Comparative analysis between analytical approximations and numerical solutions describing recession flow in unconfined hillslope aquifers

Recession flow of aquifers from a hillslope can be described by the non-linear Boussinesq equation. Under strong assumptions and for specific conceptual formulations, different authors derived analytical approximations or linearized versions to this partial differential equation. A comparative analysis between some analytical approximations of the Boussinesq equation and the numerical solution of the recession flow of an unconfined homogeneous aquifer (horizontal, inclined and concave aquifer floor) was carried out. The objective was to define the range where the analytical solutions approximate the numerical solution. The latter was considered in this study as the reference method, because it requires fewer assumptions. From the considered analytical approximations, exponential decay relationships were found to be mainly valid for fine domain materials when horizontal, mild slopes (less than 2%) and concave aquifer floors were considered, but failed to reproduce coarse aquifer numerical model outflows, in contrast to the quadratic decay relationship, which better reproduce outflows in such domains. On the basis of the comparative analysis, it has been found that recession flows obtained with the considered analytical approximations yield similar values only for certain ranges of aquifer properties and geometries.

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Résumé L’écoulement de décrue dans les aquifères de versant peut être décrit par l’équation non-linéaire de Boussinesq. En considérant d’importantes hypothèses et des formulations conceptuelles spécifiques, différents auteurs ont dérivé des approximations analytiques ou des versions linéarisées de cette équation partielle différentielle. Une analyse comparative entre des approximations analytiques de l’équation de Boussinesq et une solution numérique de la décrue d’un aquifère homogène libre (limite inférieure de l’aquifère horizontale, inclinée et concave) a été effectuée. L’objectif était de définir la plage pour laquelle les solutions analytiques approchent la solution numérique. Cette dernière a été considérée dans cette étude comme la méthode de référence car elle demande moins d’hypothèses. Parmi les approximations analytiques considérées, les relations de décroissance exponentielle se sont avérées être principalement valides pour des matériaux fins lorsque des couches aquifères horizontales, concaves et de pentes douces (moins de 2%) étaient considérées; en revanche, celles-ci n’ont pas réussi à reproduire les écoulements sortants d’un modèle numérique pour un aquifère à matériaux grossiers, contrairement à la relation de décroissance quadratique qui reproduit mieux les écoulements sortants dans de tels milieux. Sur la base de l’analyse comparative, il s’est avéré que les écoulements en décrue obtenus avec les approximations analytiques considérées présentent des valeurs similaires seulement pour certains critères géométriques et propriétés de l’aquifère.

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Resumen Puede describirse el flujo de recesión de acuíferos de una ladera mediante la ecuación no-lineal de Boussinesq. Bajo fuertes supuestos y para formulaciones conceptuales específicas, distintos autores han derivado aproximaciones analíticas o versiones lineales para esta ecuación parcial diferencial. Se realizó un análisis comparativo entre algunas aproximaciones analíticas de la ecuación de Boussinesq y la solución numérica del flujo de recesión de un acuífero homogéneo no confinado (piso de acuífero cóncavo, inclinado, y horizontal). El objetivo consistió en definir el rango donde las soluciones analíticas aproximan la solución numérica. En este estudio se consideró la solución numérica como el método de referencia debido a que requiere menores supuestos. De las aproximaciones analíticas consideradas se encontró que las relaciones de desintegración exponencial eran principalmente válidas para materiales de ámbitos finos donde se consideraron pisos de acuíferos horizontal, pendientes medianas (menores a 2%) y cóncavos pero fallaron en reproducir flujos de salida de modelos numéricos de ámbito grueso, en contraste con la relación de desintegración cuadrática la cual reproduce mejor los flujos de salida en tales ámbitos. En base al análisis comparativo se encontró que los flujos de recesión obtenidos con las aproximaciones analíticas consideradas aportaron valores similares solo para ciertos rangos de propiedades y geometrías de acuíferos.

     

Nouvelles données structurales et datations 40K–40Ar sur les roches métamorphiques de la région de Neyriz (zone de Sanandaj–Sirjan,Iran méridional). Leur intérêt dans le cadre du domaine néo-téthysien du Moyen-Orient

The metamorphic rocks of the Neyriz area (Sanandaj–Sirjan zone) represent a Palaeozoic sequence, the upper part of which being palaeontologically dated from the Carboniferous and the Permian. Field structural analysis of the whole sequence, detailed in laboratory by microstructural one and ⁴⁰K–⁴⁰Ar dating carried on separated minerals, lead to establish that the whole sequence, from gneisses to Permian rocks, has suffered a unique synmetamorphic deformation, of variable intensity, marked by a foliation. Isotopic ages measured on extracted amphiboles and micas, clustered in four groups between 300 and 60 Ma, show the successive stages of their slow exhumation, which ended by the end of the Cretaceous. To cite this article: R. Sheikholeslami et al., C. R. Geoscience 335 (2003).     

Signification des ferruginisations des formations néoprotérozoı̈ques du Nord-Burkina Faso (Afrique de l'Ouest)Meaning of ironstones in the sedimentary Neoproterozoic formations of the northern Burkina Faso (western Africa)

There are many small ferruginous outcrops of different facies, often breccia-like, in the Neoproterozoic sedimentary formations in northern Burkina. These outcrops are made up of goethite and quartz, and are often along with high grades of various elements. It could be a question of gossans. Their large distribution in this part of the Taoudéni Basin offers it prospects as a province geochemically rich in Cu, Pb, Zn, Mo, As, Cd, Co... This basin would be a geochemical bin for ancient formations, which would have been evacuated before the Neoproterozoic. The ironstones would be the mark of further concentrations. To cite this article: A. Blot, C. R. Geoscience 334 (2002) 909–915.     

巢湖北部地区海相地层黏土矿物特征及其地质意义

对巢湖北部地区以碳酸盐岩为主的寒武系—下三叠统海相地层的黏土矿物分析及伊利石结晶度测定的结果表明:伊利石是本区海相地层的主体矿物,平均体积分数为73%。黏土矿物的成岩转化特征表现为蒙皂石的伊利石化。按照伊利石结晶度的成岩-浅变质程度划分,本区寒武系—下三叠统的成岩作用已达到最高成岩阶段。伊利石结晶度的纵向变化总体上可分为上、下两部分:下部寒武系至志留系的结晶度为0.41～0.60°Δ2θ,结晶度与埋深具有一定的相关性,成岩作用是下部地层黏土矿物特征的主控因素;上部泥盆系至下三叠统的结晶度为0.45～0.77°Δ2θ,呈波动变化,蒙皂石连续出现,地层没有被显著埋藏,沉积因素是上部地层黏土矿物特征的主控因素。下三叠统地层中蒙皂石的分布特征以及上泥盆统五通组顶部高岭石的普遍富集具有重要的古气候研究意义。     

Detecting a Defective Casing Seal at the Top of a Bedrock Aquifer

An improperly sealed casing can produce a direct hydraulic connection between two or more originally isolated aquifers with important consequences regarding groundwater quantity and quality. A recent study by Richard et al. (2014) investigated a monitoring well installed in a fractured rock aquifer with a defective casing seal at the soil–bedrock interface. A hydraulic short circuit was detected that produced some leakage between the rock and the overlying deposits. A falling‐head permeability test performed in this well showed that the usual method of data interpretation is not valid in this particular case due to the presence of a piezometric error. This error is the direct result of the preferential flow originating from the hydraulic short circuit and the subsequent re‐equilibration of the piezometric levels of both aquifers in the vicinity of the inlet and the outlet of the defective seal. Numerical simulations of groundwater circulation around the well support the observed impact of the hydraulic short circuit on the results of the falling‐head permeability test. These observations demonstrate that a properly designed falling‐head permeability test may be useful in the detection of defective casing seals.     

Use of a Sodar to Improve the Forecast of Fogs and Low Clouds on Airports

A sodar was deployed at Roissy–Charles de Gaulle airport near Paris, France, in 2008 with the aim of improving the forecast of low visibility conditions there. During the winter of 2008–2009, an experiment was conducted that showed that the sodar can effectively detect and locate the top of fog layers which is signaled by a strong peak of acoustic reflectivity. The peak is generated by turbulence activity in the inversion layer that contrasts sharply with the low reflectivity recorded in the fog layer below. A specific version of the 1D-forecast model deployed at Roissy for low visibility conditions (COBEL-ISBA) was developed in which fogs’ thicknesses are initialized by the sodar measurements rather than the information derived from the down-welling IR fluxes observed on the site. It was tested on data archived during the winters of 2008–2009 and 2009–2010 and compared to the version of the model presently operational. The results show a significant improvement—dissipation times of fogs are better predicted.     

Detrital transport of metals by glaciers, an example from the Pinchi Mine, central British Columbia

 Abundant cinnabar (HgS) mineralization is associated with the Pinchi Fault in central British Columbia. Two formerly producing mercury mines have been developed on this fault: Pinchi and Bralorne Takla. The mercury content of till (a sediment type directly deposited by glaciers) in the area of this fault is primarily controlled by the occurrence of cinnabar mineralization in bedrock and the direction of ice flow. Cinnabar-bearing bedrock was eroded by glaciers, transported in the direction of ice flow, and deposited "down-ice" from its source. An example of such a dispersal train is documented for the Pinchi Mine area where mercury ore was transported over a distance of 12 km, as measured in the clay-sized fraction (< 0.002 mm) of till, and could have been transported over 24 km according to heavy mineral concentrates (specific gravity >3.3) of this same sediment. Antimony, chromium, and nickel dispersal trains were also detected in the region. These data indicate that natural glacial processes can result in the "mobilization" of metals in the surficial environment, a factor which has to be considered at mine sites in glaciated terrain, where mine reclamation and remediation measures are now required. Received: 31 October 1996 · Accepted: 27 May 1997     

Cassini UVIS observations of Jupiter's auroral variability

The Cassini spacecraft Ultraviolet Imaging Spectrograph (UVIS) obtained observations of Jupiter's auroral emissions in H₂ band systems and H Lyman-α from day 275 of 2000 (October 1), to day 81 of 2001 (March 22). Much of the globally integrated auroral variability measured with UVIS can be explained simply in terms of the rotation of Jupiter's main auroral arcs with the planet. These arcs were also imaged by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope (HST). However, several brightening events were seen by UVIS in which the global auroral output increased by a factor of 2-4. These events persisted over a number of hours and in one case can clearly be tied to a large solar coronal mass ejection event. The auroral UV emissions from these bursts also correspond to hectometric radio emission (0.5-16 MHz) increases reported by the Galileo Plasma Wave Spectrometer (PWS) and Cassini Radio and Plasma Wave Spectrometer (RPWS) experiments. In general, the hectometric radio data vary differently with longitude than the UV data because of radio wave beaming effects. The 2 largest events in the UVIS data were on 2000 day 280 (October 6) and on 2000 days 325-326 (November 20-21). The global brightening events on November 20-21 are compared with corresponding data on the interplanetary magnetic field, solar wind conditions, and energetic particle environment. ACE (Advanced Composition Explorer) solar wind data was numerically propagated from the Earth to Jupiter with an MHD code and compared to the observed event. A second class of brief auroral brightening events seen in HST (and probably UVIS) data that last for ∼2 min is associated with auroral flares inside the main auroral ovals. On January 8, 2001, from 18:45-19:35 UT UVIS H₂ band emissions from the north polar region varied quasiperiodically. The varying emissions, probably due to auroral flares inside the main auroral oval, are correlated with low-frequency quasiperiodic radio bursts in the 0.6-5 kHz Galileo PWS data.