The effect of single vegetation elements on wind speed and sediment transport in the Sahelian zone of Burkina Faso

Soil loss caused by wind erosion is a widespread phenomenon in the Sahelian zone of West Africa. According to Sahelian farmers, scattered vegetation standing in amongst the crop has the potential for a wind erosion control strategy. This study was conducted to study the effect of single vegetation elements on the pattern of average wind speed and sediment transport. This was done by two experiments that were carried out during the rainy seasons of 2002 and 2003 in north Burkina Faso, West Africa. Wind speeds were measured using three sonic anemometers, at a sampling frequency of 16 Hz. Sediment transport was determined by calculating the mass fluxes from 17 MWAC catchers. In this study, a shrub was defined as a vegetation element with branches until ground and a tree as a vegetation element with a distinctive trunk below a canopy. Behind shrubs wind speed near the soil surface was reduced up to approximately seven times the height of the shrub. The observed reduction in wind speed in the area where wind speed was reduced was 15 per cent on average. At the sides of the shrub, wind speed was increased, by on average 6 per cent. As the area of increase in wind speed is one‐third of the area of decrease in wind speed, the net effect of a shrub is a reduction in wind speed. A similar pattern was visible for the pattern of sediment transport around a shrub. Downwind of a shrub, sediment transport was diminished up to seven times the height of the shrub. Probably most of this material was trapped by the shrub. Trees showed a local increase of wind around the trunk, which is expected to relate to an increase in sediment transport around the trunk. Mass flux measurements of sediment transport were not made, but visual observations in the field substantiate this. Behind the canopy of a tree, a tree acts similarly to a shrub regarding its effects on average wind speed, but as a tree is generally a larger obstacle than a shrub the extent of this effect is larger than for shrubs. Thus, whereas shrubs are more effective than trees regarding their direct effect on soil loss by trapping sand particles near the soil surface, trees are more effective in affecting soil loss indirectly by reducing the wind speed downwind more effectively than shrubs. Therefore, to reduce soil loss in an area, the presence of both trees and shrubs is crucial. Copyright © 2007 John Wiley & Sons, Ltd.     

Evaluation of the SEDD model for predicting sediment yield at the Sicilian experimental SPA2 basin

In this paper a spatially distributed model of the hillslope sediment delivery processes, named the sediment delivery distributed (SEDD) model, is initially reviewed; the model takes into account the sediment delivery processes due to both the hillslope sediment transport and the effects of slope curvature. Then the rainfall and sediment yield events measured at the experimental SPA2 basin, in Sicily, are used both to calibrate the SEDD model and to verify the predictive capability of the distributed sediment delivery approach at event scale. For the SPA2 basin discretized into morphological units and stream tubes, the SEDD model is calibrated at event scale using the measurements carried out at the outlet of the experimental basin in the period December 2000–January 2001. The model calibration is used to determine a relationship useful for estimating the unique coefficient β_e of the model by rainfall erosivity factor R_e at event scale. To test the predictive capability of the β_e = f(R_e) relationship, 20 events measured in the period September 2002–December 2005 are used; the comparison between measured sediment yield values and calculated ones for all monitored events shows that the sediment delivery distributed approach has a good predictive ability at event scale. The analysis also shows that estimating β_e by the relationship β_e = f(R_e) gives a better agreement between measured and calculated sediment yields than obtained with the median value β_e,m of all 27 calculated β_e values. Finally the analysis at annual scale, for the period 2000–2005, allows the estimation of the median value β_a,m representative of the annual behaviour. This analysis shows that the sediment delivery distributed approach also has a good predictive ability at annual scale. Copyright © 2006 John Wiley & Sons, Ltd.     

Impact of root architecture on the erosion‐reducing potential of roots during concentrated flow

Many studies focus on the effects of vegetation cover on water erosion rates, whereas little attention has been paid to the effects of the below ground biomass. Recent research indicates that roots can reduce concentrated flow erosion rates significantly. In order to predict this root effect more accurately, this experimental study aims at gaining more insight into the importance of root architecture, soil and flow characteristics to the erosion‐reducing potential of roots during concentrated flow. Treatments were (1) bare, (2) grass (representing a fine‐branched root system), (3) carrots (representing a tap root system) and (4) carrots and fine‐branched weeds (representing both tap and fine‐branched roots). The soil types tested were a sandy loam and a silt loam. For each treatment, root density, root length density and mean root diameter (D) were assessed. Relative soil detachment rates and mean bottom flow shear stress were calculated. The results indicate that tap roots reduce the erosion rates to a lesser extent compared with fine‐branched roots. Different relationships linking relative soil detachment rate with root density could be established for different root diameter classes. Carrots with very fine roots (D < 5 mm) show a similar negative exponential relationship between root density and relative soil detachment rate to grass roots. With increasing root diameter (5 < D < 15 mm) the erosion‐reducing effect of carrot type roots becomes less pronounced. Additionally, an equation estimating the erosion‐reducing potential of root systems containing both tap roots and fine‐branched roots could be established. Moreover, the erosion‐reducing potential of grass roots is less pronounced for a sandy loam soil compared with a silt loam soil and a larger erosion‐reducing potential for both grass and carrot roots was found for initially wet soils. For carrots grown on a sandy loam soil, the erosion‐reducing effect of roots decreases with increasing flow shear stress. For grasses, grown on both soil types, no significant differences could be found according to flow shear stress. The erosion‐reducing effect of roots during concentrated flow is much more pronounced than suggested in previous studies dealing with interrill and rill erosion. Root density and root diameter explain the observed erosion rates during concentrated flow well for the different soil types tested. Copyright © 2007 John Wiley & Sons, Ltd.     

Feedbacks between erosion and sediment transport in experimental bedrock channels

Natural bedrock rivers flow in self‐formed channels and form diverse erosional morphologies. The parameters that collectively define channel morphology (e.g. width, slope, bed roughness, bedrock exposure, sediment size distribution) all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes. To explore the mechanics of river incision, we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate (sand and cement), a single sediment size primarily transported as bedload, a single erosion mechanism (abrasion) and sediment‐starved transport conditions. We find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field, sediment transport and bottom roughness. Abrasion only occurs where sediment impacts the bed, and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows. However, the spatial focusing of erosion results in tortuous flow paths and erosional forms (inner channels, scoops, potholes), which dissipate flow energy. This energy dissipation is a negative feedback that reduces sediment transport capacity, inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load. Copyright © 2007 John Wiley & Sons, Ltd.     

A device for measuring downwearing rates on cohesive shore platforms

The device described in this paper, known as the traversing erosion beam, is believed to be the first to be designed specifically for measuring rates of downwearing of cohesive foreshores, such as clay shore platforms, under high‐energy wave conditions. It is primarily constructed of Flexlink Aluminium Structural System components and consists of a horizontal beam with a main leg and two further support legs. A sliding ‘dolly’ module is moved along the beam at precise intervals using a longitudinal scale as a reference. The topographic profile of the measurement surface is transferred to the dolly by a vertical sliding steel alloy rod, from which the relative height differences are measured by a separate engineer's digital height gauge placed on the dolly. The main leg of the device fits into a marine grade stainless steel box, which is deeply embedded into the foreshore or shore platform to act a permanent datum. The box is robust and can be expected to have a lifetime of at least a year, and possibly even five years or more. Sample data are provided from a clay shore platform at Warden Point in the Isle of Sheppey, Kent (UK), which demonstrate that the TEB can provide information on seasonal as well as annual rates of downwearing. The device has also been tested successfully on a peat and clay foreshore at Pett Level in Sussex and on a shore platform cut in glacial till at Easington, Yorkshire. Copyright © 2007 John Wiley & Sons, Ltd.     

In situ measurements of surficial mud strength: A new vane tester suitable for soft intertidal muds

A vane fitted to a standard shear vane tester is described, the purpose of which is to obtain better measurements of the rheological properties of soft, surficial, intertidal muds (and consequently, variations in their erodability). Specifications and geometry of this vane are presented. The computation procedure is detailed, so that the methodology can be applied to any vane tester. The main improvement is a greater vertical resolution, typically less than 1 cm. Sets of direct measurements with this equipment and correlation with dry densities for three intertidal mudflats (estuaries and bay) are presented.     

不同尺度条件下的土壤侵蚀实验监测及模型研究

土壤侵蚀的实验和监测是获取水土流失资料的重要手段,土壤侵蚀模型研究是水土保持规划、管理的重要内容。本文对点、小区、田间和坡面、流域及区域尺度条件下的土壤侵蚀的实验和监测研究进行了综述,分析了当前国际上实验研究和监测研究的侧重点及其不足;并对当前国际上土壤侵蚀模型的总体发展趋势进行了评述,特别分析了这些模型在全球气候变化背景下的作用;最后对当前土壤侵蚀、试验及检测研究作出展望。     

PHYSICAL MODELING OF HYDRAULIC DESILTATION IN TAPU RESERVOIR

1 INTRODUCTIONIn recent years, the concePt of long-term sustained use of reservoirs has been addressed because areservoir is very much considered to be a nonrenewable resource (Morris and Fan, l998). Technically,many options for reservoir sedAnentation control can be utlized to pursue the sustainable develoPment ofwater resources. In general, reduction of incoming sedimen yields from watersheds is often emPloyedin conjunction with hydraulic methods such as flushing or density currnt vot…     

长江上游土地利用变化与冲沟发生和演变

冲沟侵蚀是中国西部山区一个重要的环境问题。小流域研究表明，黄河和长江中的泥沙主要是冲沟侵蚀引起的。目前，长江上游80%的冲沟因被幼林草植被覆盖而处于非活跃期。到底这些冲沟系统是什么时候形成的？未来的暴雨和径流是否会激活这些冲沟系统？土地利用与景观结构对冲沟系统的发生、演变有何关系？基于这些问题，近几年来在中科院山地环境学“百人计划”和“国外引进杰出人才”项目及德国洪堡基金的资助下，中德科学工作者进行了合作研究。本文介绍了长江上游地区土地利用变化与冲沟发生、演变的部分研究成果。我们在西昌安宁河干暖河谷选择了两个冲沟系统一个位于西溪乡长山岭，一个位于大青梁子。为了定量评价土地利用对冲沟发育的影响，我们对研究小流域的航片、土壤剖面、泥沙堆积与土地利用历史等进行了详尽的综合对比分析。同时，对两个冲沟系统的大小和土壤流失的体积进行了详细的野外调查、测定和计算。据此，获得了研究流域的冲沟侵蚀速率。研究结果表明，长江上游数百年的持续景观演变，尤其是20世纪中期人为强烈的不合理农业垦殖和利用土地，造成了四川西南部大量冲沟系统的形成和发展，研究流域的冲沟侵蚀速率高达375t/hm2*a-1。采取成功的水土保持措施以控制冲沟侵蚀，对中国西南山地退化环境的生态修复具有重大实践意义。