Intro to Geomorphology (Geos 450/550) Lecture 9: hillslope processes FT#4 preview slope transport processes Mohr-Coulomb slope stability criterion HW#2 out on web site – due next week read ch. 7 of Anderson 2

NorthSouth

Erosion in hillslope&fluvial systems: two basic regimes 1.Weathering (detachment) limited low weathering rates relative to high transport rates thin regolith or soil slope landsliding bedrock rivers (downcutting) 2.Transport limited low transport rates relative to high weathering rates thick regolith or soil slope creep, bioturbation, etc. (diffusive processes) alluvial rivers (transporting or depositional)

Typical slopes of arid and humid environments Slide courtesy of Mike Poulos

Classic arid slope

Hillslope transport processes creep: heave+gravity, diffusive, low-slope, high veg continuous creep (solifluction), seasonal drainage mass movements: gravity, weathering-limited bioturbation: movement of animals and growth of plants, coarse-grained raindrop impact: diffusive, fine-grained overland flow: semi-diffusive, low veg rill wash: low veg (badlands) solution: karst piping: the movement of water and sediment in interconnected networks of cracks in the near-surface. fine-grained, low- perm spring sapping: outflow of water at a spring where surface and water table meet. structural control

When and where do hillslopes “fail”?

Landslide hazard map – U.S.

slope angle =  potential failure plane Stability Index for a buried water table at a vertical distance h w above the base of the soil layer (depth h s ): Stability Index =  f /  C’ + tan  [  – (  w /  s )(h w /h s )] cos 2  = where C’ = C/  s gh s = ratio of cohesion to weight of soil sin  cos  water table hshs hwhw

slope angle =  potential failure plane x unit area on surface block exerts: normal force per unit area on buried plane =  gx cos  shear force per unit area on buried plane =  gx sin   gx  = density g = gravitational acceleration h = depth x = thickness perpendicular to surface force/area components

h slope angle =  potential failure plane normal stress acting on plane =  =  gh cos 2  shear stress acting on plane =  =  gh sin  cos  in terms of vertical thickness h, x = h cos  hence    = density g = gravitational acceleration h = depth x = thickness perpendicular to surface Stress on buried plane parallel to infinite sloping surface x

hshs slope angle =  potential failure plane  eff  Coulomb slope failure: effect of pore pressure Coulomb failure:  =  f = C + tan  eff ) where  eff = effective normal stress =  –   = (  s gh s –  w gh w ) cos 2   = pore pressure  s = wet soil density,  w = water density tan  = coefficient of friction of the material C = cohesion of material water table hwhw

h slope angle =  =  potential failure plane  = normal stress acting on plane =  gh cos 2   = shear stress acting on plane =  gh sin  cos    angle of repose For c = 0 and  = 0 (cohesionless material without pore pressure)  =  f =  tan  sin  cos  = cos 2  tan  tan  = tan   =  = angle of repose

h slope angle =  potential failure plane Stability Index In general, for c and  not = 0, one can compute the “safety factor” or “Stability Index” = SI =  f / 

slope angle =  potential failure plane Stability Index for a buried water table at a vertical distance h w above the base of the soil layer (depth h s ): Stability Index =  f /  C’ + tan  [  – (  w /  s )(h w /h s )] cos 2  = where C’ = C/  s gh s = ratio of cohesion to weight of soil sin  cos  water table hshs hwhw

slope angle =  potential failure plane predicting slope failure Stability Index =  f /  C’ + tan  [  – (  w /  s )(h w /h s )] cos 2  = sin  cos  water table hshs hwhw This is a basis for predicting slope instability by knowing the slope angle and water table. These can be estimated by knowing the topography. See SINMAP* manual for more discussion of this approach.SINMAP

Cinder cones provide unusually well-constrained initial condition Slide courtesy Craig Rasmussen

South FacingNorth Facing ~70 kyr ~300 kyr ~2,000 kyr ~70 kyr ~300 kyr ~2,000 kyr Slide courtesy Craig Rasmussen

Use both topo and soil variations with aspect to discriminate better among competing models In water-limited environments: Bioturbation is 1-2 orders of magnitude more important that freeze-thaw-driven creep. North-facing slopes have greater maximum steepness. Regolith is thicker on north-facing slopes but clay clay accumulation more well developed on south- facing slopes (Rech, 2001; Rasmussen et al.) Modern biomass density is greater on north-facing slopes BUT paleo-biomass density was greater on south-facing slopes (glacial climates are 80% of Quaternary).

Blue – North facing Red – South facing Soils more well developed (e.g. clay accum.) on south-facing slopes Slide courtesy Craig Rasmussen

McGuire et al., in rev. Biomass greater on north-facing slopes now, but what about the past?

McGuire et al., in rev. Paleovegetation a key element of the story – South-facing slopes had greater biomass for 80% of Quaternary

Pelletier et al. (2013); McGuire et al. (2014) conceptual model For water-limited environments and slopes (>2 km a.s.l. in warmer parts of western U.S., 1-2 km a.s.l. in colder parts): Greater soil water availability on north-facing slopes drives faster regolith production. Greater biomass on south-facing slopes (during glacial climates) increased colluvial transport rates compared with north-facing slopes, leading to lower max gradients on north-facing slopes over time. Greater biomass led to greater dust (clay, quartz) accumulation in soils on south-facing slopes.