A transient phreatic surface mound,evidenced by a strip of vegetation on an earth dam

Abstract

On the slopes of the embankment of the Al-Khod groundwater recharge–flood protection dam (Oman), a band of scrub vegetation community emerged after torrential rains and temporary filling of the dam reservoir. Species composition differs markedly on both sides of the embankment, with many exotics found on the reservoir side and more typical gravel-desert species on the outside. Hydro-ecologically, the vegetation is interpreted as the footprint of a temporary storage of water, which is a small-sized groundwater mound within the permeable shoulder of the levee. The levee, as an anthropogenic landform, induces a U-turn (gravitational slumping–lateral seepage–transpirational moisture ascent) topology of seepage. The Lembke method of successive variations of steady states is used in modelling the water table dynamics. In the early stage of the mound decay, outflow through a seepage face of the shoulder is modelled by the Barenblatt slumping parabola of the phreatic-zone part of the flow domain, which is perfectly matched with the Youngs exact solution for a purely horizontal flow through a porous wedge. At the stitching cross-section, the flow rates and saturated depths in the two zones coincide. The late stage of mound evolution is characterized by transpiration by the plant roots projected onto a shrinking free surface, with the Barenblatt and Youngs solutions conjugated but without the outcrop of the saturated mound on the levee slope. Ordinary differential equations for the sliding or descending locus of the intersection of the parabola and the triangle hypotenuse are integrated in a closed form or by the Runge-Kutta method. The dwindling saturated volume and the rate of drainage are obtained. They can be used in assessments of the hydro-ecological sustainability of slope-rooted shrubs (vegetation survival between rare rainfall episodes).

Editor D. Koutsoyiannis; Associate editor A. Porporato     

Improving rainfall–runoff modelling through the control of uncertainties under increasing climate variability in the Ouémé River basin (Benin,West Africa)

ABSTRACT

The objective of this paper is to understand how the natural dynamics of a time-varying catchment, i.e. the rainfall pattern, transforms the random component of rainfall and how this transformation influences the river discharge. To this end, this paper develops a rainfall–runoff modelling approach that aims to capture the multiple sources and types of uncertainty in a single framework. The main assumption is that hydrological systems are nonlinear dynamical systems which can be described by stochastic differential equations (SDE). The dynamics of the system is based on the least action principle (LAP) as derived from Noether’s theorem. The inflow process is considered as a sum of deterministic and random components. Using data from the Ouémé River basin (Benin, West Africa), the basic properties for the random component are considered and the triple relationship between the structure of the inflowing rainfall, the corresponding SDE that describes the river basin and the associated Fokker-Planck equations (FPE) is analysed.

EDITOR D. Koutsoyiannis; ASSOCIATE EDITOR D. Gerten     

Remarks upon numerical solutions of infiltration into a soil profile

Abstract

A methodology of time-step estimation for numerically solving the Richards equation is discussed. Its importance in simulating water movement in unsaturated—saturated soils is shown for infiltration into a soil profile by applying various time-step estimations and boundary conditions for different soils. In order to test the results of the computations, infiltration theory was applied. According to infiltration theory, the pressure head in the initially unsaturated part will not take positive values as long as the moisture front has not reached the phreatic level, or, in the case of a profile with a free-draining lower boundary, it is not saturated at the base. In other cases, the appearance of positive values of the pressure head produces incorrect values for the inflow rate q.     

Timescales of seawater intrusion and retreat

Quantifying the timescales associated with moving freshwater–seawater interfaces is critical for effective management of coastal groundwater resources. In this study, timescales of interface movement in response to both inland and coastal water level variations are investigated. We first assume that seawater intrusion (SWI) and retreat (SWR) are driven by an instantaneous freshwater-level variation at the inland boundary. Numerical modelling results reveal that logarithmic timescales of SWI (lnT_i) and SWR (lnT_r) can be described respectively by various simple linear equations. For example, SWI timescales are described by lnT_i = a + blnh′_f–s, where a and b are linear regression coefficients and h′_f–s is the boundary head difference after an instantaneous drop of inland freshwater head. For SWR cases with the same initial conditions, but with different increases in freshwater head, lnT_r = c + dΔX_T, where c and d are regression coefficients and ΔX_T is the distance of toe response that can be estimated by a steady-state, sharp-interface analytical solution. For SWR cases with the same freshwater head increase, but with different initial conditions, in contrast, lnT_r = e + flnΔX_T, where e and f are regression coefficients. The timescale of toe response caused by an instantaneous variation of sea level is almost equivalent to that induced by an instantaneous inland head variation with the same magnitude of water level change, but opposite in direction. Accordingly, the empirical equations of this study are also applicable for sea-level variations in head-controlled systems or for simultaneous variations of both inland and coastal water levels. Despite the idealised conceptual models adopted in this study, the results imply that for a particular coastal aquifer, SWI timescales are controlled by the boundary water levels after variations, whereas SWR timescales are dominated by the distance of toe response.     

准P波方程紧致交错网格井间地震波场模拟及边界条件

研究井间地震波场的形成过程以及波场的传播机理、规律,对于指导实际井间地震勘探有着重要的意义.基于具有垂直对称轴的横向各向同性(VTI)介质中的一阶准P波方程,应用具有无条件稳定性质的紧致交错网格隐式差分格式求解该方程.重点研究了紧致交错网格求解该方程的完全匹配层(PML)吸收边界条件,在此基础上实现了VTI介质中一阶准P波方程的井间地震波场的正演模拟.数值算例表明:紧致交错网格能精准模拟VTI介质中准P波的传播过程,得到高精度的正演结果.一阶准P波方程能以足够的精度描述VTI介质中准P波特征.完全匹配层吸收边界能有效地解决人工边界问题,是一种高效的边界吸收算法.     

页岩气成藏动力特点及其平衡方程

针对页岩气吸附加游离的成藏方式对其成藏动力特点进行了系统分析。与特定的储集物性条件与相应的气源压力相匹配时,页岩气藏中游离气具有与根缘气或常规储层气相似的作用力类型,包括生烃膨胀力、连续气柱顶界静水柱压力、连续气柱底界静水柱压力、连续气柱重力、毛细管压力和动水头压力等,而造成气体分子在有机质颗粒及岩石矿物表面进行吸附与游离相态转换的作用力类型则为吸附作用力和解吸作用力。天然气动力类型存在运移动力与成藏动力的区别,页岩气作为源储一体的聚集模式,其运移动力和成藏动力具有方向相反特点。根据页岩热演化程度、生气强度、构造条件、物性条件等,将页岩气成藏划分为4个阶段,对每一阶段的成藏动力特点进行了系统分析,据此建立了成藏(运移)动力平衡方程,并采用权重系数赋值方法建立了页岩气成藏(运移)动力平衡方程通式。     

混凝土结构缺陷检测的探地雷达资料波场反演方法

本文针对工程建筑中的混凝土无损探测问题,提出了一种新的反演成像方法.它避免了传统方法观察偏移探测数据的情况.这种方法以探地雷达为探测手段,以电磁场原理为理论依据,其结果更具定量化和可视性.具体算法是将阻尼高斯牛顿法和GCV方法(Generalized Cross Validation)结合起来,其中阻尼牛顿法可以避免传统牛顿法计算Hessian矩阵的困难,GCV方法可以自适应地选择正则化参数,既减小了计算量又克服了噪声及不适定性的影响.数值算例验证了算法的有效性.     

正压大气模式下具有β效应与地形效应的Rossby孤立波

正压大气模式下,采用摄动方法和时空伸长变换推导了具有β效应、地形效应和耗散的mKdV-Burgers方程,得到Rossby孤立波振幅的演变满足带有β效应,地形与耗散的mKdV-Burgersm方程的结论.说明β效应、地形效应是诱导Rossby孤立波的重要因素.     

川东气田盐岩、膏盐岩蠕变特性试验研究

对川东气田盐岩、膏盐岩进行了矿物组分测定,采用TAW-1000深水孔隙压力伺服试验系统进行了蠕变试验,分析了固有的矿物组分及偏应力、温度、围压对蠕变的影响,结合蠕变曲线和岩石参数,提出了稳态蠕变速率本构方程。研究表明：川东气田盐岩、膏盐岩矿物组分以NaCl、CaSO4为主,其中NaCl含量最高可达93%以上;岩石固有的矿物组分是影响岩石蠕变特性的内因,随NaCl含量增加,瞬态蠕变、稳态蠕变速率相应增大;随偏应力、温度的增加稳态蠕变速率增大,围压对其影响不大;通过试验数据拟合得出赫德（Heard）蠕变本构方程中的参数,求出不同偏应力、温度下盐岩、膏盐岩的稳态蠕变速率,为川东气田盐岩、膏盐岩地层安全钻进泥浆密度窗口计算和套管非正常变形分析提供了理论依据。     

盾构掘进速度及非正常停机对地面沉降的影响

软土中盾构隧道施工不可避免地扰动周围地层,进而引起地面沉降,沉降过大时将危及邻近建（构）筑物的正常使用和结构安全。全面理解盾构隧道施工引起的地面沉降的影响因素及对沉降的准确预测,对于减少施工环境危害十分重要。考虑盾构压重后,引入Mindlin解计算盾构下卧土层中的附加应力,采用单向压缩分层总和法计算盾构下卧土层的总固结沉降,由盾构掘进速度及停机时间确定附加应力作用时间后,应用太沙基一维固结理论计算在该作用时间内的固结沉降,应用Peck公式建立了盾构下卧土层沉降与地面沉降的关系,并以杭州庆春路过江隧道地面沉降的实测验数据对上述理论进行了验证。分析表明,考虑盾构掘进速度及停机时间的地面沉降计算理论基本合理;盾构掘进速度及停机时间会对隧道施工引起的地面沉降产生显著影响;在其他施工条件相同的前提下,提高盾构掘进速度和减少停机时间有利于减少地面沉降。