Showing papers on "Siltation published in 1985"

The political economy of soil erosion in developing countries

Abstract: Argues for combining the study of physical and social processes to study the political economy of soil erosion; the study must include a 'place-based' analysis of soil erosion, where it actually occurs, where flooding and siltation caused by soil erosion in one place affects another, and where land users have been spatially displaced to and from areas. It must also include 'non-place- based' analysis of the relations of production under which land is used including land tenure, rents, prices of agricultural inputs and outputs. Bringing these two analyses together, a 'bottom-up' approach is outlined in which the focus is first directed to the smallest unit of decision making in the use of land, the family and the household, up to the government and administration. At the latter level, it looks at where power lies and how it is used.

A depth-integrated model for suspended sediment transport

Abstract: A model for suspended sediment transport in unsteady and non-uniform flow is derived, in which the vertical dimension is eliminated by means of an asymptotic solution. The resulting depth-integrated model is tested for unidirectional flow cases. Also, an application is given to the siltation of a dredged channel.

Fish Communities of Midwestern Rivers: A History of Degradation

Abstract: Since humans first crossed the Bering Strait land bridge, growing human populations and their increased technological impact have degraded water resources in North America. Initially, this degradation was transitory and local, but as population centers developed and urbanization increased, many areas developed chronic water resource problems. In the Laurentian Great Lakes, for example, sport and commercial fish resources declined following selective overfishing; extensive watershed modifications, including drainage of lake-margin wetlands; introduction of exotics, especially the sea lamprey, alewife, and salmonids; and progressive chemical modification of lake environments (Emery 1981, Francis et al. 1979, Smith 1972, Wagner and Stauffer 1982). Chemical change began with an acceleration of natural processes (e.g., nutrient enrichment and siltation of shallow areas) and was compounded by drainage of wetlands, discharges of synthetic chemicals and chemical wastes, and increased water temperatures stemming from other technological changes. Each lake's fishery was more or less altered, depending on the size, geographic location, and relative influence of these factors on each lake. Watershed modification and chemical contamination, in particular, have severely altered tributary streams

Soil erosion and sediment transport research in tropical Africa

Abstract: This paper reviews the magnitude of soil erosion in tropical Africa and relates it to erodibility, erosivity and landform in different ecological regions There are few direct measurements of erosivity and erodibility in tropical Africa and the relevance of using the Universal Soil Loss Equation in estimating these parameters is reviewed. Soil erodibility is not a fixed parameter and changes with time. Although localized rates of soil erosion can be high, the erosion rates derived from sediment loads in rivers are often low. Most of the available data on sediment loads of African rivers are 10–20 years old, and little research information is available on the delivery ratios associated with different catchments. Rapidly changing land use is one of the major factors responsible for accelerated soil erosion, and the effects of deforestation, grazing, fire, and of cultural practices are discussed. The economics of soil erosion is reviewed in terms of loss in productivity and siltation of reservoirs. R...

Sediment yield estimated from reservoir siltation in the Ochil hills, Scotland

Abstract: Measurements made on the floors of the temporarily-drained Glenfarg and Glenquey Reservoirs indicate that sediments with wet volumes of 63.94 × 103 m3 and 12.64 × 103 m3 were deposited in 56 and 73 years respectively. These figures represent 2.5 per cent and 1.1 per cent losses of original storage capacity. When corrected for water, organic, and diatom skeleton contents, and reservoir trap efficiency inorganic sediment yields of at least 31.3 tonnes km−2 yr−1 and of 9.0 tonnes km−2 yr−1 are suggested. The difference is probably related to contrasts of land use.

Basic Characteristics of Sediment Transport in Hangzhou Bay

Abstract: Entering such a funnel-shaped Hang zhou Bay, the actions of both tide and tidal current are strong.The Bay is connected with the Yangtse Estuary whose annual sediment discharge of 480×10~7 tons is carried ocenaward, and, through analysising the temporal and spacial distri-butions of suspended sediment and bottom sediment,the sediments of the Bay come mainly from the Yangtse Estuary. Sediments exchange between the Yangtse Estuary and the north-bank of Hangzhou Bay can also be analysed from salinity, tide, tidal currents and residual currents.Sediments of the Yangtes Estuary diffuse toward the south through the mouth of Hangzhou Bay when sediments is carrieda in or out of the Bay, and under the flood and ebb currents from the east and the west, its total amount may reach 1×10~9 tons per day.So, this sediments exchande is enormous, but the amount of sediment carried in or out of the Bay is relatively small, only 4×10~6 to 11×10~6 to tons per day.Thus,in general, there seems a rough balance exists between erosion and siltation in the Hangzhou Bay.However, the local change with erosion and siltation in the Bay is adjusted by the sediments′longshore transport.

Management of watersheds and water resources planning

Abstract: Siltation of reservoirs is a consequence of poor management of watersheds. Four case studies have been highlighted in this study to indicate the reasons for increased siltation, such as population growth, deforestation, extensive agricultural operations, cattle grazing and soil erosion. Nizamsagar and Sriramsagar are reservoirs heavily silted due to lack of proper management practices in the watershed. Machkund and Tungabhadra are reservoirs in which siltation has been contained through systematic management of the watersheds. Procedures needed for managing watersheds in water resources projects have been derived based on the four case studies

Further studies of the field settling velocities of Thames mud

Abstract: This report describes work undertaken to meet an important need in the successful modelling of siltation processes and prediction of siltation rates. Most previous work on settling of solids from suspension has been laboratory based with little or no attempt to corroborate results in the field. Mr M W Owen developed an appropriate field instrument in 1969 and carried out a limited amount of field measurement, which indicated that settling velocities of cohesive sediments were 10 times higher than previously measured in laboratory tests.

A mathematical model of mud transport in deep partially mixed canalized estuaries

Abstract: The report describes the theory and first application of a new type of multi-layer model which can simulate the interaction of tidal and fluvial flows on saline intrusion, mud transport and siltation in deep partially mixed canalised estuaries. The physics of the various interacting processes are described. Novel aspects include the method of calculating vertical mixing and the modelling of the formation of a thin layered slack water deposit.

A Subtidal Survey of the Tasmanian North West Coast for Tioxide Pty Ltd.

Abstract: There is definite evidence of environmental disturbance at the Tioxide sites with neasurable effects extending to Burnie and Sulphur Creek.Species richness of both animals and plants is scmewhat reduced and there is a clear indication of a change in cqnnunity structure within this region. We suggest that the causes are primarily siltation ard turbidity which reduce light levels.