Intensiv nederbörd och pluviala översvämningar i Umeå

Sammanfattning: During intensive rainfall, the ground is at risk of flooding if the water has no opportunity to infiltrate into the ground or drain. Cities are most heavily affected by such pluvial floods due to their predominantly solid surfaces. An example of such a city is Umeå, which experienced extensive floods causing up to 40 million Swedish kronor in damages. During the period 1970 to 2020, the population of Umeå has increased from 70,000 to 130,000 inhabitants, which means that both housing demand and thus the proportion of hardened surfaces in the municipality increased rapidly. To avoid future flooding problems, studying intensive precipitation trends as well as factors that affect the risk of pluvial floods is of the utmost importance. The purpose of this research is thus to investigate heavy precipitation trends as well as study how climate change and hardened surfaces affect the risk of flooding in Umeå. This study shows that the frequency of intense rainfall in Umeå has increased compared to the mean of the period 1963-1987 and that climate change could lead to an even higher frequency. Increased frequency of intensive precipitation in combination with an increased proportion of hardened surfaces increases the risk of flooding problems. Furthermore, heavy rainfalls, defined as precipitation above ten millilitres a day, occurred eighteen times a year during the period 1996-2020, which is four days more than the 1963-1987 average. By the end of the twenty-first century, climate change is expected to increase these number of days by a further 20-30% (equivalent to 7-12 days) according to SMHI predictions. Intense rainfall is likely to become more common in the future and population growth in Umeå will likely lead to an increase in the number of paved areas. These changes, in combination, place high demands on Umeå municipality to work efficiently with urban planning and climate adaptation.