ISSN:0706-652X    

DOI:10.1139/f83-264

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

A depth-averaged hydrodynamical numerical model is used to evaluate tidal stresses, currents, and mixing in the Bristol Channel and Severn Estuary. Benthic macrofaunal associations and sediment bed types are shown to depend on the magnitude of the tidal stress, and the direction of sediment transport (as bed-load) in the central parts of the Channel is shown to be a consequence of ebb dominated stress. This asymmetry in the tidal stress is mainly caused by M4currents, and computed M4elevations and currents are compared with observed values at a number of stations. Residual flows and horizontal mixing are deduced from the hydrodynamical model, and used to construct transport relationships for an ecosystem model. Agreement between observed salinity over a number of years and that computed by the ecosystem model is generally good.Key words: Bristol Channel, hydrodynamical model, salinity model, tidal stress, M4tides, sediment movement

ISSN:0706-652X    

DOI:10.1139/f83-265

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

Two recent independent studies of the Minas Basin have been combined to produce a numerical simulation of suspended sediments. One study devised a numerical, barotropic tidal model of the Bay of Fundy and Gulf of Maine, which produced a good simulation of the tides and currents in the Minas Basin. The second, an extensive program of sediment sampling and monitoring addressed the nature of suspended sediments, bottom sediments, their various sediment sources and the postglacial evolution of the system. The only significant source of suspended sediment was from the open boundary to the Minas Basin and was of relatively uniform concentration through the water column. Consequently, the barotropic tidal model was well suited to modeling this material. Sediment laws describing sediment settling, resuspension, and the critical stress for entrainment were obtained from the literature and evaluated. The settling velocity of the suspended sediment was obtained by experimentation using actual water/sediment samples. Because the various parameters and critical stresses show a considerable variation among experimenters, runs were planned to examine a range of parameters. A close fit to existing conditions was obtained with the model generating mudflats in those regions where indeed they exist. Using the various parameters and critical stresses reflecting maximum sedimentation, the model was run to include the effects of a tidal power barrage off Economy Point. It is concluded that suspended sediment would not inhibit the operation of such a barrage through siltation directly in front of or behind the barrier. Increased siltation can be expected in the intertidal areas behind the barrage. Sediment budget considerations further limit the degree of post barrier siltation, insofar as the resident volume of suspended material (30 × 106 m3) is not enough to significantly reduce the headpond area.Key words: suspended sediment, sediment transport, numerical model, tide

ISSN:0706-652X    

DOI:10.1139/f83-266

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

A brief review has been made of five hydraulic investigations of port siltation carried out by various international laboratories. The ports studied are distributed throughout the tropics from South America to Indonesia and the factors affecting sediment supply and distribution either directly or indirectly have been summarized. In spite of local variations, the similarity of many features suggests the existence of a common mechanism of sediment transport.Twenty months of field measurements at one of the ports demonstrated that turbidity in the water column is low from surface to 0.15 m above the bed. These measurements, together with those from flume tests on the behavior of local mud in currents, indicated that siltation is not solely due to deposition of silt suspended in the main body of flow. Further wave flume tests showed that under certain conditions, a shallow, highly turbid layer forms on the bed, capable of being transported by weak tidal currents without significant vertical mixing. Supplementation of field measurements of water movement with more detailed hydraulic model measurements permitted the formulation of a siltation mechanism.Appraisal of results suggests the most important immediate source of the silt to be the shallow coastal mud banks. Waves produce the turbid, near-bed layer which is transported by littoral currents into port approach channels for redistribution by the prevailing residual current, itself a function of fluvial/tidal interaction.Key words: siltation, dredging, silt movement, flocculation, waves, tides, river flow, residual currents

ISSN:0706-652X    

DOI:10.1139/f83-267

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

Various independent sets of data are used to investigate near-surface suspended sediment concentrations. The data include satellite (LANDSAT) imagery, field verification and near-synoptic surveys of specific areas and integrated, longer-term, surveys of the Channel. The information is used to identify areas of high turbidity, which relate to tidal and wave resuspension of the mobile sediments within the Estuary and Channel system. Thematic maps produced from the satellite imagery are comparable directly with the survey results and are combined with them, through a radiance/logeconcentration relationship. Derived estimates of material present within the waters of the Channel range from 4.5 × 106 t to 13.4 × 106 t. Fluvial contributions are estimated to be of the order of 1.6 × 106 t/yr. Other evidence is available for the supply of material from the Celtic Sea. Isopleths of suspended sediment concentration are used to infer residual water and fine-grained sediment transport directions; these are compared with a more general model, which includes observed and predicted near-bed sand transport directions.Key words: Bristol Channel, Severn Estuary, suspended sediment, satellite imagery, sediment budget, sediment transport, residual water movement

ISSN:0706-652X    

DOI:10.1139/f83-268

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

Progradation of Atchafalaya Delta, one of the most dynamic geological events in historical times within the Mississippi Delta complex, has produced a 230 km2new sediment body on the Louisiana coast of which 28 km2is subaerial. Sedimentation in Atchafalaya Bay was initiated in the early 1950s, and by 1973 a complex subaerial feature had evolved. Between 1972 and 1980 the area of Atchafalaya Delta dominated by coarse sedimentation increased. The greatest increase occurred along the delta's western flank as a result of introduction of dredge spoil from a navigation channel, as well as greater discharges in this area. Detailed field studies in the natural (spoil-free) eastern Atchafalaya Delta show that discharge and suspended load vary greatly between flood and nonflood conditions. Recognition of sedimentary environments, each characterized by sedimentary structures and grain size, indicates rates and modes of deposition. Processes of deposition in this fluvially dominated environment are characterized by rapid settling of suspended loads generally under waning flood conditions.Key words: new Mississippi delta, Atchafalaya River, suspended sediment, river-dominated deposition

ISSN:0706-652X    

DOI:10.1139/f83-269

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

The Yangtze River, fourth largest in the world in terms of sediment discharge, contributes about 500 × 106 t annually to the East China Sea. A disproportionate part of the load is carried after peak summer discharge, presumably the result of draining the rice fields. During a week-long study of the estuary, greatest water discharge was observed in the North Passage. Suspended sediment concentrations were highest in the South Channel of the South Passage, although in contrast to the North Passage little of the suspended sediment was coarse. The high sediment concentrations in the South Channel appear to be the result of oscillatory (net transport upstream) movement of material, in contrast to the marked seaward transport measured in the North Passage.Key words: Yangtze River, sedimentation, discharge, transport, estuary

ISSN:0706-652X    

DOI:10.1139/f83-270

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

During the 10 yr, 1970–80 a variety of field measurements using some new techniques have allowed the distribution of muddy sediment in the Severn Estuary and Inner Bristol Channel to be defined. Based on these measurements, a behavioral model has been adopted comprising mobile and stationary suspensions, and settled mud. The regional distribution of mobile suspensions shows a suspended solids front located along the main channel axis with the more turbid water on the southern, English, side. At maximum current velocities on spring tides, vertical suspended sediment profiles are homogeneous. As velocity decreases, a stepped structure develops due to settling, which is then remixed on the next accelerating semidiurnal tidal phase. Over the lunar timescale these steps become more stable, eventually settling to the bed. The steps are unrelated to salinity, temperature, grain-size, or mineralogy. On neap tides, layered, acoustically detectable stationary suspensions, which occasionally consolidate to form settled mud, develop from the mobile suspensions. An idealized "three-zone" model has been devised characterizing the structures developed in the mobile suspensions.Key words: Severn Estuary, vertical profiling, siltmeter, densimeter, suspended solids, fronts, stratification

ISSN:0706-652X    

DOI:10.1139/f83-271

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

Characteristics of suspended cohesive sediment from the benthic boundary layer of the Bristol Channel/Severn Estuary were examined at one location (51°27.66′N. 2°53.20′W). Samples were collected in vertical profiles over the bottom 2 m of the water column at times near to, and at low and high slack water periods on spring and neap tides. Primary particle size distributions obtained using a Coulter Counter (range 1.59–50 μm) and electron microscopy (range 0.1–20 μm) reveal bimodal (volume) frequency distributions with a lower mode fixed at 1.59 μm and a higher mode variable between 6 and 30 μm. When these data are converted to number frequency distributions, the higher mode effectively vanishes, while the lower is retained. Suspended sediment mineralogy, of dispersed samples, was determined by individual and multiple particle examination using analytical electron microscopy. Average suspended sediment composition was found to be illite 44%, quartz 23%, chlorite 19%, calcium carbonate 5%, kaolinite 4%, and organic carbon 3% (by weight). Floc diameters estimated from hindered settling experiments were ≈ 140 μm possessing a sedimentation velocity in the range 1.8–2.0 mm∙s−1, in good agreement with field observations of temporal changes in the vertical distribution of turbidity.Key words: suspended cohesive sediment, flocculation, Bristol Chan

ISSN:0706-652X    

DOI:10.1139/f83-272

出版商:NRC Research Press    

年代:1983

数据来源: NRC

摘要:

This study extended from the Delaware River down the Delaware Bay into the upper Delaware Bay. The suspended material concentration in the lower Delaware River and upper Delaware Bay was determined by filtration, and the size and nature of flocs were determined by optical microscope. The principal turbidity maximum, at a salinity of 1‰ was composed of the largest flocs (up to 130 μm diameter), and sizes found in the water column decreased progressively down the Bay. This maximum was probably related to the formation of flocs. A second turbidity maximum, not related to floc formation or resuspension, may have occurred at a salinity of 5‰.Key words: flocs, turbidity maximum, Delaware Bay

ISSN:0706-652X    

DOI:10.1139/f83-273

出版商:NRC Research Press    

年代:1983

数据来源: NRC