GEOMORPHOLOGY REPORT - CRC LEME

6.1 Granulometry
6.1.1 Methodology
Granulometrey provides a quantitative measurement of the particle size distribution in a soil
or sediment. As a result much more accurate modelling of parameters such as porosity and
permeability, recharge, and salt load can be made. It also provides a check on the accuracy of
field estimates of soil texture.
The grainsize was determined using a Malvern Instruments Mastersizer 2000 instrument. The
laser diffraction instrument consists of three parts, a laser source (He-Ne gas or diodes
emitter), detectors, and sample chamber that allows suspended particles to recirculate in front
of the laser beam. The Mie theory (Rawle, 2001) was used to solve the equations for
interaction of light with matter and calculates the volume of the particle. This technique
calculates the % volume of a range of particle sizes (0.05 – 2000 μm), and the results are
grouped according to the Wentworth scale. To standardise with other analytical data, SI units
(μm) were reported instead of Phi units.
6.1.2 Results
Samples from each site had fairly similar distributions, indicating that within the sampled
depth range there were only minor differences in grainsize distribution. Some surface
samples were less sandy than those at depth, indicating more abundant silt and clay at the
surface. This is interpreted to be from the draping of older land surfaces by fine-grained
material deposited by flood waters. Overall, however the samples were of silty clays to clayey
silts, usually with minor sand. This is consistent with deposition by overbank flow during
river floods for all geomorphic units including floodplains, levee banks, and channel plugs.
Particle size analysis showed a general trend of fining with distance from the main river
channel within the Murray River Trench (Figure 10). Soils were generally sandier on the Fm1
unit and more clay-rich on the Terrace unit. The particle size distribution of the Uplands unit
is most likely the result of mixing of sediments from a number of sources, including windblown
sand from dunes.
90
Sand (%)
80
70
60
50
40
30
20
10
0
0 10 20 30 40 50 60
Clay (%)
Fm1
Fm2
Fm3
T
U
Figure 10. Sand and clay percentage from each geomorphic unit.
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