1 I.B Soil Conservation Systems Rabi H. Mohtar Professor, Environmental and Natural Resources...
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1 I.B Soil Conservation Systems Rabi H. Mohtar Professor, Environmental and Natural Resources Engineering Executive Director, Strategic Projects, Research & Development Qatar Foundation [email protected]or [email protected]July 2013
1 I.B Soil Conservation Systems Rabi H. Mohtar Professor, Environmental and Natural Resources Engineering Executive Director, Strategic Projects, Research
1 I.B Soil Conservation Systems Rabi H. Mohtar Professor,
Environmental and Natural Resources Engineering Executive Director,
Strategic Projects, Research & Development Qatar Foundation
[email protected]@purdue.edu or
[email protected]@qf.org.qa July 2013
Slide 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
Slide 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. 1989. 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. http://cobweb.ecn.purdue.edu/~abe325/: Mohtar soil
and water resources conservation course.
http://cobweb.ecn.purdue.edu/~abe325/ I.B Mohtar
Slide 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
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
Slide 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
Slide 33
33 USLE/RUSLE approach Lookup Maps, tables, figures Databases
Process-based calculations Show changes over time Where dont have
good data I.B Mohtar
Slide 34
34 R factor rainfall erosivity Maps R(customary SI) = 17.02 *
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
Slide 35
35 K factor soil erodibility Soil surveys, NASIS, Haan et al
(1994) 261-262; 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
Slide 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
Slide 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
Slide 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; 0.035
typical; 0.025 interrill SR = roughness; set by tillage, reduces
over time SM = soil moisture; used only in NWRR I.B Mohtar
Slide 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
Slide 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
Slide 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
Slide 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
Slide 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 = 0.025 SC 1 = exp(-0.025 * 40%) = 0.368 SC 2 = exp(-0.025
* 80%) = 0.135 Reduction = (0.368 0.135)/0.368 = 63% I.B
Mohtar
Slide 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
Slide 45
45 Sediment Delivery 2 I.B Mohtar
Slide 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
Slide 47
47 Sediment Delivery Ratio Haan et al (1994) pp:293-299 SDR =
SY / HE SDR = sediment delivery ratio SY = sediment yield at
watershed exit HE = hillslope erosion over watershed I.B
Mohtar
Slide 48
48 Sediment Delivery Ratio 2 Area-delivery relationship Haan et
al (1994) pp:294 I.B Mohtar
Slide 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
Slide 50
50 Sediment Delivery Ratio 4 Forest Service Delivery Index
Method Haan et al (1994) pp:295 I.B Mohtar
Slide 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
54 Channel Erosion-Deposition Modeling Process-based small
channel models Foster-Lane model Haan et al (1994) pp285-289
Complicated and process-based Ephemeral Gully Erosion Model EGEM
Fit to Foster-Lane Model results I.B Mohtar
56 Sediment Transport Settling ( Haan et al (1994) pp:204-209 )
Stokes Law V s = settling velocity d = particle diameter g = accel
due to gravity SG = particle specific gravity = kinematic viscosity
Simplified Stokes Law SG = 2.65 Quiescent water at 68 o F d in mm,
V s in fps I.B Mohtar
Slide 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
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
Slide 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 = 0.00994mm I.B
Mohtar
Slide 61
BREAK I.B Mohtar61
Slide 62
62 Soil Strength and Mechanics From McCarthy (1982) pages
233-237,373-379 Soil stresses Normal Stress = F n /A = Shear Stress
= F t /A = F n = normal force F t = tangential or shear force As
normal stress () , sheer stress ( ) to cause failure ( f ) i.e.
shear strength tan = f / ; where = angle of internal friction I.B
Mohtar
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
Slide 65
65 Calc.: Soil Strength 2 From table 10-1, for well-graded
sand, = 32-35 o = 33.5 o Normal stress = (124 lb/ft 3 )(6 ft) = 744
lb/ft 2 tan = f / ; f = * tan = f = 744 lb/ft 2 * tan(33.5 o ) =
492 lb/ft 2 I.B Mohtar
Slide 66
66 Footing bearing loads q ult = a 1 *c*N c + a 2 *B* 1 *N + 2
*D f *N q total support=soil cohesiveness+ below footing +soil
bearing c = soil cohesion beneath footer 1,, 2 = effective soil
unit weight above and below footer B = footer size term N c, N , N
q = capacity factors D f = footing depth below surface q design = q
ult / FS Length/width Ba1a1 a2a2 1 (square)Width1.20.42
2Width1.120.45 3Width1.070.46 4Width1.050.47 6Width1.030.48
StripWidth1.000.50 CircularRadius1.20.60 McCarthy (1982) page
374-379 I.B Mohtar
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
Slide 69
69 Calc.: Footing Load 2 a1 = 1.0, a2 = 0.5, B = width = 3 1 =
125/2 = 62.5 lb/ft3; 2 = 125 lb/ft3 c = 400 lb/ft2 N c = 30, N =
18, N q = 20 q design = 23,700/3 = 7900 lb/ft 2 I.B Mohtar
Slide 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 > 56000 ft-lbs of energy exerted. I.B
Mohtar
Slide 71
71 Slope Stability & Failure Possible Forms of Failure
McCarthy (1982) page 440 McCarthy (1982) page 437-455 I.B
Mohtar
