Upslope Side Research Articles

Land use, management and climate effects on runoff and soil loss responses in the highlands of Ethiopia

Soil erosion by water is a major cause of land degradation in the highlands of Ethiopia and anywhere else in the world, but its magnitude and variability are rarely documented across land uses and climatological conditions. The purpose of this study was to examine runoff and soil loss responses under cropland (CL) and grazing land (GL) management practices in three climatic regions of the Ethiopian highlands: semi-arid (Mayleba), dry sub-humid (Gumara), and humid (Guder). We measured runoff and soil loss using runoff plots with and without soil and water conservation (SWC) measures (trenches, stone/soil bunds [embankments] with trenches on the upslope side, and exclosure) during the rainy season (July–September). The results revealed significant variation in runoff and soil loss amounts across land uses, SWC measures, and climatic regions. At Mayleba, seasonal runoff and soil loss in control plot were far higher from GL (280 mm, 26.5 t ha−1) than from CL (108 mm, 7.0 t ha−1) largely due to lack of protective vegetation cover and soil disruption because of intense grazing. In contrast, at Gumara and Guder, seasonal soil loss values were much higher from CL (21.4–71.2 t ha−1) than from GL (0.6–24.2 t ha−1) irrespective of runoff values. This was attributed to the excessive tillage/weeding operations involved in cultivation of teff (cereal crop) at Gumara and potato at Guder. Although SWC measures (practices) substantially reduced runoff and soil loss (decreased by 23%–86%) relative to control plot, seasonal soil loss under GL uses with trenches at Mayleba (12.6 t ha−1), CL with soil bunds and trenches at Gumara (22.1 t ha−1), and Guder (21.4 t ha−1) remained higher than the average tolerable soil loss rate (10 t ha−1 year−1) proposed for the Ethiopian highlands. This suggests that SWC measures should be carefully designed and evaluated specific to land use and climatic conditions. Overall, the results of this study can help improve SWC planning in regions where land use and climate impact on soil erosion vary across geographical areas, as they do in Ethiopia and anywhere else. However, further investigation is crucial with replication of measurements over years and locations to provide more accurate information on land use, management and climate controls on hydrological and soil erosion processes.

Intruding gravity currents and their recirculation in a rotating frame: Numerical results

Using a set of direct numerical simulations (DNS) we investigate the circulation of a buoyancy driven experiment in a circular rotating tank. The initial density structure is a two-layer stable-stratification. The water is densified at 32 points at the upslope side of a circular incline and flows downward as a gravity current before it intrudes to the interior along the interface between the two layers. The water above recirculates to the densification points.When the buoyancy production is lower than the maximal Ekman transport, the gravity current proceeds in the laminar Ekman layer. In the opposite case, the water cascades down the incline in a no-stationary dynamics, turbulent transport becomes important and the boundary layer is thickened. The thermal wind relation applied to the tangential velocity averaged-along-the-slope predicts the density structure averaged-along-the-slope in the domain.When the densified water arrives at the interface it flows along the slope in the cyclonic direction, in a geostrophically adjusted deep boundary current. The distance of the boundary current from the slope is given by inertial overshoot of the Ekman current due to the self advection by the inertia. When this distance is small the boundary current is close to the slope and subject to boundary friction. In the opposite case the boundary current evolves detached from the boundary. The intrusion along the interface, in the (negative) radial direction, is slow, as the intruding distance is proportional to the square-root of time.The recirculation in the upper layer is close to a geostrophic equilibrium. Its quasi-geostrophic evolution is due to the compression of the water column, caused by the gravity and intruding currents, below. This leads to a large-scale anti-cyclonic circulation in the upper layer. When the associated geostrophic surface pressure gradient exceeds the pressured gradient due to the dense water on the slope, the Ekman transport is upslope.A special emphasis is put on the interactions of the three phases of the circulation: gravity-current, intrusion and recirculation. The circulation and the mixing of density and momentum in the gravity current determine the water-mass properties in the deep boundary current. Both determine the recirculation. The recirculation leads to an increasing anti-cyclonic circulation above the gravity current which, through the corresponding surface pressure gradient, reduces and finally blocs the downslope movement in the Ekman layer.

Hydrochemical Assessment of Groundwater around Lapite Dumpsite for Irrigation Water Quality in Ibadan, Southwestern Nigeria

Due to the increase in population and industrialization growth, most countries in the world depend on groundwater to meet agriculture demands for food production. The increase in water contamination due to indiscriminate solid wastes has necessitated the assessment of water quality and its suitability for agricultural usage. Twenty four groundwater and ten stream water samples were randomly collected from the downslope and upslope side of the dumpsite for all the major physio-chemical parameters. The pH of water samples indicates slightly acidic to alkaline in nature. High concentrations of nitrate, total dissolved solids and electrical conductivity suggest the impact of the waste on the water resource. Assessment of irrigation water quality based on Sodium Adsorption Ratio (SAR), Soluble Sodium Percentage (SSP), Permeability Index (PI) and Magnesium content (MC) indicated that most of the water samples fall within the permissible standard for irrigation water. Thus, the water should be used with caution on crops for long time agricultural sustainability.

Dynamic interaction of soil and end-bearing piles in sloping ground: Numerical simulation and analytical solution

The effects of sloping ground on the vertical pile-to-pile interaction for end-bearing piles embedded in a homogenous soil are investigated through three-dimensional finite element simulations. The wave propagation mechanism and pile to pile interaction factor are investigated in detail from displacement and axial force perspectives. For contrast, soil attenuation and wave diffraction induced by a vertically vibrating end-bearing pile in horizontal ground are also studied. The highlight is put on the amplitude and phase lag in soil-pile system to reveal the essential characteristics of dynamic pile-pile interaction. Numerical results show that the dynamic displacement of a free field is position dependent. Moreover, the interaction factor of receiver pile on the upslope side is larger than the one on the downslope, since the longer embedded segment absorbs a greater amount of energy from the surrounding soil. On the basis of the numerical findings, an analytical model is established to calculate the dynamic responses of pile groups in sloping ground.

How roots of Picea abies and Fraxinus excelsior plantations contribute to soil strength and slope stability: evidence from a study case in the Hyrcanian Forest, Iran

Although the effects of different vegetation cover on soil reinforcement due to their roots have been addressed in the literature, there remains a lack of assessment and comparison of root biomechanical properties of hardwood and softwood plantations for similar site conditions. To address this gap, two adjacent hillslopes with similar site conditions but different forest plantations, a Picea abies (softwood) and a Fraxinus excelsior (hardwood), were selected to assess and compare their effectiveness in protecting sloping soils. The profile trench method was used to obtain root distribution from both upslope and downslope sides of tree samples and on each side at two horizontal distances from the tree stems. Root tensile strength of live root samples was measured using a standard Instron Universal Testing Machine. A modified Wu and Waldron root reinforcement model was used to calculate root cohesion for the two plantations. The root tensile strength for was significantly greater for softwood than for hardwood trees (19.31 ± 2.64 vs 16.98 ± 1.01 MPa). Interestingly, the number of roots, root area ratio values, and the root tensile strength of the two species did not significantly differ between the upslope and downslope sides of trees. The results also showed a higher root cohesion for the softwood than the hardwood species (1.56 ± 0.34 vs 1.03 ± 0.21 kPa). In addition, softwood trees extended their contribution to soil protection to a larger horizontal distance compared with hardwood. However, our findings generally revealed that the values of root cohesion of both studied plantations were surprisingly lower than those found in earlier reports.

A landslide induced by the 2016 Kumamoto Earthquake adjacent to tectonic displacement - Generation mechanism and long-term monitoring

The 2016 Kumamoto earthquake, which occurred in the central Kyushu region of Southwest Japan, caused numerous landslides. Mass movements from the events damaged many structures in the Minami-aso village residential areas. We conducted geological and geophysical surveys and monitoring of landslides in one of these residential areas. It was found that the landslide occurred directly adjacent to the earthquake's main surface fault-ruptures. While the landslide slip surface formed within a layer of hydrothermally altered rhyolite at about 15 m depth for the downslope side, the slip surface for the upslope side formed within intact tephra layers due to the shallow dip of the rhyolite layer. Geophysical survey results show that fault ruptures in the tephra layers along with the tectonic faulting induce a decrease in soil strength. It was inferred that the landslide head scarp and upper slip surface formed inside this weak soil. Long-term monitoring of the landslide demonstrated that deformation continued for at least four months after the main slide. Although the initial and the major slide was activated by the earthquake, the landslide did not immediately stabilize and continued to pose risks after the main event. Toward the mitigation for earthquake-induced landslides, post seismic monitoring as well as the relationship between surface fault traces and landslide blocks should be taken into consideration.

Mating trackways of a fossil giant millipede

Three closely associated arthropleurid trackways, Diplichnites cuithensis , from the Lower Carboniferous of Fife, Scotland, exhibit signs of interaction between track-makers. An extra file of footprints is found on the downslope side of two trackways (A and C), the upslope side of another (B). These additional files of footprints suggest that either: each trackway resulted from two arthropleurids of different sizes walking in tandem, matching their footprints exactly on one side for some distance; or that one arthropleurid was partially mounted on the back of another producing the three parallel files. It is here argued that the latter is correct and that this represents evidence of mating behaviour in arthropleurids.

Solutions for Soil-Pile-Soil Forces in Pile Stabilized Slopes

This paper introduces a mathematical procedure to analyze slope/pile systems. Limiting equilibrium method of slices is extended to account for the pile in the slope. Force and moment equilibrium for each individual slice is satisfied. The proposed procedure allows two separate predefined failure slip surfaces (one in the upslope side and the other in the downslope side) with a unique factor of safety for each slip surface. An illustrative example is presented to elucidate the use of the solution in comprehending the interrelationships among the pile location, the desired factor of safety of the slope/pile system, and the interactive soil/pile forces.

Low-velocity impact cratering experiments in granular slopes

Low-velocity impact cratering experiments are conducted in sloped granular targets to study the effect of the slope angle θ on the crater shape and its scales. We use two types of granular matter, sand and glass beads, former of which has a larger friction coefficient μs=tanθr, where θr is the angle of repose. Experiments show that as θ increases, the crater becomes shallower and elongated in the direction of the slope. Furthermore the crater floor steepens in the upslope side and a thick rim forms in the downslope side, thus forming an asymmetric profile. High-speed images show that these features are results of ejecta being dispersed farther towards the downslope side and the subsequent avalanche which buries much of the crater floor. Such asymmetric ejecta dispersal can be explained by combining the Z-model and a ballistic model. Using the topographic maps of the craters, we classify crater shape regimes I-III, which transition with increasing θ : a full-rim crater (I), a broken-rim crater (II), and a depression (III). The critical θ for the regime transitions are larger for sand compared to glass beads, but collapse to close values when we use a normalized slope θ^=tanθ/tanθr. Similarly we derive θ^-dependences of the scaled crater depth, length, width and their ratios which collapse the results for different targets and impact energies. We compare the crater profiles formed in our experiments with deep craters on asteroid Vesta and find that some of the scaled profiles nearly overlap and many have similar depth / length ratios. This suggests that these Vestan craters may also have formed in the gravity regime and that the formation process can be approximated by a granular flow with a similar effective friction coefficient.

On the Initiation of an Isolated Heavy-Rain-Producing Storm near the Central Urban Area of Beijing Metropolitan Region

Abstract An isolated heavy-rain-producing thunderstorm was unexpectedly initiated in the afternoon of 9 August 2011 near the central urban area of the Beijing metropolitan region (BMR), which occurred at some distance from BMR’s northwestern mountains and two preexisting mesoscale convective systems (MCSs) to the west and north, respectively. An observational analysis shows the presence of unfavorable quasigeostrophic conditions but a favorable regional environment for the convective initiation (CI) of thunderstorms. A nested-grid cloud-resolving model simulation of the case with the finest 1.333-km resolution is performed to examine the CI of the thunderstorm and its subsequent growth. Results reveal that the growth of the mixed boundary layer, enhanced by the urban heat island (UHI) effects, accounts for the formation of a thin layer of clouds at the boundary layer top at the CI site and nearby locations as well as on the upslope sides of the mountains. It takes about 36 min for the latent-heating-driven updraft to penetrate through a 1-km “lid” layer above before the formation of the thunderstorm. However, this storm may not take place without sustained low-level convergence of a prevailing southerly flow with a northerly flow ahead of a cold outflow boundary associated with the northern MCS. The latter is driven by the latent heating of the shallow layer of clouds during the earlier CI stage and then a cold mesohigh underneath the northern MCS. This study indicates the important roles of the urban effects, mountain morphology, and convectively generated pressure perturbations in determining the CI location and timing of isolated thunderstorms during the summer months.

Effects of Hurricane-Felled Tree Trunks on Soil Carbon, Nitrogen, Microbial Biomass, and Root Length in a Wet Tropical Forest

Decaying coarse woody debris can affect the underlying soil either by augmenting nutrients that can be exploited by tree roots, or by diminishing nutrient availability through stimulation of microbial nutrient immobilization. We analyzed C, N, microbial biomass C and root length in closely paired soil samples taken under versus 20–50 cm away from large trunks of two species felled by Hugo (1989) and Georges (1998) three times during wet and dry seasons over the two years following the study conducted by Georges. Soil microbial biomass, % C and % N were significantly higher under than away from logs felled by both hurricanes (i.e., 1989 and 1998), at all sampling times and at both depths (0–10 and 10–20 cm). Frass from wood boring beetles may contribute to early effects. Root length was greater away from logs during the dry season, and under logs in the wet season. Root length was correlated with microbial biomass C, soil N and soil moisture (R = 0.36, 0.18, and 0.27, respectively; all p values < 0.05). Microbial biomass C varied significantly among seasons but differences between positions (under vs. away) were only suggestive. Microbial C was correlated with soil N (R = 0.35). Surface soil on the upslope side of the logs had significantly more N and microbial biomass, likely from accumulation of leaf litter above the logs on steep slopes. We conclude that decaying wood can provide ephemeral resources that are exploited by tree roots during some seasons.

Domination of hillslope denudation by tree uprooting in an old-growth forest

Razula forest preserve in the Carpathian Mountains of the Czech Republic is an unmanaged forest that has not been logged or otherwise anthropically disturbed for at least 83years, preceded by only infrequent selective logging. We examined this 25ha area to determine the dominant geomorphological processes on the hillslope. Tree uprooting displaces about 2.9m3 of soil and regolith per year, representing about 1.5uprootedtreesha−1yr−1, based on forest inventory records dating back to 1972, and contemporary measurements of displaced soil and pit-mound topography resulting from uprooting. Pits and mounds occupy >14% of the ground surface. Despite typical slope gradients of 0.05mm−1, and up to 0.41, little evidence of mass wasting (e.g., slump or flow scars or deposits, colluvial deposits) was noted in the field, except in association with pit-mound pairs. Small avalanche and ravel features are common on the upslope side of uproot pits. Surface runoff features were rare and poorly connected, but do include stemwash erosion associated with stemflow. No rills or channels were found above the valley bottom area, and only small, localized areas of erosion and forest litter debris indicating overland flow. Where these features occurred, they either disappeared a short distance downslope (indicating infiltration), or indicate flow into tree throw pits. Surface erosion is also inhibited by surface armoring of coarse rock fragments associated with uprooting, as well as by the nearly complete vegetation and litter cover. These results show that the combination of direct and indirect impacts of tree uprooting can dominate slope processes in old-growth, unmanaged forests. The greater observed expression of different hillslope processes in adjacent managed forests (where tree uprooting dynamics are blocked by management activities) suggests that human interventions can change the slope process regime in forest ecosystems.

Assessing and correcting topographic effects on forest canopy height retrieval using airborne LiDAR data.

Topography affects forest canopy height retrieval based on airborne Light Detection and Ranging (LiDAR) data a lot. This paper proposes a method for correcting deviations caused by topography based on individual tree crown segmentation. The point cloud of an individual tree was extracted according to crown boundaries of isolated individual trees from digital orthophoto maps (DOMs). Normalized canopy height was calculated by subtracting the elevation of centres of gravity from the elevation of point cloud. First, individual tree crown boundaries are obtained by carrying out segmentation on the DOM. Second, point clouds of the individual trees are extracted based on the boundaries. Third, precise DEM is derived from the point cloud which is classified by a multi-scale curvature classification algorithm. Finally, a height weighted correction method is applied to correct the topological effects. The method is applied to LiDAR data acquired in South China, and its effectiveness is tested using 41 field survey plots. The results show that the terrain impacts the canopy height of individual trees in that the downslope side of the tree trunk is elevated and the upslope side is depressed. This further affects the extraction of the location and crown of individual trees. A strong correlation was detected between the slope gradient and the proportions of returns with height differences more than 0.3, 0.5 and 0.8 m in the total returns, with coefficient of determination R2 of 0.83, 0.76, and 0.60 (n = 41), respectively.

Effect of terrace forms on water and tillage erosion on a hilly landscape in the Yangtze River Basin, China

In long-term agricultural practices, cultivated land on long slopes in hilly areas is sometimes terraced to diminish soil loss caused by water and facilitate farming operations. In the Upper Yangtze River Basin, the forms of the terraces vary with the landform, soil resources and traditional techniques used in different areas. This study assessed the pattern of soil redistribution and soil particle size redistribution for two major types of terraces: with and without embankments. Samples of 137Cs and soil texture were collected at the upper and lower parts of terraces, and at an interval of 5m along the transect of the toposequence of individual terraces. The non-embankment terrace (NET) landscape showed increasing 137Cs downslope, whereas the embankment terrace (ET) landscape exhibited an abrupt change in 137Cs at the embankment. Tillage erosion dominated the soil redistribution, with smaller contributions made by water erosion in the ET landscape. In addition to tillage erosion, water erosion played an important role in the soil redistribution in the NET landscape, resulting in a net soil loss at both the upper and lower parts of the terrace. Soil fine particle fractions exhibited a trend of gradual increase along the transect of the toposequence in the NET landscape, and a similar fraction of fine particles was found at the upper and lower parts of the terrace. The establishment of embankments at the lower end of the terrace obstructed the formation and development of overland water flow, thereby creating a line of null downslope soil transport leading to tillage-induced soil accumulation on the upslope side of the embankment with little granulometric sorting. In the NET landscape, soil loss seemed to increase as a result of tillage erosion (an important delivery mechanism for water erosion), accelerating the on-site soil loss. The embankments at the lower end of the sloping terrace played a crucial role in the soil redistribution patterns of the toposequence, resulting in a shift from a water-dominated erosion process to a tillage-dominated process and a distinct pattern of soil particle size distribution.

Assessment and analysis of rockfall-caused tree injuries in a Turkish fir stand: A case study from Kastamonu-Turkey

Rockfalls can cause serious damage to people, property, facilities and transportation corridors. Furthermore, rockfalls are major hazards in mountain areas with negative impacts on individual trees and forested ecosystems. We conducted a study of rockfall events on 117 mapped (91% of total trees in the stand with > 1.3 m in height and > 5 cm diameter at breast height) Turkish fir trees (Abies bornmuelleriana Mattf.) in a stand within the Kayaarkasi-Topcular Village, Inebolu district, Kastamonu province of Turkey. The study site of 0.35 Ha is located on the transition zone of frequently passing rockfall fragments (∼ 40 cm in diameter) generally causing healable injuries. Parameters of trees and injuries were recorded and analysed as to injury number, height and size. Bivariate correlation analysis were used to investigate the relationships between: a) diameter at breast height and number of injuries per tree, b) diameter at breast height and total injury size, c) the number of injuries and total injury size and d) the number of injuries per tree and distance from the source of the rockfall area. Results indicate that the average height of injury, average number of injuries and average injury area to be 81.3 cm (STDEV: 49.8), 7.46 (STDEV: 4.4) and 628.6 cm2 (STDEV: 678.2), respectively. In total 84% of all injuries were recorded within 160° sector at the upslope side of trees and callus tissue that had closed wounds was observed in 79.1% of all injuries. Furthermore 14.5% of injured trees had callus tissue in the process of closing wounds while 6.4% of injuries had not formed any callus tissue. The most common injury types were bark and wood injuries. Bivariate correlation analysis indicated strong relationships between diameter at breast height and the number of injuries (rs = 0.524), injury number and total injury area (rs = 0.653) and distance from rockfall area and injury numbers relations (rs = −0.518). A weaker relationship was found between diameter at breast height and total injury area (rs = 0.363). These results indicate that bigger trees are more prone to rockfall injuries. As expected, trees further from the rockfall area seem to be less prone to rockfall injuries. From our results, it can be inferred that the protection of people and property can be increased through the maintanance of forest in areas immediately below areas prone to rockfall. This stand is still managed in selective forest management system. In order to protect the settlement at the deposition zone it has to manage for protective goals with uneven-aged and multilayered stand structure.

Attack Pattern of Platypus koryoensis (Coleoptera: Curculionidae: Platypodinae) in Relation to Crown Dieback of Mongolian Oak in Korea

The ambrosia beetle, Platypus koryoensis (Murayama), vectors the Korean oak wilt (KOW) pathogen, Raffaelea quercus-mongolicae K.H. Kim, Y.J. Choi, & H.D. Shin, in Korea, which is highly lethal to Mongolian oak, Quercus mongolica Fisch., and is considered a major threat to forest ecosystem health. We characterized the attack pattern of P. koryoensis along the lower trunk of 240 Mongolian oaks in relation to tree decline symptoms on Mt. Uam in Gyeonggi-Do Province, Korea during June-July 2009. For each tree, we recorded diameter at breast height (dbh) (DBH) and P. koryoensis entrance hole density at two heights along the lower trunk (near groundline and at 1.5 m above groundline) and on opposite sides (downslope side and upslope side). Trees were assigned to one of three dieback classes: 1) apparently healthy, no or practically no wilted foliage, and no obvious platypodine frass near the base of the tree; 2) no or only partial wilting with obvious frass near the base of the tree; and 3) apparently recently killed by KOW with all foliage wilted and mostly retained with obvious frass near the base of the tree. As dieback class increased from 1 to 3, P. koryoensis entrance hole density increased at all four trunk locations. Attack density was highest on the downslope side of the trunk near groundline, and principal component analysis indicated that this trunk location was the best indicator of tree dieback. In addition, DBH tended to increase with dieback class suggesting that larger trees were infested first.

Predicting the impact of linear landscape elements on surface runoff, soil erosion, and sedimentation in the Wahnbach catchment, Germany

AbstractModel predictions concerning the endangerment of on‐site and off‐site damages due to runoff, soil erosion and sedimentation under alternative design and operation policies are of particular importance in recent catchment planning and management. By using the raster‐based model approach, linear landscape elements, such as streets and roads, and their impacts on flow paths are often neglected. Therefore, the aim of this study was to analyse the effects of linear landscape elements on patterns of soil erosion, sediment transport and sedimentation. To accomplish this, roads are considered while determining flow paths. Simulations in the well‐investigated catchment of the Wahnbach River (54 km²) in a low mountain range in Germany were carried out using a combination of different models for hydrology and soil erosion. Although the study focuses on the catchment scale of the Wahnbach River, detailed investigations concerning the sub‐catchment scale (21 ha) were also conducted. The simulation results show that these spatial structures mainly affect the pattern of soil erosion and sedimentation. On the sub‐catchment scale, improved identification of active zones for sediment dynamic becomes possible. On the catchment scale, the predicted runoff is about 20% higher, and sediment outputs were four times larger than predicted when roads were considered. Soil erosion increases by 37% whereas sedimentation is reduced by 29%. The model improvement could not be evaluated on the catchment scale because of the high variability and heterogeneity of land use and soils, but road impacts could be explained by simulations on the sub‐catchment scale. It can be concluded that runoff concentration due to rerouted flow paths leads to lower non‐concentrated and higher concentric‐linear surface runoff. Thus, infiltration losses decline and surface runoff and soil erosion increase because sedimentation is reduced. Further, runoff concentration can cause soil erosion hot spots. In the model concept used in this study, buffering of runoff and sediments on the upslope side of roads and in local depressions adjacent to roads cannot be simulated. Flow paths will only be rerouted because of road impacts, but the temporal ponding of water is not simulated. Therefore, the drastic increase of predicted sediment output due to road impact does not seem to be reliable. However, results indicate that the consideration of roads when determining flow paths enabled more detailed simulations of surface runoff, soil erosion and sedimentation. Thus, progress in model‐based decision‐making support for river catchment planning and management can be achieved. Copyright © 2011 John Wiley & Sons, Ltd.

The Effect of Slide Mass and Pile Stiffness on Residual Thrust on Stabilizing Piles

As for stabilizing piles used to reinforce landslides, landslide thrust acts on their upslope sides but slope resistance acts on their downslope sides at the same time. In fact, the results that the former subtracts the latter are effective loads on piles, which can be called residual thrust on piles. The interaction between pile and rock or soil mass is simulated by elastoplastic finite element method in the paper. And the effect of slide mass and pile stiffness on residual thrust on stabilizing piles is analyzed. The results show that residual thrust on a stabilizing pile become more and more with elastic modulus of slide mass or the pile being increased. Meanwhile, action point of the resultant of residual thrust on the pile is lower and lower with elastic modulus of slide mass being increased but it is higher and higher to some extent with elastic modulus of the pile being increased. These results can be of theoretical reference for comparison and selection of various design plans of stabilizing piles.

Biogeomorphological influence of slope processes and sedimentology on vascular talus vegetation in the southern Cascades, California

The vascular vegetation of alpine talus slopes between 2035 and 3095 m altitude was studied at Lassen Volcanic National Park (California) in the Cascade Range. Taluses show a diverse flora, with 79 plant species; growth forms include coniferous trees, shrubs, suffrutices, herbs, graminoids, and ferns. Spatial patterns of plant distribution were studied along 40 point-intercept transects. Plant cover was low (0–32.7%) on all slopes, spatially variable, and showed no consistent trends. Sedimentological characteristics were determined by photosieving next to 1500 plants; this census indicated preferential plant growth on blocks and cobbles, with 43.2% and 23.3% of the plants growing on these stones, respectively; fewer specimens were rooted on pebbles (13%) or on stone-free gravel areas (20.5%). Growth forms displayed different substrate preferences: 92.5% of the shrubs and 83% of the suffrutices colonized blocks or cobbles, but only 57.2% of the herbs and 59.8% of the graminoids grew on large stones. Plants are associated with large clasts because (1) coarse talus is more stable than fine sediment areas, which are more frequently disturbed by various geomorphic processes, and (2) large stones help conserve substrate water beneath them while moisture quickly evaporates from fine debris. Root patterns were studied for 30 plant species; 10 specimens for each species were excavated and inspected, and several root growth ratios calculated. All species exhibited pronounced root asymmetry, as roots for most plants grew upslope from their shoot base. For 23 species, all specimens had 100% of their roots growing upslope; for the other 7 species, 92.2–99.3% of below-ground biomass extended uphill. This uneven root distribution is ascribed to continual substrate instability and resulting talus shift; as cascading debris progressively buries roots and stems, plants are gradually pushed and/or stretched downhill. Various disturbance events affect root development. Slope erosion following rubble removal often exposes plant roots. Debris deposition can completely bury plants; some may survive sedimentation, producing new shoots that grow through accumulated debris. Shrubs may propagate by layering, as adventitious roots develop along buried stems; or produce new clones along their roots. Slope processes may damage and transport plant pieces downhill; some species can sprout from severed, displaced root or stem fragments. Vegetation interacts with several geomorphic processes, including debris flows, grain flows, rockfall, snow avalanches, frost creep, and runoff. Larger plants may alter local patterns of debris movement and deposition, damming cascading debris on their upslope side and deflecting sediments laterally to plant margins, where they form narrow elongated stone stripes.

Mixing in the stratified interface of the Faroe Bank Channel overflow: The role of transverse circulation and internal waves

The overflow of cold water across the Faroe Bank Channel sill is a significant volume flux of dense water to the North Atlantic Ocean. Using observations of hydrography, current and microstructure from a 1 week cruise and 2 month long time series from moored instruments, we address the role of transverse circulation and internal waves in mixing in the stratified 100 m thick plume‐ambient interface. The streamwise momentum budget is dominated by a balance between the pressure gradient and bottom friction; the entrainment stress is negligible. The transverse momentum budget is in geostrophic balance, and the transverse velocity variability is governed by the internal streamwise pressure gradient. The transverse geostrophic flow in the interfacial layer is opposed by the bottom Ekman transport. The shear associated with the interfacial jet lowers the Richardson number and enhances dissipation rates. Convective overturning events observed on the upslope side suggest a link between the transverse circulation and the vertical mixing on the upper slope. Several independent threads of evidence support the transverse circulation as an important mixing mechanism for the overflow plume. In the ambient, dissipation rates inferred from fine‐scale shear and density profile measurements are in good agreement with direct measurements, supporting internal wave breaking as a dominant mechanism for dissipation of turbulent energy. In the interfacial layer, spectral distribution of internal wavefield is energetic. In addition to shear‐induced mixing and entrainment in the interfacial layer, internal wave breaking is likely to be important for the dissipation of turbulent energy and should not be ignored.