Round Island is a 219 hectares islet north of the Mauritian mainland and has been
classified as a Nature Reserve since 1957. The island has been subjected to human
influence in the form of land degradation through introduced grazing animals (goats and
rabbits) which has detrimentally affected the floral and faunal ecology of the island. Since
the removal of the grazers, intensive conservation management has been undertaken to
restore Round Island’s unique ecological status.
The aim of this research project was to investigate erosion phenomena at specific
study sites on Round Island through field-based classification and mapping procedures and
describe physical soil characteristics. A modified version of the SARCCUS (1981) Erosion
Classification system was used to classify linear erosion forms in the field, based on
morphometric parameters. The effect of rock hardness was also assessed to determine
lithological controls on bedrock-incised erosion features.
Given the size of Round Island and the extensive nature of erosion, five study sites
were chosen for the soil and erosion assessment. An additional two gully networks, ‘camp’
and ‘big’ gully were chosen to allow the investigation of an entire erosion system. Soils are
thin and discontinuous, with a sandy texture and are poorly sorted. The Helipad habitat has
the coarsest soils indicative of wind erosion where the deflation of fines leaves a coarser
gravel pavement. No significant differences are found between sites for soil physical
properties, with the exception of pH where the Summit has a significantly lower pH than the
Helipad (Mann- Whitney U test, z= -2.21, p= 0.03) and Rock Slab (Mann- Whitney U test, z=
-2.93, p< 0.01) habitat regions. No linear erosion forms are found on the soils of Round Island, however bedrock
incised rills and gullies extensively occur. The Summit, Rock Slab and Palm Savannah
habitats represent erosion processes along a profile gradient on the steep, convex western
slope. The Summit habitat is subject predominantly to sheetwash and wind erosion, with the
presence of two bedrock-incised rills of moderate severity. The Rock Slab region is
predominated by parallel, shallow bedrock rills and gullies running downslope with moderate
and slight severity, respectively. Soil and vegetation cover is highly variable within the
region. Downslope, the Palm Savannah region is subject to moderate gully erosion with an
irregular morphology. Soil is transported during rainfall within the gully channels where it is
ultimately lost to sea.
The two large gully systems, ‘camp’ and ‘big’ gully represent erosion of the highest
severity on Round Island. The gullies have their starting points on the mid- upslope regions
as rills, which increase in width and depth downslope, as indicated by decreasing width:
depth ratios. The gullies have their end point at sea, both with a severity of very severe
bedrock-gully erosion. During periods of intense rainfall the bedrock-incised gullies act as
transport channels for sediment which is ultimately lost to sea. Little sediment is able to
remain and this is exemplified by a lack of vegetation. This is a natural cycle where
conservation efforts will remain ineffective.
In addition to morphology, rock hardness was assessed using a Schmidt Hammer for
the bedrock incised forms. The rate of erosion of the bedrock dominated channels depends
on various factors such as rock strength, sediment supply and grain size. The predominant
rock type on Round Island is tuff which is a relatively weak volcanic rock, as indicated by low
mean Schmidt Hammer R-values, implicating higher expected bedrock erosion rates.
Mini-dissertation (MSc)--University of Pretoria, 2015.