Sediment transfer in an extremely low-gradient,low-relief and highly buffered system: Darwin Harbour catchment,northern Australia

Abstract

Sediment yields from and sediment transfer within catchments of very low relief and gradient, which make up about 50% of Earth’s surface, are poorly documented and their internal sediment dynamics are poorly known. Sediment sources, their proportionate contributions to valley floors and sediment yield, and storage are estimated using fallout radionuclides ²¹⁰Pb_(ex) and ¹³⁷Cs in the catchments that drain into Darwin Harbour, northern Australia, an example of this understudied catchment type that appears to be globally at the extreme end of this category of catchments. Unchannelled grassy valley floors (dambos, or seasonal wetlands) trap ～90% of the sediment delivered from hillslopes by sheet and rill erosion. Further down valley, small channels transport ～10% of the sediment that escapes from the dambos, and the remaining sediment comes from erosion of the channels. In this case, the fractional sediment storage is very high as a result of the existence of dambos, a landform that depends for its existence on low gradients.     

Limitations of suction‐controlled triaxial tests in the characterization of unsaturated soils

Behavior of unsaturated soils is influenced by many factors, and the influences of these factors are usually coupled together. Suction‐controlled triaxial (SCTX) tests are considered to allow researchers to investigate influences of individual variables on unsaturated soils under specified stress path with controls of stresses, pore water, and air pressures. In the past 50 years, SCTX testing method has been established as a standard approach to characterize constitutive behavior of unsaturated soils. Most important concepts for modern unsaturated soil mechanics were developed upon results from the SCTX tests. Among these, one of the most important contributions in the constitutive modeling of elasto‐plastic behavior for unsaturated soils is the Barcelona basic model (BBM) proposed by Alonso et al. in 1990. The BBM successfully explained many features of unsaturated soils and received extensive acceptance. However, the SCTX tests are designed based upon the divide‐and‐conquer approach in which an implicit assumption is used: soil behavior is stress‐path independent. However, it is well‐established that unsaturated soil behavior is elasto‐plastic and stress‐path dependent. It is found that the SCTX tests in fact cannot control the stress path of an unsaturated soil during loading. This incapability, in combination with complicated loading/collapse behavior of unsaturated soils, makes the SCTX tests for characterizing unsaturated soil questionable. This paper discusses the limitations of the SCTX tests in the characterization of unsaturated soils. A possible solution to the problem was proposed based on a newly developed modified state surface approach. The discussions are limited for isotropic conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

Elastoplastic model with loading memory surfaces (LMS) for monotonic and cyclic behaviour of geomaterials

A new elastoplastic model called loading memory surface based on the critical state concept and the multi‐surface framework is proposed for geomaterials. The model uses a hypoelastic formulation and two plastic mechanisms. The formulations of the model are made in three‐dimensional stress–strain space and work under both monotonic and cyclic loadings. A newly introduced formalism makes it possible to obtain the cyclic response directly from the monotonic loading one. This formalism gives a three‐dimensional generalization of the well‐known Masing rule. The model has been validated against test results of Hostun sand under several conditions: monotonic and cyclic, drained and undrained, tests in compression and in extension, and at different confining pressures and different densities. Copyright © 2014 John Wiley & Sons, Ltd.     

Higher tritium concentrations measured in permafrost thaw lakes in northern Alberta

Tritium concentrations were measured in a survey of 24 lakes, 15 wetlands, and 133 groundwaters in the oil sands region of northeastern Alberta and compared with both recent precipitation and precipitation sampled during the 1960s tritium peak caused by atmospheric thermonuclear weapons testing. Water samples from lakes included a group of 14 thaw lakes that had higher runoff attributed to melting of permafrost in peat plateaus within their watersheds. While tritium in all lakes was found to be intermediate between recent and 1960s concentrations, the thaw lakes were found to be significantly enriched in tritium compared with other lakes, as were unfrozen wetlands characterized by a thick sequence of low‐hydraulic conductivity peat. The results provide further evidence of different water sources to the thaw lakes and may indicate that melting of modern permafrost in part formed since the 1950s is occurring in these systems. Copyright © 2015 John Wiley & Sons, Ltd.     

陕北风沙区含砾石工程堆积体坡面产流产沙试验

采用室内人工模拟降雨方法,研究了陕北风沙区含砾石工程堆积体边坡的产流产沙过程。结果表明:①砾石存在改变了坡面入渗速率,径流系数受入渗速率的影响,随砾石含量的增加先线性递减后线性递增,并在10%砾石含量处存在阈值;径流系数随降雨强度的增加线性递增。②含砾石堆积体坡面流速较纯土堆积体降低,且随雨强增大,砾石延缓径流流动的作用越显著;雨强对径流流速的影响随砾石含量增加持续减弱。③土壤剥蚀率在产流24~33 min后显著增加,砾石主要对显著增加后的平均剥蚀率产生影响。④雨强1.0 mm/min时,砾石存在促进降雨侵蚀,产沙量增大;雨强大于1.0 mm/min时,砾石具有显著的减沙效应。     

SoFISHticated policy – social perspectives on the fish conflict in the Northeast Atlantic

Ecosystem changes currently question the traditional allocation of fishing rights and quotas in the fishery of Northeast Atlantic mackerel and Norwegian spring-spawning herring in the Northeast Atlantic. Variability in the distribution of these highly migratory species escalated in a political conflict between member states of the European Union, Iceland, the Faroe Islands and Norway, which is a driving force for unsustainable fishery. The aim of this paper is to investigate this conflict by outlining the social understandings of diverse stakeholders by using the Q methodology. The method reduced the complexity of numerous opinions, detected four distinct perspectives and simultaneously categorised the participating stakeholders. Although the perspectives differ in various elements, the protection of economic interests seems to dominate over the quest for sustainability. The call of all stakeholders in this study to clarify the fishing rights in the Northeast Atlantic reveals a clear deficiency of the current international fishery management in handling abrupt ecological changes and the necessity to acknowledge this as a complex adaptive system.     

1991—2015年三江源河曲高寒草甸干湿状况及牧草产量变化的气候归因研究

以三江源东部河曲高寒草甸为研究对象,通过分析1991—2015年气温、降水、潜在蒸散、湿润指数和牧草产量变化特征,探讨了地区干湿状况对牧草产量的影响。研究表明：1991—2015年河曲高寒草甸潜在蒸散以3.5 mm·a^-1的速率增加（P<0.01）,在年降水量按2.3 mm·a^-1呈非显著性（P>0.05）增加的趋势下,地区干湿状况基本保持平稳（多年均值为0.52）,隶属于半湿润气候区。25年来牧草干重产量平均为303.7 g·m^-2,并以3.0 g·m^-2·a^-1的速率下降。分析牧草产量与影响干湿状况的气候因素之间的相关性发现,气温对牧草产量影响不明显（P>0.05）,降水量表现为正相关关系（P>0.10）,说明该区域降水是牧草产量提高与否的主导因素;牧草产量与潜在蒸散表现为负相关关系（P>0.10）,与湿润指数表现为正相关关系（P>0.10）;在生长季时期,牧草产量与降水量、潜在蒸散和湿润指数的相关性关系达到了显著水平（P<0.10）,说明牧草产量在生长季对地区环境条件湿润与否较为敏感。     

成层软土地基预固结处理后桩基水平承载力估算方法

目前通过对软土地基预加固处理来提高桩基水平承载力已被工程界认可,但如何在工程前期设计过程中估算软土地基预处理后桩基水平承载力提高值仍是技术难点。基于此,参考Bowles[1]的地基土水平抗力计算式,同时考虑成层软土地基预排水固结处理影响,通过数学推导,推求出根据原状软土室内土工试验抗剪强度指标及预加固处理时间,估算软土地基预处理后桩基水平承载力提高值的实用计算方法。考虑桩侧土弹塑性屈服影响,推导出成层软土中水平受荷桩弹塑性解析解及塑性区深度的计算式,给出了桩顶水平位移、桩身最大弯矩的无量纲计算式及相关计算源代码。依托于浙江省某水闸桩基工程案例,根据提出的计算方法对桩基水平承载力、桩顶水平位移及桩身最大弯矩等性状进行预估计算,并与地基预处理前、后现场试桩检测值进行验证对比,认为桩基水平承载力、桩顶水平位移及桩身最大弯矩等预估计算成果与工程现场试桩的检测值较接近,对类似工程设计具有较好的参考价值。     

Effects of commercial timber harvesting on streamflow regimes in the Plynlimon catchments,mid‐Wales

This paper presents the first large‐scale British study of the impacts of commercial forest cutting on stream‐flow regimes. The 70% forested headwaters of the River Severn are part of the intensively instrumented long‐term Plynlimon catchment study into the impact of land use on stream flow. The forest area, comprising predominantly Sitka spruce (Picea sitchensis), was planted mainly in the 1930s and 1940s. Harvesting commenced in the mid‐1980s and over the study period about half the forest has been felled. Changes in annual water yield and extreme flows were studied in four nested catchments ranging in area from about 1 to 10 km² and compared with an adjacent benchmark grassland catchment. As expected from earlier process studies the cutting of the forest increased total annual flows. Less expected was the clear evidence that the felling augmented low flows. This informs a long‐standing debate whether upland forestry increases or reduces baseflows. A particularly notable result was the lack of impact of the harvesting on storm peak flows. This may result from the application of forest management guidelines designed to reduce soil damage and erosion during the harvesting, and indicates that the forest itself has a limited impact on flooding. These findings are timely because British forest expansion peaked in the 30 years following the Second World War, and large areas of these woodlands are now approaching economic maturity and will be harvested in the next two decades. Copyright © 2004 John Wiley & Sons, Ltd.     

Seismic stability analysis of piled embankments: A multiphase approach

The seismic stability analysis of an embankment lying over a soft foundation soil reinforced by a group of vertical piles is performed within the framework of the upper bound kinematic approach of yield design. The analysis is based on a previously developed ‘multiphase’ model of the reinforced ground, which explicitly accounts for the shear and bending resistances of the piles. Making use of appropriate failure mechanisms involving shear zones across which the reinforcements are continuously deforming, along with ‘plastic hinge’ surfaces, upper bound estimates to the critical seismic coefficient of the structure are derived. The results, which are confirmed by the simulations obtained from a finite element elastoplastic code, give clear evidence of the key role played by the bending strength capacities of the piles in ensuring the stability of the pile reinforced embankment. Copyright © 2009 John Wiley & Sons, Ltd.