The relative role of constructive and destructive processes in dune evolution on Cape Lookout National Seashore,North Carolina,USA

Coastal dunes are dynamic features that are continuously evolving due to constructive (e.g., wind- and wave-driven sediment transport) and destructive (e.g., elevated total water levels during storm events) processes. However, the relative importance of these processes in determining dune evolution is often poorly understood. In this study, ten lidar datasets from 1997 to 2016 are used to determine the relative role of erosion and accretion processes driving foredune change on the coast of Cape Lookout National Seashore, North Carolina, USA. Beach and dune morphometrics reveal that dune toe locations have generally retreated since 1997, while dune crest heights accreted by 0.01–0.02 m/year. We develop three univariate metrics that represent (1) the potential for erosion, i.e., total water level impact hours per year, (2) accretion, i.e., dune building hours per year, and (3) the relative net effect of foredune accretion and erosion processes, i.e., constructive–destructive dune forcing (CDDF) ratio, and test the correlative power of these metrics in explaining changes in foredune morphology. The total water level impact hours per year metric explained as much as 66% and 67% of the variance in dune crest and toe elevations, respectively, across the nearly two decades of dune evolution. The greatest number of dune building hours per year and largest dunes within the study site co-occurred at locations exposed to the dominant cross-shore wind direction as a result of varying shoreline orientation. The CDDF ratio was positively correlated to changes in the dune toe elevation in approximately 70% of dunes within the study site, outperforming the impact and dune building hours per year metrics. Our results show that these three metrics can provide first-order estimates of dune morphometric change across multiple spatial and temporal scales, which may be particularly useful at sites where lidar acquisition is intermittent.     

一种利用数学形态学提取地形轮廓的方法

提出了一种利用数学形态学提取图像中地形轮廓的方法。首先分析运用传统单一边缘检测算子提取地形轮廓的不足,然后着重介绍了数学形态学提取地形轮廓的原理,最后通过实验分析,采用开运算与闭运算相结合的方法提取地形轮廓,不仅有效地去除了各种噪声,而且极大地保持了地形轮廓的细节特征。     

Morphological changes on meander point bars associated with flow structure at different discharges

This study investigates the impact of flow structure of different discharges on meander point bar morphology. We carried out mobile and terrestrial laser scanning campaigns before and after a flood on two sandy‐bed point bars. Between the scans, the flow structure was examined using an Acoustic Doppler Current Profiler at three flow stages. The results indicated that a meander point bar both affects and in turn, is itself modified by the flow at different discharges. The lower flow stages also have a significant effect on point bar morphology, especially on deposition over the bar head. Secondary circulation is responsible for scroll bar formation on the point bar margin beyond the apex. Flow separation at the inner bank, by contrast, does not require secondary circulation, but is dependent on flow depth over the point bar. A sudden increase in depth beyond the point bar top causes decreased stream power over the bar tail. The flow separation and decreased stream power cause a slow flow zone and net deposition over point bar tail. The backwater effect, if evident, may strengthen the process. Thus, filling over the bar tail seems generic for point bars and independent on secondary flow. Chutes and chute bars, scroll bars, bar head filling and bar platform filling, by contrast, require special fluvio‐morphological circumstances discussed in this paper. Whilst this paper confirms that the three‐dimensional flow structure has a major effect on point bar morphology, the flow structure seems to depend on how the point bar affects the flow trajectory which, in turn, depends upon the flow stage. Finally, the shape of the bend and the grain size distribution control the impacts of the flow structure, leading to divergent morphologies of point bars with certain generic features. Copyright © 2012 John Wiley & Sons, Ltd.     

Reduced channel morphological response to urbanization in a flood‐dominated humid tropical environment

Urbanization through the addition of impervious cover can alter catchment hydrology, often resulting in increased peak flows during floods. This phenomenon and the resulting impact on stream channel morphology is well documented in temperate climatic regions, but not well documented in the humid tropics where urbanization is rapidly occurring. This study investigates the long‐term effects of urbanization on channel morphology in the humid sub‐tropical region of Puerto Rico, an area characterized by frequent high‐magnitude flows, and steep coarse‐grained rivers. Grain size, low‐flow channel roughness, and the hydraulic geometry of streams across a land‐use gradient that ranges from pristine forest to high density urbanized catchments are compared. In areas that have been urbanized for several decades changes in channel features were measurable, but were smaller than those reported for comparable temperate streams. Decades of development has resulted in increased fine sediment and anthropogenic debris in urbanized catchments. Materials of anthropogenic origin comprise an average of 6% of the bed material in streams with catchments with 15% or greater impervious cover. At‐a‐station hydraulic geometry shows that velocity makes up a larger component of discharge for rural channels, while depth contributes a larger component of discharge in urban catchments. The average bank‐full cross‐sectional area of urbanized reaches was 1.5 times larger than comparable forested reaches, and less than the world average increase of 2.5. On average, stream width at bank‐full height did not change with urbanization while the world average increase is 1.5 times. Overall, this study indicates that the morphologic changes that occur in response to urban runoff are less in channels that are already subject to frequent large magnitude storms. Furthermore, this study suggests that developing regions in the humid tropics shouldn't rely on temperate analogues to determine the magnitude of impact of urbanization on stream morphology. Copyright © 2012 John Wiley & Sons, Ltd.     

蓄滞洪区洪水资源化的益损定量分析——以大黄堡洼为例

为在蓄滞洪区利用洪水资源,探讨洪水资源化收益和损失的函数关系及其定量分析方法。以大黄堡洼为例,建立收益和损失的准量化函数,基于水文和土地利用信息,采用水力学数学模型,澄清3种特定洪水重现期条件下收益与损失的时空转换关系。研究表明,临界水深是实施洪水资源化规划和管控的关键参数;洪水资源化的入流面积、受益面积、平均水深、最大水深与入流洪量成非线性正比关系,并受到地形地貌和土地利用变化等因素的综合影响。     

南海北部陆坡分类及成因分析

陆坡形态隐含了丰富的地质信息,其差异性是大陆边缘沉积、侵蚀过程长期交互作用的结果。利用横跨南海北部的二维地震测线,采取曲线拟合的研究手段,在南海北部识别出了下凹型、平直型、"S"型等三种类型的陆坡。下凹型陆坡发育在莺歌海-琼东南西部、珠江口中部两个陆坡区,但其成因不同,前者主要受控于快速的沉积物供给,而后者受到陆架边缘三角洲进积和海底峡谷侵蚀的联合作用;平直型陆坡仅见于琼东南东部地区,其主控因素为弱的沉积物供给和较快的构造沉降;"S"型陆坡发育在珠江口地区的两翼,其形成明显受到海流和内波等外作用的改造。不同类型的陆坡具有特定的地层叠置样式、陆架坡折迁移轨迹类型和沉积体系分布特征。对陆坡类型的研究有助于建立沉积过程和产物的预测模式,从而指导古代陆坡的深水油气勘探。     

Objective landslide detection and surface morphology mapping using high-resolution airborne laser altimetry

A map of extant slope failures is the most basic element of any landslide assessment. Without an accurate inventory of slope instability, it is not possible to analyze the controls on the spatial and temporal patterns of mass movement or the environmental, human, or geomorphic consequences of slides. Landslide inventory maps are tedious to compile, difficult to make in vegetated terrain using conventional techniques, and tend to be subjective. In addition, most landslide inventories simply outline landslide boundaries and do not offer information about landslide mechanics as manifested by internal deformation features. In an alternative approach, we constructed accurate, high-resolution DEMs from airborne laser altimetry (LIDAR) data to characterize a large landslide complex and surrounding terrain near Christchurch, New Zealand. One-dimensional, circular (2-D) and spherical (3-D) statistics are used to map the local topographic roughness in the DEMs over a spatial scale of 1.5 to 10 m. The bedrock landslide is rougher than adjacent unfailed terrain and any of the statistics can be employed to automatically detect and map the overall slide complex. Furthermore, statistics that include a measure of the local variability of aspect successfully delineate four kinematic units within the gently sloping lower half of the slide. Features with a minimum size of surface folds that have a wavelength of about 11 to 12 m and amplitude of about 1 m are readily mapped. Two adjacent earthflows within the landslide complex are distinguished by a contrast in median roughness, and texture and continuity of roughness elements. The less active of the earthflows has a surface morphology that presumably has been smoothed by surface processes. The Laplacian operator also accurately maps the kinematic units and the folds and longitudinal levees within and at the margins of the units. Finally, two-dimensional power spectra analyses are used to quantify how roughness varies with length scale. These results indicate that no dominant length scale of roughness exists for smooth, unfailed terrain. In contrast, zones with different styles of landslide deformation exhibit distinctive spectral peaks that correspond to the scale of deformation features, such as the compression folds. The topographic-based analyses described here may be used to objectively delineate landslide features, generate mechanical inferences about landslide behavior, and evaluate relatively the recent activity of slides.     

俯冲板片形貌特征和活动大陆边缘演化体制的关系

当大洋板块向大陆板块下俯冲时,上覆板块的边缘可以以沟—弧—盆体制发育,也可以不发育弧后拉伸盆地．为什么同属上覆板块边缘但可以这二种完全不同的体制演化是现代地球科学研究的一个热点．本文在查阅大量最新文献的基础上认为造成这二种不同演化体制除了与俯冲作用的年代学特征有关外还可能主要与俯冲板片的形貌不同所导致局部地幔对流方式不一有关．由于俯冲的倾角,俯冲达到的最大深度以及俯冲板片在６７０ｋｍ上下地幔过渡带处保存的形态等因素不同,造成仰冲板块边缘之下软流圈对流方式不一．从而,造成弧体近陆一侧是否将发生岩石圈拉伸的动力学过程．     

Supervised change detection in VHR images using contextual information and support vector machines

In this paper we study an effective solution to deal with supervised change detection in very high geometrical resolution (VHR) images. High within-class variance as well as low between-class variance that characterize this kind of imagery make the detection and classification of ground cover transitions a difficult task. In order to achieve high detection accuracy, we propose the inclusion of spatial and contextual information issued from local textural statistics and mathematical morphology. To perform change detection, two architectures, initially developed for medium resolution images, are adapted for VHR: Direct Multi-date Classification and Difference Image Analysis. To cope with the high intra-class variability, we adopted a nonlinear classifier: the Support Vector Machines (SVM). The proposed approaches are successfully evaluated on two series of pansharpened QuickBird images.