More torrential rain

Simple measures can protect against cloudbursts and floods

As climate change takes hold, cloudbursts will become more frequent. We will experience more days with heavy rain, and the amount of rain will increase.

As experienced, among other things, in the capital area of Copenhagen during the July cloudburst in 2011, the public sewage system could not divert such large amounts of rain. The experience has helped to strengthen efforts to expand and restructure the public sewerage systems. Nevertheless, there is also good reason to assess whether the individual properties are sufficiently equipped to withstand increasing amounts of rain and cloudbursts. Simple measures can protect basements, low-lying floors, and decks against flooding.

Terrain regulation can help divert rainwater in a controlled and efficient manner. Establishing a perimeter drainage system can protect foundations and basement walls against dampness.

Construction of fascines, combined with a possible collection and recycling of the rainwater, can help solve the problem locally and relieve the area’s drainage systems. A combination of fascines with ground heating systems can be advantageous, according to the report ‘Development of fascines with ground heating hoses’ by The Danish Nature Agency, which is available below. Please note that not all areas are suitable for establishing fascines and usually require permission from the municipality unless the roof or surface water only originates from a smaller area.

It may be necessary to rethink and improve gutter and drainage systems to ensure effective rainwater diversion away from the roof and building in the event of a cloudburst. Establishing emergency drains may be an option for large roof surfaces, where an accumulation of water can occur in the event of cloudbursts, which can be critical for load-bearing structures.

Some roofs and decks with vegetation, called green roofs, can absorb and retain some of the water when it rains. In optimal situations, green roofs, properly executed, can absorb and evaporate up to 60 to 80 % of the precipitation that falls on them over the year.

Temporary or permanent bulkheads can prevent surface water from flowing through doors or down via stairs at terrain level. Protection of basement stairs, light shafts, and other vulnerable openings is possible with raised edges.

If flooding threatens specific installations, such as basement drains, it can be necessary to establish a backflow prevention valve in the wastewater line near the installation subject to flooding. There should not be wastewater supply upstream of the backflow prevention valve, and rainwater supply and drainage must occur downstream. Backflow prevention valves are limited to a small group of users, such as in single-family houses.

Another option for protection against flooding in buildings is a bypass system. It consists of a backflow prevention valve and a pump system. During normal operation, gravity diverts the wastewater to the main line. During flooding, the high-water valve closes, and an overflow directs the wastewater to the pumping system. The use of bypass systems is possible for small and large user groups, e.g. for apartment buildings and in both single and two-line sewer systems.