Sediment generation and soil mound denudation in areas of high-density tree throw along a river valley in the Jura Mountains, Switzerland

A preliminary field-based investigation was undertaken in a small(<10 km^(2))river valley located in the mountainous Jura region of northwest Switzerland.The aims of the work were to assess sediment generation and annual sediment transport rates by tree throw on forested hillslopes,and to document surface hydrology characteristics on four fresh soil mounds associated with recent tree throws over a 24-day monitoring period.For the soil mounds,average sediment recovery ranged from 7.7-28.2 g(dry weight),equivalent to a suspended sediment concentration of 145.2-327.8 g L^(-1),and runoff coefficients ranged from 1.0%-4.2%.Based on a soil bulk density value of 1,044 kg m^(-3),upslope runoff generation areas were denuded by an average 0.14 mm by the end of the 24-day monitoring period,representing an erosion rate equivalent to 2.1 mm yr^(-1).A ca.50 cm high soil mound could therefore feasibly persist for around 200-250 years.For tree throw work,the dimensions of 215 individual tree throws were measured and their locations mapped in 12 separate locations along the river valley representing a cumulative area equivalent to 5.3 ha(av.density,43 per ha).Tree throws generated a total of 20.1 m^(3) of fine-sediment(<2 mm diameter),or the equivalent of 3.8×10^(-4) m^(3) m^(-2).The process of tree throw was originally attributed to two extreme weather events that occurred in west and central Europe in late December 1999.Taking the 18-year period since both storms,this represents an annual sediment transport rate of 2.7×10^(-5) m^(3) m^(-1) yr^(-1).Exploring the relationship with wind on fall direction,65.5%of tree throws(143)generally fell in a downslope direction irrespective of hillslope aspect on which they were located.This infers that individual storms may not have been responsible for the majority of tree throws,but instead,could be associated with root failure.Given the high density of tree throws and their relative maturity(average age 41 years),we hypothesise that once trees attain a certain age in this river valley,their physiognomy(i.e.height,mass and centre of gravity)compromises their ability to remain securely anchored.We tentatively attribute this possibility to the presence of bedrock close to the surface,and to the shallow soil profile overlaying steep hillslopes.

A two-dimensional numerical model coupled with multiple hillslope hydrodynamic processes and its application to subsurface flow simulation

A numerical hillslope hydrodynamic model is of great importance in facilitating the understanding of rainfall-runoff mechanism.However,most of the currently existing models do not consider the effect of coupled hydrodynamic processes as runoff,subsurface flow or groundwater flow.In this study,the Tsinghua Hillslope Runoff Model based on multiple hydrodynamic process,THRM model,is developed,which couples with Saint Venant equation for surface runoff and Richards equation for variably saturated soil water movement(including subsurface flow and groundwater flow).A finite difference scheme with improved boundary conditions is adopted in this research.It is revealed from the simulation that the THRM model has a high computational efficiency and stability in simulating subsurface flow of the experimental hillslope,which is valuable in assessing the hillslope runoff generation mechanism.A model based sensitivity analysis is also carried out.The impact of boundary condition,grid size and initial soil moisture on simulation result and model stability are revealed,which provides insightful references to understand the mechanism of subsurface flow.

Field experimental study on the effect of thawed depth of frozen alpine meadow soil on rill erosion by snowmelt waterflow

Soil erosion by snow or ice melt waterflow is an important type of soil erosion in many high-altitude and high-latitude regions and is further aggravated by climate warming.The snowmelt waterflow erosion process is affected by soil freeze-thaws and is highly dynamically variable.In this study,a methodology was developed to conduct in situ field experiments to investigate the effects of the thawed depth of the frozen soil profile on snowmelt waterflow erosion.The method was implemented on an alpine meadow soil slope at an altitude of 3700 m on the northeastern Tibetan Plateau.The erosion experiments involved five thawed soil depths of 0,10,30(35),50,and 80(100)mm under two snowmelt waterflow rates(3 and 5 L/min).When the topsoil was fully frozen or shallow-thawed(≤10 mm),its hydrothermal and structural properties caused a significant lag in the initiation of runoff and delayed soil erosion in the initial stage.The runoff and sediment concentration curves for fully frozen and shallow-thawed soil showed two-stage patterns characteristic of a sediment supply limited in the early stage and subject to hydrodynamic-controlled processes in the later stage.However,this effect did not exist where the thawed soil depth was greater than 30 mm.The deep-thawed cases(≥30 mm)showed normal hydrograph and sedigraph patterns similar to those of the unfrozen soil.The findings of this study are important for understanding the erosion rates of partially thawed soil and for improving erosion simulations in cold regions.