EXTREME HEAVY RAINFALL

Extreme heavy rainfall is defined as precipitation of more than 25 millimetres per hour or more than 35 millimetres in six hours. In contrast to riverine flooding, extreme heavy rainfall does not necessarily have anything to do with bodies of water (streams, rivers, etc.), but is caused by the breaking of storm clouds.

Extreme heavy rainfall events are localised and can have catastrophic consequences: the soil cannot absorb the large volumes of water all at once. Sewage systems are also quickly overwhelmed.

Possible consequences

• Flooding
• Slope water
• Flash floods
• Landslides

Such events are difficult or almost impossible to predict, and very dangerous because of their suddenness.

Slope water

Slope water is flood water that is not caused by bodies of water but accumulates on otherwise dry ground. During intense rainfall, there is high run-off from slopes within a short period of time. This leads to erosion damage in agricultural areas and damage caused by water and mud in residential areas.

Flash floods

A flash flood is a sudden inundation that is usually associated with a subsequent sudden overflow of water from water bodies. Many factors can lead or contribute to flash floods – a small stream that is normally no more than a trickle can become a raging river in a short space of time.

In the lowlands, extreme heavy rainfall can cause the build up of high water levels, particularly in depressions and low-lying areas, and overload the drainage system. In steeper areas, on the other hand, there is a risk of water running off at high speed, and carrying large amounts of floating debris with it. Here in particular, even small bodies of water can become a considerable danger.

Precipitation can collect in hollows and underpasses during downpours, causing the water level to rise quickly. This leads to flooding if the water cannot seep into the ground quickly enough or drain away via a sewer system. Watercourses and even entire lakes can appear suddenly above ground.

The other, often greater part of the rainfall channels its way above ground, usually in uncontrolled fashion, flooding roads and properties. This leads to damage to and in buildings, unless adequate protective measures are implemented.

Flash floods can occur anywhere here with us, regardless of whether there are streams or other flowing bodies of water nearby.

If a flash flood occurs, there is no time to take protective measures. After a few hours, the event is over, and the water subsides – leaving behind deserts of mud, high levels of damage to property and, more and more often, unfortunately also injuries to life and limb.

The extent of the damage caused by extreme heavy rain depends on various factors, including:

• The location (topography, landscape, degree of soil sealing, building density, relief)
• The intensity and duration of the rainfall
• The absorption capacity of the soil and the public drainage system.
• Building design and construction
• The scope of precautions and protective measures implemented by municipalities, property developers and also individual citizens

Unsuitable building materials and their consequences

One factor influencing the extent of damage is the building materials used. Building elements with water-soluble or swellable components or materials such as wood are usually no longer usable after coming into contact with water. Typical damage to the fabric of the building includes:

• Moisture penetration and water level lines
• Efflorescence on building component surfaces
• Changes in shape and volume
• Consequential damage such as mould formation or infestation by other micro-organisms
• Coatings that have peeled off
• Short circuits in the electrical installations
• Corrosion of pipes and machines

When water penetrates building materials, not only moisture damage but also pollutants can be a serious problem. While precipitation running off the surface contains few pollutants, water flowing back up from the sewer can cause considerable hygiene problems.

Heating oil & toxic substances

Another problem is leaking heating oil. Major damage can occur if the oil tanks are not secured during heavy rainfall and the relevant cellar rooms are flooded. The oil is difficult to remove and also damages the fabric of the building, sometimes irreversibly.

Toxic substances such as paints and varnishes stored in flooded rooms can contaminate the building

Here are some factors that can influence your individual risk or the potential impact on your home.

Surface water

Small changes to the topography at the construction stage can help to keep water away. Depressions in the ground or swales can be used or created to divert water away from buildings. Ground sills are just as effective; on slopes, they can prevent water from flowing down the slope on to the property or into the building. Mobile systems such as stop log barrier systems can also keep the water out – but beware: flooding caused by extreme heavy rainfall usually occurs without much warning time, and you always have to be on site to set the systems up in time.

Entrances at ground level should be avoided, and cellar stairs and light wells should be protected by small upstands. The light wells should also have drains connected to the water removal system (or a drainage system). Entrance canopies also help to keep the water out.

Backwater from the sewage system

The backwater level is the highest level to which water can rise in a drainage system before it overflows.

During heavy rainfall, the sewer quickly fills up with a lot of water, which seeks to reach the same level everywhere. If the house connections in the building are located below the public drainage system, so-called backflow protection devices are required to prevent water from entering the house.

Backwater valves seal the pipework, preventing the backflow of draining into your house. They can be used in laundry rooms, cellars or hobby rooms, for example, if there is a slope from the drainage point to the public sewage system, the use of the drainage point during the backflow is not absolutely necessary, and there is no risk to the occupants or their property.

Toilets, showers or washing machines in the basement are usually located below the backflow level (upper road surface). The accumulating water cannot be drained away via a slope. Instead, it must be pumped out with a sewage lifting station so that it can flow into the sewage system.

Groundwater or seepage water and soil moisture

Vertical waterproofing prevents the ingress of water, while horizontal waterproofing prevents the rise of the water within the building component once water has penetrated. A drainage system diverts the water from the soil layers above it, thus helping to remove water from the soil. This prevents the build-up of seepage water and thus the build-up of water pressure.

There are various types of waterproofing for building structures and components exposed to soil moisture, seepage or groundwater. There are three different types of waterproofing, known in German as “black”, “white” and “brown” “tanks”.

A black tank is a skin-like external casing (barrier protection). All components in contact with the ground are coated or enclosed in a barrier membrane such as bitumen or plastic sheeting.

White tanks are made of concrete with a high degree of water resistance (waterproof concrete) (integral protection).

In the case of brown tanks, a waterproofing layer is applied to the ground side of a structure made of waterproof concrete. The material used is a sealing membrane based on bentonite, a naturally occurring, highly absorbent swelling clay that has a strong sealing effect against water even in thin layers.

Retention measures

When water is channelled from sealed surfaces into storage tanks (retention basins, underground tanks or cisterns), we refer to “retention measures”. Storage on the roof is also possible.

Special roofs can prevent water overflowing from gutters during heavy rainfall (green, blue or retention roofs).

Delaying runoff

Infiltration of surface water runoff and evaporation can be increased by unsealing and planting sealed surfaces. The effect depends on the type of soil. There are also various types of infiltration systems that promote the release of surface water runoff into the ground.

Before and during the extreme heavy rainfall

In your own home:

• Follow weather maps, severe weather warnings and instructions from the authorities (radio, TV, Internet, etc.)
• If possible, monitor your roof and floor inlets so that you can react if water accumulates
• Switch off the power supply (circuit breakers) to the areas affected by the flooding in good time
• Keep away from flooded areas
• Do not enter flooded cellars (risk of electric shock from damp electrical appliances, sockets or power lines)
• If water is entering the cellar or basement, be careful when opening doors or windows – there may be a sudden and massive ingress of water
• In the event of sudden water ingress into living areas, move quickly to the upper floors

In the neighbourhood:

• Only walk on well-lit areas and paths in the dark
• Do not walk along flooded roads (danger of displaced, open manhole covers)

Vehicles:

• Refrain from making non-essential journeys by car
• Keep access routes to deployment sites clear for emergency services
• Refrain from making non-essential journeys by car
• Take care not to drive into flooded roads or underpasses

After the extreme heavy rainfall

• Keep access routes to deployment sites clear for emergency services
• Dry affected areas as quickly as possible to avoid structural damage, mould or pest infestation
• Have damaged buildings, heating oil tanks and electrical appliances checked by a specialist
• Avoid direct skin contact with the water and the objects that have been submerged
• If, despite all precautionary measures, harmful substances (e.g. heating oil or chemicals) have contaminated the water, inform the fire brigade immediately