Frost sorting on slopes by needle ice: A laboratory simulation on the effect of slope gradient

Following a previous attempt to reproduce miniature sorted patterns on a level surface, we report the results of a full‐scale laboratory simulation on frost sorting produced by needle ice activity on inclined surfaces. Four models, with different slope gradients (5°, 7°, 9°, 11°), were designed. Stones 6 mm in diameter placed in a grid covered 20% of the surface of frost‐susceptible water‐saturated soil. These models were subjected to 20–40 freeze–thaw cycles between 10°C and ?5°C in 12 hours. The evolution of surface patterns was visually traced by photogrammetry. Needle ice growth and collapse induced downslope movement and concentrations of stones. A model produced incipient sorted circles on a 5° slope, whereas it resulted in three distinct sorted stripes on a 7° slope. The average diameter or spacing of these forms is 9.7–19.4 cm, comparable to those in the field dominated by diurnal freeze–thaw cycles. Surface parallel displacements of stone markers were traced with motion analysis software. The observed downslope stone displacements agree with those expected assuming that surface soil and stones move by repeated heaving perpendicular to the surface and vertical settlement due to gravity, although the growth of curved needle adds complexity to the overall displacements. Copyright © 2017 John Wiley & Sons, Ltd.     

Effects of slope angle and aspect on plant cover and species richness in a humid Mediterranean badland

Soil erosion is one of the most severe land degradation processes in the Mediterranean region. Although badlands occupy a relatively small fraction of the Mediterranean area, their erosion rates are very high. Many studies have investigated to what extent vegetation controls soil erosion rates. This study, however, deals with the impact of erosion on vegetation establishment. In semi‐arid badlands of the Mediterranean, soil water availability constitutes the main limiting factor for vegetation development. As a consequence, south‐facing slopes are typically less vegetated due to a very large water stress. However, these findings do not necessarily apply to humid badlands. The main objective of this paper is to determine the topographic thresholds for plant colonization in relation to slope aspect and to assess the spatial patterns of vegetation cover and species richness. We surveyed 179 plots on highly eroded badland slopes in the Central Pyrenees. We defined four aspect classes subdivided into slope angle classes. Colonization success was expressed in terms of vegetation cover and species richness. Slope angle thresholds for plant colonization were identified for each slope aspect class by means of binary logistic regressions. The results show that a critical slope angle exists below which plants colonize the badland slopes. Below this critical slope angle, plant cover and species richness increase with a decreasing slope angle. The largest critical slope angles in humid badlands are observed on south‐facing slopes, which contrasts with the results obtained in semi‐arid badlands. North‐facing slopes however are characterized by a reduced overall vegetation cover and species richness, and lower topographic threshold values. The possible underlying processes responsible for this slope‐aspect discrepancy in vegetation characteristics are discussed in terms of environmental variables that control regolith development, weathering and erosion processes. Moreover, possible restoration strategies through the use of vegetation in highly degraded environments are highlighted. Copyright © 2014 John Wiley & Sons, Ltd.     

The influence of freeze–thaw cycles of active soil layer on surface runoff in a permafrost watershed

As a result of global warming, the discharges from rivers in permafrost regions have varied significantly. However, its mechanism remains unclear. One of possible factors is active soil freeze–thaw cycle, which may influence surface runoff in the variation of permafrost water cycle processes. In this study, a typical permafrost watershed in the Qinghai-Tibet plateau was selected, its hydrological processes were monitored from 2004 to 2007, and the effects of the freezing and thawing depth of the soil active layer on runoff processes were assessed. The runoff modulus, runoff coefficient, direct runoff ratio, recession gradient and their seasonal variations were estimated and analyzed. The active soil dynamics and water budget were analyzed to prove the features of the surface runoff and the influences of active soil freeze–thaw processes. The primary factors influencing surface runoff processes during different seasons were analyzed by Principal Component Analysis (PCA) and statistical regression methods. The results showed that the high runoff coefficient and low direct runoff ratio were the main characteristics during the spring flood period (May–June) and during the autumn recession period (September). The runoff modulus and its year-to-year variability were the greatest in the summer flood period. The direct runoff ratio decreased from 0.43 in May to 0.29 in September, with the exception of the highest ratio, which occurred during the summer recession period (July). The active soil thawing in the upper layer of depth of 60 cm had contributed to increase in discharge, but the increase in thawing depth deeper than 60 cm led to a decrease in surface runoff and slowness in the recession process. Precipitation played a small role in the spring flood runoff and the autumn runoff. The soil active layer freeze–thaw variation, which affected seasonal soil water dynamic and water budget and reformed seasonal runoff characteristics, along with vegetation cover changes, is considered the potential major factor in control of the hydrological processes in the permafrost region.     

Quantifying the effect of a freeze–thaw cycle on soil erosion: laboratory experiments

In this paper we quantitatively test the hypothesis that soil freeze–thaw (FT) processes significantly increase the potential for upland hillslope erosion during run‐off events that follow thaw. We selected a highly frost‐susceptible silt to obtain an upper bound on FT effects, and completed three series of six experiments each to quantify differences in soil erosion and rill development in a bare soil following a single FT cycle. Each series represented a specific soil moisture range: 16–18 per cent, 27–30 per cent and 37–40 per cent by volume, with nominal flow rates of 0·4, 1·2 and 2·4 L/min and slopes of 8° and 15°. Each experiment used two identical soil bins: one a control (C) that remained unfrozen, and another that was frozen and thawed once. Standard soil characterization tests did not detect significant differences between the FT and C bins. We measured cross‐sectional geometry of an imposed straight rectangular rill before each experiment, sediment load during and rill cross‐sections after. Changes in cross section provided detailed measures of erosion at specific locations, while sediment load from time series run‐off samples integrated the rill erosion. Several parameters, including average maximum rill width, average maximum rill depth, rill cross‐section depth measures and sediment load, all followed similar trends. Each was greater in the FT than in the C, with values that generally increased with slope and flow. However, soil moisture was the only parameter that affected the FT/C ratios. Average sediment load grouped by soil moisture provided FT/C ratios of 2·4, 3·0 and 5·0 for low, mid and high moisture, respectively. In contrast, a ‘dry’ experiment at 4–5 per cent soil moisture had FT/C of 1·02 for sediment load. These results show a dramatic increase with soil moisture in the rate and quantity of bare soil eroded due to the FT cycle. As both FT and C results were highly sensitive to initial conditions, minimum differences in soil weight, bulk density and soil moisture through each series of experiments were required to achieve consistent results, indicating that rill erosion may be chaotic. Published in 2005 by John Wiley & Sons, Ltd.     

An experimental evaluation of ice cover effects on the dynamic behaviour of a concrete gravity dam

An extensive forced‐vibration testing programme has been carried out on an 84‐m concrete gravity dam located in northeastern Québec, Canada. The dam was subjected to a harmonic load on the crest in summer and severe winter conditions with temperatures ranging from ?10°C to ?15°C and a 1.0–1.5m ice cover. Acceleration and hydrodynamic frequency responses were obtained in different locations on the dam and in the reservoir. The main objective of the repeated tests was to investigate the effects of the ice cover on the dynamic behaviour of the dam–reservoir–foundation system, by comparing summer and winter results. Modifications in damping and resonance frequencies were observed, as well as an additional resonance that was attributed to an interaction of the dam with the ice cover. These findings provided a reliable and unique database for the investigations of dam–reservoir–foundation interaction and, in particular, the ice‐cover effects for dams located in northern regions. Copyright © 2002 John Wiley & Sons, Ltd.     

Quantifying the spatial variability of melting seasonal ground ice and its influence on potential evapotranspiration spatial variability in a boreal peatland

Melting seasonal ground ice (SGI) in western Boreal Plains (WBP) peatlands can reduce the available energy at the surface by reducing potential evapotranspiration (PET). PET often exceeds annual precipitation in the WBP. Including this effect in hydrological models may be important in assessing water deficits. However, SGI melt and the timing of ice-free conditions vary spatially, which suggests PET spatial variability could be influenced by SGI. Understanding this potential linkage can help improve site scale PET in peatland hydrological models. The objectives of this paper were (a) to quantify the effect of ice thickness and melt rate on peatland PET; (b) quantify the spatial variability of SGI thickness and melt rate across spatial scales; and (c) assess how/if spatial variability in SGI thickness/melt rate affects site scale PET. Results from the sensitivity analysis indicated that SGI thickness had a bigger impact on reducing PET compared with the melt rate. Two SGI thickness values were used that were observed on site: 0.32 m, which was measured in a more treed area, and 0.18 m, which was in a more open area. The 0.32 m had an average PET reduction of 14 mm (±0.7), over the month of May, compared with 9 mm (±1 mm) when there was 0.18 m of SGI, which are 13.7 and 8.8% reductions, respectively. SGI thickness and melt rate, both exhibited large- and small-scale spatial variability. At the large scale, spatial patterns in SGI thickness appeared to be influenced by extensive shading from the adjacent hillslopes. Small scale, SGI thickness may be a function of tree proximity and the snowpack. Finally, net radiation, rather than SGI, appeared to be the main driver behind PET spatial variability. This work enhances our conceptual understanding of the role of SGI in WBP peatlands. Future work can use the findings to better inform peatland hydrological models, allowing for better representation of peatlands in regional-scale models.     

The influence of ground ice distribution on geomorphic dynamics since the Little Ice Age in proglacial areas of two cirque glacier systems

Holocene glaciers have contributed to an abundance of unstable sediments in mountainous environments. In permafrost environments, these sediments can contain ground ice and are subject to rapid geomorphic activity and evolution under condition of a warming climate. To understand the influence of ground ice distribution on this activity since the Little Ice Age (LIA), we have investigated the Pierre Ronde and Rognes proglacial areas, two cirque glacier systems located in the periglacial belt of the Mont Blanc massif. For the first time, electrical resistivity tomography, temperature data loggers and differential global positioning systems (dGPS) are combined with historical documents and glaciological data analysis to produce a complete study of evolution in time and space of these small landsystems since the LIA. This approach allows to explain spatial heterogeneity of current internal structure and dynamics. The studied sites are a complex assemblage of debris‐covered glacier, ice‐rich frozen debris and unfrozen debris. Ground ice distribution is related to former glacier thermal regime, isolating effect of debris cover, water supply to specific zones, and topography. In relation with this internal structure, present dynamics are dominated by rapid ice melt in the debris‐covered upper slopes, slow creep processes in marginal glacigenic rock glaciers, and weak, superficial reworking in deglaciated moraines. Since the LIA, geomorphic activity is mainly spatially restricted within the proglacial areas. Sediment exportation has occurred in a limited part of the former Rognes Glacier and through water pocket outburst flood and debris flows in Pierre Ronde. Both sites contributed little sediment supply to the downslope geomorphic system, rather by episodic events than by constant supply. In that way, during Holocene and even in a paraglacial context as the recent deglaciation, proglacial areas of cirque glaciers act mostly as sediment sinks, when active geomorphic processes are unable to evacuate sediment downslope, especially because of the slope angle weakness. Copyright © 2014 John Wiley & Sons, Ltd.     

Uranium series dates from the windy pits of the North York Moors,United Kingdom: implications for late Quaternary ice cover and timing of speleogenesis

Windy pits are open fissures within the Corallian strata of the North York Moors. Here we present nine thermal ionization mass spectrometry (TIMS) U–Th dates on speleothem from the windy pits, the first radiometric dates on calcite for these features and for this region. The dates cluster within three time periods: the interglacial peaks of marine isotope stage (MIS) 7 (～208 to ～190 ka), MIS 5 (～127 ka to ～109 ka) and the Holocene, correlating with the marine isotope record and with the broad pattern of interglacial calcite deposition observed for northern England. However, of the two high‐resolution studies available for comparison, the windy‐pit dates match only the Lancaster Hole dates. The location of the windy pits outside of the Devensian ice limits, within the ice‐free but periglaciated region, suggests that the region has remained ice‐free since some time before MIS 7. The prevalence of widespread and prolonged permafrost conditions suggests a plausible mechanism for these open rift caves in slope‐side periglacial mass movement. Copyright © 2008 John Wiley & Sons, Ltd.     

Pressure and shear stress caused by raindrop impact at the soil surface: Scaling laws depending on the water depth

Raindrop impact is an important process in soil erosion. Through its pressure and shear stress, raindrop impact causes a significant detachment of the soil material, making this material available for transport by sheet flow. Thanks to the accurate Navier–Stokes equations solver Gerris, we simulate the impact of a single raindrop of diameter D, at terminal velocity, on water layers of different thickness h: , , D, 2D, in order to study pressures and shear stresses involved in raindrop erosion. These complex numerical simulations help in understanding precisely the dynamics of the raindrop impact, quantifying in particular the pressure and the shear stress fields. A detailed analysis of these fields is performed and self‐similar structures are identified for the pressure and the shear stress on the soil surface. The evolution of these self‐similar structures are investigated as the aspect ratio h/D varies. We find that the pressure and the shear stress have a specific dependence on the ratio between the drop diameter and the water layer thickness, and that the scaling laws recently proposed in fluid mechanics are also applicable to raindrops, paving the road to obtain effective models of soil erosion by raindrops. In particular, we obtain a scaling law formula for the dependence of the maximum shear stress on the soil on the water depth, a quantity that is crucial for quantifying erosion materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Wind flow and wind loads on the surface of a towershaped building: Numerical simulations and wind tunnel experiment

Flow structure and wind pressure distribution caused by obtuse obstacles are usually the focuses in Computational Wind Engineer researches (CWE). By solving the non-hydrostatical dynamic equations, PUMA model (Peking University Model of Atmospheric Environment) was developed and applied to simulating the flow structure and wind pressure distribution around a tower-shaped building. Evaluation about the wind environment and wind loads around the building was obtained through the analysis of the numerical simulation results and wind tunnel data. Comparisons between the simulation and wind tunnel study indicate that numerical simulation results agree well in the flow field and wind pressure distribution around the tower-shaped building. On the other hand, the horizontal grid interval of 2 m and the vertical grid of 3 m were still too crude to simulate the flow structure and wind pressure distribution on the building surface more exactly in detail; and the absence of suitable pressure perturbation parameterization scheme between the solid and the adjacent space also limits the accuracy of the numerical simulation. The numerical simulation model can be used to evaluate the wind environment and wind load around high buildings.     

Impacts of sediment retention by dams on delta shoreline recession: evidences from the Krishna and Godavari deltas,India

River deltas are the major repositories of terrestrial sediment flux into the world's oceans. Reduction in riverine inputs into the deltas due to upstream damming might lead to a relative dominance of waves, tides and currents that are especially exacerbated by coastal subsidence and sea‐level rise ultimately affecting the delta environment. Analysis of multi‐date satellite imagery and maps covering the Krishna and Godavari deltas along the east coast of India revealed a net erosion of 76 km² area along the entire 336‐km‐long twin delta coast during the past 43 years (1965–2008) with a progressively increasing rate from 1·39 km² yr^?1 between 1965 and 1990, to 2·32 km² yr^?1 during 1990–2000 and more or less sustained at 2·25 km² yr^?1 during 2000–2008. At present the Krishna has almost become a closed basin with decreased water discharges into the delta from 61·88 km³ during 1951–1959 to 11·82 km³ by 2000–2008; and the suspended sediment loads from 9 million tons during 1966–1969 to as low as 0·4 million tons by 2000–2005. In the case of the Godavari delta, although the water discharge data do not show any major change, there was almost a three‐fold reduction in its suspended sediment loads from 150·2 million tons during 1970–1979 to 57·2 million tons by 2000–2006. A comparison of data on annual sediment loads recorded along the Krishna and Godavari Rivers showed consistently lower sediment quantities at the locations downstream of dams than at their upstream counterparts. Reports based on bathymetric surveys revealed considerable reduction in the storage capacities of reservoirs behind such dams. Apparently sediment retention at the dams is the main reason for the pronounced coastal erosion along the Krishna and Godavari deltas during the past four decades, which is coeval to the hectic dam construction activity in these river basins. Copyright © 2010 John Wiley and Sons, Ltd.     

Analytical and statistical approach for evaluating the effects of a river barrage on river–aquifer interactions

The construction of a river barrage can increase groundwater levels upstream of the barrage during the rainy season. Analytical and statistical approaches were applied to evaluate the relationship between groundwater and river water at the Changnyeong–Haman river barrage in Korea using time series data of water level and electrical conductivity from June 2011 to September 2014. An artificial neural network based time series model was designed to filter out the effect of rainfall from the groundwater level data in the study area. Aquifer diffusivity and river resistance were estimated from the analytical solution of a one‐dimensional unit step response function by using the filtered groundwater level data. River resistance increased in response to groundwater level fluctuations. Cross‐correlation analyses between the groundwater and the river water showed that the lag time increased during the observation period for both the water level and the electrical conductivity while the cross‐correlation function declined for the same period. The results indicated that a constant river stage maintained at the river barrage can weaken the hydrologic stress and reduce the exchange of material between the river and the adjacent aquifer because of the deposition of fine sediment on the river bottom and walls. Copyright © 2016 John Wiley & Sons, Ltd.     

Food web manipulation by extreme enhancement of piscivory: an invertebrate predator compensates for the effects of planktivorous fish on a plankton community

By enhancing the stock of piscivorous fish in a whole-lake experiment in Gräfenhain (Germany) since 1981 to such an extent that almost all planktivorous fish were exterminated, we examined the concept of “over-biomanipulation”. This hypothesis predicts that (a) extremely strong piscivory will allow uncontrolled development of large invertebrate predators and (b) these invertebrates can exert the same strong predation pressure on large herbivorous zooplankton as planktivorous fish. The hypothesis is tested the first time by a cross-comparison of the long-term response of the plankton community structure in the experimental lake (Piscivore L.) with (1) the intermediate response in the same lake and with (2) that of the long-term state in a nearby reference lake (Planktivore L.) densely inhabited by planktivorous fish (Leucaspius delineatus, a small cyprinid). The intermediate (1989–1992) response in Piscivore L. revealed a strong increase of the abundance of the invertebrate predator Chaoborus flavicans. Large daphnids were able to coexist with C. flavicans so that edible phytoplankton were suppressed and water transparency increased.As part (a) of the hypothesis predicts, the long-term response in Piscivore L. was characterized by the immigration of the larger predator Chaoborus obscuripes which displaced the smaller C. flavicans completely. The results support also part (b) of the hypothesis of “over-biomanipulation”: C. obscuripes-dominated Piscivore L. showed not much difference in biomasses of daphnids and total and edible phytoplankton as well as Secchi depth compared with fish-dominated Planktivore Lake. On the other hand, C. obscuripes-dominated Piscivore L. was characterized by distinctly lower biomass of daphnids, mean body volume of all crustaceans and Secchi depth as well as by higher biomass of edible phytoplankton compared with C. flavicans-dominated Piscivore Lake. We conclude that long-lasting success of biomanipulation cannot be achieved by extremely high piscivory leading to the almost complete extermination of planktivorous fish.     

Topographical effects on the distributions of rainfall and 18O distributions: a case in Miyake Island,Japan

In this paper, we try to calculate precipitation in Miyake Island, Japan. In order to know the temporal and spatial variations of precipitation, we have set 15 rain gauges randomly in the island to collect the monthly precipitation data since June 1994. It is found that the precipitation is very different from point to point. First, we used statistical methods to get the correlations between the monthly precipitation at our survey points and that at the weather station. Next, regression analyses were used to establish formulae to calculate precipitation as a function of altitude, aspect of the geomorphological surface and wind direction. Based on these results, distributions of monthly and yearly precipitation and δ¹⁸O over the island were assessed. The results show that landscape patterns strongly influence precipitation distribution over the island, with the highest precipitation being found on the windward side, about 400–600 m above sea level. Even at places at the same altitude, the precipitation was different because of the aspect of the landscape. At the same time, altitude effects for δ¹⁸O on both the windward and leeward sides were −0·10‰/100 m and −0·15‰/100 m, respectively. Comparing with the distribution of precipitation distribution, it was also found that δ¹⁸O for the windward and leeward sides was different from that for precipitation, which means that both topographical effects must be considered separately. © 1998 John Wiley & Sons, Ltd.     

From soil aggregates to riverine flocs: a laboratory experiment assessing the respective effects of soil type and flow shear stress on particles characteristics

Particles eroded from hillslopes and exported to rivers are recognized to be composite particles of high internal complexity. Their architecture and composition are known to influence their transport behaviour within the water column relative to discrete particles. To‐date, hillslope erosion studies consider aggregates to be stable once they are detached from the soil matrix. However, lowland rivers and estuaries studies often suggest that particle structure and dynamics are controlled by flocculation within the water column. In order to improve the understanding of particle dynamics along the continuum from hillslopes to the lowland river environment, soil particle behaviour was tested under controlled laboratory conditions. Seven flume erosion and deposition experiments, designed to simulate a natural erosive event, and five shear cell experiments were performed using three contrasting materials: two of them were poorly developed and as such can not be considered as soils, whilst the third one was a calcareous brown soil. These experiments revealed that soil aggregates were prone to disaggregation within the water column and that flocculation may affect their size distribution during transport. Large differences in effective particle size were found between soil types during the rising limb of the bed shear stress sequence. Indeed, at the maximum applied bed shear stress, the aggregated particles median diameter was found to be three times larger for the well‐developed soil than for the two others. Differences were smaller in the falling limb, suggesting that soil aggregates underwent structural changes. However, characterization of particles strength parameters showed that these changes did not fully turn soil aggregates into flocs, but rather into hybrid soil aggregate–floc particles. Copyright © 2013 John Wiley & Sons, Ltd.