The extreme flood from mid-July 2021 in a historical context of land-use development

Extreme rainfall in mid-July 2021 led to flash floods in several catchments in Western Germany, such as the Inde catchment in the Meuse basin and the Ahr, Erft, and Wupper catchments in the Rhine basin. Record water levels exceeded historical data from gauging stations and extreme flood predictions. However, all four catchments experienced similar flooding in the past. Land-use and land-cover data (LULC) are key drivers for hydrological models. This study gathers historic LULC information for the 19^th century to quantify LULC-changes in the catchments and analyse their effects on flooding.

In the Erft catchment, flood damages are located in the upper catchment, as in Erftstadt-Blessem a 60 m deep gravel open pit mine near the river was flooded, and retained large amounts of discharge [1]. The upper catchment has been dominated by cropland since the early 19^th century (58%, vs. 48% today). Flooded areas in mid-July 2021 changed from mostly pastures (44%), to mostly urban fabric (41%).

The Inde catchment is characterized by industrial and mining activities. Urban fabric increased from less than 1% to 16% catchment-wide and to 37% in flooded areas. A lignite open pit mine at the mouth of the Inde River was flooded similarly to Erftstadt-Blessem [2].

Although reservoirs regulate the Wupper catchment, severe flooding also occurred in July 2021. In total, 11 out of 15 reservoirs exceeded their capacity limits. In this densely populated catchment, urban fabric increased from 5% to 29%, and reached 47% in flooded areas.

In the rural Ahr catchment, forest increased from 34% to 56%, and urban areas increased from less than 1% to almost 7%. In flooded areas, land-use changed from cropland to a 10-fold increase in urban fabric [3].

The four catchments differ in their geographical location (low mountain range vs, loess-dominated lowland) and their LULC-changes since the 19^th century. All catchments experienced severe flooding and damages during the July 2021 flood. Results indicate that catchment-wide land-use is not a key factor for flood severity. Rather, topographical and geological conditions and other factors, such as river regulation and damming, play a more significant role. However, increased anthropogenic pressure on floodplains led to higher water levels and an increased damage potential. In particular, urbanization and mining have emerged as critical contributors to flood severity. Thus, flood protection should consider land-use on floodplains and provide more space for the river.

[1] Lehmkuhl, F., & Stauch, G. (2023). Anthropogenic influence of open pit mining on river floods, an example of the Blessem flood 2021. Geomorphology, 421, 108522. https://doi.org/10.1016/j.geomorph.2022.108522

[2] Keßels, J., Wolf, S., Römer, W., Dörwald, L., Schulte, P., & Lehmkuhl, F. (2024). Enormous headward and gully erosion in a floodplain area reclaimed for open-cast lignite mining during the July 2021 flood in the Inde River valley (Western Germany). Environmental Sciences Europe, 36(1). https://doi.org/10.1186/s12302-024-00997-4

[3] Vélez Pérez, M., Wolf, S., Klopries, E.-M. (2023). Quantifizierung des Einflusses der Landnutzung an der Ahr auf das Abflussverhalten. Korrespondenz Wasserwirtschaft 7, 435-441