A Critical Reappraisal of the Average Degree of Consolidation

Since its development in 1925, Terzaghi’s expression for the average degree of consolidation has been extensively used in teaching, research and consulting to determine the consolidation settlement that has taken place at a specific time in a geotechnical problem. Recent research by the authors has revealed that the term degree of consolidation is not very meaningful when the initial pore pressure distribution is highly skewed. In view of these findings, a review of all aspects of Terzaghi’s consolidation theory is required, particularly when considering asymmetric initial pore pressure distributions. An exact method for estimating the consolidation settlement of a clay stratum using the mass flux per unit area out of the drainage boundaries is proposed. Graphical representations of both exact and traditional methods reveal identical average degree of consolidation curves. In light of this, Terzaghi’s expression for the average degree of consolidation was re-examined. Upon application of the original governing one-dimensional consolidation equation, it was found that the traditional and exact methods for calculating the average degree of consolidation are identical. Thus, the widespread proclivity for geotechnical engineers to use Terzaghi’s average degree of consolidation equation to estimate the consolidation settlement of a clay stratum has been validated.     

The effects of sea salts on the forms of nitrogen released from estuarine and freshwater sediments: Does ion pairing affect ammonium flux?

In sediments with oxidized surface layers, the percentage of mineralized nitrogen that is nitrified/denitrified, compared with that released directly as ammonium, appears to be affected by the presence of sea salts. In estuarine systems, a significant portion of the nitrogen is released as ammonium, whereas in freshwater systems, most of the mineralized nitrogen is often released from the sediments as nitrogen gas. We hypothesized that this discrepancy is caused by differential competition between physical diffusion and nitrification/denitrification in the two systems. The vertical migration (by Fickian diffusion) of ammonium out of the oxic layer may be hindered by cation exchange (or sorption) interactions with sediment particles to a greater extent in fresh water than in estuarine systems. The resulting relatively long residence time, and potentially high levels of particle-bound ammonium in the freshwater sediments, would favor nitrification as the major ammonium removal process. By contrast, ion pair formation of ammonium with seawater anions and blockage of sediment cation exchange sites with seawater cations may allow a sizable fraction of the ammonium to diffuse out of estuarine sediments before it is nitrified. A salt effect, consistent with this hypothesis, has been demonstrated in experimental systems by changing the ionic composition of water flowing above intact cores of freshwater and estuarine sediments. Steady-state ammonium release from Lake Michigan sediments was substantially enhanced in the presence of 30% seawater over that in the presence of lake water alone. Likewise, steady-state ammonium release, from Ochlockonee River and Bay sediments (Florida) and from Toms River and Barnegat Bay sediments (New Jersey), was usually higher in the presence of diluted synthetic seawater than it was in the presence of fresh water.     

Phenolic aldehydes as indicators of terrestrially derived organic matter in the sea

The concentration of lignin degradation products (vanillin, $\text{p-}\text{hydroxybenzaldehyde}$ and syringealdehyde) generated by oxidizing sediment samples with nitrobenzene were considered in an approach to follow the seaward transport of terrestrially derived organic matter. The concentrations of vanillin and syringealdehyde ranged from about 1 per cent of the total ‘organic matter’ in samples taken from inland rivers along the Georgia coast to < 0.1 per cent in samples taken from the continental shelf off the coast of Georgia and Texas.     

Coastal seas as a context for science teaching: a lesson from Chesapeake Bay

Lessons that employ authentic environmental data can enhance the ability of students to understand fundamental science concepts. This differs from traditional "environmental education" in that school curricula need not set aside time for educators to teach only environmental topics. Rather, the "environment" is used to advance student learning in science and technology. The success of this approach depends on programs that encourage scientists to communicate more effectively with teachers at all education levels. The expanding diversity of research and monitoring activities on the world's marine waters constitutes an outstanding potential education resource. Many of these projects involve remote sensing with sophisticated instrumentation and employ Internet technology to compile measurements, interpret data using graphs and satellite imagery, and share the results among scientific colleagues and the general public alike. Unfortunately, these resources, which constitute a much shortened path between research findings and textbook presentation, are seldom interpreted for use by K-12 educators. We have developed an example that uses the Chesapeake Bay as a paradigm to demonstrate how such interpretation can assist educators in teaching important principles in physical oceanography and marine ecology. We present this example using PowerPoint to conduct a virtual tour of selected Internet sources. Our example begins with the conceptual "salt wedge" circulation model of Chesapeake Bay as a partially mixed estuary. Teachers have the opportunity to explore this model using salinity, temperature, and dissolved oxygen data taken from a research vessel platform during summer professional development programs. This source of authentic data, originally obtained by teachers themselves, clearly demonstrates the presence of a picnocline and deep-water anoxia. Our lesson plan proceeds to interpret these data using additional Internet-based resources at increasing scales of time and space. The "salt wedge," picnocline, and anoxia are examined using graphics derived from data taken by researchers using "ScanFish," a towed instrument that samples temperature, salinity, and dissolved oxygen at a resolution of only a few meters vertically and horizontally. The seasonal dynamics of these parameters at a given location are interpreted using biweekly monitoring data obtained as part of the state-federal Chesapeake Bay Program. The influence of annual variations in freshwater input is examined using stream flow data from US. Geological Survey gauging stations. Satellite remote sensing images from the TOPEX/Poseidon project are used to show how El Ni?o and La Ni?a events in the mid-Pacific affect the Chesapeake Bay system via rainfall on its watershed. Finally, the life cycle of the blue crab (Callinectes sapidus) is presented to show how an estuarine organism has adapted to this truly unique and dynamic coastal environment.     

Short‐term microscale topographic changes of coastal bedrock on shore platforms

We report a series of short‐term (diurnal) rock surface monitoring studies on inter‐ and supra‐tidal shore platforms using a traversing micro‐erosion meter at two sites, Kaikoura Peninsula, New Zealand, and Apollo Bay, Victoria, Australia. Statistically signi?cant day‐to‐day changes were measured. Surface rise and lowering occurred at rates above instrument error, with a maximum range of 3·378 mm between 1·697 mm (lowering) and ‐1·681 mm (rise). Individual measurements showed rises greater than 2 mm. These daily variations reveal that surface lowering and rise occur at a much shorter time scale than previously reported from other studies. The patterns observed suggest wetting and drying is the most likely process causing surface changes at these temporal scales. We argue that traversing micro‐erosion meter studies operating at a short‐term time scale of day‐to‐day provide meaningful results that open new opportunities for studying rock weathering and erosion in a coastal environment. Copyright © 2004 John Wiley & Sons, Ltd.     

Modelling discharge from a coastal watershed in southeast Sweden using an integrated framework

The field hydrology model DRAINMOD integrated with Arc Hydro in geographical information system (GIS) framework (Arc Hydro–DRAINMOD) was used to simulate the hydrological response of a coastal watershed in southeast Sweden. Arc Hydro–DRAINMOD uses a distributed approach to route water from each field edge to the watershed outlet. In the framework the Arc Hydro data model was used to describe the stream network in the watershed and to connect the individual simulated DRAINMOD‐field outflow time series from each plot using Arc Hydro schema‐links features, which were summed at Arc Hydro schema‐nodes features along the stream network to generate the stream network flow. Hydrology data collected during six periods between 2003 and 2008 were used to test Arc Hydro–DRAINMOD and its performance was evaluated by considering uncertainties in model inputs using generalized likelihood uncertainty estimation (GLUE). The GLUE estimates obtained (uncertainty bands 5% and 95%) agreed satisfactorily with measured monthly discharges. The percentage of time in which the observed discharges were bracketed by the uncertainty bands was 88% in calibration periods and 75% in validation periods. Although monthly time step simulations showed good agreement with observed discharges during the two main discharge events in spring, the contradictory daily time step results indicate that the watershed response simulations on a daily basis need to be improved. The uncertainty analysis showed that in periods of higher discharge, such as spring periods, the uncertainty in prediction was higher. It is important to note that these uncertainty estimations using the GLUE procedure include the uncertainties in measured discharge values, model inputs, boundary conditions and model structures. It was estimated that stream baseflow represented 42% of the total watershed discharge, but further research is needed to confirm this. These results show that the new Arc Hydro–DRAINMOD framework is applicable for predicting discharge from artificially drained watersheds in southeast Sweden. Copyright © 2010 John Wiley & Sons, Ltd.     

Evaluation of stony coral indicators for coral reef management

Colonies of reef-building stony corals at 57 stations around St. Croix, US Virgin Islands were characterized by species, size and percentage of living tissue. Taxonomic, biological and physical indicators of coral condition were derived from these measurements and assessed for their response to gradients of human disturbance—a requirement for indicators used in regulatory assessments under authority of the Clean Water Act. At the most intensely disturbed location, five of eight primary indicators were highly correlated with distance from the source of disturbance: Coral taxa richness, average colony size, the coefficient of variation of colony size, total topographic coral surface area, and live coral surface area. An additional set of exploratory indicators related to rarity, reproductive and spawning mode and taxonomic identity were also screened. The primary indicators demonstrated sufficient precision to detect levels of change that would be applicable in a regional-scale regulatory program.     

Transitional, entraining, cloudy, and coastal boundary layers

Atmospheric boundary layers are marvelously varied and complex. Recent research has examined some of that variety. Boundary layers over land undergo drastic changes throughout the day as the sun rises and sets, and as clouds form and dissipate. Air is entrained at the top of the boundary layer at varying rates. As air moves over the coast, the boundary layer reacts to changes in surface forcing. All of these changes affect pollutant transport and weather formation. In this paper, research attempting to understand transitional, cloud-topped, and coastal boundary layers, and boundary-layer top entrainment, is reviewed.     

Accuracy of Flowmeters Measuring Horizontal Groundwater Flow in an Unconsolidated Aquifer Simulator

Borehole flowmeters that measure horizontal flow velocity and direction of groundwater flow are being increasingly applied to a wide variety of environmental problems. This study was carried out to evaluate the measurement accuracy of several types of flowmeters in an unconsolidated aquifer simulator. Flowmeter response to hydraulic gradient, aquifer properties, and well‐screen construction was measured during 2003 and 2005 at the U.S. Geological Survey Hydrologic Instrumentation Facility in Bay St. Louis, Mississippi. The flowmeters tested included a commercially available heat‐pulse flowmeter, an acoustic Doppler flowmeter, a scanning colloidal borescope flowmeter, and a fluid‐conductivity logging system. Results of the study indicated that at least one flowmeter was capable of measuring borehole flow velocity and direction in most simulated conditions. The mean error in direction measurements ranged from 15.1° to 23.5° and the directional accuracy of all tested flowmeters improved with increasing hydraulic gradient. The range of Darcy velocities examined in this study ranged 4.3 to 155 ft/d. For many plots comparing the simulated and measured Darcy velocity, the squared correlation coefficient (r²) exceeded 0.92. The accuracy of velocity measurements varied with well construction and velocity magnitude. The use of horizontal flowmeters in environmental studies appears promising but applications may require more than one type of flowmeter to span the range of conditions encountered in the field. Interpreting flowmeter data from field settings may be complicated by geologic heterogeneity, preferential flow, vertical flow, constricted screen openings, and nonoptimal screen orientation.