The bubble bursts for cavitation in natural rivers: laboratory experiments reveal minor role in bedrock erosion

The erosion of rock‐bedded channels generally is considered a slow process caused mainly by abrasion due to bedload or suspended sediments, but the mechanisms of rapid erosion remain unclear. Cavitation is a clear‐fluid erosive process, well known for its effect on engineering structures, when water vapour bubbles collapse and the resultant pressure shocks erode the boundary. However, although the occurrence of cavitation erosion in natural watercourses has long been a matter of debate, as yet there are no incontrovertible examples of cavitation damage to natural river beds. Using flume experiments, we show for the first time that only weakly‐cavitating clear‐water flows can occur for the range of flow velocities observed in rivers, and these do not erode medium‐hardness rocks after 68 hours. During this time period, only a very soft rock featured erosional marks due to dissolution. Thus, our results cast significant doubt on the likelihood of identifying cavitation damage in most rivers, and provide pointers to those river systems that might be investigated further to identify cavitation erosion. Copyright © 2016 John Wiley & Sons, Ltd.     

Distribution of erosion across bedrock channels

Lateral erosion in bedrock rivers is an important control on the shape of channel cross‐sections, and the coupling of channels and hillslopes. Recent observations link lateral erosion to the variability of flow. We propose two mechanisms to explain this. One is based on changing shear stress distributions within the channel with varying flood level, the other on the competition between cover and tool effects in fluvial bedrock erosion. We assess these processes for the Liwu River, Taiwan, and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case. Copyright © 2007 John Wiley & Sons, Ltd.     

Field techniques for measuring bedrock erosion and denudation

Bedrock erosion rates in natural landscapes are usually slow, of the order of millimeters per year or less, and sophisticated techniques have been developed to measure them. Different techniques have proved to be valuable depending on the spatial and temporal scale on which information is needed, on the environment and on the scientific question that is asked. Here, we give an overview of the various methods that have been developed. We introduce their working principles and outline their advantages and disadvantages. Further, we provide comprehensive references to relevant literature, both on the methods and on scientific examples of their application. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Multiple‐station neural network for modelling tidal currents across Shinnecock inlet,USA

This paper presents the development of a multiple‐station neural network for predicting tidal currents across a coastal inlet. Unlike traditional hydrodynamic models, the neural network model does not need inputs of coastal topography and bathymetry, grids, surface and bottom frictions, and turbulent eddy viscosity. Without solving hydrodynamic equations, the neural network model applies an interconnected neural network to correlate the inputs of boundary forcing of water levels at a remote station to the outputs of tidal currents at multiple stations across a local coastal inlet. Coefficients in the neural network model are trained using a continuous dataset consisting of inputs of water levels at a remote station and outputs of tidal currents at the inlet, and verified using another independent input and output dataset. Once the neural network model has been satisfactorily trained and verified, it can be used to predict tidal currents at a coastal inlet from the inputs of water levels at a remote station. For the case study at Shinnecock Inlet in the southern shore of New York, tidal currents at nine stations across the inlet were predicted by the neural network model using water level data located from a station about 70 km away from the inlet. A continuous dataset in May 2000 was used for the training, and another dataset in July 2000 was used for the verification of the neural network model. Comparing model predictions and observations indicates correlation coefficients range from 0·95 to 0·98, and the root‐mean‐square error ranges from 0·04 to 0·08 m s^?1 at the nine current locations across the inlet. Copyright © 2007 John Wiley & Sons, Ltd.     

Weathering and erosion of fractured bedrock systems

We explore the contribution of fractures (joints) in controlling the rate of weathering advance for a low‐porosity rock by using methods of homogenization to create averaged weathering equations. The rate of advance of the weathering front can be expressed as the same rate observed in non‐fractured media (or in an individual block) divided by the volume fraction of non‐fractured blocks in the fractured parent material. In the model, the parent has fractures that are filled with a more porous material that contains only inert or completely weathered material. The low‐porosity rock weathers by reaction‐transport processes. As observed in field systems, the model shows that the weathering advance rate is greater for the fractured as compared to the analogous non‐fractured system because the volume fraction of blocks is < 1. The increase in advance rate is attributed both to the increase in weathered material that accompanies higher fracture density, and to the increase in exposure of surface of low‐porosity rock to reaction‐transport. For constant fracture aperture, the weathering advance rate increases when the fracture spacing decreases. Equations describing weathering advance rate are summarized in the ‘List of selected equations’. If erosion is imposed at a constant rate, the weathering systems with fracture‐bounded bedrock blocks attain a steady state. In the erosional transport‐limited regime, bedrock blocks no longer emerge at the air‐regolith boundary because they weather away. In the weathering‐limited (or kinetic) regime, blocks of various size become exhumed at the surface and the average size of these exposed blocks increases with the erosion rate. For convex hillslopes, the block size exposed at the surface increases downslope. This model can explain observations of exhumed rocks weathering in the Luquillo mountains of Puerto Rico. Published 2017. This article is a U.S. Government work and is in the public domain in the USA     

The sensitivity of hillslope bedrock erosion to precipitation

Decoupling the impacts of climate and tectonics on hillslope erosion rates is a challenging problem. Hillslope erosion rates are well known to respond to changes in hillslope boundary conditions (e.g. channel incision rates) through their dependence on soil thickness, and precipitation is an important control on soil formation. Surprisingly though, compilations of hillslope denudation rates suggest little precipitation sensitivity. To isolate the effects of precipitation and boundary condition, we measured rates of soil production from bedrock and described soils on hillslopes along a semi‐arid to hyperarid precipitation gradient in northern Chile. In each climate zone, hillslopes with contrasting boundary conditions (actively incising channels versus non‐eroding landforms) were studied. Channel incision rates, which ultimately drive hillslope erosion, varied with precipitation rather than tectonic setting throughout the study area. These precipitation‐dependent incision rates are mirrored on the hillslopes, where erosion shifts from relatively fast and biologically‐driven to extremely slow and salt‐driven as precipitation decreases. Contrary to studies in humid regions, bedrock erosion rates increase with precipitation following a power law, from ～1 m Ma^?1 in the hyperarid region to ～40 m Ma^?1 in the semi‐arid region. The effect of boundary condition on soil thickness was observed in all climate zones (thicker soils on hillslopes with stable boundaries compared to hillslopes bounded by active channels), but the difference in bedrock erosion rates between the hillslopes within a climate region (slower erosion rates on hillslopes with stable boundaries) decreased as precipitation decreased. The biotic‐abiotic threshold also marks the precipitation rate below which bedrock erosion rates are no longer a function of soil thickness. Our work shows that hillslope processes become sensitive to precipitation as life disappears and the ability of the landscape to respond to tectonics decreases. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical models of continental and submarine erosion: application to the northern Ligurian Margin (Southern Alps,France/Italy)

We present a new numerical surface process model allowing us to take into account submarine erosion processes due either to submarine landslides or to hyperpycnal currents. A first set of models show that the frequency of hyperpycnal flows influences the development of submarine canyons at the mouth of continental rivers. Further experiments show that an increase in submarine slope leads to faster regressive canyon erosion and a more dentritic canyon network, whereas increasing the height of the unstable sediment pile located on the shelf break leads to wider and less dendritic canyons. The models are then applied to the western segment of the north Ligurian margin (northwestern Mediterranean), which displays numerous submarine canyons with various sizes and morphologies. From west to east, canyon longitudinal profiles as well as margin‐perpendicular profiles progressively change from moderately steep, concave‐up shapes to steeper linear to convex‐up shapes suggesting increasing eastward margin uplift. Moreover, the foot of the margin is affected by a marked slope increase with evidences of mass transport due to landslides. Numerical models which reproduce well the North Ligurian margin morphologic features indicate that the western part of the margin is submitted to rather low (i.e. 0.4 mm yr^?1) uplift and intense submarine erosion due to frequent hyperpycnal currents, whereas the eastern part bears more rapid (i.e. 0.7 mm yr^?1) uplift and has little or no hyperpycnal currents. Copyright © 2014 John Wiley & Sons, Ltd.     

Intertidal mudflat properties, currents and sediment erosion in the partially mixed Tamar Estuary, UK

Results are presented from a 1-year campaign to measure the seasonal variability of some key physical and biological properties of intertidal mudflats over a section of the central Tamar Estuary and to relate these to the physical environment. Seasonal variations in physical mudflat properties, such as grain size, density and moisture content were relatively small. With the exception of the particulate organic carbon content in the upper 0.002 m of surface sediment, biological variations were large. Redox potential exhibited considerable seasonal variation and showed that the sediments were less reduced in winter and more reduced in summer. Chlorophyll a and extracellular polymeric substances (EPS) within the surface 0.002 m of sediment (due to the presence of benthic diatoms) were strongly correlated and exhibited a pronounced seasonal pattern, with smallest values during winter and greatest values during late summer and early autumn. EPS had a dominating influence on sediment erosion, as determined from annular flume measurements. Velocity measurements and velocity modelling indicated that during the flood, and for much of the time during benthic diatom bloom conditions of high chlorophyll a and EPS sediment contents, the stresses exerted by tidal currents were too small to cause significant suspension of sediments over much of the middle and upper mudflats. Suspended fine sediment in the turbidity maximum zone was transported down-estuary and deposited in the main channel at low-water (LW) slack. Some of this sediment, in the form of relatively large aggregates, was subsequently transported onto the mudflats during the flooding tide, where slack currents and fast settling velocities may have enhanced sediment deposition there.Responsible Editor: Hans Burchard     

Episodic bedrock erosion by gully‐head migration,Colorado High Plains,USA

This study explores the frequency of bedrock exposure in a soil‐mantled low‐relief (i.e. non‐mountainous) landscape. In the High Plains of eastern Colorado, gully headcuts are among the few erosional features that will incise through the soil mantle to expose bedrock. We measured the last time of bedrock exposure using optically stimulated luminescence dating of alluvial sediment overlying bedrock in gully headcuts. Our dating suggests that headcuts in adjacent gullies expose bedrock asynchronously, and therefore, the headcuts are unlikely to have been triggered by a base‐level drop in the trunk stream. This finding supports the hypothesis that headcuts can develop locally in gullies as a result of focused scour in locations where hydraulic stress during a flash flood is sufficiently high, and/or ground cover is sufficiently weak, to generate a scour hole that undermines vegetation. Alluvium dating also reveals that gullies have been a persistent part of this landscape since the early Holocene. Copyright © 2016 John Wiley & Sons, Ltd.     

Transient hillslope erosion in slow evolution landscapes

Transient evolution and adjustment to changing tectonic and climatic boundary conditions is an essential attribute of landscapes, and characterizing transient behavior is a key to understanding their dynamics and history. Developing new approaches to detect such transience has been explored by various methods, in particular to identify landscape response to Late Cenozoic and Quaternary climatic changes. Such studies have often focused on regions of high relief and/or active tectonic activity where interferences between tectonic and climatic signals might complicate the interpretation of the observations. We investigated the case of the hillslopes of the Serra do Cipó quartzitic range in SE Brazil in order to detect and quantify transience in a tectonically quiescent landscape over 100-ka timescales. We determined hilltop curvature from a high-resolution digital surface model derived from Pléiades imagery and measured cosmogenic nuclide (¹⁰Be and ²⁶Al) concentrations at these hilltop sites. We compare both observations with predictions of hillslope diffusion theory, observing a distinctive signature of an acceleration of denudation. We performed a joint inversion of topographic and isotopic data to retrieve an evolution of the hillslope sediment transport coefficient through time. The timing of the increase in denudation cannot be unequivocally associated with a single climatic event but is consistent with important, climatically modulated fluctuations in precipitation and erosion in this area during the Middle and Late Pleistocene.     

Variability of the currents in the Luzon Strait during spring of 2002 obtained from observations and satellite geostrophic currents and spectral analyses

The structure and variability of the currents in the Luzon Strait during spring of 2002 are studied, based on the current measurements at the average position of the mooring station (20°49′57"N, 120°48′12"E) from March 17 to April 15, 2002, satellite geostrophic currents in the Luzon Strait, and the spectral analyses, using the maximum entropy method. The subtidal currents at the mooring station show de-creased amplitudes downward with an anti-cyclonic rotation, suggesting that the currents enter and exit t...     

The dynamics of mountain erosion: Cirque growth slows as landscapes age

Glacial cirques are widely used palaeoenvironmental indicators, and are key to understanding the role of glaciers in shaping mountain topography. However, notable uncertainty persists regarding the rate and timing of cirque erosion. In order to address this uncertainty, we analyse the dimensions of 2208 cirques in Britain and Ireland and model ice accumulation to investigate the degree of coupling between glacier occupation times and cirque growth. Results indicate that during the last ~120 ka, cirques were glacier-free for an average of 52.0 ± 21.2 ka (43 ± 18%); occupied by small (largely cirque-confined) glaciers for 16.2 ± 9.9 ka (14 ± 8%); and occupied by large glaciers, including ice sheets, for 51.8 ± 18.6 ka (43 ± 16%). Over the entire Quaternary (i.e. 2.6 Ma), we estimate that cirques were glacier-free for 1.1 ± 0.5 Ma; occupied by small glaciers for 0.3 ± 0.2 Ma; and occupied by large glaciers for 1.1 ± 0.4 Ma. Comparing occupation times to cirque depths, and calculating required erosion rates, reveals that continuous cirque growth during glacier occupation is unlikely. Instead, we propose that cirques attained much of their size during the first occupation of a non-glacially sculpted landscape (perhaps during the timeframe of a single glacial cycle). During subsequent glacier occupations, cirque growth may have slowed considerably, with the highest rates of subglacial erosion focused during periods of marginal (small glacier) glaciation. We propose comparatively slow rates of growth following initial cirque development because a ‘least resistance’ shape is formed, and as cirques deepen, sediment becomes trapped subglacially, partly protecting the bedrock from subsequent erosion. In support of the idea of rapid cirque growth, we present evidence from northern British Columbia, where cirques of comparable size to those in Britain and Ireland developed in less than 140 ka. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Lateral bedrock erosion and valley formation in a heterogeneously layered landscape,Northeast Kansas

In this study, we present direct field measurements of modern lateral and vertical bedrock erosion during a 2-year study period, and optically stimulated luminescence (OSL) ages of fluvial material capping a flat bedrock surface at Kings Creek located in northeast Kansas, USA. These data provide insight into rates and mechanisms of bedrock erosion and valley-widening in a heterogeneously layered limestone-shale landscape. Lateral bedrock erosion outpaced vertical incision during our 2-year study period. Modern erosion rates, measured at erosion pins in limestone and shale bedrock reveal that shale erosion rate is a function of wetting and drying cycles, while limestone erosion rate is controlled by discharge and fracture spacing. Variability in fracture spacing amongst field sites controls the size of limestone block collapse into the stream, which either allowed continued lateral erosion following rapid detachment and transport of limestone blocks, or inhibited lateral erosion due to limestone blocks that protected the valley wall from further erosion. The OSL ages of fluvial material sourced from the strath terrace were older than any material previously dated at our study site and indicate that Kings Creek was actively aggrading and incising throughout the late Pleistocene. Coupling field measurements and observations with ages of fluvial terraces can be useful to investigate the timing and processes linked to how bedrock rivers erode laterally over time to form wide bedrock valleys.     

Feedbacks between erosion and sediment transport in experimental bedrock channels

Natural bedrock rivers flow in self‐formed channels and form diverse erosional morphologies. The parameters that collectively define channel morphology (e.g. width, slope, bed roughness, bedrock exposure, sediment size distribution) all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes. To explore the mechanics of river incision, we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate (sand and cement), a single sediment size primarily transported as bedload, a single erosion mechanism (abrasion) and sediment‐starved transport conditions. We find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field, sediment transport and bottom roughness. Abrasion only occurs where sediment impacts the bed, and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows. However, the spatial focusing of erosion results in tortuous flow paths and erosional forms (inner channels, scoops, potholes), which dissipate flow energy. This energy dissipation is a negative feedback that reduces sediment transport capacity, inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load. Copyright © 2007 John Wiley & Sons, Ltd.     

Towards an improved understanding of erosion rates and tidal notch development on limestone coasts in the Tropics: 10 years of micro‐erosion meter measurements,Phang Nga Bay,Thailand

Knowledge and understanding of shore platform erosion and tidal notch development in the tropics and subtropics relies mainly on short‐term studies conducted on recently deposited carbonate rocks, predominantly Holocene and Quaternary reef limestones and aeolianites. This paper presents erosion rates, measured over a 10 year period on notches and platforms developed on the Permian, Ratburi limestone at Phang Nga Bay, Thailand. In so doing it contributes to informing a particular knowledge gap in our understanding of the erosion dynamics of shore platform and tidal notch development in the tropics and subtropics – notch erosion rates on relatively hard, ancient limestones measured directly on the rock surface using a micro‐erosion meter (MEM) over time periods of a decade or more. The average intertidal erosion rate of 0.231 mm/yr is lower than erosion rates measured over 2–3 years on recent, weaker carbonate rocks. Average erosion rates at Phang Nga vary according to location and site and are, in rank order from highest to lowest: Mid‐platform (0.324 mm/yr) > Notch floor (0.289 mm/yr) > Rear notch wall (0.228 mm/yr) > Lower platform (0.140 mm/yr) > Notch roof (0.107 mm/yr) and Supratidal (0.095 mm/yr). The micro‐relief of the eroding rock surfaces in each of these positions exhibits marked differences that are seemingly associated with differences in dominant physical and bio‐erosion processes. The results begin to help inform knowledge of longer term shore platform erosion dynamics, models of marine notch development and have implications for the use of marine notches as indicators of changes in sea level and the duration of past sea levels. Copyright © 2014 John Wiley & Sons, Ltd.     

Bowhead whales feed on plankton concentrated by estuarine and tidal currents in Academy Bay,Sea of Okhotsk

Academy Bay in the Sea of Okhotsk is an important summertime feeding ground for pelagic-feeding Bowhead whales (Balaena mysticetus) in the western subarctic North Pacific. The present work combines satellite observations with physical (CTD, currents, tides) and biological (zooplankton sampling) measurements. Data obtained aboard the RV Lugovoe in August–September 2003 and July–August 2004 show dense populations of zooplankton (such as copepods Calanus glacialis, Pseudocalanus sp., pteropods Limacina helicina, and chaetognaths Sagitta sp.) that are concentrated by physical mechanisms within this critically important ecosystem. We show that near-bottom, cold water intrusions from the northern Sea of Okhotsk associated with the estuarine circulation advect arctic herbivorous calanoids (Calanus glacialis) and mollusks Limacina helicina into the region.     

The effect of bidirectional flow on tidal channel planforms

Salt marsh tidal channels are highly sinuous. For this project, ?eld surveys and aerial photographs were used to characterize the planform of tidal channels at China Camp Marsh in the San Francisco Bay, California. To model the planform evolution, we assume that the topographic curvature of the channel centreline is a key element driving meander migration. Extraction of curvature data from a planimetric survey, however, presents certain problems because simple calculations based on equally distanced points on the channel axis produce numerical noise that pollutes the ?nal curvature data. We found that a spline interpolation and a polynomial ?t to the survey data provided us with a robust means of calculating channel curvature. The curvature calculations, combined with data from numerous cross‐sections along the tidal channel, were used to parameterize a computer model. With this model, based on recent theoretical work, the relationship between planform shape and meander migration as well as the consequences of bidirectional ?ow on planform evolution have been investigated. Bank failure in vegetated salt marsh channels is characterized by slump blocks that persist in the channel for several years. It is therefore possible to identify reaches of active bank erosion and test model predictions. Our results suggest that the geometry and evolution of meanders at China Camp Marsh, California, re?ect the ebb‐dominated regime. Copyright © 2004 John Wiley & Sons, Ltd.     

A demonstration of the importance of bedload transport for fluvial bedrock erosion and knickpoint propagation

We provide field evidence for the role of bedload in driving fluvial incision and knickpoint propagation. Using aerial photographs, field surveys, and hydrological data, we constrain the incision history of a bedrock gorge 1200 m long and up to 20 m deep cut by Da'an River in western Taiwan. This reach of the river experienced 10 m of uplift during the 1999 Chi‐Chi earthquake. For five years following the earthquake, bedload was prevented from entering the uplift zone, the knickpoint was static and little incision took place. Bedload transport across the uplift zone resumed in 2004, initiating extremely rapid incision, with 620 m of knickpoint propagation and up to 20 m of downcutting by 2008. This change highlights the relative inefficiency of suspended sediment and the dominant role of bedload as a tool for fluvial erosion and knickpoint propagation. Once bedload tools became available, knickpoint propagation was influenced by geological structure, lithology, and drainage organization. In particular, a change in dip of the sandstone beds at the site caused a decrease of knickpoint propagation velocity. Copyright © 2012 John Wiley & Sons, Ltd.     

Excavation of subglacial bedrock channels by seasonal meltwater flow

Subglacial water flow drives the excavation of a variety of bedrock channels including tunnel valleys and inner gorges. Subglacial floods of various magnitudes – events occurring once per year or less frequently with discharges larger than a few hundred cubic metres per second – are often invoked to explain the erosive power of subglacial water flow. In this study we examine whether subglacial floods are necessary to carve bedrock channels, or if more frequent melt season events (e.g. daily production of meltwater) can explain the formation of substantial bedrock channels over a glacial cycle. We use a one‐dimensional numerical model of bedrock erosion by subglacial meltwater, where water flows through interacting distributed and channelized drainage systems. The shear stresses produced drive bedrock erosion by bed‐ and suspended‐load abrasion. We show that seasonal meltwater discharge can incise an incipient bedrock channel a few tens of centimetres deep and several metres wide, assuming abrasion is the only mechanism of erosion, a particle size of D=256 mm and a prescribed sediment supply per unit width. Using the same sediment characteristics, flood flows yield wider but significantly shallower bedrock channels than seasonal meltwater flows. Furthermore, the smaller the shear stresses produced by a flood, the deeper the bedrock channel. Shear stresses produced by seasonal meltwater are sufficient to readily transport boulders as bedload. Larger flows produce greater shear stresses and the sediment is carried in suspension, which produces fewer contacts with the bed and less erosion. We demonstrate that seasonal meltwater discharge can excavate bedrock volumes commensurate with channels several tens of metres to a few hundred metres wide and several tens of metres deep over several thousand years. Such simulated channels are commensurate with published observations of tunnel valleys and inner gorges. Copyright © 2018 John Wiley & Sons, Ltd.