天然气水合物稳定带内流体压裂计算的程序耦合方法

海洋天然气水合物稳定带气烟囱结构中存在被水合物充填的裂隙, 表明在自然条件下沉积物中曾发生过流体压裂以及相关的流体流动和水合物形成。在水合物稳定带内实施人为的流体压裂工程, 并联合其他方法(如降压或注热)进行水合物开采, 有望提高开采效率。水合物稳定带内, 无论是自然条件下发生的流体压裂过程, 还是人为实施的流体压裂工程, 都存在水合物反应和沉积物裂隙变形之间的耦合响应。当前, 已有不少数值程序对水合物反应与沉积物弹塑性变形的耦合过程进行了定量研究, 但尚没有数值程序能够计算水合物反应和离散裂隙变形之间的耦合过程。文章将TOUGH+Hydrate程序、IC-FERST和Solidity两者的耦合程序进行了进一步耦合, 为水合物稳定带内的流体压裂计算提供了一种耦合计算方法, 同时通过一个算例初步验证了该耦合计算方法的可行性。验证结果表明, 该耦合计算方法经进一步改进后有望应用于定量研究水合物稳定带内的裂隙变形和水合物反应过程。     

A stabilized Smoothed Particle Hydrodynamics,Taylor–Galerkin algorithm for soil dynamics problems

Modelling of failure under dynamic conditions in geomaterials with finite elements presents a series of complex problems, among which we can mention those of (i) volumetric locking, which results on higher failure loads, (ii) influence of mesh alignment, resulting to unrealistic failure surfaces, (iii) diffusion of the shear band over some element widths, (iv) nonoptimal propagation properties (numerical diffusion and dispersion), (v) fulfilling Babuska–Brezzi conditions when using the same order of interpolation for displacement and pressures in coupled problems and (vi) large deformation analysis. This paper is based on previous work done by the authors, who developed a mixed approximation based on (i) casting the dynamic problem in the form of a system of first order PDEs and (ii) using stresses and velocities as nodal variables. The equations were discretized following a Taylor–Galerkin algorithm, first in time using a Taylor expansion and then in space using Galerkin method. The model was limited to small deformations. The purpose of this paper is to show how Taylor–Galerkin method can be extended to meshless formulations, such as the SPH method. The algorithm consists of (i) discretizing in time using a Taylor series expansion complemented with integration of source terms using a Runge–Kutta scheme and then (ii) discretizing in space using the SPH method. It is shown how the proposed method keeps the advantages of the Taylor–Galerkin method in Finite Elements (good propagation properties and capturing of shear bands) and avoids the tensile instability. A set of test problems ranging from elastic propagation of a wave in a bar to failure of a slope on a cohesive softening material are used to assess the performance of the method. Copyright © 2011 John Wiley & Sons, Ltd.     

Three dimensional elasto‐plastic interface for embedded beam elements with interaction surface for the analysis of lateral loading of piles

This paper presents a numerical formulation for a three dimensional elasto‐plastic interface, which can be coupled with an embedded beam element in order to model its non‐linear interaction with the surrounding solid medium. The formulation is herein implemented for lateral loading of piles but is able to represent soil‐pile interaction phenomena in a general manner for different types of loading conditions or ground movements. The interface is formulated in order to capture localized material plasticity in the soil surrounding the pile within the range of small to moderate lateral displacements. The interface is formulated following two different approaches: (i) in terms of beam degrees of freedoms; and (ii) considering the displacement field of the solid domain. Each of these alternatives has its own advantages and shortcomings, which are discussed in this paper. The paper presents a comparison of the results obtained by means of the present formulation and by other well‐established analysis methods and test results published in the literature. Copyright © 2016 John Wiley & Sons, Ltd.     

Volunteered geographic information systems: Technological design patterns

Volunteered geographic information (VGI) has been studied extensively within various contexts, however, generally focusing on isolated aspects of data. Therefore, we still need to advance the structured knowledge of the mechanisms that affect and support its creation. This article aims to contribute to the integrated study of VGI systems (VGIS) in order to enhance design processes that lead to a more systematic and efficient VGI management. We define VGIS, their main components, and general architectures. Then, we use the service blueprinting technique to conduct the case study of several systems, identifying system actions to find the processes and technologies forming the different functional components of VGIS: elements of the technological infrastructure that implement diverse processes to facilitate participation and data management. Integrated in the data collection component, we also propose a typology of crowdsourcing tasks that—coupled with the functional components—establishes a foundation for VGIS research and design. We characterize these components, arguing that they are necessary in the development of VGIS, however, there is still a need for a more detailed study of the flow of processes and subprocesses and how they work under the criteria of different types of projects to continue facilitating their design.     

Comparison of streamflow recession between plantations and native forests in small catchments in Central-Southern Chile

In central Chile, many communities rely on water obtained from small catchments in the coastal mountains. Water security for these communities is most vulnerable during the summer dry season and, from 2010 to 2017, rainfall during the dry season was between 20% and 40% below the long-term average. The rate of decrease in stream flow after a rainfall event is a good measure of the risk of flow decreasing below a critical threshold. This risk of low flow can be quantified using a recession coefficient (α) that is the slope of an exponential decay function relating flow to time since rainfall. A mathematical model was used to estimate the recession coefficient (α) for 142 rainstorm events (64 in summer; 78 in winter) in eight monitored catchments between 2008 and 2017. These catchments all have a similar geology and extend from 35 to 39 degrees of latitude south in the coastal range of south-central Chile. A hierarchical cluster analysis was used to test for differences between the mean value of α for different regions and forest types in winter and summer. The value of α did not differ (p < 0.05) between catchments in winter. Some differences were observed during summer and these were attributed to morphological differences between catchments and, in the northernmost catchments, the effect of land cover (native forest and plantation). Moreover, α for catchments with native forest was similar to those with pine plantations, although there was no difference (p < 0.05) between these and Eucalyptus plantations. The recession constant is a well-established method for understanding the effect of climate and disturbance on low flows and baseflows and can enhance local and regional analyses of hydrological processes. Understanding the recession of flow after rainfall in small headwater catchments, especially during summer, is vital for water resources management in areas where the establishment of plantations has occurred in a drying climate.     

Aggregate performance in managing marine ecosystems of 53 maritime countries

Fourteen indicators of marine living resource management performance by country, reflecting both their intention to sustainably use the resource within their Exclusive Economic Zones and the effectiveness of their policies, were developed and the performances of 53 maritime countries were assessed. Four rankings of the countries, which jointly account for over 95 percent of the world's marine fisheries landings, are presented here as aggregated scores of the fourteen indicators, using different schemes for weighting the indicators, each reflective of the management preferences identified by the Global Environment Outlook 4 (GEO4) future development scenarios: Market First; Policy First; Security First; and Sustainability First. The resulting rankings differed substantially between the weighting schemes for the top performing countries but less so for the countries performing poorly.     

What do biphasic flow experiments reveal on the variability of exposure on alluvial fans and which implications for risk assessment result from this?

Sudden avulsions, unexpected channel migrations and backfilling phenomena are autogenic phenomena that can considerably change the propagation patterns of sediment-laden flows on alluvial fans. Once the initial and boundary conditions of the hazard scenario with a given return period are determined, the assessment of the associated exposed areas is based on one numerical, essentially deterministic, process simulation which may not adequately capture the underlying process variability. We generated sediment-laden flows on an experimental alluvial fan by following a “similarity-of-process concept”. Specifically, we considered a convexly shaped alluvial fan model layout featuring a curved guiding channel. As loading conditions, we defined a reference, an increased and a reduced level for the released water volume and the predisposed solid fraction, respectively. Further, we imposed two different stream power regimes and accomplished, for each factor combination, eight experimental runs. The associated exposure areas were recorded by video and mapped in a GIS. We then analysed exposure data and determined exposure probability maps superposing the footprints of the eight repetitions associated with each experimental loading condition. The patterns of exposure referred to the specific loading conditions showed a noticeable variability related to the main effects of the total event volume, the solid fraction, the interactions between them, and the imposed stream power in the feeding channel. Our research suggests that adopting a probabilistic notion of exposure in risk assessment and mitigation is advisable. Further, a major challenge consists in adapting numerical codes to better reflect the stochastics of process propagation for more reliable flood hazard assessments.

     

Challenges for hydropower-based nationally determined contributions: a case study for Ecuador

Hydropower is the dominant renewable energy source to date, providing over two-thirds of all renewable electricity globally. For countries with significant hydropower potential, the technology is expected to play a major role in the energy transition needed to meet nationally determined contributions (NDCs) for greenhouse gas (GHG) emission reductions as laid out in the Paris Agreement. For the Republic of Ecuador, large hydropower is currently considered as the main means for attaining energy security, reducing electricity prices and mitigating GHG emissions in the long-term. However, uncertainty around the impacts of climate change, investment cost overruns and restrictions to untapped resources may challenge the future deployment of hydropower and consequently impact decarbonization efforts for Ecuador’s power sector. To address these questions, a partial equilibrium energy system optimization model for Ecuador (TIMES-EC) is used to simulate alternative electricity capacity expansion scenarios up to 2050. Results show that the share of total electricity supplied by hydropower in Ecuador might vary significantly between 53% to 81% by 2050. Restricting large hydropower due to social-environmental constraints can cause a fourfold increase in cumulative emissions compared to NDC implied levels, while a 25% reduction of hydropower availability due to climate change would cause cumulative emissions to double. In comparison, a more diversified power system (although more expensive) which limits the share of large hydropower and natural gas in favour of other renewables could achieve the expected NDC emission levels. These insights underscore the critical importance of undertaking detailed whole energy system analyses to assess the long-term challenges for hydropower deployment and the trade-offs among power system configuration, system costs and expected GHG emissions in hydropower-dependent countries, states and territories.

Key policy insights

• Ecuador’s hydropower-based NDC is highly vulnerable to the occurrence of a dry climate scenario and restrictions to deployment of large hydropower in the Amazon region.

• Given Ecuador’s seasonal runoff pattern, fossil-fuel or renewable thermoelectric backup will always be required, whatever the amount of hydropower installed.

• Ecuador’s NDC target for the power sector is achievable without the deployment of large hydropower infrastructure, through a more diversified portfolio with non-hydro renewables.

     

Sex change and morphological transitions in a marine ectoparasite

The fitness of parasitic organisms is strongly driven by their ability to infect potential hosts. Although transmission to a host organism is a key component to the parasitic lifestyle, surviving and reproducing within a host poses additional challenges. Cymothoa excisa is a parasitic isopod that infects Atlantic croaker, Micropogonias undulatus, along the Texas coast and has evolved mechanisms to successfully survive and reproduce within its host. Cymothoa excisa is known to exhibit sex‐change strategies but limited information exists on morphological changes, reproductive output and the timing at which sex change becomes optimal. This study collected Atlantic croaker during a 22‐month survey period and identified parasite prevalence and intensity in the host fish population. Infection rates were constant throughout the year at 19.88% and intensity increased through the season up to a maximum of four parasites. Following collection, isopod morphological parameters were quantified for each life stage (including female, transitional, male and juvenile), identifying shape and size transitions through ontogeny and sex change. Transitional C. excisa isopods only occurred when only one isopod was present in a fish, suggesting that isopods change sex from male to female if they are the first to recruit to an uninfected host. As isopods transition to females they have a large increase in size, legs, and pleotelson (which influence fecundity and anchoring ability), whereas the gonopod, eyes and uropod show a reduction (which are no longer needed for swimming and finding hosts). Data suggest that C. excisa sex change is related to the timing of infection and brood size increases with female size and host size. Therefore, it would be advantageous to be the first isopod to infect a host, as it could change into a female and increase reproductive potential. We discuss hypotheses that could explain the mating behavior of parasitic isopods.