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Chapter 5 Sediment Yield

Chapter 5 Sediment Yield. 学而时习之，不亦说乎！. 5.1 Introduction. Soil erosion rate is the gross amount of soil moved by water and wind. Soil loss is the soil moved off a particular slope or field. Sediment yield is soil delivered to a point under evaluation.

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Chapter 5 Sediment Yield

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  1. Chapter 5 Sediment Yield

  2. 学而时习之，不亦说乎！ 5.1 Introduction • Soil erosionrate is the gross amount of soil moved by water and wind. • Soil loss is the soil moved off a particular slope or field. • Sediment yield is soil delivered to a point under evaluation. Kirkby, M.J. and R.P.C. Morgan. Edited. Soil Erosion. John Wiley & Sons. Watershed management-- 14

  3. 5.1 Introduction • 75% of upland erosion is deposited at the base of hillslopes, in flood plains, and within river channels. • 25% of sediment is discharged into the ocean. Read table 9.1 on page 173 calculate the sediment concentration (kg/m2) Watershed management-- 14

  4. 5.1 Introduction • Sediment concentration of Yellow river is 61 times larger than that of the Mississippi river. Why? Watershed management-- 14

  5. 5.1 Introduction Watershed management-- 14

  6. 5.2 Sediment movement and measurement • Sediment discharge is defined as the quantity of sediment per unit time carried past any cross section of a stream. • Measurement: Sampling and relating it to streamflow discharge Sediment deposit surveys Watershed management-- 14

  7. 5.2 Sediment movement and measurement Factors affect the sediment yield • Supply of material-erosion • Physical characteristics of the sediment • Characteristics of the channel-morphological-stage roughness steepness • Streamflow discharge Watershed management-- 14

  8. +++ Total sediment transport Transport capability   Sediment transport  Supply rate    Wash load  Bed material load Sediment size 5.2 Sediment movement and measurement Watershed management-- 14

  9. Fine Coarse Flat Steep Sediment Streamflow 100 100 0 Degradation Aggradation 5.2 Sediment movement and measurement Watershed management-- 14

  10. 5.2 Sediment movement and measurement • By particle size Bed material load Wash load: 0.0625mm or smaller • By method of transport Suspended load Bed load Watershed management-- 14

  11. 5.2 Sediment movement and measurement • Suspended load is the sediment supported yb the upward components of turbulent currents and transported in suspension. Watershed management-- 14

  12. 5.2 Sediment movement and measurement • The settling velocity of particles less than 0.1 mm in diameter is proportional to the square of the grain diameter. Settling velocity  diameter2 d< 0.1mm • The settling velocity of particles larger than 0.1 mm in diameter is proportional to the square root of the grain diameter. Settling velocity  diameter1/2 d> 0.1mm Watershed management-- 14

  13. 5.2 Sediment movement and measurement Settling velocity W = (s-  )D3/6 F = CD D2 2/8 2 = 4 (s-  )gD / 3CD  G. G. Stokes 1851 F = 3 D Watershed management-- 14

  14. 5.2 Sediment movement and measurement Settling velocity = (s-  )gD2 / 18  D<0.076 R<0.4 = 1.72[(s-  )g D / ] Watershed management-- 14

  15. 5.2 Sediment movement and measurement • Bed load Bed load is the sediment that moves by saltation, rolling, or sliding in the bed layer, on or near the bed. Bed layer: a flow layer, several grain diameters thick (usually taken as two grain diameters thick immediately above the bed). Watershed management-- 14

  16. 5.2 Sediment movement and measurement • Duboy’s (1879) equation for low velocities and small grains: Tf = WWDS Where Tf = tractive force; WW = specific weight of water; D = depth of water; and S = stream gradient. for high velocities and large particles: Competence = CV6 Where C is a constant. Watershed management-- 14

  17. 5.3 Sediment delivery ratio A percentage of the onsite eroded material that reaches a given measuring point. Df = Ys/Te Where Df = sediment delivery ratio; Ys = sediment yeidl at a point (wt·area-1·yr-1); Te = total erosion from the watershed above the point at which sediment yield is measured (wt·area-1·yr-1). Watershed management-- 14

  18. Further reading 钱宁，万兆惠. 1983，泥沙运动力学，科学出版社。 American Society of Civil Engineers. 1975. Sedimentation Engineering. M.Selim Yalin. 1977. Mechanics of sediment transport. Pergamon Press. Watershed management-- 14

  19. 钱 宁 Ning Chien 1922 born 1943 graduate 1948 master degree 1951 doctor degree 1955 back to China 1979 cancer 1986.12.6 pass away H. A. Einstein Watershed management-- 14

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