River mouth bar formation, riverbed aggradation and channel migration in the modern Huanghe (Yellow) River delta, China   总被引：4，自引：1，他引：4  

Hui Fan  Haijun Huang  Thomas Q. Zeng  Kairong Wang 《Geomorphology》2006,74(1-4):124-136

This paper addresses the recent (1970s–1990s) processes of river mouth bar formation, riverbed aggradation and distributary migration in the Huanghe River mouth area, in the light of station-based monitoring, field measurements and remote sensing interpretation. The results show that the morphological changes of the river mouth bar have been closely associated with the largely reduced fluvial discharge and sediment load. Landform development such as bar progradation occurred in two phases, i.e. before and after 1989, which correspond to faster and lower bar growth rates, respectively. Fast riverbed aggradation in the mouth channel was strongly related to river mouth bar progradation. During 1976–1996, about 2.8% of the total sediment loads were deposited in the river channel on the upper to middle delta. Therefore, the river water level rose by a few meters from 1984 to 1996. The frequent distributary channel migration, which switched the radial channel pattern into the SE-directed pattern in the mid-1980s, was linked with mouth bar formation. Marine conditions also constrain seaward bar progradation. Furthermore, the history of river mouth bar formation reflects human impacts, such as dredging and dyking in order to stabilize the coastal area.  相似文献   

Disturbance, stream incision, and channel evolution: The roles of excess transport capacity and boundary materials in controlling channel response   总被引：4，自引：0，他引：4  

Andrew Simon  Massimo Rinaldi 《Geomorphology》2006,79(3-4):361

Channel incision is part of denudation, drainage-network development, and landscape evolution. Rejuvenation of fluvial networks by channel incision often leads to further network development and an increase in drainage density as gullies migrate into previously non-incised surfaces. Large, anthropogenic disturbances, similar to large or catastrophic “natural” events, greatly compress time scales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power. Field examples of channel responses to antrhopogenic and “natural” disturbances are presented for fluvial systems in the mid continent and Pacific Northwest, USA, and central Italy. Responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems. Similar disturbances are shown to result in varying relative magnitudes of vertical and lateral (widening) processes, and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks. This apparent contradiction is explained through an analysis of temporal adjustments to flow energy, shear stress, and stream power with time. Numerical simulations of sand-bed channels of varying bank resistance and disturbed by reducing the upstream sediment supply by half, show identical adjustments in flow energy and the rate of energy dissipation. The processes that dominate adjustment and the ultimate stable geometries, however, are vastly different, depending on the cohesion of the channel banks and the supply of hydraulically-controlled sediment (sand) provided by bank erosion.The non-linear asymptotic nature of fluvial adjustment to incision caused by channelization or other causes is borne out in similar temporal trends of sediment loads from disturbed systems. The sediments emanating from incised channels can represent a large proportion of the total sediment yield from a landscape, with erosion from the channel banks generally the dominant source. Disturbances that effect available force, stream power or flow energy, or change erosional resistance such that an excess of flow energy occurs can result in incision. Channel incision, therefore, can be considered a quintessential feature of dis-equilibrated fluvial systems.  相似文献   

Distribution of macrophytes and condition of the physical environment of streams flowing through agricultural landscape in north-eastern Slovenia     

Urška Kuhar  Tatjana Gregorc  Miran Ren?elj  Nina Šraj-Kr?i?  Alenka Gaberš?ik 《Limnologica》2007,37(2):146-154

The distribution and abundance of macrophytes, land-use beyond the riparian zone, characteristics of the riparian zone and selected channel properties have been studied in 9 streams flowing through an agricultural landscape in the north-eastern part of Slovenia. The streams studied supported a rich macrophyte community. Altogether, 53 plant taxa were observed on 93 km of the streams. Canonical correspondence analysis indicated that 10 out of the 11 parameters examined significantly influenced macrophyte distribution. Of these, substrate characteristics and riparian vegetation type were the most influential. Species composition analysis revealed that the majority of species indicated moderate nutrient load while, in the more strongly modified reaches, species indicating eutrophic conditions, i.e. Myriophyllum spicatum and different species of genus Potamogeton, were more abundant.  相似文献   

IMPACTS OF FLOOD EVENTS IN COARSE SEDIMENT-PRODUCING AREAS ON CHANNEL SILTATION AND FLUVIAL PROCESS OF THE LOWER YELLOW RIVER   总被引：1，自引：0，他引：1  

Ou-yang ZHANG Xiufu FENG Jiong-xin XU 《国际泥沙研究》2007,22(2):142-149

The method of multiple regression is used to analyze the influences of flood events from the coarse sediment producing areas on the channel siltation and fluvial process of the lower Yellow River based on the flood events from 1950 to 1985. The results showed that the flood events from the coarse sediment producing areas carry larger amounts of sediment load and coarser particle sizes than from other source areas, which increases deposition in the lower river channel. And there exist good correlations between channel siltation of the lower reaches of the Yellow River and the coming water and sediment of flood events from the coarse sediment producing areas. Through these correlations, the amounts of sediment deposition in the lower river channel could be roughly estimated based on the runoff and sediment load of flood events from the coarse sediment producing areas. The sediment deposition caused the fluvial process. There exists a complex response of channel form change to the coming water and sediment load of flood events from the coarse sediment producing areas. When the sediment concentration is smaller than 200kg/m3, the ratio between wide-depth ratio after flood and wide-depth ratio before flood((B/h)a / (B/h)b) will increase with the increase of the maximum sediment concentration; when the sediment concentration is near 200kg/m3, (B/h)a / (B/h)b reaches the maximum value; and when the sediment concentration reaches the limits of hyperconcentrated flow, (B/h)a / (B/h)b will decrease with the increase of the maximum sediment concentration. Generally, flood events from the coarse sediment producing areas made channel form of the lower Yellow River deeper and narrower, but a large amount of sediment deposition simultaneously occurs. So, the impacts of flood events from the coarse sediment producing areas on the channel of the lower Yellow River are lessened.  相似文献   

VHF radar observations of surface currents off the northern Opal coast in the eastern English Channel     

Alexei Sentchev  Max Yaremchuk 《Continental Shelf Research》2007

Two very high-frequency radars (VHFR) operating on the Opal coast of eastern English Channel provided a nearly continuous 35-day long dataset of surface currents over a 500 km² area at 0.6–1.8 km resolution. Argo drifter tracking and CTD soundings complemented the VHFR observations, which extended approximately 25 km offshore. The radar data resolve three basic modes of the surface velocity variation in the area, that are driven by tides, winds and freshwater fluxes associated with seasonal river discharge. The first mode, accounting for 90% of variability, is characterized by an along-shore flow pattern, whereas the second and third modes exhibit cross-shore, and eddy-like structures in the current velocity field. All the three modes show the dominant semi-diurnal variability and low-frequency modulation by the neap-spring tidal cycle. Although tidal forcing provides the major contribution to variability of local currents, baroclinicity plays an important role in shaping the 3D velocity field averaged over the tidal cycle and may strongly affect tracer dynamics on larger time scales. An empirical orthogonal function (EOF) decomposition and a spectral rotary analysis of the VHFR data reveal a discontinuity in the velocity field occurring approximately 10 km offshore which was caused by the reversal in the sign of rotation of the current vector. This feature of local circulation is responsible for surface current convergence on ebb, divergence on flood and strong oscillatory vertical motion. Spectral analysis of the observed currents and the results of the Agro drifter tracking indicate that the line of convergence approximately follows the 30-m isobath. The most pronounced feature of the radar-derived residual circulation is the along-coast intensification of surface currents with velocity magnitude of 0.25 m/s typical for the Regions of Freshwater Influence (ROFI). The analysis has provided a useful, exploratory examination of surface currents, suggesting that the circulation off the Opal coast is governed by ROFI dynamics on the hypertidal background.  相似文献   

Anthropogenic ¹²⁹I in seawaters along the north-central part of the English Channel: Levels and tracer applications     

Peng He  Zheng Yang  Hongying Pang  Ala Aldahan  Xiaolin Hou  G?ran Possnert  Xiangjun Pei  Yi Huang 《海洋学报(英文版)》2022,41(11):73-80

The English Channel (the Channel) represents a major sink and transport pathway of anthropogenic radioactive 129I. Despite this important role, data concerning the distribution of 129I in seawater of the Channel are scarce, and most of existing data are restricted to the eastern part of the Channel. The advection and dispersion of 129I from the French coast toward the central and further the English coast, especially in the Channel west of Cap de La Hague, are not fully investigated. We present results of iodine isotopes (127I and 129I) analyses of surface water samples collected along the central English Channel in October, 2010. The data show high 129I concentrations between Dover Strait and La Hague, followed by a dramatic drop towards the Celtic Sea and reveal the dispersal of 129I towards central and northern part of the Channel. Our observation also implies that the entire British coast is contaminated by 129I. 129I levels in the westernmost English Channel, close to the English coast, may reflect combined influences from La Hague and Sellafield. Evolution of 129I between 2005 and 2010 suggests a strong link to temporal marine discharges from La Hague plant. The discharges from the nuclear reprocessing facility have continued since 2010 and thus an ecological evaluation of 129I radioactive hazards in the environment of the Channel may be needed.  相似文献   

Morphological evolution of the channel-shoal system in the South Channel of the Changjiang Estuary during 1958-2018: Causes and future trends     

LUAN Hualong  YAO Shiming  QU Geng  LEI Wentao 《地理学报(英文版)》2022,32(11):2291-2310

The stability of estuarine channel-shoal systems is important for port utilization, navigation maintenance, habitat protection and ecosystem service functions. This paper uses the South Channel of the Changjiang (Yangtze River) Estuary as a typical example to investigate the channel-shoal adjustment mechanism and its future trend. The combined approaches of bathymetric data analysis and process-based modeling (Delft3D) are applied. Quantitative analysis of morphological changes indicates that the South Channel experienced remarkable channel-shoal adjustment during 1958-2018. Periodic evolution was identified, including shoal migration, incision and emergence under natural conditions before the mid-1980s. Since then, fluvial sediment decline and local human intervention have interrupted the periodic processes. After 1986, as river sediment discharge started to decline, the South Channel converted to net erosion, and both the mid-channel shoal at the bifurcation node and the tail of the Ruifeng Shoal showed significant scour. Process-based hydrodynamic simulations revealed that the northern rotation of the mainstream downstream of Wusong triggered the erosion of the Ruifeng Shoal, while unordered sand mining at the shoal tail in approximately 2002 enhanced shoal shrinkage. In addition, the self-adjustment of the transverse section shape resulted in abnormal accretion in 2002-2007. Afterward, the South Channel underwent overall erosion as sediment discharge decreased to a low level (<150 Mt/a). Five stages of channel-shoal pattern adjustment and accretion/erosion status during the past 60 years were defined, i.e., the accretion stage (1958-1965), remarkable channel-shoal adjustment stage (1978-1986), slow erosion stage (1986-1997), shoal scour and shrinkage stage (1997-2007) and overall channel-shoal erosion stage (2007-2018). Model prediction of the evolutionary trend indicates that overall erosion within the South Channel is most likely to continue in 2015-2050. Further adjustment of the South Channel under extremely low sediment discharge may threaten the riverbed stability and the sustainable development of this large-scale estuary. Future work on adaptive strategies for varying conditions is recommended.  相似文献   

Stratigraphic architecture and evolution of a deep-water slope channel-levee and overbank apron: The Upper Miocene Upper Mount Messenger Formation,Taranaki Basin     

《Marine and Petroleum Geology》2014

This paper re-examines the Upper Miocene Upper Mount Messenger Formation, Taranaki Basin, to characterize its architecture and interpret its environmental evolution. Analysis of stratal architecture, lithofacies distributions, and paleotransport directions over the 250 m thick formation shows the outcrops provide a nearly dip parallel section displaying the lateral relationships between contemporaneous channel-levee and overbank depositional environments. At least five 30–40 m thick upward fining units are recognized in the north-central parts of the outcrop and are interpreted as large-scale overbank avulsion cycles. Each unit consists of thick- to medium-bedded predominantly planar laminated sandstone turbidites at the base that fine upward into thin- to very thin-bedded, planar laminated and ripple cross-laminated mud-rich turbidites. The units are traceable laterally over a distance exceeding 3 km where they are cut by channels that show basal mudstone draped by medium- to thin-bedded sandstone, and onlapped by thick-bedded planar laminated sandstone at the margin. The channels are separated by tapered packages of medium- to thin-bedded turbidites containing climbing-ripple cross-lamination interpreted as levees. The individual channel-levee and overbank avulsion cycles formed through four stages: 1) a channel avulsion spread sand into the overbank as an unconfined splay, 2) preferential scouring in one area of the splay led to development of a channel with small levees that prograded across the splay, 3) a deep incision followed by abandonment of the channel deposited a mud lining. Alternatively, the mud lining was formed during the first stage as the downdip portion of the channel was abandoned. 4) The channel filled at first by thick-bedded planar laminated and then by climbing-ripple cross-laminated sand. At this time, the growth of constructional levees progressively limited sand into the overbank. Ratios of Bouma division thicknesses calculated over a stratigraphic interval present a new method to distinguish deep-water depositional environments.  相似文献   

Experiments on Channel Evolution due to Dam Removal in Taiwan     

WANG Hsiao-Wen  CHENG Ying-Che  LIN Chung-Yi 《山地科学学报》2014,(6):1396-1405

Due to safety concerns and habitat restoration for landlocked salmon,a 13-m high check dam on Chijiawan Creek was removed in late May 2011 in Taiwan.We conducted experiments to understand channel evolution of different scenarios.We further compared our experimental results of riverbed elevation changes with the analytical solutions derived from the diffusion equation and field dynamics as well after the creek experienced the first flood event.The results indicated that magnitude of discharges and notch size are dominant factors in resulting channel evolution.While the largest differences between grain size distribution are associated with discharge,the largest differences in net change in upstream volume are associated with notch size.While the theoretical equation could help understand the channel change after dam removal,it only explained the evolution closer to the dam.The physical experiments,on the other hand,provided insights especially with regard to comparing alternative proposed management actions.The discrepancies between predicted and actual outcome highlight more needed inputs for future dam-removal assessments.  相似文献   

地质遗迹旅游资源化：概念、动力及途径          下载免费PDF全文

李如友 《江苏地质》2012,36(1):107-112

地质遗迹是具有独特观赏价值和科普价值的自然遗产,旅游资源化是地质遗迹保护与利用协调发展的最佳方式。提出地质遗迹旅游资源化的概念,分析地质遗迹旅游资源化的动力和途径,认为地质遗迹旅游资源化是旅游需求层次的提高、地质遗迹的保护、地质产业转型、国家政策的支持、地质公园建设的成功5种因素共同作用的结果。地质遗迹向旅游资源的转化需经过地质遗迹调查、地质遗迹评价、旅游产品开发、完善解说系统等步骤。  相似文献