Showing papers on "Siltation published in 2000"

Pollution by Mud of Great Barrier Reef Coastal Waters

Abstract: Oceanographic data from the coastal waters of the Great Barrier Reef near Cairns were obtained for two months in the 1997 dry season when river runoff was negligible. A muddy coastal zone was found with suspended sediment concentration reaching 1000 mg/L during trade winds. These high turbidity events are very stressful conditions for coral reefs and were caused by resuspension of sediment by wind waves. In calm weather a nepheloid layer prevailed, extending about 30 km offshore and carrying coastal mud toward offshore coral reefs. In this layer, muddy marine snow with flocs exceeding 1000 µm in diameter was found inshore, while only small flocs < 100 µm were found offshore. Near-surface visibility may have decreased by 50% in the last 70 years. High turbidity appears to be due to man and ultimately threatens the main body of the Great Barrier Reef.

Hydraulic Resistance Induced by Deposition of Sediment in Porous Medium

Abstract: The deposition of the instantaneously released sediment into a gravel matrix and the hydraulic resistance induced by the rapid siltation are investigated herein. The experimental results reveal that the depositional patterns of the sediment are governed by the gravel-sediment size ratio, the amount of sediment released, and the seepage flowrate. The observations also indicate that the stable stage of sediment deposition is reached shortly after the occurrence of sudden slump. A regression relation is developed to quantify the stable-stage hydraulic resistance with the major governing factors. Given the hydraulic resistance, one can use the siltation equation to evaluate the hydraulic conductivity of the silted porous medium. With such information, planning of the engineering alternatives to meet the seepage flow requirements is made possible.

Soil erosion evaluation in a small watershed in Brazil through 137 Cs fallout redistribution analysis and conventional models

Abstract: An investigation of rates and patterns of soil erosion on agricultural land cultivated with sugarcane was undert a ken using the 137Cs technique, USLE (Universal Soil Loss Equation) and WEPP (Water Erosion Prediction Project) model. The study was carried out on a representative catchment of a small watershed of the Piracicaba river basin, State of Sao Paulo, Brazil, called Ceveiro watershed, well known for its severe soil degradation caused by erosion. The results from the 137Cs technique indicate that most part of the studied area (94%) are eroded at erosion rates that go up to 59 Mg ha-1 y-1, with a weighted average rate of 23 Mg ha-1 y-1 . The weighted average rate of infield deposition and sediment retrieval that occurs in only 6% of the total area was estimated to be around 12 Mg ha-1 y-1 . These values led to very high net soil loss from the field, with rates of the order of 21 Mg ha-1 y-1 , which represents a sediment delive ry ratio of 97%. A linear correlation between soil erosion rate estimated by the 137Cs technique and the amount of available K in the top soil layer (0-20 cm) was observed. Based on this correlation the estimated amounts of net and gross K loss in the grid area due to soil erosion were of 0.2 and 1.52 kg ha-1 y-1, respectively. The erosion rate estimated by USLE was 39 Mg ha-1 y-1 and by WEPP model 16.5 Mg ha-1 y-1 with a sediment delivery of 12.4 Mg ha-1 y-1 (75%). The results are a confirmation that the soil conservation practices adopted in the area are very poor and can explain the high siltation level of water reservoirs in the watershed.

Hydraulic resistance induced by deposition of sediment in porous medium. technical note

Abstract: This note investigates the deposition of the instantaneously released sediment into a gravel matrix and the hydraulic resistance induced by the rapid siltation. The experimental results reveal that the depositional patterns of the sediment are governed by the gravel-sediment size ratio, the amount of sediment released, and the seepage flowrate. The observations also indicate that the stable stage of sediment deposition is reached shortly after the occurrence of sudden slump. A regression relation is developed to quantify the stable-stage hydraulic resistance with the major governing factors. Given the hydraulic resistance, one can use the siltation equation to evaluate the hydraulic conductivity of the silted porous medium. With such information, planning of the engineering alternatives to meet the seepage flow requirements is made possible.

The Loano Artificial Reef

Abstract: In the past 40 years the coastal marine environment in the Western Ligurian Riviera has been affected by three major human activities: (1) Illegal fishing, especially otter-trawling, in the shallow (< 50 m) coastal zone where this type of gear is banned. The problem is often compounded by the use of undersize net mesh in the cod end of the trawls. (2) Pollution, mainly sewage discharges into the sea. Such discharges increased considerably in recent decades following an increase in the number of inhabitants in the Italian Riviera and the success of the tourist industry. (3) Siltation caused by badly planned beach replenishment schemes designed to repair sandy shores. These schemes have used fine silty material rather than sand or gravel with the result that the seabed has become muddy and marine flora has been destroyed. Where the result is the decline or disappearance of the seabed-stabilizing Posidonia (seagrass) meadows a chain reaction may be involved in which storms become even more destructive to beaches and so more material has to be discharged to maintain the sandy shore for tourism. The result is that the natural rocky seabed is disappearing while the sandy seabed is becoming abiotic at a macroscopic level. The effect of this situation on small coastal fisheries has already resulted in a significant decrease in quality and quantity of fish caught.

Geomorphic and sedimentary evidence of human impact on the new zealand coastal landscape

Abstract: The coastal landscape of New Zealand has been utilized heavily by humans for the last 600 to 800 years, first by Polynesian settlers who disturbed native forests through burning and later by Europeans who continued forest burning and introduced logging and grazing in the mid-19th century. Whangape Harbor and its catchment in Northland, North Island, is an example of a heavily used coastal landscape where the impacts of human use are clearly evident on the deforested and eroding slopes of the catchment, and in the harbor where siltation is contributing to expansion of mangrove forests and a deterioration in the quality and quantity of seafood stocks. This paper documents the physical condition of Whangape Harbor and its catchment and uses sedimentological data (grain size, magnetic susceptibility, pollen) to establish links between sediment sources, pathways, and sinks. Radiocarbon dating of in situ estuarine shells suggests that sedimentation rates in the estuary have increased by an order of magnitude du...

Floating islands: a unique fish aggregating method

Abstract: A fish aggregating device made of aquatic weeds and grass (Phoom) is used in Loktak lake in the northeastern region of India. It has been used successfully in this very productive fishery for centuries. Today, the fishery itself is under pressure from overexploitation, soil erosion leading to siltation and a hydroelectric project that has blocked the migratory route of the fish that used the lake as a breeding ground.

Entrance flow control to reduce siltation in tidal basins

Abstract: A means of controlling the flow in the entrance of a tidal basin, which reduces the siltation rate in the basin mouth, is described. The flow is controlled by a structure close to the basin wall that reduces the strength of the mixing layer across the entrance and the entrance eddy. Measurements of velocity and suspended solids concentration in a real tidal basin are presented together with results from hydraulic model studies. Results are also presented from a prototype installation in the Port of Hamburg. These results show siltation rates were reduced by 50% as a result of entrance flow control.

Using 137 Cs as a tool for the assessment and the management of erosion/sedimentation risks in view of the restoration of the Rainbow Smelt (Osmerus mordax) fish population in the Boyer River basin (Québec, Canada)

Abstract: The Boyer River (Quebec, Canada) drains a 217 km 2 watershed that is under cultivation at 60%. The last 2 km of the river bed has always been used as a spawning ground by Rainbow Smelts (Osmerus mordax). This fish population, which plays an important ecological role in the St.Lawrence River estuary, has dramatically declined over the last decades. Siltation and excessive algal growth in the spawning area were identified as the most probable causes of the fish population decline; suggesting that soil erosion, nutrient and sediment transport are major factors underlying the environmental problem . In this context, 137Cs provides an effective tool for investigating the magnitude and spatial distribution of long-term soil redistribution taking place in the watershed. Sampling of cultivated fields, riverbanks, bottom sediments and forested sites were thus undertaken to help understand the erosive behaviour of the watershed. Results obtained so far suggest in-field erosion rates of up to 13 t ha-1 yr-1 with net outputs reaching 11 t ha-1 yr-1. These results agree well with estimates obtained from the USLE. The 137Cs data indicate that fields located in the upstream half of the basin produce smaller sediment loadings than those in the downstream portion, despite higher soil erodibilities and more frequent ose for annual crops. They also suggest that more than 75% of the sediment deposited in the spawning area originates from cultivated fields, and less than 25% from streambanks.

Fish Fauna of the Chaliyar River, North Kerala

Abstract: The Chaliyar river is one of the west flowing rivers of Western Ghats. It has many tributaries such as Karimpuzha. Punnappuzha, Kuruvanpuzha, Tiruanchipuzha, Cherupuzha. etc. with a catchment area of 1535 sq. km. The total discharge of the river is 7775 Mm3, The river which was in a pristine condition before 4 to 5 decades has become highly degraded in the lower stretches by the effluents of Gwalior Rayons and in the upper stretches by various anthropogenic factors like deforestation, high siltation, dynamite fishing and use of copper sulphate for fishing. During the summer months, the water in the river is very low due to high run off during the wet months.

Siltation analysis in the Neyyar reservoir and forest degradation in its catchment: a study from Kerala state, India.

Abstract: Impact of catchment erosion and the resultant reservoir siltation from a tropical environment is reported here to discuss the effect of a reservoir in the natural degradation of an evergreen-forested segment. While an area of 8.01 km2 has been affected by direct inundation at the full reservoir level, another 2.6 km2 area once under thick forest cover, had also lost its identity over the last 38 years by indirect degradation. This zone mainly falls in the confluence of tributaries, namely Neyyar and Mullayar, with the reservoir. The capacity of the reservoir was found to be reduced by 28.8 Mm3 during this period and the annual average loss is calculated as 0.75 Mm3 (0.71%), indicating the intensity of erosion from the catchment zone. In case the proposed upper dam in the reservoir comes into existence, an additional area of 2.4 km2 from the virgin forest would be submerged and more area would face degradation around the inundated zone.

Silt transform characteristics and latent effect on fluvial system environment in yangtze river

Abstract: Silt, as an important physical flux, has a direct effect on fluvial environment system, Over the last forty years, the vegetation coverage has declined almost a half, and the soil erosion has expanded. Although in the 80th the silt concentration in main stream and tributary of Jinshajiang River increased 12-60 percent compared with that in the 60th, yet the sediment runoff per year through Yichang hydrological station did not increase, In the upper reaches, the large amount of silt coming from the soil erosion and the mountain gravity disaster was brought into fluvial system and piled up in middle and lower courses of the tributary streams. In over 50 % affluent channel, there has occurred severe silt accetion with its yaring ratio 5 - 20 centimeters per year. As a result, parts of the tributary are the charactersistic of braided river because of over-silting, and this has become the great incipient fault of environment in the Yangtze River. When the silting reaches to a limit or the environment changes abruptly, the silt deposited in tributaries must flow into main channel, which will greatly affect the safety of the environment in the Yangtze River. It is thus important to investigate the current conditions and the patterns of silting and silt movement in tributaries in the upreaches of the Yangtze River.

Factors accounting for the rapid siltation of hazelmere dam, kwazulu-natal

Abstract: Hazelmere Dam, located on the Mdloti river 12 km from the KwaZulu Natal coast, was completed in 1975. Since then it has lost more than 25% of its original design capacity through siltation, a fact which could threaten its economic life. This has occurred partly because of high annual average sediment accumulation rates under normal climatic conditions, and partly as a result of the 1987 floods, an extreme event with a return period of between 50 and 200 years. The objective of this work was to indicate why sediment production rates per km- of catchment area as indicated by dam siltation were higher than in most other KwaZuku Natal dam catchments. Basic rainfall, soil and slope conditions cause the Mdloti catchment to have a high erosion potential, which has been exacerbated in recent years by large scale population increases, vegetal degradation and road construction activities. These together created conditions for massive sediment transport during the 1987 floods. In terms of the theories of fl...

Impact of Changes in Mud Siltation of Dongting Lake on the Endemic Factors of Scllistosomiasis After Building Three-Gorge Dam

Abstract: Objective] To predict the impact of changes in mud siltation of Dongting Lake after building Three-Gorge dam on the endemic factors of schistosomiasis. [Methods] Prospective studies were carried out by sampling regionally on lake beaches. [Results] About 80% of mud and sand in Dongting Lake came from the Yangtzc River. The three inlets through which water flows the River to the Lake contain great amount of mud and sand. On the beaches along which the water passes, the main vegetation is reed and fewer domestic animals are herded there, and so the density of infected snails is low. Water from the four rivers to the Lake in the Province contains lower amount of mud and sand, and on the river beaches the main vegetation is cypcrus sp and willow. Many domestic animals are herded there and the density of infected snails is high. It is estimated that with in 50 years after Three-Gorge project the mud silting into the Lake would be decreased by more than 80%. [Conclusion] Within 50 years after Three-Gorge project, the snail-ridden area on the beaches of Dongting Lake tends to increase slowly; some reed beaches would be degencrated to grass and weed beaches; number of infeeting soures of domestic animaly will increase; area of endemic toci will be enlarged; and infection rate of schistosomiasis among the population who enter these areas for different activities would possiblly increase.

Sediment Characteristics and Siltation Analyses of Shanghai Yangshan Harbor and Its Approach Channel Waters

Abstract: Analyses are made on the sediment characteristics and natural conditions of water of Shanghai Yangshan Harbor and its approach channel to be constructed, and yearly siltation strength and siltation volume of basins and channels of several design schemes are calculated by siltation formnulae.$$$$

Siltation and Its Control in Downstream Tidal Gates in North of Jiangsu Province

Abstract: After the construction of tidal gates on estuaries, siltation usually occurs in downstream tide gates, which seriously affects the gate operation and management Such siltation is often caused by many factors, such as the decreasing in runoff or tidal flow, extension of falling tide period, variations of tide waves and waterways, and inappropriate tideland cultivation The siltation should be harnessed comprehensively through desilting and control measures, involving water flushing, mechanical desilting, engineering measures, and vegetation measures

Water Intake Sediment Problems in Thermal Power Plants

Abstract: Various sediment-related problems encountered in cooling water pump intakes for power plants are addressed. The extent and type of sediment problems at water intakes is different for coarse sediment moving as bed load and fine sediment moving as suspended load. Coarse sediments tend to settle in the intake, cause blockage to the flow and are more abrasive. If coarse sediments reach the pump, they may cause pump impeller damage and erosion of piping. Fine suspended sediments at high concentrations tend to cause siltation inside the pump intake structure, heat exchangers, and pipes with low flow velocities, thus adversely affecting the cooling water system performance. Also described are the desired approaches to intake design that both satisfy requirements for reliable pump operation and minimize sediment-related problems. Mitigation measures to minimize sediment deposition inside the cooling water intake layout are also presented.

Assessing the Impact of Reclamation Activities on Recent Sedimentation in Victoria Harbour, Hong Kong

Abstract: The sediments that accumulate in an estuarine environment recorded the information about the history of changes in the source of the material, the rate of deposition and the influence of man. Man's influences on Victoria Harbour are significant. To create flat land and cope with the rapid development in Hong Kong, reclamation activities have taken place about every 10～20 years in Victoria Harbour began from a few decades ago. And to maintain the navigation channel, get fill material for reclamation, many dredging activities were carried out in the Harbour. These activities changed the coastline seaward and also complicated the recent sedimentation process in the Harbour. To study the extent and history of man's influence on the sediments and recent sedimentation process in the Harbour, four vibrocores were collected in three designated locations within Victoria Harbour, Hong Kong in 1996. Sampling locations were chosen from a range of settings where reclamation or construction is planned or taking place. Each of the cores was 76 mm in diameter and 6 m in length. The sediment type and colour were describe from visual and photographic observations and by matching Munsell (R) Soil Color Charts. The 210 Pb activities for the sediments were determined by 210 Pb method and ICP-AES was employed for the determination of Zn, Cu, while FAAS for that of Pb concentrations. Sedimentation rates in Victoria Harbour have been determined to be about 0.3～2 cm/a in open areas based on 210 Pb dating and heavy metal distributions. This value can represent the natural sedimentation rates in the Harbour. Since reclamation activities have changed the coastline and affect the sedimentation process in the Harbour, sediments have been contributed by effluent discharge, materials lost from dredging and reclamation work. In some nearby enclosed coastal port and dumping areas, the rates were found to be somewhat higher in these areas, in the range 3～5 cm/a. This result, that the sedimentation rates are high in some places in the Harbour, is also in keeping with the more serious heavy metal pollution in these places and reflects man's influence on the Harbour. This means that changes in the coastline caused by reclamation may have a significant effect on siltation in the Harbour.

Reclamation of a sandy desert through floodwater spreading: ii. characterization of clay minerals in the watershed and the freshly-laid sediment

Abstract: The artificial recharge of groundwater (ARG), where practicable, is an easy and economical method for desertification control in the arid and semi-arid zones where overpumping has critically lowered the watertable. Although desiltation of floodwaters, which is a prerequisite for the ARG, leads to rehabilitation of the eroded sites that house the sedimentation basins (SBs), the gradual impermeability of these basins and infiltration ponds by migrating phyllosilicates shortens the economic life of such projects. Therefore, characterization of these minerals is necessary if treatments for their removal are to be found. As soil erosion in the watershed that supplies the ARG systems with floodwater deteriorates the land, and threatens the recharge facilities and other hydraulic structures downstream with siltation, identification of the erosion source facilitates prioritization of soil conservation activities. Clay mineralogical analysis has been developed in recent years as a powerful tool in sediment source attribution. However, applicability of such methods to cases in Iran requires regional studies. Sediment samples were collected at 0-10,10-20, and 20-30 cm increments on two transects in the 1st, 4th and 6th SBs in a Gareh Bygone Plain ARG system. Rock samples were collected on the southern flank of the Bisheh Zard Basin that supplies floodwater to the ARG systems. All samples were characterized by the XRD method. Transmission--electron micrograph (TEM) of one sample was also obtained. Chlorite, mica, smectite, and possibly kaolinite were detected in all samples. The presence of palygorskite was revealed only in a TEM. The non - clay minerals were calcite, dolomite, alkali and plagioclase feldspars, gypsum, halite and quartz. The common origin of these inherited clay minerals precludes their utility in sediment source detection.

Experimental Study on the Use of Salt Water to Prevent Siltation Caused by Density Flow

Abstract: The deposition caused by density flow in the navigation channel at the lower reach of the ship lock of the Three Gorges Project would affect the navigation of the channel. Except from the measure of using dredger to dig the mud out, there is a thought that through adding salt to improve the water density inside the “cecum part” of the navigation, the invasion of density flow would be prevented. The result of the experiment shows that the effect is notable. The result can be of help in treating siltation caused by density flow under similar circumstances.

Siltation at a capital dredged site in Cochin harbour, SW coast of India

Abstract: A capital dredging site at Cochin harbour was closely monitored for topographic variability and the role of natural processes in reprieving the scenario. The textural characteristics points out the abundance of silt and clay (76-95%) and sparingly sand; on conduct of capital dredging, the prominence of sand is noteworthy. The bathymetric contours revealed the periodic changes on dredging and the resultant sedimentation leading to the gradual fill up of the dredged sites. The amount of dredging performed to reclaim the adjacent land was quantified using a computer aided analysis.

Initial Study on Fluid Mud in North Passage of the Yangtze Estuary

Abstract: Based on the data of site observation and indoor experiment, it gives a thorough description on the distribution, physical characteristics of fluid mud in North Passage of the Yangtze Estuary, the formation conditions and influencing factors, its rheological properties, settlement＆ consolidation characteristics, its influence on waterway siltation and dredging, as well as utilization of navigable depth, etc. Moreover, the source and evolutionary mechanism are initially discussed and the way for further study is proposed.