Flash-flooding of Ephemeral Streams in the Context of Climate Change

Ana M Camarasa-Belmonte

doi:10.18172/cig.4838

Abstract

Ephemeral streams, which are more extended than expected, entail a significant flood risk. Historically they have been underestimated due to their intermittent flow and the lack of knowledge on their hydro-geomorphology. Currently, European legislation recognizes their associated risk and supports research into them, adapting the scale and methodology to their characteristics. Based on the compilation of various works carried out in four Valencian catchments (Eastern Spain), this paper approaches the key questions of rainfall-runoff conversion and flood generation in ephemeral streams, taking into account their hydro-geomorphological specificity. Moreover, the consequences which derive from current environmental changes are addressed in the wider scale of Júcar River Water Authority.The study is based on 5-minute data, registered by the SAIH-Júcar network (Authomatic Hydrological Information System). The investigation has been conducted in two phases. Firstly, key issues determining flash-flood generation at basin scale have been addressed, based on the study of 138 floods, registered between 1989 and 2018, in four Valencian ephemeral streams (Barranc del Carraixet, Rambla de Poyo, Riu Vernissa and Rambla de Gallinera). Secondly, concerning a broader scale (Júcar River Water Authority), the evolution of 698 rain episodes (1989-2007) has been analysed. Finally, the consequences that environmental changes (climatic, anthropogenic and morphogenetic) might mean for flash-flood generation have been discussed.The results show how environmental changes point towards an increase in risk to the detriment of resource. Rain episodes tend to increase in intensity and decrease accumulated precipitation. As a consequence, hydrological connectivity will become more dependent on rain intensity, thus reducing runoff thresholds and basin response times. Anthropic changes enhance this behaviour, reducing infiltration and increasing surface runoff and erosion, while accelerating the hydrological cycle. An increase in process-form disequilibrium in Mediterranean catchments can be expected due to the increase in morphogenetic phases (because of the intensification of events) and a decrease in the efficiency of low-magnitude recovery episodes.Consequently, the behaviour of ephemeral-streams under current climate change conditions points firstly to an increase in intense flash-flood events, which will be difficult to manage with the current flood control measures, and secondly an increase in the general aridity conditions of catchments.

Highlights

Ephemeral streams are more frequent than assumed since, globally, more than 50% of the river network is intermittent (Skoulikidis et al, 2017)
The results show how environmental changes point towards an increase in risk to the detriment of resource
The present study addresses both phases: that of flash flood generation, and its trend under the current context of climate change in a wide area of the Spanish Mediterranean coast

Summary

Introduction

Ephemeral streams are more frequent than assumed since, globally, more than 50% of the river network is intermittent (Skoulikidis et al, 2017). Gómez et al (2005) estimate that more than 70% of fluvial systems are ephemeral streams in the south-east of Spain. An increase in this percentage is to be expected over the century, because as a consequence of climate change, drylands are predicted to expand approximately by 10% before the 22nd century (Feng and Fu, 2013). In the Mediterranean region, ephemeral streams exhibit certain climatic, geomorphological and anthropic characteristics which condition the hydro-geomorphological processes of runoff generation. Seasonal rainfall regime is very irregular as it is subject to severe summer droughts and autumn episodes of high intensity (Camarasa-Belmonte and Soriano, 2014).

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