Showing papers on "Siltation published in 2011"

Combining suspended sediment monitoring and fingerprinting to determine the spatial origin of fine sediment in a mountainous river catchment

Abstract: An excess of fine sediment (grain size <2 mm) supply to rivers leads to reservoir siltation, water contamination and operational problems for hydroelectric power plants in many catchments of the world, such as in the French Alps. These problems are exacerbated in mountainous environments characterized by large sediment exports during very short periods. This study combined river flow records, sediment geochemistry and associated radionuclide concentrations as input properties to a Monte Carlo mixing model to quantify the contribution of different geologic sources to river sediment. Overall, between 2007 and 2009, erosion rates reached 249 ± 75 t km−2 yr−1 at the outlet of the Bleone catchment, but this mean value masked important spatial variations of erosion intensity within the catchment (85-5000 t km−2 yr−1). Quantifying the contribution of different potential sources to river sediment required the application of sediment fingerprinting using a Monte Carlo mixing model. This model allowed the specific contributions of different geological sub-types (i.e. black marls, marly limestones, conglomerates and Quaternary deposits) to be determined. Even though they generate locally very high erosion rates, black marls supplied only a minor fraction (5-20%) of the fine sediment collected on the riverbed in the vicinity of the 907 km2 catchment outlet. The bulk of sediment was provided by Quaternary deposits (21-66%), conglomerates (3-44%) and limestones (9-27%). Even though bioengineering works conducted currently to stabilize gullies in black marl terrains are undoubtedly useful to limit sediment supply to the Bleone river, erosion generated by other substrate sources dominated between 2007 and 2009 in this catchment.

Sediment redistribution due to a dense reservoir network in a large semi-arid Brazilian basin

Abstract: This paper investigates for a 25-year period the sediment distribution in a semi-arid Brazilian basin (2 × 104 km2) with a network containing more than 4000 surface reservoirs. The methodology is based on rating curves and fitted parameters derived from field data. The results showed that suspended load corresponded to 70% of the total sediment yield (148 t km-2 year-1). The relatively low contribution of the suspended load (compared with other semi-arid regions) was attributed to the impact of the numerous upstream reservoirs, which retained 235 t km-2 year-1. The micro ( 50 hm3) reservoirs responded to, respectively, 5, 17, 30 and 48% of the total sediment retention by the reservoir network. This indicates that retention in the non-strategic reservoirs has a positive impact on water availability, since siltation of the strategic reservoirs would be expected to more than double if only such reservoirs existed. Citation ...

Morphodynamic characteristics of the dextral diversion of the Yangtze River mouth, China: tidal and the Coriolis Force controls

Abstract: This paper examines the morphological development of the Yangtze River mouth, which has been diverting southeasterly (dextrally), according to historical (150 years) chart-based digital evolution model and on-site measured tidal flow data. We reveal a significantly narrowing of the northern river mouth branch from formerly >30 km wide to presently 10 km wide due to rapid siltation. Net siltation there, however, decreases gradually, which largely contrasts with the fact that the siltation has shifted to the southern river mouth area, as shown by many newly-emerged estuarine islands, sandy shoals and bifurcated branches. Our data have further demonstrated that the ebb flow that dominates in the study area changes its direction gradually from east to southeast from the inner to outer river mouth area, and its duration is much longer than the flood flow in the inner river mouth area, but nearly equal at the river mouth area. Accordingly, the sediment transport pathway has been diverted from east to southeast. We examine whether the Coriolis Force could explain the dextral diversion of the ebb flow and the altered morphodynamical processes. Although too weak to strengthen the tidal flows, the Coriolis Force can drag the ebb flow southeasterly, and so influence sediment transport paths at the estuarine scale. The Coriolis Force is limited in the inner river mouth, but substantial at and in the outer river mouth area when gradually free of estuarine topographic constraints. The Coriolis Force causes an offset in propagation of in-out flow directions at the river mouth area to form a slack water setting prone to estuarine siltation. Using the present approach also enables explanation of the morphological development of the Holocene Yangtze delta-coast that extends to the southeast. Copyright © 2010 John Wiley & Sons, Ltd.

Optimal parameter selection for qualitative regional erosion risk monitoring: A remote sensing study of SE Ethiopia

Abstract: Arid and semi-arid regions are susceptible to high levels of erosion. A rapid and cost effective methodological erosion assessment for these regions is required to describe and monitor the processes that control erosion. This study uses remote sensing to describe the contribution of several factors that control erosion. Topography, land use, vegetation density, soil properties and climatic proxies are used to determine erosion risk and to provide basic maps of water and soil conservation practices. A hierarchical decision tree is used to sum and combine the weight of parameters controlling the erosion. The assigned weights of each spatial unit express the susceptibility to erosion. We focus on the catchment basin of the Maleka Wakena reservoir, located in the southeastern portion of the main Ethiopian Rift, where erosion is the major environmental problem. Three different combinations of the dominant controlling factors are yielded in this study. In order to optimize the qualitative erosion risk assessment, each combination is discussed and evaluated depending on the contribution of parameters involved in the erosion process. Maps derived from this qualitative analysis allow the identification and ranking of erosion intensity. These maps enable us to highlight areas in immediate need of conservation strategies in order to control siltation of the SE Ethiopian reservoir and to control further soil degradation.

Estimating sediment yield risk of reservoirs in northern Ethiopia using expert knowledge and semi-quantitative approaches

Abstract: Reservoir siltation is a serious problem that threatens the productivity and sustainability of water-harvesting schemes. Quantification of sediment deposition in reservoirs and understanding of its major drivers are needed to apply targeted management interventions. Most of the techniques used to estimate sediment deposition in reservoirs require extensive measurements on a frequent time basis, as well as being costly and time-consuming. Thus, a rapid and relatively economical means of assessing the erosion susceptibility of catchments and predicting their sediment yield potential is necessary. In this study, expert-based rankings and semi-quantitative factorial scoring approaches were applied to assess the siltation severity of 25 reservoirs in the Tigray region of northern Ethiopia. The results were then correlated with quantitative sediment yield estimates acquired for representative sites, and a sediment yield predication model was developed for the region. The calibrated model has an efficiency and relative root mean square error (RRMSE) of 79 and 36%, respectively, which is considered adequate to assess erosion susceptibility and siltation risk of reservoirs in similar environments. The study demonstrates that expert knowledge and rapid characterization of catchments, in terms of susceptibility to erosion, are viable options for assessing siltation risks and for analysing controlling factors at a larger number of sites, with minimum costs and acceptable accuracy.

Detection and quantification of sediment remobilization processes using a dynamic penetrometer

Abstract: Sediment remobilization processes have a high impact on coastal morphology as well as offshore engineering. In many cases countermeasures are required to preserve the stability of the construction or coastline. To apply such countermeasures in the most effective manner, it is crucial to (i) locate and map areas of sediment erosion and deposition and to (ii) quantify the mobilized sediment with a high spatial and temporal resolution. In this study we tested the small dynamic penetrometer Nimrod as a tool for rapid detection and quantification of sediment remobilization processes. More than 20 surveys with about 5000 deployments covered natural-induced sediment remobilization processes such as shifting sandbars and sorted bedforms as well as man-induced processes such as the siltation in navigation channels and scour. The surveys were carried out in areas of different sediment textures ranging from soft clay to hard quartz and carbonate sands. Additionally, preliminary experiments under controlled boundary conditions were accomplished in a wave channel (Franzius Institut, Hannover, Germany).

Effects of erosion and deposition on particle size distribution of deposited farmland soils on the chinese loess plateau

Abstract: Particle size distribution (PSD) in the soil profile is strongly related to erosion, deposition, and physical and chemical processes. Water cycling and plant growth are also affected by PSD. Material sedimented upstream of the dam constructions formed large areas of deposited farmland (DF) soils on the Chinese Loess Plateau (CLP), which has been the site of the most severe soil erosion in the world. Two DFs without tillage on the CLP were chosen to study the combined effect of erosion and check dams on PSD. Eighty-eight layers (each 10 cm thick) of filled deposited farmland (FDF) soils and 22 layers of silting deposited farmland (SDF) soils of each studied soil profile were collected and 932 soil samples were investigated using laser granulometry. The particle sizes were stratified in both DFs based on soil properties and erosion resistance. The obtained results of clay and silt fractions showed similar horizontal distribution, indicating parallel characteristics of erosion and deposition processes. Fine sand represented the largest fraction, suggesting the preferential detachment of this fraction. The most erodible range of particle sizes was 0.25-0.5 mm, followed by 0.2-0.25 mm in the studied soil profiles. The correlation between particle size and soil water contents tended to increase with increasing water contents in FDF. Due to the abundant shallow groundwater, the relationship between particle size and soil water content in SDF was lost. Further studies on PSD in the DF area are needed to enhance the conservation management of soil and water resources in this region.

A review of sediment quantity issues: Examples from the River Ebro and adjacent basins (Northeastern Spain)

Abstract: Sediment flows naturally through the drainage network, from source areas to deposition zones. Sedimentary disequilibrium in rivers and coastlines is related to the imbalance within the fluvial system caused mostly by dams, instream mining, and changes in land use. This phenomenon is also responsible for ecological perturbations in rivers and streams. A broad need exists to establish comprehensive management strategies (soft measures) that would go beyond site-specific engineering practices (technical measures) typically taken to solve particular problems. Long-term programs are also required to monitor sediment transport in river basins, in order to assess the magnitude and variability of sediment transfer and potential deficits. This paper shows examples of rivers with important sediment disequilibrium in the Ebro and adjacent basins. These basins, like most in the Iberian Peninsula, experience sediment discontinuity in the catchment–river–coast system. Reservoir siltation is the main quantitative issue. Land use change and especially gravel mining downstream from dams accentuate the process. We also present and discuss recent developments on water and sediment management undertaken to improve the morphosedimentary dynamics of rivers. Integr Environ Assess Manag 2011;7:256–268. © 2010 SETAC

Predicting the Morphodynamic Response of Silt-Laden Rivers to Water and Sediment Release from Reservoirs: Lower Yellow River, China

Abstract: The high sediment load of the Yellow River results in rapid infilling of its reservoirs when sediment is not regularly flushed. Simultaneously, the downstream reaches of the Yellow River experience extremely high siltation rates, which are reduced when sediment is retained in its reservoirs. To minimize siltation in the reservoirs and the downstream river bed, water and sediment are released from the reservoir in a controlled way through flushing experiments. In this paper, we analyze the effect of such a flushing event on the downstream river bed through data analysis and numerical modeling. Sedimentation may be minimized by relating the amount of sediment released from the reservoir to the sediment available for release through operational monitoring and by releasing relatively clear water after turbid water. Despite this flushing of sediment, the reservoir will eventually fill up, and more sediment released again into the lower Yellow River. The change in discharge magnitude and frequency brought about by the reservoir will then probably result in increased siltation rates in the lower Yellow River compared to the predam situation.

Predicting the effect of a Current Deflecting Wall on harbour siltation

Abstract: A new container dock in the Port of Antwerp, the Deurganckdok, was recently constructed in a location close to the Estuarine Turbidity Maximum (ETM) of the Scheldt River. In order to minimise sedimentation in the new dock, a Current Deflecting Wall (CDW) will be constructed. The aim of this paper is to assess the effect of the CDW on exchange flows and sediment fluxes. Determining this effect requires (1) understanding and reproduction of the sediment dynamics close to the Deurganckdok, and (2) accurate reproduction of the exchange flows between the dock and the Scheldt, and the effect of the CDW thereon. The sediment dynamics close to the Deurganckdok are dominated by a high concentration suspension (near-bed concentrations around 1 g/l) which is advected along the dock by the tidal currents. The sediment concentration is limited by the supply of sediment rather than the flow velocity. Most sediment will temporarily settle from suspension around slack tide. Hence, flood and ebb sediment concentrations are comparable, although the ebb and flood velocities are asymmetric. The peak sediment concentration occurs at the left or right bank of the Scheldt River, depending on the clockwise and counter-clockwise residual cross-channel currents. Exchange flows between the dock and the Scheldt are mainly a combination of horizontal eddies and salinity-driven flows, with velocities around 0.4 m/s. Most sediment enters the dock in a near-bed horizontal eddy, occurring around HW. The main effect of a CDW is a deflection of this sediment patch towards the river, reducing the sediment influx into the dock. The estimated reduction of siltation by the CDW is 18%. Alternative CDW configurations also reduce sedimentation in the dock, but less than computed with the design CDW.

Siltation in Reservoirs

Abstract: In most recent times, both nation with sound technological background and those without are beginning to show great concern on issues related to reservoir siltation, especially those nation with limited available water and those whose water sources are mainly from dams created reservoirs. This research work tries to highlight the implications of siltation on reservoir and consequently its eects on human resources. It is also intended to make known to the general public that though siltation cannot be totally eliminated, it can be controlled by employing some practices. This work goes further to make some recommendations towards minimization of reservoir siltation to a very manageable extent. Calls have been made to the government through various media to assist its populace in combating this nagging problem. It was concluded that sediment maximum accumulation is experienced in reservoir during the periods of maximum low.

Sediment Transport Circulation Pattern through Mesotidal Channels System

Abstract: There are three modes of sediment in motion, which are recognized as rolling and/or sliding; jumped and suspension particles motion. These particles motion include two essential sediment transport modes, which are bedload and suspended load. The first corresponds to the part of the total load which is moving very close to the bed. On this opportunity, the sediment is transported by rolling and/or sliding along the floor or jumping immediately above the bed. The suspended load comprises the particles that are carried away at suspension in the moving fluid. Bedload transport occurs in low velocity flow and/or large grain sizes, while suspended load take place at high velocity flow and/or small grain sizes. The sediment transport theory to point out that sediment capacity flow and sediment availability in channels regulates the motion of sediments. The sediment transport capacity is the maximum load that can be transported by flow and the sediment availability is the material supply that the flow can put in motion. In general, the sediment load that moves in a rolling or saltating mode is smaller compared with the suspended load transported in a natural flow. However, the bedload sediment is important for its contribution to the morphological change of the channel. Moreover, it is the most important factor in determining the stability of natural channel, the bed topography, and therefore, the cause of the hydraulic resistance and some properties of the flow. Accordingly, on this chapter we focus on the bed sediment transport pointing to the pathway migration rather than the sediment transport rate itself. The sediments transport as bedload in estuarine environment controls the bottom morphology. Therefore, estimating the transport of non-cohesive sediments, mobilized by the channel floor, has relevance to the understanding of many pressing environmental problems including eutrophication, contaminant transport, sediment bed erosion, siltation and waste disposal, which impacts on seabed stability and at times on ecosystem distribution. Also, sediments that are moving on the bed generate different bedforms such as shoal, dunes, point bar, etc., which can be hazards to navigation and to affect the flow conditions and the flanks channel stability.

Natural and human-induced effects on grain size of surface sediments along the Lianyungang muddy coast, China

Abstract: To examine spatial variations of grain-size characteristics caused by both natural and human-induced processes along the Lianyungang muddy coast, China, 129 samples were collected and measured using standard sieving and sedimentation techniques. Results show that sediment diameter tends to increase with increasing water depth from nearshore to offshore. Size-frequency distributions indicate a gradual mixing process of coarse and fine diameter material. Grain size trend analysis indicates that a man-made structure, the West Breakwater, along with Liandao Island itself, has resulted in severe siltation in Haizhou Bay and the Lianyungang port area, where sediment quality is also poor. Results demonstrate that grain size can be used as a natural tracer to infer how sediments respond to the effects caused by both natural and human-induced processes.

Technical Note: Assessing the Risks of Soil Erosion and Small Reservoir Siltation in a Tropical River Basin in Mali Using the SWAT Model under Limited Data Condition

Abstract: This article presents a modeling-based assessment of soil erosion and small reservoir siltation risks in a middle-sized river basin in southern Mali, West Africa. An integrated watershed model, the Soil and Water Assessment Tool (SWAT), was applied to simulate the watershed hydrology and the sediment detachment and transport processes on land and in streams. According to the estimated land sediment yields and the reservoir lifespans, we concluded that soil erosion and reservoir siltation risks in the study river basin are not significant. The limited data availability posed challenges to the application of the SWAT model and lead to uncertainty in this study. The data available only allows for calibrating and evaluating the SWAT-Banifing model in hydrologic simulation. However, it was found with the hydrologically-calibrated SWAT model that the river system in the Banifing River basin is dominated by the combined baseflow and lateral flow. The low surface runoff intensity provides a possible causal and physical explanation for this study's findings, which were arrived at under circumstances in which there is a lack of monitoring data on sedimentation processes.

Palaeoenvironmental Evolution of Subaqueous Yangtze Delta Inferred from Sedimentary Records

Abstract: A ~60 m long core HYZK5 was taken from the subaqueous Yangtze River delta.It consists of fluvial,coastal marsh,estuarine-shallow marine(prodelta)and distributary facies deposited since the latest glacial maximum.Peat was formed during the last glaciation when the climate was cold and wet.Foraminifera abundance shows that sea level had risen rapidly since 12 000 cal.aBP,but paused during 11 200 ~ 10 000 cal.aBP.The data of grain size,magnetic susceptibility,organic carbon and nitrogen show obvious variations in sedimentary facies in the core.Magnetic susceptibility can reflect hydrodynamics to certain extent,and indirectly suggest the evolution of sedimentary environment.But it is not a good indicator of climatic change.Parameters including grain size,magnetic susceptibility,organic carbon and nitrogen in the estuarine-shallow marine(prodelta)facies are very stable,reflecting a relatively stable environment.By contrast,coarser particle size,higher magnetic susceptibility and lower organic elements as well as intensive fluctuations occurred at the top layer of the core indicate a high-energy environment under the joint action of tidal current,runoff and other dynamics.The above phenomena have also provided important evidence for the siltation of the North Branch of Yangtze River and the decline of its mouth bar.Owing to the intensive land-sea interaction and complicated post-depositional changes in the Yangtze River Estuary,it is necessary to seek other sensitive and reliable proxies for postglacial climatic evolution.

Promoting siltation effects and impacts of Hengsha East Shoal on the Yangtze River Estuary

Abstract: For the Hengsha East Shoal Promoting Siltation Project in the Yangtze River Estuary, this work developed a formula for calculating sediment carrying capacity based on variable coefficients and a mathematical model for suspended sedimentation using variable saturation recovery coefficients. Not only does the model yield good verification, preferably give the hydrodynamics and the sediment concentration, but also reach a good agreement between the simulation results and the measured topographical changes in the promoting siltation zone of the Hengsha East Shoal. Moreover, this article proposes a method for examing the net sediment tran-sportation flux by tide for studying the effects of the project of promoting siltation and comparatively analyzes the current siltation of the Hengsha East Shoal and siltation mouth layouts, entrance widths, entrance bottom elevations, and the impact of the South Main Dike on promoting siltation through calculating the net sediment transportation flux by tide over the fixed bed and the direct movable bed numerical simulations. The results are in good agreement with each other, indicating that the method for calculating the net sediment transportation flux by the tide is also useful for assessing the project in promoting siltation. Finally, we use the modeling results to analyze the water and sediment diversion ratios and the changes in silting and scouring in riverbeds and draw the conclusion that the completion of the Hengsha East Shoal Promoting Siltation Project would not adversely affect the Yangtze River Estuary Deepwater Channel Project.

Monitoring of Soil Surface under Wind and Water Erosion by Photogrammetry

Abstract: Soil degradation resulting from accelerated water and wind-induced erosion is a serious problem in drylands, and will remain so throughout this century. The detachment and transport of soil particles degrade the fertility of agricultural land and consequently reduce its productivity (Lyles and Tartako 1986 ).Many of the particles involved in soil erosion processes, such as raindrops, wind velocity, soil aggregates, sediment, and siltation have characteristic dimensions on the millimeter scale (Huang 1998). The addition of organic matter increases the connection between aggregate by physical and chemical bounding. The strongly bonding aggregation induces the increase of soil porosity and permeability, which result the decrease of water erosion. The bigger aggregate also decrease the wind erosion due to their heaviness. The modeling and quantification of such processes require detailed measurements of the physical, chemical, and biological properties of soils (Soil Conservation Service 1976). However, these measurements are too slow, tedious, and expensive for routine or regular monitoring. Several researchers have already used aerial photography to assess soil erosion. A precise form of this photography, photogrammetry, has the advantage of very efficiently and cost effectively providing detailed information about a large area. Together with aerial photography, the use of remotely sensed data forms the basis for land use mapping and change detection (Pellikka et al. 2004). In particular, for inaccessible areas, photogrammetry is far superior to traditional ground surveys. The subsequent convergence in recent years of photogrammetry and digital imaging technology has led to an increase in the use of digital elevation models (DEMs) in modern studies involving the monitoring of landscape changes (Prosser and Aberneathy 1996; DeRose et al. 1998). The areas measured experimentally in microtopographical studies of soil erosion range from 1 to approximately 20 m2. In general, the DEMs used for analysis have grid resolutions of 1 to 15 mm (Elliot et al. 1997; Darboux and Huang 2003). A variety of instruments and methods are used by soil scientists to acquire measurement coordinates, including mechanical point gauges (Elliot et al. 1997) that make contact with the soil surface,

Multi-scale Variability of Flow Discharge and Sediment Load of Yellow River to Sea and Its Impacts on the Estuary during the Past 60 Years

Abstract: Based on the hydrological data of Lijin station from 1950 to 2008,the temporal change of flow discharge and sediment load of Yellow River to sea is analyzed by the wavelet analyses.What's more,the impacts of their changes on the estuary are studied based on the measured coastline and bathometric data.The results show that there are three significant periodic variations for flow discharge and sediment load,which may be related with the periodic variations of ENSO,additionally,the variations of flow discharge and sediment load are varying in different time scales.The long-term variation of flow discharge and sediment load are stepwise decreasing because of the combined influences of human activities and precipitation.The evolution of the Yellow River Estuary is in relation with the flow discharge and sediment load,the stretch speed of its coastline in different periods is significantly correlated with the incoming flow discharge and sediment load,and the subaqueous topography near the estuary has exhibited siltation generally since 1976.Furthermore,the siltation extent is closely related with the flow discharge and sediment load into sea.

Farmers’ Perception and Reponses to Soil erosion in Zing Local Government Area of Taraba State, Nigeria

Abstract: Soil erosion by water has remained a serious threat especially in the mountainous regions and sloppy lands where agriculture is practice, causing loss of valuable soil resources and attendant loss of agricultural productivity as well as siltation of the various water bodies across the continent of Africa and other regions of the world. This study is an appraisal of farmers’ perception and responses to soil erosion on their farmlands in Zing Local Government Area of Taraba state. This was in realization that farmers in the study area are farming on erosion prone areas such as hill slopes and flood plains, disregarding the flatlands provided by nature. To arrive at the results obtained, a total of 312 questionnaires were administered to solicit for responses of the farmers on their perception of soil erosion on their farmlands in the area. Descriptive statistics using frequencies converted to percentages was used in analyzing the data. The result shows that 87.2% of the respondents have their farmlands either located on the steep or gentle slopes, and 35.2% of the respondents which constitutes the majority cultivate the hill slopes because of historical reason. It was further established that more than 76 % of the respondents are aware of the consequences of soil erosion on farms located on slopes, but, they do not agree that erosion is an individual problem caused by them. And to some great extent, they farmers have knowledge on measures of controlling soil erosion on their farm lands especially terracing, inter-cropping and molding ridges along contours. Key Words: Perception, Erosion, farmers, Zing

Deposition characteristics of beach profile in strong-tidal environment-A case study of Yintan,Guangxi during spring

Abstract: Beach morphodynamic state is one of the important contents to understand beach processes and land-ocean interac-tionFew studies on geomorphology and deposit for sandy beaches in response to strong-tidal action in China have ever been conductedBased on the measured profile change and associated depositions of Yintan Beach in March,2010,deposition cha-racteristics of strong-tidal beach profile in response to externally-driven actions were analyzedThe results show that: 1) It is of simple geomorphologic composition for Yintan Beach,and its geomorphologic state is attributed to one of the dissipated types2) Deposition of Yintan Beach is a kind of sand with main compositions of fine sand and medium sandThe sorting of depositions is relatively wellIn addition,the percentage of both medium sand and the median size in deposition gradually decreased from land to sea,and the percentage of the super-fine sand in depositions from land to sea gradually increased3) Depositions along the transverse profile for Yintan Beach can be divided into three types: backshore cell,beachface,and in-shore partSediments in the backshore part consist of mainly saltation load under the control of wind actionSediments in the beachface are of bilateral movement resulted from swash actions driven by broken wavesHowever,sediments in the inshore part consist of mainly siltation load with the determination of combination of tidal current and wave

Simulation of sorting sedimentation in the channel of huanghua harbor by using 3d multi-sized sediment transport model of efdc

Abstract: Huanghua Harbor, located in the south-west coast of Bohai Bay, China, has frequently encountered severe channel siltation with sorting sedimentation along the channel during storm events since its construction. For prediction of channel siltation, a 3D numerical model of multi-fraction sediment transport based on the coupling of modified EFDC model and SWAN model is developed to investigate the sediment transport. It is shown from simulated results that the sorted sedimentation in the channel was well simulated in storm events, and the high sediment concentration near sea bed for silty coast during storm process can also be basically reflected by the model.

Economic loss of soil erosion in Linyi City

Abstract: Soil erosion causes both on-site and off-site impacts that contribute to economic loss of different extent. On-site impacts include losses of sediments, soil nutrients and water; whereas off-site impacts include soil detainment and siltation, respectively. Based on the date of the remote sensing survey on the soil erosion in Linyi City, monetary loss caused by soil erosion were calculated using different methods derived from environmental economics. Results indicate that economic loss caused by soil erosion in Linyi reaches 327.7069×106 yuan in 2008. The loss of soil sediment accounts for 37% of the total loss. Results indicate that physical factors, mainly county size, topography and location are decisive in determining the economic loss due to soil erosion, although human factors may also play an important role.

Hydrography of Dhaka City Catchment and Impact of Urbanization on Water Flows: A Review

Abstract: Dhaka has one of the fastest urban growth rates of the world with around 12 million people in 2006. It is located on extensive sub-continental floodplains of the Ganges and the Brahmaputra with average elevation of 6 m above mean sea level (MSL). It is surrounded by four rivers with well distributed streams, drainage channels, inland and open waters. Physiographical, spatial and temporal environments of these waters profoundly influence inhabitants' life. Mass shrinkage has occurred during the last four decades due to encroachment and filling-up, man-made changes in elevation pattern and siltation with debris from urban development activities. Groundwater storage is on the verge of mass threat because of increasing pollution of surface waters. Water logging has become every-rainy season phenomenon due to unplanned urbanization. Development activities are required to enhance the water environments safe and sustained, and regulations are needed to be imposed to protect these environments.

Developing Sediment Yield Prediction Equations for Small Catchments in Tanzania

Abstract: Sediment yield refers to the amount of sediment exported by a basin over a period of time, which is also the amount which will enter a reservoir or pond located at the downstream limit of the basin (Morris and Fan, 1998). Estimate of long-term sediment yield have been used for many decades to size the sediment storage pool and estimate reservoir life. However, these estimates are often inaccurate especially for small catchments. Besides, it is known from literature that long term period sampling programmes are required to capture the high variability of sediment fluxes in these catchments (Horowitz, 2004; Thodsen et al., 2004). The correlation of sediment yields to erosion is complicated by problem of determining the sediment delivery ratio, which makes it difficult to estimate the sediment load entering a reservoir/pond on the basis of erosion rate within the catchment (Morris and Fan, 1998). Sediment yield from the dam catchment is one of the parameters controlling sedimentation of small dams. This has to be estimated if future sedimentation rates in a dam are to be predicted. Non Governmental Organizations (NGOs) and Government Agencies (GAs) have constructed thousands of small dams in semi-arid regions of East and Southern Africa including Tanzania to provide water for livestock and small-scale irrigation (Lawrence et al, 2004; Faraji, 1995). In Tanzania, in particular, at present it is not known whether the original storage capacities of these dams still exist as a result of many years of operations. Besides, irrigation/water supply schemes ponds/reservoirs are normally draining small catchments. Most of the small catchments are characterized as ungauged. The effective life of many of these dams is reduced by excessive siltation – some small dams silt up after only 2 years. This issue is poorly covered in the many small dam design manuals that are available, which mostly focus on civil engineering design and construction aspects. While a capability to estimate future siltation is needed to ensure that dams are sized correctly, and are not constructed in catchments with very high sediment yields, little guidance is available to small dam planners and designers (Lawrence et al, 2004). Therefore, prediction of sediment yields from catchments is very important where water resources sedimentation is a serious problem like Tanzania and construction of dams is needed (Mulengera, 2008). This chapter discusses also the findings of a few previous representative and related research works in Tanzania and the region at large as this study is a follow up research. These studies were selected in order to cover a wide range of study methods.