Recent trends in the frequency and duration of global floods

Nasser Najibi,Naresh Devineni

doi:10.5194/esd-9-757-2018

Nasser Najibi, Naresh Devineni

Open Access

https://doi.org/10.5194/esd-9-757-2018

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Abstract

Abstract. Frequency and duration of floods are analyzed using the global flood database of the Dartmouth Flood Observatory (DFO) to explore evidence of trends during 1985–2015 at global and latitudinal scales. Three classes of flood duration (i.e., short: 1–7, moderate: 8–20, and long: 21 days and above) are also considered for this analysis. The nonparametric Mann–Kendall trend analysis is used to evaluate three hypotheses addressing potential monotonic trends in the frequency of flood, moments of duration, and frequency of specific flood duration types. We also evaluated if trends could be related to large-scale atmospheric teleconnections using a generalized linear model framework. Results show that flood frequency and the tails of the flood duration (long duration) have increased at both the global and the latitudinal scales. In the tropics, floods have increased 4-fold since the 2000s. This increase is 2.5-fold in the north midlatitudes. However, much of the trend in frequency and duration of the floods can be placed within the long-term climate variability context since the Atlantic Multidecadal Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation were the main atmospheric teleconnections explaining this trend. There is no monotonic trend in the frequency of short-duration floods across all the global and latitudinal scales. There is a significant increasing trend in the annual median of flood durations globally and each latitudinal belt, and this trend is not related to these teleconnections. While the DFO data come with a certain level of epistemic uncertainty due to imprecision in the estimation of floods, overall, the analysis provides insights for understanding the frequency and persistence in hydrologic extremes and how they relate to changes in the climate, organization of global and local dynamical systems, and country-scale socioeconomic factors.

Highlights

Higher levels of vulnerabilities to extreme events, especially floods, are becoming a “new normal” in both developing and developed countries (Mirza, 2003; Thomalla et al, 2006)
In order to investigate the association between flood duration and damage at the country scale, we present a linear model for flood damage (Fdamage) as a function of flood duration (FD) in the log space as follows: Fdamage = αFDβ ⇒ log Fdamage = log(α) + β log (FD), (6)
In our analysis of H3, we find that there is no significant trend in the frequency of short-duration events across all latitudinal scales (Table 7), but a significant trend can be seen across the tropics for moderate- and long-duration flood events

Summary

Introduction

Higher levels of vulnerabilities to extreme events, especially floods, are becoming a “new normal” in both developing and developed countries (Mirza, 2003; Thomalla et al, 2006). While flood-related fatalities have substantially decreased in recent decades mainly due to improved early warning systems and better flood control infrastructure, statistics still point out that there are people (in)directly affected by these events. GuhaSapir et al (2016) in their annual disaster statistical review of 2016 reported that the number of people affected by hydrologic disasters (floods or landslides) is 78.1 million, approximately 13.7 % of all people affected in 2016. It is striking to note that 60 million of these 78.1 million people were affected by one flood in China. Other impacts of floods include various deteriorations of social services, economic disruptions, health-related issues, and consequences of population displacement

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Discussion

Conclusion