Three hypotheses on changing river flood hazards

Abstract

There is serious concern that the hazard, or probability, of river floods is increasing over time. Starting from narratives that are sometimes discussed in public, the article addresses three hypotheses. The first suggests that land-use changes, such as deforestation, urbanisation and soil compaction by agriculture, increase flood hazards. This review finds that land-use effects on floods are particularly pronounced in small catchments since soil permeability plays an important role in infiltration at this scale. For regional floods, and the most extreme events, land use is usually not the most important control, since areas of soil saturation play a greater role in runoff generation, which are less dependent on soil permeability. The second hypothesis suggests that hydraulic interventions and structures, such as river training, levees and dams, increase flood hazards. This review finds that hydraulic structures have the greatest impact on events of medium magnitude, associated with return periods of tens to hundreds of years, and that their effects are usually local. Long-term interactions between humans and floods must be taken into account when predicting future flood hazards. The third hypothesis suggests that climate change increases flood hazard. This review finds that, in small catchments of a few hectares, flood hazards may increase due to convective storms. In large catchments, where regional floods occur, changes are not necessarily directly related to precipitation, nor are they directly related to rising air temperatures, but are determined by the seasonal interplay of soil moisture, snow and extreme precipitation via runoff generation. Increases and decreases in flood hazards have been observed worldwide. It is concluded that significant progress has been made in recent years in understanding the role of land use, hydraulic structures and climate in changing river flood hazards. It is crucial to consider all three factors of change in flood risk management and communicate them to the general public in a nuanced way.