Showing papers on "Siltation published in 1987"

Effect of siltation on stream fish communities

Abstract: The effect of siltation on stream fish in northeast Missouri was evaluated using community structural measurements and a functional approach that emphasized feeding and reproductive guilds. As the percentage of fine substrate increased, the distinction among riffle, run, and pool communities decreased, primarily because the number of individuals of typical riffle species decreased. Within the riffle communities the abundance of fish of two feeding guilds — benthic insectivores and herbivores — was reduced as the percent of fine substrate increased. The abundance of fish in other feeding guilds was not affected. The only reproductive guild to be similarly affected was the simple and lithophilous, whose members require a clean gravel substrate for spawning. Species within each guild affected by siltation had significantly similar trends in abundance. The guild analysis indicated that species with similar ecological requirements had a common response to habitat degradation by siltation.

Flow dynamics and suspended sediment properties in arid zone flash floods

Abstract: Hydrological process in arid zones differs substantially from that in better documented humid environments. The ponding point for infiltration is reached within 10 mins of first rain and overland flow forms the major component of basin runoff. Drainage densities are high, approaching 100 km.km−2, maximising the opportunity for both water and eroded soil to reach the channel network. The typical flood bore is not as abrupt as the mythology of desert streams would suggest. Nevertheless, the time of rise of the flood hydrograph is usually between 4 and 16 mins, giving credance to the notion of ‘flash flood’. Measured flows remain subcritical in the main, though Froude number exceeds unity for short periods around peak discharge. Flow is exceedingly turbulent, with Reynolds number > 105 even for much of the recession limb. As a result, suspended sediment concentrations by size grade are shown to be hydraulically controlled. However, the high degree of turbulence and wide availability of sediment from hillslope and channel sources also means that the mean size of the suspended load varies systematically with flow parameters. In this respect, ephemeral streams differ from perennial counterparts in humid environments where no clear-cut relationships exist. There is greater prospect of deriving a physically deterministic model of suspended sediment transport in desert streams. Implications for soil erosion and reservoir siltation are discussed, and sediment is traced from its source to its various sinks within the drainage basin.

Ecological effects of groundwater pumping and a natural drought on the upper reaches of a chalk stream

Abstract: The ecological effects are examined of a three-month test of a groundwater pumping scheme, which augmented flow in a chalk stream in autumn 1975. The impact on the macrophytes and invertebrates in the upper perennial stream which received pumped water were found to be minimal. After pumping ceased, a dry winter and spring led on to a major drought in summer 1976. This prompted operational use of the groundwater pumping scheme in late summer 1976 prior to heavy winter rains which resulted in a return to the normal pattern of discharge in 1977. The ecological effects of the drought, the operational pumping and its aftermath are assessed on three channel reaches: the intermittent zone of the stream, which remained dry throughout 1976; the upper perennial channel where changes in flow regime were most severe; and the lower perennial section of the stream. Drying of the intermittent section for over one year had more severe effects on the invertebrates and fish populations than on the macrophytes, which recovered rapidly after the return of flow. In the upper perennial section, the drought led to siltation of the river-bed, loss of macrophytes and limited habitat diversity for the invertebrate fauna. Further downstream, effects were still detectable, though less severe. Operational pumping brought immediate benefit to the perennial stream by increasing the river width, removing silt and encouraging growth of macrophytes, which provided habitat and food resources for invertebrates. However, in the upper perennial reach, where siltation and loss of macrophytes had been severe, regrowth of macrophytes was slow and the effects of the drought on both macrophytes and invertebrates were still apparent in autumn 1977. It is proposed that a pumping policy that maintains river flow above the level at which siltation and subsequent loss of macrophytes occurs could minimize undesirable ecological effects of extreme low flows.

Short and long term changes in estuary capacity

Abstract: Changes in capacity have been examined in the Lune, Ribble, Mersey and Humber estuaries to assess the role played by land-derived sediments in coastal accretion. The engineering concept of dynamic equilibrium is introduced and shown to imply that land-derived sediments should contribute exclusively to coastal accretion. The various factors responsible for changing estuary capacity are also introduced. Capacity changes over the last 100 years show that all the estuaries considered have suffered large scale siltation, with the majority of the sediment being derived from coastal and offshore sources. In the case of the Lune, Ribble and Mersey estuaries, engineering works have been responsible for large permanent changes in capacity. The situation in the Humber is less clear and siltation is probably a result of natural infilling; the estuary having been over-deepened by glacial and fluvial action in the past. A detailed study of the Mersey suggests that the estuary was also over-deepened and over-widened and has only been tidal for the last 9000 years or so. Calculations suggest that a new equilibrium will be achieved in about 250 years time and that the estuary may then be in a state of dynamic equilibrium. The much smaller Lune estuary may already have achieved this state, although detailed evidence is scarce.

Ecology and conservation of Dal lake, Kashmir

Abstract: Studies carried out on the physicochemical characteristics of water and sediments, biological features and nutrient dynamics of Dal Lake by various investigators are reviewed The results indicate that the direct discharge of sewage from houseboats, reduction of plant cover in the catchment area, interruptions to the flow of water, human encroachments, increase in population within the catchment area, etc, have resulted in the deterioration of water quality, prolific growth of aquatic macrophytes and siltation Based on the data available some measures are proposed for conservation of the lake's ecosystem

Stream regulation in Argentina, including the superior Paraná and Paraguay rivers

Abstract: Argentine water resources are located in three hydrologically contrasting regions: the so-called Del Plata basin, the Arid Central Andean area, and Patagonia. About 150 dams and impounding reservoirs that exceed a capacity of 150 × 103 m3, have been built. Most limnological studies of regulated rivers have been conducted recently. In the Del Plata basin, Argentine dams are scarce, although many have been built or are projected upstream, mainly in Brazil. The ecological problems of the reservoirs are related to decomposition of inundated terrestrial vegetation; algae blooms, dissolved oxygen depletion phenomena, eutrophication, impaired fish migrations, overproduction of water hyacinth Eichhornia spp., and schistosomiasis. The importance of examining the influence of the projected middle Parana dams on downstream sediment regimes, especially as it affects the delta, is emphasized. In the Arid or Central Andean region, the main development of the Grande-Tercero rivers includes dams, reservoirs, and the operation of a nuclear power plant. One of the most important effects will be a slight increase in the water temperature of the Rio Tercero reservoir as a consequence of water use in the cooling system of the nuclear power plant. In the Patagonian region, the main dams are those built and projected for the Colorado and Negro basins. In the Negro basin studies are being carried out mainly in the Ramos Mexia Reservoir, on different aspects of zooplankton and phytoplankton, benthos, and fishes. Commercial fishing is not permitted and fishing is based mainly on the introduced salmonids which show symptoms of undernourishment because of a lack of forage fishes. Indigenous fishes have been decimated by the introduced salmonids. Although studies have been conducted in several areas, much work remains to be done in relation to the environmental effects of dams on streams in Argentina. Results from ecological studies are still insufficient to solve the multiple problems associated with them, such as climatic influences, retention of suspended solids, deforestation, schistosomiasis and other aquatic diseases, pollution, eutrophication, fish-ways and fish migrations, accumulations of the water-hyacinth, salt and nutrient regimes, and siltation. Even partial solutions to these problems will require an ensemble of urgent measures to assure the maximum benefits with minor risks and ecological costs.

Channel sedimentation within urban gravel bed rivers

Abstract: Fine substrate sediments are considered to be important in the management of urban river systems. Urban construction activities have been reported to increase sediment loads causing the temporary siltation of channel substrates within the urban area. Nevertheless fine sediment derived from urban areas frequently carry toxic material well in excess of background concentration levels. While the soluble phase of heavy metals and the importance of their association with suspended sediment has received considerable attention, longer term studies of fine urban river-bed sediments are limited. Furthermore studies of heavy pollutants in active stream sediments, below mine waste tips, have shown the channel substrate can provide a long term store for heavy metals in association with fine sediments. This thesis investigates the variety of impacts that urbanisation has upon the sedimentation of gravel bed rivers. A freeze coring technique and infIltration baskets have been used to study the textural-geochemical properties of fme matrix sediment and its development within an urban river-bed framework, within and below a number of contrasting urban catchments in the U.K. Complex urban hydrological and sedimentological regimes are shown to have a variable influence upon matrix sedimentation. The actual volume of matrix present within the urbanised substrate is influenced by the degree of urbanisation within the catchment. Furthermore this sediment is finer in size and associated heavy metal concentrations are well in excess of natural background levels. Although heavy metal levels do correlate slightly with textural characteristics, the presence of maximum concentrations at depth in the substrate indicate possible mobilisation of metals within the urbanised river-bed. The temporal behaviour of matrix development within an urbanised substrate is shown to differ from natural river-beds. Despite high suspended sediment concentrations the magnitude of the potential rate of supply is lower, by 50 percent, and dominated by organic material. This sediment also contains elevated heavy metal cocnentrations. This contrasts to the inorganic sediment ingress of natural river substrates. It is concluded that fine matrix sediments within urban gravel bed rivers should be at least of concern to public health engineers, water authorities and conservationists.

Mining Pollution: the Case of the Baguio Mining District, the Philippines

Abstract: Environmental problems caused by improper mine tailings disposal in the Baguio district include pollution of the Lower Agno River system and its watershed and siltation of irrigation canals in the Pangasinan plains. Direct economic losses are from reduced agricultural production due to siltation of irrigation works and farmlands. To check the adverse ecological effects of improper mine tailings disposal, government regulations have been imposed on mining firms. Several disposal schemes have been proposed, including the use of the reservoir of a multipurpose project to be sited in the watershed where the mines are located. Because of siltation problems, however, trapping the tailings in the reservoir will diminish the economic benefits that can be derived from the project.

Siltation of the Tamar River

Abstract: The results of an investigation to study the causes of siltation of the Tamar River at Launceston, Tasmania, are summarised and possible remedial measures to reduce the problems caused by siltation are discussed.

Physico-chemical limnology of Zoo Negara Lake, Malaysia

Abstract: A physico-chemical Iimnological survey on Zoo Negara Lake and its incoming water from the upstream of Kemansah River was carried out from May to October 1981. Generally the water in both the stream and lake is weakly acidic with low alkalinity and low nutrient concentrations. The physical and chemical parameters in the lake are higher than that observed in the upstream reaches of Kemansah River especially in turbidity and total residues. The high turbidity and residues were due-to the high loading of erosion particles resulting from the housing development in the watershed. The lake is threatened by erosion and siltation problems, but it is free from biogenic pollution.

Siltation and Ocean Dumping Problems at Dalhousie, New Brunswick

Abstract: Dredging of the Dalhousie Harbour and the ocean disposal of dredge spoils had been ongoing for many years prior to the implementation of the Ocean Dumping Control Act of 1975. Due to discharge from local industries and the open storage of bulk lead/zinc ore concentrates at the port, pollution levels in the Harbour were elevated and by the early 1980's open water disposal of the dredgate was no longer permitted. As a result of the elimination of Ocean Disposal and continued high rates of siltation, a siltation study has been commissioned by the Federal Government to determine ways and means to reduce or eliminate the maintenance dredging requirement through the development of engineering concepts. This paper will address the methodology developed to reduce the maintenance dredging requirement through the findings of the Siltation Study.

Sedimentation Problems and Management Implications in Hydro-Electric Storage Lakes of South Island, New Zealand

Abstract: Studies of hydrodynamics and sedimentation in the large natural and man-made lakes of the South Island have identified potential problems of sedimentation and management options to minimise their effects. Problems have arisen by controlling the natural range of lake levels causing erosion on shorelines with small sediment volumes and low natural sediment supply. Deep-water sedimentation is dominated by lake head rivers, from which bottom flowing turbidity currents and underflows are common, both of which transport sediment downlake to be ponded behind the dam face. On low angle muddy deltas rotational slumping is common and transports very large volumes of sediment downlake. Operational guidelines have been implemented on lakes to minimise beach erosion. Bottom openings in dams can be scheduled to utilise the naturally occurring bottom flows to scour sediment out of the lakes. The natural lakes trap most of the sediment from upstream catchments and man-made lakes built downstream do not have siltation problems unless water is sourced from catchments bypassing the natural lakes.

Silt Concentration Measurements

Abstract: : In the area of the Port of Rotterdam a number of possible means of reducing the costs which arise from the need for maintenance dredging and of reducing costs resulting from environmental problems are being investigated. The limit or concentrate siltation or to improve the efficiency and effectiveness of dredging and disposal operations. To this end a number of possibilities are being considered. In order to investigate means of limiting siltation or concentration, simulation of all conditions must be conducted. It is therefore essential that the relevant processes and boundary conditions be described and that of these parameters, silt concentration, it appeared that there were very few records available and that there was no system for analysis and presentation. An information system has therefore been designed, which enables determination of the silt concentration over an extended time period. This paper indicates the present state of development of this system and shows that the knowledge which has been gained so far can contribute to the solution of other problems in which the same parameters are significant.