RUSLE2

1I.B Soil Conservation SystemsRabi H. Mohtar

Professor, Environmental and Natural Resources EngineeringExecutive Director, Strategic Projects, Research & DevelopmentQatar Foundationmohtar@purdue.edu or rmohtar@qf.org.qa

July 201312Materials To Be Covered Principles of Soil PhysicsSediment TransportErosion ControlSoil MechanicsSlope Stabilization

This review will provide you with an overall understanding and not necessarily makes you an expert! I.B Mohtar23SourcesEnvironmental Soil Physics; Hillel; 1998 Hillel (1998)Essentials of Soil Mechanics & Foundations, 7th ed.; McCarthy; 2007; McCarthy (2007)Soil and Water Conservation Engineering4th ed. Schwab, Fangmeier, Elliott, Frevert: Schwab et al (1993)5th ed. Fangmeier, Elliott, Workman, Huffman, Schwab: Fangmeier et al (2006)Design Hydrology & Sedimentology for Small Catchments; Haan, Barfield, Hayes: Haan et al (1994)USLE/RUSLE: USDA Agricultural Handbook No. 537 (1978)Cuenca, R. H. 1989. Irrigation System Design - An Engineering Approach. Prentice-Hall, Inc., Englewood Cliffs, NJ. 552 pp. Cuenca (1989). Ward, Elliot 1995 (Environmental Hydrology, Lewis Publishers).http://cobweb.ecn.purdue.edu/~abe325/: Mohtar soil and water resources conservation course.I.B Mohtar34Soil Physics & MechanicsSoil classes and particle size distributionsBasics of soil waterWater ContentWater PotentialWater FlowSoil strength & mechanicsI.B Mohtar45Soil Classes & Particle Sizes

Hillel (1998) page 61I.B Mohtar56Soil Classes & Particle Sizes - 2ISSS classification is easiest

Sand 0.02-2.0mm (20-2000)Silt 0.002-0.02mm (2-20)Clay 10000 plot-years of dataInternational useUnit Plot basis; LS = C = P = 1Near worst-case managementR from good fit rainfall-erosionK from K = A / RC and P from studiesSub-factors in later versionsI.B Mohtar3233USLE/RUSLE approachLookupMaps, tables, figuresDatabasesProcess-based calculationsShow changes over timeWhere dont have good dataI.B Mohtar3334R factor rainfall erosivityMapsR(customary SI) = 17.02 * R(customary US)

S4I.B MohtarHaan et al (1994) pp:251; Haan et al (1994) Appendix 8A; Schwab et al (1993) 99(SI); Fangmeier et al (2006) pp:143(SI); USDA (1978) pp:1-53435K factor soil erodibilitySoil surveys, NASIS, Haan et al (1994) 261-262; USDA 6Erodibility nomograph: Haan et al (1994) 255; Schwab et al (1993) 101; Fangmeier et al (2006) pp144; USDA (1978) pp: 7No short-term OM

I.B Mohtar3536LS Topography FactorNew tables & figuresHaan et al (1994) 264; USDA (1978) 8Know susceptibility to rillingHigh for highly disturbed soilsLow for consolidated soilsI.B Mohtar3637C cover-management factorPart of normal management schemeLookup: Schwab (1993) 102; Fangmeier et al (2006) pp: 146; Haan et al (1994) 266; Hillel (1998) Appendix 8; USDA (1978) 9It Changes over timeI.B Mohtar3738C Cover-Management Factor - 2Subfactor approach (RUSLE)C = PLU * CC * SC * SR * SM; all 0-1PLU = prior land useroots, buried biomass, soil consolidationCC = canopy cover; % cover & fall heightSC = exp(-b * % cover)b = 0.05 if rills dominant; 0.035 typical; 0.025 interrillSR = roughness; set by tillage, reduces over timeSM = soil moisture; used only in NWRRI.B Mohtar3839P Conservation Practice FactorCommon practicesContouring, strip cropping, terracesChange flow patterns or cause depositionLookup tablesSchwab (1993) pp:103; Fangmeier et al (2006) pp:146; Haan et al (1994) pp: 281; USDA (1978) pp:10I.B Mohtar3940Calc.: USLE/RUSLEExample 9:Given:Materials in handout3-Acre construction site near ChicagoStraw mulch applied at 4 T/AAverage 20% slope, 100 lengthLoamy sand subsoilFill (loose soil)Find:Erosion rate in T/A/YI.B Mohtar4041Calc: USLE/RUSLE 2R = 150 (HO.1)K = 0.24 (HO.7)LS = 4 (HO.8-high rilling)C = 0.02 (HO.9)P = 1.0A = R * K * LS * C * P = 2.9 T/A/YI.B Mohtar4142Calc: USLE/RUSLE 2.1Example 10:Given:Materials in handout16-A site near Dallas, TXSilty clay loam subsoilAverage 50% slope, 75 lengthCut soilFind:By what percentage will the erosion be reduced if we increase our straw mulch cover from 40% cover to 80% cover?I.B Mohtar4243Calc: USLE/RUSLE 2.2Only thing different is COnly subfactor different is SCSC = exp(-b * %cover)For consolidated soil, b = 0.025SC1 = exp(-0.025 * 40%) = 0.368SC2 = exp(-0.025 * 80%) = 0.135Reduction = (0.368 0.135)/0.368 = 63%I.B Mohtar4344Sediment DeliveryUSLE/RUSLE for hillslopesErosionDeliveryErosion critical for soil resource conservationDelivery critical for water qualityMovement through channel systemI.B Mohtar4445

Sediment Delivery 2

I.B Mohtar4546Sediment Delivery 3SDR (Sediment Delivery Ratio)Hillslope erosionEmpirical fit for watershed deliveryChannel erosion/deposition modelingErosionTransportDepositionI.B Mohtar4647Sediment Delivery RatioHaan et al (1994) pp:293-299SDR = SY / HESDR = sediment delivery ratioSY = sediment yield at watershed exitHE = hillslope erosion over watershedI.B Mohtar4748Sediment Delivery Ratio 2Area-delivery relationship

Haan et al (1994) pp:294I.B Mohtar4849

Sediment Delivery Ratio 3Relief-length ratioRelief = elev change along main branchLength = length along main branchHaan et al (1994) page .294I.B Mohtar4950

Sediment Delivery Ratio 4Forest Service Delivery Index MethodHaan et al (1994) pp:295I.B Mohtar5051Sediment Delivery Ratio 5MUSLE (Haan et al (1994) pp: 298 and 298Y = 95(Q * qp)0.56 (Ka)(LSa)(Ca)(Pa)Y = storm yield in tonsQ = storm runoff volume in acre-inq = peak runoff rate in cfsK, LS, C, P = area=weighted watershed valuesSDR = 95(Q * qp)0.56/(R * area)R = storm erosivity in US unitsRouting for channel deliveryI.B Mohtar5152Calc: SDRExample 11:Given:Flow path length in watershed = 4000ftElevation difference = 115ftFind?SDRI.B Mohtar5253Calc.: SDR 2R/L = 115/4000 = 0.029From figure SDR = 0.45I.B Mohtar5354Channel Erosion-Deposition ModelingProcess-based small channel modelsFoster-Lane modelHaan et al (1994) pp285-289Complicated and process-basedEphemeral Gully Erosion ModelEGEMFit to Foster-Lane Model resultsI.B Mohtar5455Channel Erosion-Deposition Modeling 2Large-channel modelsSediment transportChannel morphologyI.B Mohtar5556Sediment TransportSettling (Haan et al (1994) pp:204-209)Stokes LawVs = settling velocityd = particle diameterg = accel due to gravitySG = particle specific gravity = kinematic viscositySimplified Stokes LawSG = 2.65Quiescent water at 68oFd in mm, Vs in fps

I.B Mohtar5657Calc.: Stokes Law SettlingExample 12:Given:ISSS soil particle size classificationFind:Settling velocities of largest sand, silt, and clay particlesI.B Mohtar5758Calc.: Stokes Law Settling 2ISSS classificationLargest particles sizeClay = 0.002mmSilt = 0.2mmSand = 2mmVs,clay = 1.12*10-4 fps = 0.04 ft/hr = 0.97 ft/dayVs,silt = 0.11 fps = 405 ft/hr = 1.83 mi/dayVs,sand = 11.24 fps = 7.66 mph = 184 mi/dayI.B Mohtar5859Calc.: Stokes Law SettlingExample 13:Given:Stokes Law settlingFind: particles larger than what size can be assumed to settle 1 ft in one hour?I.B Mohtar5960Calc.: Stokes Law Settling 2Vs = [(1 ft)/(1 hr)](1 hr/3600s) = 2.778*10-4 fpsd = (Vs/2.81)1/2 = 0.00994mmI.B Mohtar60BREAKI.B Mohtar6162Soil Strength and MechanicsFrom McCarthy (1982) pages 233-237,373-379Soil stressesNormal Stress = Fn/A = Shear Stress = Ft/A = Fn = normal forceFt = tangential or shear forceAs normal stress () , sheer stress () to cause failure (f) i.e. shear strength tan = f / ; where = angle of internal frictionI.B Mohtar6263Soil strength and mechanics 2

McCarthy (1982) page 234I.B Mohtar6364Calc.: Soil strengthExample 6:

Given:Well-graded sand; density 124 lb/ft3Find:Ultimate shear strength 6 ft below surface?I.B Mohtar6465Calc.: Soil Strength 2From table 10-1, for well-graded sand, = 32-35o = 33.5oNormal stress = (124 lb/ft3)(6 ft) = 744 lb/ft2tan = f / ; f = * tan = f = 744 lb/ft2 * tan(33.5o) = 492 lb/ft2I.B Mohtar6566Footing bearing loadsqult = a1*c*Nc + a2*B*1*N + 2*Df*Nqtotal support=soil cohesiveness+ below footing +soil bearingc = soil cohesion beneath footer1,, 2 = effective soil unit weight above and belowfooterB = footer size termNc, N, Nq = capacity factorsDf = footing depth below surfaceqdesign = qult / FSLength/widthBa1a21 (square)Width1.20.422Width1.120.453Width1.070.464Width1.050.476Width1.030.48StripWidth1.000.50CircularRadius1.20.60McCarthy (1982) page 374-379I.B Mohtar6667

Footing Bearing Loads 2McCarthy (1982) page 375I.B Mohtar6768Calc.: Footing LoadExample 7:Given:Strip footing 3 ft wideWet soil with density of 125 lb/ft3Angle of internal friction = 30oCohesive strength of 400 lb/ft2Use factor of safety of 3Find:qdesign in lb/ft2I.B Mohtar6869Calc.: Footing Load 2a1 = 1.0, a2 = 0.5, B = width = 31 = 125/2 = 62.5 lb/ft3; 2 = 125 lb/ft3c = 400 lb/ft2Nc = 30, N = 18, Nq = 20

qdesign = 23,700/3 = 7900 lb/ft2

I.B Mohtar6970Soil Compaction and DensitySoil compactionGreater strength and reduced permeabilityDependent on water content dry soil cannot be compacted wellProctor testPack soil into mold with pounding at various moistures. Find soil moisture for maximum compaction and density.Modified Proctor > 56000 ft-lbs of energy exerted.I.B Mohtar7071

Slope Stability & FailurePossible Forms of FailureMcCarthy (1982) page 440McCarthy (1982) page 437-455I.B Mohtar7172Slope stability & failure 2Terms=max. slope angle before sliding=angle of internal frictionCohesionless soiltan() = tan()Saturated: tan() = (1/2)tan()I.B Mohtar7273Slope Stability & Failure 3Cohesive soil*z*sin()*cos() = c + *tan()z = assumed depthc = cohesive force = effective compressive stressRotational or sliding blockI.B Mohtar7374Slope Stability & Failure 4

For clay soilFor soil with cohesion and internal friction > 0McCarthy (1982) page 474I.B Mohtar7475Slope Stability & Failure 5Ns = c / ( * Hmax)c = cohesion force = soil unit weightHmax = max depth without slidingI.B Mohtar7576Calc.: Slope StabilityExample 8:Given:Cohesion strength = 500 lb/ft2Unit weight = 110 lb/ft3Slope steepness = 50oInternal friction angle = 15oFind:Max. slope heightI.B Mohtar7677Calc.: Slope Stability 2Fig. b, = 15o, i = 50oHmax = c / ( * Ns) =

(500 lb/ft2)(1ft3/ 10)(1/ 0.095) = 48 ftI.B Mohtar7778Materials CoveredPrinciples of Soil PhysicsSediment TransportErosion ControlSoil MechanicsSlope StabilizationI.B Mohtar78Thank You and Best LuckI.B Mohtar79