The impact of exceptional events on erosion, bedload transport and channel stability in a step‐pool channel

Abstract

Sediment transport in the Erlenbach, a small stream with step-pool morphology in the canton of Schwyz, Switzerland, has been monitored for more than 20 years. During this time three exceptional events (events with high sediment yield and long return times that have a large effect on channel morphology) have impacted the stream and partly or completely rearranged the existing step-pool morphology. In the aftermath of the events, sediment transport rates at a given discharge and total sediment yield remained elevated for about a year or longer. For the last event, dated on the 20 June 2007, observations of boulder mobility and step destruction were used to interpret channel stability. Boulders with median diameters of up to 135 cm and estimated weights of more than 2·5 tons have moved during the 2007 event. Using hydraulic observations and shear stress calculations boulders up to 65 cm in diameter were predicted to have been fully mobile in peak conditions, even if form resistance and increased critical stresses needed for the initiation of motion in steep streams were taken into account. For two of the events, estimated peak shear stresses at the bed exceeded 1000 Pa, calculated both from observations of the flow hydraulics and from boulder mobility. This suggests that highly energetic flows occur relatively frequently in small, steep streams and that large boulders can be transported by fluvial processes in such streams. The observations have potential significance for hazard risk mitigation, stream engineering and restoration.

Contributing sources to baseflow in pre‐alpine headwaters using spatial snapshot sampling

“…The annual precipitation is 2300 mm year -1 and is relatively equally distributed but slightly skewed to the summer season (Turowski et al, 2009). On average there is precipitation on almost every second day.…”

The influence of sediment thickness on energy delivery to the bed by bedload impacts

“…To measure the energy delivered by an impact, we used a Swiss plate geophone system sensor unit (Figure 1), which is a sensor system developed for the field monitoring of bedload transport (Rickenmann, Turowski, Fritschi, Klaiber, & Ludwig, 2012;Rickenmann et al, 2014;Turowski, Böckli, Rickenmann, & Beer, 2013;Turowski & Rickenmann, 2011;Turowski, Yager, Badoux, Rickenmann, & Molnar, 2009). The sensor unit contains a geophone fixed in the centre under a steel plate with the dimensions 360 × 500 × 6 mm.…”

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

“…The channel shows a pronounced step-pool morphology with some cascading and riffle reaches and has an average bankfull channel width of 3.7 m (Molnar et al, 2010). Its banks are prone to hillslope creep and actively supply the channel with sediment and organic matter, including wood of a wide range of sizes (Schuerch et al, 2006;Turowski et al, 2009Turowski et al, , 2013a. There have been no signs of debris flow occurrence in the channel, and sediment transport was observed to be fluvial even under the highest discharges (Turowski et al, 2009).…”

“…Its banks are prone to hillslope creep and actively supply the channel with sediment and organic matter, including wood of a wide range of sizes (Schuerch et al, 2006;Turowski et al, 2009Turowski et al, , 2013a. There have been no signs of debris flow occurrence in the channel, and sediment transport was observed to be fluvial even under the highest discharges (Turowski et al, 2009). Around 40 % of the catchment area is covered by multi-level subalpine forest, dominantly Norway spruce (Picea abies) and silver fir (Abies alba), and the remaining 60 % of the area is grass and wetlands (Burch, 1994).…”