Floodplain sedimentation and sensitivity: summer 1993 flood,Upper Mississippi River Valley

Patterns of overbank sedimentation in the vicinity of, and far removed from, levee breaks that occurred in response to the >100 year, summer 1993 flood in the upper Mississippi River valley are elucidated. Two suites of overbank deposits were associated with the failure of artificial levees within a 70 km long study reach. Circumjacent sand deposits are a component of the levee break complex that develops in the immediate vicinity of a break site. As epitomized by the levee break complex at Sny Island, these features consist of an erosional, scoured and/or stripped zone, together with a horseshoe-shaped, depositional zone. At locales farther removed from the break site, the impact of flooding was exclusively depositional and was attributed to the settling of suspended sediment from the water column. The overall picture was one of modest scour at break sites and minimal suspended deposition (<4 mm) at locales farther removed from the breach. Downriver from the confluence with the Missouri River, suspended sediment deposition was of a similar magnitude to that observed within the study reach and levee break complexes exhibited a similar morphology, but scour at break sites was greatly enhanced and the excavated sand formed extensive deposits on the floodplain surface. The different erosional response was probably engendered by the higher sand content and reduced aggregate cohesion of the floodplain soils downriver from the confluence with the Missouri River. A qualitative comparison serves to highlight the influence that the resistance threshold may have on the sensitivity of floodplains bordering large low-gradient rivers to high magnitude floods. © 1997 John Wiley & Sons, Ltd.     

Improved 2-D attenuation analysis for Northern Italy using a merged dataset from selected regional seismic networks

A merged, high-quality waveform dataset from different seismic networks has been used to improve our understanding of lateral seismic attenuation for Northern Italy. In a previous study on the same region, Morasca et al. (Bull Seismol Soc Am 98:1936–1946, 2008) were able to resolve only a small area due to limited data coverage. For this reason, the interpretation of the attenuation anomalies was difficult given the complexity of the region and the poor resolution of the available data. In order to better understand the lateral changes in the crustal structure and thickness of this region, we selected 770 earthquakes recorded by 54 stations for a total of almost 16,000 waveforms derived from seismic networks operating totally or partially in Northern Italy. Direct S-wave and coda attenuation images were obtained using an amplitude ratio technique that eliminates source terms from the formulation. Both direct and early-coda amplitudes are used as input for the inversions, and the results are compared. Results were obtained for various frequency bands ranging between 0.3 and 25.0 Hz and in all cases show significant improvement with respect to the previous study since the resolved area has been extended and more crossing paths have been used to image smaller scale anomalies. Quality-factor estimates are consistent with the regional tectonic structure exhibiting a general trend of low attenuation under the Po Plain basin and higher values for the Western Alps and Northern Apennines. The interpretation of the results for the Eastern Alps is not simple, possibly because our resolution for this area is still not adequate to resolve small-scale structures.     

Tectonic influences on ground water quality: insight from complementary methods

A study using multiple techniques provided insight into tectonic influences on ground water systems; the results can help to understand ground water systems in the tectonically active western United States and other parts of the world. Ground water in the San Bernardino Valley (Arizona, United States and Sonora, Mexico) is the main source of water for domestic use, cattle ranching (the primary industry), and the preservation of threatened and endangered species. To improve the understanding of ground water occurrence, movement, and sustainability, an investigation was conducted using a number of complementary methods, including major ion geochemistry, isotope hydrology, analysis of gases dissolved in ground water, aquifer testing, geophysics, and an examination of surface and subsurface geology. By combining information from multiple lines of investigation, a more complete picture of the basin hydrogeology was assembled than would have been possible using fewer methods. The results show that the hydrogeology of the San Bernardino Valley is markedly different than that of its four neighboring basins in the United States. The differences include water quality, chemical evolution, storage, and residence time. The differences result from the locally unique geology of the San Bernardino Valley, which is due to the presence of a magmatically active accommodation zone (a zone separating two regions of normal faults with opposite dips). The geological differences and the resultant hydrological differences between the San Bernardino Valley and its neighboring basins may serve as a model for the distinctive nature of chemical evolution of ground water in other basins with locally distinct tectonic histories.     

Biofilm Detection in a Model Well‐Bore Environment Using Low‐Field NMR

This research addresses the challenges of the lack of non‐invasive methods and poor spatiotemporal resolution associated with monitoring biogeochemical activity central to bioremediation of subsurface contaminants. Remediation efforts often include growth of biofilm to contain or degrade chemical contaminants, such as nitrates, hydrocarbons, heavy metals, and some chlorinated solvents. Previous research indicates that nuclear magnetic resonance (NMR) is sensitive to the biogeochemical processes of biofilm accumulation. The current research focuses on developing methods to use low‐cost NMR technology to support in situ monitoring of biofilm growth and geochemical remediation processes in the subsurface. Biofilm was grown in a lab‐scale radial flow bioreactor designed to model the near wellbore subsurface environment. The Vista Clara Javelin NMR logging device, a slim down‐the‐borehole probe, collected NMR measurements over the course of eight days while biofilm was cultivated in the sand‐packed reactor. Measured NMR mean log T₂ relaxation times decreased from approximately 710 to 389 ms, indicating that the pore environment and bulk fluid properties were changing due to biofilm growth. Destructive sampling employing drop plate microbial population analysis and scanning electron and stereoscopic microscopy confirmed biofilm formation. Our findings demonstrate that the NMR logging tool can detect small to moderate changes in T₂ distribution associated with environmentally relevant quantities of biofilm in quartz sand.     

Radar observations of prevailing winds and waves in the Southern Hemisphere mesosphere and lower thermosphere

HF radar stations (utilizing the spaced-antenna partial-reflection technique) located at Adelaide (35°S, 138°E) and Mawson Station (67°S, 63°E) have observed horizontal mesospheric winds continuously since mid-1984. Observations in the period 1984–87 are compared with the Northern Hemisphere [latitude conjugate] stations of Kyoto (35°N, 136°E) and Poker Flat (65°N, 147°W), and with satellite-derived circulation models. Particular reference is made to the equinoctial changeovers in zonal flow and to the temporal and altitude variations in the planetary wave activity at Mawson and Adelaide.     

An evaluation of the health status of the Lavaca Bay, Texas ecosystem using Crassostrea virginica as the sentinel species

Locational risks for compromised ecosystem health for the eastern oysters (Crassostrea virginica) harvested from Lavaca Bay, Texas were estimated. Flow cytometric evaluation of variations in DNA content and the lysosomal destabilization assay were used for evaluation of genotoxicity and stress, respectively. Bayesian geo-statistical methods were utilized to estimate and evaluate spatial effects. For models with spatial risks, continuous surface maps of predicted parameter values were created to evaluate risk location. Lysosomal destabilization assay results were spatially oriented whereas flow cytometry results were fit best with the random effects model. While not spatially oriented, the highest levels of variations in DNA content were also present near industrial facilities. Locational risks of increased biomarkers of genotoxicity and stress in the eastern oyster (Crassostrea virginica) were increased with proximity to industrial facilities     

Avulsion regimes in southeast Texas rivers

Avulsions – relatively sudden changes in course, or establishment of new anabranches – are an important process in alluvial rivers. Their key role in floodplain construction and alluvial architecture, and the general conditions favouring avulsions, are well known. However, avulsion processes and evolution, and the factors controlling avulsion regimes, are poorly understood. In the southeast Texas coastal plain, where avulsions are common features of the river valleys, avulsions were studied on the lower Brazos, Navasota, Trinity, Neches and Sabine rivers using a combination of aerial imagery, digital elevation models and field surveys. Avulsions have important influences on the surface morphology and contemporary processes in all five rivers. Features associated with avulsions are active and distinct throughout the study area, and all the rivers have experienced geologically (if not historically) recent avulsions. However, no two of the study rivers have the same contemporary avulsion regime. First‐order differences in avulsion style are controlled by the stage of valley filling, and within the three rivers characterized by an unfilled incised valley, antecedent morphology associated with late Quaternary and Holocene coastal and fluvial‐deltaic processes accounts for the major differences. In the Navasota (27 avulsions in 185 km) and Neches (21 in 340 km) rivers, subchannels associated with avulsions exist in all stages of development from active to infilled, and some have occurred in recent decades. The other rivers have fewer avulsions, but both the Sabine and Trinity have experienced historic channel shifts. Only the Brazos River has experienced no avulsions within the past c. 300 years. Results show that even within a region of similar environmental controls and geological history local variations in inherited morphology can result in different avulsion regimes. Copyright © 2008 John Wiley & Sons, Ltd.     

Induced Microearthquake Patterns in Hydrocarbon and Geothermal Reservoirs: Six Case Studies

—?The injection or production of fluids can induce microseismic events in hydrocarbon and geothermal reservoirs. By deploying sensors downhole, data sets have been collected that consist of a few hundred to well over 10,000 induced events. We find that most induced events cluster into well-defined geometrical patterns. In many cases, we must apply high-precision, relative location techniques to observe these patterns. At three sedimentary sites, thin horizontal strands of activity are commonly found within the location patterns. We believe this reflects fracture containment between stratigraphic layers of differing mechanical properties or states of stress. At a massive carbonate and two crystalline sites, combinations of linear and planar features indicate networks of intersecting fractures and allow us to infer positions of aseismic fractures through their influence on the location patterns. In addition, the fine-scale seismicity patterns often evolve systematically with time. At sedimentary sites, migration of seismicity toward the injection point has been observed and may result from slip-induced stress along fractures that initially have little resolved shear. In such cases, triggering events may be critical to generate high levels of seismic activity. At one crystalline site, the early occurrence of linear features that traverse planes of activity indicate permeable zones and possible flow paths within fractures. We hope the continued development of microseismic techniques and refinement of conceptual models will further increase our understanding of fluid behavior and lead to improved resource management in fractured reservoirs.     

Consequences of short‐term changes in coastal processes: a case study

In 1997–98, unique critical beach erosion led to structural failure along the Penarth, South Wales, UK coastline and anthropogenic activities, such as the construction of the Cardiff Bay Barrage and offshore marine aggregate dredging, were suggested as causes. The time‐frame of significant erosion was between 1995 and 1997 and forcing agents (extreme sea level and wind direction) and shoreline indicators (mean beach level and MHW) were analysed in order to assess change. Water level analysis showed that although there was no significant difference between actual and predicted mean sea levels, extreme sea levels at that time were significantly higher (t = 3·305; d.f. = 8; p < 0·05). Three wind direction analyses (annual mean, mean annual maximum gust and mean annual maximum gust ≥28 kn) between 1995 and 1997 also showed significant differences (p < 0·05). All comprised more easterly components which meant they approached the beach from the sea. Furthermore, gusts ≥28 kn from the northeast quadrant, that is, 0° to 90° true, were significantly more frequent during these years (t = 3·674; d.f. = 8; p < 0·01). Justification of statistical significances was established and there was supporting evidence of unusual meteorological conditions at that time. Relationships showed correlation between forcing agents (extreme sea level and wind direction) and shoreline indicators (mean beach level and Mean High Water). Furthermore, regression analysis showed winds from the northeast quadrant resulted in steeper longshore gradients, as a consequence of beach material loss. Therefore, it was concluded that the critical erosion of Penarth beach between 1995 and 1997 was caused by increased wave attack from the northeast and southeast quadrants, generated by unique significant changes in wind direction and extreme sea levels. Copyright © 2008 John Wiley & Sons, Ltd.