Heavy precipitation, a risk to be considered

09 September 2012
Lessons from Losses 4/2012. A cloudburst in Copenhagen in July 2011 caused extensive damage in the area. Events like this are becoming more frequent, and while authorities may provide improved infrastructure to address the issue, your own preparation for an extreme rain event may prove valuable sooner than you may wish to know.

The cloudburst in Copenhagen

Copenhagen was hit by a cloudburst on the 2nd of July 2011; more than 120 millimetres of rain fell over the city in the course of 2 hours. Main roads were rapidly transformed into rivers of water and many ground floor and basement areas were flooded. Additionally, traffic in the capital was dramatically affected when flood water blocked several major roads into and around the city. It took several days to normalize traffic in the effected areas.

According to the records of the insurance industry the event caused close to 91,000 losses at a total expense of approximately 4.9 BDKK. Almost 21,000 of these losses were reported by businesses, totalling losses worth approximately 3.2 BDKK.

Heavy precipitations on the rise

The Intergovernmental Panel on Climate Change (IPCC) has analysed weather observations from countries all over the world, collected since 1950. According to the IPCC a likely trend can be observed in this data for an increase in heavy precipitation events within many land regions. The same trend is seen even in areas where the annual precipitation is believed to be in decline.

Indeed, the same conclusion has been derived by the Danish Meteorological Service (Danish abbreviation: DMI) by analysis of Danish records on precipitation since 1874. During this period annual precipitation has gradually increased by 100 millimetres to an average of approximately 700 millimetres and, interestingly, heavy rain has become a more intense and at the same time more frequent event. A cloudburst of the intensity that Copenhagen experienced in July 2011 had not been recorded in the previous 55 years.

The intensity of the cloudburst on the 2nd of July 2011 in Copenhagen is unlikely, albeit not less likely than last year, to be repeated any time soon, but data still suggests that we should prepare both for more extreme precipitation of higher intensity as well as higher frequency on a global scale. This is worth bearing in mind when planning your property protection.

The response of the authorities

Authorities generally design national sewer systems for specific severities. In Denmark the design is for a 10 year event, i.e. an intensity that is not likely to be repeated more frequently than every 10 years. At the design severity water will accumulate in the sewer system to a height level with the ground. On rarer events the capacity of the sewers will be overcharged and water will remain above ground until the sewer system has had time to drain the affected areas.

With the prospect of individual rain events becoming still more severe in future, existing sewer systems would be expected to become overcharged more often than today. The expectation of heavy rain becoming more common adds further to the problem. From a macroeconomic point of view it makes sense to enhance infrastructure to mitigate the exposure to flood losses, and, indeed, governments are looking to develop systems for handling surface water.

The time frame for the implementation of such infrastructural responses is likely to be many years into the future in the developed world. In poorly developed countries it is unlikely the authorities are planning any relief in this respect.

Planning your response

Although the efforts of the authorities are likely to reduce your exposure to losses following cloudbursts in your area, your own control efforts can prove invaluable sooner that you may wish to know. You have to start by understanding your exposure to this kind of loss. These are some of the questions  you may want to be answered in this process:

Is there history of flooding in the local area?

In this respect history is likely to repeat itself. You may want to learn from history.

Where is surface water likely to flow in your area?

Water bodies nearby may give you a clue, but you have to bear in mind that even if your location has a commanding view, it may still be at a topographic depression locally. In that case you may be exposed to water draining from a fair proportion of the local area.

How much water is the local system able to handle?

The local authorities should be able to provide details on the design criteria of the local sewer system. It may be designed for 10 year or 5 year events. Your location is safer the more conservative the design is. The system’s maintenance is also an important factor.

Does the local or national government have an analysis?

Does the local or national government have an analysis of the exposure in your area?Systems are now available that can predict water flows and accumulation levels locally upon specified rain events. Many municipalities have bought access to such systems and may be able to provide you with information on your location or, at least, with a list of most exposed areas in the municipality. If not, you can obtain this data directly from the provider.

How are your buildings constructed?

It is not uncommon in modern designs to leave the first floor or stairways to the basement areas flush with the surrounding ground. This design feature is pleasing to the eye and convenient to the handicapped, but unfortunately would also allow for the easy access of water to the buildings. Ramps for underground parking and store rooms have also proved to be effective conduits for water. Matters would be further aggravated if your land slopes toward the buildings.

What do you store in the basement?

It is cheaper to develop the basement areas for new purposes, than it is to add new buildings next door, and this is probably the reason why we see still more expensive occupancies in these vulnerable areas. Restaurants and bars open up in basements, and in industry we come across a large number of archives and server rooms there. Perhaps cloudbursts have not been considered when establishing such areas.

Depending on answers that you have acquired above, you may want to reconsider how you use your basement areas. Our risk management specialists can provide estimates for your loss exposures as well as valuable advice in the design of the engineering solutions that will provide the safety and stability to your operation that you need. 

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