Relative quantification of wind erosion in argan woodlands in the Souss Basin,Morocco     

Miriam Marzen  Mario Kirchhoff  Irene Marzolff  Ali Aït Hssaine  Johannes B. Ries 《地球表面变化过程与地形》2020,45(15):3808-3823

The endemic argan woodlands cover large parts of South Morocco and create a characteristic landscape with areas of sparsely vegetated and bare soil surfaces between single trees. This unique ecosystem has been under extensive agrosilvopastoral management for centuries and is now at risk of degradation caused by overgrazing and increasing scarcity and variability of rainfall. To investigate susceptibility to wind erosion, we conducted an experimental–empirical study including wind tunnel tests and a drone-generated digital elevation model and quantified wind-erodible material on five different associated surface types by means of sediment catchers. The highest emission flux was measured on freshly ploughed surfaces (1875 g m^–2 h^–1), while older ploughed areas with a re-established crust produced a much lower emission flux (795 g m^–2 h^–1). Extensive tillage may have been a sustainable practice for generations, but increasing drought and uncertainty of rainfall now lead to an acute risk of severe soil erosion and dust production. The typical crusted surfaces characterized by residual rock fragment accumulation and wash processes produced the second highest emission flux (1,354 g m^–2 h^–1). Material collected from tree-shaded areas (933 g m^–2 h^–1) was revealed to be a considerable source of organic material, possibly affecting substrate conditions positively on a larger regional scale. The lowest flux was measured on rock fragment-covered surfaces (301 g m^–2 h^–1). The data show that open argan woodland may be a considerable source for wind erosion and dust production, depending on surface characteristics strongly related to management. An adapted management must include the conservation of argan trees to offer a promising approach to prevent severe wind erosion and dust production and mitigate possible impacts of land-use change and climate change related shifts in wind and rainfall patterns. © 2020 The Authors. 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   

What controls submarine channel development and the morphology of deltas entering deep-water fjords?     

Jenny A. Gales  Peter J. Talling  Matthieu J.B. Cartigny  John Hughes Clarke  Gwyn Lintern  Cooper Stacey  Michael A. Clare 《地球表面变化过程与地形》2019,44(2):535-551

River deltas and associated turbidity current systems produce some of the largest and most rapid sediment accumulations on our planet. These systems bury globally significant volumes of organic carbon and determine the runout distance of potentially hazardous sediment flows and the shape of their deposits. Here we seek to understand the main factors that determine the morphology of turbidity current systems linked to deltas in fjords, and why some locations have well developed submarine channels while others do not. Deltas and associated turbidity current systems are analysed initially in five fjord systems from British Columbia in Canada, and then more widely. This provides the basis for a general classification of delta and turbidity current system types, where rivers enter relatively deep (>200 m) water. Fjord-delta area is found to be strongly bimodal. Avalanching of coarse-grained bedload delivered by steep mountainous rivers produces small Gilbert-type fan deltas, whose steep gradient (11°–25°) approaches the sediment's angle of repose. Bigger fjord-head deltas are associated with much larger and finer-grained rivers. These deltas have much lower gradients (1.5°–10°) that decrease offshore in a near exponential fashion. The lengths of turbidity current channels are highly variable, even in settings fed by rivers with similar discharges. This may be due to resetting of channel systems by delta-top channel avulsions or major offshore landslides, as well as the amount and rate of sediment supplied to the delta front by rivers. © 2018 John Wiley & Sons, Ltd.  相似文献   

The impact of submersed aquatic vegetation on the development of river mouth bars     

Sara Lera  William Nardin  Lawrence Sanford  Cindy Palinkas  Roberto Guercio 《地球表面变化过程与地形》2019,44(7):1494-1506

This work is inspired by the sudden resurgence of the submersed aquatic vegetation (SAV) bed in the Chesapeake Bay (USA). Because the SAV bed occurs at the mouth of the Bay's main tributary (Susquehanna River), it plays a significant role in modulating sediment and nutrient inputs from the Susquehanna to the Bay. Previous model studies on the impact of submersed aquatic vegetation on the development of river mouth bars lacked a complete mechanistic understanding. This study takes advantage of new advances in 3D computational models that include explicit physical-sedimentological feedbacks to obtain this understanding. Specifically, we used Delft3D, a state-of-the-art hydrodynamic model that provides fine-scale computations of three-dimensional flow velocity and bed shear stress, which can be linked to sediment deposition and erosion. Vegetation is modeled using a parameterization of hydraulic roughness that depends on vegetation height, stem density, diameter, and drag coefficient. We evaluate the hydrodynamics, bed shear stresses, and sediment dynamics for different vegetation scenarios under conditions of low and high river discharge. Model runs vary the vegetation height, density, river discharge, and suspended-sediment concentration. Numerical results from the idealized model show that dense SAV on river mouth bars substantially diverts river discharge into adjacent channels and promotes sediment deposition at ridge margins, as well as upstream bar migration. Increasing vegetation height and density forms sandier bars closer to the river mouth and alteration of the bar shape. Thus, this study highlights the important role of SAV in shaping estuarine geomorphology, which is especially relevant for coastal management. © 2019 John Wiley & Sons, Ltd.  相似文献   

Geomorphological controls on fluvial carbon storage in headwater peatlands     

D. M. Alderson  M. G. Evans  J. J. Rothwell  E. J. Rhodes  S. Boult 《地球表面变化过程与地形》2019,44(9):1675-1693

Geomorphological controls and catchment sediment characteristics control the formation of floodplains and affect their capacity to sequester carbon. Organic carbon stored in floodplains is typically a product of pedogenic development between periods of mineral sediment deposition. However, in organically-dominated upland catchments with a high sediment load, eroded particulate organics may also be fluvially deposited with potential for storage and/or oxidation. Understanding the redistribution of terrestrial carbon laterally, beyond the bounds of river channels is important, especially in eroding peatland systems where fluvial particulate organic carbon exports are often assumed to be oxidised. Floodplains have the potential to be both carbon cycling hotspots and areas of sequestration. Understanding of the interaction of carbon cycling and the sediment cascade through floodplain systems is limited. This paper examines the formation of highly organic floodplains downstream of heavily eroded peatlands in the Peak District, UK. Reconstruction of the history of the floodplains suggests that they have formed in response to periods of erosion of organic soils upstream. We present a novel approach to calculating a carbon stock within a floodplain, using XRF and radiograph data recorded during Itrax core scanning of sediment cores. This carbon stock is extrapolated to the catchment scale, to assess the importance of these floodplains in the storage and cycling of organic carbon in this area. The carbon stock estimate for the floodplains across the contributing catchments is between 3482-13460 tonnes, equating on an annualised basis to 0.8-4.5% of the modern-day POC flux. Radiocarbon analyses of bulk organic matter in floodplain sediments revealed that a substantial proportion of organic carbon was associated with re-deposited peat and has been used as a tool for organic matter source determination. The average age of these samples (3010 years BP) is substantially older than Infrared Stimulated Luminesence dating which demonstrated that the floodplains formed between 430 and 1060 years ago. Our data suggest that floodplains are an integral part of eroding peatland systems, acting as both significant stores of aged and eroded organic carbon and as hotspots of carbon turnover. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.  相似文献   

From Grain to Floodplain: Evaluating heterogeneity of floodplain hydrostatigraphy using sedimentology,geophysics, and remote sensing     

H.F. Malenda  N.A. Sutfin  G. Guryan  S. Stauffer  J.C. Rowland  K.H. Williams  K. Singha 《地球表面变化过程与地形》2019,44(9):1799-1815

Stratigraphy is a fundamental component of floodplain heterogeneity and hydraulic conductivity and connectivity of alluvial aquifers, which affect hydrologic processes such as groundwater flow and hyporheic exchange. Watershed-scale hydrological models commonly simplify the sedimentology and stratigraphy of floodplains, neglecting natural floodplain heterogeneity and anisotropy. This study, conducted in the upper reach of the East River in the East River Basin, Colorado, USA, combines point-, meander-, and floodplain-scale data to determine key features of alluvial aquifers important for estimating hydrologic processes. We compare stratigraphy of two meanders with disparate geometries to explore floodplain heterogeneity and connectivity controls on flow and transport. Meander shape, orientation, and internal stratigraphy affected residence time estimates of laterally exchanged hyporheic water. Although the two meanders share a sediment source, vegetation, and climate, their divergent river migration histories resulted in contrasting meander hydrofacies. In turn, the extent and orientation of these elements controlled the effective hydraulic conductivity and, ultimately, estimates of groundwater transport and hyporheic residence times. Additionally, the meanders’ orientation relative to the valley gradient impacted the hydraulic gradient across the meanders—a key control of groundwater velocity. Lastly, we combine our field data with remotely sensed data and introduce a potential approach to estimate key hydrostratigraphic packages across floodplains. Prospective applications include contaminant transport studies, hyporheic models, and watershed models. © 2019 John Wiley & Sons, Ltd.  相似文献   

Geomorphological monitoring of a highly dynamic estuary using oblique aerial photographs     

《International Journal of Digital Earth》2013,6(2):109-121

Abstract

Bad weather in many countries limits the use of optical satellite imageries in spatial and temporal monitoring of the environment. In this paper, a series of low-altitude oblique aerial photos taken on daily, weekly and monthly intervals were used to monitor the geomorphological changes in the upper part of the Mersey Estuary, northwestern England. This low-altitude aerial photo methodology reveals itself to be a satisfying compromise between cost, accuracy and difficulty of implementation. It offered a large amount of information on a spatial and temporal scale aiding in the understanding of channel mobility. This was an important consideration in the sitting and installation of new bridge pier foundations. This series of oblique aerial photos was used in a dynamic model to determine the migration of the ebb channel and was effective in identifying the main route of flow. Few uncertainties were encountered and the level of accuracy achieved in resolving these uncertainties in the images was in the range from 40 cm to a maximum of 1.7 m. This was compared with historical navigation charts and showed good correlation. Further applications are required to improve the quality of the data output from these images and the development of the technique.  相似文献   

MULTIVARIATE GEOMORPHIC ANALYSIS OF FOREST STREAMS: IMPLICATIONS FOR ASSESSMENT OF LAND USE IMPACTS ON CHANNEL CONDITION     

RICHARD D. WOOD-SMITH  JOHN M. BUFFINGTON 《地球表面变化过程与地形》1996,21(4):377-393

Multivariate statistical analyses of geomorphic variables from 23 forest stream reaches in southeast Alaska result in successful discrimination between pristine streams and those disturbed by land management, specifically timber harvesting and associated road building. Results of discriminant function analysis indicate that a three-variable model discriminates 10 disturbed from 13 undisturbed reaches with 90 per cent and 92 per cent correct classification respectively. These variables are the total number of pools per reach, the ratio of mean residual pool depth to mean bankfull depth, and the ratio of critical shear stress of the median surface grain size to bankfull shear stress. The last variable can be dropped without a decrease in rate of correct classification; however, the resulting two-variable model may be less robust. Analysis of the distribution of channel units, including pool types, can also be used to discriminate disturbed from undisturbed reaches and is particularly useful for assessment of aquatic habitat condition. However, channel unit classification and inventory can be subject to considerable error and observer bias. Abundance of pool-related large woody debris is highly correlated with pool frequency and is an important factor determining channel morphology. Results of this study yield a much needed, objective, geomorphic discrimination of pristine and disturbed channel conditions, providing a reference standard for channel assessment and restoration efforts.  相似文献   

Ice-sheet dynamics through the Quaternary on the mid-Norwegian continental margin inferred from 3D seismic data     

《Marine and Petroleum Geology》2017

Reconstructing the evolution of ice sheets is critical to our understanding of the global environmental system, but most detailed palaeo-glaciological reconstructions have hitherto focused on the very recent history of ice sheets. Here, we present a three-dimensional (3D) reconstruction of the changing nature of ice-sheet derived sedimentary architecture through the Quaternary Ice Age of almost 3 Ma. An extensive geophysical record documents a marine-terminating, calving Fennoscandian Ice Sheet (FIS) margin present periodically on the mid-Norwegian shelf since the beginning of the Quaternary. Spatial and temporal variability of the FIS is illustrated by the gradual development of fast-flowing ice streams and associated intensification of focused glacial erosion and sedimentation since that time. Buried subglacial landforms reveal a complex and dynamic ice sheet, with converging palaeo-ice streams and several flow-switching events that may reflect major changes in topography and basal thermal regime. Lack of major subglacial meltwater channels suggests a largely distributed drainage system beneath the marine-terminating part of the FIS. This palaeo-environmental examination of the FIS provides a useful framework for ice-sheet modelling and shows that fragmentary preservation of buried surfaces and variability of ice-sheet dynamics should be taken into account when reconstructing glacial history from spatially limited datasets.  相似文献   

祁连山东段童子坝河阶地对气候变化与新构造运动的响应^*   总被引：1，自引：1，他引：0       下载免费PDF全文

钟岳志  李有利  熊建国  雷惊昊  辛伟林  胡秀  刘飞 《古地理学报》2017,19(6):1075-1086

通过对祁连山东段童子坝河各级阶地的年代和变形程度进行测定,得到了河流阶地的形成年代和过程,推算出民乐—大马营逆断裂的活动强度和速率,并分析了阶地形成和气候变化之间的关系。童子坝河5级阶地(年龄分别为16.70±1.81 cal,ka BP、 10 092.5±27.5 cal a BP、8127.5±72.5 cal a BP、2900±60 cal a BP、282.5±17.5 cal a BP)均形成于气候由冷转暖的阶段,属于气候成因阶地。基于断裂两侧阶地面的平面几何形态并结合上、下盘阶地横剖面同级阶地的高差,得到T4、T3、T2和T1阶地在民乐—大马营断裂处的垂直位错分别为10.6±3.1,m、5.0±2.6 m、2.0±1.9 m和1.9±1.3 m,推算出全新世以来民乐—大马营断裂的垂直滑动速率为1.05±0.31 mm/a,水平缩短速率为1.02±0.60 mm/a。  相似文献