After a couple of months of absence, GeoLog is once again hosting the Geosciences column. This month we have no less than two posts highlighting recent research in the Earth sciences. In the second of this month’s columns, Eline Vanuytrecht writes about recent research on flood risk published in the EGU journal Natural Hazards and Earth System Sciences.
If you’d like to contribute to GeoLog, please contact EGU’s Media and Commmunications Officer, Bárbara T. Ferreira at firstname.lastname@example.org.
Floods can cause serious damage in residential areas. Recent records show that the damage has increased over the last decades, placing floods as one of the most severe natural hazards. But what exactly was the main cause for this increase in damage? And how will the relative contribution of drivers of flood-risk change such as meteorological phenomena, land use and socio-economic developments evolve in the future?
To answer these questions, a group of German researchers, led by Florian Elmer of the GFZ Research Centre for Geosciences in Potsdam, analyzed what drives changes in flood risk in the lower part of the Mulde River basin in Eastern Germany. They concluded, rather surprisingly, that land-use changes, not meteorological phenomena, are the main drivers of flood risk change.
“Consequently, the potential influence of local and regional land-use policies is substantial and could contribute significantly to (…) risk mitigation,” the authors write in the Natural Hazards and Earth System Sciences paper.
The study focused on the lower Mulde River catchment but it can serve as bench-mark for complete risk analysis for other river systems. Flooding is common in this catchment, which comprises several municipalities and an area of approximately 1000km² of which around 8% is inhabited.
The scientists surveyed the change of flood risk in terms of expected annual damage in residential areas between 1990 and 2020 in 10-year time steps. The analysis was based both on observations (for the past period) and model projections (for future times). They further quantified how the cost and impact of floods are modified by each of these drivers: climate change, land-use change and changes in building values.
The results of the study show that, under changing meteorological conditions, including altering rainfall patterns and rising temperatures, only few changes in return time of floods of different magnitude are experienced. The return time of a flood is a measure for its frequency, which reflects the estimated average time between two events.
Predicted increases in flood risk are thus mainly related to land-use changes including paving of previously permeable surfaces. Since 1990, the region has undergone major socio-economic changes after the reunification of Germany, including population decrease. However, at the same time, the area saw urban sprawl and residential structure change towards single-family dwellings. This expansion of urban areas increases the area covered by impermeable pavements, hence increasing flood risk. The scientists expect to see more urbanization, and thus increased risk, in the future.
Interestingly, the monetary value of the estimated annual flood damage decreased from 1990 to 2000. This is due to a combination of factors after Germany’s reunification leading to an exceptional situation of high inflation after 1990. In the future, however, the authors predict increasing damage values.
Another interesting conclusion is that small to moderate flood events dominate the risk expectation. These events combine relatively small flood volumes with a high return time (less than 20 years).
The results of the research hold an important message for flood-risk policy. Since land-use change is identified as the main driver of flood-damage change, a key role is reserved for land-use policies in risk mitigation. Further, since the majority of the annually expected damage can be attributed to small to moderate floods occurring frequently, relatively easy-to-install protection measures can erase substantial part of the damage.
By Eline Vanuytrecht, freelance writer & PhD student, KU Leuven