Flooded cities: A flood-regulating ecosystem services assessment for heavy rainfalls in urban areas

Scientists from Germany developed a framework with indicator suggestions to quantify and compare flood-regulating ecosystem services supply and demand.

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Extreme weather events – like heavy rainfall – are a major environmental risk. Only recently after the Ecosystem Services Partnership (ESP) 2022 Europe Conference on Crete (Greece) some conference attendees were able to directly experience the effects of heavy rainfall, when air traffic was stopped in Heraklion for many hours, streets were flooded, properties damaged, and even people died.

Heavy rainfall can occur anywhere and are usually highly localized. Cities are particularly vulnerable to pluvial flooding because of the high degree of surface sealing, the high population density and the high potential of socio-economic damage in urban areas. In this light, ecosystems are important natural flood-regulating elements that can relieve grey infrastructure such as sewer systems. They can buffer rain events and prevent flooding as their functions turn into a flood-regulating ecosystem service (ES) to protect society.

So far, flood-regulating ES supply and demand for heavy rainfall in urban areas have rarely been studied. Therefore, scientists from the Climate Service Center Germany and the Leibniz University Hannover (Germany) developed a framework with indicator suggestions to quantify and compare flood-regulating ES supply and demand. Interception by canopies and infiltration in the soil serve as essential indicators for urban flood-regulating ES supply. The indicators can be quantified based on the outputs of a hydrological model that has explicitly been developed for this study. The model is based on single, individual landscape elements. It considers vegetation-hydrological interaction, and 2D surface water routing. Social-economic indicators and the surface flooding indicate the related ES demand.

A flooded neighbourhood. Photo by U.S. Geological Survey

In their study, published in the journal One Ecosystem, they assessed the flood-regulating ES of an urban district in the City of Rostock (Germany) for a one-hour heavy rainfall event. They found the highest mean ES supply on greened areas of forests, woodlands and green areas, resulting in a supply surplus. Whereas, sealed areas (paved surface where water cannot infiltrate into the soil), such as settlements, urban dense areas, traffic areas and industry, showed an unmet demand resulting from both low supply and relatively high actual demand. The results indicated that vegetation plays an important part in flood regulation, if the soils are saturated or sealed and, thus, should be considered in urban flood-regulating ES assessments.

Budget of the flood-regulating ecosystem services supply and demand resulting in unmet demand and supply surplus.

Analysing the supply and demand for flood-regulating ES is particularly important for urban planning in order to identify ES supply-demand mismatches. Based on this information, adaptation measures such as Nature-based Solutions can be planned and their possible ES contributions can be quantified. Since heavy precipitation events are projected to become more frequent and intense in the future, the future functionality of current flood-regulating ES and the benefits of adaptation measures under changing climate conditions need to be assessed. This provides information about changing ES supply and the development of ES demand.

Research article:

Wübbelmann T, Bouwer LM, Förster K, Bender S, Burkhard B (2022) Urban ecosystems and heavy rainfall – A Flood Regulating Ecosystem Service modelling approach for extreme events on the local scale. One Ecosystem 7. https://doi.org/10.3897/oneeco.7.e87458

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