Empirical atmospheric thresholds for debris flows and flash floods in the southern French Alps

Abstract

Abstract. Debris flows and flash floods are often preceded by intense, convective rainfall. The establishment of reliable rainfall thresholds is an important component for quantitative hazard and risk assessment, and for the development of an early warning system. Traditional empirical thresholds based on peak intensity, duration and antecedent rainfall can be difficult to verify due to the localized character of the rainfall and the absence of weather radar or sufficiently dense rain gauge networks in mountainous regions. However, convective rainfall can be strongly linked to regional atmospheric patterns and profiles. There is potential to employ this in empirical threshold analysis.This work develops a methodology to determine robust thresholds for flash floods and debris flows utilizing regional atmospheric conditions derived from ECMWF ERAInterim reanalysis data, comparing the results with raingauge-derived thresholds. The method includes selecting the appropriate atmospheric indicators, categorizing the potential thresholds, determining and testing the thresholds. The method is tested in the Ubaye Valley in the southern French Alps (548 km 2 ), which is known to have localized convection triggered debris flows and flash floods. This paper shows that instability of the atmosphere and specific humidity at 700 hPa are the most important atmospheric indicators for debris flows and flash floods in the study area. Furthermore, this paper demonstrates that atmospheric reanalysis data are an important asset, and could replace rainfall measurements in empirical exceedance thresholds for debris flows and flash floods.

Assessing debris flow activity in a changing climate

“…The differences may be due to the proxies being associated with different atmospheric conditions. The QCAPE-proxy is representative of short-lasting convective rainfall systems existing in a mesoscale humid environment triggering debris flows (Turkington et al 2014). The rain-proxy includes all synoptic conditions producing sufficient rainfall to trigger a debris flow as it is based on observational records.…”

“…As rain gauges are very local and often non-catchment representative, the QCAPE-proxy was used to mimic the mesoscale atmospheric conditions leading to heavy local convective precipitation. Turkington et al (2014) showed that a QCAPE-based proxy could outperform daily precipitation and multiple day precipitation amounts as a proxy for debris flows in the Barcelonnette Basin. QCAPEproxy was derived from ERA-Interim reanalysis dataset, for the period 1979-2011 (Dee et al 2011).…”

“…However, while debris flows are often preceded by intense rainfall, capturing the rainfall in mountain catchments can be difficult due to the lack of rain gauges or poor coverage by weather radar in mountainous regions (Nikolopoulos et al 2014;Pavlova et al 2014). Therefore, other meteorological proxies for debris flow occurrence can account for deficiencies in precipitation records, either without (Turkington et al 2014) or alongside rainfall (Paranunzio et al 2015;Rulli et al 2007). Temperature has been used where snow and thaw can contribute to debris flow occurrence (e.g.…”

“…van den Heuvel et al (2016)). Previous work demonstrated that atmospheric variables, such as specific humidity (Q) combined with atmospheric instability through Convective Available Potential Energy (CAPE), can be used as proxies for debris flow occurrence, especially for those triggered by intense convective rainfall unrecorded by a rain gauge (Turkington et al 2014).…”

Debris flows in the eastern Italian Alps: seasonality and atmospheric circulation patterns

“…Furthermore, Toreti et al (2013) showed that debris-flow occurrence in the southern Swiss Alps, exhibits a distinct pattern in large-scale atmospheric circulation and suggested that this information can be used to improve existing warning systems. On this line, Turkington et al (2014), in a study centered on the southern French Alps, showed that empirical thresholds can be directly identified based on regional atmospheric patterns. Second, linking debris-flow occurrence with seasonal and meteorological characteristics may provide indications on the potential impact of climate change on debris-flow activity .…”