Rainfall intensity–duration thresholds for bedload transport initiation in small Alpine watersheds

Abstract

Abstract. Although channel discharge represents one of the primary controls of bedload transport rates in mountain streams, it is rarely measured in small, steep catchments. Thus, it is often impossible to use it as a predictor of hazardous bedload events. In this study, the characteristics of rainfall events leading to bedload transport were investigated in five small Alpine catchments located in different geographical and morphological regions of Switzerland, Italy and France. Using rainfall data at high temporal resolution, a total of 370 rainfall events were identified that led to abundant sediment transport in the different catchments, and corresponding threshold lines were defined using a power law in intensity-duration space. Even though considerable differences in the distribution of the rainfall data were identified between catchments located in various regions, the determined threshold lines show rather similar characteristics.Such threshold lines indicate critical conditions for bedload transport initiation, but rainfall events that do not cause transport activity (so called no-bedload events) can still plot above them. With 0.67 overall in the Erlenbach (Swiss Prealps) and 0.90 for long-duration, low-intensity rainfall, the false alarm rate is considerable. However, for short-duration, high-intensity events, it is substantially smaller (0.33) and comparable to values determined in previous studies on the triggering of Alpine debris flows. Our results support the applicability of a traditional, generalized threshold for prediction or warning purposes during high-intensity rainfall. Such (often convective) rainfall events are unfortunately (i) difficult to measure, even by dense rain gauge networks, and (ii) difficult to accurately predict, both due to their small spatial and temporal scales. Still, for the protection of human life (e.g. along transportation infrastructure such as roads and railway) automated alerts based on power law threshold lines may be useful.

Runoff generation in a pre-alpine catchment: A discussion between a tracer and a shallow groundwater hydrologist

“…The University of Zurich started research in the headwater catchments neighboring the Erlenbach in 2009 to (i) better understand the hydrological processes that result in the very fast runoff response and their spatiotemporal variation, and to (ii) determine the differences in the runoff response in catchments with different fractions of forest, meadows and wetlands. This research complements the existing research in the Erlenbach on snow processes (e.g., Stähli and Gustafsson, 2006;Stähli et al, 2009) and sediment (e.g., Badoux et al, 2012;Roth et al, 2016) and nutrient transport (e.g., Hagedorn et al, 2001;Schleppi et al, 2004;Providoli et al, 2006;Smith et al, 2013) This paper builds on an earlier overview paper by Hegg et al (2006) and reviews the main results of the measurements between 2009-2016 in the catchments neighboring the Erlenbach. In particular, this review summarizes findings of two recent PhD theses (Rinderer, 2015;Fischer, 2016) and six Master theses (Kollegger, 2011;Zehnder, 2013;Herrmann, 2014;Sjöberg, 2015;Grunder, 2017;Sauter, 2017).…”

The role of log jams and exceptional flood events in mobilizing coarse particulate organic matter in a steep headwater stream

“…With a catchment area of 0.7 km 2 and a mean channel slope of 18 %, its elevation ranges from 1100 to 1655 m a.s.l. (Badoux et al, 2012). Discharge and meteorological variables are recorded at 10 min intervals, while sediment export can be monitored with various types of equipment (Badoux Rickenmann et al, 2012;Turowski et al, 2013b).…”

Selection of intense rainfall events based on intensity thresholds and lightning data in Switzerland

“…Storms associated with high rainfall intensities often lead to severe flooding in catchments and urban areas, accelerated hillslope and channel erosion, triggering of landslides, mud and debris flows (e.g. Llasat, 2009;Brunetti et al, 2010;Badoux et al, 2012). The severity of storms and their damage potential is dependent not only on the total rainfall amount but also on other storm characteristics such as peak rainfall intensity and event duration.…”