Hazardous processes and events from glacier and permafrost areas: lessons from the Chilean and Argentinean Andes

Glacier and permafrost hazards such as glacial‐lake outburst floods and rock–ice avalanches cause significant socio‐economic damages worldwide, and these processes may increase in frequency and magnitude if the atmospheric temperature rises. In the extratropical Andes nearly 200 human deaths were linked to these processes during the twentieth century. We analysed bibliographical sources and satellite images to document the glacier and permafrost dynamics that have caused socio‐economic damages in this region in historic time (including glacial lake outburst floods, ice and rock–ice avalanches and lahars) to unravel their causes and geomorphological impacts. In the extratropical Andes, at least 15 ice‐dammed lakes and 16 moraine‐dammed lakes have failed since the eighteenth century, causing dozens of floods. Some floods rank amongst the largest events ever recorded (5000 × 10⁶ m³ and 229 × 10⁶ m³, respectively). Outburst flood frequency has increased in the last three decades, partially as a consequence of long‐term (decades to centuries) climatic changes, glaciers shrinkage, and lake growth. Short‐term (days to weeks) meteorological conditions (i.e. intense and/or prolonged rainfall and high temperature that increased meltwater production) have also triggered outburst floods and mass movements. Enormous mass failures of glaciers and permafrost (> 10 × 10⁶ m³) have impacted lakes, glaciers, and snow‐covered valleys, initiating chain reactions that have ultimately resulted in lake tsunamis and far‐reaching (> 50 km) flows. The eruption of ice‐covered volcanoes has also caused dozens of damaging lahars with volumes up to 45 × 10⁶ m³. Despite the importance of these events, basic information about their occurrence (e.g. date, causes, and geomorphological impact), which is well established in other mountain ranges, is absent in the extratropical Andes. A better knowledge of the processes involved can help to forecast and mitigate these events. Copyright © 2014 John Wiley & Sons, Ltd.     

Impact of a rock avalanche on a moraine‐dammed proglacial lake: Laguna Safuna Alta,Cordillera Blanca,Peru

Moraines that dam proglacial lakes pose an increasing hazard to communities in the Andes and other mountain ranges. The moraines are prone to failure through collapse, overtopping by lake waters or the effect of displacement waves resulting from ice and rock avalanches. Resulting floods have led to the loss of thousands of lives in the Cordillera Blanca mountains of Peru alone in the last 100 years. On 22 April 2002 a rock avalanche occurred immediately to the south‐west of Laguna Safuna Alta, in the Cordillera Blanca. The geomorphic evidence for the nature, magnitude and consequences of this event was investigated in August 2002. Field mapping indicated that the avalanche deposited 8–20 × 10⁶ m³ of rock into the lake and onto the surface of the frontal region of Glaciar Pucajirca, which flows into the lake. Repeated bathymetric surveying indicated that ～5 × 10⁶ m³ of this material was deposited directly into the lake. The immediate effect of this event was to create a displacement wave that gained in height as it travelled along the lake basin, overtopping the impounding moraine at the lake's northern end. To achieve overtopping, the maximum wave height must have been greater than 100 m. This, and subsequent seiche waves, caused extensive erosion of both the proximal and distal faces of the impounding terminal moraine. Further deep gullying of the distal face of this moraine resulted from the supply of pressurized water to the face via a relief overflow tunnel constructed in 1978. Two‐dimensional, steady‐state analysis of the stability of the post‐avalanche moraine rampart indicates that its proximal face remains susceptible to major large‐scale rotational failure. Copyright © 2005 John Wiley & Sons, Ltd.     

Debris avalanches and debris flows transformed from collapses in the Trans-Mexican Volcanic Belt, Mexico – behavior, and implications for hazard assessment

Volcanoes of the Trans-Mexican Volcanic Belt (TMVB) have yielded numerous sector and flank collapses during Pleistocene and Holocene times. Sector collapses associated with magmatic activity have yielded debris avalanches with generally limited runout extent (e.g. Popocatépetl, Jocotitlán, and Colima volcanoes). In contrast, flank collapses (smaller failures not involving the volcano summit), both associated and unassociated with magmatic activity and correlating with intense hydrothermal alteration in ice-capped volcanoes, commonly have yielded highly mobile cohesive debris flows (e.g. Pico de Orizaba and Nevado de Toluca volcanoes). Collapse orientation in the TMVB is preferentially to the south and northeast, probably reflecting the tectonic regime of active E–W and NNW faults. The differing mobilities of the flows transformed from collapses have important implications for hazard assessment. Both sector and flank collapse can yield highly mobile debris flows, but this transformation is more common in the cases of the smaller failures. High mobility is related to factors such as water content and clay content of the failed material, the paleotopography, and the extent of entrainment of sediment during flow (bulking). The ratio of fall height to runout distance commonly used for hazard zonation of debris avalanches is not valid for debris flows, which are more effectively modeled with the relation inundated area to failure or flow volume coupled with the topography of the inundated area.     

Why permafrost rocks become unstable: a rock–ice‐mechanical model in time and space

In this paper, we develop a mechanical model that relates the destabilization of thawing permafrost rock slopes to temperature‐related effects on both, rock‐ and ice‐mechanics; and laboratory testing of key assumptions is performed. Degrading permafrost is considered to be an important factor for rock–slope failures in alpine and arctic environments, but the mechanics are poorly understood. The destabilization is commonly attributed to changes in ice‐mechanical properties while bedrock friction and fracture propagation have not been considered yet. However, fracture toughness, compressive and tensile strength decrease by up to 50% and more when intact water‐saturated rock thaws. Based on literature and experiments, we develop a modified Mohr–Coulomb failure criterion for ice‐filled rock fractures that incorporates fracturing of rock bridges, friction of rough fracture surfaces, ductile creep of ice and detachment mechanisms along rock–ice interfaces. Novel laboratory setups were developed to assess the temperature dependency of the friction of ice‐free rock–rock interfaces and the shear detachment of rock–ice interfaces. In degrading permafrost, rock‐mechanical properties may control early stages of destabilization and become more important for higher normal stress, i.e. higher magnitudes of rock–slope failure. Ice‐mechanical properties outbalance the importance of rock‐mechanical components after the deformation accelerates and are more relevant for smaller magnitudes. The model explains why all magnitudes of rock–slope failures can be prepared and triggered by permafrost degradation and is capable of conditioning long para‐glacial response times. Here, we present a synoptic rock‐ and ice‐mechanical model that explains the mechanical destabilization processes operating in warming permafrost rocks. Copyright © 2013 John Wiley & Sons, Ltd.     

GIS‐assisted modelling for debris flow hazard assessment based on the events of May 1998 in the area of Sarno,Southern Italy: Part I. Maximum run‐out

Based on the debris flow events that occurred in May 1998 in the area of Sarno, Southern Italy, this paper presents an approach to simulate debris flow maximum run‐out. On the basis of the flow source areas and an average thickness of 1·2 m of the scarps, we estimated debris flow volumes of the order of 10⁴ and 10⁵ m³. Flow mobility ratios (ΔH/L) derived from the x, y, z coordinates of the lower‐most limit of the source areas (i.e. apex of the alluvial fan) and the distal limit of the flows ranged between 0·27 and 0·09. We performed regression analyses that showed a good correlation between the estimated flow volumes and mobility ratios. This paper presents a methodology for predicting maximum run‐out of future debris flow events, based on the developed empirical relationship. We implemented the equation that resulted from the calibration as a set of GIS macros written in Visual Basic for Applications (VBA) and running within ArcGIS. We carried out sensitivity analyses and observed that hazard mapping with this methodology should attempt to delineate hazard zones with a minimum horizontal resolution of 0·4 km. The developed procedure enables the rapid delineation of debris flow maximum extent within reasonable levels of uncertainty, it incorporates sensitivities and it facilitates hazard assessments via graphic user interfaces and with modest computing resources. Copyright © 2007 John Wiley & Sons, Ltd.     

Geomorphic Evidence of Ancient Catastrophic Flow Type Landslides in the Mid-mountain Ridges of the Western Flysch Carpathian Mountains

Ancient flow type landslides are relatively frequent fossil forms of the relief in mid-mountain conditions of the Czech Carpathian Mountains.Sixty rather distinctive displays of debris flows,rock avalanches,and debris avalanches have been mapped in the uppermost part of the territory.Unlike contemporary sporadic and low volume debris flows,ancient (Pleistocene and Lower Holocene) accumulations are a few orders of magnitude more extensive and were of considerable geomorphologic significance in forming the steep sections of mountain valleys and slopes.This geomorphic pattern does not hold for flow type slides,the source of which is material released as a consequence of numerous deep-seated landslides.Due to deep disruption of slopes,a few high-magnitude flow type landslides (e.g.,rock avalanches),quite rare in flysch mid-mountain conditions,also occurred in the Late Holocene.     

Internal structure of an alpine rock glacier based on crosshole georadar traveltimes and amplitudes

Rapid melting of permafrost in many alpine areas has increased the probability of catastrophic rock slides. In an attempt to provide critical structural information needed for the design and implementation of suitable mitigation procedures, we have acquired low frequency (22 MHz) cross‐hole radar data from within a fast‐moving rock glacier, an important form of alpine permafrost. Since the ice, rock and pockets of water and air found in the underground of high alpine areas have very different dielectric permittivities and electrical conductivities, the radar method was well‐suited for investigating the structure and state of the rock glacier. Our interpretation of the radar velocities and attenuations was constrained by geomorphological observations, borehole lithological logs and the results of a surface seismic survey. The radar data revealed the existence of a discontinuous 7–11 m thick ice‐rich zone distinguished by high velocities (0.14–0.17 m/ns) and low attenuations (0.04–0.09 m^?1) and a thin underlying ice‐free zone characterized by moderate velocities (0.11–0.12 m/ns) and low attenuations (0.04–0.09 m^?1). Beneath these two zones, we observed a prominent band of high velocities (0.14–0.17 m/ns) and moderately high attenuations (0.10–0.20 m^?1) associated with unconsolidated glacial sediments and numerous large air‐filled voids, which in the past were probably filled with ice. At greater depths, the variably dry to water‐saturated sediments were represented by generally lower velocities (0.08–0.10 m/ns) and higher attenuations (0.16–0.24 m^?1). The bedrock surface was represented by an abrupt ～0.03 m/ns velocity increase. We speculate that the disappearance of ice, both laterally and with depth, occurred during the past one to two decades.     

Evaluating Titan2D mass-flow model using the 1963 Little Tahoma Peak avalanches,Mount Rainier,Washington

The Titan2D geophysical mass-flow model is evaluated by comparing its simulation results and those obtained from another flow model, FLOW3D, with published data on the 1963 Little Tahoma Peak avalanches on Mount Rainier, Washington. The avalanches, totaling approximately 10×10⁶ m³ of broken lava blocks and other debris, traveled 6.8 km horizontally and fell 1.8 km vertically (H/L=0.246). Velocities calculated from runup range from 24 to 42 m/s and may have been as high as 130 m/s while the avalanches passed over Emmons Glacier.Titan2D is a code for an incompressible Coulomb continuum; it is a depth-averaged, ‘shallow-water’, granular-flow model. The conservation equations for mass and momentum are solved with a Coulomb-type friction term at the basal interface. The governing equations are solved on multiple processors using a parallel, adaptive mesh, Godunov scheme. Adaptive gridding dynamically concentrates computing power in regions of special interest; mesh refinement and coarsening key on the perimeter of the moving avalanche. The model flow initiates as a pile defined as an ellipsoid by a height (z) and an elliptical base defined by radii in the x and y planes. Flow parameters are the internal friction angle and bed friction angle. Results from the model are similar in terms of velocity history, lateral spreading, location of runup areas, and final distribution of the Little Tahoma Peak deposit. The avalanches passed over the Emmons Glacier along their upper flow paths, but lower in the valley they traversed stream gravels and glacial outwash deposits. This presents difficulty in assigning an appropriate bed friction angle for the entire deposit. Incorporation of variable bed friction angles into the model using GIS will help to resolve this issue.     

地形对碎屑流冲出的影响作用(英文)

传统的碎屑流冲出预测经验模型主要基于冲出物体积与冲出物到达角的假设关系式,并没有考虑地形的影响,其中到达角指滑坡后壁顶点与堆积体末端点连线的角度。本文通过小尺寸模型槽实验研究不同地形条件对碎屑流冲出的影响,发现冲出路径上的缓地形将增加到达角,对近期的两个地震诱发型碎屑流场点的调查结果也表明了缓地形对到达角的影响。还发现冲出物受地形限制而改变冲出方向时可能增加冲出角。考虑上述两种地形约束作用的冲出距离预测值将小于传统的预测方法。     

Transport and emplacement mechanisms of channelised long-runout debris avalanches,Ruapehu volcano,New Zealand

The steep flanks of composite volcanoes are prone to collapse, producing debris avalanches that completely reshape the landscape. This study describes new insights into the runout of large debris avalanches enhanced by topography, using the example of six debris avalanche deposits from Mount Ruapehu, New Zealand. Individual large flank collapses (>1 km³) produced all of these units, with four not previously recognised. Five major valleys within the highly dissected landscape surrounding Mount Ruapehu channelled the debris avalanches into deep gorges (≥15 m) and resulted in extremely long debris avalanche runouts of up to 80 km from source. Classical sedimentary features of debris avalanche deposits preserved in these units include the following: very poor sorting with a clay-sand matrix hosting large subrounded boulders up to 5 m in diameter, jigsaw-fractured clasts, deformed clasts and numerous rip-up clasts of late-Pliocene marine sediments. The unusually long runouts led to unique features in distal deposits, including a pervasive and consolidated interclast matrix, and common rip-up clasts of Tertiary mudstone, as well as fluvial gravels and boulders. The great travel distances can be explained by the debris avalanches entering deep confined channels (≥15 m), where friction was minimised by a reduced basal contact area along with loading of water-saturated substrates which formed a basal lubrication zone for the overlying flowing mass. Extremely long-runout debris avalanches are most likely to occur in settings where initially partly saturated collapsing masses move down deep valleys and become thoroughly liquified at their base. This happens when pore water is available within the base of the flowing mass or in the sediments immediately below it. Based on their H/L ratio, confined volcanic debris avalanches are two to three times longer than unconfined, spreading flows of similar volume. The hybrid qualities of the deposits, which have some similarities to those of debris flows, are important to recognise when evaluating mass flow hazards at stratovolcanoes.     

The joint driving effects of climate and weather changes caused the Chamoli glacier-rock avalanche in the high altitudes of the India Himalaya

Science China Earth Sciences - Ice avalanches are one of the most devastating mountain hazards, and can pose a great risk to the security of the surrounding area. Although ice avalanches have been...     

Slope failures and erosion rates on a glacierized high‐mountain face under climatic changes

In this study, rapid topographic changes and increased erosion rates caused by massive slope failures in a glacierized and permafrost‐affected high‐mountain face were investigated with respect to the current climatic change. The study was conducted at one of the highest periglacial rock faces in the European Alps, the east face of Monte Rosa, Italy. Pronounced changes in ice cover and repeated rock and ice avalanche events have been documented in this rock wall since around 1990. The performed multi‐temporal comparison of high‐resolution digital terrain models (DTMs) complemented by detailed analyses of repeat photography represents a unique assessment of topographic changes and slope failures over half a century and reveals a total volume loss in bedrock and steep glaciers in the central part of the face of around 25 × 10⁶ m³ between 1988 and 2007. The high rock and ice avalanche activity translates into an increase in erosion rates of about one order of magnitude during recent decades. The study indicates that changes in atmospheric temperatures and connected changes in ice cover can induce slope destabilization in high‐mountain faces. Analyses of temperature data show that the start of the intense mass movement activity coincided with increased mean annual temperatures in the region around 1990. However, once triggered, mass movement activity seems to be able to proceed in a self‐reinforcing cycle, whereby single mass movement events might be strongly influenced by short‐term extreme temperature events. The investigations suggest a strong stability coupling between steep glaciers and underlying bedrock, as most bedrock instabilities are located in areas where surface ice has disappeared recently and the failure zones are frequently spatially correlated and often develop from lower altitudes progressively upwards. Copyright © 2013 John Wiley & Sons, Ltd.     

Field observations,rheological testing and numerical modelling of a debris‐flow event

Debris flows generated from landslides are common processes and represent a severe hazard in mountain regions due to their high mobility and impact energy. We investigate the dynamics and the rheological properties of a 90 000 m³ debris‐flow event triggered by a rapid regressive landslide with high water content. Field evidence revealed a maximum flow depth of 7–8 m, with an estimated peak discharge of 350–400 m³ s^?1. Depositional evidence and grain‐size distribution of the debris pose the debris flow in an intermediate condition between the fluid‐mud and grain‐flow behaviour. The debris‐flow material has silt–clay content up to 15 per cent. The rheological behaviour of the finer matrix was directly assessed with the ball measuring system. The measurements, performed on two samples at 45–63 per cent in sediment concentration by volume, gave values of 3·5–577 Pa for the yield strength, and 0·6–27·9 Pa s for the viscosity. Based on field evidence, we have empirically estimated the yield strength and viscosity ranging between 4000 ± 200 Pa, and 108–134 Pa s, respectively. We used the Flo‐2D code to replicate the debris‐flow event. We applied the model with rheological properties estimated by means of direct measurements and back‐analyses. The results of these models show that the rheological behaviour of a debris‐flow mass containing coarse clasts can not be assessed solely on the contribution of the finer matrix and thus neglecting the effects of direct grain contacts. For debris flows composed by a significant number of coarse clasts a back‐analysis estimation of the rheological parameters is necessary to replicate satisfactorily the depositional extent of debris flows. In these cases, the back‐estimated coefficients do not adequately describe the rheological properties of the debris flow. Copyright © 2006 John Wiley & Sons, Ltd.     

The effect of glaciation on the intensity of seismic ground motion

Seismicity is known to contribute to landscape denudation through its role in earthquake‐triggered slope failure; but little is known about how the intensity of seismic ground motions, and therefore triggering of slope failures, may change through time. Topography influences the intensity of seismic shaking – generally steep slopes amplify shaking more than flatter slopes – and because glacial erosion typically steepens and enlarges slopes, glaciation may increase the intensity of seismic shaking of some landforms. However, the effect of this may be limited until after glaciers retreat because valley ice or ice‐caps may damp seismic ground motions. Two‐dimensional numerical models (FLAC 6.0) were used to explore how edifice shape, rock stiffness and various levels of ice inundation affect edifice shaking intensity. The modelling confirmed that earthquake shaking is enhanced with steeper topography and at ridge crests but it showed for the first time that total inundation by ice may reduce shaking intensity at hill crests to about 20–50% of that experienced when no ice is present. The effect is diminished to about 80–95% if glacier ice level reduces to half of the mountain slope height. In general, ice cover reduced shaking most for the steepest‐sided edifices, for wave frequencies higher than 3 Hz, and when ice was thickest and the rock had shear stiffness well in excess of the stiffness of ice. If rock stiffness is low and shear‐wave velocity is similar to that of ice, the presence of ice may amplify the shaking of rock protruding above the ice surface. The modelling supports the idea that topographic amplification of earthquake shaking increases as a result of glacial erosion and deglaciation. It is possible that the effect of this is sufficient to have influenced the distribution of post‐glacial slope failures in glaciated seismically active areas. Copyright © 2012 John Wiley & Sons, Ltd.     

Large debris avalanches and associated eruptions in the Holocene eruptive history of Shiveluch Volcano, Kamchatka, Russia

 Shiveluch Volcano, located in the Central Kamchatka Depression, has experienced multiple flank failures during its lifetime, most recently in 1964. The overlapping deposits of at least 13 large Holocene debris avalanches cover an area of approximately 200 km² of the southern sector of the volcano. Deposits of two debris avalanches associated with flank extrusive domes are, in addition, located on its western slope. The maximum travel distance of individual Holocene avalanches exceeds 20 km, and their volumes reach ∼3 km³. The deposits of most avalanches typically have a hummocky surface, are poorly sorted and graded, and contain angular heterogeneous rock fragments of various sizes surrounded by coarse to fine matrix. The deposits differ in color, indicating different sources on the edifice. Tephrochronological and radiocarbon dating of the avalanches shows that the first large Holocene avalanches were emplaced approximately 4530–4350 BC. From ∼2490 BC at least 13 avalanches occurred after intervals of 30–900 years. Six large avalanches were emplaced between 120 and 970 AD, with recurrence intervals of 30–340 years. All the debris avalanches were followed by eruptions that produced various types of pyroclastic deposits. Features of some surge deposits suggest that they might have originated as a result of directed blasts triggered by rockslides. Most avalanche deposits are composed of fresh andesitic rocks of extrusive domes, so the avalanches might have resulted from the high magma supply rate and the repetitive formation of the domes. No trace of the 1854 summit failure mentioned in historical records has been found beyond 8 km from the crater; perhaps witnesses exaggerated or misinterpreted the events. Received: 18 August 1997 / Accepted: 19 December 1997     

Morphology and evolution of supraglacial hummocks on debris-covered Himalayan glaciers

Thick supraglacial debris layers often have an undulating, hummocky topography that influences the lateral transport of debris and meltwater and provides basins for supraglacial ponds. The role of ablation and other processes associated with supraglacial debris in giving rise to this hummocky topography is poorly understood. Characterizing hummocky topography is a first step towards understanding the feedbacks driving the evolution of debris-covered glacier surfaces and their potential impacts on mass balance, hydrology and glacier dynamics. Here we undertake a geomorphological assessment of the hummocky topography on five debris-covered glaciers in the Everest region of the central Himalaya. We characterize supraglacial hummocks through statistical analyses of their vertical relief and horizontal geometry. Our results establish supraglacial hummocks as a distinct landform. We find that a typical hummock has an elongation ratio of 1.1:1 in the direction of ice flow, length of 214 ± 109 m and width of 192 ± 88 m. Hummocky topography has a greater amplitude across-glacier (15.4 ± 10.9 m) compared to along the glacier flow line (12.6 ± 8.3 m). Consequently, hummock slopes are steeper in the across-glacier direction (8.7 ± 4.3°) than in the direction of ice flow (5.6 ± 4.0°). Longer, wider and higher-amplitude hummocks are found on larger glaciers. We postulate that directional anisotropy in the hummock topography arises because, while the pattern of differential ablation driving topography evolution is moderated by processes including the gravitational redistribution of debris across the glacier surface, it also inherits an orientation preference from the distribution of englacial debris in the underlying ice. Our morphometric data inform future efforts to model these interactions, which should account for additional factors such as the genesis of supraglacial ponds and ice cliffs and their impact on differential ablation.     

Seismic constraints on rock damaging related to a failing mountain peak: the Hochvogel,Allgäu

Large rock slope failures play a pivotal role in long-term landscape evolution and are a major concern in land use planning and hazard aspects. While the failure phase and the time immediately prior to failure are increasingly well studied, the nature of the preparation phase remains enigmatic. This knowledge gap is due, to a large degree, to difficulties associated with instrumenting high mountain terrain and the local nature of classic monitoring methods, which does not allow integral observation of large rock volumes. Here, we analyse data from a small network of up to seven seismic sensors installed during July–October 2018 (with 43 days of data loss) at the summit of the Hochvogel, a 2592 m high Alpine peak. We develop proxy time series indicative of cyclic and progressive changes of the summit. Modal analysis, horizontal-to-vertical spectral ratio data and end-member modelling analysis reveal diurnal cycles of increasing and decreasing coupling stiffness of a 260,000 m³ large, instable rock volume, due to thermal forcing. Relative seismic wave velocity changes also indicate diurnal accumulation and release of stress within the rock mass. At longer time scales, there is a systematic superimposed pattern of stress increased over multiple days and episodic stress release within a few days, expressed in an increased emission of short seismic pulses indicative of rock cracking. Our data provide essential first order information on the development of large-scale slope instabilities towards catastrophic failure. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Digital landscapes of deglaciation: identifying Late Quaternary glacial lake outburst floods using LiDAR

High resolution DEMs obtained from LiDAR topographic data have led to improved landform inventories (e.g. landslides and fault scarps) and understanding of geomorphic event frequency. Here we use airborne LiDAR mapping to investigate meltwater pathways associated with the Tweed Valley palaeo ice‐stream (UK). In particular we focus on a gorge downstream of Palaeolake Milfield, previously mapped as a sub‐glacial meltwater channel, where the identification of abandoned headcut channels, run‐up bars, rock‐cut terrace surfaces and eddy flow features attest to formation by a sub‐aerial glacial lake outburst flood (GLOF) caused by breaching of a sediment dam, likely an esker ridge. Mapping of these landforms combined with analysis of the gorge rim elevations and cross‐section variability revealed a two phase event with another breach site downstream following flow blockage by higher elevation drumlin topography. We estimate the magnitude of peak flow to be 1–3 × 10³ m³/s, duration of the event to range from 16–155 days, and a specific sediment yield of 10⁷–10⁹ m³/km²/yr. We identified other outburst pathways in the lower Tweed basin that help delineate an ice margin position of the retreating Tweed Valley ice stream. The results suggest that low magnitude outburst floods are under‐represented in Quaternary geomorphological maps. We therefore recommend regional LiDAR mapping of meltwater pathways to identify other GLOFs in order to better quantify the pattern of freshwater and sediment fluxes from melting ice sheets to oceans. Despite the relatively low magnitude of the Till outburst event, it had a significant impact on the landscape development of the lower Tweed Valley through the creation of a new tributary pathway and triggering of rapid knickpoint retreat encouraging new regional models of post‐glacial fluvial landscape response. Copyright © 2015 John Wiley & Sons, Ltd.     

Large wood inhibits debris flow runout in forested southeast Alaska

Due to their potentially long runout, debris flows are a major hazard and an important geomorphic process in mountainous environments. Understanding runout is therefore essential to minimize risk in the near-term and interpret the pace and pattern of debris flow erosion and deposition over geomorphic timescales. Many debris flows occur in forested landscapes where they mobilize large volumes of large woody debris (LWD) in addition to sediment, but few studies have quantitatively documented the effects of LWD on runout. Here, we analyze recent and historic debris flows in southeast Alaska, a mountainous, forested system with minimal human alteration. Sixteen debris flows near Sitka triggered on August 18, 2015 or more recently had volumes of 80 to 25 000 m³ and limited mobility compared to a global compilation of similarly-sized debris flows. Their deposits inundated 31% of the planimetric area, and their runout lengths were 48% of that predicted by the global dataset. Depositional slopes were 6°–26°, and mobility index, defined as the ratio of horizontal runout to vertical elevation change, ranged from 1.2 to 3, further indicating low mobility. In the broader southeast Alaskan region consisting of Chichagof and Baranof Islands, remote sensing-based analysis of 1061 historic debris flows showed that mobility index decreased from 2.3–2.5 to 1.4–1.8 as average forest age increased from 0 to 416 years. We therefore interpret that the presence of LWD within a debris flow and standing trees, stumps, and logs in the deposition zone inhibit runout, primarily through granular phenomena such as jamming due to force chains. Calibration of debris flow runout models should therefore incorporate the ecologic as well as geologic setting, and feedbacks between debris flows and vegetation likely control the transport of sediment and organic material through steep, forested catchments over geomorphic time. © 2020 John Wiley & Sons, Ltd.