The causes and types of flooding in Calderdale

Why does Calderdale flood?

Our geography - Calderdale has very steep hills. When it rains, water quickly rushes down the hillsides and pours into the River Calder. This makes the river rise fast.
Moorland fires – Healthy moors act like big sponges. They hold rainwater and stop it from rushing into the valleys below. When fires burn the moors, they can no longer hold this water. Sadly, these fires are increasing due to both climate change and human carelessness.
Climate change - We have seen many more floods over the last ten years, and they are getting worse. This is because the earth is warming up, which brings more extreme and dangerous weather.

A low-angle, wide shot taken from just above the water's surface shows the river flowing toward an old stone arch bridge in Brighouse. The foreground is dominated by the dark, rippling water, which appears slightly blurred by movement, with patches of white foam floating on the surface. To the left, damp, dark rocks line the immediate bank. In the background, the heavy stone bridge spans the river, its central pier reinforced with a white metal structure. Dense, dark green trees hang over the water from the right bank, creating a tunnel-like effect that frames the bridge. The lighting is soft and diffused under a cloudy sky, giving the scene a quiet, secluded feel despite its industrial undertones.

Types of Flooding

In the UK, there are five main types of flooding. All of them can cause serious damage. Knowing how they differ helps you protect yourself and know who to call for help.

River flooding

River flooding happens when heavy rain fills small streams, which then pour into larger rivers. How a river reacts depends on how big it is and whether it has flat land (a floodplain) around it to soak up the extra water.

+ More information about warnings and responsibilities

When a river is at risk of flooding, the Environment Agency (EA) sends out a warning. You can sign up to get these alerts sent straight to your phone, computer, or landline.

If you own land next to a river, you are a ‘riparian owner.’ This means you must keep the water moving safely. You must clear away any blocks, such as fallen trees, thick bushes, or dumped rubbish.

The EA has the power to clean and improve main rivers. They are also in charge of building and looking after major flood defenses.

Localised or flash flooding

Flash floods happen when an exceptional amount of rain falls in a very short time. Heavy rainstorms can mean that drainage systems and sewers fill to capacity very quickly, and then ‘over-top’.

Flash flooding can be a major problem if the weather has been very dry for some time, as the land becomes hard and can’t absorb the rain. Rainwater can then run off the land and lead to serious flooding.

Flash flooding can be very localised, but it can do as much damage as flooding from rivers or other watercourses. Localised flooding can also happen after a period of prolonged rainfall, when the ground becomes saturated and unable to absorb any more water.

+ Warning signs and what makes flash flooding worse

Flash flooding can also be made worse when the capacity of small watercourses is reduced by blockages of rubbish. To help avoid this, people who own or manage land next to watercourses are encouraged to keep those watercourses clear of debris that might cause problems further downstream.

Flash flooding can happen suddenly. It is important to know what signs to look for in your location so you can ACT FAST. Those signs are:

Heavy rain or severe weather reports
Rising water levels with churning dark water
A build-up of debris in rivers or streams

Climate change and an increase in thunderstorms means that flash floods are more and more likely to happen. Flash floods are extremely destructive, so being prepared could save property, belongings and even lives.

Cars driving through flooded road caused by burst water main.

Flooding from sewers

Localised flash flooding from blocked or overloaded drainage systems can happen at times of heavy rainfall. This type of flooding is unpredictable, and it often occurs in unexpected locations depending on the location and intensity of rainfall.

Where flooding occurs from foul sewers, the floodwater will be contaminated with sewage. In some cases, contaminated floodwater can flow back through foul sewers causing flooding inside buildings.

+ Health risks, responsibilities and protecting your property

The water companies are responsible for the foul sewerage system, and the surface water sewerage system if it has been adopted. They do not have responsibility for any other watercourses or land drainage systems.

If floodwater has been contaminated, a filthy layer of silt is usually left behind when the water recedes. Contamination can add to the cost of cleaning and disinfecting buildings that have been flooded and it also poses risks to health. Protective clothing should be worn while cleaning after the flood, and other hygiene precautions should also be taken.

General flooding can create blockages in drains and sewers, and this can lead to the backflow of sewage into properties through low-level drain gullies, toilets and washing machine/dishwasher outlets. Backflow through drains is particularly likely where floodwater has been prevented from entering the property by using temporary barriers. It is also more likely where the flooding depth outside is higher than the internal level of the drain entry points. This flooding can often be effectively controlled by installing ‘non-return valves’ within the private sewer of a property upstream of the public sewerage system.

Some properties have private sewage systems and flooding of these systems can also create a back-up of sewage into the property. Damaged septic tanks, cesspools or small package sewage systems should be inspected by a qualified engineer before the drainage system is used after a flood.

Surface and highway flooding

Roads and paths are usually drained into highway drains that run alongside them. In rural areas, roads may drain into ditches along their edges. In heavy rain and storm conditions, or following periods of prolonged rain, these drains can sometimes find it difficult to cope with run-off from roads and paths. This can lead to flooding.

In rural areas, keeping drainage ditches clear of obstructions is the responsibility of the landowners whose land they flow through. Landowners are encouraged to maintain land ditches.

Highway drains are the responsibility of the highway authorities, who maintain them and attempt to keep them clear so that they can cope with run-off.

Reservoir flooding

Reservoirs hold large volumes of water above ground level, contained by dams. Although the safety record for reservoirs is excellent, there is a very small possibility that a dam could fail, causing a large volume of water to be released very quickly. Reservoirs are regularly inspected, and essential safety work is carried out. Local authorities are responsible for coordinating emergency plans for reservoir flooding as well as ensuring communities are well prepared. They work with other organisations to develop these plans for your area.