Stopping basements from becoming ponds: Drainage of buildings in heavy rainfall events

These two example demonstrate impressively the consequences heavy rainfall events can have. In the last 30 years more than 4000 flooding events occurred, out of which 1000 took place in the temperate zone of the northern hemisphere. An analysis of climate model calculations shows that – with some regional exceptions – the intensity of precipitation in the 21st century has been rising in most parts of the world, especially in northern Europe and other regions.*

What does this mean for planning and selecting building services drainage systems? How can real estate be effectively protected against torrential rainfall?

Batting down the hatches is not enough: Water can also enter the building through backflow

One thing is for sure: If you want to protect your real estate from heavy rainfall and flooding, you should first of all take classic measures, making sure the windows in the cellar and basement as well as on the ground floor are sealed as much as possible. Mobile protection systems, such as moveable wall panels can be used in addition to cover light wells or entrance areas to stop water from entering the house.

However, these measures are not sufficient as waste water can also reach the inside of the house in other ways in such weather events – and that is through the drains of toilets, showers, washbasins or washing machines located at a lower level or through floor drains, for example in laundry rooms. This happens when a so-called backflow occurs due to overloaded sewers. Unfortunately, many buildings are not or only insufficiently protected against backflow. Often, nothing happens for many years. But then, extreme storms bring torrential rain with them, the sewer system exceeds its capacity, and lower-level rooms are flooded through their own drainage systems. In most cases, remedying such damage is hard work, comes at a high cost and is rarely covered by insurance.

The problem of backflow: When stormwater shoots out of the basement toilet

Backflow occurs when waste water from the discharging sewer is pressed back into the buildings connected. This happens when waste water cannot run off fast enough. Such a sewer overload can be caused when more and more buildings are built and then connected to the existing sewers without the sewer system being enlarged accordingly, for example.

Of course, municipalities are aware of this problem. However, it is hardly feasible to implement sewer networks of sufficient dimensions to transport the water from torrential rainfall events without any backflow occurring whatsoever. When thousands of cubic metres of water fall from the sky within a short time in a small localised area, sewers quickly exceed their capacities.

Water as high as the flood level

What actually happens when backflow occurs? When excessive volumes of stormwater enter the sewer system through gullies on the road, the water in the affected sewers and premises’ drainage systems rises up to the so-called flood level, which is usually the upper edge of the road. If protection is insufficient, the waste water then spills out of lower-level discharge points (e.g. toilets, showers or drains in the basement). Backflow causes many million euros of damage to property every year. The liability lies with the property owner, not with the municipality. This is why backflow protection devices are compulsory when taking out building insurance.

Keeping your feet dry during torrential rain: Effective protection against backflow

To keep the basement dry, also during torrential rain, a house not only has to be be protected against water entering from the outside but also against waste water pressed back into the building. Two options are generally available for the latter:

• Anti-flooding devices
• Lifting units

The use of anti-flooding devices is limited

Anti-flooding devices are simply fitted in the relevant drain pipes. As a rule, swing check valves are used for this purpose which work like this: A flap allows water to drain off to the outside but closes when backflow occurs. Such anti-flooding devices to DIN EN 13564-1 are available in the most diverse of design variants – for stormwater, for waste water with or without faeces, with manual actuation only or with electric as well as manual actuation. 

Naturally, these devices are a lot less costly than a lifting unit with pump(s), valves, etc. However, they do not provide complete protection. Malfunctions, insufficient maintenance, foreign objects etc. may compromise or even fully eliminate the protective function. Even with intensive maintenance, absolute safety against water entering the property cannot be ensured unless the flooding device’s obturator has been closed manually. Opening and closing the obturator is cumbersome and easily forgotten. This is why this solution is not very practical – especially since washbasins, toilets, etc. in the basement cannot be used when the obturator is closed. In brief: Anti-flooding devices carry significant risks and may lead to unnecessary claims for damages in the event of flooding.

For the above reasons the use of anti-flooding devices is limited. In accordance with DIN EN 12056 they are permitted in the following specific cases:

• When a natural gradient to the sewer is given, waste water containing faeces from rooms of subordinate use may be discharged via an anti-flooding device – provided that few people use the sanitary installations and an additional toilet is available above the flood level.
• In addition, faecal-free waste water may be discharged via an anti-flooding device when the connected installations (shower, washbasin, etc.) do not need to be used in the event of backflow.

Lifting units: the only truly reliable backflow protection

Backflow is inevitable in many cases but its disastrous consequences are not. Only a waste water lifting unit with backflow loop offers absolutely reliable backflow protection, even in the event of a power failure. During normal operation a pump-operated lifting unit “lifts” waste water above a so-called backflow loop, so it can flow off into the sewer system. When backflow occurs, no water can push back into the house as the loop is located above the flood level. If the sewer system is overloaded, the water will only spill out of the gully on the road. Only a backflow loop combined with a lifting unit offers absolute protection. Another benefit: With a lifting unit, the installations connected to it can even be used during backflow conditions.

To ensure continuous protection DIN EN 12056-4 specifies regular, professional checks and maintenance of the entire system: for sewage lifting units in single-family houses once a year; for multiple dwelling buildings twice a year, and in commercially used buildings four times a year. The corresponding maintenance costs should be taken into account.

Dry basement, also during heavy rainfall: Summary and conclusions

When, during heavy rainfall events, large volumes of water enter the sewer and exceed its capacity, backflow may occur: The water pushes back into houses through the discharge points, flooding lower-level rooms if no protection is provided. For this reason, DIN EN 12056 specifies that backflow protection for discharge points below the flood level is not voluntary but compulsory. In some exceptional cases anti-flooding devices are permissible. Technically speaking, complete lifting units are the clearly better solution. They pump the waste water generated in the house, lifting it above the flood level, from where it runs off into the sewer system. Selecting a matching lifting unit should be part of designing a house.

*https://wiki.bildungsserver.de/klimawandel/index.php/Starkniederschläge_und_Hochwasser