Showing papers on "Siltation published in 1996"

Fine-sediment Dynamics in the Mekong River Estuary, Vietnam

Abstract: Field studies of fine-sediment transport were carried out in the Mekong River estuary, Vietnam, in the high-flow season of November 1993. In the freshwater region, erosion and deposition of suspended sediment occurred at tidal frequency with a strong down-river transport at a mean velocity of 1 m s−1and mean suspended-solid concentration of about 250 mg l−1. Also, the suspended sediment was mostly fine silt, and the clay fraction accounted for only 15%. The suspended sediment was transported either as individual particles or agglomerated with organic detritus. At the mouth of the estuary, a salt wedge was present but was flushed out of the estuary at low tide. Flow reversal occurred across the pycnocline. A turbidity maximum zone was present at the toe of the salt wedge at flood tide. Most of the suspended sediment was coagulated with little organic matter. The bulk of the suspended sediment was exported to coastal waters, but some sediment returned to the estuary in the salt wedge. Data for the low-flow season are sparse but suggest that partially well mixed estuarine conditions prevail, with salinity penetrating about 40 km inland, carrying fine-sediment up-river to a turbidity maximum zone. Most of this sediment would have been deposited in shallow coastal waters in the previous high-flow seasons. The construction of large hydro-electric dams further upstream on the Mekong River may exacerbate siltation patterns in the estuary.

Mechanical soil conservation with contour ridges: Cure for, or cause of, rill erosion?

Abstract: Based on the results of an erosion damage assessment in Southern Zimbabwe, where mechanical conservation work has been carried out since the 1940s, this paper describes the impact of mechanical conservation systems on processes leading to rill erosion. In a study of a catchment area, it was found that influxes of water from roads and waterways as well as contour ridges that were originally designed to control rill erosion had a major role in the formation of rills. Existing rills and depressions which cause water concentration, siltation of contour drains and overflowing of contour ridges were the main factors leading to excessive rill erosion. The study showed that particularly during a highly erosive year like 1992/93, the damage due to rill erosion can be excessive, causing an abrupt degradation. It is concluded that the present conservation system is insufficient to control rill erosion effectively and is often the cause of this erosion. Effective control of rill erosion is a pre-condition for optimal implementation of land management systems such as conservation tillage. Therefore, an integrated approach to land husbandry must be developed jointly with farmers and promoted in order to improve crop production and sustainable management of natural resources. This should consider improved mechanical conservation as well as agronomic and biological soil and water conservation techniques.

Numerical Modeling for Sediment-Pass-Through Reservoirs

Abstract: Sediment accumulation in reservoirs on the North Fork Feather River will soon affect hydroelectric power generation. For sediment control, low-level outlets through the dams have been considered to allow Sediment-Pass-Through (SPT) the reservoirs. The objective of SPT is to maintain sediment balance through the reservoirs with no net erosion or deposition over an extended operating cycle. The velocity required for bed sediment to pass through a reservoir will be achieved by reservoir level drawdown during large floods. A numerical modeling study, using FLUVIAL-12, was made to evaluate the feasibility and effectiveness of SPT for the system. The study developed the following features: (1) rating curves that specify the reservoir drawdown in relation to the changing discharge; and (2) reservoir operation rules that specify the operational procedure and settings for control gates. Simulated results are presented to demonstrate that mass balance for the river/reservoir system can be achieved, without adverse siltation in the river channel to affect fish spawning. This approach for reservoir sediment control should be useful for similar problems.

The GCTE Soil Erosion Network: A multi-participatory research program

Abstract: One of the major threats for sustainable land management is soil erosion. Erosion is a major type of human-induced land degradation; while recognizing that an accurate figure is hard to establish, Oldeman et al. (1991) suggest that about one-sixth of the world's usable land has already been degraded by-water or wind erosion. Soil erosion can also cause off-site environmental problems such as increased dust in the air (Stetler et al. 1994; EPA 1990), increased transport of sediments to rivers and lakes (Robinson 1979), and siltation of reservoirs (Tagwira 1992). Global change may exacerbate current problems. The “Global Change and Terrestrial Ecosystems Core Project (GCTE) of the International Geosphere-Biosphere Program (IGBP) has established an international research network to improve predictive capability for soil erosion under global change. The network involves experimentalists undertaking process-level experiments and monitoring programs and modelers. This paper provides an overview of the network. Other papers in this issue address specific topics being addressed by the network”. GCTE background Global change presents a formidable and unique research challenge. The world's terrestrial ecosystems are being subjected to rapidly changing environmental conditions, both in their rate and in …

Modelling the long-term course of non-flushed reservoir sedimentation and estimating the life of dams

Abstract: An analysis of hydrologic and geodetic data from numerous reservoirs lying in different climatic zones, has allowed the two main phases in filling the reservoir during its life to be distinguished, firstly silting of an originally deep reservoir, and secondly silting of a reservoir which has become shallow or initially was not deep. During the first phase, the reservoir plays a role as an accumulator of sediment, so that its trap efficiency for total mineral material is much reduced, and its trap efficiency for suspended load decreases to zero. This phase ends when the mean depth reaches a critical value which is specific for each reservoir. During the second phase, shallowing of the reservoir is much slower, and over short time periods it can play a role as a net exporter of sediment. A general model of reservoir sedimentation is proposed and is tested by data on the long-term and seasonal courses of siltation in selected reservoirs in the Vistula drainage basin, Poland. The rate of sedimentation is analysed for both phases of siltation, and the useful lifetime of a reservoir, which corresponds to the first phase of siltation, has been computed according to a methodology proposed.

Environmental impact of mining, erosion and sedimentation in Sri Lanka

Abstract: Erosion and sedimentation have a devastating effect on the environment of Sri Lanka. The Central Highlands with very steep slopes and a radial network of rivers that begin in these mountainous regions cause major erosional hazards, often resulting in disastrous landslides. Mining, particularly gem mining, has been responsible for serious problems of soil erosion and river siltation. A large number of illegal mines leave mine spoils resulting in damage to nearby paddy fields, crops, rivers and streams. Extensive sand mining in some rivers has lead to the collapse of river banks, destabilising structures such as bridges and causing influx of salt water. Landslides and coastal erosion are other factors that have had a negative impact on the environment in Sri Lanka.

The Unionid Mollusks of the Big and Little Nemaha River Basins of Southeastern Nebraska and Northeastern Kansas

Abstract: A qualitative survey of the unionid mollusks of the Nemaha basins resulted in the recovery of twenty-seven taxa and the confirmation of one additional taxon through museum records. The channelization of the basins, combined with siltation and possibly the impacts of various pollutants, have severely impacted the bivalve fauna ofthe region, eliminating of as many as 61 percent of the documented unionid taxa of the basins.