Showing papers on "Siltation published in 2006"

Reservoirs in Tigray (Northern Ethiopia): characteristics and sediment deposition problems

Abstract: In Tigray (Northern Ethiopia, significant achievements were made, mainly from 1994 to 2002), on the development of agriculture through irrigation by employing seasonally harvested runoff using earth dams. However, most of the implemented schemes are not serving the intended purpose well because of constraints associated with both pre- and post-implementation. Sediment deposition in reservoirs is a serious off-site consequence of soil erosion in the region. However, the extent of the problem is not well studied. Moreover, there are no sufficient and reliable sediment-yield data for Northern Ethiopia, which are important for designing new reservoirs and for implementing soil conservation practices. This study addresses those problems by undertaking: (1) a survey and evaluation of the general characteristics and problems of 54 recently built reservoirs and the characteristics of their respective catchments; and (2) a detailed sediment survey in ten reservoirs and related catchment characteristics. A field survey in 2002–2003, a review of technical reports and interviews were the bases for this study. Most of the reservoirs are under risk of insufficient inflow, excessive seepage and sediment deposition. These problems are mainly attributed to the use of a poor database on hydrology and sediment yield, and the lack of adaptable methodologies for assessing controlling factors at the planning stage. The reservoir survey, which is the first of its kind in Ethiopia, also indicates that specific sediment yield (SSY) varies significantly between catchments: i.e. from 237 to 1817 t km−2 y−1 with an average of 909 ( ± 500) t km−2 y−1. The high spatial variability is mainly associated with differences in lithology, cover, extent of gully network and human activities. Therefore, adapting an average SSY value for the whole region is not recommended for future planning purposes. The sediment deposition problem is significant: i.e. 70 per cent of the study reservoirs have important siltation problems so that they will end their useful life well before the dam design period. Hence, sediment management in reservoirs could be an effective approach towards maintaining the existing storage capacity. For the realization of sustainable land and water development in Tigray, sufficient and reliable database building, development and standardization of appropriate methodologies for predicting sediment yield and capacity building of designers needs to be given top priority. Moreover, awareness creation among policy makers, donors and beneficiaries is also important for action. Copyright © 2005 John Wiley & Sons, Ltd.

Assessment of gully erosion rates through interviews and measurements: a case study from northern Ethiopia

Abstract: Gullying has been widespread in the Ethiopian Highlands during the 20th century. It threatens the soil resource, lowers crop yields in intergully areas through enhanced drainage and desiccation, and aggravates flooding and reservoir siltation. Knowing the age and rates of gully development during the last few decades will help explain the reasons for current land degradation. In the absence of historical written or photographic documentation, the AGERTIM method (Assessment of Gully Erosion Rates Through Interviews and Measurements) has been developed. It comprises measurements of contemporary gully volumes, monitoring of gully evolution over several years and semi-structured interview techniques. Gully erosion rates in the Dogu'a Tembien District, Tigray, Ethiopia, were estimated in three representative case-study areas. In Dingilet, gullying started around 1965 after gradual environmental changes (removal of vegetation from cropland in the catchment and eucalyptus plantation in the valley bottom); rill-like incisions grew into a gully, which increased rapidly in the drier period between 1977 and 1990. The estimated evolution of the total gully volume in the other areas show patterns similar to those of the Dingilet gully. Average gully erosion rate over the last 50 years is 6·2 t ha−1 a−1. Since 1995, no new gullies have developed in the study area. Area-specific short-term gully erosion rates are now on average 1·1 t ha−1 a−1. The successful application of the AGERTIM method requires an understanding of the geomorphology of the study area and an integration of the researchers with the rural society. It reveals that rapid gully development in the study area is some 50 years old and is mainly caused by human-induced environmental degradation. Under the present-day conditions of ‘normal’ rain and catchment-wide soil and water conservation, gully erosion rates are decreasing. Copyright © 2006 John Wiley & Sons, Ltd.

Soil erosion prediction at the basin scale using the revised universal soil loss equation (RUSLE) in a catchment of Sicily (southern Italy)

Abstract: Soil erosion by water is a serious problem in southern Italy, particularly in Sicily which is one of the Italian administrative regions prone to desertification. Soil erosion not only affects soil quality, in terms of agricultural productivity, but also reduces the availability of water in reservoirs. This study was conducted in the Comunelli catchment in south-central Sicily, to predict potential annual soil loss using the revised universal soil loss equation (RUSLE) and to test the reliability of this methodology to predict reservoirs siltation. The RUSLE factors were calculated for the catchment using survey data and rain gauge measurement data. The R-factor was calculated from daily, monthly and annual precipitation data. The K-factor was calculated from soil samples collected in May and November 2004. The LS topographic factor was calculated from a 20 m digital elevation model. The C- and P-factors, in absence of detailed data, were set to 1. The results were compared with those obtained from another soil loss estimation method based on 137Cs and with the soil loss estimated from the sediment volume stored in the Comunelli reservoir between 1968 and 2004.

Reservoir siltation in the semi-arid highlands of northern Ethiopia: sediment yield–catchment area relationship and a semi-quantitative approach for predicting sediment yield

Abstract: Due to shortage of rainfall and its increasing variability, moisture stress is identified to be one of the most critical factors affecting agricultural productivity in the drylands of Ethiopia. To circumvent this problem, a strategy of supplemental irrigation through surface water harvesting was adopted by the government and several micro-dams have been built in the semi-arid parts of the country. However, the benefits from the water harvesting schemes are not sustainable because of rapid water storage loss due to siltation. There is, therefore, an urgent need for improved catchment-based erosion control and sediment management strategies. The design and implementation of such strategies require data on the rate and magnitude of sediment deposition. To this end, reservoir surveys were conducted to estimate sediment deposition rate for 11 reservoirs identified to be representative of catchments in the Tigray region of northern Ethiopia. Two approaches were employed during the survey: one was based on measurement of sediment thickness in reservoirs while the other was based on comparing the original and existing topography of the reservoir-beds. The average annual sediment yield estimated for the study sites was about 19 t ha−1 y−1. An equation of the type SSY = 3a36A0a67 (with SSY = area specific sediment yield in t ha−1 y−1 and A = catchment area in km2) was also established for the study region, which is opposite to the ‘universal’ SSY–A relationship. In order to improve the sediment yield predictive capability of A, it was integrated with a factorial index that assesses the catchment's propensity to erosion and potential sediment yield. The effect of accelerated sediment deposition on water storage loss of reservoirs and possible controlling factors of the SSY–A relationship are outlined. The potential semi-quantitative scoring approach to characterize catchments in terms of erosion sensitivity and the significance of the A-index approach to predict SSY of similar catchments are also highlighted. Copyright © 2006 John Wiley & Sons, Ltd.

Physical pollution: effects of gully erosion on benthic invertebrates in a tropical clear-water stream

Abstract: 1.Many tropical streams are situated in geologically old, weathered landscapes that are prone to erosion. Permanent or seasonally pulsing inputs of eroded material into the streams may have significant effects on benthos and habitats, even if chemical water quality remains unaltered. 2.This study presents data from the Cerrado Region, Brazil, where the semideciduous forests have been largely converted into agricultural areas with considerable erosion problems. In order to quantify impacts of erosion on the benthic community, a simple, inexpensive, standardized artificial substrate was exposed on the stream bed above and below the mouth of an erosion gully in two experiments at the beginning and at the end of the rainy season. 3.While the abundance and biomass of most of the biota in the reference sites did not change during the rainy season, it decreased significantly at the impact sites below the mouth of the erosion gully. It was the combination of rainfall-driven flood pulses and increased load of suspended particles from erosion which caused the decline of the benthic colonization in these streams rather than the hydrological disturbance alone. 4.Site-to-site comparisons revealed a highly significant reduction in density, biomass and taxon richness of the benthic invertebrates caused by siltation. All benthic insect taxa studied showed the same pattern, indicating a general impact of erosion on habitat quality and food sources. Semi-aquatic insects adapted to shifting habitat conditions, terrestrial food sources and aerial respiration were the most resistant invertebrate group. 5.Restoration schemes for the stream catchments are urgently needed to reduce local population extinctions due to impassable stream sections. Copyright © 2006 John Wiley & Sons, Ltd.

Hydrodynamics and sedimentation induced by large-scale coastal developments in the Keum River Estuary, Korea

Abstract: The macro-tidal Keum River Estuary located in the eastern Yellow Sea has been suffering siltation and morphological change since 1994. To understand the effects of the large-scale coastal developments on the sedimentation processes in the estuary, hydrodynamic and sedimentary data collected from 1985 to 2002 were analyzed and numerical experiments of hydrodynamics were performed. The sedimentation rate in the estuary increased by a factor of 1.9, from 3.5 × 10 6 to 6.7 × 10 6 m 3 y −1 , after the construction of a dam in the upper reaches of the estuary in 1994. Large part of the estuary is veneered by the muddy sediments noticeably, which were rarely found before dam construction. Since then, siltation has concentrated in the upper estuary rather than the lower. The upstream transport and accumulation of fine-grained sediments is due to: (1) the change to flood-dominance in the main channel, i.e. the relative intensification of flood current and the flood-directed residual current; and (2) the decrease in transport capacity in the upper estuary, i.e. the marked decrease in current velocity, which was induced by dam construction. The former has resulted in the ebb-dominance of the Gaeya channel, a distributary in the north of the main channel. The tidal pumping of fine sediments was reinforced not by the freshwater/saltwater interaction but by the residual tidal circulation. The sediment fluxes observed in 2001–2002 demonstrate year-round net inflow both at the entrance of the jetties and at the Gaeya channel, which implies that the sediments delivered by the Keum River are entirely confined to the estuary, incapable of escaping to the sea. The net inward transport of fine sediments may accumulate pollutants adsorbed to or absorbed in the sediment grains in the estuary, thus deteriorating the benthic environment gradually and the water quality eventually.

Watershed Prioritization and Reservoir Sedimentation Using Remote Sensing Data

Abstract: To limit siltation, it is essential that soil conservation measures are undertaken in the drainage basin upstream of the reservoir. In this study, the catchment for the Ramganga reservoir has been divided into nine sub‐watersheds to determine the sub‐watershed most prone to soil erosion. Also, temporal IRS‐1B LISS‐III images between years 2000-2001 are used on ILWIS image processing and GIS software for the assessment of reservoir sedimentation in Ramganga catchment. An approach, based on land surface factors mainly responsible for erosion, which include slope, landuse, brightness and greeneness etc., are used in this study. The catchment of reservoir is divided into 9 sub‐watersheds to assess the sub‐watershed contributing maximum sediments to the reservoir. Palain sub‐watershed of Ramganga catchment is identified as being most susceptible to sedimentation. The LISS III images are used to compute the water index, while the GIS system is used to analyse the topography. The integrated effect of al...

Palaeoenvironmental and Archaeological Implications of a Sediment Core from Polje Cepic, Istria, Croatia

Abstract: Palaeoenvironmental and archaeological records provide an invaluable framework for land and water management in karstic areas of the Mediterranean realm. We present the results from analyses carried out on three segments of a sediment core extracted in 2004 from a portion of Polje Cepic (Istria, Croatia), a tectono-karstic depression covered in water until artificial drainage took place in 1932. We used bulk sediment analyses, pollen analysis and radiocarbon dating to assess the possible contribution of people to the progressive siltation of the lake, and compared our results with recent archaeological discoveries made on the polje margins. The dating of the sequence points to the presence of a predominantly wet landscape at the coring location since at least 7000 years cal BP. The coincidence of cereal type pollen grains with several open-air archaeological sites indicates that smallscale agricultural practices possibly developed around the polje in Neolithic times, ca 6500 years cal BP. Concentrations of charcoal in the sediment core at this time suggest the use of fire clearance as part of the agricultural practice. During the Bronze and Iron Ages, the possible intensification of forest clearance and agricultural practices is indicated by: (1) an increase in the number and type of archaeological sites found around the polje; (2) a more open plant landscape in the upper part of the analysed sequence; and (3) a three-fold increase in the sedimentation rate, possibly starting as early as 4000 years cal BP. Wetter climate conditions and higher erosion of the surrounding slopes probably led to the progressive siltation of the lake. Finally, in 1932 the lake was artificially drained and Istria lost its largest natural basin of fresh water.

Sediment yields in the Exe Basin: a longer-term perspective

Abstract: In the UK, fine sediment is viewed increasingly as a diffuse pollutant due to its role as a vector for the transport of potential contaminants, and in causing siltation, which may have adverse effects on river and estuarine habitats. There is a need, therefore, for river managers to have reliable information on sediment budgets in order to plan measures that will achieve good status under the EU Water Framework Directive. As part of a wider sediment-budget investigation in the EU-funded Cycleau Project, detailed records of fine sediment yield over the 10-year period from 1994-2003 have been analysed for the Exe Basin (1500 km 2 ), a principal river system of southwest England. The longer-term average yields in the three major tributaries of the Exe Basin are discussed and results of monitoring of sediment loads at a site near the tidal limit over a one-year period confirm the importance of the River Exe in contributing sediment to the Estuary.

Stratification and fine sediment transport mechanisms in a semi-enclosed tidal lagoon (Pagham Harbour, West Sussex)

Abstract: Preliminary analysis of data collected at a macrotidal semi-enclosed lagoon (Pagham Harbour, UK) has revealed useful information about long-term patterns of siltation and some of the related mechanisms. Sediment surface-level measurements made over 2 years at different sites within Pagham Harbour have shown a steady siltation, in common with earlier measurements, which is moderated by seasonal effects due to erosion by locally generated waves. Furthermore, inspection of vertical profiles of salinity and turbidity over individual tidal cycles has revealed that the degree of sediment transport on the flood tide is related to the vertical salinity gradient. Thus, at the Ferry Pool site, which is characterised by episodic pumped discharges from a nearby sewage treatment plant, landward sediment transport is enhanced by the high degree of salinity stratification observed during the flood tide. The mobility of the sediment, and the greater distribution of softer, less well-consolidated sediment deposits, is greater here than at the other significant freshwater inflow at the Salthouse site, where the fresh water flow is instead moderated by a tidal flap gate. Preliminary analyses suggest that the higher the salinity stratification, the greater the landward sediment transport during the flood tide. Such analyses could help inform future policy on the methods of land drainage to macrotidal lagoons, and on the potential for managed realignment at such sites.

Observations of Tsunami Impact on the Coast of Kerala, India

Abstract: The tsunami generated by the December 2004 Sumatra-Andaman earthquake had a devastating effect on some parts of Kerala coast, which is a coast located in southwest India. Results of post-tsunami field surveys carried out to understand the changes in coastal morphology and sediment characteristics in the worst affected Kayamkulam region of Kerala coast are documented in this study. Analysis of offshore bathymetric data indicates the shifting of depth contours towards shore, indicating erosion of sediments and deepening of innershelf due to the tsunami. Depth measurement along the backwater (T-S canal) in the hinterland region indicates siltation due to the inundation of the canal.

On the relationship between maintenance condition of Yangtze Estuary deepwater channel and water & sediment transport from the valley

Abstract: With the completion of the first and second phases of the improvement project of Yangtze Estuary deepwater channel,the quantity of back silting is under control.Analyzing the reasons,we noticed that the total quantity of sediment passing through the Yangtze Estuary from the Yangtze Valley has decreased obviously in recent 10 years,while the sediment concentration around the river mouth hasn't reduced along with it.The total amount of movable sediment around the river mouth is larger than the total quantity of sediment from the Yangtze Valley due to the resuspension process of the sediment,which results in that the decrease of sediment from the Yangtze Valley can't improve the maintenance condition of the channel.On the other hand,the analysis of river facies relation and data of bathymetric survey show that the area of shoals in the Yangtze Estuary is decreasing in recent 10 years,which is relevant to the reducing and thinning of sediment transport from the Yangtze Valley.Within a period of time,the fluvial processes are getting active,which increases the movement of sediment and results in unfavorable effect on the maintenance of the channel.

Reservoir Siltation in India: Latest Studies Revealing Results, a Wake up Call

Abstract: Most parts of India being dependent for water on 3-4 month long monsoon, Reservoirs are created to store water for use in non monsoon months. The reservoirs, created by dams on rivers, also get silt in the water of the rivers that enters the reservoirs and a significant proportion of the silt settles down in the reservoir, thus reducing the space available for storage of water. Moreover, studies over the years have shown that the silt gets deposited in both the dead storage (the storage at the bottom, below the Minimum Draw Down Level, which is not used under normal circumstances) and in the Live Storage (LS). This process of accumulation of silt in the reservoirs is called siltation. Siltation results in reduction in benefits from the projects constructed at huge costs to the nation. Siltation of reservoirs can also have a number of other impacts, including increased evaporation losses, increased backwater flooding and also could damage the power house turbines.

Study on deposition characteristics and channel siltation in offshore zone of Qinzhou Bay

Abstract: From analysis of dynamic property,deposition characteristics,sources of the sediment and its transport trends,sediment conditions and siltation status after the development of East Channel,it's concluded that sediment is not large in Outer Qinzhou Bay,silt content in the sea water is slight,riffle and pool is in stable stateEast Channel axial line is basically coinsident with the direction of flood current,and the siltation after channel's excavation will not be heavy

Delta processes and management strategies in China——II The Yellow and Haihe River deltas

Abstract: The present Yellow River delta has developed over the past 150 years as a consequence of siltation and avulsions of the river channel,flooding and sedimentation.Human activities have also sharply changed the delta processes.Oil fields and a new city with a modern infrastructure have been developed on the newly created land.The river channel within the delta is very dynamic.Humans stabilize the river channel by dredging and building levees for construction of the oil field infrastructure and residences.Moreover,the sediment is being used for land creation.To control flash flood humans have dug many drainage channels to discharge flood from the Haihe River and its tributaries.Nevertheless,the number of river mouths is proportional to the ratio of water and sediment under natural conditions.The number of river mouths is too large and tidal flow carries sediment into the river mouths and caused the river mouths shrinking.Dredging is only a temporary solution to this problem.Double guiding dykes,long enough to cut off the high turbidity belt along the coast,are being examined as the main strategy to control sedimentation of the river mouths.The shallow high turbidity belt along the Bohai coast can be changed into land if numerous guiding dykes are constructed.

Causes of Floods in Past Fifty Years in Weihe River Basin and Their Control Measures

Abstract: The yearly and the monthly changes of floods happened in the last fifty years in Weihe river basin and the change of water level corresponding to 1 000 m~(3)\5s~(-1) discharge at Tongguan hydrologic station height have been synthetically analyzed,results show that the factors leading to flood disaster in this river basin include: uneven-distributed precipitation inner year and inter year,more severely siltation in river bed,silting-up of branch river mouth,and the worse of river terrain and water flowing regime,etc.This paper,according to the characteristics of flood disaster in Weihe river basin,puts forward relevant control measures such as lowering height of the lower reaches,decreasing siltation,improving the ability of river course releasing floodwater,applying reservoirs to control water and sand,diverting far-away water to wash the lower reaches of Weihe;consolidating flood-preventing engineering and eliminating danger,enhancing their flood-controlling standard and flood-resisting ability;reconstructing vegetation,preventing water and soil erosion,especially preventing mud and sand entering into river in headstream region.

Regime theory on river bed evolution and its application in the Lower Yellow River

Abstract: Based on the systemic analysis of theories about riverbed evolution in regime,the principle and formula for minimum rate of available energy dissipation deduced from theory of fluvial entropy is considered to completely express the basic principle of automatic regulation about riverbed evolution of alluvial river.A mathematic model of riverbed evolution in regime was developed with the formula for minimum rate of available energy dissipation to close the equations of riverbed evolution.The model was applied to compute the channel sections in regime under various conditions of flow and sediment in the Lower Yellow River,and the influences were calculated and analyzed for several factors,such as scour of clear flow released from Xiaolangdi reservoir,detaining coarse sediment and releasing fine sediment by the reservoir,change of water temperature, scour at flood rising and siltation at flood recession.Based on analyses of critical indexes for optimal transport of water and sediment,the measures to improve the management of Xiaolangdi reservoir were presented,which include the regulation of unsteady flow,control of the disadvantage sediment concentration of 60～100kg/m~3,and storing coarse sediment by flood detention.

Analysis on Foreslope Evolution of Gudong Sea Area of the Yellow River Delta

Abstract: The analysis on foreslope evolution of Gudong sea area for the period of 1979~2002 shows that a) the maximum erosion recession rate of time interval is 314 m/a when shallow water region in erosion recession statement;b) the accumulated deposition of 23 a is 6 516 ~ 7 925 m when deep water area is continuously silting;c) the maximum erosion rate of time interval is 0.272 m/a,the accumulated deposition of 23 a 3.625 m and the thickness of deposition of deep water area 3~5 m when the maximum thickness of erosion is in the shallow water region and;d) each slope of bathymetric contour changes from steep to gentle and the erosion limiting depth is about 7.6 m after obtaining 40 a of foot of dyke slope by theoretical analysis and graphic method.It also points out that waving and tidal flowing are the main dynamic factors of coastal erosion.

A Necessidade da Consideracao do Processo de Assoreamento no Planejamento e Operacao de Reservatorios

Abstract: This work aims at discussing the importance of estimating sediment volume in reservoir planning and operation. Currently, stage vs area vs volume curves have been updated for only a few reservoirs in Brazil. Therefore reservoir volumes measured by limnimeter are usually overestimated Thus, it is necessary to estimate accumulated sediment volume in reservoirs, by means of transport and deposition sediment models or reservoir bathymetry. This study introduces some of the models presently used in estimating reservoir siltation and demonstrates the need for more sediment information data in Brazil in order to allow the use of computational fluid dynamics models. During the feasibility study of the implementation of a reservoir, active volume reduction is also an important factor to be considered in studying the economic feasibility of the project. Finally, another goal of this paper is to encourage Brazilian hydrologists to consider siltation in reservoir planning and operation models

Dynamic Monitoring of Newborn Estuary Wetland since Channel Diversion of the Yellow River to Qingshuigou

Abstract: Using remote sensing data from Dec.1976 to Jan.2003 and spatial analysis of GIS,this paper analyses the dynamic change of the newborn estuary wetland since channel diversion of the Yellow River to Qingshuigou.The research area is a sector area,composing of three evident objects:a Yellow River inflexion near Kenli County government,northwest inflexion of Guhe reservoir and northeast inflexion of Guangbei reservoir.By the visual interpretation of remote sensing images at different phases,we can get the borderline between land and ocean.Comparing the borderlines of the research area supported by GIS,the areas of siltation and erosion can be identified and their scope can be computed.The results indicate that the increase and decrease of newborn estuary wetland are ongoing alternately because of the Yellow River siltation and the ocean erosion,and the area of net increase is declining gradually.Within 26 years,the newborn estuary wetland got a net increase of 17 154.41ha,and the annual average net increase is 657.76ha.The wetland area was a net increase before 1993,while this net increasing trend declined gradually after 1993.At the same time,the alternative periods of increase and decrease of the newborn wetland become shorter and shorter from 2.52 years to 1.06 years.Before 1993,the increased parts are located in the new estuary region and southeast coastal zone,and the decreased area located in the old estuary region.After 1993,the variational trend of the above two regions are reversed.The main reason is due to the influence of the Yellow River siltation and the ocean erosion.This study will provide suggestions on decision,making for the wetland protection of the Yellow River delta.

Coastal erosion and its main cause in the qinhuangdao area

Abstract: The paper analyzes coastal erosion and silting up in the Qinhuangdao area and discusses the constraints on coastal erosion and silting and their harm to coastal resources and environment based on information obtained from on-site investigationThe main conclusions are as follows:in the past 50-odd years,the 1235km long coastline of Qinhuangdao has undergone six activities of erosion and silting up in different forms,of which the 8269km long coastline has undergone erosion to various degrees,accounting for 67% of its totalBesides the storm tide impact,the main causes for coastal erosion are due to the continued decrease of material supply and manual extraction of sand and gravelCoastal erosion can seriously destruct tourism resources and land resources and aggravate invasion of seawater besides threatening the safety of coastal engineering facilities

Desilting Technology and Application in Flushing Gallery

Abstract: To remove the siltation and the bed load, auto-desilting technology has been applied in Donglei. From 2004-2006, after 8 irrigation periods' tests, we found the technique showed features as being small dropping variance, being "intermittence" discharge of silt; large adaptation range for water current, the silt, the topographical variation; not consuming the principal part of a project the water head energy; having no special request to the main body current of water depth, the current capacity, the speed of flow; which could widely used in channels, river courses, settling basins, reservoirs and power plants for desilting.

Analysis of probability of channal rapid siltation of Yangkou Harbour

Abstract: By the measured data of "9711" Typhoon wave,the process of bed load resuspending under the effect of "9711" Typhoon wave around the channal of Yangkou Harbour is calculated.It's obtained that there is no probability of channal rapid siltation of Yangkou Harbour.