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.     

大气环流的变化与南海夏季风活动的关系

通过对近20年观测资料的计算分析表明，在南海夏季风活跃与中断期，南北半球副热带西风急流和热带东风急流以及青藏高压和墨西哥高压有明显的差异。从南半球向北传播的散度场低频振荡以及从北半球高纬向南传播的西风场低频振荡等的有利位相同时传入南海并共同作用可引起南海夏季风的活跃或中断。各种低频振荡的有利位相传播到南海并发生锁相的时间决定着南海夏季风明显的活跃或中断发生的时间。     

高压旋喷技术在基坑护壁与止水工程中的应用

长沙天都大厦工程围护墙先后采用2套方案施工,实践证明,采用高压旋喷单管双液分喷法具有设备简单;投资少;操作容易,施工速度快;桩间搭接严密,固结体强度高;适用范围广等优点,尤其在处理软弱地基、基坑支护和止水工程中显示出有效性、可靠性和优越性。该方法再经进一步完善,将具广阔的推广前景。     

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。     

高空急流入口区次级环流在一次突发性强降水过程中的作用

通过对 1999年秋季一次突发性降水过程高空急流的分析 ,发现急流入口区南侧辐散 ,其低层辐合上升 ,当低层有印缅槽活动的时候 ,对流加强 ,印缅槽发展 ;急流入口区北侧辐合 ,其低层辐散下沉 ,有向南的非地转风     

吐鲁番一场罕见降雪天气个例分析

对“2000·2”罕见降雪天气过程分析表明,低层强烈的辐合上升运动和低空急流的突然增强是产生降雪的关键.     

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…