Now showing items 21-40 of 139

    • Taking the pulse of debris-flows: extracting debris-flow dynamics from good vibrations in southern California and central Colorado

      Michel, Abigail; Kean, Jason W.; Smith, Joel B.; Allstadt, Kate E.; Coe, Jeffrey A. (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      The destructive nature of debris flows makes it difficult to quantify flow dynamics with direct instrumentation. For this reason, seismic sensors placed safely away from the flow path are often used to identify the timing and speed of debris flows. While seismic sensors have proven to be a valuable tool for event detection and early warning, their potential for identifying other aspects of debris flows (such as sediment concentration) is less studied. Here, we use two monitoring sites to investigate the extent to which debris-flow dynamics can be decoded from ground vibrations. One site is a bedrock channel in a steep semiarid basin in central Colorado (Chalk Cliffs), and the other is in a debris-flow channel incised in alluvium in a recently burned area in southern California (Van Tassel). At both sites, seismic data are measured with geophones (4.5 Hz) mounted next to the channels and sampled at high frequencies (500-1000 Hz). Independent constraints on flow dynamics are provided by laser distance meters to record flow stage (at 10 Hz) and high-definition video cameras to record flow velocity and qualitative estimates of sediment concentration. The observed debris flows at Chalk Cliffs typically consist of a series of short-duration (~30 second) surges with total durations of <40 minutes and have coarse-grained fronts and fluid-rich tails. In contrast, the events at Van Tassel are longer duration flows (>40 minutes) that begin as debris flows and transform into more steady debris floods. The arrangement of sensors at both sites allows us to identify correlations between vertical ground velocity, frequency, flow stage, and qualitative estimates of sediment concentration.
    • Observations on the development and decay processes of debris-flows

      Murasawa, Masaki; Imaizumi, Fumitoshi; Yokota, Yushi (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      It is important to understand the development and decay processes of debris-flows in order to plan effective debris-flow countermeasures. However, few studies have successfully observed the development and decay processes of debris-flows. This study aimed to reveal changes in characteristics of debris-flow surges as they flow down, based on observation using time lapse cameras installed at multiple sites along a debris-flow torrent in the upper Ichinosawa catchment within the Ohya landslide, central Japan. Observation results showed that debris-flow surge volume and flow velocity tended to increase in the section just below their initiation point. In the subsequent section, debris-flow surges tended to maintain their volume and flow velocity while descending. Increases in flow velocity were observed in sections with a fixed bed, the channel bed consists of exposed bedrock with no sediment cover. Debris-flow surge volume and velocity tended to decrease in these sections, in which channel gradient decreases abruptly. These observation results can be explained by the theory of equilibrium concentration, which states that sediment concentration in the flow approaches the equilibrium concentration given from the channel gradient by the erosion and deposition of sediment. At the same time, small debris-flow surges tended to terminate with a short travel distance, which cannot be explained fully by the theory of equilibrium concentration.
    • Monitoring of sediment runoff and observation basin for sediment movements focused on active sediment control in Jo-Gan-Ji River

      Nagayama, Takahiko; Furuya, Tomohiko; Matsuda, Satoru; Itoh, Takahiro; Fujita, Masaharu; Mizuyama, Takahisa (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Continuously measuring sediment runoff along multiple sections of the Jo-Gan-Ji River is necessary to understand both the propagation of sediment as well as the changing of grain sizes in order to appropriately evaluate sediment yielding from debris flows temporally and spatially. The present study proposes a combination of sediment monitoring tools and appropriate equipment to identify various sediment transport modes from wash load to bedload in mountainous torrents. As a result of monitoring runoff volume and grain sizes, sediment management can be achieved. In the Jo-Gan-Ji River basin in Japan, temporal and longitudinal sediment runoff has been measured continuously since the 1990's. Previous studies help determine the proper instrumentation suite for this type of sediment runoff monitoring. Bedload is measured with a Reid-type bedload slot sampler and by use of the hydrophone to survey acoustic waves. In addition, hydrophones and a velocity meter (vertically installed on a side wall) are used to quantify suspended loads. A turbidity meter is also used to measure wash load. Propagation of sediment particles can be observed during flooding in mountainous torrents. Specifically, bedload discharge rates of each particle are evaluated using of the hydrophone. Monitoring of the Jo-Gan-Ji river also identifies inactive bedload movements such as large boulders. Previous installations of this type monitoring equipment make it clear that the destructive nature of bedload collisions indicate a need for robust instruments. Alternate instrumentation methods, that are robust, are explored here. Moreover, in order to actively control sediment runoff in flooding, we developed a sabo dam with shutter and pilot operations that activate during flooding. Differences of those sediment transport characteristics with/without the shutter also shown through the sediment monitoring along the Jo-Gan-Ji River.
    • Measurements of velocity profiles in natural debris flows: a view behind the muddy curtain

      Nagl, Georg; Huebl, Johannes; Kaitna, Roland (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      The internal deformation behavior of natural debris flows is of interest for model development and model testing for debris-flow hazard mitigation. Up to now, only a view attempts were made to measure velocity profiles in natural debris flows due to low predictability and high destructive power of these flows. In this contribution we present recent advances of measuring in-situ velocity profiles together with flow parameters like flow depth, basal normal stress, and pore fluid pressure. For that a fin-shaped monitoring barrier was constructed in the Gadria creek (IT), laterally carrying an array of paired conductivity sensors. We present results from two debris-flow events with volumes of around 5,000 m³ each. Compared to the first event on July 10th, 2017, the second event on August 19th, 2017, was visually more liquid. Both debris flows exhibited significant longitudinal changes of flow properties like flow depth and density. The liquefaction ratios reached values up to unity in some sections of the flows. Velocity profiles for the July event were mostly concave up, while the profiles for the more liquid event in August were linear to convex. Though limited by boundary roughness at the wall and occasional sediment deposition on the force plates and pressure sensors, these measurements gain new insights of the dynamics of real-scale debris flows.
    • Debris-flow early warning system at regional scale using weather radar and susceptibility mapping

      Palau, Rosa M.; Hürlimann, Marcel; Berenguer, Marc; Sempere-Torres, Daniel (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Risk mitigation for debris flows at regional scale is a challenge. Early warning systems are helpful in depicting the time and the location of future debris flows so that emergency responders can act in advance before the disaster takes place. Herein we present a prototype real-time regional early warning system for rainfall induced shallow landslides and debris flows for the region of Catalonia (northeastern Spain). The model issues a warning level combining susceptibility information and real-time rainfall triggering conditions obtained from weather radar observations and forecasts. Susceptibility maps have been derived using a fuzzy-logic approach and two input variables, terrain slope and land cover. These maps have been obtained using (i) grid cells of different resolutions, and (ii) physical catchments (of first order) as terrain units. Although high resolution grid-cell maps show a more accurate representation of susceptibility over the region, maps based on catchments are more intuitive and better characterize the area affected by future debris flows. Rainfall triggering conditions are assessed by means of probabilistic intensity-duration thresholds obtained from literature. Finally, we have validated the early warning system and tested its performance for some important events from the last ten years that were either monitored in specific catchments, or were reported in unmonitored catchments. In general, the system has been able to satisfactorily forecast the time of occurrence of most of the analyzed past debris-flow events.
    • Real-time monitoring of debris-flow velocity and mass deformation from field experiments with high sample rate lidar and video

      Rengers, Francis K.; Rapstine, Thomas D.; Allstadt, Kate E.; Olsen, Michael; Bunn, Michael; Iverson, Richard M.; Kean, Jason W.; Leshchinsky, Ben; Logan, Matthew; Sharifi-Mood, Mahyar; et al. (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Debris flows evolve in both time and space in complex ways, commonly starting as coherent failures but then quickly developing structures such as roll waves and surges. These processes are readily observed, but difficult to study or quantify because of the speed at which they occur. Many methods for studying debris flows consist of point measurements (e.g., of flow height or basal stresses), which are inherently limited in spatial coverage and fail to fully capture the spatiotemporal evolution of a flow. In this study, we use terrestrial lidar to measure debris-flow profiles at high sample rates to examine debris-flow movement with high temporal and spatial precision and accuracy. We acquired measurements during gate-release experiments at the USGS debris-flow flume, a unique experimental facility where debris flows can be simulated at a large scale. In this study, the laser scanner was placed at the bottom of the steep flume and recorded topography of the entire flume bed and debris flow at a rate of 60 Hz along a narrow profile (~1mm in width), providing a detailed, two-dimensional cross-section of the debris flow through time. The high- resolution profiles enabled us to quantify flow front and surge velocities of the debris flow and provide an unprecedented record of the development and evolution of the flow structure over time. The profiles also preserve a record of the highly variable deposition pattern of the debris flow on a downstream fan with a time resolution of hundredths of a second. In addition, video imagery from the experiment was used to track debris-flow movement through time. By acquiring high-resolution topographic data and video imagery during a controlled experiment, we have been able to obtain unusually complete quantitative measurements of debris-flow movement. Such measurements may help constrain future modeling efforts.
    • Exploring controls on debris-flow surge velocity and peak discharge at Chalk Cliffs, Colorado, USA

      Smith, Joel B.; Kean, Jason W.; Coe, Jeffrey A. (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      We present a series of debris-flow events and use combined sensor and video data to explore how sediment concentration and triggering rainfall intensity affect the velocity and discharge of debris-flow surges generated by surface-water runoff. We analyze an initial data set of 49 surges from four debris-flow events recorded by a monitoring system at Chalk Cliffs, Colorado, and compare measurements of surge height, velocity, peak discharge, triggering rainfall intensity, and qualitative estimates of sediment concentration. Measurements of sediment concentration and velocity were obtained using an automated camera system with a high resolution and frame rate. We find that the triggering rainfall intensity of the debris flows, which affects the sediment-to-water ratio, is a strong control on surge velocity and peak discharge. While surges with high and low sediment concentrations both exceed the peak discharge of water-only flow, fluid-rich surges generated by high rainfall intensities have much greater velocities and peak discharges than sediment-rich surges generated by lower rainfall intensities. These observations suggest that rainfall intensity may be an important predictive variable in empirical relationships for estimating the velocity and peak discharge of runoff-generated debris flows, which are common in alpine areas and burned steeplands.
    • Dynamic characteristics of extreme superelevation of debris flows observed by laser profile scanners in Sakura-jima volcano, Japan

      Takahashi, Yuya; Fujimura, Naoki; Akita, Hiromi; Mizuno, Masaki (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Supercritical flows in natural open channel show very complicated behavior due to shockwaves, roll waves, and superelevations. Traces of debris flows often show large difference in height at inner and outer sides of curved channels, and not only that, superelevation of debris flow is larger than that of pure water. This behavior of debris flow may cause underestimation of channel section design or unexpected bank erosion. Therefore, accumulation of observation data is important to estimate hydraulic characteristics of debris flows at curved channels. We conducted continuous field observations at Sakura-jima volcano, Japan, introducing laser profile scanners (LPSs) to acquire surface geometry of debris flows, especially superelevation at curved channels. The LPS can measure distance from sensor to obstacles with sufficiently high resolution to document superelevations and spillway flow profiles both in space and time on a survey line. A debris flow that occurred at Arimura River, Sakura–jima volcano on January 17, 2018 is well documented by LPSs. Data from the debris flow compared with coefficient in current model, which indicates traction/viscosity and potential superelevation of the debris–flow front is under-predicted assuming the flow is super-critical pure water flow.
    • Monitoring and early warning of debris-flow in an earthquake impacted area, Baishahe catchment, southwest China

      Tian, Hongling; Yang, Zongji; Qiao, Jianping; Shi, Lili (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      After 10 years of the Wenchuan earthquake of China, the post-seismic landslides are turning weak. However, the debris flows in earthquake-stricken area continue to be threat. In order to reduce the risk from debris flows in this region, we discuss the operation of an alert system, monitoring objectives, and early-warning policies. Three gullies in the Baishahe catchment, Dujiangyan, Southwest China were selected and rain-gauge, tilt-sensor, mud-meter and ground acoustic meter combined to a warning system in April, 2014. The data collected from these events shows changes in susceptibility levels and triggering rainfall complicating the application of a fixed rainfall threshold for any alert system.
    • Deciphering debris-flow seismograms at Illgraben, Switzerland

      Wenner, Michaela; Walter, Fabian; McArdell, Brian; Farinotti, Daniel (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Mass wasting, such as rockfalls, landslides and debris flows in steep mountain terrain, has a high destructive potential, and plays a key role in both erosion and landscape evolution. As an alternative to many conventional approaches, seismology allows monitoring of such mass movements at safe distances, provides estimates of event location and timing, and can give insights into dynamics and rheology granular flows. Here, we analyze seismic data recorded during the 2017 and 2018 debris-flow seasons at Illgraben, a steep canyon located in Switzerland. Yearly precipitation is controlled by summer rainstorms with high rainfall intensity during which mass wasting including rock-slope failure and debris flows occur regularly. The frequent debris-flow occurrence (on average three events per year) makes the Illgraben an ideal site for cross-validating a seismically-derived event catalog of mass movements with “ground-truth data”, such as digital terrain models, flow depths estimates and other in-torrent measurements. We present seismic frequency characteristics of the Illgraben debris-flow series and investigate how the seismic signature depends on actual debris-flow characteristics, such as grain sizes, and on propagation effects of the generated seismic waves. Whereas these two effects are usually difficult to separate, the source component contains valuable information on the flow’s material composition. Stations that are close to the torrent, we find that dominant frequencies in the recorded signal reflect the distance to the dominant source. For one particular station, this is shown on recordings of several events, where a dominant frequency of about 5.5 Hz indicates the passing of the flow at a 48m check dam. Power spectral densities at that instance give an estimate of the particle content of the debris flow. We also find that a jump in dominant frequency does not necessarily reflect the location of the flow front. Seismic studies of debris-flow dynamics and material composition should therefore not be limited to entire debris-flow seismograms, but instead focus on individual time windows and consider different sensors separately. The presented analysis underlines the use of seismic data in torrent and landscape studies.
    • Reproducibility of debris-flow fan physical modeling experiments

      Adams, Kailey; Wasklewicz, Thad; de Haas, Tjalling; Lecce, Scott; Gares, Paul (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Debris-flow fans are common in steep mountainous terrain. The complexity of these environments makes it difficult to be onsite at the time of the debris flow, which has led many researchers to turn to physical models. While physical models have played an integral role in unraveling how alluvial fans have been developed, little work has been done to test and validate the approaches and results reported in physical models. Here, we replicated a debris-flow physical model developed at Utrecht University in a laboratory at East Carolina University (ECU). The ECU physical modeling experiment was done in an independent laboratory from the original study and was conducted with different equipment and different operators. Sediment size and water volume varied slightly from the original experiments. Fifty-six debris flows were released to form the ECU fan. Each debris flow was recorded with video. Terrestrial laser scanning recorded the topographic changes on the ECU fan throughout the experiment. Despite sediment size and water volume differences, ECU’s physical model replicated the autogenic processes promoting flow avulsion patterns in a debris-flow fan with simulated unlimited accommodation space (the trap door in feeder channel and table edge did confine the maximum extent the fan could grow). These results corroborate previous findings from the Utrecht fan and support the repeatability of processes from the different models despite differences in the materials used as the debris-flow medium. Reproducibility also permits future collaborative efforts to run multiple concurrent physical modeling experiments to increase the sample size of the study populations on debris-flow processes and resulting forms.
    • Influence of momentum correction factor and friction factor on flow models of debris-flow related to flow surface deformation

      Arai, Muneyuki (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Intermittent debris-flow surges may be considered as roll wave phenomenon of shallow water flow that naturally develop in the mountain basins. The shape of the water surface with discontinuous depth change is one manifestation of flow wave phenomena. Here we obtained a wave equation representing the wave phenomenon of shallow water flow by using the Laplace equation, the bottom condition, a surface conservation condition, a flow surface fluctuation condition, and the perturbative expansion method. The derived equation is a kind of KdV-Burgers equation in which a nonlinear term includes a momentum correction factor b, a dissipation term includes a friction factor f’ and a third term does not include either dispersion terms. The derived equation offers an explanation of the discontinuity of the water surface in shallow water flow that may be useful for debris flow modeling.
    • Constraining parameter uncertainty in modeling debris-flow initiation during the September 2013 Colorado Front Range storm

      Baum, Rex L.; Scheevel, Caroline R.; Jones, Eric S. (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      The occurrence of debris flows during the September 2013 northern Colorado floods took the emergency management community by surprise. The September 2013 debris flows in the Colorado Front Range initiated from shallow landslides in colluvium. Most occurred on south- and east-facing slopes on the walls of steep canyons in crystalline rocks and on sedimentary hogbacks. Previous studies showed that most debris flows occurred in areas of high storm-total rainfall and that strength added by tree roots accounts for the low number of landslides in densely forested areas. Given the lack of rainfall thresholds for debris flow occurrence in northern Colorado, we want to parameterize a numerical model to assess potential for debris flows in advance of heavy rainfall. Natural Resources Conservation Service (NRCS) soil mapping of the area, supplemented by laboratory testing and field measurements, indicates that soil textures and hydraulic properties of landslide source materials vary considerably over the study area. As a step toward modeling storm response, available soil and geologic mapping have been interpreted to define zones of relatively homogeneous properties. A new, simplified modeling approach for evaluating model input parameters in the context of slope and depth of observed debris flow source areas and recorded debris-flow inducing rainfall helps narrow the range of possible parameters to those most likely to produce model results consistent with observed debris flow initiation. Initial results have narrowed the strength parameters to about one third of possible combinations of cohesion and internal friction angle and narrowed hydraulic conductivity to a range spanning slightly more than one order of magnitude.
    • Evaluation of debris-flow runout model accuracy and complexity in Montecito, California, An: towards a framework for regional inundation-hazard forecasting

      Bessette-Kirton, Erin K.; Kean, Jason W.; Coe, Jeffrey A.; Rengers, Francis K.; Staley, Dennis M. (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Numerous debris-flow inundation models have been applied retroactively to noteworthy events around the world. While such studies can be useful in identifying controlling factors, calibrating model parameters, and assessing future hazards in specific study areas, model parameters tailored to individual events can be difficult to apply regionally. The advancement of debris-flow modeling applications from post-event model validation of individual case studies to pre-event forecasting that can be implemented rapidly and at regional scales is critical considering the fatalities and extensive infrastructure damage caused by debris flows that inundated a developed fan in Montecito, California, following heavy rain on 9 January 2018. In this study, we evaluated the tradeoffs between model accuracy and simplicity in the context of the need for a framework that can be used in conjunction with initiation models and storm predictions for rapid, large-scale inundation hazard mapping as a component of post-fire debris-flow hazard assessments. We used numerical (FLO-2D) and empirical (LAHARZ) models to simulate debris flows from one of the drainages upstream of Montecito that was burned in the Thomas fire in December 2017 and compared model results with field observations and building damage assessments collected immediately following the event. Initial testing demonstrated that LAHARZ can simulate channelized flow but is not able to replicate flow bifurcations or avulsions, which are critical aspects of flows travelling over populated fans. FLO-2D simulations matched well with observed inundation area data, but variably under- and overpredicted inundation height, deposit depth, and velocity. We found that FLO-2D and LAHARZ had true positive rates of 0.84 and 0.6, respectively, and both models had similar false positive rates (0.3 and 0.35, respectively). Our model evaluation framework allowed us to compare model results with detailed field observations and will serve as a platform for more extensive model testing in the future.
    • Discrete-element investigation of granular debris-flow runup against slit structures

      Du, Junhan; Zhou, Gordon G. D. (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Runup of granular debris flows against slit dams on slopes is a complex process that involves deceleration, deposition and discharge. It is imperative to understand the runup mechanism and to predict the maximum runup height for the engineering designs and hazards mitigation. However, the interaction between granular flows and slit dams, which affects the runup height significantly, is still not well understood. In this study, a numerical investigation of granular debris flow impacting slit dams by the discrete element method (DEM) was then conducted. The influence of the opening size of slit dams characterizing by the relative post spacing R=b/d (b: post spacing; d: particle diameter) on runup height was studied. Numerical study illustrates that there is a critical value of relative post spacing (RC): within the critical value, the maximum runup height is insensitive to the relative post spacing; once b/d exceeds the critical value, the maximum runup height decreases rapidly as the relative post spacing increases.
    • Method for predicting debris-flow occurrence based on a rainfall and sediment runoff model, A

      Fujita, Masaharu; Yamanoi, Kazuki; Suzuki, Gohta (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Based on a basin scale rainfall runoff model, we proposed a prediction method of debris-flow occurrence on steep mountain slopes related to hydrological processes such as the rainfall infiltration, the surface flow and the slope stability. For example, in one case that the soil layer is unsaturated and a landslide does not occur in the slope even though the groundwater level rises in the slope soil layer during a rainfall event, it is unlikely for a debris flow to occur on the slope. However, if the soil layer is more unstable due to fully saturation and a surface flow also takes place on the slope, the possibility of debris-flow occurrence gets much higher. According to such a consideration, the slope conditions on hydrological processes during heavy rainfalls were classified into six patterns. For these patterns, the possibility of debris-flow occurrence was investigated qualitatively. Then, SiMHiS (Storm Induced Multi-Hazards Information Simulator) by Yamanoi and Fujita was employed as a rainfall runoff model. A slope stability model has been already installed in SiMHiS. Therefore, this model can simulate the time variations of the safety factors for landslides as well as the saturation degrees and the hydrographs of the surface flow for the slopes. SiMHiS was applied to the sediment disasters due to a heavy rainfall in July 2017 in the Akatani river basin to examine the occurrence patterns of debris flow. Also, the differences in the occurrence patterns were shown for other two rainfall events. Using the simulation result on the safety factor, the saturation degree and the surface flow discharge, it was noted whether debris flows took place or not, and the debris-flow occurrence patterns on the slopes in the basin could be identified.
    • Seamless numerical simulation of a hazard cascade in which a landslide triggers a dam-breach flood and consequent debris flow

      George, David L.; Iverson, Richard M.; Cannon, Charles M. (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Numerical simulations of hazard cascades downstream from moraine-dammed lakes commonly must specify linkages between models of discrete processes such as wave overtopping, dam breaching, erosion, and downstream floods or debris flows. Such linkages can be rather arbitrary and can detract from the ability to accurately conserve mass and momentum during complex sequences of events. Here we describe an alternative methodology in which we use high-resolution lidar topography and 2-D, two-phase conservation laws to seamlessly simulate all stages of a hazard-cascade that culminates in a debris flow. Our simulations employ our depth-integrated numerical model D- Claw to evaluate hazards from prospective breaching of a moraine dam that impounds Carver Lake on the eastern flank of South Sister volcano in central Oregon, USA. We simulate a “worst-case scenario” sequence of events that begins with a hypothetical 1.6 million m3 landslide that originates near the summit of South Sister and enters Carver Lake. Wave generation and displacement of lake water then leads to dam overtopping, breach erosion, and a downstream debris flow that funnels into Whychus Creek and eventually reaches the community of Sisters, Oregon, about 20 km away. Notably, our simulations predict that much of the debris is directed away from Sisters as a result of natural avulsion and flow diversion that occurs near the head of a low-gradient alluvial fan upstream from Sisters. Consequently, predicted hazards to downtown Sisters are less severe than those predicted by 1-D shallow-water simulations of a Carver Lake dam breach that were performed in the 1980s.
    • Woody debris blocking conditions at bridges in mountainous streams

      Hasegawa, Yuji; Nakatani, Kana; Satofuka, Yoshifumi (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      When debris flows occur, bridges in mountainous streams may become dangerous when blocked by woody debris. When bridges are blocked with accumulated woody debris, high flow depths can cause the flow to spread widely. However, not all bridges become blocked with woody debris. Many studies have examined bridges blocked with woody debris for gentle slopes, but few studies have been conducted on steep mountain streams with supercritical flow. To better understand the interaction of woody debris and bridges across steep streams, we conducted laboratory experiments using one-pier bridge model and considered factors for the blockage of bridges by woody debris. We used straight rectangular channel flume 7 m in length, 0.3 m width, and with a variable slope. We supplied steady water from upstream end. We supplied woody debris model to the upstream end of the flume at approx.1 second. We set the bridge model 1.5 m upstream from the downstream end of the flume. We used ABS plastic material with a specific weight of 1.05 for the experimental woody debris. We varied the flume slope, water discharge, supply of woody debris, length of wood, height of bridge piers and Froude number. When the woody debris reached the bridge model, the total time of woody debris to pass through became longer from the supplied upstream condition due to rotational motion and diffusion. Because the total time changed, the amount of woody debris per unit time at the bridge was smaller than supplied condition. When examining the blocking conditions, we applied the woody debris condition at bridge model. From the results, we proposed methods to estimate the threshold condition of woody debris blocking at bridge from dimensional analysis. We applied parameters combining the experimental conditions of the bridge model, woody debris model, and hydraulic conditions, as well as the amount of woody debris per unit time required for bridge blocking.
    • Flume experiments and numerical simulation focused on fine sediments in stony debris flow

      Hina, Junya; Uchida, Taro; Matsumoto, Naoki; Sakurai, Wataru; Nishiguchi, Yuki; Murakami, Masato (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      In stony debris flow, it has been considered that the gravels move like laminar flow, but the interstitial water behave as turbulent flow. Moreover, fine particles can behave with the interstitial water as fluid and many previous studies call this process of fine sediment as shifting solid phase to fluid phase, “phase-shift”. Phase-shifted sediment affect the fluidity of debris flow. Therefore, it is necessary to consider fine sediments behavior to describe run-out processes of debris flow. However, the hydraulic conditions that fine sediment can behave as a fluid are not well understood. Here, we analyzed this hydraulic condition through flume experiments and numerical simulations. We examined effects of grain size distribution on the equilibrium sediment concentration, which has been defined as the sediment concentration that in which there is neither erosion nor deposition on the experimental flume bed. We found that for the same hydraulic conditions the equilibrium sediment concentration differed due to variations in the grain size distribution. Based on these experimental results, we tested the following three models for describing the conditions that fine sediment can behave as a fluid. First, we fixed fine sediment concentration in interstitial fluid (Model 1), then, we fixed the maximum diameter of phase-shifted sediment (Dc) (Model 2). In Model 3, Dc is assumed to be variable according to the ratio of the friction velocity to the settling velocity of Dc. As the result, the experimental relationship between grain size distribution and longitudinal gradient of deposited sediment surface under steady-state condition can be described by using the Models 2 and 3, but Model 1 could not describe.
    • On the regression of velocity distribution of debris flows using machine learning techniques

      Huang, Li-Jeng; Hsiao, Darn-Horng (Colorado School of Mines. Arthur Lakes LibraryAssociation of Environmental and Engineering Geologists, 2019)
      Five machine learning techniques-- classical nonlinear regression (NLR), multi-layer perceptrons (MLP), support vector machines (SVM) with radial-basis function (RBF) kernel, k nearest neighbour (kNN) and decision tree (DT) schemes-- were applied for regression of velocity distribution along the depth of debris flows by using experimental data of steady uniform open-channel flows. Programs coded in Python and package scikit-learn were developed for machine learning analyses. Experimental results of two cases conducted and published by Matsumura and Mizuyama (1990) were adopted for training and prediction curves of the velocity distributions using the five different machine learning techniques. Three theoretical formulas were employed for comparison and investigation, the power-law derived by Takahashi (1978) based on Bagnold dilatant flow, theory modified by Matsumura and Mizuyama (1990), and the two-region formula derived by Su et al. (1993). R-squared scores for each case were calculated to check the fitness of the machine learning results to the experimental data and then to verify the fitness of the theoretical formulas to the machine learning predictions. The quantified results revealed that machine learning schemes provide powerful approaches for building prediction models for velocity distribution of debris flows.