Bare Slope Research Articles

Soils in the Mediterranean region: what makes them different?

Mediterranean climates are characterized by winter rains — with some months of excess rainfall over evatranspiration, warm and dry summer months with moisutre deficits — drying out soils and their annual vegetation (xeric moisture regime). They are found on western parts of all continents, between the cooler temperature zone and the hot dry desert zone. The largest Mediterranean region, surrounding the Mediterranean Sea extends over 4,300,000 km 2 and exhibits a wide variety of soils and geo-ecosystems. Characteristics landscape attributes are the high proportion of mountains with sleep slopes, significant additions of Saharan desert dust to practically all soils of the region, and a large proportion of limestone and other calcareous rocks as soil parent materials. Characteristics soil behavior features are moderate weathering with pervection (leaching, lessivage) of mostly 2:1 clays into B horizons (Xeralfs:Luvisols), hematite-induced reddening of the clays due to summer dehydration of free iron oxyhydroxides, carbonate dissolution and reprecipitation with prevalence of calcic horizons (Xerolls; Calcisols) in semiarid regions, and development of Vertisols, mostly in lowlands, where deep layers of swelling/cracking clays have sedimented. Shallows soils on nearly bare slopes, mostly a result of erosion subsequent to deforestation, are frequent (Leptosols, Cambiosols; Inceptisols, Entisols). Red (or Brown) Mediterranean soils are no longer used as a separate classification group in modern, well defined, soil property-controlled taxonomies (Soil Taxonomy; FAO system), but were partially replaced by Duchaufour's term Fersiallitic soils in some classification systems. Terra Rossa continues to be used in some classification for hard limestone derived red soils, mostly shallow. The effects of man's past and current interference with the lanscape are pervasive in most regions, and predictions for possible future effects on the soils and ecosystems, due to greenhouse gas induced warming and rainfall change, are partly negative, partly still too uncertain.

Effect of rainfall on matric suctions in a residual soil slope

A slope stability study involving shallow slip surfaces should include the effect of negative pore-water pressures in a slope. A field instrumentation program was carried out to monitor negative pore-water pressure (i.e., in situ matric suction) in a residual soil slope in Singapore. Variations in matric suction and the matric suction profiles under (1) a canvas-convered grassed surface, (2) a grassed surface, and (3) a bare ground surface, in response to rainfalls were investigated. Changes in matric suction due to changes in climatic conditions decrease rapidly with depth. The change was found to be most significant in the bare slope and least significant under the canvas-covered slope. The amount of decrease in matric suction after a rainstorm was observed to be a function of the initial matric suction just prior to the rainstorm. Positive pore-water pressures were observed above the groundwater table, suggesting the development of a perched water table within the slope. These observations are also typical of other regions experiencing high seasonal rainfalls. The field monitoring program presented can be adopted for investigating rainfall-induced landslides in other parts of the world. Key words: matric suction, negative pore-water pressure, field instrumentation, rainfall, residual soil, slope stability.

USING A COMBINED SLOPE HYDROLOGY/STABILITY MODEL TO IDENTIFY SUITABLE CONDITIONS FOR LANDSLIDE PREVENTION BY VEGETATION IN THE HUMID TROPICS

The susceptibility of cut slopes to landsliding can be reduced in certain circumstances by the establishment of a vegetation cover. However, the hydrological implications of allowing a cover to develop may offset the mechanical benefits of soil reinforcement by roots. The balance between hydrological and mechanical effects is critical on slopes which are susceptible to the development of an infiltration-induced transitory perched water table, a common cause of landslides in deep, tropical residual soils. This balance is likely to change both between slopes of different types as well as temporally on any given slope. The net effect of a vegetation cover must be predicted either before natural vegetation covers are allowed to encroach on bare slopes, or if engineers are considering the use of trees as a protective measure. This paper presents a method of calculating the impact of a vegetation cover on slope stability. Simulations carried out on a wide range of slope types suggest that where failure is most likely to be triggered by infiltration rather than ground water rise, large-scale vegetation covers may contribute to instability. Whether vegetation had a positive or negative impact on slope stability was controlled by the permeability of the soil matrix, whilst the magnitude of impact was controlled by the soil strength and the slope height.

Armor Coat Formation and Fractional Sediment Discharge of Sand Mixtures caused by Rainfall on Bare Slope

The sediment transport process on the slope caused by rainfall impact and overland flow is the fundamental phenomenon, and many sediment discharge equations were proposed. Though most of them are proposed for the uniform sand, they are usually used for the estimation of the total sediment discharge of sand mixtures which is one of characteristics of the natural slopes. In this study, we experimentally dealt with the armor coat and the fractional sediment discharge on the slope caused by rainfall impact. Next, in order to simulate the non-equilibrium process of sediment transport we have applied the stochastic model which includes the effect of the sediment pick-up rate by rainfall impact. Since the simulated results have explained the experimental results, the validity of the proposed model has been clarified.

Fundamental Study on Soil Erosion along Bare Slope by Rainfall Impact and Overland Flow

The sediment transport process on the steep slope caused by rainfall and its runoff process can be divided into two: I. process casused by rainfall impact and II. process by overland flow. However, it was very rare to deal with two processes simultaneously, and some equilibrium sediment disdcharge equations were proposed by emphasizing either of two processes. In this study, we have proposed a unified model which includes the effects of rainfall impact and overinad flow, and examined the validity of the model by the experiment. Especially, we clarified the contribution of the rainfall impact to the non-equilibrium process of sediment transport.

Effect of rainfall on matric suctions in a residual soil slope

A slope stability study involving shallow slip surfaces should include the effect of negative pore-water pressures in a slope. A field instrumentation program was carried out to monitor negative pore-water pressure (i.e., in situ matric suction) in a residual soil slope in Singapore. Variations in matric suction and the matric suction profiles under (1) a canvas-convered grassed surface, (2) a grassed surface, and (3) a bare ground surface, in response to rainfalls were investigated. Changes in matric suction due to changes in climatic conditions decrease rapidly with depth. The change was found to be most significant in the bare slope and least significant under the canvas-covered slope. The amount of decrease in matric suction after a rainstorm was observed to be a function of the initial matric suction just prior to the rainstorm. Positive pore-water pressures were observed above the groundwater table, suggesting the development of a perched water table within the slope. These observations are also typical of other regions experiencing high seasonal rainfalls. The field monitoring program presented can be adopted for investigating rainfall-induced landslides in other parts of the world. Key words: matric suction, negative pore-water pressure, field instrumentation, rainfall, residual soil, slope stability.

Dynamics of fluvial and slope processes in the changing geomorphic environment of singapore

AbstractThe nature and rates of fluvial and slope processes change over time and space as urbanized areas replace forested land in Singapore. Storm‐based and time‐based data, from undisturbed rainforests, heavily disturbed construction sites, urban grass‐covered slopes and an experimental plot, are collected to observe the impact of rainwater on the soil moisture conditions, surface microtopography, runoff generation, sediment movement, and ground lowering in the three different categories of land use.The undisturbed forested environment is characterized by high throughfall (58% of total rainfall) and frequent negative soil moisture suctions. The slow and unconcentrated overland flow during heavy storms is restricted by the forest floor microtopography. No rills develop. Ground lowering is recorded as 3·2–3·4 mm a−1. But sediment movement is episodic and suspended sediment concentrations in overland flow are 172–222 mg l−1. During urban construction, gully development is rapid on the bare slopes, runoff generation, voluminous, and sediment‐laden discharges (5200–75498 mg l−1) lead to sediment plumes at channel mouths. Ground lowering rates are measured at 132·4 mm a−1. Once grass‐covered, runoff carries less suspended sediment (800 mg l−1) and ground lowering rates are reduced, but depend on the condition of the cover, ranging from 0·2 to 8·2 mm a−1.As urban development continues, environments are altered both in time as well as spatially.

Algal crust formation in the inland dune area, Laarder Wasmeer, the Netherlands

The aim of this study is to acquire insight in the initial vegetation development on active drifting sands in relation to geomorphological processes. For this purpose the algal vegetation and surface dynamics were monitored on a sand hill with active drifting sand, and on a bare slope within a terrain covered with Polytrichum piliferum located in the Laarder Wasmeer (The Netherlands).

Communist Legacies in the Albanian Landscape

THE elements of the socialist landscape are disappearing fast, and documenting them is important to discern the legacies of communism. Unlike other East European countries, Albania chose nationalistic or self-reliant approach to modernization and shut itself off from the rest of the world for several decades. This circumstance makes it even more important to analyze the geographical aspects of social change. Between 1944 and 1985 Albania may have made considerable economic progress under Enver Hoxha, but it remains the most backward country in Europe (Pano 1992). Examination of the current landscape reveals four significant legacies from the communist regime: reclamation of the Myzeqe Plain, establishment of forty-one new urban centers as focal points of economic development, transformation of the Drin River as source of hydroelectrical power, and promotion of Tirana as socialist capital. As student in France during the interwar years, Hoxha became aware of the extent to which Albanian underdevelopment resulted from Turkish and Italian exploitation (Biberaj 1990, 17). Alter 1945 Hoxha embraced Stalinist program of forced industrialization that depended on economic aid from other communist countries. However, his obsession with overdependence on external powers and his rigid Stalinist ideology led to rejection of aid successively from Yugoslavia, the Soviet Union, and China, with heavy economic consequences for Albania. His nationalistic emphasis justified not only the hardships of isolation and self-reliance but also rural-development program that stabilized urbanization at approximately 35 percent. He also reduced the fragmenting effects of tribal and religious loyalties. Most significant was the attempt to integrate the northern Gegs and southern Tosks, whose differences in way of life and dialect had long been divisive forces in Albania. THE MYZEQE PLAIN The Myzeqe Plain, the largest on the eastern Adriatic coast, illustrates the transformation from malarial swamp to irrigated fields that has been characteristic of the northern Mediterranean coast since 1945. Two factors are important in explaining the patterns in Albania. The first is the misuse of land that started in antiquity (Semple 1931, 289-291; Braudel 1972, I, 67). Landscape deterioration was noted as long ago as 1920, when an Austrian geographer described the plain as a desolate wasteland covered with spreading water areas and impassable swamp forests . . . inundated in winter and dried up in summer (Veith 1920, 77). The cycle of deterioration included overcutting and overgrazing followed by loss of soil from the steep, bare slopes during winter rains; this transported sediment then filled the beds of the main streams as their gradients were radically reduced on reaching the plain; and the result was flooding, swamp conditions, and malaria. The deposits of the Shkumbin, Seman, and Vijose rivers eventually expanded the plain outward into the sea. The second factor was land tenure, in which the consequences of Ottoman rule and its decline were paramount (Stavrianos 1958, 138). With the weakening of Ottoman power during the eighteenth century, serfdom was introduced as fiefs were converted into private, heritable estates. Rather than accept the new status of serfs, many peasants fled to the mountains and left areas of potentially productive land abandoned and untilled. A result was the paradox of an underpopulated plain of swamp, fallow, and pasture next to overpopulated mountains. An analytical framework for this pattern is devastation theory, which holds that physical factors, hydrographical in this case, are inseparable from human ones, in this case the exploitive Turkish land-tenure system (Busch-Zantner 1938, 124-134). Little improvement in the conditions of the plain occurred in the interwar period, when Albania was not viable state. Italy, which initially dominated and then occupied the country, was not interested in land improvement and left the large estates mostly intact. …

Sediment Production and Movement from Bare Slope due to Landslide in Mountains Composed of Weathered Granite

Sediment Production and Movement from Bare Slope due to Landslide in Mountains Composed of Weathered Granite

Book Review: Cold Hearths and Barren Slopes: The Woodfuel Crisis in the Third World

Book Review: Cold Hearths and Barren Slopes: The Woodfuel Crisis in the Third World

Soil loss process by surface erosion on a loamy bare slope

Soil loss process by surface erosion on a loamy bare slope

The relevance of soils and paleosols in interpreting past and ongoing climatic changes

Mediterranean climates are characterized by winter rains — with some months of excess rainfall over evatranspiration, warm and dry summer months with moisutre deficits — drying out soils and their annual vegetation (xeric moisture regime). They are found on western parts of all continents, between the cooler temperature zone and the hot dry desert zone. The largest Mediterranean region, surrounding the Mediterranean Sea extends over 4,300,000 km2 and exhibits a wide variety of soils and geo-ecosystems.Characteristics landscape attributes are the high proportion of mountains with sleep slopes, significant additions of Saharan desert dust to practically all soils of the region, and a large proportion of limestone and other calcareous rocks as soil parent materials. Characteristics soil behavior features are moderate weathering with pervection (leaching, lessivage) of mostly 2:1 clays into B horizons (Xeralfs:Luvisols), hematite-induced reddening of the clays due to summer dehydration of free iron oxyhydroxides, carbonate dissolution and reprecipitation with prevalence of calcic horizons (Xerolls; Calcisols) in semiarid regions, and development of Vertisols, mostly in lowlands, where deep layers of swelling/cracking clays have sedimented. Shallows soils on nearly bare slopes, mostly a result of erosion subsequent to deforestation, are frequent (Leptosols, Cambiosols; Inceptisols, Entisols). Red (or Brown) Mediterranean soils are no longer used as a separate classification group in modern, well defined, soil property-controlled taxonomies (Soil Taxonomy; FAO system), but were partially replaced by Duchaufour's term Fersiallitic soils in some classification systems. Terra Rossa continues to be used in some classification for hard limestone derived red soils, mostly shallow.The effects of man's past and current interference with the lanscape are pervasive in most regions, and predictions for possible future effects on the soils and ecosystems, due to greenhouse gas induced warming and rainfall change, are partly negative, partly still too uncertain.

Sediment yield in a bare Slope

Sediment yield is characterized by many regional factors such as geology, geomorphology, climate and vegetation. The object of this study is to establish a method of surveying sediment yield process in a bare slope, and of analysis and evaluation of the characteristics of the slope. They clarified the characteristics of sediment yield: in particular that the erosion rate is affected by slope gradient, compressive strength of the bare slope, rainfall intensity and the freezing and thawing effect.

Hydrodynamic description of the erosional development of drainage patterns

The erosional development of drainage networks is analyzed by modeling two‐dimensional flow of water and sediment over a bare slope. Deterministic equations of mass and momentum conservation are coupled with random fluctuations of sediment pickup and transportation processes acting at the hydrodynamic scale. These fluctuations are then shown to be able to provide an active source of channelization at the hillslope scale. Controlled experiments are also carried out by simulation runs aimed at investigating model performance (1) to describe formation of natural drainage patterns, (2) to account for different factors (i.e., soil characteristics, hillslope geometry, and rainfall intensity) which control network formation and development, and (3) to reproduce differentiation of drainage patterns due to above factors.

Numerical simulation of seepage in a forested hillslope by finite element method

In order to examine the effect of forest on runoff process in a hillslope, numerical simulation of seepage was carried out under several conditions associated with canopy interception and evapotranspiration. As a result the following has been suggested; 1) Discharge recesses more quickly in case of a forested slope than in case of a bare slope because of different sensitivity to evaporation suppression. 2) Water uptake by tree roots causes larger moisture deficit than soil evaporation and it moderates the response of storm hydrographs as an antecedent moisture condition. 3) Canopy interception decreases amount and intensity of net rainfall and it causes reduction of peak flow and delay of peak time of stormflow.

The use of manual surface seeding, liming & fertilization in the reclamation of acid metal‐contaminated land in the Sudbury, Ontario mining and smelting region of Canada

Abstract A method has been devised for the establishment of a grass‐legume cover on steep, eroded, barren slopes by the manual application of limestone, fertilizer and seeds without prior site‐preparation. Over a four year period, more than one thousand ha of acid, copper‐ and nickel‐contaminated land has been vegetated in this way, using government‐funded summer student employment programmes. Native woody plants are showing rapid colonization of these grassed sites giving rise to optimism that a stable, self‐sustaining plant community will develop.

Caching of Japanese Stone Pine Seeds by Nutcrackers at the Shiretoko Peninsula, Hokkaido

In early September of 1982 clumps of seedlings of Japanese stone pine Pinus pumila germinated from caches were observed on the slopes cut along the road (Route 334), between the Shiretoko Pass and Utoro (Fig. 1, Photos 1-3), at 750 to 400 m above sea level. At 3 caching sites, 8 species of woody seedlings were growing (Figs. 2-5). Most seedlings of the pine and a maple Acer tschonoskii were in clumps, and to a lesser extent seedlings of Todo-fir Abies sachalinensis were also in clumps. The number of seedlings was up to 23 per clump in the pine, 40 in the maple, and 6 in the fir. They were 1 to 3 years old.The clumping of seedlings appeared to be caused by animals. The disseminules of the pine are wingless, but those of the maple and the fir are winged (Fig. 6, Table 1). The mature cones of the pine are easily removed from the branchlets, but they remain closed and do not release their seeds. Although several species of small mammals and birds store the seeds of the pine (Table, 2), the main disseminator appeared to be the Nutcrackers Nucifraga caryocatactes, because the distances between seed sources situated in the alpine zone and caching sites in the subalpine zone are not close, and the caches exist on the bare parts of upper slopes.These caching sites are suitable both for the Nutcrackers to retrieve seeds cached and for the bushy pines to establish themselves in less competition with tall trees. The Nutcrackers are the best planters of the pine for the bare slopes cut along the mountain roads in alpine and subalpine zones. The large, winged, nut-like disseminules, such as of the maple and the fir, at first appear to be dispersed by wind. However, they may also be originated from the caches by the Nutcrackers. In the dispersal of seeds of trees and shrubs, ornithochoryl, especially synzoochoryl and endozoochoryl, seems to play an important role.

Detachment and transportation of loose sediments by raindrop splash: Part II Detachability and transport ability measurements

Laboratory experiments with simulated rainfall and using a recently developed weighing technique were conducted in order to obtain information on the detachability and transportability of 9 loose sediments as a time-dependent phenomenon. The effect of three different initial states on these two erodibility parameters was also studied. During a rain erosion experiment, detachability was significantly influenced by surface water content changes while transportability was not. Both erodibility parameters decreased as soon as a surface water film developed. After transforming the measurements into absolute data, using the nomogram developed in part I, the effect of grain size on detachment and transportation was studied. The relation between resistance to detachment by raindrop impact and grain size was very similar to the relation between resistance to detachment by turbulent runoff or wind and grain size. The lowest resistance was found for a fine sandy sediment. Transportability was negatively related to grain size, and this relation could be represented by a power function. For the same range of sediments detachability variations were more important than variations in transportability. For the coarse sediments, transportation by raindrop impact was a selective process. Finally, the data obtained in this and other studies were combined in order to make a rough estimate of the net downslope splash erosion unit discharge on a bare slope. Des expériences en laboratoire ont été effectuées afin d'obtenir des renseignements sur la détachabilité et la transportabilité de neuf sédiments meubles en fonction de la durée de l'expérience. A cet effet, une pluie simulée ainsi qu'une technique de pesage récemment développée ont été employées. L'effet de l'état physique initial du sol sur ces deux paramètres d'érodibilité a également été étudié. La détachabilité était nettement influencée par les changements en teneur d'eau en surface, tandis que la transportabilité ne l'était pas. Les deux paramètres d'érodibilité décroissaient aussitôt qu'une pellicule d'eau se développait à la surface. Après transformation des résultats des expériences en données absolues, en utilisant l'abaque développé dans la Première Partie, l'effet de la taille des grains sur le détachement et le transport a été étudié. Il y avait une forte ressemblance entre la relation entre la résistance au détachement par les gouttes de pluie et la taille des grains d'une part, et la relation entre la résistance au détachement par le ruissellement turbulent ou le vent et la taille des grains d'autre part. Un sable fin offrait la plus petite résistance. Entre la transportabilité et la taille des grains, il y avait une relation négative, relation qui a pu être représentée par une fonction de puissance. Pour la même gamme de sédiments, les variations de la détachabilité étaient plus importantes que les variations de la transportabilité. Pour les sédiments grossiers, le transport par l'impact des gouttes d'eau était un processus sélectif. Finalement, les données obtenues par cette série d'expériences ont été combinées avec d'autres données, afin d'effectuer une première estimation du débit solide unitaire net déplacé en aval par l'impact des gouttes de pluie sur une pente découverte.

A STUDY OF RILL DEVELOPMENT PROCESS BASED ON FIELD EXPERIMENTS

Two rectangular experimental slopes are set on a bare slope in order to study the development process of rills (Figs. 1 and 2, Photo. 1). One slope is aimed to observe mainly the development pattern of rills (A-slope in Fig. 2) and another to measure the hydraulic conditions (B-slope in Fig. 2). Both have concrete partition walls except the lower end to eliminate the inflowing of water and sediment from outside the experimental slopes. The electrical resistivity survey, the cone penetration test and the grain-size analysis are also carried out to survey the soil conditions (Figs. 3, 4, 5 and 7). The results of them show that the soil conditions are nearly homogeneous in the traverse and change remarkably from the middle part to the lower part along the longitudinal profile. As for the hydraulic conditions, iL is recognized that the surface erosion occurrs when the rainfall intensity is above 1 mm/10 min (Fig. 8) and surface discharge is more closely related to the rainfall in 30 min than the rainfall in 60 min (Figs. 9 and 10). It is also observed that the sediment discharge per unit width at the end of the slope is nearly proportional to the square of the surface discharge (Fig. 13). Therefore, it can be considered that the sediment discharge per unit area at a certain place in such slopes is proportional to the length from the upper end to the place in a given rainfall intensity if the surface run-off coefficient is constant on the whole slope. Next, we shall discuss the erodibility of the slope by using _??_ where _??_ is the average erodibility coefficient, _??_the sediment discharge and the surface discharge at the end of the slope respectively and S the gradient of the slope. Using the values of _??_ and S obtained from the field experiment for the equation, we can obtain values of the coefficient key (Fig. 15). The figure shows that the value (_??_) decreases almost exponentially with the passing of time. This means that the erosible material of the slope decreases in the course of time. Then, let us investigate the relationship between the slope of the experiment plot and the model slope in order to examine the theoretical model which can validly apply to the development of rills in the model slope (Kashiwaya, 1979). The relationship between variables which play important roles for the rill morphology can be expressed as follows ;_??_(8-2) with D; drainage density, υ; flow velocity, ke; erosion proportionality factor, w; width of rill, ρ; density fluid, μ, viscosity of fluid and g; acceleration of gravity. Assuming that p, p and g take the same values both in the prototype and in the model, we can obtain_??_(9-1) and_??_ with RA; stream area number and Ho; Morton number, from eq. (8-2) by employing dimensional analysis. Therefore, it is said that the law of similarity can be established by using the above two equations. As for the number of rills in the steady state, let us introduce the next equation which is valid for the model slope; _??_ with k; the number of rills, p ; the coefficient expressing the relationship between the number of rills and the total width of rills, r; the ratio of the branching coefficient to the joining coefficient and N; the number of initial rills. It is derived from the stochastic differential equation based on the two hypotheses; the joining probability is proportional to the number of rills and the branching probability is proportional to the relative width (width/depth). In the present slope, assuming that the number of initial rills N is proportional to the slope length and r is in inverse to N (Fig. 17) and using the value of p given in the field experiment (Fig. 16), we can obtain the theoretical number of rills in the steady state of this slope.