Numerical modelling of aeolian erosion over rough surfaces

The presence of non‐erodible roughness elements on erodible surfaces has the effect of absorbing part of the wind shear stress and thus protecting the erodible surface from wind erosion. This paper examines the shear stress distribution over roughness arrays of varying density, representing the progress of erosion on a bed of erodible and non‐erodible particles. Three‐dimensional numerical simulations, simulating wind flow over a bed of particles covered by roughness elements, were conducted in order to investigate the effect of roughness elements on the shear stress near the surface. The results of these simulations confirm that the erosion of soil by wind is strongly attenuated by the presence of roughness elements on the surface and depends on the geometric properties of the roughness elements. Based on the new numerical results obtained, a refinement of existing theoretical approaches is developed to describe the dependence of the friction velocity upon roughness frontal area and real exposed cover rate. The new formulation proposed will allow a more accurate evaluation of shear stress partitioning as a function of topographic changes. Copyright © 2010 John Wiley & Sons, Ltd.     

Influence of lithologic erodibility on alluvial fan area,western white mountains,California and Nevada

Differences in lithologic erodibility and sediment storage within a drainage basin affect the relationship between alluvial fan area and drainage basin area along the western White Mountains. Large fans are produced by basins underlain by resistant rocks, which have steep and narrow trunk stream canyons with little sediment in storage. Small fans are produced by basins composed of erodible lithologic units, which have wider valley floors, lower valley-side slopes, and considerably more sediment stored along trunk stream canyons than is the case in basins underlain by resistant rocks.     

Laboratory soil piping and internal erosion experiments: evaluation of a soil piping model for low‐compacted soils

Mechanistic models have been proposed for soil piping and internal erosion on well‐compacted levees and dams, but limited research has evaluated these models in less compacted (more erodible) soils typical of hillslopes and streambanks. This study utilized a soil box (50 cm long, 50 cm wide and 20 cm tall) to conduct constant‐head, soil pipe and internal erosion experiments for two soils (clay loam from Dry Creek and sandy loam from Cow Creek streambanks) packed at uniform bulk densities. Initial gravimetric moisture contents prior to packing were 10, 12 and 14% for Dry Creek soil and 8, 12, and 14% for Cow Creek soil. A 1‐cm diameter rod was placed horizontally along the length of the soil bed during packing and carefully removed after packing to create a continuous soil pipe. A constant head was maintained at the inflow end. Flow rates and sediment concentrations were measured from the pipe outlet. Replicate submerged jet erosion tests (JETs) were conducted to derive erodibility parameters for repacked samples at the same moisture contents. Flow rates from the box experiments were used to calibrate the mechanistic model. The influence of the initial moisture content was apparent, with some pipes (8% moisture content) expanding so fast that limited data was collected. The mechanistic model was able to estimate equivalent flow rates to those observed in the experiments, but had difficulty matching observed sediment concentrations when the pipes rapidly expanded. The JETs predicted similar erodibility coefficients compared to the mechanistic model for the more erodible cases but not for the less erodible cases (14% moisture content). Improved models are needed that better define the changing soil pipe cross‐section during supply‐ and transport‐limited internal erosion, especially for piping through lower compacted (more erodible) soils as opposed to more well‐compacted soils resulting from constructing levees and dams. Copyright © 2013 John Wiley & Sons, Ltd.     

REFLEXIONS SUR LA CONSERVATION DES RESSOURCES D'EAU DANS LES REGIONS ARIDES

Summary

A 0- to 6-inch sample of residual mineral soil was taken from 258 locations in northern California's mountain lands. Standard textural, organic matter, and erodibility analyses were made on these soil samples. These variables were then related to climatic parameters, parent material, and cover types by regression analysis. Variation in erodibility was found to be significantly correlated with parent material, cover type and climate. The acid igneous and schist rock types were more erodible than the more basic and ultramafic types. Soils developed under fir were found to be less erodible than those developed under pine, brush, or grass.     

Soil pipe collapses in a loess pasture of Goodwin Creek watershed,Mississippi: role of soil properties and past land use

Little is known about the association of soil pipe collapse features with soil properties or land use history. Three loess covered catchments in northern Mississippi, USA were characterized to investigate these relationships. Soil pipe collapses were characterized for their size, type feature and spatial location along with soil properties across the three catchments. Although mapped as the same soil, one of the catchments did not contain pipe collapse features while the other two had 29.4 and 15.4 pipe collapses per hectare. These loess soils contained fragipan layers that are suspected of perching water, thereby initiating the piping processes. Pipe collapses associated with subsurface flow paths were not always consistent with surface topography. The surface layer tended to be non‐erodible while layers below, even the upper fragipan layers, were susceptible to erosion by pipeflow. Soil properties of the lowest fragipan layer were highly variable but tended to prevent further downward erosion of soil pipes and thus formed a lower boundary for gullies. Middle to lower landscape positions in one of the piped catchments contained anthropic soils that were highly erodible. These anthropic soils were previously gullies that were filled‐in in the 1950s when forested areas, assumed to have been established when land was previously converted from crop to forest land, were converted to pasture. Three decades after this land use change from forest to pasture, pipe collapses became evident. In contrast, the adjacent catchment that does not exhibit pipe collapse features experienced severe sheet and rill erosion prior to the 1930s while in cotton production. The surface horizons above the lower fragipan layer were completely removed during this period, thus the top‐soil layer that tends to form a bridge above soil pipes in the more erodible subsoil layers was removed. This study showed that knowledge of soil characteristics or topography alone do not explain the distribution of soil pipe collapses as past land use can play a definitive role. Copyright © 2015 John Wiley & Sons, Ltd.     

Cogitation on developing a dynamic model of soil wind erosion

Studies on soil wind erosion began with single factors affecting soil wind erosion; with increasing quantities of data being accumulated,the wind erosion equation(WEQ),the revised wind erosion equation(RWEQ),the wind erosion prediction system(WEPS),and other soil wind erosion models have been successively established,and great advances have been achieved.Here we briefly review the soil wind erosion research course and analyze the advantages and disadvantages of the current soil wind erosion models.From the perspective of the dynamics of wind erosion,we classified the factors affecting soil wind erosion into three categories,namely,wind erosivity factors(WEF),soil antierodibility factors(SAF),and roughness interference factors(RIF).We proposed the concept of a standard plot of soil wind erosion to solve the problem of uncertainty of the soil wind erosion modulus on a spatial scale,and provided methods to set similarity conditions in wind tunnel simulation experiments and to convert the spatial scale of the wind erosion modulus from the standard plot to a large scale field.We also proposed a conceptual model on the basis of the dynamics of soil wind erosion with the theoretical basis that wind produces a shear force on the soil surface.This shear force is partitioned by barely erodible soil surfaces and roughness elements on the ground,and the amount of soil loss by wind should be calculated by comparing the shear force of the wind on barely erodible soil surfaces with the anti-erosion force of the surface soil.One advantage of this conceptual model is that the calculated soil wind erosion modulus is not subject to changes of spatial scale.Finally,we recommended continual improvement of the existing models while also establishing new models.     

The erodibility of upland soils and the design of preafforestation drainage networks in the United Kingdom

Hydraulic thresholds for erosion of fourteen upland mineral and organic soils were determined in a hydraulic flume. These soils are from areas to be afforested in the United Kingdom. Some of the group are erosion resistant but others are susceptible to erosion once denuded of vegetation; for example, by preafforestation ploughing. These threshold data were required to calibrate a hydraulic model for effective design of preafforestation drainage networks on a variety of soils. However, simple field measures of soil properties indicative of erosion potential would be of value to the forestry industry for management purposes. Consequently, hydraulic threshold data were related by linear regression methods to basic soil properties, including organic content, grain size, bulk density, compression strength and penetration resistance. The investigation concluded that four peat soils are not eroded by clear water velocities up to 5·7 m s⁻¹, although a mineral bedload might induce erosion at lesser current speeds. Penetration resistance is a good field indicator of the degree of humification of the peat soils. Although selected physical parameters contribute resistance to water erosion, an increased organic content is pre-eminent in reducing erosion susceptibility in both organic and mineral soils. Although compressive strength was not indicative of soil erodibility, field measurements of penetration resistance on a variety of soils could be related to hydraulic thresholds of erosion; albeit through the construction of discriminant functions interpolated by eye. Consequently, organic content (laboratory) or penetration resistance (field) might form the basis of classifying upland soils in terms of erodibility. Mineral soils differ widely in terms of their erodibility, so that subject to further consideration, the use of ploughing for forestry cultivation might be appropriate in wider circumstances than presently recommended by the Forests and Water Guidelines. Ploughing should be acceptable on deep peat providing the underlying mineral soil is not exposed in the bottom of the furrow, and furrows are not led from mineral soils on to deep peat. © 1997 John Wiley & Sons, Ltd.     

Time-dependency of runoff velocity and erosion the effect of the initial soil moisture profile

The paper reports on experiments carried out to evaluate the effect of the initial soil moisture profile on temporal variations in runoff erosion rate. The moisture profile was varied by applying infrared heating to the soil sample surface over various time periods, while runoff erosivity was varied by varying the slope of the flume. The experiment confirms that dry loamy soils are very erodible: on a slope length of only 4.3 m long sediment concentrations are near transporting capacity in case of a dry soil sample. It appears that temporal variations in sediment concentrations can be well simulated using a simple relationship between runoff erosion resistance and initial soil moisture content, thereby implicitly assuming that the effect of initial moisture content is persistent over the whole duration of the experiment. The implications of these findings with respect to the modelling of sediment output from larger catchments and the design of experiments on rill erodibility are discussed. The experiments also show that, under the present circumstances, mean velocities in the rills appear to be independent of slope. This finding may be of importance with respect to overland flow routing and deterministic erosion modelling.     

Coupled modeling of rainfall-induced floods and sediment transport at the catchment scale

Rainfall-induced floods may trigger intense sediment transport on erodible catchments, especially on the Loess Plateau in China, which in turn modifies the floods. However, the role of sediment transport in modifying floods has to date remained poorly understood. Concurrently, traditional hydrodynamic models for rainfall-induced floods typically ignore sediment transport, which may lead to inaccurate results for highly erodible catchments. Here, a two-dimensional (2D) coupled shallow water hydro-sediment-morphodynamic (SHSM) model, based on the Finite Volume Method on unstructured meshes and parallel computing, is proposed and applied to simulate rainfall-induced floods in the Zhidan catchment on the Loess Plateau, Shaanxi Province, China. For six historical floods of return periods up to 2 years, the numerical results compare well with observations of discharge hydrographs at the catchment outlet. The computed runoff-sediment yield relation is quantitatively reasonable as compared with other catchments under similar geographical conditions. It is revealed that neglecting sediment transport leads to underestimation of peak discharge of the flood by 14%–45%, whilst its effect on the timing of the peak discharge varies for different flood events. For 18 design floods with return periods of 10–500 years, sediment transport may lead to higher peak discharge by around 9%–15%. The temporal pattern of concentrated rainfall in a short period may lead to a larger exponent value of the power function for the runoff-sediment yield relation. The current finding leads us to propose that incorporating sediment transport in rainfall-induced flood modeling is warranted. The SHSM model is applicable to flood and sediment modeling at the catchment scale in support of risk management and water and soil management.     

A diagnosis of sub-surface water table dynamics in low hydraulic conductivity soils in the sugar cane fields of Pongola,South Africa

Water and land are the two natural resources restraining crop production in South Africa. With the increasing demand for food, emphasis has shifted from the sole reliance on rain fed crop production, to irrigation. The deterioration in irrigation water quality from surface water sources is, however, posing a big challenge to the sustainability of irrigated crop production. This is because more water is required for leaching, resulting in shallow water tables in agricultural lands. The installation of well designed subsurface drainage systems alone is not enough; the provision of timely maintenance is also necessary. In this study, the extent and severity of problems as a consequence of shallow water tables and their possible causes were investigated at three sugarcane fields in Pongola, South Africa, having low hydraulic conductivity soils. Also investigated were soil salinity levels and the temporal variation in the salinity of the irrigation water. A water table map of a 32 ha sugarcane field was generated, using observed water table depth (WTD) data from 36 piezometers monitored from September 2011 to February 2012. Out of the total 32 ha under cultivation, 12% was found to be affected by shallow WTDs of less than the 1.0 m design WTD. The inability of natural drainage to cope with subsurface drainage needs and the poor maintenance of subsurface drainage systems contributed to the shallow water tables in the area. Furthermore, the currently adopted drainage design criteria also proved unsatisfactory with mean observed water table depth and drainage discharge (DD) of 20% and 50%, respectively, less than their respective design levels. The salinity of the irrigation water was, on average, 32% higher than threshold tolerance level of sugarcane. The root zone soil salinity levels at the three study sites were greater than the 1.7 dS m⁻¹ threshold for sugar cane. The subsurface drainage design criteria adopted at the site needs to be revisited by ensuring that the slope of the land is taken into consideration in the drainage design in addition to adhering to a recommended maintenance schedule.     

Sediment budgeting: A case study in the Katiorin drainage basin,Kenya

The primary objective of this study was to compute a detailed budget for a small semiarid tropical drainage basin in Kenya. Results indicated that transfer of sediments (‘inputs’) from primary source areas was minor in comparison to changes in storage. The major sediment source area within the Katiorin drainage basin was the colluvial hillslope zone. The net change in storage within this zone was approximately 2100 Mg yr^?1. Surface wash and rilling were the dominant transport processes responsible for the remobilization of colluvial sediments. Sediment storage within the in-channel reservoir increased by 60 Mg yr^?1, which was minor when compared to the total store of sediment in this reservoir. During 1986, the channel network stored only a small fraction ( < 3 per cent) of the sediment delivered from the hillslope subsystem. Therefore, the in-channel reservoir had limited influence on sediment conveyance to the basin outlet. These data indicate that a static equilibrium condition cannot be assumed within the Katiorin drainage basin. Such an assumption would result in erosion estimates of approximately 5.5 mm yr^?1 for the entire basin (based on a sediment output of 7430 Mg km^?2 yr^?1 and a measured bulk density of 1.35 Mg m^?3). However, this masked the actual rates of 1.2 to 7.1 mm yr^?1 in subbasin primary source areas, and rates of 0.6 to 17 mm yr^?1 for colluvial material in the various subbasins. The extreme accelerated erosion rates resulted from minimal ground vegetation, steep slopes, soil crust formation, an erodible substrate, and a well-integrated drainage network for rapid conveyance of sediments from the hillslope subsystem to the basin outlet.     

The effect of forestry drainage operations on upland sediment yields: A case study

Forestry is generally considered to be a land use giving low sediment yields. However, in Britain most forestry is on land that requires extensive drainage, and it was found that the drainage of a small upland catchment resulted in sediment yields over the following five years equivalent to nearly half a century's load at pre-drainage rates. Subsequent sediment yields did not decline to pre-drainage levels, but remained about four times higher, as a result of erosion of the drains.     

Effect of spatial variability and seasonality in soil moisture on drainage thresholds and fluxes in a conceptual hydrological model

This paper uses soil moisture data from 17 recording sensors within the 50 km² Mahurangi catchment in New Zealand to determine how measured variability in soil moisture affects simulations of drainage in a typical lumped conceptual model. The data show that variability smoothes the simulated field capacity threshold such that a proportion of the catchment contributes to drainage even when mean soil moisture content is well below field capacity. Spatial variability in soil moisture controls by extension the catchment drainage behaviour: the resulting smoothed shape of the catchment‐scale drainage function is demonstrated and is also determined theoretically under simplifying assumptions. The smoothing effect increases the total simulated discharge by 130%. The analysis explains previous findings that different drainage equations are required at point scale versus catchment scale in the Mahurangi. The spatial variability and hence the emergent drainage behaviour are found to vary with season, suggesting that time‐varying parameters would be warranted to simulate drainage. Copyright © 2012 John Wiley & Sons, Ltd.     

Volcanic ash soils in New Zealand

Ash and lapilli derived from Holocene and Late Pleistocene volcanic cruptions cover approximately half of the North Island of New Zealand. Deposits from 16 separate series of showers are recognised in the surface soils and areas with 3 inches and more of each ash bed have been mapped out during soil surveys. These and other shower deposits are also recognised and described in fossil soils. The effects of the volcanic ashes on soil properties are related to the thickness and frequency of deposition, to mineral composition, and to the period of exposure to soil formation. Soils formed from shower deposits of Holocene age are extensive and have characteristic properties of friability, fine granular structure, deep humic topsoils and slippery non-sticky clays. The clays are chiefly amorphous and responsible for the notable practical properties of free drainage, high moisture retention, low bulk density, and critical limits of stability under pressure. In New Zealand the development of these characteristies increases with time up to about 15,000 years and then decreases with the crystallisation of the clays to halloysite. Soils formed from Late Pleistocene volcanic ashes are high in halloysite clay and with increasing age resemble soils formed from marine sedimentary rocks. Differing degrees of development of the characteristics are shown in the soil classification. The soils from volcanic ash are widely used for crop, pasture or forestry production. More intensive permanent usage depends on more detailed knowledge of the materials which will be obtained through combined volcanological and pedological research.     

Hydrological and salinity impacts of a bio‐drainage strategy application in the Yizre'el Valley,Israel

The bio‐drainage–commercial forestry strategy was applied in five plots in the Yizre'el Valley, northern Israel, to evaluate the hydrological and salinity impacts of eucalypt plantations. Each plot contained a mix of 11 selected eucalyptus species/ecotypes. Two plots (Nahalal and Genigar), representing the two extreme waterlogging/salinity conditions in the valley, were selected for in‐depth monitoring over a 10‐year period to assess the likely environmental improvement through bio‐drainage. Despite impressive growth rates of genetically improved Eucalyptus camaldulensis in the year‐round waterlogged, slightly saline Nahalal site (650 mm annual rainfall), the water uptake by the trees was insufficient to control the rising water table caused by excessive water inputs, both natural and human. In the more saline, alkaline and drier Genigar plot (450 mm annual rainfall), where rainfall is the only water input, the ground water dropped to below 3 m from soil surface in the fourth year after planting, i.e. deeper than the adjacent ground water levels. Both sites showed appreciable rise in wells that penetrated the 3‐ to 4‐m confining layer. The 10‐year salinity (EC) trend of the top layer in Nahalal varied because the drainage was limited by the positive water balance and the above‐average spells of dry winters. In and below the 4 m deep layer the EC remained below 1·5 dS m^?1 throughout the entire 10‐year study. The last EC measurement, taken in 2003, showed values not higher than 4 dS m^?1 throughout the 6 m soil profile. In Genigar, there was significant leaching of salts from the top layer (1 m) during the 9‐year monitoring period, but recently a salts ‘bulge’ was gradually developed in the 1–5 m strata indicating that the expected downward movement of leached salts was impeded by the 3–4 m deep low‐permeability clayey layer that lies over a coarser, far more conductive and notably confined layer, which leads to a perched water body. The last EC measurement at the end of 2003 showed a maximum value of 5·5 dS m^?1 at 3 m depth. No signs of tree stress were observed in either site, at any soil depth during the 10 years of monitoring. Theoretical considerations do not support the hypothesis that there would be a fatal long‐term accumulation of salts in the root zone. The Israeli experience has shown that the bio‐drainage technique can effectively lower a shallow water table and reverse salinity trends, provided that the overall water balance is negative, i.e. that the water inputs match the water use by the tree plantation and local drainage characteristics. However, the rate of improvement of the hydraulic, salinity, sodicity and soil physical properties is site specific. Excess fresh water inputs into the plantation, although they create waterlogging conditions, supply unlimited water to the trees, which, in turn, show exceptional growth rates, with usable commercial value. Copyright © 2007 John Wiley & Sons, Ltd.     

B.—U. N. E. S. C. O.

ABSTRACT

Sir Charles Cotton (1964) has pointed out that in an earlier paper (Carlston, 1963) which related drainage density to hydrology, there was insufficient emphasis on the role of climate in its effect on drainage density. Re-examination of the relation of drainage density to base flow in the 15 basins originally described has revealed additional evidence that base flow is affected by precipitation or recharge (a climatic variable), while varying inversely with drainage density.

Within the climatic region studied in the earlier paper (the Humid Subtropical Climate of the eastern U. S.), no evidence could be found that amount or intensity of rainfall affected the intensity of flood runoff or the scale of drainage density. In comparison with other climates, however, such as the Marine West Coast Climate, it is possible that the less intense precipitation of a marine climate may result in lower runoff intensities and lower drainage densities, however the lower mean temperatures of such climates may develop soils of generally higher infiltration capacity which would produce lower drainage densities.

A progressive increase in aridity results in a decrease in soil and vegetal cover which greatly magnifies the range of drainage densities characteristic of semi-arid regions. In such regions, where the land sur-face has a good infiltration capacity rainfall sinks readily into the dry soil (although recharge to ground water may be negligible), and runoff is virtually zero, as is drainage density, Impermeable terranes devoid of vegetal and soil cover reject the rain, runoff is briefly total and drainage density may be greatly magnified, as in the South Dakota Badlands, where drainage density runs into the hundreds. Arid or Desert Climates should produce erosional landforms with generally high drainage densities, though not reaching the magnitudes of drainage density found in the semi-arid badlands where rainfall intensities are much higher.     

Water exclusion from tunnel cavities in the saturated capillary fringe

The problem of water flow around a tunnel cavity located in the saturated capillary fringe on top of a very permeable, freely draining substratum is considered for the critical non-leakage condition when there is uniform vertical downward flow through the upper surface of the saturated region. In this critical condition the soil–water pressure is equal to zero everywhere on the cavity wall that is also a streamline. The conditions at the upper fringe boundary are that the soil–water pressure is equal to the air-entry value of the soil and the flux through this surface is the uniform infiltration rate. The cavity surface and the fringe boundary which is elevated above the cavity position, are found through conformal mapping and the use of integral representations of non-standard mixed boundary-value problems. They are calculated for a range of infiltration rates and compared with those obtained by assuming the upper fringe boundary to be horizontal. The exact analysis given here gives larger tunnel cavities than those given by the approximate treatment of the problem. The results have application in the design of underground repositories against entry of seepage water, the construction of protective capillary barriers and in the design of interceptor drainage systems.     

Hydrological consequences of artificial drainage of grassland

Soil water regimes and water balances are presented for a series of drained and undrained experimental grassland plots, intended to examine the agronomic consequences of drainage. Although drainage has lowered the watertables and reduced the duration of waterlogging in the drained plots, its effects in terms either of the total water quantities leaving the site or of peak flows is quite small. The major effect of drainage is to alter the route of water loss from the site. In its undrained state, the soil is waterlogged for the majority of the winter, incident rainfall cannot infiltrate, and water leaves as surface runoff or near-surface flow. The introduction of fissures by mole drainage both provides an outlet and enhances the macroporosity, so that the rain moves rapidly through the soil and appears as drainflow. Consequently, the additional delay in generating peak runoff through the drainage system is only of the order of 30 minutes on this site.     

考虑碎石桩排水能力复合地基中孔压长消解析解

由于地震作用时间较短,且碎石桩渗透能力和土体渗透能力相比并不是无限大,因此本文考虑碎石桩排水能力研究了碎石桩桩体材料由地震引起的孔压的长消规律。根据比奥固结理论综合考虑碎石桩的排水能力和相应的初始条件及边界条件,推导出了能够真实反映碎石桩排水减压作用在地震期超孔隙水压力产生、扩散、消散过程中的贡献作用的一般解析解公式。同时讨论了碎石桩渗透能力的不同对抗震液化的影响作用。     

Effects of macro-pores on water flow in coastal subsurface drainage systems

Leaching through subsurface drainage systems has been widely adopted to ameliorate saline soils. The application of this method to remove salt from reclaimed lands in the coastal zone, however, may be impacted by macro-pores such as crab burrows, which are commonly distributed in the soils. We developed a three-dimensional model to investigate water flow in subsurface drainage systems affected by macro-pores distributed deterministically and randomly through Monte Carlo simulations. The results showed that, for subsurface drainage systems under the condition of continuous surface ponding, macro-pores increased the hydraulic head in the deep soil, which in turn reduced the hydraulic gradient between the surface and deep soil. As a consequence, water infiltration across the soil surface was inhibited. Since salt transport in the soil is dominated by advection, the flow simulation results indicated that macro-pores decreased the efficiency of salt leaching by one order of magnitude, in terms of both the elapsed time and the amount of water required to remove salt over the designed soil leaching depth (0.6 m). The reduction of the leaching efficiency was even greater in drainage systems with a layered soil stratigraphy. Sensitivity analyses demonstrated that with an increased penetration depth or density of macro-pores, the leaching efficiency decreased further. The revealed impact of macro-pores on water flow represents a significant shortcoming of the salt leaching technique when applied to coastal saline soils. Future designs of soil amelioration schemes in the coastal zone should consider and aim to minimize the bypassing effect caused by macro-pores.