Over the last 25 years the heaviest rainfalls have become 20-25% heavier. Heavy rainfalls are becoming more frequent and when they come they are more intense.
This is the result of a new analysis carried out by DHI Water & Environment and the consulting engineering company, COWI A/S.
The results support an earlier study that showed that Western Denmark has more frequent heavy rainfalls and heavier rainfalls of short duration, whereas Eastern Denmark has heavier rainfalls of longer duration.
This most recent study covers the years 1979 to 2005 and includes considerably more data than previous studies.
An increasing trend
While the previous studies used data from 41 rain gauge stations, the data for the new study is based on 66 rain gauge stations and includes data from the past nine years.
"The new study shows that there is an increasing trend over the whole of Denmark towards heavier rainfall events and for the first time puts numbers on the changes. The large rainfall events are 5-10% heavier and the largest events are 20-25% heavier. Furthermore, the frequency of the heavy rainfall events has increased by about 10%." says Henrik Madsen, senior hydrologist at DHI and adds "Previous analysis of precipitation climate by the Danish Meteorological Institute has shown a general increase in precipitation in Denmark. The average annual rainfall for the standard normal period 1961-1990 was, for the country as a whole, 7% larger than in the period 1931-1960. Our analysis shows that the increasing trend in heavy rainfall is much more pronounced than the general precipitation climate".
Problems possible
"It surprises me that the rainfall has become so heavy. We are seeing the effects of climate change now, which we five years ago thought would be changes over the next 100 years or so. We can certainly handle the changes we have seen so far. However, if we continue to see increasingly heavier rainfalls, we will have problems of more frequent flooding both in cities and in some rural areas." says Karsten Arnbjerg, the project leader at COWI.
The results of this study and how this new data should be used in design of urban drainage systems will be discussed in the beginning of April by the Danish Water Pollution Control Committee.
If this tendency towards heavier rainfalls continues, then it will require more resources, points out Karsten Arnbjerg: "The current sewer system can handle the extra water without causing excessive flood damage. However, if we are going to build new sewer systems, then we need to estimate how much water to expect during the typical 100-year lifetime of the sewers. At the moment, we can't answer this question. It is a large uncertainty."
Cities need to reserve areas for flooding
"The extreme rainfall has implications for the capacity of the sewer system in the future, but also for the way we develop our urban areas. It is important that we begin to investigate ways to supplement the sewer system" says Karsten Arnbjerg and adds "we need to find out; how to develop urban areas so that very heavy rainfalls do not cause serious damage; what level of service we can expect; how good the sewers should be; and what we can expect for the money we are willing to pay"
One possibility is to reserve special areas that can be flooded during heavy rains. These areas need to be designed to handle floods without sustaining damage, but at the same time can be used when they are not flooded, for example as a football field, playground, or an open-air theatre.
For further information please contact: Dr. Jacob Hoest Madsen jhm@dhigroup.com
Source: www.cowi.dk