Rough Basement Research Articles

Unexpectedly High Magma Productivity Inferred From Crustal Roughness and Residual Bathymetry on the Eastern Part of the Ultra‐Slow Spreading Gakkel Ridge Since ∼45 Ma, Eurasian Basin, Arctic Ocean

AbstractThe Gakkel Ridge in the Eurasian Basin has the slowest seafloor spreading worldwide. The western Gakkel Ridge (3°W–85°E; 14–11 mm/a) alternate between magmatic and sparsely magmatic zones, while the eastern Gakkel Ridge (85–126°E; 11–6 mm/a) appears to be dominated by magmatic zones despite ultraslow spreading. Little is known about the seafloor spreading conditions in the past along the entire ridge. Here, we exploit the residual bathymetry and basement roughness to assess the crustal accretion process of the Gakkel Ridge over time using 23 published regional multichannel seismic reflection profiles. Full seafloor spreading rates were faster (20–24 mm/a) up to ∼45 Ma, and residual bathymetry for the older crust is deeper than the world average in the entire Eurasian Basin. There is a sharp transition to 300–400 m shallower residual bathymetry for seafloor <45 Ma in the eastern Eurasian Basin. The crustal roughness versus spreading rate of the western Eurasian Basin is on the global trend, while that of the eastern is significantly below. Both low roughness and shallow residual bathymetry of the eastern Eurasian Basin is close to that of oceanic crust for spreading rates above 30 mm/a, demonstrating increased magmatic production of the eastern Gakkel Ridge since ∼45 Ma. A recent mantle tomography model predicts partial melting in the upper mantle based on the low Vs anomaly underneath. The sedimentary pattern toward the Lomonosov Ridge indicates that this hot mantle anomaly started to cause dynamic uplift of the area at ∼45 Ma.

A Mechanistic Model and Experiments on Bedrock Incision and Channelization by Rockfall

AbstractRockfall and rock avalanches are common in steep terrain on Earth and potentially on other planetary bodies such as the Moon and Mars. Since impacting rocks can damage exposed bedrock as they roll and bounce downhill, rockfall might be an important erosive agent in steep landscapes, even in the absence of water. We developed a new theory for rockfall‐driven bedrock abrasion using the ballistic trajectories of rocks transported under gravity. We calibrated this theory using laboratory experiments of rockfall over an inclined bedrock simulant. Both the experiments and the model demonstrate that bedrock hillslopes can be abraded by dry rockfall, even at gradients below the angle of repose, depending on the bedrock roughness. Feedback between abrasion and topographic steering of rockfall can produce channel‐like forms, such as bedrock chutes, in the absence of water. Particle size has a dominant influence on abrasion rates and runout distances, while the hillslope angle has a comparatively minor influence. Rockfall transport is sensitive to bedrock roughness; terrain with high friction angles can trap rocks creating patches of rock cover that affect subsequent rockfall pathways. Our results suggest that dry rockfall can play an important role in eroding and channelizing steep, rocky terrain on Earth and other planets, such as crater degradation on the Moon and Mars.

Imaging the source region of recent megathrust earthquakes along the Chile subduction zone: A summary of results from recent experiments

In addition to the largest documented earthquake, the Mw 9.5 Valdivia earthquake of 1960, the Chilean subduction zone has experienced three great (>Mw 8) earthquakes since 2010 for which detailed models of deformation prior to, during and after the earthquake are available. The 1960 and 2010 Mw 8.8 Maule earthquake affected the south-central Chilean margin, where the trench is sediment filled, whereas the 2014 Mw 8.2 Iquique and 2015 Mw 8.3 Illapel earthquakes occurred farther north where there is little sediment in the trench. We summarize recent results from three marine seismological projects designed to elucidate the impact that geologic structure may have had on slip during these events. Because analysis of these datasets is ongoing, this report should be considered as an interim summary rather than a comprehensive synthesis. None-the-less, some patterns can be discerned. That we can resolve 5 m of margin uplift by differencing swath bathymetric data highlights the value of obtaining ground truth marine geophysical data from potential seismic gaps. Controlled source seismic imaging indicates that subduction of nearly all incoming sediment in the sediment-filled section of the trench is a widespread characteristic of the subduction zone segment that ruptured in the two largest events and is likely a generalizable characteristic that can be used to anticipate whether a subduction zone is likely to rupture in great tsunamigenic earthquakes. While this is not a new idea, our data provide additional insights into plate boundary evolution at depth in this scenario. We also find that subduction of anomalously rough basement topography ∼2 million years ago may have led to indentation, large scale internal deformation, and erosion of the overlying wedge north of the 2010 rupture zone and that the process of wedge recovery may have resulted in an abrupt offset along strike of the plate boundary that served as a barrier to slip propagation in 2010. In general, structures generating wedge disruption may no longer be detectable following their subduction deep beneath the forearc wedge, but their passage can be inferred from residual structures mapped within the wedge, and their impact on slip during large earthquakes may be long lasting. We anticipate that further analysis of the data from these experiments, and from future experiments that build on these results, will further elucidate the relationship between geologic structure and slip such that images of subsurface structure become an additional tool in the toolbox of observations used to evaluate future hazard.

Dating seafloor spreading of the southwest sub-basin in the South China Sea

Marine magnetic lineations play important role in interpreting the age and spreading processes of the oceanic crust. However, it is difficult to identify magnetic lineations from sea surface observations in a deep and narrow low-latitude slow-spreading inactive ocean basin for many reasons, such as the highly filtered low-amplitude magnetic anomalies, highly oblique magnetization, contamination from narrow stripes, etc., which may hamper accurate lineation discrimination and identification. The spreading history of the Southwestern Sub-basin of the South China Sea (SW-SCS), which is located in such a low-latitude tectonic setting, has been debated for a long time. Near-seafloor deep-tow magnetic measurements provide high-resolution reversal records, making accurate lineation discrimination feasible. Corrections, including diurnal variation correction, regional field correction, and de-skewing, are explored. The results, calibrated with drill sites from the International Ocean Discovery Program (IODP) Expedition 349, show that the spreading ages of the SW-SCS range from ∼ 21 to 15 Ma (from C6A to C5B) and the full spreading rate ranges from 26 to 46 mm/yr, with an average rate of 37 mm/yr. Spreading was initially symmetric but later became asymmetric, with a rate differential reaching 10%. Combined with two seismic profiles, it is found that the roughness of the oceanic basement to the south of the extinct ridge is mainly influenced by faulted blocks, whereas to the north it is mainly influenced by magmatism. The RMS seafloor roughness is estimated to be 211 m in the south and 191 m in the north respectively. This indicates that SW-SCS seafloor spreading tends to be a transition type between axial high and axial low at the intermediate-slow spreading rate. There is a large valley in the spreading center, but the roughness far away from the center is low. the RMS basement roughness is obviously lower than that of the typical slow spreading oceanic basin. we conjecture that the larger than normal syn-spreading magma may be the reason for the low roughness. A long history of being subducted might be the deep-seated reason for all these behaviors where, and the mantle is rich in water and carbon.

Quantifying Basal Roughness and Internal Layer Continuity Index of Ice Sheets by an Integrated Means with Radar Data and Deep Learning

Understanding englacial and subglacial structures is a fundamental method of inferring ice sheets’ historical evolution and surface mass balance. The internal layer continuity index and the basal roughness are key parameters and indicators for the speculation of the relationship between the ice sheet’s internal structure or bottom and ice flow. Several methods have been proposed in the past two decades to quantitatively calculate the continuity index of ice layer geometry and the roughness of the ice–bedrock interface based on radar echo signals. These methods are mainly based on the average of the absolute value of the vertical gradient of the echo signal amplitude and the standard deviation of the horizontal fluctuation of the bedrock interface. However, these methods are limited by the amount and quality of unprocessed radar datasets and have not been widely used, which also hinders further research, such as the analysis of the englacial reflectivity, the subglacial conditions, and the history of the ice sheets. In this paper, based on geophysical processing methods for radar image denoising and deep learning for ice layer and bedrock interface extraction, we propose a new method for calculating the layer continuity index and basal roughness. Using this method, we demonstrate the ice-penetrating radar data processing and compare the imaging and calculation of the radar profiles from Dome A to Zhongshan Station, East Antarctica. We removed the noise from the processed radar data, extracted ice layer continuity features, and used other techniques to verify the calculation. The potential application of this method in the future is illustrated by several examples. We believe that this method can become an effective approach for future Antarctic geophysical and glaciological research and for obtaining more information about the history and dynamics of ice sheets from their radar-extracted internal structure.

The Impact of Basal Roughness on Inland Thwaites Glacier Sliding.

Swath radar technology enables three‐dimensional mapping of modern glacier beds over large areas at resolutions that are higher than those typically used in ice‐flow models. These data may enable new understanding of processes at the ice‐bed interface. Here, we use two densely surveyed swath‐mapped topographies (<50 m2 resolution) of Thwaites Glacier to investigate the sensitivity of inferred basal friction proxies to bed roughness magnitude and orientation. Our work suggests that along‐flow roughness influences inferred friction more than transverse‐flow roughness, which agrees with analytic form‐drag sliding theory. Using our model results, we calculate the slip length (the ratio of internal shear to basal slip). We find excellent agreement between the numerically derived slip lengths and slip lengths predicted by analytic form‐drag sliding theory, which suggests that unresolved short wavelength bed roughness may control sliding in the Thwaites interior.

Identification of Unstable Glacier Flow in the Western Tibetan Plateau and Karakoram Using Machine Learning

AbstractA novel method was developed for rapidly and accurately identifying unstable glacier flows, including pulse and surge events. The identification method was based on machine learning and high‐frequency glacier velocities derived from short time interval Sentinel‐1 Synthetic Aperture Radar (SAR) data. The method was built and tested using 32 glaciers in the eastern Pamir Plateau and applied it to 13 glaciers in western Karakoram. The test results showed that the success rate of identifying speedup events was more than 90% compared to manual interpretation results derived using velocity change maps and DEM differencing. The application results showed that there were some neglected pulse events, including two pulses on the Kukuar Glacier in 2017 and 2019, one pulse on the Karambar Glacier in 2017, and one pulse on the Ghulkin Glacier in 2016. Moreover, a complete surge event with multiple scattered pulses on the Shispare Glacier in 2018–2019 was also detected. The speedup events of the Shispare Glacier indicated the possibility of mutual transformation between pulse and surge events due to the influence of glacier basal roughness. In all unstable flows occurring within the test and application areas, pulse events could be observed on the nonsurging glaciers during the active phase and quiescent phase of the surge‐type glaciers, which might suggest the randomness of pulse events. Machine learning method made it possible to identify glacier speedup events in large areas with high efficiency and low manpower.

Deposition morphology in large-scale laboratory stony debris flows

Debris flows affect vulnerable areas by unloading their destructive impact and depositing sediments in debris-flow lobes in the runout area. The debris flow deposition morphology characteristically displays a complex process and is of particular interest for hazard mapping, economic activities planning, construction of civil structures, and settlements in vulnerable areas. This laboratory study is based on large-scale stony debris flow experiments, focusing on the runout distance, mobility, and inundated area. The experiments were conducted under different initial and boundary conditions like particle size, solid volume fraction, and basal roughness. Our experimental results reveal that runout distance and lobe shape change dramatically under different solid volume fractions and are highly dependent on basal roughness. On the contrary, the particle size has a marginal effect on deposit morphology. The newly introduced dimensionless runout capacity allows the evaluation of the momentum transfer of the debris flows to the end of the flume into a runout distance. The experimental results show that the runout capacity is inversely proportional to the Froude number of the stony debris flow. We deduce a relationship for the runout distance from this concept, which reveals that flow velocity contributed to the runout distance on a par with the flow depth. A weak velocity can be counterbalanced by an increased front depth to achieve a similar runout distance. Furthermore, we evaluate flow mobility, deposited volume, as well as the inundated area, and compare our results with other experimental and field data. The presented data match existing hypermobility models in-between small-scale laboratory and field-scale events. Consequently, this study complements missing data to the previous dataset of runout distance, elevation difference, mobility, and inundated area, contributing to better understand debris flow dynamics.

Study of the systematic fully grouted rock bolts performance in tunnels considering installation condition of bolt head

ABSTRACT A two-dimensional parametric numerical study is conducted to investigate some unknown aspects of the systematic fully grouted rock bolts performance in the stability of tunnels. The influence of the bolt head constraint conditions, i.e. a stiff steel plate on a smooth rigid basement or a rough basement (the rigid and flexible complexes, respectively) on the convergence confinement, development of the plastic band, and the maximum bolt force is studied in a tunnel located at a poor or good rock mass quality. As well, the difference of the tunnel convergence on the bolt head location and in the middle between the two bolts is the other investigated parameter. The results show that for a tunnel excavated in a poor rock mass quality at great depths (very squeezing condition), the difference of convergences must be considered, particularly for large openings in which the spacing of the bolts is not small.

Oceanic crustal structures and temporal variations of magmatic budget during seafloor spreading in the East Sub-basin of the South China Sea

Following magma-poor continental rifting, the South China Sea (SCS) experienced seafloor spreading from ~33 Ma to ~15 Ma with an intervening southward ridge jump at ~25 Ma, generating the oceanic East, Northwest and Southwest Sub-basins. However, the magmatic processes during spreading remain poorly known owing to the lack of oceanic crustal images. Here we present a composite profile comprising three seismic lines that run from the northern continent-ocean transition (COT) to the extinct spreading center of the East Sub-basin to reveal the crustal structures. In the northernmost oceanic portion, the first oceanic crust featuring rough basement with numerous faults, and diffuse and weak Moho reflector, is only 3.9–4.5 km thick, implying relatively low magmatic budget and protracted tectonic extension continued from the magma-poor continental rifting to initial spreading. From the northern COT ocean-ward to the abandoned ridge where the jump event occurred, the basement becomes smooth with subdued faults. The Moho reflector tends to be continuous and strong, and the igneous crust gradually becomes thick (locally up to 8.4 km), suggesting considerably increasing magmatic budget during the early spreading stage. However, in the central portion spanning from the abandoned ridge to the extinct spreading axis, the basement becomes rough with crustal faults again. And the crust gradually becomes thin, to only 3.7 km at the extinct spreading center where a deep median valley is still visible, though severely altered by post-spreading volcanism, implying decreasing magmatic budget and increasing tectonic extension during the late spreading stage. Therefore, the crustal structure variations in the East Sub-basin reflects a prominent increasing to decreasing change of magmatic budget during spreading with the peak around the intervening ridge jump. Thin, tectonized crust and deep median valley around the extinct spreading center further support slow seafloor spreading during the last spreading stage of the SCS which ended at ~15–16 Ma, and also suggest that magma contribution from the Hainan mantle plume might be very limited if any.

Velocity and Volume Fraction Measurements of Granular Flows in a Steep Flume

ABSTRACTLaboratory experiments on granular flows remain essential tools for gaining insight into several aspects of granular dynamics that are inaccessible from field-scale investigations. Here, we report an experimental campaign on steady dry granular flows in a flume with inclination of 35°. Different flow rates are investigated by adjusting an inflow gate, while various kinematic boundary conditions are observed by varying the basal roughness. The flume is instrumented with high-speed cameras and a no-flicker LED lamp to get reliable particle image velocimetry measurements in terms of both time averages and second-order statistics (i.e., granular temperature). The same measuring instruments are also used to obtain concurrent estimations of the solid volume fraction at the sidewall by employing the stochastic-optical method (SOM). This innovative approach uses a measurable quantity, called two-dimensional volume fraction, which is correlated with the near-wall volume fraction and is obtainable from digital images under controlled illumination conditions. The knowledge of this quantity allows the indirect measurement of the near-wall volume fraction thanks to a stochastic transfer function previously obtained from numerical simulations of distributions of randomly dispersed spheres. The combined measurements of velocity and volume fraction allow a better understanding of the flow dynamics and reveal the superposition of different flow regimes along the flow depth, where frictional and collisional mechanisms exhibit varying relative magnitudes.

Failure mechanism of gently inclined shallow landslides along the soil-bedrock interface on ring shear tests

There were many gently inclined shallow landslides along the soil-bedrock interface at a depth of 3–5 m in Nanjiang County, Sichuan Province, China. To explain why slope failure occurs at the soil-bedrock interface at a depth of 3–5 m, we conducted ring shear tests on samples of the slip zone soils, slip zone soil-to-the smooth bedrock, slip zone soil-to-the natural bedrock, and slip zone soil-to-the rough bedrock interfaces under various normal stresses. Twelve specimens of the slip zone soils and the slip zone soil-to-the bedrock of the gently inclined shallow landslides were tested by means of a ring shear apparatus. The results show that the peak shear strength and residual strength of the slip zone soils were larger than those of the slip zone soil-to-bedrock interface under a normal stress of 60–100 kPa. Thus, the slope slides along the slip zone soil-bedrock interface at a depth of 3–5 m. This can explain why the slope fails along the slip zone soil-bedrock interface at a depth of 3–5 m. The peak shear strength and residual strength of the slip zone soil-to-bedrock were larger than those of the slip zone soils under a normal stress of more than 100 kPa. Thus, the slope slides along the slip zone at a depth of more than 5 m. This can explain why the slope does not fail along the interface between the slip zone soils and the bedrock at a depth of more than 5 m.

Complex Basal Conditions and Their Influence on Ice Flow at the Onset of the Northeast Greenland Ice Stream

AbstractThe ice stream geometry and large ice surface velocities at the onset region of the Northeast Greenland Ice Stream (NEGIS) are not yet well reproduced by ice sheet models. The quantification of basal sliding and a parametrization of basal conditions remains a major gap. In this study, we assess the basal conditions of the onset region of the NEGIS in a systematic analysis of airborne ultra‐wideband radar data. We evaluate basal roughness and basal return echoes in the context of the current ice stream geometry and ice surface velocity. We observe a change from a smooth to a rougher bed where the ice stream widens, and a distinct roughness anisotropy, indicating a preferred orientation of subglacial structures. In the upstream region, the excess ice mass flux through the shear margins is evacuated by ice flow acceleration and along‐flow stretching of the ice. At the downstream part, the generally rougher bed topography correlates with a decrease in flow acceleration and lateral variations in ice surface velocity. Together with basal water routing pathways, this hints to two different zones in this part of the NEGIS: the upstream region collecting water, with a reduced basal traction, and downstream, where the ice stream is slowing down and is widening on a rougher bed, with a distribution of basal water toward the shear margins. Our findings support the hypothesis that the NEGIS is strongly interconnected to the subglacial water system in its onset region, but also to the subglacial substrate and morphology.

Subducting oceanic basement roughness impacts on upper-plate tectonic structure and a backstop splay fault zone activated in the southern Kodiak aftershock region of the Mw 9.2, 1964 megathrust rupture, Alaska

AbstractIn 1964, the Alaska margin ruptured in a giant Mw 9.2 megathrust earthquake, the second largest during worldwide instrumental recording. The coseismic slip and aftershock region offshore Kodiak Island was surveyed in 1977–1981 to understand the region’s tectonics. We re-processed multichannel seismic (MCS) field data using current standard Kirchhoff depth migration and/or MCS traveltime tomography. Additional surveys in 1994 added P-wave velocity structure from wide-angle seismic lines and multibeam bathymetry. Published regional gravity, backscatter, and earthquake compilations also became available at this time.Beneath the trench, rough oceanic crust is covered by ∼3–5-km-thick sediment. Sediment on the subducting plate modulates the plate interface relief. The imbricate thrust faults of the accreted prism have a complex P-wave velocity structure. Landward, an accelerated increase in P-wave velocities is marked by a backstop splay fault zone (BSFZ) that marks a transition from the prism to the higher rigidity rock beneath the middle and upper slope. Structures associated with this feature may indicate fluid flow. Farther upslope, another fault extends &amp;gt;100 km along strike across the middle slope. Erosion from subducting seamounts leaves embayments in the frontal prism.Plate interface roughness varies along the subduction zone. Beneath the lower and middle slope, 2.5D plate interface images show modest relief, whereas the oceanic basement image is rougher. The 1964 earthquake slip maximum coincides with the leading and/or landward flank of a subducting seamount and the BSFZ. The BSFZ is a potentially active structure and should be considered in tsunami hazard assessments.

Overview on the subsurface structure-tectonic controls in the northwest Bahariya, Western Desert, Egypt

A regional structural investigation was done on the northwestern part of Bahariya Oasis, using the potential field maps aided by geo-seismic sections and wells data. The gravity and magnetic maps were reinterpreted in context of geotectonic evolution of the north Western Desert to throw more light on the source of subsurface structures. Several processing methods and filtering techniques were applied to delineate locations, trends and depths of main source structures. Qualitatively, the study indicates that Bahariya horst is the highest in gravity, Sitra platform has the strongest magnetism, whereas Misawage graben is the largest in depth to the basement surface. The results were integrated to construct a tentative tectonic map, which exhibits an older folding in the NW direction intersected by a younger ENE trend, which seems to be crossed by a set of NW-trending faults. The map underlines regional folding in NE-SW trend (Syrian arc), forming four major structural features (Misawage graben, Sitra platform, Bahariya uplift and Dirrut basin), each has distinctive characteristics. The tectonic model suggests two different forces (NNW-compression and ENE-tension) affected the area, forming two main fold trends and five cycles of deformation. The models indicate a rough basement surface with a regional dip regime toward the north, overlain by a sedimentary cover ranges between 1 and 4.5 km in thickness. The basement is generally of acidic composition with some basic intrusions penetrate the boundary faults.

Evolution of stony debris flows in laboratory experiments

The run out and destructive potential of gravitational multi-phase flows is largely determined by the mixture composition, the material properties of the solid particles and the fluid. One instrument to expand the understanding of the governing processes of flow is laboratory experiments. In this study, we concentrate experimentally on landslide-induced stony debris flows as a particular type of flow-like mass movement. We aim to observe different natural flow types for varying initial and boundary conditions. In a laboratory flume, 12.0 m long, 1.3 m wide and 0.3 m deep, we initiate stony debris flows and measured flow variables such as flow depth, mass, bulk density, front velocity and front shape for varying particle size, solid volume fraction and basal roughness. Our experimental results reveal that flow type and evolution changes significantly for different solid volume fractions, as well as for different basal roughness. The particle size had a noticeable effect on flow velocity and front shape. The smooth surface facilitated rapid, shallow, and turbulent flows. In contrast, experiments with rough beds showed relatively lower velocities and dense flow behaviour. Although the flow parameters covered only a small spectrum of the naturally possible parameter space, flow phenomena such as phase-separation, longitudinal sorting, steep front surges, or front overtopping were observed. We group our observed flows regarding the flow properties and classify them into common flow types: (1) debris flood (hyperconcentrated flow), (2) debris flow, and (3) non-liquefied debris flow. To compare our results with natural events and other experimental results, we analyse the data with several dimensionless numbers. The flows were generally dominated by grain collision on the smooth surface. Naturally, frictional forces gain more influence on the rough surface but did not overrule collisional forces. Viscous forces played only a minor role in our experiments, due to the lack of highly viscous fluid. Overall, we infer, that our well-controlled experiments mimicked natural stony debris flow and give new profound insights into the causal relationship of how the initial and boundary conditions affect the flow evolution.

RADAR-DERIVED INTERNAL LAYERING AND BASAL ROUGHNESS CHARACTERIZATION ALONG A TRAVERSE FROM ZHONGSHAN STATION TO DOME A, EAST ANTARCTICA

Abstract. The internal layers of ice sheets from ice-penetrating radar (IPR) investigation preserve critical information about the englacial conditions and ice-flow field. This paper presents a new detailed analysis of the spatial distribution characteristics of internal layers and subglacial topography of East Antarctic ice sheet (EAIS) from Zhongshan station to Dome A. Taking the internal layering continuity index (ILCI) and basal roughness as indicators, it provides an opportunity to evaluate the past internal environment and dynamics of ice sheet. The radar data of 1244 km along a traverse between Zhongshan Station and Dome A of EAIS was collected during the 29th Chinese National Antarctic Research Expedition (CHINARE 29, 2012/2013). Except for the upstream of Lambert Glacier, the patterns of the folds in the internal layers are basically similar to the bed topography. The relatively flat basal topography and the decrease of ILCI with the deepening of the depth provide evidence for identifying previous rapid ice flow areas that the satellite cannot obtain, especially in the upstream of Lambert Glacier. Well continuous internal layers of Dome A almost extend to the bed, with high ILCI and high roughness characteristics. There are three kinds of basal roughness patterns in the whole traverse. The characteristics of the internal layer and basal topography of the traverse between Zhongshan Station and Dome A provide new information for understanding the ancient ice-flow activity and the historical evolution of EAIS.

Hydraulic resistance in mixed bedrock-alluvial meandering channels

ABSTRACT We present an experimental analysis of hydraulic roughness variations due to changes in alluvial cover in a mixed bedrock-alluvial meandering channel with larger bedrock roughness than alluvial roughness. Three sets of experiments were conducted in a highly sinuous flume: one with bare bedrock, and two with enough sediment to cover 21% and 78% of the bedrock respectively. We compare our results with data from experiments previously conducted in the same flume with flat- and smooth-bed, and data from experiments with fully alluvial conditions. Our results show that: (1) hydraulic resistance in a mixed bedrock-alluvial channel changes with the degree of alluviation; (2) hydraulic resistance is greater for bare-bedrock conditions, and decreases as sediment supply increases; (3) if bedforms appear, hydraulic resistance may be larger than for bare bedrock conditions due to form drag; (4) fluctuations in alluvial cover due to freely-migrating bars lead to instantaneous changes in hydraulic resistance.

Alluvial cover on bedrock channels: applicability of existing models

Abstract. Several studies have demonstrated the importance of alluvial cover; furthermore, several mathematical models have also been introduced to predict the alluvial cover on bedrock channels. Here, we provide an extensive review of research exploring the relationship between alluvial cover, sediment supply and bed topography of bedrock channels, describing various mathematical models used to analyse the deposition of alluvium. To test one-dimensional theoretical models, we performed a series of laboratory-scale experiments with varying bed roughness under simple conditions without bar formation. Our experiments show that alluvial cover is not merely governed by increasing sediment supply and that bed roughness is an important controlling factor of alluvial cover. A comparison between the experimental results and the five theoretical models shows that (1) two simple models that calculate alluvial cover as a linear or exponential function of the ratio of the sediment supplied to the capacity of the channel produce good results for rough bedrock beds but not for smoother bedrock beds; (2) two roughness models which include changes in roughness with alluviation and a model including the probability of sediment accumulation can accurately predict alluvial cover in both rough and smooth beds; and (3), however, except for a model using the observed hydraulic roughness, it is necessary to adjust model parameters even in a straight channel without bars.

Basal roughness of the East Antarctic Ice Sheet in relation to flow speed and basal thermal state

AbstractBasal motion of ice sheets depends in part on the roughness and material properties of the subglacial bed and the occurrence of water. To date, basal motion represents one of the largest uncertainties in ice-flow models. It is that component of the total flow velocity that can change most rapidly and can, therefore, facilitate rapid variations in dynamic behaviour. In this study, we investigate the subglacial properties of the East Antarctic Ice Sheet by statistically analysing the roughness of the bed topography, inferred from radio-echo sounding measurements. We analyse two sets of roughness parameters, one derived in the spatial and the other in the spectral domain, with two roughness parameters each. This enables us to compare the suitability of the four roughness parameters to classify the subglacial landscapes below the ice sheet. We further investigate the relationship of the roughness parameters with observed surface flow velocity and modelled basal temperatures of the ice sheet. We find that one of the roughness parameters, the Hurst exponent derived in the spatial domain, coincides with the thermal condition at the base of the ice sheet for slow flow velocities and varies with flow velocity.