Controls on the lateral channel-migration rate of braided channel systems in coarse non-cohesive sediment     

Aaron Bufe  Jens M. Turowski  Douglas W. Burbank  Chris Paola  Andrew D. Wickert  Stefanie Tofelde 《地球表面变化过程与地形》2019,44(14):2823-2836

Lateral movements of alluvial river channels control the extent and reworking rates of alluvial fans, floodplains, deltas, and alluvial sections of bedrock rivers. These lateral movements can occur by gradual channel migration or by sudden changes in channel position (avulsions). Whereas models exist for rates of river avulsion, we lack a detailed understanding of the rates of lateral channel migration on the scale of a channel belt. In a two-step process, we develop here an expression for the lateral migration rate of braided channel systems in coarse, non-cohesive sediment. On the basis of photographic and topographic data from laboratory experiments of braided channels performed under constant external boundary conditions, we first explore the impact of autogenic variations of the channel-system geometry (i.e. channel-bank heights, water depths, channel-system width, and channel slope) on channel-migration rates. In agreement with theoretical expectations, we find that, under such constant boundary conditions, the laterally reworked volume of sediment is constant and lateral channel-migration rates scale inversely with the channel-bank height. Furthermore, when channel-bank heights are accounted for, lateral migration rates are independent of the remaining channel geometry parameters. These constraints allow us, in a second step, to derive two alternative expressions for lateral channel-migration rates under different boundary conditions using dimensional analysis. Fits of a compilation of laboratory experiments to these expressions suggest that, for a given channel bank-height, migration rates are strongly sensitive to water discharges and more weakly sensitive to sediment discharges. In addition, external perturbations, such as changes in sediment and water discharges or base level fall, can indirectly affect lateral channel-migration rates by modulating channel-bank heights. © 2019 The Author. Earth Surface Processes and Landforms published by John Wiley & Sons, Ltd. © 2019 The Author. Earth Surface Processes and Landforms published by John Wiley & Sons, Ltd.  相似文献   

Assessment of geomorphological bank evolution of the alluvial threshold rivers based on entropy concept parameters     

Azadeh Gholami  Majid Mohammadian  Amir Hossein Zaji  Bahram Gharabaghi 《水文科学杂志》2019,64(7):856-872

The complex stream bank profiles in alluvial channels and rivers that are formed after reaching equilibrium has been a popular topic of research for many geomorphologists and river engineers. The entropy theory has recently been successfully applied to this problem. However, the existing methods restrict the further application of the entropy parameter to determine the cross-section slope of the river banks. To solve this limitation, we introduce a novel approach in the extraction of the equation based on the calculation of the entropy parameter (λ) and the transverse slope of the bank profile at threshold channel conditions. The effects of different hydraulic and geometric parameters are evaluated on a variation of the entropy parameter. Sensitivity analysis on the parameters affecting the entropy parameter shows that the most effective parameter on the λ-slope multiplier is the maximum slope of the bank profile and the dimensionless lateral distance of the river banks.  相似文献   

南海北部水合物富集区水道特征对比与成因分析     

付超  于兴河  董亦思  何玉林  梁金强  匡增桂 《海洋学报(英文版)》2019,38(3):103-113

天然气水合物作为现今乃至未来的重要清洁能源之一，其沉积环境和成矿条件的研究一直是国内外地质学家关注的热点问题，我国2007年、2013年及2015年已先后在神狐、和东沙海域多次成功钻获了天然气水合物实物样品。由此南海北部陆坡成为探讨天然气水合物沉积成因与成矿条件的重要试验区。然而，南海北部陆坡区的神狐、东沙及琼东南三个海域均有不同程度的天然气水合物发现，其各自的水深、沉积特征、气源成因及水合物成矿条件各有特点。本文利用地震沉积学原理，结合不同沉积相和沉积演化和古地貌、海平面变化和构造运动等因素，识别出不同类型的地震相。通过对比南海北部陆坡区域的水道系统的MTDs，认为其发育位置、展布和控制因素的不同影响了沉积展布。其中东沙区域位于近物源的上陆坡，神狐区域位于正常的缓陆坡区域，琼东南区域位于远离物源的海底平原区域。并且，水道系统可以分成侵蚀型，侵蚀-加积型和加积型，MTDs也可以分成头部拉张型，中部过渡型和趾部挤压型。  相似文献   

Mid-late Holocene anthropogenic and natural variations in geochemistry and sedimentation in the Yazoo Basin: Clark Lake,Sharkey County,Mississippi     

Stan Galicki  Maria Bujenovic  Theresa Woehnker  Jeb Galtney  Trevor Galicki 《自然地理学》2019,40(3):209-226

A 4000-yr history at Clark Lake reveals natural variations in sediment geochemistry and nutrient levels and anthropogenic influence on twentieth century sedimentation rates. Sediment texture and ridge and swale topography indicate that the channel system creating Clark Lake is now occupied by the Yazoo River. Between 1.24 and 0.60 m (2522–865 cal yr BP), total carbon percentages and the C/N ratio average 42% and 17, respectively. The base of the 1650-yr interval and onset of high organic activity in the lake coincides with the abandonment of the Yazoo Meander Belt. The top of the interval is marked by a drop in C/N ratio to 11 and a geochemical transition zone. No change in the rate of sediment accumulation or clay mineralogy was observed despite increased cumulative percentages of Al, Fe, K, Mg, Mn, Ca, and Na. The decrease in organic activity at 865 cal yr BP is attributed to unfavorable growth conditions related to the entrenchment of the Yazoo River and changes in the hydroperiod of the area. Significant floodplain alteration was completed in 1978, but the only major effect was increased sedimentation in the late twentieth century due to the completion of the Whittington Auxiliary Channel.  相似文献   

Predicting gravel bed river response to environmental change: the strengths and limitations of a regime‐based approach          下载免费PDF全文

Brett Eaton  Robert Millar 《地球表面变化过程与地形》2017,42(6):994-1008

Rivers respond to environmental changes such as climate shifts, land use changes and the construction of hydro‐power dams in a variety of ways. Often there are multiple potential responses to any given change. Traditionally, potential stream channel response has been assessed using simple, qualitative frameworks based largely on professional judgement and field experience, or using some form of regime theory. Regime theory represents an attempt to use a physically based approach to predict the configuration of stable channels that can transport the imposed sediment supply with the available discharge. We review the development of regime theory, and then present a specific regime model that we have created as a stand‐alone computer program, called the UBC Regime Model (UBCRM). UBCRM differs from other regime models in that it constrains its predictions using a bank stability criterion, as well as a pattern stability criterion; it predicts both the stable channel cross‐sectional dimensions as well as the number of anabranches that the stream must have in order to establish a stable channel pattern. UBCRM also differs from other models in that it can be used in a stochastic modelling mode that translates uncertainty in the input variables into uncertainty in the predicted channel characteristics. However, since regime models are fundamentally based on the concept of grade, there are circumstances in which the model does not perform well. We explore the strengths and weaknesses of the UBCRM in this paper, and we attempt to illustrate how the UBCRM can be used to augment the existing qualitative frameworks, and to help guide professionals in their assessments. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

Assessing the relative contributions of the flood tide and the ebb tide to tidal channel network dynamics     

Liang Geng  Zheng Gong  Zeng Zhou  Stefano Lanzoni  Andrea D'Alpaos 《地球表面变化过程与地形》2020,45(1):237-250

Flood and ebb currents provide different contributions to the initiation and evolution of tidal channel networks, generating diverse network structures and channel cross-sections. In order to separate the effects of these contributions, a physical model of a sloping tidal-flat basin was set up in the laboratory. Depending on the degree of tidal asymmetry imposed offshore, either flood or ebb currents can be enhanced. The experimental results show that the ebb current has a higher capability to initiate and shape tidal networks than the flood current. Headward erosion is mainly induced by the ebb flow. The slightly inclined flat surface tends to reduce the energy of the flood current and to enhance the ebb current, thus prolonging the duration of morphodynamic activity as well as sediment motion. Overall, flood-dominated tides favour the formation of small-scale channel branches in the upper basin zone, while long lasting ebb-dominated tides result in more complex, wider and deeper tidal networks. © 2019 John Wiley & Sons, Ltd.  相似文献   

The influence of channel morphology on bedload path lengths: Insights from a survival process model     

Conor McDowell  Marwan A. Hassan 《地球表面变化过程与地形》2020,45(12):2982-2997

Tracer studies are a commonly used tool to develop and test Einstein-type stochastic bedload transport models. The movements of these tracers are controlled by many factors including grain characteristics, hydrologic forcing, and channel morphology. Although the influence of these sediment storage zones related to morphological features (e.g., bars, pools, riffles) have long been observed to “trap” bedload particles in transport, this influence has not been adequately quantified. In this paper we explore the influence of channel morphology on particle travel distances through the development of a Bayesian survival process model. This model simulates particle path length distributions using a location-specific “trapping probability” parameter (p_i ), which is estimated using the starting and ending locations of bedload tracers. We test this model using a field tracer study from Halfmoon Creek, Colorado. We find that (1) the model is able to adequately recreate the observed multi-modal path length distributions, (2) particles tend to accumulate in trapping zones, especially during large floods, and (3) particles entrained near a trapping zone will travel a shorter distance than one that is further away. Particle starting positions can affect path lengths by as much as a factor of two, which we confirm by modelling “starting-location-specific” path length probability distributions. This study highlights the importance of considering both tracer locations and channel topography in examinations of field tracer studies. © 2020 John Wiley & Sons, Ltd.  相似文献   

Deducing the spatial variability of exchange within a longitudinal channel water balance          下载免费PDF全文

Noah M. Schmadel  Bethany T. Neilson  Tamao Kasahara 《水文研究》2014,28(7):3088-3103

Developing an appropriate data collection scheme to infer stream–subsurface interactions is not trivial due to the spatial and temporal variability of exchange flowpaths. Within the context of a case study, this paper presents the results from a number of common data collection techniques ranging from point to reach scales used in combination to better understand the spatial complexity of subsurface exchanges, infer the hydrologic conditions where individual influences of hyporheic and groundwater exchange components on stream water can be characterized, and determine where gaps in information arise. We start with a tracer‐based, longitudinal channel water balance to quantify hydrologic gains and losses at a sub‐reach scale nested within two consecutive reaches. Next, we look at groundwater and stream water surface levels, shallow streambed vertical head gradients, streambed and aquifer hydraulic conductivities, water chemistry, and vertical flux rates estimated from streambed temperatures to provide more spatially explicit information. As a result, a clearer spatial understanding of gains and losses was provided, but some limitations in interpreting results were identified even when combining information collected over various scales. Due to spatial variability of exchanges and areas of mixing, each technique frequently captured a combination of groundwater and hyporheic exchange components. Ultimately, this study provides information regarding technique selection, emphasizes that care must be taken when interpreting results, and identifies the need to apply or develop more advanced methods for understanding subsurface exchanges. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Dynamic,discontinuous stream networks: hydrologically driven variations in active drainage density,flowing channels and stream order          下载免费PDF全文

S. E. Godsey  J. W. Kirchner 《水文研究》2014,28(23):5791-5803

Despite decades of research on the ecological consequences of stream network expansion, contraction and fragmentation, surprisingly little is known about the hydrological mechanisms that shape these processes. Here, we present field surveys of the active drainage networks of four California headwater streams (4–27 km²) spanning diverse topographic, geologic and climatic settings. We show that these stream networks dynamically expand, contract, disconnect and reconnect across all the sites we studied. Stream networks at all four sites contract and disconnect during seasonal flow recessions, with their total active network length, and thus their active drainage densities, decreasing by factors of two to three across the range of flows captured in our field surveys. The total flowing lengths of the active stream networks are approximate power‐law functions of unit discharge, with scaling exponents averaging 0.27 ± 0.04 (range: 0.18–0.40). The number of points where surface flow originates obey similar power‐law relationships, as do the lengths and origination points of flowing networks that are continuously connected to the outlet, with scaling exponents averaging 0.36–0.48. Even stream order shifts seasonally by up to two Strahler orders in our study catchments. Broadly, similar stream length scaling has been observed in catchments spanning widely varying geologic, topographic and climatic settings and spanning more than two orders of magnitude in size, suggesting that network extension/contraction is a general phenomenon that may have a general explanation. Points of emergence or disappearance of surface flow represent the balance between subsurface transmissivity in the hyporheic zone and the delivery of water from upstream. Thus the dynamics of stream network expansion and contraction, and connection and disconnection, may offer important clues to the spatial structure of the hyporheic zone, and to patterns and processes of runoff generation. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

Morphodynamic assessment of side channel systems using a simple one‐dimensional bifurcation model and a comparison with aerial images          下载免费PDF全文

R. Pepijn van Denderen  Ralph M. J. Schielen  Astrid Blom  Suzanne J. M. H. Hulscher  Maarten G. Kleinhans 《地球表面变化过程与地形》2018,43(6):1169-1182

Side channel construction is a common intervention applied to increase a river's conveyance capacity and to increase its ecological value. Past modelling efforts suggest two mechanisms affecting the morphodynamic change of a side channel: (1) a difference in channel slope between the side channel and the main channel and (2) bend flow just upstream of the bifurcation. The objective of this paper was to assess the conditions under which side channels generally aggrade or degrade and to assess the characteristic timescales of the associated morphological change. We use a one‐dimensional bifurcation model to predict the development of side channel systems and the characteristic timescale for a wide range of conditions. We then compare these results to multitemporal aerial images of four side channel systems. We consider the following mechanisms at the bifurcation to be important for side channel development: sediment diversion due to the bifurcation angle, sediment diversion due to the transverse bed slope, partitioning of suspended load, mixed sediment processes such as sorting at the bifurcation, bank erosion, deposition due to vegetation, and floodplain sedimentation. There are limitations to using a one‐dimensional numerical model as it can only account for these mechanisms in a parametrized manner, but the model reproduces general behaviour of the natural side channels until floodplain‐forming processes become important. The main result is a set of stability diagrams with key model parameters that can be used to assess the development of a side channel system and the associated timescale, which will aid in the future design and maintenance of side channel systems. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献