Key concepts Failure of slopes depends on direction of groundwater flow, but in many cases surface parallel flow is an adequate assumption Failure also depends on a balance between the intensity and duration of rainfall Larger landslides have relatively longer runout distances, and therefore lower ‘effective’ friction. Wet material can have extremely long runout distances

Key concepts Debris flows vs. landslides: It is all about the water! Critical state porosity in soils Characteristics of debris flows Long runout Kinematic sorting Erosion of landscapes Supply and storage of sediment Colluvial hollow cycling and triggering

Hillslope sediment transport Eroding Landscapes Hillslope sediment transport

What follows is a long derivation of the mass balance of a hillslope soil Q: Why go through this? A1: It serves as the basis for all quantitative hillslope studies A2: The thought process is the same for channels, glaciers, etc. Alan Howard

Meaningful predictions require quantitative analyses! Q: Suppose IPCC says rainfall will increase by 25% in 2080. How much more sediment is going to come from these slopes? Alan Howard

Meaningful predictions require quantitative analyses! Q: Suppose IPCC says rainfall will increase by 25% in 2080. How much more sediment is going to come from these slopes? Why would you want to know? Landslide risk Soil loss Changing soil properties changes hydrology Downstream effects Alan Howard

Meaningful predictions require quantitative analyses! Q: Suppose IPCC says rainfall will increase by 25% in 2080. How much more sediment is going to come from these slopes? A1: Qualitative: More sediment will come out. A2: Quantitative: There is a 50% chance that erosion rates will increase from 0.1mm/yr to 0.17mm/yr, leading to 30% greater risk of flooding in your town. Alan Howard

What is this course about? Many kinds of eroding landscapes Chinese Loess Plateau Chinese karst terrain Upper Mekong River Basin What is this course about? Many kinds of eroding landscapes

Basic observations about hillslopes and rivers Hillslopes tend to be round. Valleys frequently have regular spacing. Roering et al. (2007), EPSL

Basic observations about hillslopes and rivers Slope area data (sometimes) shows a ‘boomerang’ pattern. Grieve, PhD (former GPG student!)

Now for the mass balance (this will be done on the board but these notes accompany the lecture) Alan Howard

Here is a landscape with some rivers and hillslopes

Zoom in

For convenience, pick a planar hillslope (i.e., not convergent, not divergent)

A generic hillslope strip

Now look from the side

Side view

Look at mass in and out

Ignore mass exchange at the surface

Aside: dimensions and units Express dimensions in M for mass L for length T for time Units can be Mass: kilograms, grams, etc. Length: metres, cm Time: seconds, hours, days, etc

Mass in box: Check dimensions Mass in box is: ρs*h*dA Dimensions of ρs: M/L3 Dimensions of h: L Dimension of dA: L2 So dimensions ρs*h*dA: M*L*L2/L3 = M In my personal experience, this is the easiest way to check if you are doing things correctly.

r is the density of the parent material

What could determine qs? What we need is a ‘sediment flux law’

Sediment flux laws A sediment flux law tells us how qs varies as a function of the properties of the landscape

Sediment flux laws So, you tell me the slope angle, or the amount of overland flow, or the number of gophers, and I’ll tell you how much dirt is moving on your hillslope

A very simple case The steeper the slope, the more sediment flux

A very simple case The steeper the slope, the more sediment flux This is called a ‘linear’ sediment flux law Proposed by Culling, W.E.H., 1960. Analytical Theory of Erosion. Journal of Geology, 68(3): 336-344.

A very simple case The steeper the slope, the more sediment flux This is called a ‘linear’ sediment flux law Proposed by Culling, W.E.H., 1960. Analytical Theory of Erosion. Journal of Geology, 68(3): 336-344.

Slightly more complex When the slope gets very steep (near the angle of repose), particles start to slide downslope. This sediment transport gets very fast as the slope approaches some critical value Proposed by Andrews, D.J. and Bucknam, R.C., 1987. Journal of Geophysical Research-Solid Earth and Planets, 92(B12): 12857-12867. Popularized by Roering, J.J., Kirchner, J.W. and Dietrich, W.E., 1999. Water Resources Research, 35(3): 853-870.

Slightly more complex When the slope gets very steep (near the angle of repose), particles start to slide downslope. This sediment transport gets very fast as the slope approaches some critical value Proposed by Andrews, D.J. and Bucknam, R.C., 1987. Journal of Geophysical Research-Solid Earth and Planets, 92(B12): 12857-12867. Popularized by Roering, J.J., Kirchner, J.W. and Dietrich, W.E., 1999. Water Resources Research, 35(3): 853-870.

Sediment flux laws: we’ll come back to this later First lets go back to the hillslope mass balance

Introduce a concept: Steady state: Steady state refers to a condition in which something doesn't change in time. What is ‘something’? Topography? Topography relative to some moving datum? Soil thickness? Soil production?

Change in the amount of soil in the box Soil production Soil coming in minus soil going out

but hnew=hold

Okay, lets go back to looking at the hillslope strip

Because it is at the divide

What came out of box 1 goes into box 2.

Sediment flux laws: reminder

Check dimensions

Implication? Slope increases as you move away from the divide!!!

A steadily eroding hillslope with linear creep is convex up!

So that explains the top part of this profile, but not the bottom

Simplest creep flux law: What about water? Probably also depends on slope. But also depends on how much water is there

Simplest sediment flux law that accounts for water: What about water? Simplest sediment flux law that accounts for water: Greater slope, more transport. More drainage area, more water More water, faster erosion

This is a fundamental result in geomorphology!!

This is a fundamental result in geomorphology!! Convex: creep like processes dominate Concave: fluvial and wash processes dominate

Roughening and smoothing Creep like processes smooth the landscape Wash and fluvial processes roughen the landscape

Roughening and smoothing are in competition!!! In balance at the channel head!

Two end members In creep-like domain: In fluvial/wash domain: Greater area means you need greater slope to erode at the same rate, since you have more sediment to transport and sediment transport depends only on slope In fluvial/wash domain: Greater area means you need a gentler slope to erode at the same rate because the water you gain is very efficient at eroding and transporting sediment

So, looking at slope area plots:

Slope-area plots can be used to compare landscapes, and the competition between creep and fluvial processes

Relationship between valley spacing and uplift rates unclear (hard to distinguish from climate, parent material, etc) But transition between channel and hillslope can now be determined for large areas using digital elevation models Roering et al. (2007), EPSL

Process and form Threshold landsliding Creep only

Process and form On a theoretical basis, one could pick out process transitions from slope – contributing area relationships

Key points Creep leads to fundamentally different topography than water transport Creep smoothes the landscape, water roughens it One can search for different process regimes by looking at S-A plots Reading: Anderson and Anderson Chapter 10 and section on page 592 about derivatives

Calculus note