over time.

Understanding the level of coastal flood risk is therefore important when purchasing property, planning development or managing land in coastal areas.

Need Advice on Coastal Flood Risk?

Assessing tidal flood risk can be complex because it depends on coastal processes, storm surge modelling and future sea level projections. National flood mapping provides an initial indication of risk, but it cannot determine the level of flood risk for an individual site.

Where tidal flooding may affect a planning application, property purchase or development proposal, a detailed Flood Risk Assessment is often required to understand potential impacts and identify appropriate mitigation measures.