Sediment Budget Methods And Applications

The sediment movement involves soil erosion, sediment transportation, andsediment deposition. Most of the sediment after the first time erosion is deposited inthe upstream gullies. A part of the deposited sediment may be eroded again. Therefore,erosion is a multiple process and the amount of sediment erosion can be countedmultiple times. Sediment budget provides a physical picture of sediment erosion andtransportation amounts for river basin management. This paper used size distributionanalysis method, sediment budget matrix, and the concept of limit velocity to study therelations of sediment amounts eroded from the basin and transportation in the streamnetworks.Based on the equation of sediment mass conservation and the representative sizedistribution of original eroded sediment, the size distribution analysis, as a method ofsediment budget, computed the amount of sediment deposition of original erodedsediment in different sediment size fractions. The representative size distribution oforiginal eroded sediment can be roughly estimated by the weighted average of anaverage for size distributions of sediment deposits and the size distribution of suspendedload at the control hydrological station, according to the ratio of sediment transported intothe control hydrological station to the total eroded sediment. Take the Xihanshui RiverBasin as an example; this study collected the samples of sediment deposits in debrisflow gullies, tributaries and hillslopes. As the results, the total soil erosion from theXihanshui River Basin is about33.38million tons, of which about16.94million tons isstored in the gullies, tributaries, and hillslopes,99.9%of the sediment coarser than0.5mm eroded from the watershed,3.58of the5.87million tons of sediment of diameterin the range of0.05-0.5mm, and2.88of the17.02million tons of sediment finer than0.05mm also deposit in the said region.A sediment budget matrix as a statistical table can be used to show the quantitativerelations among sediment erosion, sediment transportation, sediment deposition, andsediment yield. This matrix as a whole is not used for calculation. Take the Liujia Gullyas an example, the values of the components in the sediment budget matrix may bedetermined by field investigation, measurement and analysis of satellite images anddigital topographical maps, the fallout137Cs technique,"3S" technology, and the equation of sediment mass conservation. The results show that, in the Liujia Gully, thetotal volume of sediment erosion is0.14million t·km^-2·yr^-1, and the annual amount oferosion due to landslides and avalanches per unit area which was omitted in previousstudies contributes to93%of the total volume of sediment erosion. The total amount ofsediment yield is1600t·km^-2·yr^-1, which is mainly produced by soil erosion from slopesand contributes to81%of the total sediment yield.92%of erosion sediment depositedin the gullies and8%was transported by flowing water after landslides and avalanches,96%of sediment deposited in the gully and on the riverbed after bank failure and gullyerosion,49%of erosion sediment from slopes was transported by flowing water to theoutlet of the Liujia Gully. The ratio of sediment transported into the control hydrologicalstation to the total eroded sediment, calculated from sediment budget matrix is smaller thanthe value from size distribution analysis for the former method takes into account theeffect of gravitational erosion.The concept of limit velocity was proposed by analyzing hydrological data fromalluvial rivers. The principle of self-adjustment, characteristics of cross section, and thecritical velocity for the coarsest sediment to be suspended were used to analyze thesediment movement and fluvial process in alluvial rivers. In general, the averagevelocity increases with increasing discharge at low discharge. As the discharge exceedsthe bankfull discharge, any further increases in the discharge do not greatly increase thevelocity. Thus, the average velocity approaches a limit. For alluvial rivers, the wet crosssection changes with the discharge so that the flow velocity does not exceed amaximum value. The limit velocity roughly equals to the critical velocity for thecoarsest sediment to be suspended. As the velocity exceeds the limit velocity, thecross-section of river may be changed and thus flow velocity is reduced.