How Dry Weather Increases Flood Risk in the UK
Long dry spells make soil hydrophobic and compacted, increasing surface water runoff and flash flooding risk when rain finally arrives.
How Dry Weather Causes Surface Water Flooding
It may seem counterintuitive, but long dry spells can make the ground more vulnerable to flooding when rain finally arrives. This is because soil can become compacted and, in some instances, hydrophobic, meaning it actively repels water. As a result, the ground loses its ability to absorb rainfall, and instead of soaking in, water rapidly runs off the surface, increasing the risk of surface water flooding.
In urban areas, where roads, pavements and buildings already prevent natural infiltration, the effect is even more severe. When heavy rain falls on hardened or hydrophobic ground, it can overwhelm local drainage systems and cause water to accumulate rapidly. This results in surface water flooding, which can sometimes lead to flash flooding if the rainfall is particularly intense and the runoff builds up quickly. A recent example occurred in Kent, where after weeks of warm, dry weather, sudden thunderstorms over the weekend of June 14th caused significant flooding in areas such as Sandwich, Thanet, Dover and Folkestone.
Surface water flooding happens when rainfall cannot drain away quickly enough. It is often fast, unpredictable and highly localised, but it can cause serious disruption and damage. Sustainable Drainage Systems (SuDS) are essential in managing this risk, helping to slow, store and absorb rainfall where it falls.
Pluvial (Surface Water) Flooding in the UK
Pluvial, or surface water, flooding occurs as a result of heavy rainfall that is unable to infiltrate into the ground or enter a drainage system, resulting in runoff flowing overland. This type of flooding is particularly common in urban areas, where impermeable surfaces such as roads, pavements and rooftops prevent water from soaking into the ground. However, it can also occur in rural locations, especially where soil has poor permeability, groundwater levels are high, or rainfall intensity exceeds the soil’s infiltration capacity.
Why Surface Water Flooding Matters
Surface water flooding has increasingly been recognised as a major flood risk. In July 2021, the National Planning Policy Framework (NPPF) was updated to emphasise the importance of assessing all sources of flood risk, including surface water, in planning decisions.
The latest NaFRA2 data has since confirmed that “surface water flooding now affects more properties across the UK than either river or coastal flooding,” underlining the need for it to be considered on equal terms.
The Environment Agency’s Flood Map for Planning (FMfP) also now shows surface water datasets, further highlighting its importance and relevance.
How SuDS Help Manage Surface Water Flooding
Sustainable Drainage Systems (SuDS) are designed to mimic natural processes in urban areas by managing rainfall close to where it falls. Their primary aim is to reduce the volume and speed of surface water entering the sewer system, which in turn lowers the risk of flooding, improves water quality and supports more liveable, greener places.
Green SuDS features such as green roofs, rain gardens, permeable paving and swales provide the most benefit. They intercept rainfall at source, reducing the burden on sewers and slowing the rate at which water enters the drainage system.