Quantifying and modelling soil armouring and erosion on steep slopes
Human induced accelerated erosion from steep slopes is often observed in deforested
areas, construction sites, and large surface mining operations. For example, in large
surface mining operations such as in Stockton, topsoil is a limited commodity and a
valuable resource. Topsoil is used for restoring vegetation and for providing an
adequate cover to protect the capping material used to cover acid rock waste. Excess
topsoil loss due to erosion is not only detrimental to the environment, but is also an
economic liability that can hinder the overall restoration effort.
To help in land use management, design of restored slopes, and to mitigate
environmental and economic losses, it is necessary to quantify the potential rate of
soil erosion from steep slopes. Under normal circumstances the application of an
erosion model such as RUSLE (Revised Universal Soil Loss Equation) or WEPP
(Water Erosion Prediction Project model) would be adequate to estimate erosion
rates; however, many steep hill soils in New Zealand and around the world have high
concentration of rock and are thus prone to soil armouring. During the soil armouring
process fine soil particles are eroded quickly, leaving a coarse surface layer of rock
and gravel (Figure 1). This coarse layer provides protection to the underlying soil,
reducing further erosion. In other words, soil armouring effectively decreases erosion
with time by preferentially stripping away fine particles. Soil armouring is a process
which is not readily simulated by WEPP or RUSLE unless changes in erodibility or
rock cover are quantified.
The main aim of the project is to quantify and model soil armouring and erosion on
steep hills. This can be done through:
1. Running a series of experiments using the steep slope flume (Figure 2) to
understand the armouring process and related changes in soil erosion by
experimenting with different:
a) slopes
b) rainfall intensities
c) soil
 2 soil types (w/ different distributions of rock sizes)
 sieved soil (<2mm particle sizes)
2. Model the change in erodibility due to armouring by modifying key parameters in
the RUSLE and WEPP models.
It is strongly recommended you take or audit the ENCI648 course (offered during
Term 1). A special permit to overcome GPA restrictions to attend the class may be
considered.
Figure 1. Topsoil armouring (2 – 4 mm in depth) following a rainfall event. The top
part of the picture shows the armouring layer, the bottom part of the picture showing
the exposed soil has been scrapped off.
Figure 2. The Steep Slope flume.