Siltation

About: Siltation is a research topic. Over the lifetime, 1420 publications have been published within this topic receiving 20983 citations.

Runoff and Sediment load of the Yan River, China: changes over the last 60 yr

Abstract: . Runoff and sediment load changes are affected by climate change and human activities in an integrated way. Historical insight into these effects can not only improve the knowledge of river processes, but also promote more effective land and water management. In this study, we looked at runoff and sediment change in the Yan River basin, Loess Plateau, China, using data sets on land use and land cover (LUC), monthly data of precipitation and temperature, and observed data on runoff and sediment load from 1952 to 2010 at the Ganguyi Hydrologic Station. Available data on soil and water conservation structures and their effect were also studied. Five main findings emerged from the data analysis. (1) The annual runoff and sediment load varied greatly during the last 60 yr, and both had coefficients of variation that were much larger than those of precipitation and temperature. (2) Annual runoff and sediment load both showed a significant trend of linear decline over the period studied. The climate data showed a non-significant decline in precipitation over the same period, and a very significant increase in temperature; both can help explain the observed declines in runoff and soil loss. (3) Based on a mass curve analysis with anomalies of normalized runoff and sediment load, 4 stages in the change of runoff and soil loss were identified: 1951 to 1971 (Stage I), 1972 to 1986 (Stage II), 1987 to 1996 (Stage III) and 1997 to 2010 (Stage IV). (4) When years were paired based on similar precipitation and temperature condition (SPTC) and used to assess the impacts of human activities, it was found that 6 sets of paired years out of 12 (50%) showed a decline in runoff, 8 (67%) a decline in sediment load, and 9 (75%) a decline in sediment concentration. The other sets show an increasing change with time. It showed the complexity of human impacts. (5) Human impacts relating to LUC change and soil and water measures in this basin were significant because of both the transfer of sloping cropland into non-food vegetation or terraces, and the siltation in the reservoirs and behind check dams. Data indicated that about 56 Mt of sediment was deposited annually from 1960–1999 as a result of the soil and water conservation structures, which is significantly more than the average 42 Mt, leaving the Yan River basin as sediment load each year. Although the effects of climate change and human action could not be separated, analysis of the data indicated that both had a significant impact on runoff and sediment load in the area.

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.

Multi‐decadal Morpho‐sedimentary Dynamics of the Largest Changjiang Estuarine Marginal Shoal: Causes and Implications

Abstract: Understanding the long‐term evolution of estuarine shoals given natural variations and human modifications is a key issue for wetland protection and shoal management. Here, the multi‐decadal (1958–2013) morpho‐sedimentary dynamics of the Nanhui Shoal (NHS), the largest Changjiang estuarine marginal shoal, are studied using a suite of hydrological, sedimentological, and bathymetric data. The results show that the tidal flow regime and sedimentary mode around the NHS changed slightly after the 1980s. Moreover, the NHS experienced a siltation‐induced volume increase of 4.1 × 10⁸ m³, concentrated in the landward region, and seaward progradation, producing an increase in gross area of 33 km², during 1958–2013. Even so, the actual tidal flat resource decreased by 29% due to the reclamation of 202 km². Transition in the development of the NHS is detected: a planar geometry transformation from a triangular cusp to an arcuate cusp during 1958–1989; vertical siltation in the landward region under a stable arcuate‐shaped geometry thereafter. Furthermore, a steeply sloping profile with grades of 2–11‰ formed in the northern section, which limits future reclamation to 80 km² there. Estuarine regime adjustment, inducing hydrodynamic alterations in the South Passage, dominated the geometric changes in the NHS during 1958–1989, whereas substantial siltation promotion projects led to the landward siltation after 1989. The decrease in sediment input downstream of the Three Gorges Dam has played a minor role in the shoal evolution. This work provides new insights into the long‐term morpho‐sedimentary responses of estuarine shoals to natural and artificial forcings and their implications for shoal exploitation.

Ecological aspects of water impoundment in the tropics

Abstract: Recognition of the potentially harmful effects of water development in the tropics had led to increasing efforts to assess the environmental impact of such projects prior to construction. Decisions regarding the development or non-development of water resources must be based on sound investigation of both the long-and short-term effects of reservoir construction and operation. The environmental effects of water impoundment vary greatly with the characteristics of the region as well as the type of reservoir to be constructed (area and depth of reservoir, ratio of water inflow to storage). Of major concern are the reduction of reservoir capacity as sediments accumulate behind the dam and the loss of these sediments to downstream agriculture and fisheries. The potential impact of altered flow regimes, siltation, reduction in beach formation and nutrient enrichment at the mouths of rivers, and the possibility of saltwater encroachment should receive careful stdy. A thorough description of the plants and animals to be affected by inundation should be made to determine the possible loss of rare or key organisms as well as the potential development of "nuisance species". Included in this survey should be a detailed study of existing fish and the potential for commercial fishery development in the proposed reservoir. Consideration should be given to vegetation removal in the reservoir basin prior to inundation, since decaying vegetation can result in deoxygenation, formation of hydrogen sulfide, possible development of suitable habitats for undesirable species and snagging of fish nets. Sanitation and land use practices as well as erosion in the watershed surrounding the reservoir must be controlled to prevent accelerated eutrophication caused by increased nutrient loading. Inundation in tropical areas can have serious sociological and human health implications including the increase of diseases, e.g., malaria, schistosomiasis, onchocerciasis, and dysentery, and the probable resettlement and alteration of land use practices. Census information and surveys concerning land use, housing and health standards and the social and economic structure of the community to be affected must be evaluated in order to anticipate and avoid potential problems. The archaeologic, historic, scenic and recreational value of the site to be inundated must also be considered. Studies of the EL Cajon site on the Sula River in Honduras and the Purari River Project in Papua, New Guinea, are utilized as examples in this report.