Showing papers on "Siltation published in 1990"

Siltation of stone-surface periphyton in rivers by clay-sized particles from low concentrations in suspention

Abstract: The observation that deposits of fine sediment are found on stream beds only in areas of slower water velocity promotes a common misunderstanding of the depositional behaviour of fine suspensoids in flowing water and a disregard for the potential for siltation effects on the biota on the surface of stones in fast flowing water. A model for deposition from turbulent water, whereby particles are lost from suspension where water currents are slowed by boundary friction, provides an explanation for silt infiltration into epilithic periphyton. Theoretically calculated deposition rates of clay sized mineral particles at low suspended concentrations (2 to 5 g m−3) were found to account for observed rates of silt accumulation in epilithic periphyton in a braided river in the South Island of New Zealand. At concentrations between 1 and 10 g m−3 of suspended mineral silt during normal flow, silt accumulation in epilithic periphyton accounted for about 50% of its dry weight. This caused a reduction in the mean organic content of the periphyton to 22% of the dry weight compared to 52% in a reference stream where the concentration of suspended mineral particles was less than 1.0 g m−3 during non-freshet flow. This reduction in proportional organic content is discussed in terms of diminished food value of the periphyton and potential interactions between periphyton and invertebrate consumers.

Micromorphological assessment of drain siltation risk indexes in a saline-sodic soil in Monegros irrigation district (Spain)

Abstract: As a result of some research conducted on a saline-sodic Xeric Torrifluvent in the irrigation district of Monegros (Huesca, Spain), it has been found that the values of risk indexes for drain siltation based on particle size determinations do not fit with the quick infilling of the pipes by fine materials. Micromorphological studies of soils, trenches, envelopes and pipes were a useful tool to check the validity of some indexes for predicting the suitability of soils to be pipe drained. Moreover, the micromorphology of infilling allowed also the characterization of drain siltation processes. In the studied soils the micromorphology of drain infillings differs from the siltation of hydromorphic soils. Moreover, the first step in siltation occurs already as a layered, fine-grained and sorted deposit (“secondary siltation”). This micromorphological approach gives information about siltation episodes, internal arrangements, particle size distribution along layers, composition and morphology of grains and fragments, as well as flow and deposition conditions.

A man's role in the degradation of soil and water resources in Sri Lanka: a historical perspective.

Abstract: The upper catchment areas of the major rivers situated in the central Highlands of Sri Lanka had been stripped of the natural vegetation to make way for the plantation agriculture during the British period. Large areas of forest on steep slopes were cleared within a period of less than 100 years and the consequences of these actions were immediately visible in the form of accelerated soil erosion, siltation of low lying areas, frequent flooding, drying out of streams, etc. Even after Independence, land clearing continued unabated. Today only small patches of forest are left, even those are not in the critical areas. Most of the ten plantations have lost the fertile upper soil layer due to erbsion. Adoption of proper conservation strategies in the upper catchment areas to protect soil and water resources from further destruction is of paramount importance now more than ever before.

Soil erosion and suspended sediment production in Hong Kong: development of an assessment strategy.

Abstract: A review of the erosion and sedimentation problem in Hong Kong is presented. It is suggested that expansion of monitoring might in the first instance concentrate upon assessing sediment yield from drainage basins. The use of rating curves to estimate yield is discussed. Rainfall and runoff records are utilised to examine the length of the monitoring period necessary to determine long term average sediment yield. Relating onsite erosion to sediment yield at the basin outlet is identified as being an important future objective while evidence is presented that any monitoring program should incorporate sediment quality in addition to quantity.

Model study of dredged channel siltation in offshore open water

Abstract: Based on the model study, the siltation of dredged channel under the condition of clear water and sediment-water mixture in Lianyun Harbor, Jiangsu Province and the experimental method under complex hydrodynamic elements such as tides, winds, waves and tidal currents are studied. The regularities of the variation of the velocity in the dredged channel in clear water are discussed as well. A group of curves such as water depth, velocity and angle has been plotted as a guide to the selection of a proper direction for a dredged channel. The intensity of siltation and the rate of deposition in the condition of sediment-water mixture, and the relationship between the dimensions of the dredged channel and the deposition amount can be estimated.