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1 I.B Soil Conservation Systems Rabi H. Mohtar Professor, Environmental and Natural Resources Engineering Executive Director, Strategic Projects, Research.

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Presentation on theme: "1 I.B Soil Conservation Systems Rabi H. Mohtar Professor, Environmental and Natural Resources Engineering Executive Director, Strategic Projects, Research."— Presentation transcript:

1 1 I.B Soil Conservation Systems Rabi H. Mohtar Professor, Environmental and Natural Resources Engineering Executive Director, Strategic Projects, Research & Development Qatar Foundation or July 2013

2 2 Materials To Be Covered 1. Principles of Soil Physics 2. Sediment Transport 3. Erosion Control 4. Soil Mechanics 5. Slope Stabilization This review will provide you with an overall understanding and not necessarily makes you an expert! I.B Mohtar

3 3 Sources 1. Environmental Soil Physics; Hillel; 1998 Hillel (1998) 2. Essentials of Soil Mechanics & Foundations, 7 th ed.; McCarthy; 2007; McCarthy (2007) 3. Soil and Water Conservation Engineering a) 4 th ed. Schwab, Fangmeier, Elliott, Frevert: Schwab et al (1993) b) 5 th ed. Fangmeier, Elliott, Workman, Huffman, Schwab: Fangmeier et al (2006) 4. Design Hydrology & Sedimentology for Small Catchments; Haan, Barfield, Hayes: Haan et al (1994) 5. USLE/RUSLE: USDA Agricultural Handbook No. 537 (1978) 6. Cuenca, R. H Irrigation System Design - An Engineering Approach. Prentice-Hall, Inc., Englewood Cliffs, NJ. 552 pp. Cuenca (1989). 7. Ward, Elliot 1995 (Environmental Hydrology, Lewis Publishers). 8. Mohtar soil and water resources conservation course. I.B Mohtar

4 4 Soil Physics & Mechanics 1. Soil classes and particle size distributions 2. Basics of soil water a) Water Content b) Water Potential c) Water Flow 3. Soil strength & mechanics I.B Mohtar

5 5 Soil Classes & Particle Sizes Hillel (1998) page 61 I.B Mohtar

6 6 Soil Classes & Particle Sizes - 2 ISSS classification is easiest 1. Sand mm ( μ) 2. Silt mm (2-20μ) 3. Clay <0.002mm (<2μ) I.B Mohtar

7 7 Soil Classes & Particle Sizes – 3 Soil Textural Triangle Example 1: Find the soil texture for this soil:  50% sand,  20% silt Hillel (1998) page 64 I.B Mohtar

8 8 Soil Classes & Particle Sizes – 4 Particle size distribution Example 2 Draw in a sandy clay loam? Hillel (1998) page 65 I.B Mohtar

9 9 Primary soil mapping unit Soil type RE V Primary soil mapping unit Pedon Soil Structure and Functionality + Primary peds and free mineral grains + Primay particles and pedological features Inter-ped pore space Clay pore space Pedostructure = Primary ped = Primary peds Clay particles Horizon = Vertical porosity (cracks, fissures) + Pedostructure Interpedal porosity (macro-porosity) Clay plasma porosity (micro-porosity) Mineral grains Geomorphological unit Pedostructure, primary peds, primary particles, are functionally defined and quantitatively determined using the shrinkage and potential curve measurement I.B Mohtar Mohtar (2008)

10 10 Soil Water Content 1. M t = M s + M w + M a 2. V t = V s + V w + V a a. t = total, s = solids, w = water, a = air 3. ρ b = bulk density = M s /V t ≈ g/cc (why dry basis?) 4. ρ p = particle density = M s /V s ≈ 2.65 g/cc 5. Porosity = (V w + V a ) / V t ≈ 25-60% 6. ρ w = water density = M w /V w = 1.0 g/cc 0 I.B Mohtar

11 11 Soil Water Content – 2 Water content wet basis: W w = M w / (M s + M w ) Water content dry basis: W = mass wetness = M w / M s Volumetric water content: θ = V w /V t = V w / (V s + V w + V a ) I.B Mohtar

12 12 Calc.: Soil Water Content Soil Water Example 3. Given: Soil with 30% water content dry basis Find? Best guess at equivalent inches of water in the top foot of soil? I.B Mohtar

13 13 Calc.: Soil Water Content – 2 1. M w / M s = 0.30 a. M w = V w * ρ w b. ρ b = M s / V t ; M s = V t * ρ b I.B Mohtar

14 14 Calc.: Soil Water Content – 3 M w / M s = (V w * ρ w )/(V t * ρ b ) = (V w / V t )(ρ w / ρ b ) θ = V w /V t θ *(ρ w / ρ b ) = 0.3; θ = (ρ b / ρ w ) * 0.3 θ = 0.3 *(1.3/1.0) = * 1 ft * 12”/ft = 4.7” I.B Mohtar

15 15 Soil Water Potential soil characteristic curve Hillel (1998) page 157 I.B Mohtar

16 16 Soil Water Potential – 2 Cuenca (1989) page 58 I.B Mohtar

17 17 Ward, Elliot 1995 (Environmental Hydrology, Lewis Publishers) Soil Water Management I.B Mohtar

18 18 Soil Water Potential – 3 Fangmeier et al (2006) page 337 I.B Mohtar

19 19 Soil Water Potential – 4 Hillel (1998) page 162 I.B Mohtar

20 20 Calc.: Soil Water Potential Soil Water Potential Example 4. Given: Mercury tensiometer SG = 13.6 Situation as shown Find: 1. Total potential at point C 2. Is point C above or below the current water table? Cuenca, (1989) page 64 I.B Mohtar

21 21 Calc.: Soil Water Potential Pick datum 2. Add pressures a. Suction b. Water depth c. Gravity 3. T = z + p + p os a. z = + 80 cm b. p = ? c. T = -86cm d. Point C is above water table. Why? I.B Mohtar

22 22 Soil Water Flow q = A*K*H/L K = (q*L)/(A*H) K values A q L H Fangmeier et al (2006) page 261; Schwab et al (1993) page 359; Haan et al (1994) page 430 I.B Mohtar

23 23 Calc.: Soil Water Flow Darcy Law Application Example 5. Given: Need gpd through a 1-ft thick sand filter with K = 8 ft/d, and a total driving head of 3 ft Find? Required diameter for circular tank? I.B Mohtar

24 24 Calc.: Soil Water Flow – 2 q = A*K*H/L; A = (q*L)/(K*H) I.B Mohtar

25 25 Soil Erosion and Sediment Yield Hillslope erosion Channel system erosion Sediment delivery to streams Sediment transport in streams Slope stability I.B Mohtar

26 26 Hillslope soil erosion Background Detachment Raindrop impact By turbulent overland flow Runoff Transport downslope By runoff Schwab et al (1993) pp:91-111; Fangmeier et al (2006) pp: ; Haan et al (1994) pp: I.B Mohtar

27 27 Hillslope Soil Erosion Background At the top of the slope Detachment by raindrop impact Transport by shallow sheet flow Sheet erosion USDA-NRCS I.B Mohtar

28 28 Hillslope Soil Erosion Background - 2 Lower on slope Small flow concentrations Start to cut small channels Rills Roughly parallel Head straight downslope Random formation Flow from sheet areas between rills Sheet and rill erosion USDA-NRCS I.B Mohtar

29 29 Hillslope Soil Erosion Background - 3 Bottom of hillslope Ends at concentrated flow channel Low area in macrotopography “ephemeral gullies” USDA-NRCS I.B Mohtar

30 30 Hillslope Erosion Factors Rainfall erosivity Intensity Total storm energy Soil erodibility Topography Slope length Steepness Management Reduce local erosion Change runoff path Slow and spread runoff => deposition I.B Mohtar

31 31 USLE/RUSLE A = R * K * LS * C * P A = average annual soil erosion (T/A/Y) R = rainfall erosivity (long empirical units) K = soil erodibility (long empirical units) R * K gives units of T/A/Y LS = topographic factor (dimensionless, 0-1) C = cover-management (dimensionless, 0-1) P = conservation practice (dimensionless, 0-1) I.B Mohtar

32 32 USLE/RUSLE – background Empirical approach been in use since 1960 >10000 plot-years of data International use Unit Plot basis; LS = C = P = 1 Near worst-case management R from good fit rainfall-erosion K from K = A / R C and P from studies Sub-factors in later versions I.B Mohtar

33 33 USLE/RUSLE – approach Lookup Maps, tables, figures Databases Process-based calculations Show changes over time Where don’t have good data I.B Mohtar

34 34 R factor – rainfall erosivity Maps R(customary SI) = * R(customary US) S4 I.B Mohtar Haan 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:1-5

35 35 K factor – soil erodibility Soil surveys, NASIS, Haan et al (1994) ; USDA 6 Erodibility nomograph: Haan et al (1994) 255; Schwab et al (1993) 101; Fangmeier et al (2006) pp144; USDA (1978) pp: 7 No short-term OM I.B Mohtar

36 36 LS – Topography Factor New tables & figures Haan et al (1994) 264; USDA (1978) 8 Know susceptibility to rilling High for highly disturbed soils Low for consolidated soils I.B Mohtar

37 37 C – cover-management factor Part of normal management scheme Lookup: Schwab (1993) 102; Fangmeier et al (2006) pp: 146; Haan et al (1994) 266; Hillel (1998) Appendix 8; USDA (1978) 9 It Changes over time I.B Mohtar

38 38 C – Cover-Management Factor - 2 Subfactor approach (RUSLE) C = PLU * CC * SC * SR * SM; all 0-1 PLU = prior land use roots, buried biomass, soil consolidation CC = canopy cover; % cover & fall height SC = exp(-b * % cover) b = 0.05 if rills dominant; typical; interrill SR = roughness; set by tillage, reduces over time SM = soil moisture; used only in NWRR I.B Mohtar

39 39 P – Conservation Practice Factor Common practices Contouring, strip cropping, terraces Change flow patterns or cause deposition Lookup tables Schwab (1993) pp:103; Fangmeier et al (2006) pp:146; Haan et al (1994) pp: 281; USDA (1978) pp:10 I.B Mohtar

40 40 Calc.: USLE/RUSLE Example 9: Given: Materials in handout 3-Acre construction site near Chicago Straw mulch applied at 4 T/A Average 20% slope, 100’ length Loamy sand subsoil Fill (loose soil) Find: Erosion rate in T/A/Y I.B Mohtar

41 41 Calc: USLE/RUSLE – 2 R = 150 (HO.1) K = 0.24 (HO.7) LS = 4 (HO.8-high rilling) C = 0.02 (HO.9) P = 1.0 A = R * K * LS * C * P = 2.9 T/A/Y I.B Mohtar

42 42 Calc: USLE/RUSLE – 2.1 Example 10: Given: Materials in handout 16-A site near Dallas, TX Silty clay loam subsoil Average 50% slope, 75’ length Cut soil Find: By what percentage will the erosion be reduced if we increase our straw mulch cover from 40% cover to 80% cover? I.B Mohtar

43 43 Calc: USLE/RUSLE – 2.2 Only thing different is C Only subfactor different is SC SC = exp(-b * %cover) For consolidated soil, b = SC 1 = exp( * 40%) = SC 2 = exp( * 80%) = Reduction = (0.368 – 0.135)/0.368 = 63% I.B Mohtar

44 44 Sediment Delivery USLE/RUSLE for hillslopes Erosion Delivery Erosion critical for soil resource conservation Delivery critical for water quality Movement through channel system I.B Mohtar

45 45 Sediment Delivery – 2 I.B Mohtar

46 46 Sediment Delivery – 3 SDR (Sediment Delivery Ratio) Hillslope erosion Empirical fit for watershed delivery Channel erosion/deposition modeling Erosion Transport Deposition I.B Mohtar

47 47 Sediment Delivery Ratio Haan et al (1994) pp: SDR = SY / HE SDR = sediment delivery ratio SY = sediment yield at watershed exit HE = hillslope erosion over watershed I.B Mohtar

48 48 Sediment Delivery Ratio – 2 Area-delivery relationship Haan et al (1994) pp:294 I.B Mohtar

49 49 Sediment Delivery Ratio – 3 Relief-length ratio Relief = elev change along main branch Length = length along main branch Haan et al (1994) page.294 I.B Mohtar

50 50 Sediment Delivery Ratio – 4 Forest Service Delivery Index Method Haan et al (1994) pp:295 I.B Mohtar

51 51 Sediment Delivery Ratio – 5 MUSLE ( Haan et al (1994) pp: 298 and 298 Y = 95(Q * q p ) 0.56 (K a )(LS a )(C a )(P a ) Y = storm yield in tons Q = storm runoff volume in acre-in q = peak runoff rate in cfs K, LS, C, P = area=weighted watershed values SDR = 95(Q * q p ) 0.56 /(R * area) R = storm erosivity in US units Routing for channel delivery I.B Mohtar

52 52 Calc: SDR Example 11: Given: Flow path length in watershed = 4000ft Elevation difference = 115ft Find? SDR I.B Mohtar

53 53 Calc.: SDR – 2 R/L = 115/4000 = From figure SDR = 0.45 I.B Mohtar

54 54 Channel Erosion-Deposition Modeling Process-based small channel models Foster-Lane model Haan et al (1994) pp Complicated and process-based Ephemeral Gully Erosion Model EGEM Fit to Foster-Lane Model results I.B Mohtar

55 55 Channel Erosion-Deposition Modeling – 2 Large-channel models Sediment transport Channel morphology I.B Mohtar

56 56 Sediment Transport Settling ( Haan et al (1994) pp: ) Stokes’ Law V s = settling velocity d = particle diameter g = accel due to gravity SG = particle specific gravity ν = kinematic viscosity Simplified Stokes’ Law SG = 2.65 Quiescent water at 68 o F d in mm, V s in fps I.B Mohtar

57 57 Calc.: Stokes’ Law Settling Example 12: Given: ISSS soil particle size classification Find: Settling velocities of largest sand, silt, and clay particles I.B Mohtar

58 58 Calc.: Stokes’ Law Settling – 2 ISSS classification Largest particles size Clay = 0.002mm Silt = 0.2mm Sand = 2mm V s,clay = 1.12*10 -4 fps = 0.04 ft/hr = 0.97 ft/day V s,silt = 0.11 fps = 405 ft/hr = 1.83 mi/day V s,sand = fps = 7.66 mph = 184 mi/day I.B Mohtar

59 59 Calc.: Stokes’ Law Settling Example 13: Given: Stokes’ Law settling Find: particles larger than what size can be assumed to settle 1 ft in one hour? I.B Mohtar

60 60 Calc.: Stokes’ Law Settling – 2 V s = [(1 ft)/(1 hr)](1 hr/3600s) = 2.778*10 -4 fps d = (V s /2.81) 1/2 = mm I.B Mohtar

61 BREAK I.B Mohtar61

62 62 Soil Strength and Mechanics From McCarthy (1982) pages , Soil stresses Normal Stress = F n /A = σ Shear Stress = F t /A = τ F n = normal force F t = tangential or shear force As normal stress (σ) ↑, sheer stress ( τ) to cause failure ( τ f )↑ i.e. shear strength ↑ tan Φ = τ f / σ; where Φ = angle of internal friction I.B Mohtar

63 63 Soil strength and mechanics – 2 McCarthy (1982) page 234 I.B Mohtar

64 64 Calc.: Soil strength Example 6: Given: Well-graded sand; density 124 lb/ft 3 Find: Ultimate shear strength 6 ft below surface? I.B Mohtar

65 65 Calc.: Soil Strength – 2 From table 10-1, for well-graded sand, Φ = o = 33.5 o Normal stress = (124 lb/ft 3 )(6 ft) = 744 lb/ft 2 tan Φ = τ f / σ; τ f = σ * tan Φ = τ f = 744 lb/ft 2 * tan(33.5 o ) = 492 lb/ft 2 I.B Mohtar

66 66 Footing bearing loads q ult = a 1 *c*N c + a 2 *B*γ 1 *N γ + γ 2 *D f *N q total support=soil cohesiveness+ below footing +soil bearing c = soil cohesion beneath footer γ 1,, γ 2 = effective soil unit weight above and below footer B = footer size term N c, N γ, N q = capacity factors D f = footing depth below surface q design = q ult / FS Length/width Ba1a1 a2a2 1 (square)Width Width Width Width Width StripWidth CircularRadius McCarthy (1982) page I.B Mohtar

67 67 Footing Bearing Loads – 2 McCarthy (1982) page 375 I.B Mohtar

68 68 Calc.: Footing Load Example 7: Given: Strip footing 3 ft wide Wet soil with density of 125 lb/ft 3 Angle of internal friction = 30 o Cohesive strength of 400 lb/ft 2 Use factor of safety of 3 Find: q design in lb/ft 2 I.B Mohtar

69 69 Calc.: Footing Load – 2 a1 = 1.0, a2 = 0.5, B = width = 3’ γ 1 = 125/2 = 62.5 lb/ft3; γ 2 = 125 lb/ft3 c = 400 lb/ft2 N c = 30, N γ = 18, N q = 20 q design = 23,700/3 = 7900 lb/ft 2 I.B Mohtar

70 70 Soil Compaction and Density Soil compaction Greater strength and reduced permeability Dependent on water content dry soil cannot be compacted well Proctor test Pack soil into mold with pounding at various moistures. Find soil moisture for maximum compaction and density. Modified Proctor > ft-lbs of energy exerted. I.B Mohtar

71 71 Slope Stability & Failure Possible Forms of Failure McCarthy (1982) page 440 McCarthy (1982) page I.B Mohtar

72 72 Slope stability & failure – 2 Terms β=max. slope angle before sliding Φ=angle of internal friction Cohesionless soil tan(β) = tan( Φ) Saturated: tan( β) = (1/2)tan( Φ) I.B Mohtar

73 73 Slope Stability & Failure – 3 Cohesive soil γ*z*sin(β)*cos(β) = c + σ*tan(Φ) z = assumed depth c = cohesive force σ = effective compressive stress Rotational or sliding block I.B Mohtar

74 74 Slope Stability & Failure – 4 For clay soil For soil with cohesion and internal friction > 0 McCarthy (1982) page 474 I.B Mohtar

75 75 Slope Stability & Failure – 5 N s = c / (γ * H max ) c = cohesion force γ = soil unit weight H max = max depth without sliding I.B Mohtar

76 76 Calc.: Slope Stability Example 8: Given: Cohesion strength = 500 lb/ft 2 Unit weight = 110 lb/ft 3 Slope steepness = 50 o Internal friction angle = 15 o Find: Max. slope height I.B Mohtar

77 77 Calc.: Slope Stability – 2 Fig. b, φ = 15 o, i = 50 o H max = c / (γ * N s ) = (500 lb/ft 2 )(1ft 3 / 10)(1/ 0.095) = 48 ft I.B Mohtar

78 78 Materials Covered Principles of Soil Physics Sediment Transport Erosion Control Soil Mechanics Slope Stabilization I.B Mohtar

79 Thank You and Best Luck I.B Mohtar79


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