Quantifying preferential flow in soils: A review of different techniques

Preferential flow (PF) in soil has both environmental and human health implications since it favours contaminant transport to groundwater without interaction with the chemically and biologically reactive upper layer of soil. PF is, however, difficult to measure and quantify. This paper reviews laboratory and field techniques, such as breakthrough curves, dye tracing, and scanning techniques, for evaluating PF in soil at different scales. Advanced technologies, such as scanning techniques, have increased our capability to quantify transport processes within the soil with minimal soil disturbance. Important issues with respect to quantifying PF concern large-scale studies, frozen soil conditions, tracing techniques for particles and gases, a lack of simple mathematical tools for interpreting field data, and the lack of a systematic approach for comparing PF data resulting from different measurement techniques. Also, more research is required to quantify the relative importance of the various PF processes that occur in soil rather than the integrated result of all PF processes in soils.     

苏北近岸水域"五条沙"侵蚀发育的卫星监测

利用2000年3月26日、5月20日和9月18日Landsat-5卫星的TM和Landsat-7卫星ETM多波段多时相图像数据,经几何、辐射校正、海洋潮汐订正和多波段合成处理后,采用图象等灰度线-海图主特征地形10 m等深线配准相关法,对覆盖区内苏北岸段(32°00′~34°00′N,120°00′~123°00′E)范围内的岸滩,水下地形,潮流沙脊等重要地貌特征及其走向进行了分析和解译,并利用减法对研究区的沙洲沙脊进行点对点的经纬度计算,进而给出研究区内沙洲沙脊的逐点侵蚀与发育量值,获得了研究区内水下沙脊侵蚀发育的分析数据。结果显示,近15 a来,位于北凌河和九洋河外的蒋家沙南部有明显的侵蚀特征,范围明显收缩,而其北部有明显的向NE方向发育的特征;位于北侧的外毛竹沙等3条沙脊向N或向NNW方向延伸的趋势明显,其范围也显著加大。对比由卫星图象解译获得的10 m等深线和1986年出版的海图(10 m等深线)发现,外毛竹沙向北延伸了30 km;蒋家沙向北偏东延伸了11 km;而毛竹沙与太平沙之间的菱形“无名沙”向北延伸了约11 km。     

Scarifying and fingering surfaces of plunging jets

The local surface deformation resulting from the oblique impact of a columnar water jet has been computed, using a three-dimensional large eddy simulation, as a model of the overturning jet of a breaking wave. The emergence of the secondary jet from the front face of the initial jet has been examined and the organisation of the vortices within the jet characterised. As the secondary jet emerges, the vorticity field becomes unstable under the action of the strong shear beneath the jet surface and pairs of longitudinal counter-rotating vortices stretched along the direction of the jet projection are formed. The presence of these longitudinal vortex pairs creates convergent surface flows, resulting in the formation of longitudinal scars on the rear face of the projecting jet. Following significant growth of the scars on both its upper and lower surfaces, the jet decouples into fingers. The lateral widths of the longitudinal vortices provide a minimum measure of the finger size. A horizontal Froude number Fr_h, representing a measure of strength of horizontal shear in a gravity-dominated impacting flow is defined, which characterises the organisation of the longitudinal vortices occurring in the shear flow, and the resultant formation of scars and fingers. For higher Fr_h, stronger longitudinal vortices and deeper scars are formed at longer lateral intervals, enhancing the fingering process during the splashing event. Fundamental features of material transport in the vicinity of the surface of jets (e.g. gas transfer across a sea surface) are related to the entrainment of surface fluid by the longitudinal vortices, and is thus also characterised by Fr_h.     

The Fernbank interglacial site near Ithaca, New York, USA

The Fernbank interglacial site, on the west side of Cayuga Lake, New York, has been recently subjected to more detailed study. To a lengthened mollusc list are added ostracodes, insects, fish, pollen, and plant macrofossils. Of these, plants are well preserved and diverse, whereas other groups are poorly preserved and incomplete. Nevertheless, all support the interglacial assignment (Sangamon), which is further supported by minimum age radiocarbon dates (> 50,000 ¹⁴C yr BP) and a TL date of 81 ± 11 ka. In the plant record near the top of the sequence, abundant tree charcoal indicates forest fires. Like the Toronto interglacial record, the plants show a declining July mean temperature from 24 to 18°C (according to transfer functions) through the sequence, from mixed deciduous forest to boreal forest.