Local- and watershed-scale controls on the spatial variability of natural levee deposits in a large fine-grained floodplain: Lower Pánuco Basin, Mexico

This study examines spatial variations in natural levee deposits within the lower reaches of a large coastal plain drainage system. The Pánuco basin (98,227 km²) drains east-central Mexico, and is an excellent setting to examine the influence of watershed and local controls on the morphology and sedimentology of natural levees. Although many fluvial systems in the U.S. Gulf Coastal Plain have been investigated, the rivers in the Mexican Gulf Coastal Plain have received comparatively little attention. Lateral and downstream characteristics of natural levee morphology and sediment texture are considered within the context of meandering river floodplain deposits. Data sources include total-stations surveying, sediment samples of surficial levee deposits, topographic maps (1:50,000), and aerial photographs (1:40,000). The slope of natural levees average 0.0049 m/m, whereas the texture (D₈₄) of levee deposits averages 0.12 mm. Natural levee characteristics vary due to local- and watershed-scale controls. The lateral reduction in levee height displays a curvilinear pattern that coincides with an abrupt change in sediment texture. The downstream pattern of natural levee texture exhibits the influence of local-scale perturbations superimposed upon a larger watershed-scale trend. Disruption to the fining trend, either by tributary inputs of sediment or reworking of Tertiary valley deposits, is retained for a limited distance. The influence of the channel planform geometry on levee morphology is examined by consideration of the radius of curvature (R_c) of meander bends, and is inversely related to natural levee width. This suggests that the planform geometry of river channels exerts a control on the dispersal of flood sediments, and is responsible for considerable local variability in the floodplain topography. The average width of natural levees increases with drainage area, from an average of 747 m in the Moctezuma to an average of 894 m in the Pánuco. However, in the lower reaches of the Pánuco valley the width of natural levees rapidly decreases, which is associated with fining of the suspended sediment load. Thus, the reduction in natural levee width signifies an abrupt change in the directionality of cause–effect relationships at the watershed-scale. Findings from this study elucidate linkages between meandering river channels and floodplains for a large lowland alluvial valley.     

Developments in triaxial testing technique

Summary A simple triaxial cell is described together with details of the technique employed for triaxial testing. Triaxial strength results for 254 specimens of eight rock types are tabulated.

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Zusammenfassung Ein einfaches Triaxial-Prüfgerät wird, zusammen mit Einzelheiten des experimentellen Verfahrens, beschrieben. Ergebnisse dreiachsiger Festigkeitsprüfung sind für 254 Proben aus acht verschiedenen Gesteinsarten in Tabellen wiedergegeben.

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Résumé Un appareil triaxial simple est décrit avec des détails sur la technique expérimentale. Des résultats de résistance triaxiale pour 254 échantillons pris parmi huit types de roche sont donnés.

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With 2 Figures     

Longitudinal characterization of a tidal marsh creek separating two hydrographically distinct estuaries

Jones Creek, South Carolina, connects two hydrographically distinct estuarine systems: North Inlet and Winyah Bay. One hundred fifty-two sets of hydrographic measurements from February 1978 indicate the existence of a Jones Creek nodal point, an effective barrier limiting flow exchange between the end regions of the creek. Morphologic and vegetation observations and water quality ratings suggest that this nodal point is a permanent feature of Jones Creek and may be present in other similar systems.     

The stability of transition metal dolomites in carbonate systems: a discussion

The instability of transition metal dolomites [CaR²⁺(CO₃)₂ where R²⁺ is Fe, Co, Ni, Cu, or Zn] and the limited substitution of transition metal cations for Mg in the dolomite structure can be accounted for by the effect of octahedral distortion. For example, trigonal elongation of the Fe octahedron, due to the Jahn-Teller effect, observed in siderite and ankerite, results in elongation of the Ca octahedron which is sensitive to distortion because the radius of Ca²⁺ is close to the upper limit for octahedral coordination. Co, Ni, Cu, Zn octahedra are also thought to be deformed, relative to Mg octahedra, in carbonates.The free energy of formation (ΔG^o_f) of R²⁺CO₃ becomes more positive with increasing octahedral distortion. Estimated ΔG^o_f(dolomite) as well as stabilities and solubility limits of R²⁺ in natural and synthetic dolomites suggest a series in order of decreasing stability: Mg >Mn >Zn >Fe >Co >Ni >Cu.ΔG^o_f(est.) for the terminal Fe-dolomite solid solution [72 mol% CaFe(CO₃)₂] in the system CaCO₃-MgCO₃-FeCO₃ may represent an empirical threshold value for dolomite stability which lies between ΔG^o_f for Mn- and Zn-dolomites. While Zn-dolomite is probably not a stable phase, very extensive solid solution toward CaZn(CO₃)₂ is to be expected in the system CaCO₃-MgCO₃-ZnCO₃. The tendency for transition metal dolomites to contain excess CaCO₃ can also be accounted for in terms of octahedral distortion and AG^o_f.     

On the influence of urbanization on the water budget in Nairobi city: A numerical study

The impact of the growth and development of the City of Nairobi on the water budget is simulated using a high-resolution limited-area numerical model. The water substance fields are modelled with full physics in a control experiment. Five sensitivity experiments are then performed by altering the land-use/cover over the domain of study to assess the influence of the city, forests and terrain undulations on the water substance fields. Results showed that the highest evaporation occurred in areas of the study domain with open grasslands/scattered bush-land's vegetation types and the least at the city centre. Deforestation would lead to a substantial increase in the loss of water effected through evaporation despite a reduction in transpiration. The observed rainfall amount and frequency were highest in the high ground portions to the northwest of the study domain. Numerical analyses showed that the urban heat island had a destabilizing effect on the flow, which enhanced convection that resulted in increased rainfall downwind of the urban area. Further growth and expansion of the city of Nairobi would increase the area and amount of rainfall received. Deforestation would decrease rainfall amounts. Massive reforestation would increase the observed rainfall. There has been a decrease in soil moisture at the current location of the city centre; the decrease is bound to increase with the expansion of the city. The City of Nairobi has resulted in a large decrease in the soil moisture through converting the natural fabric to concrete/asphalt material. Deforestation would result in a marginal decrease in the soil moisture. Further growth and development of Nairobi City would modify the water substance budget appreciably. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rock mass characterization using photoanalysis

Summary Rock formations are distinguished from each other by measuring first the properties of the intact rock, and second those of the jointing. Whereas simple methods are available for measuring intact rock properties, those available for measuring jointing remain slow, expensive, and sometimes dangerous. Digitized photographs (photoanalysis) may provide a solution. In this paper, the new techniques of photoanalysis are reviewed together with applications, promising areas for research, and also some obstacles that remain to be overcome. Aspects of the rock mass that lend themselves to photoanalytical measurement include those of individual joints, such as persistence, orientation and roughness, and those relating to the mass as a whole, such as block size and the spacing or intensity of jointing. Photoanalysis can also be applied to measurement of blasting. It allows characterization of the rock about to be blasted, helping the engineer to predict fragmentation and to design an appropriate blasting pattern. Afterwards, the same methods can be used to measure fragmentation, overbreak and backbreak, for quality control and for diagnosis of problems.Presented at the 28th US ROck Mechanics Symposium, Tucson, Arizon, 29 June–1 July 1987.     

Guest Editorial: Spatial demography in regional science

Journal of Geographical Systems -     

The demographic burden of population loss in US cities, 2000–2010

Journal of Geographical Systems - Although the effects of urban shrinkage on quality of life and the built environment have received a great deal of attention, the characteristics of those...