When water and sediment supply to a river change, the short-term channel response will be a combination of adjustments in bed grain size (texture) and the accumulation or evacuation of sediment in the channel (topography). This response is well documented for gravel bed rivers, but little attention has been given to sand bed rivers, the focus of this paper. If the channel response is predominantly textural, subsequent changes in channel geometry may be minor. If the channel response is predominately aggradation or degradation, long-term changes in channel dimension, planform, and slope may occur. Here, we use a mixed-size morphodynamic model to explore the interaction between the textural and topographic responses to an increase in sediment supply in sand bed rivers. First, we consider how the steady-state transport condition varies as a function of sediment supply rate and grain size. We then evaluate the path to steady-state using numerical morphodynamic experiments. We find that bed aggradation in response to an increase in sediment supply can be reduced, eliminated, or even reversed depending on the sediment supply grain size. The path to the new steady-state condition involves two adjustment phases: the first is textural-dominated, and the second is topography-dominated. Under the common condition of an increased supply of finer sediment, rapid textural adjustments can produce a transport capacity that is a large fraction of the new supply rate long before the system has reached complete equilibrium. Under conditions of supply coarsening, the initial textural adjustment is less dominant, and aggradation is immediate.
Seismic instruments placed outside of spatially extensive hazard zones can be used to rapidly sense a range of mass movements. However, it remains challenging to automatically detect specific events of interest. Benford's law, which states that the first non-zero digit of given data sets follows a specific probability distribution, can provide a computationally cheap approach to identifying anomalies in large data sets and potentially be used for event detection. Here, we select vertical component seismograms to derive the first digit distribution. The seismic signals generated by debris flows follow Benford's law, while those generated by ambient noise do not. We propose the physical and mathematical explanations for the occurrence of Benford's law in debris flows. Our finding of limited seismic data from landslides, lahars, bedload transports, and glacial lake outburst floods indicates that these events may follow Benford's Law, whereas rockfalls do not. Focusing on debris flows in the Illgraben, Switzerland, our Benford's law-based detector is comparable to an existing random forest model that was trained on 70 features and six seismic stations. Achieving a similar result based on Benford's law requires only 12 features and single station data. We suggest that Benford's law is a computationally cheap, novel technique that offers an alternative for event recognition and potentially for real-time warnings.
Low sea levels during the last Ice Age exposed millions of square kilometers of Arctic shelves which have been subsequently submerged, creating subsea permafrost. In onshore settings, permafrost can also exist beneath water bodies such as coastal lagoons, rivers, and thermokarst lakes. We explored passive seismology as a method for mapping unfrozen sediment thickness above subsea and sub-aquatic permafrost. We present passive seismic data collected with the Mobile Ocean Bottom Seismic Instrument (MOBSI) from the Beaufort Sea near Tuktoyaktuk in Canada, Ivashkina Lagoon on the Bykovsky Peninsula, as well as a lake and river in the Lena Delta, Siberia, Russia. We use borehole data and frost probe measurements to identify permafrost-related H/V measurement peaks and calibrate shear wave velocities for frequency-to-depth conversion. We employ the shortest path and maximum signal amplitude to connect peaks and generate geological profiles. The MOBSI detected the ice-bonded permafrost table beneath the Beaufort Sea, as well as beneath a Siberian lake and lagoon. At Tuktoyaktuk, an ocean bottom seismometer revealed a 5% scatter about the peak frequency for three-minute time windows and over 8 hr of recording time. With peak frequencies ranging from 4.9 ± 0.2 Hz to 27.6 ± 1.4 Hz, the depth to subsea permafrost ranged from 1.4 ± 0.1 m bsl at the shoreline to 14.0 ± 0.4 m bsl 240 m offshore. Given an accurate shear wave velocity, our findings highlight that MOBSI deployment times as short as 3 min are adequate for detecting Arctic subsea and sub-aquatic permafrost.
Seasonal sea ice impacts Arctic delta morphology by limiting wave and river influences and altering river-to-ocean sediment pathways. However, the long-term effects of sea ice on delta morphology remain poorly known. To address this gap, 1D morphologic and hydrodynamic simulations were set up in Delft3D to study the 1500-year development of Arctic deltas during the most energetic Arctic seasons: spring break-up/freshet, summer open-water, and autumn freeze-up. The model focused on the deltaic clinoform (i.e., the vertical cross-sectional view of a delta) and used a floating barge structure to mimic the effects of sea ice on nearshore waters. From the simulations we find that ice-affected deltas form a compound clinoform morphology, that is, a coupled subaerial and subaqueous delta separated by a subaqueous platform that resembles the shallow platform observed offshore of Arctic deltas. Nearshore sea ice affects river dynamics and promotes sediment bypassing during sea ice break-up, forming an offshore depocenter and building a subaqueous platform. A second depocenter forms closer to shore during the open-water season at the subaerial foreset that aids in outbuilding the subaerial delta and assists in developing the compound clinoform morphology. Simulations of increased wave activity and reduced sea-ice, likely futures under a warming Arctic climate, show that deltas may lose their shallow platform on centennial timescales by (a) sediment infill and/or (b) wave erosion. This study highlights the importance of sea ice on Arctic delta morphology and the potential morphologic transitions these high-latitude deltas may experience as the Arctic continues to warm.
Fragmentation is a common phenomenon in complex rock-avalanches. The fragmentation intensity and process determines exceptional spreading of such mass movements. However, studies focusing on the simulation of fragmentation are still limited and no operational dynamic simulation model of fragmentation has been proposed yet. By enhancing the mechanically controlled landslide deformation model (Pudasaini & Mergili, 2024, https://doi.org/10.1029/2023jf007466), we propose a novel, unified dynamic simulation method for rock-avalanche fragmentation. The model includes three important aspects: mechanically controlled rock mass deformation, momentum loss while the rock mass fiercely impacts the ground, and the energy transfer during fragmentation resulting in the generation of dispersive lateral pressure. We reveal that the dynamic fragmentation, resulting from the overcoming of the tensile strength by the impact on the ground, leads to enhanced spreading, thinning, run-out and hypermobility of rock-avalanches. Thereby, the elastic strain energy release caused by fragmentation becomes an important process. Energy conversion between the front and rear parts caused by the fragmentation results in the enhanced forward movement of the front and hindered motion of the rear of the rock-avalanche. The new model describes this by amplifying the lateral pressure gradient in the opposite direction: enhanced for the frontal particles and reduced for the rear particles after the fragmentation. The main principle is the switching between the compressional stress and the tensile stress, and therefore from the controlled deformation to substantial spreading of the frontal part in the flow direction while backward stretching of the rear part of the rock mass. Laboratory experiments and field events support our simulation results.
Knowledge of the rates of carbonate rock denudation, the relative apportionment of chemical weathering versus physical erosion, and their sensitivity to climate, vegetation, and tectonics is essential for disclosing feedbacks within the carbon cycle and the functioning of karst landscapes that supply important services to humans. Currently, however, for carbonate lithologies, no method exists that allows to simultaneously partition denudation into erosion and weathering fluxes at spatial scales ranging from soil to watersheds. To determine total denudation rates in carbonate landscapes from an individual soil or river sample, we adapted a published framework that combines cosmogenic meteoric 10Be as an atmospheric flux tracer with stable 9Be that is released from rocks by weathering, to the limestone-dominated French Jura Mountains. By analyzing water, soil, sediment, travertine, and bedrock for 10Be/9Be, major and trace elements, carbon stable isotopes and radiogenic strontium, we quantified contributions of Be from primary versus secondary carbonate phases and its release during weathering from carbonate bedrock versus silicate impurities. We calculated partitioning of Be between solids and solutes, and rates of catchment-wide (from sediment) and point source (from soil) denudation, weathering and erosion. Our results indicate that average denudation rates are 300–500 t/km2/yr. Denudation is dominated by weathering intensity (W/D) ratios of >0.92, and a non-negligible contribution from deeper (below soil) weathering. Our rates agree to within less than a factor of two with decadal-scale denudation rates from combined suspended and dissolved fluxes, highlighting the substantial potential of this method for future Earth surface studies.
We developed a robust InSAR processing strategy that can effectively mitigate severe decorrelation noise in a large volume of InSAR data. We mapped the average land subsidence rate (2017–2020) over the 131,572 km2 Upper Texas and Louisiana coasts from Sentinel-1 data, with ∼2 mm/yr accuracy based on independent GPS and tide gauge validation at 189 locations. The improved InSAR observations reveal widespread subsidence that was previously undetected in coastal wetlands and rural areas with small communities. Our InSAR surface deformation map is at the spatial scale that overlaps with the scale of hydrodynamic model grids. This allows us to integrate InSAR observations into operational storm surge models to analyze future flooding risks due to relative sea level change. We found that these subtle millimeter-to-centimeter subsidence features can substantially increase hurricane-induced inundation, and passive flood mapping (known as the “bathtub” approach) can lead to inaccurate flood risk predictions.
River channels are maintained by coordination between flow hydraulics, sediment supply, riparian vegetation, and sediment transport. This coordination is challenging to understand in natural flow regimes, where climatic and environmental drivers produce episodic flood and sediment supply events. To better understand the response of channels to flood sequences, we have undertaken laboratory flume experiments on sediment storage and export across a sequence of alternating hydrographs. Our experiments indicate that accumulated sediment storage before floods predicts sediment transport during floods, with sediment storage depletion during floods causing a nonlinear variation of sediment-transport rates through time. Likewise, sediment storage between floods follows a growth-to-saturation pattern, whereby the sediment transport gradually increases toward the sediment feed rate depending on the occupation of available sediment storage zones. To describe these non-linear variations, we developed a mathematical model which represents sediment transport and storage as a coupled dynamical system. This work highlights the crucial role that within-channel sediment storage and its history play in determining sediment export in rivers.
The rate of channel incision in bedrock rivers is often described using a power law relationship that scales erosion with drainage area. However, erosion in landscapes that experience strong rainfall gradients may be better described by discharge instead of drainage area. In this study, we test if these two end member scenarios result in identifiable topographic signatures in both idealized numerical simulations and in natural landscapes. We find that in simulations using homogeneous lithology, we can differentiate a posteriori between drainage area and discharge-driven incision scenarios by quantifying the relative disorder of channel profiles, as measured by how well tributary profiles mimic both the main stem channel and each other. The more heterogeneous the landscape becomes, the harder it proves to identify the disorder signatures of the end member incision rules. We then apply these indicators to natural landscapes, and find, among eight test areas, no clear topographic signal that allows us to conclude a discharge or area-driven incision rule is more appropriate. We then quantify the distortion in the channel steepness index induced by changing the incision rule. Distortion in the channel steepness index can also be driven by changes to the assumed reference concavity index, and we find that distortions in the normalized channel steepness index, frequently used as a proxy for erosion rates, is more sensitive to changes in the concavity index than to changes in the assumed incision rule. This makes it a priority to optimize the concavity index even under an unknown incision mechanism.
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