SedimentTransportbyWater

1、Sediment Transport by WaterTheory ProcessesRainsplashoverland flow transportRilling and gullyingMass movementsWeathering limited versus supply limitedThe extreme event1TheoryMechanics of flowStream energyEntrainment2Mechanics of flowWater is subject to two forces:gravity (Wa = g sin )frictionDefines

2、 ability of water to erode and transport sediment3Types of flow in open channels4Laminar FlowEach fluid element moves along a specific path with no significant mixing between layersBoundary layer in contact with the bed has no forward velocityEach layer can slip past each other5Turbulent flowAt a cr

3、itical velocity or depth laminar flow becomes unstable and the parallel streamlines are destroyedAdjacent layers mix, transferring momentum by large scale eddiesVelocity more evenly distributed with depthSteeper near bed velocity gradient6Reynolds Number (Re)Re = h u/where = fluid densityh = flow de

4、pthu = fluid viscosity = viscositylarger values, larger turbulence7EntrainmentMovement of material depends on its physical properties;grain sizeshapedensitystructual arrangementBasic distinction;cohesive (silt-clay size)non-cohesive8Shear stressCauses initial movement Shear stress = estimate of forc

5、e exerted on the bed by the fluid9cr Dbut doesnt include lift forcesLift due to:eddiesdifference in velocity at top and bottom of grain10Critical shear stress11Shields (1936)Dimensionless critical shear stressPlot against particle Reynolds no. (ratio of grain size to thickness of laminar sublayer)12

6、Factors producing scatteruse of average or spatial variability over bedchannel sizeirregularity of eddiesdegree of exposurepivot anglesimbrication degree of packinggrain shapemicrotopography13ErosionEntrainment/detatchmentTransport14Detatchment vs TransportRainsplashWeatheringTillageTramplingRunoffR

7、ainsplashOverland flowRill flowGully flow15Rainsplashvaries with rainfall intensityvaries with land covervaries with slopevaries with % of area which is rilledvaries with lithologycrusting?16Surface Washparticles detatched and transported by surface flowing waterforce = velocity x mass (i.e. Q)contr

8、ols relate to character of materials, especially ability to produce rainfall excess17Resistance to detatchmentnon-uniformvaries with particle sizecyclic variation with seasonsand/silt clay ratiostoniness18RillsImpermanent channelsvary in lateral position year to yeardevelop once threshold exceeded i

9、n a single event19Gulliespermanent incised X-sectional formdevelop once threshold exceeded over longer term average conditionsmay be discontinuousgully / arroyo / donga20Mass Movementpossibly only important in extreme eventsdirectly contribute to load or rills/gullies4 main typesshallow slidesslab f

10、ailurerockfallsdeep seated slides21Soil ErosionSoil loss = R K L S P CR = rainfall erosivityK = erodibility of soilL = slope lengths = slope angleP = coefficient of cultivation methodsC = crop management factor22Weathering LimitedWhen unlimited capacity for transport occurs, removal of material is l

11、imited by the rate at which material is detatched.23Transport LimitedWhen there is an abundant supply of material and erosion depends on the efficiency of forces transporting the material away.24Equilibrium conditionRemoval of material = supply of materialContionuous range between extremesOccurs ove

12、r different timescales:CyclicGradedSteady-state25Transport vs Weathering26TimescalesCyclicperiod over which an effective change in basin elevation can be measuredGraded (equilibrium)a change in any factor will cause a displacement of the equilibrium in a direction which will absorb the effect of cha

13、ngeSteady statea measurement can be taken and the system assumed to be in a constant condition27Timescales28Magnitude-Frequency conceptWolman and Miller, 1960majority of work carried out by events which occur on average 1 or 2 times per yearbasin characteristics adjusted to these events29Different in semi-arid channels:stress-strain reln more complicatedlarge spatial variationmorphology adjusted to extreme events30Extreme events Do majority of work because:larger particle sizetransmission lossespoor sortingvegetation31Themes of dryland floodsGet scour and fi