Pebble analysis

• its purpose is to measure the size, shape, and fabric (orientation and disposition) of particles to learn about the processes involved in the erosion, transportation, and deposition of material.
• This technique can be used to measure sediments on a beach, in a riverbed, within glacial deposits and in weathered rock.
• Equipment:

  • A clear ruler
  • A pebbleometer or stoneboard
  • The Cailleux scale of roundness and/or a Power’s scale of angularity
  • Tape measure
  • Quadrat
  • Compass
  • Recording sheet
  • Random number table
• Sediment should be sampled randomly, and pebble axes can be measured using callipers and Callieux roundness cards.
• Methodology 1:

  • Reach down with your index finger extended and select the first pebble it touches.
  • Use the callipers,pebbleometer or stoneboard to measure the long (a), intermediate (b) and short (C) axes of the pebble. The three axes must be at right angles to each other. Average the lengthof the axes to give the mean particle size.
  • Record the measurements on the recording sheet
• Methodology 2:

  • If appropriate for your research, write down as many additional background comments and observations as possible (e.g. grain size, presence of bedforms, debris lines or berm crests).
  • Repeat this process, perhaps ten times per location, ensuring that the distance from the bank or shore line is recorded.
• Considerations and limitations:

  • Deciding on the sampling strategy is very important in reducing subjectivity and increasing the validity of your results.
  • To avoid bias, consideration needs to be given to the size of the sample and the method used to select the pebbles.
  • The sample size should be large enough to provide a representative sample of the parent population, yet not too large to be unmanageable.
  • The sharpest point of a stone is measured using the Cailleux scale of roundness; and judgement of this may vary from person to person, creating subjectivity.
• Considerations and limitations:

  • The use of visual charts such as Power's scale of angularity can also be subjective.
  • In reality, using Power's scale will mostly give an angularity of class five or six.
  • Anything that may affect the results should be noted - for example, recent storms or management structures that might have altered the composition of beach material.
• Considerations and limitations:

  • Inaccessible and overgrown sample sites
  • Rate of flow (flood conditions) can make sampling dangerous
  • Differences in flow rates across a meander (deeper on outside of bend)
• Processing techniques:

  • Scatter graphs and spearman’s/Pearson’s to find relationships such as from source to mouth and erosion.
  • Dispersion graphs for comparison