Introduction 2

Introduction 2

1. Shear Stress - the force that is responsible for entrainment and transport

   • Stress is measured in = force per unit area

2. Particle Entrainment

   • When sediment is picked up and incorporated into the flow of water, being transported.
   • Experiments: sediment at the bottom of a flume, passed different amounts of water through the flume to see when sediment became entrained. (Shields, 1936)
     • Particles first became entrained at different flow levels, depending on size.
     • Higher flows entrained larger particles
   • When applied to real streams not experiments there are other factors effecting entrainment other than size:
     • Shape
     • Density
     • Structural Arrangement

3. Thresholds

   • A condition at which there is significant landform change without a change in external controls e.g. climate or land use
     • Broadened to - sudden landform change as a result of progressive change of external controls
     • (Schumm, 1979)
   • "the condition, level or stage at which an effect is produced"
   • Examples in Geomorphology
     • Boundaries that lead to changes in the rate of a process e.g. the point at which a particle is entrained into transport
     • Seperation between unstable/stable systems e.g. an increase in sediment transport resulting in dune erosion

4. Reaction & Relaxation

   • Reaction time: time taken to react to a change in input. Most geomorphological systems trail behind a disturbance
   • Relaxation time: how long it takes for a system to adjust to the changed input

5. Uniformitarianism

   • Dictionary: "the theory that changes in the earth's crust during geological history have resulted from continuous & uniform processes.
   • Uniformity of Law - natural laws do not change between places or over time
   • Principle of Simplicity of Landforms - the most straightforward process should be used to explain landforms

6. Made & Worked Ground

   • Made - constructional anthropogenic landforms
   • Worked - erosional anthropogenic features
   • "Over the last 200 years, people have excavated, moved & constructed a volume of soil and rock: 6times the volume of Ben Nevis, Scotland" (Brown et al, 2017)
   • Worked ground is a category on geological maps but is now categorised into subcategories" (Brown et al, 2017)