A modeling methodology to study the tributary-junction alluvial fan connectivity during a debris flow event

Abstract

Abstract. Traditionally, interactions between tributary alluvial fans and the main river have been studied in the field and in the laboratory, giving rise to different conceptual models that explain their role in the sediment cascade. On the other hand, numerical modeling of these complex interactions is still limited because the broad debris flow transport regimes are associated with different sediment transport models. Even though sophisticated models capable of simulating many transport mechanisms simultaneously exist, they are restricted to research purposes due to their high computational cost. In this article, we propose a workflow to model the response of the Crucecita Alta alluvial fan in the Huasco Valley, located in the Atacama Desert, Chile, during an extreme storm event. Five different deposits were identified and associated with four debris flow surges for this alluvial fan. Using a commercial software application, our workflow concatenates these surges into one model. This study depicts the significance of the mechanical classification of debris flows to reproduce how an alluvial fan controls the tributary–river junction connectivity. Once our model is calibrated, we use our workflow to test if a channel is large enough to mitigate the impacts of these flows and the effects on the tributary–river junction connectivity.

Rheology of hail-debris flow and implications in flow mobility

“…After calibration, we obtained values for viscosity and yield stress in the order of ~4 Pa s and ~50 Pa, respectively, and maximum flow velocities in the same order than estimated from video records. Viscosities are in the lower limit compared to the debris flow literature, and probably near one order magnitude below expected values for similar debris mixtures [12]. Calibrated yield stress values are in the range of 30-50 Pa, that is one or two order magnitude lower than reported in the literature.…”

“…As starting point, we used typical rheological parameters obtained by different authors for debris flows mixtures in laboratory tests (see references in [10]), and by calibrating debris flows models using field post-event data [2,11,12]. However, this set of rheological parameters overestimate frictional stress, resulting in lower flow velocities and flooded areas compared to observation.…”

“…Thus, rheological model parameters were calibrated against observed flooded areas following Zegers et al [2] and Garces et al [12] and validated against flow velocities estimations obtained from videos recorded by inhabitants. Results are shown in Figure 3 and Table 2.…”

faDebrisFOAM validation using field data surveyed in Crucecita (Chile) alluvial fan for the event of 13th May 2017

“…Crucecita basin has an area of 13 km 2 , being a lateral tributary to the El Carmen River in the southern limit of the Atacama Desert. This creek has been previously studied due to the debris flows that occurred in 2015 [1,3]. As a result of the previous studies, high-resolution topography was surveyed in the confluence zone with the river, including Crucecita's alluvial fan.…”

“…In the following years, smaller events occurred in more specific parts of Atacama. Garces et al [1] characterized the behavior of Crucecita creek for the debris flow event that occurred in 2015 using the FLO-2D commercial software. The authors concluded that, despite the correct representation of the event in general terms, this numerical model should improve how it resolves the erosion and deposition processes and changes in rheology, to achieve better representations of the sediment transport processes in alluvial fans.…”