Coping with Cloudbursts 2020

On the evening of July 2, 2011, downtown Copenhagen, Denmark, was hit with 136 millimeters of rain (nearly 5 inches) in an hour and a half. The overwhelmed sewer system backed up, causing thousands of basements to be flooded with a mixture of rain and wastewater. In the aftermath of the storm, insurance companies  paid about $1 billion  in compensation to some 90,000 policyholders.

The Danish Meteorological Institute  defines an extreme rainfall event , or cloudburst, as 15 millimeters of rain (about 0.6 inches) in a 30-minute period. The Copenhagen downpour qualified, with buckets of water to spare.

Standard climate models predict a rainstorm of such intensity for Copenhagen  once in a thousand years . Yet, a number of other extreme rain events have occurred in the same area.

It seems that climate change has upset standard predictive models. Cloudbursts in Denmark should probably now be expected with increased frequency.

Both the city of Copenhagen and the entire country of Denmark are actively preparing for future cloudbursts. At national and municipal levels, plans being considered or implemented include new surface channels to lead rainwater away from buildings, upgraded sewer systems, and the establishment of  Sustainable Drainage Systems (SuDS)  and pumping stations to pump excess water to the sea. The City of Copenhagen's  Cloudburst Management Plan 2012  considers these issues in more detail. 

A simple but critical planning precaution is not to build in places where water flows in but doesn't flow out.

But this simple solution is not always practical. Low-lying areas—sometimes called depressions, sinks, or hollows—are everywhere. They come in all shapes and sizes. On farmland and in parks, fields that look flat may actually contain shallow depressions that trap rainfall.

Some residential areas are situated in places where the low-lying landscape goes unnoticed under dry conditions—it only becomes apparent when so much rain falls that the ground can't absorb it, and the sewer system can't take it away. Roads and other types of infrastructure, such as subway stations and railroad tracks, are also vulnerable.

Almost all low areas, whether on cultivated or developed land, pose some risk. On farmland, the low areas may be a threat to crops and equipment. In residential areas, unless buildings are constructed on high bases or pillars, the low areas can lead to flooding.

Structures that are not located in depressions may still be at risk. When so much rain falls that a depression overflows at its so-called pour point, adjacent areas may be flooded.

Historically, the risks of building in low-lying terrain weren't foreseen. Perhaps they couldn't have been. But in Denmark, at least, several developments are located on land that was drained after the 1850s. Such areas were once peat bogs, wetlands, or small lakes that became fine agricultural land when ditched or reclaimed. If the drainage was efficient, these areas may have gone without flooding for long periods in the absence of heavy rain. Unfortunately, over the past decades, many residential developments were constructed in such areas due to increased demands for housing.

Meanwhile, today, in times of climate change, these still low-lying areas act as water pools during more frequently occurring cloudbursts, so homeowners living on such former abandoned farmland face the challenge of frequent floods. Additionally, infrastructures are affected if they’re located in flooded landscape depressions, too, affecting trade and commuting, and rescue teams can be trapped for hours until the water has been drained.

The combination of the satellite navigation systems and lidar technology has made it possible in recent years to create detailed, high-resolution landscape models. These landscape models make it possible to predict with very high accuracy where water is trapped in sinks in storm water situations. Although bluespot is simply another term for a depression or sink in the landscape, it puts a special emphasis on flood risk: a bluespot is an area likely to fill or overflow in a rainstorm, endangering structures within or near it.

In these lessons, you will create bluespot maps similar to those used by the Danish government. In addition, you will determine where the bluespots spill over when above capacity. You will also discover where the excess water flows downstream, shaping an often highly complex network of streams contributing to the fill-up of other bluespots until the spillover finally reaches the ocean.

When the bluespots and the interconnecting flow paths are identified, you will add various uniform rainfall scenarios to see the impacts for the homeowners and infrastructure at various flooded locations.