Week 3 Sediment Characterisation Flashcards
(24 cards)
Weathering definition
The process of breaking down rocks and minerals into sediment and ions.
Importance: The released sediment forms diverse environments across the Earth’s surface.
Types of weathering
Types of Weathering:
Physical Weathering: Breaks rocks into smaller pieces without altering composition.
Biological Weathering: Involves living organisms (e.g., root wedging, rock boring).
Chemical Weathering: Alters the chemical composition of rocks, e.g., acid rain reactions.
Physical Weathering Mechanisms
Abrasion: Rocks grind against each other, wearing down surfaces.
Freeze-Thaw: Water freezes in rock cracks, expands, and breaks the rock apart.
Thermal Expansion: Rock expands with heat and contracts when cooled, causing fractures.
Unloading: Reduction of pressure on buried rocks causes them to expand and crack at the surface.
Chemical Weathering Process
Carbon dioxide dissolved in rainwater forms carbonic acid.
Reaction: Carbonic acid dissociates into hydrogen ions and carbonate ions, making rainwater acidic.
Result: The acidic water reacts with minerals, altering the rock’s chemical structure.
Sediment Characterization Methods
Non-Cohesive Sediments: Sand and gravel.
Cohesive Sediments: Silt and clay (fines).
Common Properties Analysed:
Grain size, particle density, bulk density, fall velocity, angle of repose, mineral composition, and organic content.
Ternary Diagrams for Mixed Sediments
Purpose: To classify environments based on the percentages of sand, gravel, and mud.
Example Classification: A habitat may be described as “Muddy Sand” or “Gravelly Muddy Sand” depending on the proportions.
Grain Size Classification and Sediment Fall velocity
If the particle size doubles, the sediment classification may change (e.g., sand to gravel).
Sediment Fall Velocity (ws):
Increases with larger particle diameter.
Increases with higher sediment density.
Decreases in more dense fluids (e.g., water vs. air).
Varies with temperature changes.
Sample Preparation Techniques
Separation Methods: Wet sieving, centrifugation, filtration, settling.
Cleaning Samples:
Washing/Drying: To remove contaminants.
Loss on Ignition: Measures organic content.
Acid Digestion: Removes carbonates.
Weighing and Sub-Sampling: Coning and quartering or using a riffle box for dividing samples.
Dry Sieving for Grain Size Analysis
Setup: Stack of sieves ordered by decreasing aperture size from top to bottom.
Procedure: Use a sieve shaker to separate sediment into size classes.
Outcome: Weigh mass retained in each sieve to construct a particle size distribution.
Settling Methods – Pipette and Hydrometer
Pipette Method: Take repeated samples from a suspension at specific times.
Hydrometer Method: Measure buoyancy of a floating device to determine particle density.
Theory: Based on Stokes’ Law, which describes the settling velocity of small particles in a fluid.
Measuring Grain Size via Settling Velocity
Empirical Methods: Use particle settling velocity (ws) to infer grain size.
Advantages: Relates to hydrodynamic behavior, offers good resolution, and aligns with accepted measurements.
Challenges: Hindered settling, convection effects, and particle concentration can introduce errors.
Stokes law
the motion of spherical particles through a viscous fluid.
It gives the relationship between the force exerted on the particle due to the fluid’s viscosity and the velocity at which the particle moves through the fluid.
Grain Size Distribution Statistics
Skewness: Describes the asymmetry of the particle size distribution.
Kurtosis: Indicates the sharpness or peakedness of the distribution, showing how particles are concentrated around the mean.
Importance of Sediment Size in Classification
Sand and Gravel: Non-cohesive and relatively stable in water, more easily characterized by grain size.
Silt and Clay (Fines): Cohesive, with significant interactions between particles, leading to complex behaviour in water.
Application: Size classification helps predict sediment behaviour under various flow conditions.
Cohesive vs. Non-Cohesive Sediments
Non-Cohesive (Sand & Gravel): Larger particles with minimal inter-particle attraction; easily mobilized by currents.
Cohesive (Silt & Clay): Small particles that stick together due to electrostatic forces, making them resistant to erosion but challenging to resuspend once settled.
Angle of Repose
The maximum angle at which sediment can pile without sliding.
Influencing Factors:
- Grain size and shape (rounded grains have lower angles).
- Moisture content (wet sediment has a higher angle of repose).
Application: Helps predict sediment stability on slopes and banks.
Fall (Settling) Velocity of Sediment Particles factors
The rate at which particles settle through a fluid.
- Particle Diameter: Larger particles settle faster.
- Particle Density: Heavier particles have a higher fall velocity.
- Fluid Density: Lower fluid density (e.g., air vs. water) increases fall velocity.
- Environmental Influence: Higher temperatures reduce fluid density, affecting settling rates.
Dimensionless Grain Diameter
Purpose: A normalized measure of grain size, factoring in density and fluid viscosity.
Use: Helps compare settling behaviors of different sediments in diverse fluid environments.
Grain Shape Characteristics
Roundness: Indicates the smoothness of a grain’s surface; increases with transport due to abrasion.
Sphericity: Degree to which a particle approximates a sphere, determined by the ratio of its axes.
Changes with Transport
Sample Division Techniques
Coning and Quartering: Form a cone of sediment, split it into quarters, discard opposite quarters, and repeat.
Sample Divider (Riffle Box): A device that splits samples evenly, particularly useful for non-cohesive samples.
Purpose: Ensures sample homogeneity for accurate analysis.
Sorting
Indicates the degree of uniformity in particle sizes. Well-sorted sediments have a narrow size range, while poorly sorted ones vary widely.
Skewness:
Measures the asymmetry of size distribution. Positive skewness means finer particles dominate; negative skewness means coarser particles dominate.
Kurtosis:
Describes the concentration of particles around the mean size; high kurtosis indicates a peaked distribution.
Applications of Sediment Characterization
Environmental Assessment: Understanding sediment dynamics in habitats like wetlands, beaches, and riverbeds.
Engineering Projects: Predicting erosion, stability, and sediment transport for coastal and offshore structures.
Habitat Classification: Using sediment properties to classify environments (e.g., muddy sand, gravelly sand) in ecological studies.
