Direct measurement of channel erosion by debris flows, Illgraben, Switzerland

Abstract

[1] The timing and magnitude of channel bed erosion by three debris flows was measured in 2008 at the Illgraben catchment, Switzerland, using a scour sensor which consisted of a vertical array of erodible sensor elements. During the largest debris flow, sediment was entrained progressively and stepwise at the flow head within 20 s after front arrival, and onset of erosion started before maximum values for flow height and normal and shear stress, measured nearby, were reached. Erosion in one of the two smaller debris flows also occurred at the head of the flow, but the magnitude of erosion was at the detection limit for the sensor. For the other small debris flow, we were not able to determine the timing of erosion due to the presence of a sediment layer covering the sensors. Measurements of pressure fluctuations along the channel sidewall, which are produced by interparticle collisions within the flow, indicate that the entrainment of sediment is coincident with the largest mean and fluctuating pressures, suggesting that interparticle collisions may drive the erosion process at the front of debris flows. After erosion at the head of the debris flows, sediment was deposited on top of the erosion sensor columns, indicating that the bed was reworked to a larger depth than directly visible at the surface after the event.Observations from elsewhere along the channel support our measurements of the magnitude of net debris flow erosion and indicate that significant erosion can be expected on debris fans when the flow is confined to a channel. Debris flow entrainment and subsequent bulking of the flow influence the flow dynamics and therefore should be considered in debris flow models and hazard assessment.

Sediment budget monitoring of debris-flow and bedload transport in the Manival Torrent, SE France

“…Without such information, it is not possible to unambiguously attribute scouring and filling phases to specific flow conditions. However, other field (Berger et al, 2011b) and experimental evidences (Mangeney et al, 2010) identify that the maximum scouring is related to the passage of the debris-flow front.…”

The importance of entrainment and bulking on debris flow runout modeling: examples from the Swiss Alps

“…Because of its high degree of debris-flow activity, this catchment became one of the most studied debris flow sites in the Alps. During the last decade, studies focused on various aspects of the debris flow process addressing the flow process itself (Hürli-mann et al, 2003;McArdell et al, 2007), developing a reliable warning system for debris flows based on flow detection (Badoux et al, 2009), studying sediment transfer (Berger et al, 2010b;Bennet et al, 2012Bennet et al, , 2013 and the erosion process (Berger et al, 2010a(Berger et al, , b, 2011Schürch et al, 2011a, b;Bennet et al, 2014). Today, the channel is equipped with several measurement installations which allow estimation of typical flow parameters such as front velocity and flow depth.…”

“…They concluded that the fragmentation of material from a matrix (gravel bed or bedrock) depends on the exposure of the particle to the basal forces acting on the matrix, and furthermore that the removal of sediment increases with increasing water content. To assess the timing and the absolute erosion depth caused by natural debris flows at the Illgraben channel, Berger et al (2010aBerger et al ( , 2011 installed a novel channel bed erosion sensor based on the concept of an electrical resistance chain. McCoy et al (2013) recently studied basal forces generated by erosive debris flows in Chalk Cliff catchment, Denver, Colorado USA, using a similar type of instrumentation consisting of erosion sensors similar to those in the Illgraben as well as erosion bolts in bedrock and a small force plate.…”

“…Today, the channel is equipped with several measurement installations which allow estimation of typical flow parameters such as front velocity and flow depth. Other debris flow properties are measured using force plates on the channel bed and on a lateral wall to determine basal and lateral stresses as well as bulk density (McArdell et al, 2007;Berger et al, 2011;Schürch et al, 2011b).…”

“…Physically based numerical models were developed to investigate runout distance and inundation patterns as well as flow heights and flow velocities (Crosta et al, 2003;D'Ambrosio et al, 2003;Medina et al, 2008;Hungr and McDougall, 2009;Christen et al, 2012). Only recently have researchers focused on better understanding the process by which debris flows entrain sediment from the bed of a torrent channel as part of the erosion process (e.g., Hungr et al, 2005;Mangeney et al, 2007;Berger et al, 2011;McCoy et al, 2010McCoy et al, , 2012McCoy et al, , 2013.…”