Modelling of meander migration in an incised channel

An updated linear computer model for meandering rivers with incision has been developed. The model simulates the bed topography, flow field, and bank erosion rate in an incised meandering channel. In a scenario where the upstream sediment load decreases （e.g., after dam closure or soil conservation）, alluvial river experiences cross section deepening and slope flattening. The channel migration rate might be affected in two ways： decreased channel slope and steeped bank height. The proposed numerical model combines the traditional one-dimensional （1D） sediment transport model in simulating the channel erosion and the linear model for channel meandering. A non-equilibrium sediment transport model is used to update the channel bed elevation and gradations. A linear meandering model was used to calculate the channel alignment and bank erosion/accretion, which in turn was used by the 1D sediment transport model. In the 1D sediment transport model, the channel bed elevation and gradations are represented in each channel cross section. In the meandering model, the bed elevation and gradations are stored in two dimensional （2D） cells to represent the channel and terrain properties （elevation and gradation）. A new method is proposed to exchange information regarding bed elevations and bed material fractions between 1D river geometry and 2D channel and terrain. The ability of the model is demonstrated using the simulation of the laboratory channel migration of Friedkin in which channel incision occurs at the upstream end.     

Toward a stepwise Kwangsian Orogeny

The Kwangsian Orogeny originated along the southeast coast of China and stepwise developed in a northwest direction. It includes two stages, a long locally varying uplift from the Late Ordovician to the early Silurian and a finally tectonic movement near the Silurian and Devonian transition. The Kwangsian uplift event shows a stepwise delay northwestwards from the southeastern coast area in Nemagraptus gracilis Biozone (Sa1) to the south side of the Xuefeng Mountains in or later than Cystograptus vesiculosus Biozone (R3) to Coronograptus cyphus Biozone (R4). In the southern of Yangtze Platform, the Yichang Uplift was droved by the Kwangsian Orogeny forming a diachronous stratigraphical break through Rhuddanian and Aeronian. The distribution of the early Telychian lower marine red beds indicates a northwestward increase of the Cathaysian Oldland. Stratigraphical evidence may explain why the Kwangsian movement was marked by an angular disconformity during the Pridoli to earliest Devonian interval.     

黄河内蒙古段头道拐断面冰期水流特征及其对气候变化与人类活动的响应

利用最新冰凌观测数据,分析了1960年以来黄河内蒙古段头道拐断面冰期流量特点及其影响因素.结果表明：头道拐断面小流量持续时间的年际变化具有明显的阶段性特征,1960-1967年、1968-1985年和1986-2010年小流量平均持续天数分别为68 d、17 d和37 d.气候变化和人类活动是影响小流量过程的主要因素,气候变化通过影响冰盖厚度和冰花堆积来影响封冻河段流量过程;河道冲淤演变则是冰期流量过程和槽蓄水增量长期变化的重要原因;刘家峡水库调度直接影响着内蒙古河段的流量过程. 最近10 a来,凌汛期气温变化剧烈,上游来水有所减少,河道淤积更加严重,河道工程增多是黄河头道拐断面小流量持续天数延长的主要原因.     

Coupling between landslides and eroding stream channels reconstructed from spruce tree rings (examples from the Carpathians and Sudetes – Central Europe)

The analysis of the positive feedback between landslides and erosion requires determination of the precise temporal and spatial relations between events of colluvium delivery and fluvial erosion. In our study we use decennial datasets on the occurrence of landsliding and erosion achieved through dendrochronological methods. Four sites covering areas of landslide slopes and adjacent valley floors with stream channels were studied. Landsliding on slopes was dated from the tree‐ring eccentricity developed in stems tilted due to bedrock instability. Erosion in channels was dated using the wood anatomy of roots exposed by erosion of the soil cover. Analysis of the temporal relations between dated landsliding, erosion and precipitation record has revealed that two types of repeating sequences can be observed: (1) rainfall → landsliding → erosion; (2) rainfall → erosion → landsliding. These sequences are an indication of the occurrence of slope‐channel positive feedback in the sites studied. In the first type, landsliding triggered by rainfall delivers colluvia into the valley floor and causes its narrowing, which in turn causes increased erosion. In the second type erosion triggered by rainfall disturbs the slope equilibrium and causes landsliding. Landsliding and erosion, once triggered by precipitation, can occur alternately in years with average precipitation and reinforce one another. Bidirectional coupling between landsliding and channel erosion was shown notably through the effects of channel shifting and forced sinuosity and by increased erosion of the slopes opposite the active landslides. Observations also suggest that the repetition of sequences described over longer periods of time can lead to a general widening of the valley floor at the expense of slopes and to a gradual change of the valley cross‐profile from narrow, V‐shaped into a wide flat‐bottomed. Thus landsliding–erosion coupling/positive feedback was recognized as an important factor shaping hillslope–valley topography of the mid‐mountain areas studied. Copyright © 2014 John Wiley & Sons, Ltd.     

断陷湖盆深水重力流水道的识别标志及沉积模式——以歧口凹陷歧南斜坡沙一段为例

以歧口凹陷歧南斜坡沙一段的深水厚层砂岩为研究对象,运用地震沉积学基本理论和方法,通过古地貌分析约束物源方向,结合岩心观测描述确定沉积相类型,应用地层切片、反演与三维可视化技术确定空间形态。结果表明:该区广泛发育的深水厚层砂岩为重力流水道沉积,包括滑塌岩、砂质碎屑流砂体与浊积岩3种类型重力流沉积的组合;顺北东向展布的断槽内呈大型条带状展布,可划分为水道中心微相、水道边缘微相和水下漫溢微相;岩心中可识别出7种岩相类型。在沉积特征分析的基础上,建立了发育于斜坡且由顺源断层控制的重力流沉积模式,分析了歧南斜坡重力流水道的成因及其油气成藏特征。研究认为重力流水道是该区一类重要的储集体,具有良好的成藏条件,该成果扩大了歧口凹陷油气勘探领域。     

一种改进的单幅图像快速去雾方法与实验

目前,物理模型的单幅图像去雾已成为图像去雾算法研究的重点。在分析了暗原色先验知识的单幅图像去雾算法基础上,针对暗原色先验去雾算法时间复杂度大的缺点,比较了目前已有的暗原色先验改进去雾算法,提出了一种新的暗原色先验单幅图像去雾改进算法。通过引入快速、各向同性的低通高斯滤波器,实现对透射率图的平滑均匀,以代替暗原色去雾方法中精妙但时间复杂度高的软抠图算法;对于图像中图层交界处,提出了以区域中值滤波方法进行修正的算法,以及满足自适应要求的全局大气光求解详细算法。实验结果表明,结合了以上3点改进的快速去雾算法在保证图像去雾效果的同时,能大幅度提高暗原色去雾算法的速度,适用于对工程上的图像、视频实时去雾。     

20世纪30年代珠江三角洲平原河网结构重建及最大槽蓄容量

基于1954年美国陆军制图局编制的中国地形图、1930―1934年日军参谋本部所绘中国地形图、广东省水文志和研究区各县县志等历史文图资料,重建研究区20世纪30年代的河网模型,对传统农业末期平原河网水系进行高精度复原,计算其最大槽蓄容量并分析其区域特点。结果表明：1）20世纪30年代珠三角河网（除干流河道）的最大槽蓄容量为6.78亿~20.75亿m³,其极值为20.75亿m³,平均值为13.38万m³/km²。其中1级河道最大槽蓄容量占总量的33.5%~49.1%,对于整个河网的容蓄洪水能力及水资源调度能力影响更大。与现阶段不同,传统农业时代末期（20世纪30年代）珠三角河网的槽蓄容量与河网密度呈正相关,槽蓄容量的大小受河网密度的影响较为明显。2）20世纪30年代珠三角河网的形成与传统时期的农业开发有着密切的关系,当时的河网水系是自然发育与该时期围田业对其开发、围垦的共同结果,围田开发的兴修堤围、挑挖河道等活动对于河网的扩展及其最大槽蓄容量的增加具有客观上的促进作用。     

Reconstruction of eroded and deposited sediment volumes of the embanked River Waal,the Netherlands,for the period ad 1631–present

In the last few centuries humans have modified rivers, and rivers have responded with noticeable changes in sedimentary dynamics. The objective of this study is to assess these responses of the sedimentary dynamics. Therefore, we calculated a sediment budget for eroded and deposited sediment volumes in a ~12‐km long floodplain section of the largest semi‐natural embanked but still dynamic lower Rhine distributary, for ~50‐years time slices between ad 1631 and present. This is the period during which embanked floodplains were formed by downstream migration of meander bends between confining dykes. Our sediment budget involves a detailed reconstruction of vertical and lateral accretion rates and erosion rates of floodplain sediment. To do so, we developed a series of historical geomorphological maps, and lithogenetic cross‐sections. Based on the maps and cross‐sections, we divided the floodplain into building blocks representing channel bed and overbank sediment bodies. Chronostratigraphy within the blocks was estimated by interpretation of heavy metal profiles and from optically stimulated luminescence (OSL) dating results. Sediment budgets were hence calculated as a change of volume of each building block between time steps. The amount of lateral accretion initially increased, as a result of island and sand bar formation following embankment. From the eighteenth century onwards, there was a decrease of lateral processes in time, which is a result of straightening of the river by human activities, and a reduction of water and sediment supply due to the construction of a new upstream bifurcation. With straightening of the river, the floodplain area grew. Artificial fixation of the channel banks after ad 1872 prevented lateral activity. From then on, overbank deposition became the main process, leading to a continuous increase of floodplain elevation, and inherent decrease of flooding frequency and sediment accumulation rate. Copyright © 2014 John Wiley & Sons, Ltd.     

Network concepts to describe channel importance and change in multichannel systems: test results for the Jamuna River,Bangladesh

Most of the largest rivers on Earth have multiple active channels connected at bifurcations and confluences. At present a method to describe a channel network pattern and changes in the network beyond the simplistic braiding index is unavailable. Our objectives are to test a network approach to understand the character, stability and evolution of a multi‐channel river pattern under natural discharge conditions. We developed a semi‐automatic method to derive a chain‐like directional network from images that represent the multi‐channel river and to connect individual network elements through time. The Jamuna River was taken as an example with a series of Landsat TM and ETM+ images taken at irregular intervals between 1999 and 2004. We quantified the overall importance of individual channels in the entire network using a centrality property. Centrality showed that three reaches can be distinguished along the Jamuna with a different network character: the middle reach has dominantly one important channel, while upstream and downstream there are about two important channels. Temporally, relatively few channels changed dramatically in both low‐flow and high‐flow periods despite the increase of braiding index during a flood. Based on the centrality we calculated a weighted braiding index that represents the number of important channels in the network, which is about two in the Jamuna River and which is larger immediately after floods. We conclude that the network measure centrality provides a novel characterization of river channel networks, highlighting properties and tendencies that have morphological significance. Copyright © 2013 John Wiley & Sons, Ltd.     

Evaluation of digital channel network derivation methods in a glaciated subalpine catchment

The digital elevation model (DEM) has become an essential tool for an increasing array of mountain runoff analyses, particularly the derivation and mapping of stream channel networks. This study examines how well commonly applied DEM‐based channel derivation methods at different spatial resolutions can represent the channel network for a glaciated Rocky Mountain headwater catchment. The specific objectives are to (1) examine how differences in gridded DEM resolution affect spatially distributed values of local slope, specific contributing area, and topographic wetness index derived from both eight and infinite directional flow algorithms, (2) map the actual stream channel network to examine the influence of surface variables on channel initiation, and (3) assess accuracy of DEM‐derived networks compared with the field surveyed network. Results show that for the same contributing area threshold, increasing grid cell size leads to increased channelization of modeled networks. A plot of local slope versus contributing area reveals a negative relationship similar to that of prior studies in un‐glaciated areas but with breaks in slope at contributing areas that are too small to represent thresholds for channelization. Field survey results and evaluation of DEM‐derived channel networks suggest that channel network formation is not clearly related to surface topographic variables at Loch Vale. Digitally derived channel networks do not accurately predict low order channel locations, but approximations of the channel network with drainage density and headward extent of channelization similar to the observed network can be derived with both a 1 m and 10 m DEM using a contributing area threshold of approximately 4x10⁴ m². Copyright © 2014 John Wiley & Sons, Ltd.