Erosion phenomena on Round Island, Mauritius

Abstract

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.