Morphodynamics of deltas under the influence of humans

A consistent database was established to characterize key environmental factors known to control delta morphology. The database includes the location, basin morphology, fluvial and sediment discharge to the deltas, delta morphology, ocean energy, and shelf depth reached by the sub-aqueous delta. Fifty-one deltas were selected to cover the global parameter range of rivers entering all major oceans and coastal seas. Seasonal satellite images of the deltas were processed (IKONOS, SPOT, LANDSAT, and MODIS). Predictive statistical relationships were obtained, suitable for hypothesis testing or to constrain/verify numerical models used to simulate the evolution of coastal systems. The area of a delta is best predicted from average discharge, the total sediment load feeding the delta, and the offshore accommodation space. The gradient of a delta plain, measured from the apex of the delta to the coast along the main channel, is best predicted with a ratio of sediment supply to sediment retention, sediment concentration used as a proxy of delta plain sedimentation, and mean water discharge. Widths of distributary channels form a lognormal distribution, with the cumulative width of the river mouths directly related to the maximum discharge, tidal and wave energy. The grain size of topset deposits scales with the river length. Hundreds of millions of people occupy deltas and human engineering is now a major influence on the growth and evolution of many deltas, through control of the flow path of distributary channels, and mitigation of the seasonal flood wave with concomitant change in the delivery of sediment load. More and more deltas are moving away from their pre-Anthropocene morphology, as influenced by pristine sediment supply and sediment dispersal.     

新疆宽浅变迁河流桥位河段壅水研究

新疆宽浅动床河流水深小、比降大、水流急,河床容易冲刷。建桥后,由于桥位断面压缩而导致的壅水与一般冲积河流不同。通过概化动床模型试验,得出了新疆宽浅变迁河流桥位河段壅水计算公式。动床模型公式充分反映了这类河流壅水的特点,因而与实际情况较为符合。     

Monitoring and predicting channel change in a free‐evolving,small Alpine river: Ridanna Creek (North East Italy)

The recent (25 years) morphodynamics of a proglacial reach of the Ridanna Creek, North‐East Italy, evolving in the absence of human constraints, has been investigated by means of an intensive field activity and of the analysis of aerial photographs. The study reach mostly displays a braided morphology, with sharp downstream variations of valley gradient, sediment size and formative conditions within the main channel. These discontinuities are associated with different processes of channel adjustment at different timescales, which have been quantified by coupling hydrological with morphological information. Several processes of channel change and variations in braiding intensity have been documented along the whole reach and highlight how a regular, weakly meandering main channel may significantly affect the morphodynamics of the braided network. A first attempt to predict the morphological instability of this main channel at the observed spatial scales through existing linear theories of curved river channels shows a good agreement with field observations. Finally, the complete hydro‐morphodynamical characterization of such an undisturbed alpine river reach can provide a relevant contribution to the definition of reference conditions for Alpine rivers required by the EU Water Framework Directive. Copyright © 2007 John Wiley & Sons, Ltd.     

吐哈盆地WM油田辫状河三角洲前缘砂体分析

湖盆可容空间较小时,辫状河三角洲相对发育,水下分支河道横向迁移频繁,易形成席状分布的砂体 (水下分支河道复合体 )。可容空间增大时,辫状河三角洲相对不发育,水下分支河道延伸不远,河道砂体较孤立,河口坝相对比较发育。文中根据水流和波浪强弱提出九种水下分支河道与河口坝的关系图,基本上总结了吐哈盆地WM油田三间房组河口坝的形态;并对水下分支河道复合体的形成机制提出了独特的推理。     

Assessment of vegetation effects on hydraulics and of feedbacks on plant survival and zonation in ephemeral channels

The interaction of vegetation and flow in channels is important for understanding the influences of forces in channels and effects on erosion, sediment flux and deposition; it has implications for channel habitats, channel instability and restoration schemes. Methods are needed for calculating forces on plants and data are required on thresholds for plant destruction and survival. A simple method of calculating the effect of hydraulics on vegetation and its zonation within ephemeral channels is described. Detailed cross section surveys of channel morphology, vegetation and estimates of Manning's n are input into the software program WinXSPRO to calculate the hydraulics of flows across the channel for a given event or flow level, incorporating subdivision into zones of differing morphology and vegetation across the section. This was applied to a number of cross sections on ephemeral channels in SE Spain and typical roughness values for Mediterranean vegetation types in channels were assessed. The method is demonstrated with reference to two well‐documented floods in SE Spain, in September 1997 on the Torrealvilla and in October 2003 along the Salada. These flows led to the mortality of herbs, reed and smaller shrub species. Some damage to larger shrubs and trees occurred, but trees such as Tamarisk (Tamarix canariensis) were shown to withstand high forces. Some grasses were highly resistant to removal and induced sedimentation. Significant erosion was limited to areas with little vegetation covering the channel floor. Further quantification of resistance of vegetation to flows and upper threshold values for removal is continuing by relating calculated hydraulic conditions using the methods outlined to measurements of vegetation responses in events at monitoring sites. Copyright © 2009 John Wiley & Sons, Ltd.     

Be data from meltwater channels suggest that Jameson Land, east Greenland, was ice-covered during the last glacial maximum

Along the northeast Greenland continental margin, bedrock on interfjord plateaus is highly weathered, whereas rock surfaces in fjord troughs are characterized by glacial scour. Based on the intense bedrock weathering and lack of glacial deposits from the last glaciation, interfjord plateaus have long been thought to be ice-free throughout the last glacial maximum (LGM). In recent years there is growing evidence from shelf and fjord settings that the northeast Greenland continental margin was more extensively glaciated during the LGM than previously thought. However, little is still known from interfjord settings. We present cosmogenic ¹⁰Be data from meltwater channels and weathered sandstone outcrops on Jameson Land, an interfjord highland north of Scoresby Sund. The mean exposure age of samples from channel beds (n = 3) constrains on the onset of deglaciation on interior Jameson Land to 18.5 ± 1.3–21.4 ± 1.9 ka (for erosion conditions of 0–10 mm/ka, respectively). This finding adds to growing evidence that the northeast Greenland continental margin was more heavily glaciated during the LGM than previously thought.     

Chute channels in the Holocene high‐sinuosity river deposits of the Firenze plain,Tuscany, Italy

This paper focuses on Holocene deposits of the Firenze alluvial plain (Northern Apennines, Italy) and deals with the sedimentary features of chute channels draining the down‐river edges of the meander neck formed by 70 to 100 m wide and 1 to 1·5 m deep sinuous channels. Two main types of chute channels have been recognized. Type 1 is represented by 3 to 6 m wide and 0·5 to 1 m deep straight channels filled with mud aggregates overlying a basal gravel lag made of reworked caliches. These channels drained the point bar top during floods, and are thought to have been initiated as small rills when a shallow flow overpassed the downstream side of the point bar. Type 2 channels, 3 to 6 m wide and 1 to 1·5 m deep, are moderately to highly sinuous and filled with well‐stratified sand and gravels sourced from nearby rocky highlands. Type 2 channels were connected to the main river channel also during the base flow stage. The transition from Type 1 to Type 2 channels is documented and is interpreted as the result of the meander cut‐off process. Type 1 chute channels represent the early stage of the cut‐off phase, when a headcut is incised on the down‐river edges of the meander neck. The headcut migrates up‐river across the meander neck during floods, when fast currents shape the chute channels into a straight route. The transition from Type 1 into Type 2 channels is linked to the connection of the up‐river migrating headcut with the main channel and the termination of the cut‐off process. At this stage, the cut‐off channel is drained permanently and receives bedload from the main channel. The progressive shaping of the newly formed channel will convert it into the main channel and lead to the formation of an oxbow lake in the abandoned meander branch. Development of chute channels in the Firenze alluvial plain is thought to have heralded a decrease in sinuosity of the main channels, triggered by a climate‐driven increase in water discharge.     

Modelling sediment transport in stream channels in the arid zone of India

Ephemeral channels in arid regions convey larger amounts of sediment than perennial channels in humid regions. Sediment graphs at the outlet of channels have been derived by the standard sediment rating curve technique and by a lumped model based on the instantaneous unit sediment graph (IUSG) concept. The IUSG gives estimates of sediment transport that are better than those derived with the sediment rating curve as it takes into account the availability of erodible material within the channel bed.