Sand particle velocities over a subaqueous dune slope using high-frequency image capturing     

Renske C. Terwisscha van Scheltinga  Giovanni Coco  Heide Friedrich 《地球表面变化过程与地形》2019,44(10):1881-1894

This work presents measurements and analysis of sand particle velocities over a subaqueous dune with median sand diameter of 0.85 mm. Time-lapse images of the mobile bed and an automated particle image velocimetry (PIV)-based cross-correlation method are used to obtain mean velocity of sand particles. This technique is shown to be consistent with measurements obtained with manual tracing. The measurements indicate an increase in mean particle velocity over a dune slope. Three regions are distinguished over the dune slope: (1) region of fluctuating particle velocity, (2) region of increasing particle velocity, and (3) region of maximum particle velocity. The observations are aligned with experimental and numerical modelling studies, indicating fluctuations in flow velocity over a dune stoss slope. We furthermore show that the standard deviation of the mean particle velocity is affected by the slope location and decreases from the lower slope towards the upper slope. The particle velocity variability is discussed in the context of general onset and cessation of sediment transport, the effect of the reattachment zone, sweep-transport events, and the existence of superimposed bedforms. With this work we bridge the gap between measurements of bedload transport at the particle-scale and at the bedform-scale. © 2019 John Wiley & Sons, Ltd.  相似文献   

SLIP OFFSET ALONG STRIKE-SLIP FAULT DETERMINED FROM STREAM TERRACES FORMATION     

XU Bin-bin  ZHANG Dong-li  ZHANG Pei-zhen  ZHENG Wen-jun  BI Hai-yun  TIAN Qing-ying  ZHANG Yi-peng  XIONG Jian-guo  LI Zhi-gang 《地震地质》2019,41(3):587-602

Slip rate is one of the most important parameters in quantitative research of active faults. It is an average rate of fault dislocation during a particular period, which can reflect the strain energy accumulation rate of a fault. Thus it is often directly used in the evaluation of seismic hazard. Tectonic activities significantly influence regional geomorphic characteristics. Therefore, river evolution characteristics can be used to study tectonic activities characteristics, which is a relatively reliable method to determine slip rate of fault. Based on the study of the river geomorphology evolution process model and considering the influence of topographic and geomorphic factors, this paper established the river terrace dislocation model and put forward that the accurate measurement of the displacement caused by the fault should focus on the erosion of the terrace caused by river migration under the influence of topography. Through the analysis of the different cases in detail, it was found that the evolution of rivers is often affected by the topography, and rivers tend to migrate to the lower side of the terrain and erode the terraces on this side. However, terraces on the higher side of the terrain can usually be preserved, and the displacement caused by faulting can be accumulated relatively completely. Though it is reliable to calculate the slip rate of faults through the terrace dislocation on this side, a detailed analysis should be carried out in the field in order to select the appropriate terraces to measure the displacement under the comprehensive effects of topography, landform and other factors, if the terraces on both sides of the river are preserved. In order to obtain the results more objectively, we used Monte Carlo method to estimate the fault displacement and displacement error range. We used the linear equation to fit the position of terrace scarps and faults, and then calculate the terrace displacement. After 100, 000 times of simulation, the fault displacement and its error range could be obtained with 95%confidence interval. We selected the Gaoyan River in the eastern Altyn Tagh Fault as the research object, and used the unmanned air vehicle aerial photography technology to obtain the high-resolution DEM of this area. Based on the terrace evolution model proposed in this paper, we analyzed the terrace evolution with the detailed interpretation of the topography and landform of the DEM, and inferred that the right bank of the river was higher than the left bank, which led to the continuous erosion of the river to the left bank, while the terraces on the right bank were preserved. In addition, four stages of fault displacements and their error ranges were obtained by Monte Carlo method. By integrating the dating results of previous researches in this area, we got the fault slip rate of(1.80±0.51)mm/a. After comparing this result with the slip rates of each section of Altyn Tagh Fault studied by predecessors, it was found that the slip rate obtained in this paper is in line with the variation trend of the slip rate summarized by predecessors, namely, the slip rate gradually decreases from west to east, from 10~12mm/a in the middle section to about 2mm/a at the end.  相似文献   

Terrestrial LiDAR monitoring of coastal foredune evolution in managed and unmanaged systems     

I. Conery  K. Brodie  N. Spore  J. Walsh 《地球表面变化过程与地形》2020,45(4):877-892

Coastal dunes provide essential protection for infrastructure in developed regions, acting as the first line of defence against ocean-side flooding. Quantifying dune erosion, growth and recovery from storms is critical from management, resiliency and engineering with nature perspectives. This study utilizes 22 months of high-resolution terrestrial LiDAR (Riegl VZ-2000) observations to investigate the impact of management, anthropogenic modifications and four named storms on dune morphological evolution along ~100 m of an open-coast, recently nourished beach in Nags Head, NC. The influences of specific management strategies – such as fencing and plantings – were evaluated by comparing these to the morphologic response at an unmanaged control site at the USACE Field Research Facility (FRF) in Duck, NC (33 km to the north), which experienced similar environmental forcings. Various beach-dune morphological parameters were extracted (e.g. backshore-dune volume) and compared with aeolian and hydrodynamic forcing metrics between each survey interval. The results show that LiDAR is a useful tool for quantifying complex dune evolution over fine spatial and temporal scales. Under similar forcings, the managed dune grew 1.7 times faster than the unmanaged dune, due to a larger sediment supply and enhanced capture through fencing, plantings and walkovers. These factors at the managed site contributed to the welding of the incipient dune to the primary foredune over a short period of less than a year, which has been observed to take up to decades in natural systems. Storm events caused alongshore variable dune erosion primarily to the incipient dune, yet also caused significant accretion, particularly along the crest at the managed site, resulting in net dune growth. Traditional empirical Bagnold equations correlated with observed trends of backshore-dune growth but overpredicted magnitudes. This is likely because these formulations do not encompass supply-limiting factors and erosional processes. © 2019 John Wiley & Sons, Ltd.  相似文献   

Response of A Gravel - Bed River To Dam Closure: Insights From Sediment Transport Processes And Channel Morphodynamics     

A. Brenna  N. Surian  L. Mao 《地球表面变化过程与地形》2020,45(3):756-770

This work deals with the impacts of dams on large gravel -bed rivers in terms of altering coarse transport regimes and the relationship with river morphodynamics. Using data collected by a tracer -based monitoring programme carried out in a 4 -km -long study sector of the Parma River (Italy), located downstream from a relatively recently established dam, we applied a virtual velocity approach to estimate the coarse bed material load at four river cross -sections. Monitoring and calculation results provided new insights into the impacts of the dam on streambed material mobility and the sediment regime over the 17 -month calculation period. A longitudinal gradient of effects was observed along the study sector. Sections located closer to the dam are characterized by more evident impacts due to deficits in coarse sediment input from upstream. Sediment mobility here is strongly altered, especially in the highly armoured main channel, and the overall bed material load is extremely low. A partial recovery of sediment dynamics was observed at the sections located further from the dam, where estimates indicate higher sediment yield. The observed longitudinal trend in the coarse sediment transport regime matches the morphology, as the river shifts downstream from a sinuous configuration with alternate bars to a wandering one. The novel insights into alteration of coarse sediment dynamics and the relationship with river morphodynamics are potentially applicable to many other fluvial contexts affected by similar impoundments. © 2019 John Wiley & Sons, Ltd.  相似文献   

Relating the depth of the water table to the depth of weathering     

M.I. Lebedeva  S.L. Brantley 《地球表面变化过程与地形》2020,45(9):2167-2178

Weathering of bedrock creates and occludes permeability, affecting subsurface water flow. Often, weathering intensifies above the water table. On the contrary, weathering can also commence below the water table. To explore relationships between weathering and the water table, a simplified weathering model for an eroding hillslope was formulated that takes into account both saturated and unsaturated subsurface water flow (but does not fully account for changes in dissolved gas chemistry). The phreatic line was calculated using solutions to mathematical treatments for both zones. In the model, the infiltration rate at the hill surface sets both the original and the eventual steady-state position of the water table with respect to the weathering reaction front. Depending on parameters, the weathering front can locate either above or below the water table at steady state. Erosion also affects the water table position by changing porosity and permeability even when other hydrological conditions (e.g. hydraulic conductivity of parent material, infiltration rate at the surface) do not change. The total porosity in a hill (water storage capacity) was found to increase with infiltration rate (all else held constant). This effect was diminished by increasing the erosion rate. We also show examples of how the infiltration rate affects the position of the water table and how infiltration rate affects weathering advance. Published 2020. This article is a U.S. Government work and is in the public domain in the USA  相似文献   

Wood retention at inclined racks: Effects on flow and local bedload processes     

Isabella Schalko 《地球表面变化过程与地形》2020,45(9):2036-2047

Large wood (LW) transport can increase greatly during floods, leading to accumulations at river infrastructures. To mitigate the potential flood hazard, racks are a common method to retain LW upstream of endangered settlements or infrastructures. The majority of LW retention racks consist of vertical bars and, therefore, disrupt bedload transport. It can be hypothesized that inclined racks reduce backwater rise and local scour, as wood will block the upper part of the rack, thereby increasing the open flow cross-section below the accumulation. Flume experiments were conducted under clear water conditions to analyse backwater rise and local scour as a function of (1) rack inclination, (2) hydraulic inflow condition, (3) uniform bed material, and (4) LW volume. In addition, the first experiments were performed under live bed scour conditions to study the effect of bedload transport on local scour and backwater rise. Based on the experiments, backwater rise and local scour decrease with decreasing rack angle to the horizontal. LW predominantly accumulated at the upper part of the rack, leading to an open flow cross-section below the accumulation. The effect of rack angle was included in existing design equations for backwater rise and local scour depth. In addition, the first experiments with bedload transport resulted in smaller backwater rise and local scour depth. This study contributes to an enhanced process understanding of wood retention and bedload transport at rack structures and an improved design of LW retention racks. © 2020 John Wiley & Sons, Ltd.  相似文献   

High-latitude dust deposition in snow on the glaciers of James Ross Island,Antarctica     

Jan Kavan  Daniel Nývlt  Kamil Láska  Zbyněk Engel  Michaela Kňažková 《地球表面变化过程与地形》2020,45(7):1569-1578

High-latitude dust (HLD) depositions on four glaciers of James Ross Island (the Ulu Peninsula) were analysed. The deposition rate on the selected glaciers varies from 11.8 to 64.0 g m⁻², which is one order of magnitude higher compared to the glaciers in Antarctica or elsewhere in the world. A strong negative relationship between the sediment amount and altitude of a sampling site was found. This is most likely caused by the higher availability of aeolian material in the atmospheric boundary layer. General southerly and south-westerly wind directions over the Ulu Peninsula – with exceptions based on local terrain configuration – help to explain the significantly lower level of sediment deposition on San Jose Glacier and the high level on Triangular Glacier. X-ray fluorescence (XRF) spectrophotometry was used to estimate the relative proportions of the main and trace (lithophile) elements in the sediment samples. Both the sediment amount and the XRF results are analysed in a depth profile at each locality and compared among the glaciers, suggesting long-range transport of fine mineral material from outside James Ross Island. The distribution of aeolian sediment among the glaciers corresponds well with the prevailing wind direction on the Ulu Peninsula. © 2020 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   

Density effects on solute release from streambeds     

Guangqiu Jin  Wenhai Yang  Huiyu Xu  Zhongtian Zhang  Saiyu Yuan  Hongwu Tang  Ling Li  David Andrew Barry 《水文研究》2020,34(5):1144-1153

Contaminants that entered the streambed during previous surface water pollution events can be released to the stream, causing secondary pollution of the stream and impacting its eco-environmental condition. By means of laboratory experiments and numerical simulations, we investigated density effects on the release of solute from periodic bedforms. The results show that solute release from the upper streambed is driven by bedform-induced convection, and that density effects generally inhibit the solute release from the lower streambed. Density gradients modify the pore water flow patterns and form circulating flows in the area of lower streambed. The formation of circulating flows is affected by density gradients associated with the solute concentration and horizontal pressure gradients induced by stream slope. The circulating flows near the bottom of the streambed enhance mixing of the hyporheic zone and the ambient flow zone.  相似文献   

Source and supply of sediment to a shoreline salient in a fringing reef environment     

Michael V.W. Cuttler  Jeff E. Hansen  Ryan J. Lowe  Julie A. Trotter  Malcolm T. McCulloch 《地球表面变化过程与地形》2019,44(2):552-564

Reef-associated landforms are coupled to the health of the reef ecosystem which produces the sediment that forms and maintains these landforms. However, this connection can make reef-fronted coastlines sensitive to the impacts of climate change, given that any decline in ecosystem health (e.g. decreasing sediment supply) or changes to physical processes (e.g. sea level rise, increasing wave energy) could drive the sediment budgets of these systems into a net erosive state. Therefore, knowledge of both the sediment sources and transport mechanisms is required to predict the sensitivity of reef-associated landforms to future climate change. Here, we examine the benthic habitat composition, sediment characteristics (composition, texture, and age), and transport mechanisms and pathways to understand the interconnections between coastal morphology and the reef system at Tantabiddi, Ningaloo Reef, Western Australia. Benthic surveys and sediment composition analysis revealed that although live coral accounts for less than 5% of the benthic cover, coral is the dominant sediment constituent (34% on average). Sediment ages (²³⁸U/²³⁰Th) were mostly found to be thousands of years old, suggesting that the primary sediment source is relic reef material (e.g. Holocene reef framework). Sediment transport across the lagoon was quantified through measurements of ripple migration rates, which were found to be shoreward migrating and responsible for feeding the large shoreline salient in the lee of the reef. The derived sediment fluxes were comparable with previously measured rates of sediment production by bioerosion. These results suggest that sediment budgets of systems dependent on old (>10³ years) source materials may be more resilient to climate change as present-day reef health and community composition (i.e. sources of ‘new’ carbonate production) have limited influence on sediment supply. Therefore, the vulnerability of reef-associated landforms in these systems will be dictated by future changes to mechanisms of sediment generation (e.g. bioerosion) and/or physical processes. © 2018 John Wiley & Sons, Ltd.  相似文献