Developing a flood typology for Denmark with practical applications for public warnings and communication

Denmark has experienced several significant compound flood events in recent years, and in parallel the Danish Meteorological Institute (DMI) has been developing a new flood warning system. This has led to a reassessment of how floods are conceptualized, predicted, and communicated. To support this, we here propose a framework for a flood typology tailored to both single-source and compound flood phenomena, with practical applications for public warnings and communication.

Our methodology builds upon the internationally acknowledged UNDRR/ISC hazard classes, which we filter for flood-related hazards relevant to Northern European coastal, lowland conditions. We then consider the organization of Denmark’s national agencies, local emergency response, and insurance structures. From this, we develop a flood typology for communication. As a part of the study, historical occurrences of compound flood phenomena are meticulously assessed by reviewing textual descriptions from a historical flood register (1990–2020) and conducting detailed case studies of recent events. Additionally, we examine the spatial and temporal overlap of flood-generating processes through a quantitative analysis of historical severe weather warning occurrences (2014–2024) addressing events of rainfall, storms, and sea levels.

Our findings reveal overlapping definitions within the UNDRR/ISC hazard classes, particularly regarding flood-generating processes and their geographic context. While DMI oversees severe weather warnings, observation networks are divided among four national agencies within the fields of: meteorology, oceanography, inland surface water, and groundwater. The emergency response in Denmark, as managed by 98 municipalities, is generally infrastructure-focused rather than flood-type-specific. For instance, urban water utilities often manage flood operations in cities. The insurance sector distinguishes between pluvial floods (private market) and fluvial or storm surge floods (covered by a national public disaster fund). We propose five general flood types for communication: (1) pluvial, (2) fluvial, (3) coastal, (4) groundwater, and (5) technological hazards (infrastructure failures of pumps, sluice gates, etc.). The historical flood register indicates two predominant compound flood types in Denmark: "coastal + fluvial" (driven by extratropical cyclones in winter) and "pluvial + fluvial" (caused by convective rainfall extremes in summer). It also shows that the key preconditioning variables include soil moisture, snow depth, and Baltic Sea water levels. The analysis of historical weather warnings reveals distinct regional patterns in compound flood risks. The detailed case studies provide storylines of how spatial compounding of flood types can overwhelm both national and local emergency responses. 

By integrating these insights, our study establishes a typology that is locally relevant for the Danish context, enhances the understanding of compound floods, and informs strategies for improved compound forecasting and communication.