Increasing river floods: fiction or reality?

Abstract

There has been a surprisingly large number of major floods in the last years around the world, which suggests that floods may have increased and will continue to increase in the next decades. However, the realism of such changes is still hotly discussed in the literature. This overview article examines whether floods have changed in the past and explores the driving processes of such changes in the atmosphere, the catchments and the river system based on examples from Europe. Methods are reviewed for assessing whether floods may increase in the future. Accounting for feedbacks within the human-water system is important when assessing flood changes over lead times of decades or centuries. It is argued that an integrated flood risk management approach is needed for dealing with future flood risk with a focus on reducing the vulnerability of the societal system.

Large-scale application of the flood damage model RAilway Infrastructure Loss (RAIL)

“…The hypothesis that frequencies of river floods in Europe may increase in the future due to climate change is widely discussed among scientists (Hall et al, 2014;Blöschl et al, 2015). Lehner et al (2006) as well as Dankers and Feyen (2008), for example, concluded that the frequency (and magnitude) of flooding in large parts of Europe is projected to increase in future, i.e.…”

“…Furthermore, future trends of climatic extremes cannot be projected with sufficient reliability, in particular with respect to heavy rainfall, which considerably contributes to raingenerated local flooding in most cases (Hanel and Buishand, 2010;IPCC, 2012). Blöschl et al (2011) presented a study addressing the issue of changing flood frequencies in Austria due to climate change. According to them, the above-mentioned complexities and uncertainties also apply on the national level, which makes a reliable projection of future trends nearly impossible against current knowledge.…”

Regional reconstruction of flash flood history in the Guadarrama range (Central System, Spain)

“…To explain the lack of correlation in the Sierra de Guadarrama mountain range, we believe that the capacity of a catchment to deliver unconsolidated material during events -which is in addition capable of damaging existing trees -is the key, but this element never appears explicitly among the geomorphic parameters used in our discipline. Blöschl et al (2015), Borga et al (2014) and Marchi et al (2009), for instance, emphasized that soil characteristics, such as hydraulic conditions, saturation and infiltration, play an important role in the activation of rapid runoff which can eventually become prevailing transfer processes. The upper part of the PU catchment, where large flash flood activity has been observed, is in fact characterized by extensive deposits of unconsolidated material (i.e., fine-grained rock and soil debris from periglacial activity) occupying steep slopes void of vegetation (Fig.…”

“…Such conditions will, in fact, favor subsuperficial runoff, especially in combination with snowmelt processes, thereby also increasing the possibility for debris to be mobilized (Costa, 1984). Specifically, Blöschl et al (2015), remark that the occurrence of river floods triggered by rainfall and snowmelt are also related with processes occurring at atmospheric and river system (i.e. flood wave propagation and superposition of flood waves) levels.…”

“…The fact that our reconstruction only contains 20% of the years for which rainfall events exceeded the defined 1-, 3-, and 5-day rainfall thresholds may be due to (i) the fact that a flood that is triggered not only depends on rainfall, but also on the temporary state of a catchment and its processes (Blöschl et al, 2015); or (ii) an incomplete reconstruction of past flash flood events and to certain limitations of dendrogeomorphology in providing a complete record of past events (see Stoffel andBollschweiler (2008, 2009);Ruiz-Villanueva et al (2010)). The assumed incompleteness of the record could be related to (i) the generalization of climatic conditions which was assumed for all catchments; (ii) the fact that only total daily rainfall sums were considered, and not precipitation intensity, due to the absence of such records, being well aware that these could have played an important role in flash flood occurrence (Quevauviller, 2014); or to (iii) the fact that the areas located next to the channels have been influenced quite heavily by forest management interventions during the last century, thus compromising the catchments' suitability for more specific dendrogeomorphic studies.…”