Modelling the impact of forest loss on shallow landslide sediment yield, Ijuez river catchment, Spanish Pyrenees

Abstract

The SHETRAN model for simulating the sediment yield arising from shallow landslides at the scale of a river catchment was applied to the 45-km 2 Ijuez catchment in the central Spanish Pyrenees, to investigate the effect of loss of forest cover on landslide and debris flow incidence and on catchment sediment yield. The application demonstrated how such a model, with a large number of parameters to be evaluated, can be used even when directly measured data are not available: rainfall and discharge time series were generated by reference to other local records and data providing the basis for a soil map were obtained by a short field campaign. Uncertainty bounds for the outputs were determined as a function of the uncertainty in the values of key model parameters. For a four-year period and for the existing forested state of the catchment, a good ability to simulate the observed long term spatial distribution of debris flows (represented by a 45-year inventory) and to determine catchment sediment yield within the range of regional observations was demonstrated. The lower uncertainty bound on simulated landslide occurrence approximated the observed annual rate of landsliding and suggests that landslides provide a relatively minor proportion of the total sediment yield, at least in drier years. A scenario simulation in which the forest cover was replaced by grassland indicated an increase in landsliding but a decrease in the number of landslides which evolve into debris flows and, at least for drier years, a reduction in sediment delivery to the channel network.

Recommendations for the quantitative analysis of landslide risk

“…Regolith depth, often referred to by geomorphologists and engineers as soil depth, is defined as the depth from the surface to more-or-less consolidated material. Despite being a major factor in landslide modelling, most studies have ignored its spatial variability by using constant values over generalised land units in their analyses (Bakker et al 2005;Bathurst et al 2007;Talebi et al 2008;Montgomery and Dietrich 1994;Santacana et al 2003). Soil thickness can be modelled using physically based methods that model rates of weathering, denudation and accumulation (Dietrich et al 1995;D'Odorico 2000) or empirical methods that determine correlations with topographical factors such as slope, or it can be predicted using geostatistical methods (Tsai et al 2001;Van Beek 2002;Penížek and Borůvka 2006;Catani et al 2007).…”

Modeling root reinforcement using a root-failure Weibull survival function

“…For example, the roots of riparian vegetation may consistently influence the morphodynamic of rivers by anchoring sediments (Edmaier et al, 2011) or stabilizing river banks (Pollen and Simon, 2005;Petrone and Preti, 2008) and represent an important factor in river restoration projects. Moreover, vegetation contributes to the mitigation of erosion and shallow landslides at the catchment scale, regulating the yield and transport of sediments (Schmidt et al, 2001;Bathurst et al, 2007). In mountain catchments, root reinforcement is one of the most important contributions of vegetation to slope stability (Phillips and Watson, 1994;Sidle, 1992;Rickli and Graf, 2009).…”

The Debris Flow of 1<sup>st</sup> August 2012 in Kakpenyi-Tinta (Akwaya Sub Division) Southwest Cameroon—I: Event Description, Causes and Impacts

“…The removal of vegetation by burning or timbering has 2 effects: 1) it removes the anchor that the tree roots have on soil particles, thus rendering them loose and easily erodible; 2) it deprives the area of rainfall-absorbing trees, thus increasing runoff and sediment removal. This reduces the threshold rain that can trigger a debris flow or landslide [27] [28]. Because of the remoteness of the area, not much of timbering activities are going on in Kakpenyi now.…”