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1.
Relationships between riverbed morphology, concavity, rock type and rock uplift rate are examined to independently unravel the contribution of along-strike variations in lithology and rates of vertical deformation to the topographic relief of the Oregon coastal mountains. Lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology. Volcanic and sedimentary rocks are the principal rock types underlying the northern Oregon Coast Ranges (between 46°30′ and 45°N) where mixed bedrock–alluvial channels dominate. Average concavity, θ, is 0·57 in this region. In the alluviated central Oregon Coast Ranges (between 45° and 44°N) values of concavity are, on average, the highest (θ = 0·82). South of 44°N, however, bedrock channels are common and θ = 0·73. Mixed bedrock–alluvial channels characterize rivers in the Klamath Mountains (from 43°N south; θ = 0·64). Rock uplift rates of ≥0·5 mm a−1, mixed bedrock–alluvial channels, and concavities of 0·53–0·70 occur within the northernmost Coast Ranges and Klamath Mountains. For rivers flowing over volcanic rocks θ = 0·53, and θ = 0·72 for reaches crossing sedimentary rocks. Whereas channel type and concavity generally co-vary with lithology along much of the range, rivers between 44·5° and 43°N do not follow these trends. Concavities are generally greater than 0·70, alluvial channels are common, and river profiles lack knickpoints between 44·5° and 44°N, despite the fact that lithology is arguably invariant. Moreover, rock uplift rates in this region vary from low, ≤0·5 mm a−1, to subsidence (<0 mm a−1). These observations are consistent with models of transient river response to a decrease in uplift rate. Conversely, the rivers between 44° and 43°N have similar concavities and flow on the same mapped bedrock unit as the central region, but have bedrock channels and irregular longitudinal profiles, suggesting the river profiles reflect a transient response to an increase in uplift rate. If changes in rock uplift rate explain the differences in river profile form and morphology, it is unlikely that rock uplift and erosion are in steady state in the Oregon coastal mountains. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
2.
An integral approach to bedrock river profile analysis 总被引：5，自引：0，他引：5
Bedrock river profiles are often interpreted with the aid of slope–area analysis, but noisy topographic data make such interpretations challenging. We present an alternative approach based on an integration of the steady‐state form of the stream power equation. The main component of this approach is a transformation of the horizontal coordinate that converts a steady‐state river profile into a straight line with a slope that is simply related to the ratio of the uplift rate to the erodibility. The transformed profiles, called chi plots, have other useful properties, including co‐linearity of steady‐state tributaries with their main stem and the ease of identifying transient erosional signals. We illustrate these applications with analyses of river profiles extracted from digital topographic datasets. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
3.
Controls on knickpoint migration in a drainage network of the moderately uplifted Ardennes Plateau,Western Europe 
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下载免费PDF全文 Much research has been devoted to the development of numerical models of river incision. In settings where bedrock channel erosion prevails, numerous studies have used field data to calibrate the widely acknowledged stream power model of incision and to discuss the impact of variables that do not appear explicitly in the model's simplest form. However, most studies have been conducted in areas of active tectonics, displaying a clear geomorphic response to the tectonic signal. Here, we analyze the traces left in the drainage network 0.7 My after the Ardennes region (western Europe) underwent a moderate 100–150 m uplift. We identify a set of knickpoints that have traveled far upstream in the Ourthe catchment, following this tectonic perturbation. Using a misfit function based on time residuals, our best fit of the stream power model parameters yields m = 0.75 and K = 4.63 × 10‐8 m‐0.5y‐1. Linear regression of the model time residuals against quantitative expressions of bedrock resistance to erosion shows that this variable does not correlate significantly with the residuals. By contrast, proxies for position in the drainage system prove to be able to explain 76% of the residual variance. High time residuals correlate with knickpoint position in small tributaries located in the downstream part of the Ourthe catchment, where some threshold was reached very early in the catchment's incision history. Removing the knickpoints stopped at such thresholds from the data set, we calculate an improved m = 0.68 and derive a scaling exponent of channel width against drainage area of 0.32, consistent with the average value compiled by Lague for steady state incising bedrock rivers. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
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Complexities of landscape evolution during incision through layered stratigraphy with contrasts in rock strength 总被引：1，自引：0，他引：1 下载免费PDF全文
下载免费PDF全文 Variation in the erodibility of rock units has long been recognized as an important determinant of landscape evolution but has been little studied in landscape evolution models. We use a modified version of the Channel‐Hillslope Integrated Landscape Development (CHILD) model, which explicitly allows for variations in rock strength, to reveal and explore the remarkably rich, complex behavior induced by rock erodibility variations in even very simple geologic settings with invariant climate and tectonics. We study the importance of relative contrasts in erodibility between just two units, the order of these units (whether hard rocks overlie soft or soft rocks overlie hard) and the orientation of the contact between the two units. We emphasize the spatial and temporal evolution of erosion rates, which have important implications for basin analysis, detrital mineral records, and the interpretation of cosmogenic isotope concentrations in detrital samples. Results of the landscape evolution modeling indicate that the stratigraphic order of units in terms of erodibility, the gross orientation of the contact (i.e. dipping away or toward the outlet of the landscape) and the contact dip angle all have measurable effects on landscape evolution, including significant spatial and temporal variations in erosion rates. Steady‐state denudation conditions are unlikely to develop in landscapes with significant contrasts in rock strength in horizontal to moderately tilted rock layers, at least at the scale of the entire landscape. Additionally, our results demonstrate that there is no general relation between rock erodibility and erosion rates in natural settings. Although rock erodibility directly controls the erosion rate constant in our models, it is not uncommon for higher erosion rates to occur in the harder, less erodible rock. Indeed erosion rates may be either greater or less than the rock uplift rate (invariant in time and space in our models) in both hard and soft rocks, depending on the local geology, topography, and the pattern of landscape evolution. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
6.
ACTIVITY CHARACTERISTICS OF THE HUASHAN PIEDMONT NORMAL FAULT: INSIGHTS FROM FLUVIAL GEOMORPHIC PARAMETERS 下载免费PDF全文
下载免费PDF全文 The Huashan piedmont fault, forming a part of the southern margin of the Weihe graben, is one of the important normal faults that control the subsidence of the intracontinental rift. Developing on the footwall of the fault, the Huashan block has experienced rapid cooling during the Cenozoic, especially since the early-middle Miocene. Mountain exhumation causes and transports a great amount of sediments to the adjacent hanging wall, setting a typical case of mountain-basin coupling system. Studies on active tectonics, historical and paleo earthquakes and field investigations reveal that the middle section(Huaxian-Huayin)of the fault is much more active than the west(Lantian-Huaxian)and east(Huayin-Lingbao)sections.
We extracted channel profiles of rivers that originate from the main water divide of the northern flank of the Huashan Mountain. Based on the method of slope-area analysis and the integral approach, we identified knickpoints, calculated channel concavity and steepness indices, and constructed paleo river profiles. Of most rivers, the concavities are within a relatively narrow range of 0.3~0.6, with no obvious correlation with tectonics. However, channel steepness and knickpoint distribution vary spatially. In the east section, rivers are under steady-state with smooth, concave-up channels and lower steepness((104±30)m0.9). In the other two sections, rivers are mainly under transient state with slope-break knickpoints. For the channel segments below knickpoints, steepness indices are much higher in the middle section((230±92)m0.9)than in the west((152±53)m0.9). Thus, the variance of fault activity can be reflected by channel steepness pattern. Above the knickpoints, channel steepness indices are much lower(middle(103±23)m0.9, west(60±14)m0.9). What's more, we found a statistically significant power-law scaling between knickpoint retreat distance and catchment drainage area. Thus, we attributed these knickpoints to be the results of recent rapid uplift of the Huashan block. The relief of paleo channels(middle(1000±153)m, west(751±170)m)accounts for~60%~80% of the relief of modern rivers(middle(1323±249)m, west(1057±231)m), which means that ~20%~40% of modern channel relief was caused by the episode of the rapid uplift. Assuming a balance between the rates of rock uplift and downstream river incision, a power-law function between uplift rates and channel steepness can be derived. According to the fault throw rates of the middle section 1.5~3mm/a(since late Pleistocene), we constrained slope exponent n~0.5 and channel erodibility K~1.5×10-4m0.55/a. Combining the knickpoint age formula, we estimated that the rapid mountain uplift/fault throw began at ~(0.55±0.25)Ma BP. Therefore, the middle of the Huashan piedmont fault is more active than the west and east sections. The fast fault throw of the west and middle sections since the middle Pleistocene has caused rapid mountain uplift and high topographic relief. 相似文献
We extracted channel profiles of rivers that originate from the main water divide of the northern flank of the Huashan Mountain. Based on the method of slope-area analysis and the integral approach, we identified knickpoints, calculated channel concavity and steepness indices, and constructed paleo river profiles. Of most rivers, the concavities are within a relatively narrow range of 0.3~0.6, with no obvious correlation with tectonics. However, channel steepness and knickpoint distribution vary spatially. In the east section, rivers are under steady-state with smooth, concave-up channels and lower steepness((104±30)m0.9). In the other two sections, rivers are mainly under transient state with slope-break knickpoints. For the channel segments below knickpoints, steepness indices are much higher in the middle section((230±92)m0.9)than in the west((152±53)m0.9). Thus, the variance of fault activity can be reflected by channel steepness pattern. Above the knickpoints, channel steepness indices are much lower(middle(103±23)m0.9, west(60±14)m0.9). What's more, we found a statistically significant power-law scaling between knickpoint retreat distance and catchment drainage area. Thus, we attributed these knickpoints to be the results of recent rapid uplift of the Huashan block. The relief of paleo channels(middle(1000±153)m, west(751±170)m)accounts for~60%~80% of the relief of modern rivers(middle(1323±249)m, west(1057±231)m), which means that ~20%~40% of modern channel relief was caused by the episode of the rapid uplift. Assuming a balance between the rates of rock uplift and downstream river incision, a power-law function between uplift rates and channel steepness can be derived. According to the fault throw rates of the middle section 1.5~3mm/a(since late Pleistocene), we constrained slope exponent n~0.5 and channel erodibility K~1.5×10-4m0.55/a. Combining the knickpoint age formula, we estimated that the rapid mountain uplift/fault throw began at ~(0.55±0.25)Ma BP. Therefore, the middle of the Huashan piedmont fault is more active than the west and east sections. The fast fault throw of the west and middle sections since the middle Pleistocene has caused rapid mountain uplift and high topographic relief. 相似文献
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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. 相似文献
9.
Jeffrey D. Niemann Nicole M. Gasparini Gregory E. Tucker Rafael L. Bras 《地球表面变化过程与地形》2001,26(12):1317-1332
Playfair's law (J. Playfair, illustrations of the Huttonian Theory of the Earth, 1802) requires any two tributaries in a river network to lower at the same rate near their junction. Although this law holds exactly at the junction, it is unclear how well it holds in the vicinity of the junction. This issue has practical importance because Playfair's law has been used to estimate parameters for detachment‐limited models of erosion. If the incision rate of a stream is modelled as βAmSn, where β is an erodibility parameter, A is the area drained by the stream, and S is the local gradient of the channel, then the ratio of the parameters m/n can be estimated from junctions by assuming that Playfair's law holds over the distance used to determine S for each tributary. In this paper, Playfair's law and associated m/n estimates are evaluated for simulated basins with constant and temporally varying uplift rates (or baselevel lowering rates). The results demonstrate that estimates of m/n may be biased for basins with upward‐concave stream profiles because the local slope must be approximated with an average upstream slope. In addition, when uplift rate varies temporally, knickpoints are shown to travel through the basins with constant vertical velocity. Because incision rates vary within the basin, Playfair's law only holds exactly at the junctions. These effects are more important when slopes are measured over longer distances. Finally, measurement techniques are presented which address these potential biases. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
10.
Stream profile analysis,tectonic geomorphology and neotectonic activity of the Damxung‐Yangbajain rift in the south Tibetan Plateau 下载免费PDF全文
下载免费PDF全文 The Damxung‐Yangbajain rift is one of the most active north–south trending rifts in the south Tibetan Plateau, and it has been playing an important role in accommodating the east–west extension of the Tibetan Plateau. Both stream profiles on the Nyainqentanglha Range adjacent to the northwest part of the Damxung‐Yangbajain rift and tectonic geomorphology in the north of the rift are analyzed to assess the spatial pattern and intensity of rock uplift which is related to neotectonic activity. A total of 85 stream profiles across the Nyainqentanglha Range are analyzed, and 111 knickpoints are interpreted. Most of these stream profiles are characterized by prominent convexities with two or more knickpoints, many of which are formed due to the strong rock uplift evidenced by abnormal concavity and extremely high steepness indices during the Quaternary. Neotectonic activity in this region is well replicated in the stream profile indices and offset landforms. Tectono‐geomorphic analysis shows that the concavity and steepness indices correlate with the fault movements at many places. The Damxung‐Yangbajain rift is characterized by left‐lateral strike‐slip in the north of Damxung and by normal movement in middle and southern parts. The middle and southern parts have been undergoing higher uplift than has the northern area. It is most likely that the strong uplift is related to the heat flow under the crust. Earthquakes occurring in the Damxung‐Yangbajain rift, including a M8 in 1411 and M6.6 in 2008, are thought to be related to heat flow activity. All of the stream profile indices and tectonic geomorphology show that the Damxung‐Yangbajain rift is not in a stable state. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
11.
The saltation–abrasion model predicts rates of river incision into bedrock as an explicit function of sediment supply, grain size, boundary shear stress and rock strength. Here we use this experimentally calibrated model to explore the controls on river longitudinal profile concavity and relief for the simple but illustrative case of steady‐state topography. Over a wide range of rock uplift rates we find a characteristic downstream trend, in which upstream reaches are close to the threshold of sediment motion with large extents of bedrock exposure in the channel bed, while downstream reaches have higher excess shear stresses and lesser extents of bedrock exposure. Profile concavity is most sensitive to spatial gradients in runoff and the rate of downstream sediment fining. Concavity is also sensitive to the supply rate of coarse sediment, which varies with rock uplift rate and with the fraction of the total sediment load in the bedload size class. Variations in rock strength have little influence on profile concavity. Profile relief is most sensitive to grain size and amount of runoff. Rock uplift rate and rock strength influence relief most strongly for high rates of rock uplift. Analysis of potential covariation of grain size with rock uplift rate and rock strength suggests that the influence of these variables on profile form could occur in large part through their influence on grain size. Similarly, covariation between grain size and the fraction of sediment load in the bedload size class provides another indirect avenue for rock uplift and strength to influence profile form. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
12.
Emmanuel J. Gabet 《地球表面变化过程与地形》2020,45(7):1579-1588
Recent studies provide a theoretical framework for understanding the incision of bedrock rivers by plucking. These studies motivated the development of a numerical model that simulates plucking to explore the evolution of channel profiles in lithologically diverse terrain. In the main governing equation, the incision rate is calculated as a function of the difference between the boundary shear stress and a threshold shear stress needed to entrain blocks from the bed. Because an earlier study suggested that plucking is the primary incisional process in the northern Sierra Nevada (CA), the model was calibrated to approximate the conditions in the region. The profiles of the simulated rivers are stair-stepped, with sharp breaks-in-slope at lithological boundaries. This characteristic is common to rivers draining the northern Sierra Nevada, suggesting that the size of blocks available for plucking, as mediated by the fracture density, may be the primary control on their gradients. Moreover, the numerical experiments highlight the role of threshold shear stresses in the post-orogenic persistence of steep reaches and relict terrain. Finally, comparisons of profiles evolved under tilting or uniform uplift scenarios provide insights into how these different uplift modes affect profile evolution. For example, whereas uniform uplift generates a single migrating knickpoint at the range front, multiple migrating knickpoints can form simultaneously along a river in a tilting landscape. © 2020 John Wiley & Sons, Ltd. 相似文献
13.
Field evidence for the influence of weathering on rock erodibility and channel form in bedrock rivers 总被引：1，自引：0，他引：1 下载免费PDF全文
下载免费PDF全文 Charles M. Shobe Gregory S. Hancock Martha C. Eppes Eric E. Small 《地球表面变化过程与地形》2017,42(13):1997-2012
Erosion processes in bedrock‐floored rivers shape channel cross‐sectional geometry and the broader landscape. However, the influence of weathering on channel slope and geometry is not well understood. Weathering can produce variation in rock erodibility within channel cross‐sections. Recent numerical modeling results suggest that weathering may preferentially weaken rock on channel banks relative to the thalweg, strongly influencing channel form. Here, we present the first quantitative field study of differential weathering across channel cross‐sections. We hypothesize that average cross‐section erosion rate controls the magnitude of this contrast in weathering between the banks and the thalweg. Erosion rate, in turn, is moderated by the extent to which weathering processes increase bedrock erodibility. We test these hypotheses on tributaries to the Potomac River, Virginia, with inferred erosion rates from ~0.1 m/kyr to >0.8 m/kyr, with higher rates in knickpoints spawned by the migratory Great Falls knickzone. We selected nine channel cross‐sections on three tributaries spanning the full range of erosion rates, and at multiple flow heights we measured (1) rock compressive strength using a Schmidt hammer, (2) rock surface roughness using a contour gage combined with automated photograph analysis, and (3) crack density (crack length/area) at three cross‐sections on one channel. All cross‐sections showed significant (p < 0.01 for strength, p < 0.05 for roughness) increases in weathering by at least one metric with height above the thalweg. These results, assuming that the weathered state of rock is a proxy for erodibility, indicate that rock erodibility varies inversely with bedrock inundation frequency. Differences in weathering between the thalweg and the channel margins tend to decrease as inferred erosion rates increase, leading to variations in channel form related to the interplay of weathering and erosion rate. This observation is consistent with numerical modeling that predicts a strong influence of weathering‐related erodibility on channel morphology. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
14.
Sean F. Gallen Karl W. Wegmann Kurt L. Frankel Stephen Hughes Robert Q. Lewis Nathan Lyons Paul Paris Kristen Ross Jennifer B. Bauer Anne C. Witt 《地球表面变化过程与地形》2011,36(9):1254-1267
The southern Appalachians represent a landscape characterized by locally high topographic relief, steep slopes, and frequent mass movement in the absence of significant tectonic forcing for at least the last 200 Ma. The fundamental processes responsible for landscape evolution in a post‐orogenic landscape remain enigmatic. The non‐glaciated Cullasaja River basin of south‐western North Carolina, with uniform lithology, frequent debris flows, and the availability of high‐resolution airborne lidar DEMs, is an ideal natural setting to study landscape evolution in a post‐orogenic landscape through the lens of hillslope–channel coupling. This investigation is limited to channels with upslope contributing areas >2.7 km2, a conservative estimate of the transition from fluvial to debris‐flow dominated channel processes. Values of normalized hypsometry, hypsometric integral, and mean slope vs elevation are used for 14 tributary basins and the Cullasaja basin as a whole to characterize landscape evolution following upstream knickpoint migration. Results highlight the existence of a transient spatial relationship between knickpoints present along the fluvial network of the Cullasaja basin and adjacent hillslopes. Metrics of topography (relief, slope gradient) and hillslope activity (landslide frequency) exhibit significant downstream increases below the current position of major knickpoints. The transient effect of knickpoint‐driven channel incision on basin hillslopes is captured by measuring the relief, mean slope steepness, and mass movement frequency of tributary basins and comparing these results with the distance from major knickpoints along the Cullasaja River. A conceptual model of area–elevation and slope distributions is presented that may be representative of post‐orogenic landscape evolution in analogous geologic settings. Importantly, the model explains how knickpoint migration and channel–hillslope coupling is an important factor in tectonically‐inactive (i.e. post‐orogenic) orogens for the maintenance of significant relief, steep slopes, and weathering‐limited hillslopes. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
15.
Knickpoint recession rate and catchment area: the case of uplifted rivers in Eastern Scotland 总被引：1，自引：0，他引：1
Knickpoint behaviour is a key to understanding both the landscape responses to a base‐level fall and the corresponding sediment fluxes from rejuvenated catchments, and must be accommodated in numerical models of large‐scale landscape evolution. Knickpoint recession in streams draining to glacio‐isostatically uplifted shorelines in eastern Scotland is used to assess whether knickpoint recession is a function of discharge (here represented by its surrogate, catchment area). Knickpoints are identified using DS plots (log slope versus log downstream distance). A statistically significant power relationship is found between distance of headward recession and catchment area. Such knickpoint recession data may be used to determine the values of m and n in the stream power law, E = KAmSn. The data have too many uncertainties, however, to judge definitively whether they are consistent with m = n = 1 (bedrock erosion is proportional to stream power and KPs should be maintained and propagate headwards) or m = 0·3, n = 0·7 (bedrock incision is proportional to shear stress and KPs do not propagate but degrade in place by rotation or replacement). Nonetheless, the E Scotland m and n values point to the dominance of catchment area (discharge) in determining knickpoint retreat rates and are therefore more consistent with the stream power law formulation in which bedrock erosion is proportional to stream power. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
16.
Normal fault growth and linkage in the Gediz (Alaşehir) Graben,Western Turkey,revealed by transient river long‐profiles and slope‐break knickpoints 下载免费PDF全文
下载免费PDF全文 Emiko Kent Sarah J. Boulton Alexander C. Whittaker Iain S. Stewart M. Cihat Alçiçek 《地球表面变化过程与地形》2017,42(5):836-852
The Gediz (Ala?ehir) Graben is located in the highly tectonically active and seismogenic region of Western Turkey. The rivers upstream of the normal fault‐bounded graben each contain a non‐lithologic knickpoint, including those that drain through inferred fault segment boundaries. Knickpoint heights measured vertically from the fault scale with footwall relief and documented fault throw (vertical displacement). Consequently, we deduce these knickpoints were initiated by an increase in slip rate on the basin‐bounding fault, driven by linkage of the three main fault segments of the high‐angle graben bounding fault array. Fault interaction theory and ratios of channel steepness suggest that the slip rate enhancement factor on linkage was a factor of 3. We combine this information with geomorphic and structural constraints to estimate that linkage took place between 0.6 Ma and 1 Ma. Calculated pre‐ and post‐linkage throw rates are 0.6 and 2 mm/yr respectively. Maximum knickpoint retreat rates upstream of the faults range from 4.5 to 28 mm/yr, faster than for similar catchments upstream of normal faults in the Central Apennines and the Hatay Graben of Turkey, and implying a fluvial landscape response time of 1.6 to 2.7 Myr. We explore the relative controls of drainage area and precipitation on these retreat rates, and conclude that while climate variation and fault throw rate partially explain the variations seen, lithology remains a potentially important but poorly characterised variable. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
17.
Rock plucking is the vertical removal of chunks of rock along intersection joint planes by flowing water. The plucking of durable jointed rock in a natural stream alters stream morphology and potentially damages a bridge substructure situated on a rock foundation by local erosion. Despite its importance, the mechanics of bedrock erosion by plucking are not well understood. In this study, we conduct laboratory experiments to investigate the threshold of block entrainment through plucking near a flow obstruction under sub-critical conditions. In a laboratory flume, we reproduce a wide range of typical lateral flow contraction scenarios that occur around a large rock or bridge substructure and investigate the process of the vertical entrainment of blocks near the obstruction. Two different modes of block removal process (impulsive plucking and accumulative plucking) are observed near the bluff structure. We find that under severe flow contraction, blocks are more likely to escape quickly via impulsive plucking because higher flow contraction and the resulting strong local turbulence around the structure generate large instantaneous uplift force that removes the block within a short period of time; however, we observe accumulative plucking under weaker flow contraction near the bluff structure. From the experimental results, we develop a formula for the threshold point of block dislocation caused by plucking with respect to the degree of lateral flow contraction. The results indicate that increasing the flow contraction ratio reduces block stability because of higher flow acceleration and the increased turbulence effect near the upstream edge of the obstruction. The results of this study should be useful to those who estimate the rate of rock erosion by plucking in natural open channels around a bluff structure. 相似文献
18.
Wouter van Gorp Arnaud J.A.M. Temme Jantiene E.M. Baartman Jeroen M. Schoorl 《地球表面变化过程与地形》2014,39(12):1587-1600
Natural damming of upland river systems, such as landslide or lava damming, occurs worldwide. Many dams fail shortly after their creation, while other dams are long‐lived and therefore have a long‐term impact on fluvial and landscape evolution. This long‐term impact is still poorly understood and landscape evolution modelling (LEM) can increase our understanding of different aspects of this response. Our objective was to simulate fluvial response to damming, by monitoring sediment redistribution and river profile evolution for a range of geomorphic settings. We used LEM LAPSUS, which calculates runoff erosion and deposition and can deal with non‐spurious sinks, such as dam‐impounded areas. Because fluvial dynamics under detachment‐limited and transport‐limited conditions are different, we mimicked these conditions using low and high erodibility settings, respectively. To compare the relative impact of different dam types, we evaluated five scenarios for each landscape condition: one scenario without a dam and four scenarios with dams of increasing erodibility. Results showed that dam‐related sediment storage persisted at least until 15 000 years for all dam scenarios. Incision and knickpoint retreat occurred faster in the detachment‐limited landscape than in the transport‐limited landscape. Furthermore, in the transport‐limited landscape, knickpoint persistence decreased with increasing dam erodibility. Stream capture occurred only in the transport‐limited landscape due to a persisting floodplain behind the dam and headward erosion of adjacent channels. Changes in sediment yield variation due to stream captures did occur but cannot be distinguished from other changes in variation of sediment yield. Comparison of the model results with field examples indicates that the model reproduces several key phenomena of damming response in both transport‐limited and detachment‐limited landscapes. We conclude that a damming event which occurred 15 000 years ago can influence present‐day sediment yield, profile evolution and stream patterns. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
19.
Sara Jakica Mark C. Quigley Mike Sandiford Dan Clark L. Keith Fifield Abaz Alimanovic 《地球表面变化过程与地形》2011,36(4):449-459
The ～900 km long Darling Scarp in Western Australia is one of the most prominent linear topographic features on Earth. Despite the presence of over‐steepened reaches in all westerly flowing streams crossing the scarp, and significant seismic activity within 100 km of the scarp, there is no historical seismicity and no reported evidence for Quaternary tectonic displacements on the underlying Darling Fault. Consequently, it is unclear whether the scarp is a rapidly evolving landform responding to recent tectonic and/or climatic forcing or a more slowly evolving landform. In order to quantify late Quaternary rates of erosion and scarp relief processes, we obtained measurements of the cosmic‐ray produced nuclide beryllium‐10 (10Be) from outcropping bedrock surfaces along the scarp summit and face, in valley floors, and at stream knickpoints. Erosion rates of bedrock outcrops along the scarp summit surface range from 0·5 to 4·0 m Myr?1. These are in the same range as erosion rates of 2·1 to 3·6 m Myr?1 on the scarp face and similar to river incision rates of 2·6 to 11·0 m Myr?1 from valley floor bedrock straths, indicating that the Darling Scarp is a slowly eroding ‘steady state’ landform, without any significant contemporary relief production over the last several 100 kyr and possibly several million years. Knickpoint retreat rates determined from 10Be concentrations at the bases of two knickpoints on small streams incised into the scarp are 36 and 46 m Myr?1. If these erosion rates were sustained over longer timescales, then associated knickpoints may have initiated in the mid‐Tertiary to early Neogene, consistent with early‐mid Tertiary marginal uplift. Ongoing maintenance of stream disequilibrium longitudinal profiles is consistent with slow, regional base level lowering associated with recently proposed continental‐scale tilting, as opposed to differential uplift along discrete faults. Cosmogenic 10Be analysis provides a useful tool for interpreting the palaeoseismic history of intraplate near‐fault landforms over 105 to 106 years. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
20.
The upper New River basin of the southern Appalachian Mountains, a major tributary of the modern Ohio River, represents the unglaciated headwaters of the Tertiary Teays River system of eastern North America. Dating of relict fluvial gravels have suggested that New River incision may be outpacing lowering of the surrounding uplands, but physical evidence of transient topographic disequilibrium has yet to be identified. We use focused topographic analysis of the upper New River basin to delineate a perched, low‐relief paleo‐landscape that is experiencing transgressive dissection due to incision by the New River and its tributaries. Accelerated incision has decoupled hillslopes from the drainage network, generating knickpoints which represent the boundary between remnants of the paleo‐landscape and actively adjusting topography downstream. Steepening of hillslopes downstream of knickpoints suggests dynamic headward migration which, along with knickpoint occurrence throughout the drainage network, is inconsistent with the development of fixed stream profile convexities atop strike‐extensive geologic contacts. In the absence of tectonic forcing, we favor a climatically‐forced drop in external base level as driver of the incision pattern we observe. Plio‐Pleistocene glacial damming and diversion of the Teays River to form the modern Ohio River lowered regional base level for the study area, potentially forcing the paleo‐landscape developed during the Teays era to adjust to the modern drainage pattern. The upper New River may therefore represent the potential for glacially‐driven drainage rearrangement to drive transient topographic evolution hundreds of kilometers away from the ice margin, long after the disappearance of ice sheets. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献