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Alluvial fans develop their semi-conical shape by quasi-cyclic avulsions of their geomorphologically active sector from a fixed fan apex. On debris-flow fans, these quasi-cyclic avulsions are poorly understood, partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas. In this study, debris-flow fans were experimentally created under constant extrinsic forcing, and autogenic sequences of backfilling, avulsion and channelization were observed. Backfilling, avulsion and channelization were gradual processes that required multiple successive debris-flow events. Debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia. In the channelization phase, debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed, resulting in longer run-out and deeper channels. Backfilling commenced when debris flows reached their maximum possible length and channel depth, as defined by channel slope and debris-flow volume and composition, after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated. The terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low-gradient zone forcing deposition. Consequently, the next debris flow was shorter which led to more in-channel sedimentation, causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout. This topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas. Debris-flow avulsion cycles are governed by the same large-scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans, although the detailed processes are unique to debris-flow fans. This novel result provides a basis for modelling of debris-flow fans with applications in hazards and stratigraphy. [ABSTRACT FROM AUTHOR]

Understanding the initial and flow conditions of contemporary flows in Martian gullies, generally believed to be triggered and fluidized by CO2 sublimation, is crucial for deciphering climate conditions needed to trigger and sustain them. We employ the RAMMS (RApid Mass Movement Simulation) debris flow and avalanche model to back-calculate initial and flow conditions of recent flows in three gullies in Hale crater. We infer minimum release depths of 1.0-1.5 m and initial release volumes of 100-200 m3. Entrainment leads to final flow volumes that are ~2.5-5.5 times larger than initially released, and entrainment is found necessary to match the observed flow deposits. Simulated mean cross-channel flow velocities decrease from 3-4 m s-1 to ~1 m s-1 from release area to flow terminus, while flow depths generally decrease from 0.5-1 m to 0.1-0.2 m. The mean cross-channel erosion depth and deposition thicknesses are ~0.1-0.3 m. Back-calculated dry-Coulomb friction ranges from 0.1 to 0.25 and viscous-turbulent friction between 100-200 m s-2, which are values similar to those of granular debris flows on Earth. These results suggest that recent flows in gullies are fluidized to a similar degree as are granular debris flows on Earth. Using a novel model for mass-flow fluidization by CO2 sublimation we are able to show that under Martian atmospheric conditions very small volumetric fractions of CO2 of <<1% within mass flows may indeed yield sufficiently large gas fluxes to cause fluidization and enhance flow mobility. [ABSTRACT FROM AUTHOR]

Landslides are destructive natural hazards that cause substantial loss of life and impact on natural and built environments. Landslide frequencies are important inputs for hazard assessments. However, dating landslides in remote areas is often challenging. We propose a novel landslide dating technique based on Segmented WAvelet-DEnoising and stepwise linear fitting (SWADE), using the Landsat archive (1985–2017). SWADE employs the principle that vegetation is often removed by landsliding in vegetated areas, causing a temporal decrease in normalized difference vegetation index (NDVI). The applicability of SWADE and two previously published methods for landslide dating, harmonic modelling and LandTrendr, are evaluated using 66 known landslides in the Buckinghorse River area, northeastern British Columbia, Canada. SWADE identifies sudden changes of NDVI values in the time series and this may result in one or more probable landslide occurrence dates. The most-probable date range identified by SWADE detects 52% of the landslides within a maximum error of 1 year, and 62% of the landslides within a maximum error of 2 years. Comparatively, these numbers increase to 68% and 80% when including the two most-probable landslide date ranges, respectively. Harmonic modelling detects 79% of the landslides with a maximum error of 1 year, and 82% of the landslides with a maximum error of 2 years, but requires expert judgement and a well-developed seasonal vegetation cycle in contrast to SWADE. LandTrendr, originally developed for mapping deforestation, only detects 42% of landslides within a maximum error of 2 years. SWADE provides a promising fully automatic method for landslide dating, which can contribute to constructing landslide frequency-magnitude distributions in remote areas. [ABSTRACT FROM AUTHOR]

The laminated sediment record from Laguna Pallcacocha, Ecuador, is widely used as a sensitive recorder of past variability in the El Niño-Southern Oscillation. However, limited knowledge of local meteorology, hydrogeomorphic processes, and the lateral variability of the lacustrine stratigraphy have resulted in some ambiguity in proxy interpretation. In this study, we report new high-resolution meteorological data, hydrogeomorphic mapping of the catchment and geochemistry of the lake's sediments. We show that the fine clastic layers are deposited from alluvial activity in the catchment related to intensive rainfall events originating from the Pacific. Frequency analyses of the geochemistry of the sediments indicates that the clastic layers in L. Pallcacocha fall into the characteristic ENSO frequency band and most likely record Eastern Pacific and Coastal Pacific El Niño events. We also illustrate that recent debris flow deposition has resulted in an abrupt avulsion of the main fluvial channels, redirecting sediment input between the lake's two basins and possibly influencing the lithostratigraphy of the sediment package of L. Pallcacocha. • Clastic sediment layers from L. Pallcacocha are deposits from Pacific rainfall-driven alluvial activity in the catchment. • The occurrence of fine clastic layers in the sediments of L. Pallcacocha fall into the characteristic ENSO frequency band. • The sediments of L. Pallcacocha most likely record Eastern Pacific and Coastal Pacific El Niño events. • Recent debris flow deposition has resulted in an avulsion of the main fluvial channels draining to the lake's two basins. • Such a redirection of sediment input possibly influences the lithostratigraphy of the sediment package of L. Pallcacocha. [ABSTRACT FROM AUTHOR]

Landslides are mass movements of rock or soil down a slope, which may cause economic loss, damage to natural resources and frequent fatalities. To support risk management, landslide dating methods can provide useful knowledge about the date of the landslide and the frequency of occurrences, and thus potential triggers. Remote sensing techniques provide opportunities for landslide dating and are especially valuable in remote areas. However, the use of optical remote sensing is frequently hampered by cloud cover, decreasing the success rate and accuracy of dating. Here, we propose a landslide dating framework that combines the advantages of optical and SAR remote sensing satellites, because optical monitoring provides spectral changes on the ground and microwave observations provide information on surface changes due to loss of coherence. Our method combines Sentinel-1 and -2 satellite data, and is designed for cases wherein the landslide causes vegetation decrease and terrain deformation resulting in changing Normalized Difference Vegetation Index (NDVI) and SAR backscatter values. This landslide dating framework was tested and evaluated against 60 published landslides across the world. We show that the mean accuracy of landslide dating reaches 23 days when using combined Sentinel-1 and -2 imagery, which is a pronounced improvement compared to using only optical Sentinel-2 images resulting in an accuracy of 51 days. This study highlights that a combination of optical and SAR remote sensing monitoring is a promising technique for dating landslides, especially in remote areas where monitoring equipment is limited or which are frequently covered by clouds. Our method contributes to identifying failure mechanism by providing reliable date ranges of landslide occurrence, assessing landslide hazard and constructing landslide early warning systems. • A landslide dating framework combining optical and SAR satellites is proposed. • The framework detects landslide dates through vegetation changes and surface variations. • Combining optical and SAR imagery results in improved accuracies relative to using only optical or SAR imagery. • Tested against 60 landslides worldwide we obtain a mean dating accuracy of 23 days when combining optical and SAR imagery. [ABSTRACT FROM AUTHOR]

River deltas commonly have a heterogeneous substratum of alternating peat, clay and sand deposits. This has important consequences for the river bed development and in particular for scour hole formation. When the substratum consists of an erosion resistant top layer, erosion is retarded. Upon breaking through a resistant top layer and reaching an underlying layer with higher erodibilty, deep scour holes may form within a short amount of time. The unpredictability and fast development of these scour holes makes them difficult to manage, particularly where the stability of dikes and infrastructure is at stake. In this paper we determine how subsurface lithology controls the bed elevation in net incising river branches, particularly focusing on scour hole initiation, growth rate, and direction. For this, the Rhine-Meuse Estuary forms an ideal study site, as over 100 scour holes have been identified in this area, and over 40 years of bed level data and thousands of core descriptions are available. It is shown that the subsurface lithology plays a crucial role in the emergence, shape, and evolution of scour holes. Although most scour holes follow the characteristic exponential development of fast initial growth and slower final growth, strong temporal variations are observed, with sudden growth rates of several meters per year in depth and tens of meters in extent. In addition, we relate the characteristic build-up of the subsurface lithology to specific geometric characteristics of scour holes, like large elongated expanding scour holes or confined scour holes with steep slopes. As river deltas commonly have a heterogeneous substratum and often face channel bed erosion, the observations likely apply to many delta rivers. These findings call for thorough knowledge of the subsurface lithology, as without it, scour hole development is hard to predict and can lead to sudden failures of nearby infrastructure and flood defence works. [Display omitted] • Geology can play a crucial role in the emergence, shape and evolution of scour holes. • Sudden increases in erosion occur of several m/yr in depth and tens of m/yr in length. • Specific types of scour growth and shape can be linked to subsurface lithology. • Including geology is essential for correct prediction of scour hole development. [ABSTRACT FROM AUTHOR]

The long-term morphodynamic evolution of estuaries depends on a combination of antecedent topography and boundary conditions, including fluvial input, sea-level change and regional-landscape interactions. Identifying effects of such boundary conditions on estuary evolution is important to anticipate future changes in specific boundary conditions and for hindcasting with numerical and physical models. A comprehensive synthesis of the evolution of the former Old Rhine estuary is presented here, together with its boundary conditions over its full lifespan from 6,500 to 1,000 cal. yr bp. This system formed during a period of sea-level high stand, during which the estuary served as the main River Rhine outlet. The estuary went through three stages of evolution: a maturation phase in a wide infilling back-barrier basin, a stable mature phase and an abandoning phase, both in a laterally confined setting. The Old Rhine River formed by a river avulsion around 6,500 cal. yr bp that connected to a tidal channel within a large back-barrier basin. Decelerating sea-level rise caused the back-barrier basin to silt up around 5,700 cal. yr bp, resulting in shoreline progradation by beach-barrier formation until ∼ 2,000 cal. yr bp. Beach-barrier formation along the coast and natural levee formation along the river triggered peat formation in the coastal plain, laterally constraining the estuary and limiting overbank deposition, which caused most sediment to accumulate offshore. The abandoning phase started around 2,200 cal. yr bp when a series of upstream avulsions led to a substantial reduction in fluvial input. This induced a period of enhanced estuarine overbank clay deposition that continued into near-complete silting up and estuary closure around 1200 ad. These findings exemplify how tidal systems, formed in wide coastal plains during sea-level high stand, depend on antecedent conditions, and how they respond to connection and disconnection of a large river over long, millennial timescales., Competing Interests: The authors declare no conflicts of interest.