Siltation

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

Darwin Harbour hydrodynamics and sediment transport

Abstract: Darwin Harbour is a large macro tidal estuary that experiences maximum tidal level variations of up to 8 metres. The mean spring tide variations are around 6 metres and the mean neap tide variations are around 3 metres. The tides are semi-diurnal, meaning there are two high tides and two low tides per day. There is some amplification of tidal range in the order of 100 mm –150 mm as the tide progresses through Darwin Harbour. The tidal range does not begin to reduce until well into the upper arms. The large tidal ranges produce strong currents that peak at speeds of up to 2 – 2.5 metres / second. Tidal flows are also large. Tidal flows have been measured along a line from East Point to Mandorah and peak spring tide flows are in the order of 120,000 cubic metres (or tonnes) per second. Over a spring tide up to 1,000,000,000 cubic metres (or tonnes) can pass through this area. Many measurements have been made throughout the harbour of tidal current distributions that contributes toward the knowledge of dynamic processes. The strong currents associated with muddy and sandy substrates also give rise to very turbid plumes that can extend over wide areas of the harbour. A numerical model of Darwin Harbour was constructed to aid management decisions. Originally built to evaluate tidal currents at East Arm wharf the computer model has now been applied to many other areas. The model simulates tidal current speed and direction along with changes in depth and can be used to track sediment plumes. The model can also be used to evaluate the fate of nutrients and other water quality constituents. Use of the model along with field studies has shown that Darwin Harbour does not readily flush particles via physical transport. Particles tend to move to and fro and often migrate from one arm of the harbour to another. More field studies are needed to quantify this movement and to analyse the importance of dilution (advection / diffusion) processes. The present seaward limit of the model is a line joining Charles Point to Lee Point. The model will be expanded in the future to account for the new definition of Darwin Harbour. To achieve this extensive field studies will be required to better map bathymetry, substrate types, tidal current speed and directions, sediment distribution and sediment transport processes. Other studies of macro-tidal estuaries in the tropics may give insights into the sediment transport processes that are active in Darwin Harbour. It is important to know about these processes as fine sediments in the water increase turbidity and siltation and erosion may change habitats. Estuarine systems trap mud within the fringing mangroves. In Australian macro-tidal estuaries it is estimated that 1,000 – 2,000 tonnes of sediment accumulate per square kilometre per year. Not all the mud remains trapped however, some is transported back into the estuary especially when wet season fresh water inflows occur. The fresh water inflows remobilise and disperse the sediments through the water column. The muds impact on plankton, by sticking to the organisms and can also smother coral reefs. Excessive siltation may impact on primary productivity and fisheries.

Assessment of the effects of land-use/land-cover changes on regional soil loss susceptibility using the RUSLE model and remote sensing data

Abstract: Water erosion often causes cultivated lands to degrade, particularly with regard to soil productivity. In addition to the reduction in arable lands lost, erosion has numerous offsite effects such as dam siltation. Erosion in the study area has been accelerated by climate regime, lithology, land-use change and topography. The study area concerns the Tlata catchment, located in northeastern Morocco. Satellite images were analysed for a 15-year period to determine the land cover and land-use changes, as well as the spatial patterns of erosion and deposition over this time period. Results of the land-use/land-cover change and meteorological data analyses indicate that the hydrological regime of the region has changed since the 1980s, following the beginning of the lasting drought, which is still ongoing over Morocco. For the Tlata River catchment, runoff coefficient increased and at the same time badlands extension also increased. This is correlated with land-use/land-cover change, which leads to the impermeabilization of the soil top layer, and the reduction of the water holding capacity.

Avaliação do assoreamento dos lagos do alto rio Doce com base em estudos morfométricos.

Abstract: The Doce river basin is an important backdrop for understanding the dynamics of landscape evolution in the Quaternary, for it is marked by rapid climate changes. The region presents a complex lacustrine system, with very peculiar geomorphological characteristics such as the presence of more than two hundred lakes in different phases, some filled with water and others partially or totally silted, distributed in a region of depression. Several studies consider that the climatic conditions are the main responsible cause for the origin of the lakes, that formed as a result of intense alluvial sedimentation from the channel tributaries of the Doce river. Although it was clear that the tectonic activity in the Cenozoic originated the "Depression Interplanaltica do Rio Doce," the verification of this influence on the origin an devolution of the lakes in the region occurred only after the study from Mello (1997).The study area presents approximately two hundred lakesin different colmatation processes, along with an other fifty containing large volumes of water. In order to understand what determines the uneven siltation processin the lakes of the region, we used morphometric analysis of lacustrine sub-basins and facies characterization based on the granulometry and mineralogy of the sediments. The morphometric analyzes indicated that there is no relationship between these dimension of the lakes and the shape of the contribution basins, nor with the shape of the lake. Thus, we deduce that the different stages of siltation of the lakes, arranged side by side and under the same environmental conditions, is due to the Cenozoic tectonics. The study of sedimentary facies in samples obtained from the silted lakes shows that different final stages of sedimentation occur within each of them.