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Mamoun C.M.,University of Mauritius | Nigel R.,Center Eau Terre Environnement | Rughooputh S.D.D.V.,University of Mauritius
Wetlands | Year: 2013

Past studies conducted on wetlands of Mauritius lead to the conclusion that half of the wetlands have been backfilled for touristic and housing development and that the ecological condition of the remaining wetlands is being seriously challenged by numerous threats, natural and anthropogenic. This research aimed to fill the information gap concerning wetland type and distribution. For this, using published maps and satellite imagery, wetlands were digitised resulting in 144 wetlands and categorised into 8 ecological units. Afterwards, a number of wetlands were ranked according to their environmental condition based on a series of Land Cover Indices (LCIs). These indices were derived by analysis of land cover types and slope gradient within a 50 m and a 950 m watershed-bounded buffer zone. Wetlands in forested areas were the least disturbed, with LCI of typically 0.30, but potentially threatened by sediment accumulation due to a high slope gradient (>20 %). Three wetlands in Mauritius are classified as Ramsar wetland. One of them, the Blue Bay Marine Park (BBMP) has moderately good condition (LCI = 0.55). The other one, the Rivulet Terre Rouge Estuary Bird Sanctuary (RTREBS) was among the most heavily impacted (LCI = 0.87) due to intensive urbanization. © 2013 Society of Wetland Scientists. Source

Nigel R.,Center Eau Terre Environnement | Rughooputh S.D.D.V.,University of Mauritius
Soil Research | Year: 2012

Soil erosion by water is one of the most important natural resources management problems in the world. The damages it causes on-site are soil loss, breakdown of soil structure, and decline in organic matter content, nutrient content, fertility, and infiltration rate. Lands with the highest erosion risk on Mauritius Island are crop cultivations (sugarcane, tea, vegetables) on erosion-susceptible terrain (slopes >20% coupled with highly erodible soils). The locations of such lands on Mauritius were mapped during previous, qualitatively based regional-scale erosion studies. In order to propose soil conservation strategies, there is a need to apply a more quantitative approach to supplement the previous, qualitatively based studies. This paper reports an application of the Revised Universal Soil Loss Equation (RUSLE) within a geographical information system in order to estimate soil loss on the island, and particularly for the high-erosion areas. Results show that total soil loss on the island is estimated at 298259tyear-1, with soil loss from high-erosion areas summing 84780tyear-1 (28% of total soil loss). If all of the high-erosion areas were afforested, their soil loss would be reduced to 10264tyear-1, i.e. a reduction of 88% for the high-erosion areas and a reduction of 25% for the island. This study thus calls for soil and water conservation programs directed to these erosion-prone areas before the land degradation and environmental damage they are causing become irreversible. The methodological approach used in this work to quantitatively estimate soil loss from erosion-prone areas can be adopted in other countries as the basis for a nationwide erosion assessment in order to better inform environmental policy needs for soil and water conservation. © 2012 CSIRO. Source

Bourgault D.,University of Quebec at Rimouski | Hamel C.,Universite de Sherbrooke | Cyr F.,University of Quebec at Rimouski | Tremblay J.-E.,Laval University | And 3 more authors.
Geophysical Research Letters | Year: 2011

Turbulence and nitrate measurements collected in the Amundsen Gulf during ice-covered conditions in fall 2007 are combined to provide mean vertical profiles of eddy diffusivity K̄ and diffusive nitrate fluxes F̄. The mean diffusivity (with 95% confidence intervals) was maximum near the uppermost sampling depth (10 m) with K̄max = 3(2, 5) × 10 -3 m2 s-1 and decreased exponentially to a depth of ∼50 m, below which it was roughly constant at the background value K$\overline{b = 3(2, 5) × 10-6 m2 s-1. The nitracline, centered around 62 m depth, was subject to an eddy diffusivity close to the background value K̄b and the mean diffusive nitrate flux across the nitracline was F̄nit = 0.5(0.3, 0.8) mmol m-2 d-1. These observations are compared with other regions and the role of vertical mixing on primary production in the Amundsen Gulf is discussed. Copyright 2011 by the American Geophysical Union. Source

Ji J.,Beijing Forestry University | Ji J.,Montpellier University | Kokutse N.,CIRAD - Agricultural Research for Development | Kokutse N.,Center Eau Terre Environnement | And 3 more authors.
Catena | Year: 2012

Vegetation is widely used for controlling shallow landslides. The mechanisms by which roots increase apparent soil cohesion is well documented and many values of root additional cohesion are available in the literature for different plant species. However, less information is given about the spatial variation of soil reinforcement by roots at a slope scale and its influence on slope stability, in particular in forest areas.The goal of this paper is to describe the spatial variability of root additional cohesion on two monospecific 17-y-old stands of Robinia pseudoacacia and Platycladus orientalis grown on slopes in the semiarid Loess Plateau of China, and to analyze numerically the effect of this variability on slopes stability. For this purpose, vertical trenches were dug at different distance and directions around trees situated at three different slope locations, i.e. up-, mid- and down-slope. Grids with a 10. ×. 10. cm mesh were placed on vertical walls. Roots were counted within each grid cell and split according to their diameter class. Root area ratio (RAR) was estimated and compared among different positions around the trees and at three different locations along the slope. Roots tensile strength was determined with laboratory mechanical tests. RAR and root tensile strength were used as inputs in six different root reinforcement models to calculate root additional cohesion. A 2D finite element model of slope stability was developed and used to calculate the increase in factor of safety (FoS) due to root additional cohesion on rectilinear and terraced slopes.Results showed that both root tensile strength and Young's modulus of R. pseudoacacia was about two times higher than tensile strength of P. orientalis. RAR distribution had a strong relationship with local soil moisture content measured in July during the raining season, and was significantly different with regards to tree location on the slope. The six theoretical models used to estimate the root additional cohesion gave different vertical profiles of root reinforcement distribution according to the underlying hypothesis on how forces are transferred to the roots. Theoretical analyses of slope stability showed that terraced slopes were 20% more stable than rectilinear slopes, disregarding the differences in hydrological regimes between these two terrain morphologies. Numerical sensitivity analyses also showed that the FoS reached an asymptotic value when increasing root additional cohesion. Actual additional cohesions of the two studied sites corresponded to FoS that were already close to this asymptotic values. Consequently variations of these actual root cohesions would not much affect slope stability. However it was showed that more attention should be given to the reinforcement of the bottom part of the actual slopes, where roots have a larger positive impact on the FoS. © 2011 Elsevier B.V. Source

Kokutse N.K.,Center Eau Terre Environnement | Temgoua A.G.T.,Center Eau Terre Environnement | Kavazovic Z.,Khalifa University
Ecological Engineering | Year: 2016

This paper is dedicated to numerical analysis of the influence of vegetation on slope's stability using the calculation of the safety factor of different slopes by a classical shear reduction method with the software PLAXIS in 2D. The soil is modelled as an elastic perfectly plastic material. The shear strength of the soil is also modelled using the Mohr-Coulomb criterion. Slope stability is influenced by vegetation mechanical parameters: plant's root matrix system and surcharge due to presence of trees. Based on 2D finite element method, this paper investigates the combined effects of different rectilinear slope geometries, soil types and vegetation mechanical parameters on the slope's factor of safety. The effects of the roots are modelled by increasing the soil cohesion following a classical Wu model. The additional cohesion depends on the vegetation type and on the depth from soil surface. The analysis examined reinforcement effects of the roots' systems of four idealised types of vegetation growing on three different types of soil composing the slope. The case study was performed on different slope configurations where slope's height and angle were allowed to vary significantly. Two key parameters of the root matrix system had been considered for the finite element analysis: additional cohesion due to the presence of roots and depth of the root matrix. Moreover, influence of geometrical parameters, height and angle of the slope, on the stability of different slope configurations had been analysed by computing slope's factor of safety. Parametric studies were performed to assess the variation of the factor of safety of a slope in case of different geometrical configurations combined with several types of vegetation coverage. Results showed that the slope angle had the greatest impact on variation of factor of safety. Additional cohesion is regarded as the second most important parameter influencing the factor of safety. These two parameters combined play an important role in shallow failures of slopes and significantly affect stability of a slope regardless of a soil type composing the slope. Results of the current study can help practitioners determine if a slope is at risk by the lack of additional roots cohesion combined with different types of soil and young vegetation. Hence, the proposed method helps evaluate slope's vulnerability and could be efficiently used as management or informative tool for ecological engineers and forest conservative practitioners. © 2015 Elsevier B.V.. Source

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