bluuebunny/crossref_metadata_2025_split huggingface dataset:
Flow competence: A criticism of a classic concept
Authors: Wilcock Peter R. 
Published: February 2007

AbstractThe largest grains found in samples of transported sediment are commonly used to estimate flow competence. With samples from a range of flows, a relationship between the flow and the largest mobile grain can be derived and used to estimate the critical shear stress for incipient motion of the different grain sizes in the bed sediment or, inversely, to estimate the magnitude of the flow from the largest grain found in a transport sample. Because these estimates are based on an extreme value of the transport grain‐size distribution, however, they are subject to large errors and are sensitive to the effect of sample size, which tends to vary widely in sediment transport samples from natural flows. Furthermore, estimates of the critical shear stress based on the largest sampled moving grain cannot be scaled in a manner that permits reasonable comparison between fractions. The degree to which sample size and scaling problems make largest‐grain estimates of fractional critical shear stress deviate from a true relationship cannot be predicted exactly, although the direction of such a deviation can be demonstrated. The large errors and unknown bias suggest that the largest sampled mobile grain is not a reliable predictor of either critical shear stress or flow magnitude. It is possible to define a single flow competence for the entire mixture, based on a central value of the transport grain‐size distribution. Such a measure is relatively stable, does not require between‐fraction scaling, and appears to be well supported by observation.
Long‐term drainage evolution in the shoalhaven catchment, southeast highlands, Australia
Authors: Nott Jonathan F. 
Published: February 2007

AbstractThe long‐term evolution of streams in the Shoalhaven catchment of southeast New South Wales has been a contentious issue for decades. Several authors have suggested that the Shoalhaven River was captured at the sharp eastward bend near Tallong: this has been used as evidence for the westward migration of the east Australian divide in this area. Other workers, however, have argued that capture did not occur and that the location of the divide has been stable throughout the Tertiary. A vast sheet of sediments which spread across and infill a palaeovalley network cut into a broad undulating plain in the middle Shoalhaven catchment provides a record of stream behaviour since at least the start of the Tertiary. This record shows that the Shoalhaven River and many of its tributaries have maintained almost the same courses since at least the very Early Tertiary. This provides strong evidence against the capture hypothesis. The record further suggests that during the Paleogene these streams were graded to a level within the southeast Australian highlands; their depths of incision thus cannot be used as evidence for the extent of uplift of the southeast Australian highlands during this time.
Timescales and the role of inheritance in long‐term landscape evolution, northern New England, Australia
Authors: Fried Alan W. , Smith Nicola 
Published: February 2007

AbstractLandscape evolution in northern New England is characterized by the persistence of landforms over long time periods. The topography in the study area was initiated by Paleozoic diapiric intrusion of granitic rocks as highs within metasedimentary and volcanic rocks. This framework was probably reinforced by differences in the resistance of the massive igneous and the fractured metasedimentary rocks to erosion. As a result, both the Tertiary and present‐day relief of the area consists of uplands developed on granitic rocks partly overlain by Paleozoic volcanic rocks, and lowlands developed on Paleozoic metasediments. Extensive volcanism in the Tertiary had only a minimal effect on this relationship. A significant impact of Tertiary volcanism, however, was the diversion of major streams across topographic highs. Despite 19‐35 Ma of incision, these streams are still far from establishing equilibrium profiles and dissecting the highs.The persistence of topography for a timescale of the same order of magnitude as that conventionally thought to be required for planation is due to reduced denudation rates in a passive tectonic setting and the presence of erosion‐resistant Paleozoic igneous units. The latter influence is indicated by knickpoint persistence for 19‐35 Ma on streams diverted across these rocks. Since it is generally considered that such disequilibria are short‐lived features formed by Quaternary or ongoing deformation, this knickpoint persistence has important implications for landscape analyses that use disequilibria to assess recent tectonism in a landscape.
Design and installation of a novel automatic erosion monitoring system
Authors: Lawler D. M. 
Published: February 2007

AbstractProcess inference in geomorphology is hindered by a lack of information on the true temporal distribution of contemporary erosional and depositional activity. To tackle this problem a low‐cost, automatic monitoring system based on the photo‐electronic erosion pin (PEEP) sensor has been developed. The PEEP is essentially an array of photosensitive cells enclosed within a transparent tube and connected by cable to a datalogger. When inserted into an eroding landform, subsequent retreat of the face exposes more photosensitive cells to light which increases PEEP voltage output. Deposition decreases sensor outputs. The logged signals thus reveal the magnitude, timing and frequency of erosion and deposition events with much greater precision than has hitherto been possible. Measurement principles, electronic and physical design, calibration, field installation, problems and prospects and pilot results from a river bank erosion site are discussed. The PEEP system appears to have great potential for disentangling competing hypotheses in geomorphological process studies, and in building and testing erosion and sediment transport models of high temporal resolution.
On the mathematical description of glaciated valley cross sections
Authors: Harbor Jonathan M. , Wheeler Dennis A. 
Published: February 2007

AbstractMorphometric analyses of glaciated valleys typically involve attempts to fit empirical data from valley cross‐profiles to quadratic (parabolic) or power‐law equations, with the choice of equation depending on the goal of the analysis. In assessing the relative merits of these two types of equations, some confusion has arisen because of apparent variations in coordinate system datums between studies which have made use of the power‐law equation. However, we show that this confusion relates to a simple mathematical error, rather than any true difference in methodology. Of more significance, however, is the fact that in fitting a power‐law equation to observed profiles, significant bias is introduced by the use of a logarithmic data transformation. Because of this implicit bias, power‐law equation parameters are influenced most strongly by data points close in elevation to the centre of the valley, and so failure to account for the depositional elements which frequently occur in these areas can lead to significant errors if the aim of the analysis is to characterize the form of the erosional profile.
A 3‐d model of quaternary terrace development, simulations of terrace stratigraphy and valley asymmetry: A case study for the allier terraces (limagne, France)
Authors: Veldkamp A. 
Published: February 2007

AbstractThe combined effects of climate and tectonism on general terrace stratigraphy and valley asymmetry during the last half million years in the Allier system (France) are simulated by a 3‐D conceptual model (LIMTER). This model allows the formulation and evaluation of long term terrace formation scenarios for the Allier system. Simulation results suggest that terrace stratigraphy in the study area is mainly the result of internal dynamics and climatic change. Local tectonism contributed to the development of unpaired terraces while the general regional uplift played a dominant role in determining terrace formation and preservation in general.
The influence of microform bed roughness elements on flow and sediment transport in gravel bed rivers: A reply
Authors: Reid Ian , Hassan Marwan A. 
Published: February 2007

AbstractClifford et al. (1992) argue that differences in hydraulic conditions at the moment of sediment entrainment in Turkey Brook, flood by flood, are attributable to variations in the form drag that arises from cluster microforms. While not denying the possibility of variable cluster concentration, we show that the time‐dependent bed consolidation and cementation that arises as a function of interstitial matrix development can also account for variation in the traction threshold. Clifford et al. further postulate that the growth of pebble clusters just prior to bedload entrainment is a cause of the observed differences in traction thresholds at the start and finish of bedload during a flood event. This merits further examination. However, we point out that clusters are not only observed at low flow and, therefore, already present and able to contribute significant form drag as the next bedload generating flood waxes, but that they are known both to survive more or less intact and to entrap mobile particles during bed disturbing events. The revelation by Clifford et al. that flow structure in the River Quarme reflects differences in bed rugosity between riffles and pools and that it relates in scale to bed roughness elements (perhaps clusters) is not surprising. There is, however, some difficulty in distinguishing the precedence of chicken and egg when using this data set to elucidate the cause of cluster spacing. Nevertheless, the inference that flow and form are dynamically linked is in sympathy with our own conclusions that cluster spacing is an equilibrium condition governed by the interplay of flow resistance and sediment transport.
Differential bedload transport rates in a gravel‐bed stream: A grain‐size distribution approach
Authors: Diplas Panayiotis 
Published: February 2007

AbstractTwo methods that were recently proposed for calculating fractional bedload transport rates in gravel‐bed streams are examined closely. Both of them employ the Oak Creek bedload data. The Diplas (PD) method is guided by dimensional analysis and, therefore, can be used to predict bedload transport in different gravel rivers. The only requirement for using this method is the knowledge of the subsurface material size distribution of the stream of interest. The expression for the fractional bedload transport obtained by the Shih and Komar (SK) method for Oak Creek cannot be used for other streams. Its use for a given stream requires information that is rarely available. For the Oak Creek case both methods demonstrate similar predictive ability.
A reply to [a discussion of shih and Komar's (1990) analysis of differential bedload transport rates in a gravel‐bed stream: A grain‐size distribution approach] by P. Diplas
Authors: Komar Paul D. , Shih Shyuer‐Ming 
Published: February 2007

AbstractSeveral points have been raised by Diplas concerning our analyses in Shih and Komar (1990) of bedload transport in gravel‐bed streams, and our comparisons with the approach developed by Diplas (1987) as a modification of the dimensional analysis first undertaken by Parker et al. (1982). We disagree with Diplas' assessment that our approach [cannot be used for other streams] and that dimensional analysis necessarily expands the range of application of his analysis approach. We argue here that the dimensional analysis of Diplas obscures the relationship between the transport rate and grain size, and in fact leads to a complex and implausible dependence. In contrast, our approach allows one to better focus on the changing grain‐size distributions as the flow discharge and mean bottom stress increase.
Relationships between deflation and near surface wind velocity in a coastal dune blowout
Authors: Pluis J. L. A. 
Published: February 2007

AbstractDetailed field measurements were made of the degree of surface level change in a blowout, situated in the coastal dune area of Meijendel, The Netherlands. A formula was established to express the erosivity of the wind for the corresponding 34 measurement intervals, over a period of about 3 years. Having established, for 12 wind sectors, the relationship between wind velocity at the nearest standard weather station and at five locations in the blowout, correlations were derived between the deflation rate in the blowout and the wind erosivity using standard hourly wind data.The winter season, although the most windy, is by far the least effective season: the threshold shear wind velocity is disproportionally increased because the moisture content of the surface sand is high. The soil moisture conditions are described as a function of the daily precipitation and evaporation rate. The two most relevant upper and lower threshold shear wind velocities are determined empirically. During the summer season, deflation rate is even higher than indicated by the net surface level change because deflation is partly compensated by deposition of sand transported by rainwash from the blowout margin.With regard to spatial variation in the deflation rate within the blowout, it appears that locations with relatively high velocities coincide with higher deflation rates. However, the spatial variation is commonly less than is expected from the horizontal wind velocity distribution. In view of this the role of wind direction, blowout morphology and algal coverage of the soil in the deflation rate is discussed.
An approximation of the sediment budget of the lower mississippi river prior to major human modification
Authors: Kesel Richard H. , Yodis Elaine G. , McCraw David J. 
Published: February 2007

AbstractA sediment budget is constructed for the Lower Mississippi River prior to the introduction of most human modifications. Components of the budget are derived from historical data including Mississippi River Commission hydrographic survey maps for the period from 1877 to 1924. A quantitative estimate of major sediment sources, sinks and storage locations in the Lower Mississippi Valley suggest that the most important sediment source was from bank caving and that at least 50 per cent more sediment was deposited in short‐term channel storage than in longer‐term overbank storage. The budget indicates that the proportion of sediment transported as bedload was significantly greater than previously estimated. The magnitude and variability of these sediment budget components are discussed. The results of the study suggest that prior to modifications the river was aggrading over much of its length.
Post‐eruption erosion and deposition in the 1980 crater of Mount St Helens, Washington, determined from digital maps
Authors: Mills Hugh H. 
Published: February 2007

AbstractThe crater of Mount St Helens shows one of the world's highest known rates of mass wasting. On many summer days, rockfall is almost continuous, and many large rock and dirty‐snow avalanches have travelled several kilometres from their sources on the crater walls. Since formation of the crater on 18 May 1980, talus cones exceeding 100 m in thickness have formed at the base of the unstable 600 m high crater walls. To estimate rates of erosion and deposition, a series of digitized topographic maps made from aerial photographs taken of the crater in 1980, 1981, 1983, 1986 and 1988 were analysed using a geographic information system. Between 1980 and 1988, 30 × 106 m3 of rock were eroded from the crater wall, representing a mean retreat rate of 2.1 m yr−1. To account for the volume increase that occurs when bedrock is transformed into scree, this volume is multiplied by 4/3; this provides an estimate of the rock‐debris volume supplied to the crater floor of 40 × 106 m3. The actual volume of deposits that accumulated during this 8 year period, however, is 68 × 106 m3. The difference of 28 × 106 m3 is presumably the volume of snow intercalated between insulating layers of rock debris. Similar calculations for each of four time intervals between 1980 and 1988 suggest that wall erosion and thus talus accumulation rates are declining, but that rates will probably remain high for decades to come.
A numerical model for shore‐normal sediment size variation on a macrotidal beach
Authors: Horn Diane P. 
Published: February 2007

AbstractThis paper describes a computer simulation model which is designed to predict the selective shore‐normal sorting of grain sizes in the nearshore environment. The model simulates wave shoaling, wave height attenuation due to frictional losses and breaking, using linear theory up to the break point and a breaker decay model in the surf zone. Peak horizontal orbital velocities at the bed are calculated from Stokes second‐order wave theory. The peak onshore and offshore velocities are used with the threshold expression of Komar and Miller (1975) to generate a spatial pattern of size variation of threshold grain diameter along a profile normal to the shore from deep water to the swash zone. The predicted grain size is used in an hydraulic interpretation of grain size distribution on the intertidal profile, based on the hydrodynamic variations over a tidal cycle on a macrotidal beach. The model is successful in predicting the broad pattern of increasing grain size in the onshore direction which has been observed in nature. Comparisons between measured and predicted grain size distributions indicate that the predictions of the model are better than those of previous models, but the model is more successful at predicting sediment size distributions than at predicting mean sizes on a beach profile.
Post‐deforestation soil loss from steepland hillslopes in Taranaki, New Zealand
Authors: Derose R. C. , Trustrum N. A. , Blaschke P. M. 
Published: February 2007

AbstractSoil erosion on steepland hillslopes in Taranaki, New Zealand, where landsliding is the dominant erosion form, was investigated by comparing mean regolith depths between first‐order basins that have had their forest cover removed for different periods of time. Regolith depth and slope angle data were collected along 19 profile lines and 30 profile lines from steepland basins that had been deforested for 10 and 85 years, respectively. These profile lines were subdivided into a total of 236 profile segments of relatively linear slope angle and uniform regolith depth, that averaged 17·5 m in length.The depth of pre‐existing regolith on post‐deforestation landslide sites is estimated from a regression of regolith depth on slope angle for undisturbed (non‐landslide) profile segments. Regolith depletion on landslide sites is in turn estimated by subtracting the depth of regolith on landslide sites from the estimate of pre‐existing regolith depth. Regolith depletion by post‐deforestation landslides, averaged over the entire length of profile lines, gives an estimate of average surface lowering. For the area deforested for 85 years, average surface lowering by post‐deforestation landslides is 0·15 ± 0·04 m, and is the same as the difference in mean depth of 0·15 ± 0·11 m between this area and the area deforested for 10 years. Erosion of regolith from hillslopes by processes other than landsliding appears to be minimal. The 0·15 m average surface lowering represents a regolith depletion rate of 1·8 ± 0±5 mm yr−1. For hillslopes steeper than 28°, where all post‐deforestation landslides occur, average surface lowering is 0·20 ± 0·05 m, and the regolith depletion rate is 2±4 · 0±6 mm yr−1. Average surface lowering is greatest at 0·23 ± 0·07 m on hillslopes steeper than 32° where most post‐deforestation landslides occur. Here, the regolith depletion rate is 2·7 ± 0·8 mm yr−1.A large‐magnitude, low‐frequency storm in March 1990, produced an average surface lowering of 0·041 m. There were proportionately more landslides in the area deforested for 10 years, illustrating the importance of previous erosion history of hillslopes on the spatial distribution of landslides. There were also comparatively few landslides on steeper hillslopes because previous lower magnitude storms had already removed much of the deeper regolith.
Monitoring alluvial sand suspension by eddy correlation
Authors: Lapointe Michel 
Published: February 2007

AbstractEddy correlation techniques are standard tools in micrometeorology and oceanography to measure momentum and contaminant transport across turbulent boundary layers. They can, in theory, be used to estimate the net vertical suspended sediment flux directly over different areas of an alluvial channel boundary, and thus disclose ongoing erosion/deposition patterns. The basic principles and main problems in applying the technique to alluvial suspension are first introduced. Results from a trial application of the method in a large sand bed river are then presented; the focus of the analysis is on the substantial (and surprising) contributions of multi‐minute flow fluctuations to suspension work in the study environment.The data were collected in a 10 m deep channel of the Fraser River near Mission, British Columbia, Canada. Turbulent fluctuations of flow components streamwise and normal to the bed, along with the output of an optical suspended sediment sensor, were monitored over 7 h, 1 m above the bed. Flow velocities averaged 0·9 ms−1 and mean suspended sediment concentrations 500 mgl−1, at sensor level above 1–5 dm high dunes.Spectral analysis of the records reveals that approximately 30 per cent of the vertical suspended sand mixing across the sensor level (and roughly as much of the momentum exchange) was linked to gradual flow oscillations with periods between 1 and 13·6 min (underlying briefer, turbulent fluctuations). Extended periods of sediment‐rich, slightly upward directed but slower mean flow alternated with periods of sediment‐poor, slightly downward and faster mean flow; these slow fluctuations involved 10–20 cms−1 changes in 5 min average flow speed, 2–4· changes in vertical flow angle and 100 mgl−1 changes in mean sand concentration. To obtain accurate eddy‐correlation estimates of the vertical suspension flux in the study conditions, hour‐scale flow and turbidity records that include many of these multi‐minute cycles appear to be necessary.The spectra of the Fraser River near‐bed signals do not conspicuously differ in overall shape (in terms of low‐frequency content and location of peak) from turbulent spectra encountered in some atmospheric boundary layers. Nonetheless, the long period fluctuations observed on the Fraser River may not be turbulent; rather they may reflect slowly evolving perturbations in the near‐bed streamlines, caused by bedform translation or gradual fluctuation within the large‐scale streamwise cells of the secondary flow.
Indicators of faulting events and periods of quiescence in desert alluvial fans
Authors: Gerson R. , Grossman S. , Amit R. , Greenbaum N. 
Published: February 2007

AbstractThe main objective of the present paper is to present an integrated approach to the study of faulting stages in coarse alluvial deposits. The study site is the alluvial fan of Nahal Shehoret, presently under an extremely arid regime. The exposure of fault scarps on the fan surfaces enables the identification of several geomorphic indicators employed in analysing and separating faulting events and periods of quiescence: (a) the nature of fault scarp topography; (b) soil catenary development on fault scarps; (c) buried paleosols in alluvial deposits; (d) colluvial bodies; (e) rotated clasts along fault planes. These indicators enable us to determine, at all sites studied, the total amount of displacement, to separate faulting events and the amount of their displacement per event, and to evaluate scarp stability and the duration of periods of tectonic quiescence.