Debris Fans Research Articles

Integration of two-phase solid fluid equations in a catchment model for flashfloods, debris flows and shallow slope failures

An integrated, modeling method for shallow landslides, debris flows and catchment hydrology is developed and presented in this paper. Existing two-phase debris flow equations and an adaptation on the infinite slope method are coupled with a full hydrological catchment model. We test the approach on the 4 km2 Scaletta catchment, North-Eastern Sicily, where the 1-10-2009 convective storm caused debris flooding after 395 shallow landslides. Validation is done based on the landslide inventory and photographic evidence from the days after the event. Results show that the model can recreate the impact of both shallow landslides, debris flow runout, and debris floods with acceptable accuracy (91 percent inventory overlap with a 0.22 Cohens Kappa). General patterns in slope failure and runout are well-predicted, leading to a fully physically based prediction of rainfall induced debris flood behavior in the downstream areas, such as the creation of a debris fan at the coastal outlet.

New Evidence on the Relief of the Southern Underwater Slope in the South Baikal Basin

A high-resolution bathymetric survey of the entire underwater slope in South Baikal was carried out for the first time by means of the multibeam echosounder. New elements were revealed in the underwater relief of the southern slope of the basin from the Kultuchnaya river to the Mysovka river, and a relevant regionalization was carried out into three areas according to the presence of ancient landforms and the intensity of the processes of their reworking, with the boundaries along the delta-front and the river fan of the Utulik and Pereemnaya. We reconstructed the formation process of the modern relief of this area of the underwater slope and determined a dominant role of the underwater-erosion processes in the formation of the modern relief, with the tectonic movements served as the “trigger” for them. The study revealed a direct correlation between the reworking of the ancient geomorphological surfaces and the occurrence of separate large underwater elevations or group ridges (remnants), canyons and linear depressions. It is determined that the pivotal role in the appearance of the modern relief of the underwater slope in the second area of extensive plano-convex (in section) and fan-shaped (in plan) delta-fronts of large rivers and debris fans enveloping the remnants. The existence of a steep underwater slope from the mouth of the Pereemnaya to the mouth of the Bol’shaya Tel’naya and of two large benches, and also the proximity of the maximum depths of the southern basin at the foot of this slope are explained by a more intense step-like lowering of this part of the lake when compared with the western part of the basin. This also accounts for the absence of large remnants in this part of the slope. The area of occurrence of lacustrine-fluvial and lacustrine-delta deposits of the Tankhoi paleobasin within the southern basin of Baikal is reconstructed. Four new hydrate-bearing structures associated with these deposits and morphologically pronounced in the vertex parts of the remnants were discovered. The morphological exploration indicators of the presence of new accumulations of gas hydrates on the underwater slopes of Baikal have been expanded.

Sedimentary architecture of a sub-lacustrine debris fan: Eocene Dongying Depression, Bohai Bay Basin, east China

The sedimentary architectures of submarine/sublacustrine fans are controlled by sedimentary processes, geomorphology and sediment composition in sediment gravity flows. To advance understanding of sedimentary architecture of debris fans formed predominantly by debris flows in deep-water environments, a sub-lacustrine fan (Y11 fan) within a lacustrine succession has been identified and studied through the integration of core data, well logging data and 3D seismic data in the Eocene Dongying Depression, Bohai Bay Basin, east China. Six types of resedimented lithofacies can be recognized, which are further grouped into five broad lithofacies associations. Quantification of gravity flow processes on the Y11 fan is suggested by quantitative lithofacies analysis, which demonstrates that the fan is dominated by debris flows, while turbidity currents and sandy slumps are less important. The distribution, geometry and sedimentary architecture are documented using well data and 3D seismic data. A well-developed depositional lobe with a high aspect ratio is identified based on a sandstone isopach map. Canyons and/or channels are absent, which is probably due to the unsteady sediment supply from delta-front collapse. Distributary tongue-shaped debris flow deposits can be observed at different stages of fan growth, suggesting a lobe constructed by debrite tongue complexes. Within each stage of the tongue complexes, architectural elements are interpreted by wireline log motifs showing amalgamated debrite tongues, which constitute the primary fan elements. Based on lateral lithofacies distribution and vertical sequence analysis, it is proposed that lakefloor erosion, entrainment and dilution in the flow direction lead to an organized distribution of sandy debrites, muddy debrites and turbidites on individual debrite tongues. Plastic rheology of debris flows combined with fault-related topography are considered the major factors that control sediment distribution and fan architecture. An important implication of this study is that a deep-water depositional model for debrite–dominated systems was proposed, which may be applicable to other similar deep-water environments.

Debris flow recurrence periods and multi-temporal observations of colluvial fan evolution in central Spitsbergen (Svalbard)

Fan-shaped accumulations of debris flow deposits are common landforms in polar regions such as Svalbard. Although depositional processes in these environments are of high interest to climate as well as Mars-analog research, several parameters, e.g., debris flow recurrence periods, remain poorly constrained. Here, we present an investigation based on remote sensing as well as in situ data of a ~0.4km2 large colluvial fan in Hanaskogdalen, central Spitsbergen. We analyzed high resolution satellite and aerial images covering five decades from 1961 to 2014 and correlated them with lichenometric dating as well as meteorological data. Image analyses and lichenometry deliver consistent results and show that the recurrence period of large debris flows (≥400m3) is about 5 to 10years, with smaller flows averaging at two per year in the period from 2008 to 2013. While this is up to two orders of magnitude shorter than previous estimates for Svalbard (80 to 500years), we found the average volume of ~220m3 per individual flow to be similar to previous estimates for the region. Image data also reveal that an avulsion took place between 1961 and 1976, when the active part of the fan moved from its eastern to its western portion. A case study of the effects of a light rain event (~5mm/day) in the rainy summer of 2013, which triggered a large debris flow, further shows that even light precipitation can trigger major flows. This is made possible by multiple light rain events or gradual snow melt pre-saturating the permafrost ground and has to be taken into account when predicting the likelihood of potentially hazardous mass wasting in polar regions. Furthermore, our findings imply a current net deposition rate on the colluvial fan of ~480m3/year, which is slightly less than the integrated net deposition rate of 576 to 720m3/year resulting from the current fan volume divided by the 12,500 to 10,000years since the onset of fan build-up after the area's deglaciation. However, the actual deposition rate, which should increase in a warmer climate including more rain, cannot be constrained due to effects like ongoing toe-cutting of the debris fan and some flows only causing internal redistributions.

EXPLODING SATELLITES—THE TIDAL DEBRIS OF THE ULTRA-FAINT DWARF GALAXY HERCULES

ABSTRACT The ultra-faint satellite galaxy Hercules has a strongly elongated and irregular morphology with detections of tidal features up to 1.3 deg (3 kpc) from its center. This suggests that Hercules may be dissolving under the Milky Way’s gravitational influence, and hence could be a tidal stream in formation rather than a bound, dark-matter-dominated satellite. Using Bayesian inference in combination with N-body simulations, we show that Hercules has to be on a very eccentric orbit ( ) within the Milky Way in this scenario. On such an orbit, Hercules “explodes” as a consequence of the last tidal shock at pericenter 0.5 Gyr ago. It is currently decelerating toward the apocenter of its orbit with a velocity of V = 157 km s−1—of which 99% is directed radially outwards. Due to differential orbital plane precession caused by the non-spherical nature of the Galactic potential, its debris fans out nearly perpendicular to its orbit. This explains why Hercules has an elongated shape without showing a distance gradient along its main body: it is in fact a stream that is significantly broader than it is long. In other words, it is moving perpendicular to its apparent major axis. In this scenario, there is a spike in the radial velocity profile created by the dominant debris component that formed through the last pericenter passage. This is similar to kinematic substructure that is observed in the real Hercules. Modeling a satellite on such a highly eccentric orbit is strongly dependent on the form of the Galactic potential. We therefore propose that detailed kinematic investigation of Hercules and other exploding satellite candidates can yield strong constraints on the potential of the Milky Way.

Genesis and provenance of a new Middle Pleistocene diamicton unit at Happisburgh, NE Norfolk, UK

Glacigenic deposits at Happisburgh, NE Norfolk, record the earliest known expansion of glaciers into lowland eastern England during the Quaternary. The sequence comprises two regionally extensive till units, the Happisburgh Till and Corton Diamicton members of the Happisburgh Glacigenic Formation, deposited during separate ice advances, and intervening glacilacustrine and outwash deposits laid down during ice-marginal retreat. During 2012, a new diamicton unit was discovered within the intervening sorted sediments and its significance is outlined here. Sedimentological and structural evidence suggests, tentatively, that the diamicton forms a small debris fan generated subaerially by a series of water-saturated hyperconcentrated or debris flows. The precise trigger mechanism for these flow deposits remains unclear, but may relate to seasonal melting of surface or buried ice followed by mass-movement, or to more abrupt geological events including periods of intense rainfall, moraine dam failure or a glacier outburst flood.

Assessing Predation Risks for Small Fish in a Large River Ecosystem between Contrasting Habitats and Turbidity Conditions

Abstract This study examined predation risk for juvenile native fish between two riverine shoreline habitats, backwater and debris fan, across three discrete turbidity levels (low, intermediate, high) to understand environmental risks associated with habitat use in a section of the Colorado River in Grand Canyon, AZ. Inferences are particularly important to juvenile native fish, including the federally endangered humpback chub Gila cypha. This species uses a variety of habitats including backwaters which are often considered important rearing areas. Densities of two likely predators, adult rainbow trout Oncorhynchus mykiss and adult humpback chub, were estimated between habitats using binomial mixture models to examine whether higher predator density was associated with patterns of predation risk. Tethering experiments were used to quantify relative predation risk between habitats and turbidity conditions. Under low and intermediate turbidity conditions, debris fan habitat showed higher relative predation...

Terminal moraines in the fjord basins of sub-Antarctic South Georgia

Sub-Antarctic South Georgia (54° 17′ S, 36° 30′ W, 3755 km2) is a large heavily glacierized island that is 170 km long, 39 km wide and rises to elevations of 2934 m. The island has extensive evidence of past glacial expansion with moraines preserved on land (Bentley et al. 2007), in the fjords (Hodgson et al. 2014) and in glacial troughs that extend 60–100 km to the edge of the continental shelf where some terminate in trough-mouth fans (Graham et al. 2008). A paucity of chronological control means that, at present, it is not possible to identify which of these sets of features mark the limit of a more extensive ice cap from the Last Glacial Maximum (LGM). The glacial geomorphology of nine of the deep glacial fjords on South Georgia has been studied from ship-based geophysical surveys. Despite the different lengths, widths and orientations of the fjords, a relatively consistent pattern of large-scale (102–103 m) submarine geomorphological features is present (Hodgson et al. 2014). Most of the fjords have shallow (30–150 m) inner basins characterized by a range of seafloor sedimentary features including sediment gravity flows, debris fans and slope failures, all of which are associated with high glacigenic sediment supply. The inner …

Sediment in Alluvial and Lacustrine Debris Fans as an Indicator for Land Degradation Around Lake Ashenge (Ethiopia)

AbstractSediments deposited by (paleo) flash floods can hold valuable information on processes of environmental change, land degradation or desertification. In order to assess the suitability of flash flood deposits as proxies for land degradation, we monitored a representative gully segment in North Ethiopia (Ashenge catchment), investigated a sequence of alluvial debris fans downstream of this segment and dated a neighbouring subaquatic debris fan using short‐lived 210Pb isotope counting. During one rainy season (July–September 2014), we measured daily rainfall, peak discharge, bedload transport, suspended sediment load and sediment deposition rates. The data show that sediment deposition in the debris fans is significantly dependent on micro‐topography (net incision in micro‐channels) (p < 0·1) and position within the sequence (net incision farther away from the lake) (p < 0·05). As sediment transfer to the lake significantly depends on the balance between available water and sediment (ratio rainfall depth/bedload transport) (p < 0·05), we could reconstruct the hydro‐sedimentary evolution of the gully over the past half century and validate it with aerial photographs and semi‐structured interviews. The findings are consistent with the short‐lived isotope count results, indicating increased sediment supply from the 1970s onwards, when little amounts of clay were deposited in the lake (<5%), and a subrecent clear water effect that resulted in increased deposition rates of clay in the lacustrine debris fan. Overall, our analysis indicates that debris fan sediments can be used to estimate past environmental degradation rates, if the contemporary water and sediment behaviour is well understood. Copyright © 2015 John Wiley & Sons, Ltd.

Deposition morphology of non-homogeneous debris flow and its energy characteristics

Non-homogeneous two-phase debris flows are widely found in the western mountainous regions of China. To investigate the characteristics of the debris flow deposition process related to the morphology and extent of the debris fan, a series of physical experiments were carried out using an experimental flume. Some useful relationships were obtained to link the flow velocity with the geometric characteristics of deposition morphology and the corresponding area or volume. Based on these, some expressions about energy dissipation process in both the transport-deposition zone and deposition zone are presented, and improved equations describing solid-liquid two-phase energy transformations in the specific deposition zone are also established. These results provide a basis for further investigating the underlying mechanisms of non-homogeneous debris flows, based upon which effective disaster control measures can be undertaken.

Mitigation planning based on the prediction of river blocking by a typical large-scale debris flow in the Wenchuan earthquake area

Due to scale amplification resulting from the blocking-bursting process associated with landslide dams in debris flow catchments, many of large-scale debris flows that occurred as a result of the Wenchuan earthquake blocked the main rivers and resulted in catastrophic dam-breaking floods. To decrease the damage caused by dam-breaking floods, mitigation works should be applied in debris flow gullies. Currently, few studies focus on how to determine the key parameters concerning the scale of debris flows (i.e., peak discharge and total volume) that need to be controlled for mitigation planning considering downstream objects affected by the flow. The Xiaojia debris flow is described and analyzed as a typical large-scale case from the Yingxiu area. A back-calculation method and numerical simulation were proposed for mitigation planning in Xiaojia gully based on predictions of river blocking. First, the maximum peak flood discharge that did not endanger the town of Yingxiu was calculated. Then, the permissible blockage from a debris flow was determined based on the back-calculation of a dam-breaking flood. Finally, the scale of the largest permissible debris flow was obtained for use in mitigation plans based on numerical simulations. The calculations showed that the peak discharge of a flood that would not endanger Yingxiu should be <1496.48 m3/s. Accordingly, based on the 1/3 and 1/2 breach modes, the permissible blocking height of a debris flow barrier dam should not exceed 43.09 and 31.64 m, respectively. The total volume and peak discharge of a single debris flow event should be controlled to not exceed 70.59 × 104 m3 and 784.59 m3/s for the 1/3 breach mode and 45.21 × 104 m3 and 551.77 m3/s for the 1/2 breach mode. Based on these determinations of the key debris flow parameters, the simulation results indicate that debris flow damage can be decreased to an acceptable level to ensure the safety of Yingxiu downstream by the implementation of two check dams in the downstream channel and a deposition works on the debris fan.

Relating the compensational stacking of debris-flow fans to characteristics of their underlying stratigraphy: Implications for geologic hazard assessment and mitigation

Compensational stacking is the tendency for sediment transport systems to fill topographic lows through avulsion. This article quantitatively relates, for the first time, compensational stacking patterns within debris fans to characteristics of their internal stratigraphy and discusses implications to geologic hazard assessment and mitigation. Three exceptionally well-exposed debris fans were selected in Colorado for quantitative stratigraphic analyses. In each fan, the cross-sectional stratigraphy was subdivided into discrete depositional units (debris-flow and stream-flow deposits). The bounding surfaces between the depositional units were used to analyze the compensation index (κcv) of the fans, which is a measure of their compensational or avulsion tendencies. In the measured datasets, κcv ranged from 0.63 to 1.03. Values close to 0.5 represent intermediate levels of compensation, whereas values approaching 1.0 reflect high levels of compensation. The compensational values (κcv) were statistically compared to some physical, observable characteristics of the fans including: (1) debris-flow size, (2) amount of stream-flow deposits, (3) debris-flow composition, and (4) longitudinal position on the fan. These parameters correlated, either positively or negatively, to κcv, supporting their use as proxies for assessing the degree of compensational stacking in settings where large-scale cross-sections of a fan are unavailable. Such empirical results can be used by geologists and engineers for avoidance and mitigation measures of land use on debris fans.

Quaternary extensional and compressional tectonics revealed from Quaternary landforms along Kosi River valley, outer Kumaun Lesser Himalaya, Uttarakhand

A portion of the Kosi River in the outer Kumaun Lesser Himalaya is characterized by wide river course situated south of the Ramgarh Thrust, where huge thickness (~200 m) of the landslide deposits and two to three levels of unpaired fan terraces are present. Brittle normal faults, suggesting extensional tectonics, are recognized in the Quaternary deposits and bedrocks as further supported by surface morphology. Trending E–W, these faults measure from 3 to 5 km in length and are traced as discontinuous linear mini-horst and fault scarps (sackungen) exposed due to cutting across by streams. Active normal faults have displaced the coarsely laminated debris fan deposits at two sites located 550 m apart. At one of the sites, the faults look like bookshelf faulting with the maximum displacement of ~2 m and rotation of the Quaternary boulders along the fault plane is observed. At another site, the maximum displacement measures about 0.60 cm. Thick mud units deposited due to blocking of the streams by landslides are observed within and above the fan deposit. Landslide debris fans and terrace landforms are widely developed; the highest level of fan is observed ~1240 m above mean sea level. At some places, the reworking of the debris fans by streams is characterized by thick laminated sand body. Along the South Almora Thrust and Ramgarh Thrust zones, the valleys are narrow and V-shaped where Quaternary deposits are sparse due to relatively rapid uplift across these thrusts. Along the South Almora Thrust zone, three to four levels of fluvial terraces are observed and have been incised by river exposing the bedrocks due to recent movement along the RT and SAT. Abandoned channel, tilted mud deposits, incised meandering, deep-cut V-shaped valleys and strath terraces indicate rapid uplift of the area. Thick mud sequences in the Quaternary columns indicate damming of streams. A ~10-km-long north–south trending transverse Garampani Fault has offset the Ramgarh Thrust producing tectonic landforms.

A comparative assessment of two different debris flow propagation approaches – blind simulations on a real debris flow event

Abstract. A detailed comparison between the performances of two different approaches to debris flow modelling was carried out. In particular, the results of a mono-phase Bingham model (FLO-2D) and that of a two-phase model (TRENT-2D) obtained from a blind test were compared. As a benchmark test the catastrophic event of 1 October 2009 which struck Sicily causing several fatalities and damage was chosen. The predicted temporal evolution of several parameters of the debris flow (such as flow depth and propagation velocity) was analysed in order to investigate the advantages and disadvantages of the two models in reproducing the global dynamics of the event. An analysis between the models' results with survey data have been carried out, not only for the determination of statistical indicators of prediction accuracy, but also for the application of the Receiver Operator Characteristic (ROC) approach. Provided that the proper rheological parameters and boundary conditions are assigned, both models seem capable of reproducing the inundation areas in a reasonably accurate way. However, the main differences in the application rely on the choice of such rheological parameters. Indeed, within the more user-friendly FLO-2D model the tuning of the parameters must be done empirically, with no evidence of the physics of the phenomena. On the other hand, for the TRENT-2D the parameters are physically based and can be estimated from the properties of the solid material, thus reproducing more reliable results. A second important difference between the two models is that in the first method the debris flow is treated as a homogeneous flow, in which the total mass is kept constant from its initiation in the upper part of the basin to the deposition in a debris fan. In contrast, the second approach is suited to reproduce the erosion and deposition processes and the displaced mass can be directly related to the rainfall event. Application of both models in a highly urbanized area reveals the limitation of numerical simulation which is inadequate in describing some disturbances of the flows that occurred during the alluvial event (e.g. the cars, the volume of debris within buildings etc.) which have a crucial influence on the evaluation of the maximum and final flow depths.

Experimental study on debris flow behavior in debris fan area : considering influence of houses

Experimental study on debris flow behavior in debris fan area : considering influence of houses

Late Cenozoic and present-day hillslope erosion dynamics in a passive margin context: stability or instability? Case studies in Northeast Brazil

Steep slopes form the marginal scarp of the semi-arid northern Brazilian “Nordeste”, above depressions or corridors connected to a low erosion plain merging seaward with a coastal erosional-gradational piedmont. Dissected pediments and sparse debris fans visible at the base of some escarpments suggest recent erosive activity and possible slope retreat, although most of these forms are decoupled from major valleys. We describe the morphostructural patterns of chosen escarpments, in the basement as well as in the sedimentary cover, and the morphodynamic conditions of their evolution since the Miocene. Only a few scarps show strong structural control. Many of them are mainly inherited landforms, initiated during the Early Cretaceous rifting or the later margin uplift, a situation which suggests long-term stability. Later on, except in one case (the Araripe scarp), only slight or local backwearing took place, associated with downwearing on low surfaces and pediments, probably in diachronic ways. The Neogene clastic sedimentation on piedmonts and coastal areas mainly reflects the occurrence of dry periods inducing widespread stripping of deep soil horizons and erosion of bare rock slopes and surfaces. Dissection stages occurred in periods of more humid climate and/or low sea level. Marks of strong recent of present activity are mainly registered in the rims of the Chapada do Araripe, owing to favourable structural, hydrogeological and climatic conditions. The moderate volumes of Neogene clastic sediments imply overall low uplift and erosion rates until the Present, favourable to morphological and lithological resistance effects in the landscapes. However, slope instabilities are not uncommon, locally leading to well characterized processes, landforms and deposits of gullying and mass wasting. Therefore, hazards related with slope processes never may be neglected, although only small scale events were recorded in recent times, and times of recurrence of larger events are probably much longer than the historic times.

Mutual interference of two debris flow deposits delivered in a downstream river reach

We investigate experimentally the depositions of two contiguous debris flows flowing into a main river reach. The aim of the present experimental research is to analyze the geometry and the mutual interactions of debris flow deposits conveyed by these tributaries in the main channel. A set of 19 experiments has been conducted considering three values of the confluence angle, two slopes of the tributary, and three different triggering conditions (debris flows occurring simultaneously in the tributaries, or occurring first either in the upstream or in the downstream tributary). The flow rate along the main channel was always kept constant. During each experiment the two tributaries had the same slope and confluence angle. The analysis of the data collected during the experimental tests indicates that the volume of the debris fan is mainly controlled by the slope angle, as expected, while the shape of the debris deposit is strongly influenced by the confluence angle. Moreover, in the case of multiple debris flows, the deposit shape is sensitive to the triggering conditions. Critical index for damming formation available in literature has been considered and applied to the present case, and, on the basis of the collected data, considerations about possible extension of such indexes to the case of multiple confluences are finally proposed.

Patterns in the Landscape and Erosion of Cultural Sites Along the Colorado River Corridor in Grand Canyon, USA

The geologic and geomorphic template of Grand Canyon influences patterns in the archaeological record, including sites where apparent increases in erosion may be related to Glen Canyon Dam. To provide geoarchaeological context for the Colorado River corridor and such issues, we explore first‐order trends in a database of field observations and topographic metrics from 227 cultural sites. The patterns revealed may be expected in other river‐canyon settings of management concern. The spatial clustering of sites along the river follows variations in width of the valley bottom and the occurrence of alluvial terraces and debris fans, linking to bedrock controls. In contrast, the pattern of more Formative (Ancestral Puebloan, 800–1250 A.D.) sites in eastern Grand Canyon and Protohistoric (1250–1776 A.D.) sites in western Grand Canyon does not follow any evident geomorphic trends. In terms of site stability, wider reaches with more terrace and debris fan landforms host a disproportionate number of sites with acute erosion. This links most directly to weak alluvial substrates, and the primary erosion process is gullying with diffusive‐creep processes also pervasive. Although Glen Canyon Dam does not directly influence these erosion processes, overall sediment depletion and the loss of major flooding leaves erosion unhampered along the river corridor.

Relationships among three repeated large-scale debris flows at Pubugou Ravine in the Wenchuan earthquake zone

The 12 May 2008 Wenchuan earthquake in China triggered numerous landslides. Loose landslide materials can easily evolve into deadly debris flows during wet seasons. During the period from 2008 to 2011, three separate large-scale debris flows occurred in the Pubugou Ravine near the epicentre of the earthquake that were among the largest repeated debris flows ever reported. Approximately 1.76 × 106 m3 of sediment was deposited during these three events. This paper aims to (i) analyze the movements of solid materials during the repeated debris flows, (ii) discuss the evolution of the initiation mechanisms of these debris flows, and (iii) evaluate the changing depositional morphology of the debris flow fans and examine the particle sizes of the debris flow materials. To achieve the above research objectives, timely field investigations were undertaken in the past 5 years after each of these debris flow events. Satellite images were used to delineate the boundaries of the initiation areas, the transportation channels, and the deposition zones of these debris flows. With the occurrence of the repeated debris flows, the hill slope deposits gradually evolved into channel deposits and the solid materials in the channels moved toward the gully mouth. Hence, channelized flows gradually became dominant. The debris fan materials of the repeated debris flows became coarser and coarser over time. The three debris flows were all characterized by coarse boulder fronts.

Reconstructing historic Glacial Lake Outburst Floods through numerical modelling and geomorphological assessment: Extreme events in the Himalaya

ABSTRACTRecession of high‐mountain glaciers in response to climatic change frequently results in the development of moraine‐dammed glacial lakes. Moraine dam failure is often accompanied by the release of large volumes of water and sediment, termed a Glacial Lake Outburst Flood (GLOF). Chukhung Glacier is a small (~3 km2) receding valley glacier in Mt. Everest (Sagarmatha) National Park, Nepal. Unlike many Himalayan glaciers, which possess a thick mantle of supraglacial debris, its surface is relatively clean. The glacier terminus has receded 1.3 km from its maximum Holocene position, and in doing so provided the space for an ice‐contact moraine‐dammed lake to develop. The lake had a maximum volume of 5.5 × 105 m3 and drained as a result of breaching of the terminal moraine. An estimated 1.3 × 105 m3 of material was removed from the terminal moraine during breach development. Numerical dam‐breach modelling, implemented within a Generalised Likelihood Uncertainty Estimation (GLUE) framework, was used to investigate a range of moraine‐dam failure scenarios. Reconstructed outflow peak discharges, including failure via overtopping and piping mechanisms, are in the range 146–2200 m3 s‐1. Results from two‐dimensional hydrodynamic GLOF modelling indicate that maximum local flow depths may have exceeded 9 m, with maximum flow velocities exceeding 20 m s‐1 within 700 m of the breach. The floodwaters mobilised a significant amount of material, sourced mostly from the expanding breach, forming a 300 m long and 100 m wide debris fan originating at the breach exit. moraine‐dam. These results also suggest that inundation of the entire floodplain may have been achieved within ten minutes of initial breach development, suggesting that debris fan development was rapid. We discuss the key glaciological and geomorphological factors that have determined the evolution of a hazardous moraine‐dammed lake complex and the subsequent generation of a GLOF and its geomorphological impact. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.