Showing papers on "Siltation published in 2007"

The combined impact on the flooding in Vietnam's Mekong River delta of local man-made structures, sea level rise, and dams upstream in the river catchment

Abstract: The Mekong River delta plays an important role in the Vietnamese economy and it has been severely impacted during this century by a series of unusually large floods. In the dry season the delta is also impacted by salinity intrusion and tides. These effects have caused severe human hardship. To mitigate these impacts, a large number of engineering structures, primarily dykes and weirs, have been built in the delta in recent years and are still being built, mainly to control floods and saltwater intrusion. These control measures are still being upgraded. A GIS-linked numerical model shows that the flood levels in the delta depend on the combined impacts of high river flows in the Mekong River, storm surges, sea level rise, and the likely, future siltation of the Mekong Estuary resulting from the construction of dams in China as well as many other dams proposed throughout the remaining river catchment. The model suggests that the engineering structures in the delta increase the flow velocities in the rivers and canals, increasing bank erosion, and cause the water to be deeper in the rivers and canals. This increases flooding in the non-protected areas of the delta and increases the risk of catastrophic failure of the dykes in the protected areas. The model also predicts that a sea level rise induced by global warming will enhance flooding in the Mekong River delta in Vietnam, and that flooding may worsen in the long term as a result of estuarine siltation resulting from the construction of dams. At the scale of the Mekong River basin, a multinational water resources management plan is needed that includes the hydrological needs of the delta. At the scale of the delta, a compromise is needed between allowing some flooding necessary for agriculture and preventing catastrophic flooding to alleviate human suffering.

Enrichment of Heavy Metals in Sediment Resulting from Soil Erosion on Agricultural Fields.

Abstract: Heavy metal pollution of soil and water is often associated with industry, but in this paper we demonstrate that water erosion on agricultural soil which has received only agrochemicals has enriched sediment metal concentrations to toxic levels which breach many accepted standards for soils and sediments. Eight 0.1 ha erosion plots with different cultivation treatments were monitored over a 6 year period for surface runoff, soil loss, and Cr, Cu, Pb, and Ni concentrations. Mean concentrations of these heavy metals were up to 3.98 times higher in the sediment than in the parent soil and in some erosion events the sediment had 13.5 times the concentration of metals in the soil. All the sediment heavy metal concentrations were significantly correlated (p < 0.01) with the clay and silt sized fractions of the sediment and with carbon content. The erosion was a highly selective process enriching the detached material in silt, clay, and organic carbon. This was particularly true in smaller erosion events. Sediment metal concentrations tended to follow the shape of runoff hydrographs, although the pattern changed from storm to storm.

Erosion rates obtained from check-dam sedimentation (SE Spain). A multi-method comparison

Abstract: The main aims of this article are to calculate the erosion rates from sediments accumulated in the check dams and to compare the results with those of other methodologies. The study was carried out in a semi-arid area in the south-east of Spain, in the basin of the river Segura, where 425 check dams have been built in order to prevent silting up of a reservoir built in the estuary. From the records of sediments deposited in the different dikes since their construction, various erosion rates have been calculated. Variations in the rates obtained are very high and explanations have been proffered based on physical parameters like the surface areas of the drainage basins to the dikes, the slope of the channel, the lithology or the year in which the dikes were constructed. The results show that, in general terms, the larger the area and the steeper the incline, the lower the erosion rates. Highest rates were found in lithologies of marls and in dikes which have been operational for only a few years. The erosion rates for the whole set, as well as by sub-basins, have been compared with those obtained through other methods (USLE, Fournier and Bathymetric). The bathymetric rates are similar to those obtained from the dikes (3.95 t/ha/year), but the USLE values are 4.5 fold higher and contrary to those of the dikes.

Hydropower production and river rehabilitation: A case study on an alpine river

Abstract: Despite the numerous benefits of hydropower production, this renewable energy source can have serious negative consequences on the environment. For example, dams act as barriers for the longitudinal migration of organisms and transport of particulate matter. Accelerated siltation processes in the receiving river reduce the vertical connectivity between river and groundwater. Hydropeaks, caused by short-term changes in hydropower operation, result in a negative impact on both habitat and organisms, especially during winter months when natural discharge is low and almost constant. In this study, we report the current deficits present in the River Rhone from two different scientific perspectives – fish ecology and hydrology. Potential rehabilitation solutions in synergy with flood protection measures are discussed. We focus on the effects of hydropeaking in relation to longitudinal and vertical dimensions and discuss local river widening as a potential rehabilitation tool. The fish fauna in the Rhone is characterized by a highly unnatural structure (low diversity, impaired age distribution). A high correlation between fish biomass and monotonous morphology (poor cover availability) was established. Tracer hydrology provided further details about the reduced permeability of the riverbank, revealing a high degree of siltation with K values of about 4.7 × 10−6 m s−1. Improving the hydrologic situation is therefore essential for the successful rehabilitation of the Rhone River. To this end, hydropeaks in the river reaches must be attenuated. This can be realized by a combination of different hard technical and soft operational measures such as retention reservoirs or slower up and down ramping of turbines.

Assessment of erosion in the Boyer River watershed (Canada) using a GIS oriented sampling strategy and 137Cs measurements

Abstract: Starting in the 1980's, the Rainbow Smelt ( Osmerus mordax ) population of the Boyer River (Canada) gradually declined due to water eutrophication and excessive siltation in the spawning area. Sediments and agricultural nutrients reach hydrosystems through runoff and soil erosion. The objectives of the study were to quantify the soil and sediment loss from agricultural fields and to identify the areas at risk, using 137 Cs measurements. Using a Geographical Information Systems (GIS), the watershed was subdivided into 6 isosectors presenting specific soil/slope combinations. Representative fields from each isosector were sampled for 137 Cs. Using GIS, the data for individual fields were extrapolated to isosectors and the whole cultivated area of the watershed. Based on this approach, it was estimated that around 30% of the arable lands of the watershed show erosion rates higher than 6 t ha − 1 yr − 1 , which is considered as a tolerable level for Canadian soils, and that 45% of the residual area presents an erosion rate close to that limit. The average sediment production at the edge of fields was estimated at 2.8 t ha − 1 yr − 1 , for an annual production of more than 60 000 t of material. Loamy soils with a slope higher than 2% were estimated to generate the highest sediment rate (6.9 t ha − 1 yr − 1 ) and nearly 40% of the overall sediment production.

Impact of water and sediment regulation by reservoirs in the upper Yellow River on Inner-Mongolia reaches

Abstract: The operation of large reservoirs in the upper Yellow River can produce great benefits in electricity,flood defense,irrigation and ice-flood control,but at the same time it can cause some negative influence to alluvial river course below the reservoirs.Water and sediment changing and fluvial processes before and after reservoir operation in Inner-Mongolia reaches are analyzed in this paper.The relationship between flow discharge and sediment transport capacity at Toudaoguai station is proposed.Based on the sediment transport law and silting reason,the impact of water and sediment regulation by Longyangxia and Liujiaxia reservoirs on the Inner-Mongolia reaches is studied.In addition,the demanding water amounts for keeping the flood conveying capacity at present state and future state with the bankfull discharge of 2 000m~3/s in the reaches are proposed.The research results can be used to guide reservoir operation and water transfer of the West-Route Project.

Analysis on the Development and Evolution of Tidal Flats and Reclamation of Land Resource along Shore of Shanghai City

Abstract: There was about 62% land resource of Shanghai developed as a result of the deposi-tion of sediment from the Yangtze River in the past 2 000 years.Especially in the latest 50 years,some efficient methods included Inning-Siltation Promotion-Inning,had been adopted to block up the sediment and to create land.These reclamated land resource amounting to 1.01×105hm2 had the land area enlarged by 15.8% in Shanghai.Obviously,it was important that the reclamation of land could alleviate shortage of land resource for the development of Shanghai,and promote industrial and agricultural development and capital construction.Based on the data of the tidal flat elevation data obgained by GIS and the long term field survey of hydrology and sediment,it was found that the abundant sediment from the Yangtze River and its adjacent areas,the hydrologic regimes around the Yangtze River estuary were advantageous to create new land resource.Many projects,which included crop cultivation works,waterway closure works,siltation promotion project on low tidal flats,piling up sediment from shallow seabed to the inner region of the dam,damming under water,etc.,had been successfully adopted around the shore of Shanghai.Different methods should be used at the corresponding phase in order to take a great advantage of the sediment supplied by the Yangtze River for promoting siltation.For example,some Inning Projects such as crop cultivation works and river branch closure works were developed at the middle and high tidal flats,which could accelerate the siltation of tidal flats and the increment of land resource.However,the projects of damming works and piling up sediment from shallow seabed to the inner region of the dam were done at the low tidal flats,which resulted in the expansion of the land resources and exposure of shallow sea topography.It was proved over the past 50 years that situations between reclaimed areas of land and changes of the areas of wetland and tidal flats could keep sustainable development.

Spatially discontinuous modern sedimentation in Georgian Bay, Huron Basin, Great Lakes

Abstract: High-resolution seismic reflection profile data show that the modern sediment cover (over the last 150 years) in Georgian Bay is thin and spatially discontinuous. Sediments rich in ragweed pollen, largely derived from siltation linked to land clearing and European settlement, form a thin, discontinuous veneer on the lakebed. Much of the lakebed consists of exposed sediments deposited during the late glacial or early postglacial. Accumulation rates of modern sediments range from < 0 mm/year (net erosion) to ∼3.2 mm/year, often within a few hundred metres spatially. These rates are much lower than those reported for the main basin of Lake Huron and the other Great Lakes, and are attributed to the low sediment supply. Only a few small rivers flow into Georgian Bay, and most of the basin is surrounded by bedrock of Precambrian gneiss and granite to the east, and Silurian dolostone, limestone and shale to the west. Thick deposits of Pleistocene drift, found on the Georgian Bay shoreline only between Meaford and Port Severn, are the main sediment source for the entire basin at present. Holocene to modern sediments are even absent from some deep basins of Georgian Bay. These findings have implications for the ultimate fate of anthropogenic contaminants in Georgian Bay. While microfossil assemblages in the ragweed-rich sediments record increased eutrophication over the last 150 years, most pollutants generated in the Georgian Bay catchment are not accumulating on the lakebed and are probably exported from the Bay.

Response of the benthic fauna of an urban stream during six years of restoration

Abstract: Okeover Stream flows through the University of Canterbury campus and has been subject to restoration since 1998. While initially spring-fed, its main source of flow is now aquifer water, which has been used for cooling university buildings. Water quality is generally good, but the low-gradient streambed includes substantial amounts of fine inorganic sediment and organic matter including deciduous tree leaves. Restoration activities include riparian plantings, channel shaping, substratum manipulations and additions, the construction of sediment traps and macrophyte management. Thirty aquatic invertebrate taxa (13-19 per year) have been recorded in annual surveys since 2000. Paracalliope fluviatilis (Amphipoda), Copepoda and Oligochaeta were most abundant in all years, whereas Mollusca and Trichoptera always made up <4% and <2% of individuals, respectively. Furthermore, cased caddisflies were found only in the two (of four) downstream reaches, whereas Copepoda were predominantly in the upper two reaches where flow was generally slower. Low annual MCI (69-84) and SQMCI (3.5-4.8) values indicated the fauna comprised mainly species that are tolerant of poor water quality or degraded habitat conditions. Our data indicate that the invertebrate fauna has yet to respond positively to the changes in physical habitat and riparian conditions made along Okeover Stream. The introduction of pulses of poor quality water during heavy rainfalls, high levels of siltation, heavy metals in bed sediments, large accumulations of slowly decomposing leaves and an inadequate source of potential colonists may all contribute to the weak response of the invertebrate fauna to restoration activities.

Effects of waves and currents on the siltation problem of Damietta harbour, Nile Delta coast, Egypt

Abstract: This study evaluates the effect of prevailing dynamic factors on the sedimentation process in Damietta Harbour along the Nile delta coast of Egypt. The monitoring program spanned the period between 1978 and 1999 and included measurements of waves, currents and bathymetric profiles. The evaluation was based on determination of erosion and accretion rates, current regime, sediment transport, wave characteristics and wave refraction. Results revealed that the predominant wave direction from N-NW sector (86 %) throughout the year is responsible for generation of a longshore eastward current. Less frequent waves from the N-NE sector generate an opposing longshore westward current. The refraction pattern for the prevailing wave direction indicates that the harbour and its navigation channel are located within a divergence of wave orthogonal and in an accretion sediment sink area. The annual net rate of littoral drift on the western side of the harbour is about 1.43 * 105 m3 (accretion), while the annual net rate of littoral drift on the eastern side is about 2.54 * 105 m3 (erosion). Currents fluctuate tremendously in speed and direction, especially during the winter months. Hence, sediment transport takes place in offshore, eastward, and onshore directions. Progressive vector diagrams show that the largest near bottom offshore, onshore and easterly net drift occurs during summer, spring and winter respectively. The onshore sediment transport generated during spring and summer plays an important role in the redistribution of eroded sediments during the winter. The overall study of dynamic factors indicated that the harbour site is characterized by eastern, western, offshore and onshore sediment movements. Therefore, the north-south orientation of the navigation channel, with its depth greater than the surrounding area, interrupts sediment drift from different directions and reduces the current speed. Consequently, the sediments sink within the navigation channel from different directions. The sources of sediments contributing to the siltation process of the harbour and its navigation channels are mainly derived from the Rosetta promontory, Burullus beaches, Damietta promontory and from offshore and the dumping area.

Depositional Environment of a Tropical Estuarine Beach near Sharavati River Mouth, Central West Coast of India

Abstract: Beach sediments of Pavinkurve and Kasarkod, located adjacent to the Sharavati River mouth at Honnavar, Central West Coast of India are medium to fine grained (1.21 - 2.30Φ) and moderate to well sorted (0.17-0.55). The convergence of long-shore circulation and interaction of river and wave processes are the main influence for the distribution of sediments along these beaches. The sediment transport land-ward during fair weather season (pre-monsoon) is mainly due to westerly wave approach. The re-suspension of sediments owing to agitation of sea during westerly wave approach, their land-ward movement during high tide, and the obstruction caused by the submerged bar formation during falling tide leading to their deposition are the main causes for siltation in the river mouth.

Effect of Forest Cover Change on Sedimentation in Lam Phra Phloeng Reservoir, Northeastern Thailand

Abstract: To predict the lifespan of Lam Phra Phloeng Reservoir, in which the capacity has been reduced by sedimentation, both sediment inflow volumes generated from upstream areas of the catchment and deposition in the reservoir were calculated. The rating curve of the relationship between discharge and sediment at station M.145 from 1996 to 2000 obtained using water level discharge and a sediment estimation (LQS) showed that the annual sediment volume resulting from inflow to the Lam Phra Phloeng catchment (820 km2) was 0.36553 Mm3 (million cubic meters). The siltation rate and annual sediment volume in the reservoir were obtained for a series of periods. During the first period, from 1970 to 1983, the annual sediment volume was 2.23 Mm3 and the erosion rate was 2.72 mm/year/km2. This period coincided with a 73.57% decrease in forest area. The second period, from 1983 to 1991, had a lower annual sediment volume of 1.625 Mm3 and a lower erosion rate of 1.98 mm/year/km2. The forest area increased 1.05% during this period. In the recent period, from 1991 to 2000, the annual sediment volume was 0.36553 Mm3 and the erosion rate was 0.445 mm/year/km2. These low rates were associated with a 4.95% increase in forest area. Since the 1960s, the agriculture of Thailand has shifted from subsistence farming to a cash crop culture to develop the social economy. This shift has resulted in the conversion of forests to cultivated lands. As a result, rapid deforestation has occurred, and soil erosion in crop fields has become a serious problem with regard to resource degradation. Soil erosion from crop fields has generally been recognized since the old days.

Minimizing Harbor Siltation (MHS)

Abstract: This paper presents methods for minimizing harbour siltation (MHS) which comprises the contents of chapter 4 of a forthcoming PIANC WG 43 report with the above title. The methods include 3 strategies: (1) KSO: Keep the sediment out, (2) KSM: Keep the sediment moving or, (3) KSN: Keep sediment navigable. 1.0 Introduction. Maintenance dredging can be minimized by locating harbors in naturally deep water or by implementing MHS measures. MHS technologies are designed to keep sediment in its natural system (KSIS). Consistent with KSIS, Krone (1987) emphasized two basic approaches to siltation minimization: (1) KSO: Keep the sediment out of a harbor, or (2) KSM: Keep the sediment moving through the harbor. A third strategy is considered here, namely: (3) KSN: Keep harbor sediment navigable. KSO strategies focus on minimizing the flow of sediment laden waters that enter harbor basins and/or navigation channels. The KSM strategies center on maximizing harbor flow velocities to prevent sediment from settling. KSN strategies are a derivative of the navigable depth concept and play on the ability of a vessel to sail through a low density fluid mud. A distinction is made between passive, i.e., those that do not require energy nor moving parts (submerged sills, flow training structures, etc.) and active measures (movable gates, locks, flow augmentation, etc.). KSO strategies are generally best suited to relatively quiescent basins where KSM is not feasible. Conversely, KSM strategies are best suited to berths located within or along relatively swiftly flowing waters, where a small increase in flow velocity will induce a significant decrease in siltation.

Impacts of flood events in coarse sediment-producing areas on channel siltation and fluvial process of the lower Yellow River

Abstract: The method of multiple regression is used to analyze the influences of flood events from the coarse sediment producing areas on the channel siltation and fluvial process of the lower Yellow River based on the flood events from 1950 to 1985. The results showed that the flood events from the coarse sediment producing areas carry larger amounts of sediment load and coarser particle sizes than from other source areas, which increases deposition in the lower river channel. And there exist good correlations between channel siltation of the lower reaches of the Yellow River and the coming water and sediment of flood events from the coarse sediment producing areas. Through these correlations, the amounts of sediment deposition in the lower river channel could be roughly estimated based on the runoff and sediment load of flood events from the coarse sediment producing areas. The sediment deposition caused the fluvial process. There exists a complex response of channel form change to the coming water and sediment load of flood events from the coarse sediment producing areas. When the sediment concentration is smaller than 200kg/m(3), the ratio between wide-depth ratio after flood and wide-depth ratio before flood((B/h)(a)/(B/h)(b)) Will increase with the increase of the maximum sediment concentration; when the sediment concentration is near 200kg/m(3), (B/h)(a)/(B/h)(b) reaches the maximum value; and when the sediment concentration reaches the limits of hyperconcentrated flow, (B/h)(a)/(B/h)(b) will decrease with the increase of the maximum sediment concentration. Generally, flood events from the coarse sediment producing areas made channel form of the lower Yellow River deeper and narrower, but a large amount of sediment deposition simultaneously occurs. So, the impacts of flood events from the coarse sediment producing areas on the channel of the lower Yellow River are lessened.

Assessment of siltation and nutrient enrichment of

Abstract: 10 Gilgel Gibe hydroelectric power dam was located in the submerging part of the Addis Ababa, Jimma highway in Ethiopia which is 11 about 55 km far from Jimma was constructed in 2004. A cross-sectional study was conducted during 2005 to assess the siltation and 12 nutrient enrichment level of the dam. It was found from the study that siltation and nutrient enrichment were the major problems in 13 this reservoir. The sheet erosion of catchment area was found to be 4.47 · 10 7 ton/year and hence 2210 ton/km 2 . The contribution of 14 the sediment load deposition of the Gilgel Gibe River to the dam was 277,437 ton/year and the total sediment load was 15 4.50 · 10 7 ton/year and this amount could cover 3.75 · 10 7 m 3 /year of the dam. From the analysis of water samples of Gilgel Gibe 16 dam, the concentrations of ammonia, chlorophyll a, total dissolved solids (TDS), dissolved oxygen (DO), biological oxygen demand 17 (BOD), pH and temperature were found within the permissible limits as prescribed by WHO standards, but other parameters like phos-

Investigating erosion and sediment transport from headwaters to catchments to reduce reservoir siltation in drylands

Abstract: Deposition in reservoirs threatens their reliability for water supply in dryland regions. We present a modelling scheme for the quantitative assessment of sediment mobilisation in catchments, transport in the river system, and retention in reservoirs, with a focus on meso-scale semi-arid catchments. An existing hydrological model tailored for specific semi-arid features is extended to represent erosion and sediment transport. Data sets from dryland catchments, rivers, and reservoirs in northeast Spain and northeast Brazil are used and expanded by own measurements. The first results concerning a representation of erosion-prone landscape units, the role of sediment transport in the river system, and the sedimentation in reservoirs are presented.

Sediment load assessment of small embankment dams in southren regions of nwfp

Abstract: Siltation has a large impact in reducing the benefi t obtained from sixty two dams constructed in the s outhern region of NWFP. The United Nations development programme (UNDP) in its poverty reduction project concentrated on the s ediment deposited in these dams to safeguards their service lives. A fie ld study was conducted to find sedimentation in nin e-sampled reservoirs of the embankment dams during August 2004 to September 2005 at Shakardara District Kohat, Pakistan. The suspe nded load through integrated approach and the total deposited sedimen t load cumulated in the reservoirs (By Grid survey) were determined separately. Staff and rain gauges for stage measure ment and precipitation in the reservoirs and area w ere installed. Siltation life of each embankment dam was determined from rate of incoming sediment, trap efficiency and reservoir ca pacity. The intensity of rainfall observed during the study period (July, 20 04 to August, 2005) ranged from 1.2 to 92.1 mm/h. Runoff from the catchments of the embankment dams ranged from 10.49 to 17.02% of the rainfall, with over all average o f 13.12%. The result of the study showed that the overall suspended sedimen t load in runoff ranged from 1478 to 11,812 mg L -1 with overall average of 3699 mg L -1 . It is also found that the average incoming total sediment load in the selected embankment dams varied from 721 to 1875 tons-km -2 year -1 . The reservoir capacity of the sampled embankment dams ranged from 5,008 to 120,190 m 3 and the siltation lives of the sampled reservoirs ranged from 11 to 9 9 years with overall average of 38 years. In genera l, the siltation lives of the sampled embankment dams increased with increase in reservoir capacity. Embankment dams with capacity less than 40,000 m 3 were found to have siltation lives of less than 40 years and their construction should be avoided.

Mangrove succession in segara anakan, cilacap

Abstract: The purpose of this research was to study primary and secondary succession of mangroves in Segara Anakan, a brackish water estuary, in Central Java, which experienced heavy siltation. This siltation created not only a lot of newly formed land, but also shoals which changed the pattern of tides and currents. In addition, the mangroves also experienced illegal cutting, which opened a gap in the forest. This study was carried out in 1997 at newly-formed lands and forest-gap areas, and it was compared to the 1981-1984 study. Permanent transect and quadrate plots were established across the selected areas. The result showed that almost all the mangrove areas in the Segara Anakan were disturbed. Much of the mangrove forest was rapidly converted into shrimp ponds. Primary succession occurred on mud flats of the newly formed land and on the progradation shorelines, and secondary succession occurred in the forest gap. The propagules of Sonneratia alba and Avicennia alba colonized the newly formed land. Then, as a sere developed the dominant species was A. alba. The development of a sere of mangrove pioneers on the newly formed lands was dictated not only by the heavy siltation, current and tide patterns, but also was forced by the availability of propagules and seeds in adjacent forest areas. Along the transects there were no species zonations. Acanthus ilicifolius and Derris heterophylla colonized the forest gap. Rhizopora spp. and Bruguiera spp. are endangered species in the Segara Anakan mangrove. Keywords: mangrove, succession, siltation, pioneer species.

Long-Term Shoreline Change and Evaluation of Total Longshore Sediment Transport Rate on Hupo Beach

Abstract: The harbor siltation by longshore sediment transports has become a serious problem on the East Coast of Korea A reasonable prediction of the longshore sediment rate is important to approach the siltation problem effectively In the recently developed 1-line model, the empirical constants of the sediment transport formula, which include the absolute quantity of sediment transport rate and the spatial distribution of breaking wave height by wave deformation, are treated as calibration parameters Since these constants should be determined by the very long-term shoreline data, the longshore sediment rates are much more reasonable values The method was applied to Hupo Beach, which has experienced heavy siltation The authors also discuss long-term shoreline change using aerial photos and the observed wave-induced current patterns According to the result, the SW-direction sediment transport rate was , and the NE direction was at Hupo Beach for the last 11 years The siltation in Hupo Harbor might be affected by the NE-direction sediment transport from Hupo Beach

Siltation mechanisms of Huanghua Port and 3D characteristics of nearshore suspended sediment concentration under waves

Abstract: Careful analysis shows that for wave dominate climate,the suspended sediment concentration is controlled by waves and showed large vertical gradients.The high deposition rates in the access channel of Huanghua Port is caused by high waves in storm events.Remote sensing and 2D horizontal numerical modeling need to be used with caution due to their lack of vertical resolution.It is recommended to apply field measurements and 3D numerical modeling whenever possible.

Soil Erosion Hazard Assessment Using Usle Model: A Case Study of Legedadi & Dire Reservoir Catchment

Abstract: Dissertation submitted for Partial Fulfillment of the Requirements for the Award of the Degree of MASTER OF SCIENCE In Remote Sensing and Geographical Information Systems (GIS) of Addis Ababa University, Addis Ababa, Ethiopia.%%%%Siltation due to erosion is the major problem facing many of the lakes and reservoirs in Ethiopia today. The erosion process in Legedadi and Dire catchment transports a lot of sediment to the reservoir and is the most important factor for reducing their storage capacity. The two-reservoir catchments are in a state of physical and biological deterioration due to over exploitation of lands by the inhabitants. A considerable part of the sediment load is due to present agricultural and livestock management practices and lack of soil conservation practices. Therefore, a reservoir catchment needs to be managed effectively in order to reduce the effect of sedimentation. This study has integrated the USLE model and GIS techniques to assess erosion hazards in Legedadi and Dire Catchment. Soil loss estimation was carried out based upon the principles defined in the USLE, which relates soil erosion process to rainfall Erosivity, soil erodibility, topographic, land cover and conservation practices. The input parameters of the model, derived from GIS analysis of primary data, were compiled in GIS database in raster data model with grid resolution of 30 meters. The spatial thematic map of the respective factors of USLE was created as following: R factor map was interpolated from point data of station; K factor map by reclassification of soil map; LS factor, relief parameters, was derived from DEM; C and P factor map by reclassification of landuse/ land-cover map of Landsat ETM+. The assessment approach involved two aspectsestimation of potential and actual erosion. After application of USLE model, the potential and actual mean annual soil loss from the catchment was found to be 120t/ha/yr and 4.8t/ha/yr, respectively. From the level of soil tolerance limits set by Hurni (1986), mean annual soil loss estimated by the model is tolerable. 26km2 or 9.1% of the catchment is prone to high erosion hazards. In terms of relative comparison of dire and Legedadi catchment the Dire catchment was found to be more endangered by erosion hazards at various intensity.

Sediment transport and morphological changes of the Xiaolangdi Reservoir in early sediment impoundment period

Abstract: Physical model tests were conducted to forecast the regime of the sediment transport and morphological changes of the Xiaolangdi Reservoir in the early sediment impoundment period.The experimental results show that the sediment transport in the early operation period took the mode of density current;in the main river channel sediment appeared in the form of delta deposition,which moved towards the dam;the deposition also occurred in the branches and gullies,especially when sand bar existed at the junctions due to the sediment-laden flow inversely running from the main river channel to the branches and gullies.When the reservoir water level sharply fell,headward erosion and bank erosion occurred in the main riverbed and the siltation area will be reduced in the branches and gullies. Although the model conditions in the experiments didn't fully agree with those of the prototype in the real case,the general agreement of the data measured in situ with those predicted in model has validated the rationality of experiment.