Field measurement of basal forces generated by erosive debris flows

Abstract

[1] It has been proposed that debris flows cut bedrock valleys in steeplands worldwide, but field measurements needed to constrain mechanistic models of this process remain sparse due to the difficulty of instrumenting natural flows. Here we present and analyze measurements made using an automated sensor network, erosion bolts, and a 15.24 cm by 15.24 cm force plate installed in the bedrock channel floor of a steep catchment. These measurements allow us to quantify the distribution of basal forces from natural debris-flow events that incised bedrock. Over the 4 year monitoring period, 11 debris-flow events scoured the bedrock channel floor. No clear water flows were observed. Measurements of erosion bolts at the beginning and end of the study indicated that the bedrock channel floor was lowered by 36 to 64 mm. The basal force during these erosive debris-flow events had a large-magnitude (up to 21 kN, which was approximately 50 times larger than the concurrent time-averaged mean force), high-frequency (greater than 1 Hz) fluctuating component. We interpret these fluctuations as flow particles impacting the bed. The resulting variability in force magnitude increased linearly with the time-averaged mean basal force. Probability density functions of basal normal forces were consistent with a generalized Pareto distribution, rather than the exponential distribution that is commonly found in experimental and simulated monodispersed granular flows and which has a lower probability of large forces. When the bed sediment thickness covering the force plate was greater than 20 times the median bed sediment grain size, no significant fluctuations about the time-averaged mean force were measured, indicating that a thin layer of sediment ( 5 cm in the monitored cases) can effectively shield the subjacent bed from erosive impacts. Coarse-grained granular surges and water-rich, intersurge flow had very similar basal force distributions despite differences in appearance and bulk-flow density. These results demonstrate that debris flows can have strong control on rates of steepland evolution and contribute to a foundation needed for modeling debris-flow incision stochastically.

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

“…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. They concluded that debris flows in the field show much broader distributions of basal force than in the laboratory, which they attributed to wider grain size distributions observed in the field.…”

“…They concluded that debris flows in the field show much broader distributions of basal force than in the laboratory, which they attributed to wider grain size distributions observed in the field. Laboratory results are, in any case, difficult to apply for investigating debris-flow erosion due to problems arising when scaling small-scale laboratory results to the field (e.g., Iverson and Denlinger, 2001;Rickenmann et al, 2003;Reid et al, 2011;McCoy et al, 2013). Therefore we focus on a field-data-based entrainment approach in our study.…”

“…Noticeable differences in modeled runout patterns are expected to emerge for larger flow heights, larger volumes, steeper (and less distinctive) channel slopes and larger bulk densities (e.g., characteristic of granular debris flows). By constraining the rate of erosion by field observations of erosion rates, we account for the suggestion that natural debris flows show rather different erosion behavior than down-scaled laboratory experiments (Iverson and Denlinger, 2001;Rickenmann et al, 2003;Reid et al, 2011;McCoy et al 2013). The empirical approach using potential erosion depths as well as erosion rates observed in the field offers the opportunity to estimate the expected amount of debris-flow entrainment and its effects on hydrograph propagation and runout.…”

“…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.…”

Application of Sensitivity Analysis for Process Model Calibration of Natural Hazards

“…Firstly, the results support previous findings (e.g., [102]) that the main limitation of the FLO-2D model is predicated on the use of simplified Bingham equations to represent complex debris flow physics. In doing so, the granular flow of heterogeneous materials such as debris and rock fragments within an interstitial mud [103] is reduced to a single-phased, homogenous flow. While the model may effectively support other types of investigations, it is not suitable for back calculating past hazardous events, where highly accurate simulated intensity outputs are required for further analysis.…”