Role of land use change in landslide-related sediment fluxes in tropical mountain regions

peer reviewe

Sediment dynamics in tropical mountain regions : influence of anthropogenic disturbances on sediment transfer mechanisms

The aim of this thesis was to improve the understanding of sediment dynamics in tropical mountain regions by analysing the influence of anthropogenic disturbances on sediment transfer mechanisms. The main objectives were (i): to determine the influence of landslides on sediment production, transport and delivery to the river network, (ii) to identify and assess the impact of land cover changes on landslide occurrence and (iii) to evaluate the extent to which land cover dynamics affect landslide area-frequency distributions and landslide-induced erosion. This study quantitatively confirms that more than fifty percent of the sediment delivered by the hillslopes to the streams in the Western Cordillera is initially mobilised by landslides. It also shows that landslide controlling factors vary through time and that topographical factors alone cannot explain the changing landslide patterns. Human disturbances significantly alter the landslide frequency-area distributions, particularly for smaller landslides. When analysing the geomorphic work realised by landslides in different environments, it becomes clear that the majority of landslide-induced sediment is coming from anthropogenic environments. However, this thesis shows that isolating human-induced processes from natural processes is not straightforward and that human impact on the geomorphic process rates may vary according to the spatial and temporal scale of study.(SC - Sciences) -- UCL, 201

Forest cover change trajectories and their impact on landslide occurrence in the tropical Andes

Tropical mountain regions are prone to landslide hazards. Given the current land pressure with increasing occupation of steep uplands, landslide hazards are expected to increase in the near future. Understanding the factors that control landslide hazards is therefore essential. Rare event logistic regression allows us to perform a robust detection of landslide controlling factors. This technique is here applied to the tropical Andes to evaluate the impact of dynamic land cover changes on landslide occurrences. Land cover change trajectories (i.e. dynamic evolution of land cover through time) were specifically included in the probabilistic landslide analysis. While natural physical processes such as slope undercutting by rivers and failure of oversteepened slopes are important in this tropical mountainous site, landslides are increasingly associated with human activities. The data show that land cover trajectories are associated with landslide patterns. In this humid mountainous site, forest degradation does not lead to a measurable increase in landslide occurrence. However, few years after forests are converted to pastures, a rapid decline of slope stability is observed. Land cover conversion from forest to pasture permanently reduces slope stability. It is assumed that major changes in soil properties and hydrology induced by the vegetation conversion play a role in accelerating landslide hazards

Landslide Controlling Factors in Catchments with High Deforestation

Flooding, landsliding and accelerated erosion are common hazards in tropical mountainous regions around the world. The aim of this study is to analyse the triggering factors of landslides in densely populated areas with different land cover dynamics. Landslide inventories and land cover data were derived from optical remote sensing data for different periods in time. Our results indicate that human activities significantly increased the landslide hazard. We observed an increase in the occurrence of landslides after deforestation and road construction. The financial and environmental losses that are associated with these landslides might induce some negative feedback mechanisms leading to a deceleration of deforestation rates

Spatio-temporal patterns of landslides and erosion in tropical andean catchments

Tropical mountain regions are prone to high erosion rates, due to the occurrence of heavy rainfall events and intensely weathered steep terrain. Landslides are a recurrent phenomenon, and often considered as the dominant erosion process on the hillslopes and the main source of sediment. Quantifying the contribution of landslide-derived sediment to the overall sediment load remains a challenge. In this study, we derived catchment-average erosion rates from sediment gauging data and cosmogenic radionuclides (CRN), and examined their reliability and validity for constraining sediment yields in tectonically active regions. Then, we analysed the relationship between catchment-average erosion rates and landslide-derived sediment fluxes. The Pangor catchment, located in the western Andean mountain front, was selected for this study given its exceptionally long time series of hydrometeorological data (1974-2009). When including magnitude-frequency analyses of the sediment yields at the measurement site, the corrected gauging-based sediment yields remain one order of magnitude lower than the CRN-derived erosion rates. The underestimation of catchment-average erosion rates from gauging data points to the difficulty of extrapolating flow frequency and sediment rating data in non-stationary hydrological regimes, and severe undersampling of extreme events. In such conditions, erosion rates derived from cosmogenic radionuclides are a reliable alternative method for the quantification of catchment-average sediment yield. Landslide inventories from remote sensing data (1963-2010) and field measurements of landslide geometries are the input data for the derivation of landslide-derived sediment fluxes. The landslide-related erosion rates of 1688 (-326;+901) and 630 (-108;+300) t.km2.y-1 are similar to the CRN-derived erosion rates, likely suggesting that landslides are the main source of sediment in this mountainous catchment