Types of Flooding Explained: Sources of Flood Risk

While many people associate flooding primarily with rivers or the sea, flood events can originate from rainfall, groundwater, drainage systems or failures in engineered infrastructure. Each source of flooding behaves differently and can affect land and property in different ways.

Within the planning system, flood risk must be considered in accordance with the National Planning Policy Framework (NPPF) in England and Technical Advice Note 15 (TAN 15) flood risk policy in Wales. Understanding the different sources of flooding is also important when interpreting national datasets such as the Flood Map for Planning and the Flood Zones used within the planning system.

The sections below explain the main types of flooding typically considered within Flood Risk Assessments and the planning process.

The Main Types of Flooding

Pluvial (Surface Water) Flooding

One of the most common types of flooding in the UK is surface water flooding, also known as pluvial flooding. This occurs when rainfall cannot drain away quickly enough and instead flows across the ground surface or collects in low-lying areas.

Surface water flooding is often associated with intense rainfall events and can occur almost anywhere, including locations far from rivers or the coast. Urban environments are particularly vulnerable because impermeable surfaces such as roads, pavements and roofs prevent rainfall from infiltrating into the ground.

Fluvial (River) Flooding

Fluvial flooding occurs when rivers or watercourses exceed the capacity of their channels and water flows onto surrounding land.

This type of flooding most commonly affects river valleys and floodplains, where water naturally spreads during periods of high river flow. Prolonged rainfall across a river catchment is one of the most common causes of river flooding.

Within the planning system, the likelihood of flooding from rivers and the sea is represented through the Flood Zones shown on the Flood Map for Planning.

Tidal (Sea) Flooding

Tidal flooding occurs when sea water moves inland from the coast, typically during periods of high tide combined with storm surge conditions.

Strong winds and low atmospheric pressure during coastal storms can raise sea levels above normal tidal conditions. When coastal defences are overtopped or breached, water can spread across low-lying coastal land and estuaries.

Groundwater Flooding

Groundwater flooding occurs when water levels within the ground rise to the surface.

This usually happens when the water table rises following prolonged periods of rainfall, allowing water to emerge through permeable soils or rock formations such as chalk and limestone. In some cases groundwater may enter basements or affect low-lying land.

Groundwater flooding is often strongly influenced by local geology and hydrogeological conditions.

Sewer Flooding

Sewer flooding occurs when drainage systems become overwhelmed or fail, allowing wastewater or stormwater to escape from the sewer network.

During intense rainfall events, sewer systems may become surcharged, meaning pipes fill completely and water is forced back up through drains, gullies or manholes.

Responsibility for managing local flood risk from sources such as surface water and ordinary watercourses often sits with Lead Local Flood Authorities (LLFAs), while Local Planning Authorities (LPAs) consider flood risk when determining planning applications.

Reservoir Flooding

Reservoir flooding refers to the potential release of water from large raised reservoirs if a dam or embankment were to fail or become overtopped.

Although such events are extremely rare due to strict safety regulations and inspection regimes, reservoirs store large volumes of water and therefore represent a potential source of flood risk.

Potential flood extents from reservoirs are mapped nationally by the Environment Agency.

Infrastructure or Artificial Source Flooding

Flooding can also arise from failures in engineered water infrastructure.

Examples include canal breaches, burst water mains, culvert failures or pumping system malfunctions. Because these sources are linked to specific infrastructure assets, the resulting flooding is often highly localised.

Why Understanding Types of Flooding Matters

Different types of flooding behave in different ways and require different approaches to flood risk assessment and mitigation.

For example, a site that is outside river flood zones may still be vulnerable to surface water flooding or groundwater emergence. Flood Risk Assessments prepared for planning applications therefore consider multiple potential sources of flooding, alongside planning policy requirements and national flood datasets.

Understanding the types of flooding affecting a site is an important step in identifying appropriate mitigation measures and determining whether development can be made safe.

Need Advice on Flood Risk?

Flood risk can arise from a range of natural and artificial sources, and interpreting national flood mapping or planning policy requirements is not always straightforward.