There is serious concern that the hazard, or probability, of river floods is increasing over time. Anticipating any change in flood hazard is extremely important for adapting flood management strategies and thereby reducing potential damage and loss of life. However, floods are the result of complex interactions of runoff generation processes within the catchment area not easy to quantify. This presentation will review recent advances in understanding how and why river floods, and their probabilities, are changing over time.
Land use change, such as deforestation, urbanisation and soil compaction resulting from more intense agriculture, modify river floods by altering the infiltration capacity and soil moisture. Locally, these processes are well understood but less so at the catchment scale. The effect of land use on floods is particularly pronounced for flash floods in small catchments because of the role of soil permeability in infiltration at this scale. For regional floods, and for the most extreme events, land use is usually not the most important control, because areas of soil saturation are more relevant in runoff generation, which are less driven by soil permeability.
Instead, hydraulic engineering works, such as river training, reservoirs and levees, are more relevant. The effect of individual hydraulic structures can be captured well by hydraulic modelling based on mass and momentum balance, and their role depends on the event magnitude. There is a tendency for all of these engineering works to exert the greatest effect on floods for events of intermediate magnitude, e.g. associated with return periods of the order of ten to one hundred years. Regional effects of engineering works are an active research topic.
Climate change can affect river floods at all catchment scales, from a few hectares to hundreds of thousands of square kilometres. Observed changes in river floods, e.g. in Europe, suggest that climate change is indeed modifying the river flood hazard, but the changes are not necessarily directly linked to precipitation, nor are they directly linked to rising air temperatures. The key to understanding climate change effects on floods is therefore the seasonal interaction between soil moisture (influenced by precipitation and evaporation), snow processes, extreme precipitation and runoff generation. In Europe, there have been a number of flood-rich periods in the past 500 yrs and we are currently in one of them. A trend of storm tracks to move further north in Europe has increased both average and extreme precipitation and thus river flood hazard in the Northwest of Europe, but floods are decreasing where snowmelt is relevant due to shallower snowpacks. There is a tendency for climate change to have the greatest effect on floods of large event magnitudes.
It is concluded that substantial progress has been made in recent years in understanding the role of land use, river works and climate in changing river flood hazards, both through data based and modelling approaches. Considering all three controls of change is essential in reliable flood risk assessment and management in order to maximise protection levels at an affordable cost.