Floods and water quality

Flooding is the most frequent and most costly natural hazard in most countries. Its water quality consequences persist long after the water has gone.

Types of flooding

Fluvial (river) flooding: A river exceeding its channel capacity. Usually predictable hours to days ahead, which is what makes flood warning systems effective.
Flash flooding: Intense rainfall on steep, small or saturated catchments. Little or no warning time, and responsible for a disproportionate share of flood deaths.
Surface water flooding: Rainfall exceeding drainage capacity, independent of any river. The most common flood type in urban areas and the hardest to map.
Groundwater flooding: Water tables rising above ground level after prolonged wet periods. Slow, persistent, and difficult to pump away because the source is continuous.
Coastal flooding: Storm surge and wave overtopping, often compounded by high tide and by sea level rise.

What flood water contains

Flood water in a developed area is not river water. It is a mixture of whatever the flood has passed through and dissolved or entrained.

Sewage, from surcharged combined sewers and inundated treatment works — the most significant public health hazard
Fuel and oil from vehicles, domestic heating tanks and garages
Agricultural runoff carrying nutrients, pesticides and animal waste
Industrial chemicals from flooded premises and storage
Sediment, which carries adsorbed metals and organic pollutants
Debris and sharps, the leading cause of physical injury during clean-up

Effects on drinking water supply

Flooding disrupts supply in several distinct ways. Treatment works and pumping stations sited near rivers can be inundated and taken offline. Raw water turbidity rises sharply, which can overwhelm filtration and force a works to shut down precautionarily. Loss of pressure in the distribution network can allow contaminated water to be drawn into mains through defects — which is why pressure loss usually triggers a boil water notice rather than the flooding itself.

Private supplies and shallow wells are far more vulnerable than mains supply, because they typically have no treatment barrier and no monitoring.

Why urbanisation makes it worse

The urban water cycle

Cities break the natural cycle. Paving stops infiltration, so rain that would have soaked away becomes runoff that arrives all at once.

Impermeability: A natural grassland catchment might turn 10% of rainfall into runoff. A dense urban catchment can turn 70% or more, and deliver it in a fraction of the time.
Why paving front gardens matters: Small individual decisions aggregate. Widespread front garden paving measurably increases peak runoff across a whole neighbourhood.
Sustainable drainage: Permeable paving, swales, rain gardens and green roofs restore infiltration and storage, reducing both flooding and sewer overflow.

A natural catchment might turn ten percent of rainfall into runoff, delivered slowly. A densely built one can turn seventy percent or more into runoff, delivered in a fraction of the time. The same storm therefore produces a far higher and far faster peak — which is why surface water flooding has grown even where rainfall has not changed much.

Sources

Environment Agency — Environment Agency open data catalogue. Open — attribution required · Open Government Licence v3.0
Copernicus Emergency Management Service — Rapid Mapping and Risk & Recovery Mapping. Open — attribution required · CC BY 4.0
UK Health Security Agency — Waterborne infectious disease surveillance and guidance. Open — attribution required · Open Government Licence v3.0