崩岗侵蚀地貌的演变过程及阶段划分

崩岗是中国南方一种常见的土壤侵蚀类型,也是一种严重的水土流失方式,多分布于低山丘陵区的花岗岩风化壳之上.崩岗的形成和发展是2个不同的阶段,并始终受到水力和重力的作用.2种外力对比关系的不同使崩岗的演变过程具有明显的阶段性.通过选取适当的指标体现这种规律性,可以对崩岗的发展阶段进一步划分,对崩岗的治理控制具有重要的意义.     

Automated suitable drainage network extraction from digital elevation models in Taiwan's upstream watersheds

Automatically extracting drainage networks from digital elevation models coupled with the constant stream threshold value is a regular method. These extracted networks can be verified by comparing the channel initiation points with those from real networks. From the results analysed, the differences in channel initiation points will affect the network geometries, geomorphological indices and hydrological responses. This paper develops two automatic algorithms, the headwater‐tracing method and the fitness index, to trace the flow paths from headwaters to the outlet and to calculate the reasonable stream threshold. Instead of the method determined by trial and error or field survey, the accurate channel initiation points can be obtained from airborne photographs coupled with high‐resolution SPOT images for suitable drainage network extraction. Copyright © 2005 John Wiley & Sons, Ltd.     

Gully position,characteristics and geomorphic thresholds in an undisturbed catchment in Northern Australia

Gullying is a significant process in the long‐term dynamics and evolution of both natural and rehabilitated (i.e. post‐mining) landscapes. From a landscape management perspective it is important that we understand gully initiation and development, as it is well recognized that catchment disturbance can result in the development of gullies that can be very difficult to rehabilitate. This study examines gully position using geomorphic statistics relating to features such as depth, width and length in a catchment undisturbed by European activity in the Northern Territory, Australia. The results demonstrate that gullying occurs throughout the catchment and that a slope–area threshold does not exist and that gully position broadly follows the catchment area–slope relationship. Simple relationships relating catchment area and slope to gully depth, width and length provide poor results, despite these relationships having been found to apply for ephemeral gullies in cropland. The results suggest that gully initiation thresholds are low as a result of an enhanced fire regime. A threshold model for gully position that uses catchment area and slope to switch between gully and hillslope was evaluated and found broadly to capture gully position. Copyright © 2006 John Wiley & Sons, Ltd.     

Source‐sink System and Sedimentary Model of Progradational Fan Delta Controlled by Restricted Ancient Gully: An Example in the Enping Formation in the Southern Baiyun Sag,Pearl River Mouth Basin,Northern South China Sea

Tectonic activity occurred during the depositional period of the Enping Formation in the southern Baiyun Sag in the Pearl River Mouth Basin,with a series of synsedimentary faults and a set of large fan delta reservoir bodies developing.There is no data from drilling,cores,etc.for this area,so this paper applies three-dimensional seismic data to study the structural style of the steep slope zone,the seismic facies characteristics of fan deltas,and the source-sink system and sedimentary model of the Enping Formation.The control action of tectonic activity on fan deltaic sedimentary systems is studied by combining interpretation of fault systems,dissection of structural styles,seismic reflection structure,seismic facies geometry,and seismic attribute analysis,together with theoretical analysis of the source-sink deposition process.The Baiyun Sag has experienced tectonic activity since the Eocene,and a series of synsedimentary faults are developed in the southern steep slope zone.Under the common control of multiple large synsedimentary faults,a large ancient gully formed in the steep slope zone in the south,which gradually widened from south to north.The uplift area in the southern part of the sag was exposed for a long time during the deposition of the Enping Formation and consequently suffered weathering and erosion.The resulting sediments were transported through a system of provenance channels composed of slopes and an ancient gully to the depression area,where they were deposited and eventually converged to form a large fan delta.The fan delta presents the overall characteristics of NS strong wedge reflection and EW strong domal reflection—thick in the middle part and thin in the wings.It displays a lobe-shaped distribution on the plane,with the fan root pointing to the south slope.According to differences in reflection intensity from bottom to top,it can be subdivided into three stages of progradational sedimentary bodies—the southern uplift and denudation zone,the large ancient gully,and the fan delta—which together constitute a complete source-sink system.This represents a sedimentary model of progradational fan delta under the overall joint control of the re stricted ancient gully and syndepositional faults.     

Formation of asymmetrical loess gullies in the northeastern loess plateau of China

Loess gullies are the most active and changeable landform unit on the Loess Plateau of China. Under the influence of inhomogeneous internal and external forces, various gully morphologies have been identified as specific forms of asymmetrical loess gullies in the northeastern Loess Plateau. Thus, the formation mechanisms of asymmetrical gullies should be examined to better understand the gully evolution processes in this area. In this study, a typical asymmetrical gully area and its geological background in the northeastern Loess Plateau are investigated. Then, the asymmetrical gullies are extracted and ordered under different watershed hierarchies using 5 m horizontal resolution digital elevation models. The asymmetrical gullies are characterized using the gully deviation index and gully asymmetrical coefficient to quantitatively and qualitatively describe the gully formation from the perspective of gully morphology. Subsequently, environmental factors, such as the bedrock, climate, vegetation and interactions with neighbouring watersheds, are combined to achieve an in-depth understanding of the mechanisms of asymmetrical gully formation. The results show that most watersheds shift to the right side of the watershed geometric centre line, thereby forming a specific asymmetrical gully morphology. The phenomenon in which the asymmetrical degree characteristics decrease with the increase in drainage area suggests evident morphological differences on both sides of the main channel on a small scale, and relatively weak morphological differences on both sides of the main channel on a large scale. The degree of loess gully asymmetry appears higher in the area where only the windward slope is covered by loess than in areas where all slopes are covered by loess. The interaction between adjacent watersheds also influences the formation of asymmetrical gullies. These results support the understanding of asymmetrical gully formation in relation to the underlying bedrock structure and gully reorganization, thereby contributing to the development of process-based gully evolution models.     

A comparison of hillslope drainage area estimation methods using high-resolution DEMs with implications for topographic studies of gullies

Topographic models provide a useful tool for understanding gully occurrence in the landscape but require reliable estimates of gully head drainage areas. Modern high-resolution topography data (collected using structure from motion photogrammetry or light detection and ranging) is increasingly used for topographic studies of gullies, but little work has been done to assess the variability of gully head drainage area estimates using different methods. This study evaluated alternative approaches to using high-resolution digital elevation models (DEMs) so that gully topographic models can be more readily applied to any area with suitably high-resolution data. Specifically, we investigated the impact of single- or multiple-direction flow routing algorithms, DEM hydrologic-enforcement procedures and spatial resolution on gully head drainage area estimation. We tested these methods on a 40 km² site centred on Weany Creek, a low-relief semi-arid landscape draining towards the Great Barrier Reef, Australia. Using a subroutine to separate gully heads into those with divergent or convergent flow patterns upslope, we found that divergent flow conditions occurred at half of 484 studied gullies. Drainage areas estimated by different flow routing algorithms were more variable in these divergent cases than for convergent cases. This variation caused a significant difference between topographic threshold parameters (slope b and intercept k) derived from single- or multiple-direction flow routing algorithms, respectively. Different methods of hydrologic enforcement (filling or breaching) also affected threshold analysis, resulting in estimates of the exponent b being ~188% higher if the DEM was filled than if breached. The testing of the methods to date indicates that a finer resolution (≤2 m) DEM and a multiple-direction flow routing algorithm achieve the most realistic drainage area estimates in low-relief landscapes. For Weany Creek we estimated threshold parameters k = 0.033 and b = 0.189, indicating that it is highly susceptible to gully erosion.     

地面3维激光技术在水土保持监测中的应用

在水电站建设工程中,需要对水土保持进行监测,本文提出了一种新的侵蚀沟监测方案,实践证明它比传统的监测结果更准确,更真实,可以很直观地反映出侵蚀沟的形态,实现3维建模。同时,在安全方面真正实现了非接触式测量,大大减少了外业工作量,降低了外业危险性。     

Channel network extent in the context of historical land use,flow generation processes,and landscape evolution in the North Carolina Piedmont

Intensive agricultural land use in the 18th to early 20th centuries on the southeastern Piedmont resulted in substantial soil erosion and gully development. Today, many historically farmed areas have been abandoned and afforested, and such landscapes are an opportunity to study channel network recovery from disturbance by gullying. Channel initiation mapping, watershed area–slope relationships, and field monitoring of flow generation processes are used to identify channel network extent and place it in hydrologic, historical and landscape evolution context. In six study areas in the North Carolina Piedmont, 100 channel heads were mapped in fully‐forested watersheds, revealing a channel initiation relationship of 380 = AS^1.27, where A is contributing area (m²) and S is local slope (m/m). Flow in these channels is generated by subsurface and overland flow. The measured relative slope exponent is lower than expected based on literature values of ~2 for forested watersheds with subsurface and overland flow, suggesting that the channel network extent may reflect a former hydrological regime. However, geomorphic evidence of recovery in channel heads within fully forested watersheds is greater than those with present day pasture. Present day channel heads lie within hollows or downslope of unchanneled valleys, which may be remnants of historical gullies, and area–slope relationships provide evidence of colluvial aggradation within the valleys. Channel network extent appears to be sensitive to land use change, with recovery beginning within decades of afforestation. Channel initiation mapping and area–slope relationships are shown to be useful tools for interpreting geomorphic effects of land use change. Copyright © 2012 John Wiley & Sons, Ltd.     

Sediment production and yield from an alluvial gully in northern Queensland,Australia

Sediment production, transport and yield were quantified over various timescales in response to rainfall and runoff within an alluvial gully (7 · 8 ha), which erodes into dispersible sodic soils of a small floodplain catchment (33 ha) along the Mitchell River, northern Australia. Historical air photographs and recent global positioning system (GPS) surveys and LiDAR data documented linear increases in gully area and volume, indicating that sediment supply has been relatively consistent over the historic period. Daily time lapse photography of scarp retreat rates and internal erosion processes also demonstrated that erosion from rainfall and runoff consistently supplied fine washload (< 63 µm) sediment in addition to coarse lags of sand bed material. Empirical measurements of suspended sediment concentrations (10 000 to >100 000 mg/L) and sediment yields (89 to 363 t/ha/yr) were high for both Australian and world data. Total sediment yield estimated from empirical washload and theoretical bed material load was dominated by fine washload (< 63 µm). A lack of hysteresis in suspended sediment rating curves, scarp retreat and sediment yield correlated to rainfall input, and an equilibrium channel outlet slope supported the hypothesis that partially or fully transport‐limited conditions predominated along the alluvial gully outlet channel. This is in contrast to sediment supply‐limited conditions on uneroded floodplains above gully head scarps. While empirical data presented here can support future modelling efforts to predict suspended sediment concentration and yield under the transport limiting situations, additional field data will also be needed to better quantify sediment erosion and transport rates and processes in alluvial gullies at a variety of spatial and temporal scales. Copyright © 2013 John Wiley & Sons, Ltd.     

Using Satellite Data to Analyze the Initiation and Evolution of Deep Convective Clouds

In this study, two deep convective cloud cases were analyzed in detail to study their initiation and evolution. In both cases, all deep convective clouds were positioned at the rear of the cold front cloud bands and propagated backward. Satellite data showed that prior to initiation of the deep convective clouds, thermodynamic and moist conditions were favorable for their formation. In the morning, a deep convective cloud at the rear of cold front cloud band propagated backward, the outflow boundary of which created favorable conditions for initiation. An additional deep convective cloud cluster moved in from the west and interacted with the outflow boundary to develop a mesoscale convective system（MCS） with large, ellipse-shaped deep convective clouds that brought strong rainfall. The initiation and evolution of these clouds are shown clearly in satellite data and provide significant information for nowcasting and short-term forecasting.