Sediments + Forces Intro

Forces, Slopes, Magnitude vs Frequency, Systems

Define shear stress:

The force per unit area exerted by the shearing action of a mass over a surface. Measured in pascals, Pa

Give the equation for shear stress and what the symbols represent:

𝜏 = 𝜌 x g x D x S

𝜏 is shear stress, Pa

𝜌 is density, kg m^-3

D is flow depth, m

S is slope m/m

Define strength:

The ability of a material to withstand deformation and fracture without significant failure along any internal plane. A material’s strength can depend on cohesion (e.g. clay is more coherent) and internal friction.

Define the threshold of stress:

The critical value of applied force to overcome the resistance to motion.

Describe the properties of rock:

A hard, coherent material made of individual particles or crystals. It has joints and fractures but is not significantly weakened when saturated with water.

Describe the properties of soil:

A weak, unconsolidated deposit, forming a continuous mass that is weakened when saturated with water.  

Define talus:

Unconsolidated material transported across/ deposited on slopes, made of large rock fragments.

Define a talus slope:

Where talus accumulates at the bottom of a cliff or free face. 

Define a debris cone:

Where debris is funnelled down a notch or gully.

Define colluvium:

The same as talus but made of finer material. This may show historical layers of the slope from periods characterised by low erosion and/or high deposition.

Define talluvium:

A term used to describe material that’s a mix of talus and colluvium.

Define the angle of internal friction (ϕ):

The frictional resistance between particles in a material, related to their size, shape and arrangement, resistance to crushing, and the number of contact points per unit volume.

Define cohesion:

The inherent strength of a material regardless of outside forces. This includes chemical bonding and adhesion of clay-sized material due to electrostatic (+magnetic) forces.

Define intact strength (rock):

In an unjointed rock, the intact strength describes how resistant the rock is to fracture and other forms of damage. e.g. low in chalk, high in basalt.

Define creep:

The slow, plastic deformation of rock or soil due to overburdening. It can often precede slide-type movement.

Define cambering:

Here weak rocks e.g. clays are extruded between more rigid strata, causing the sides of a form to bulge.

Define flow:

Avalanches, debris flows, earthflows and mudflows are all examples. Water is usually present and there is no well-defined shear plane, although it’s usually greatest at the base.

Define slide:

In a pure slide failure occurs along a well-defined shear plane. Resistance to movement falls immediately as the slide occurs, and downslope movement continues until friction is high enough to slow it.

Define heave:

Cycles of expansion and contraction, where expansion occurs normal to the plane and contraction occurs vertically under gravity, so there is very slow creep downslope.

Define fall:

The downward motion of rock or, more rarely, soil through the air. Occurs on steep surfaces e.g. cliffs. 

 Toppling is a specific subcategory where there is rotation of the material whilst it falls. 

Define subsidence:

Subsidence can occur either as the more or less instantaneous collapse of material into a cave or other cavity (cavity collapse) or as a progressive lowering of the ground surface (settlement).

Give some factors affecting landscape development:

• Location on tectonic plates and type of tectonic boundary can affect vertical movement of landscapes 

• Rock types (lithologies) have different vulnerabilities to erosion (e.g. sandstone vs granite) 

• Climactic conditions affect weathering (e.g. freeze-thaw cycles), as well as transportation via water 

• Ecological influences can play a role (e.g. tree roots stabilising some soil) 

Describe the Kosi River event:

The Kosi River in India moved more than 113km westward over the last 2 centuries, but then in Aug 2008, it moved 120km east in one event! 

Describe how frequency of occurrence and sediment transport rate are related:

In fluvial events, events of slightly higher magnitude that occur moderately less often than the most frequent events have the greatest impact on sediment transport.

Define a system:

A set of objects or characteristics which are related to one another and operate together as a complex entity.

Define equilibrium:

A system where form and process are balanced, either in static or dynamic equilibrium. 

Define threshold:

A condition that must be achieved for a system to pass from one state to another. 

Give some examples of types of system state:

• Steady state

• Periodic

• Damped

• Explosive

• Unsystematic