The logarithmic profile in wind erosion: An algebraic solution

This paper presents a simple algebraic approach to the interpretation of wind erosion data that allows for density variation close to the surface due to the presence of large amounts of saltating material. The effect explains nonlinearities in the logarithmic velocity profile near the surface and extends the profile into the saltating layer.     

Effect of discharge on the chlorophylla distribution in the tidally-influenced Potomac River

In the tidal Potomac River, high river discharges during the spring are associated with high chlorophylla concentrations in the following in the following summer, assuming that summertime light and temperature conditions are favorable. Spring floods deliver large loads of particulate N and P to the tidal river. This particulate N and P could be mineralized by bacteria to inorganic N and P and released to the water column where it is available for phytoplankton use during summertime. However, during the study period relatively low concentrations of chlorophylla (less than 50 μg l^?1 occurred in the tidal river if average monthly discharge during July or August exceeded 200 m³s^?1. Discharge and other conditions combined to produce conditions favorable for nuisance levels of chlorophylla (greater than 100 μg l^?1 approximately one year out of four. Chlorophylla maxima occurred in the Potomac River transition zone and estuary during late winter (dinoflagellates) and spring (diatoms). Typical seasonal peak concentrations were achieved at discharges as high as 970 m³ s^?1, but sustained discharges greater than 1,100 m³ s^?1 retarded development. Optimum growth conditions occurred following runoff events of 10 to 15 d duration which produced transit times to the transition zone of 7 to 10 d. Wet years with numerous moderate-sized runoff events, such as 1980, tend to produce greater biomass in the transition zone and estuary than do dry years such as 1981.     

The Cold Bay Volcanic Center,Aleutian Volcanic arc

The widespread abundance of Hi-Alumina Basalt (HAB) lavas in most volcanic arcs has been suggested by some as evidence for a primary, parental HAB magma generated by the high pressure melting of subducted oceanic crust (quartz eclogite). Others suggest a parental, mantle-derived olivine tholeiite magma which produces HAB magmas through fractionation of olivine, clinopyroxene, chrome-spinel +/– plagioclase. The petrology and geochemistry of seven HAB lavas from the Aleutian Cold Bay Volcanic Center have been studied in order to specifically address these two possibilities. All lavas show mineralogical and compositional features typical of most Aleutian HAB lavas. Coexisting opx and cpx in a closely associated basaltic-andesite indicate a minimum pre-eruption temperature of 1,110° C. A comparison of the observed (plag-tmag-olivcpx) and experimentally determined crystallization sequences yields a minimum pre-eruption pressure estimate of 7 kb and estimated H₂O contents of 0.7 wt.%. Maximum pre-eruption f _{o 2} values have been estimated at NNO+0.6 log units.Mass balance calculations demonstrate that the HAB compositions are satisfied by the fractionation of olivine, clinopyroxene +/– plagioclase from a primitive (Mg-# > 65) parental tholeiite. Plagioclase accumulation does not play a significant role in their origin. Many of the same compositional characteristics are also satisfied by high pressure melting of altered ocean ridge tholeiite +5 v.% pelagic sediment (quartz eclogite). The available HAB phase equilibria data do not support a fractionation origin but do support an origin involving high pressure melting of quartz eclogite. The lack of compositional zonation in the HAB phenocrysts, and the complete absence of disequilibrium MgO-rich mafic phenocrysts further argue against a tholeiite fractionation origin.Consideration of all these features indicates that the geochemical data are permissive in their interpretation. A process involving tholeiite fractionation successfully predicts the compositions of the HAB lavas but is at odds with the mineralogical and phase equilibria evidence. With some exceptions (notably Ni, Cr and Sr abundances), a process of high pressure quartz eclogite melting is consistent with the compositional, mineralogical and phase equilibria characteristics of these HAB lavas. When the relative merits of both origins are weighed it is apparent that a quartz eclogite source satisfies more of important features of these HAB lavas.Extrusive rocks have been grouped on a basis of SiO₂ content into basalt (<52 wt.%), basaltic-andesite (52–56 wt.%) and andesite (>56 wt.%) after Ewart (1982)     

Pressure dependence of melt viscosities on the join diopside-albite

The pressure dependence of melt viscosities on the join diopside-albite has been studied using falling-sphere viscometry. The five melt compositions investigated are: diopside, Ab₂₅Di₇₅, Ab₅₀Di₅₀, Ab₇₅Di₂₅ and albite. Experiments were performed at 1500° and 1600°C and at pressures of 5, 10, 15, 20 and 25 kbar. The positive and negative pressure dependence of the viscosity of diopside and albite, respectively, were confirmed. All intermediate compositions show an initial decrease in viscosity with increasing pressure; however, melt of Ab₂₅Di₇₅ composition passes through a minimum viscosity at approximately 12 kbar and 1600°C. This behavior is analogous to the variation in the viscosity of water with pressure at low temperature.

It is suggested that the three-dimensional, fully polymerized, albite structure dominates flow at low pressures. With increasing pressure, disruption of this structure and decrease in the average size of the flow units leads to domination by the diopside structure. The variation in viscosity with composition along the join at one atmosphere can be adequately modelled using the and (1965) configurational entropy model with an additional two-lattice configurational entropy of mixing term. The pressure dependence of viscosity in the diopside-albite system, however, cannot be predicted by the model, because there is an absence of information on the pressure dependence of the model parameters.

It is probable that relatively polymerized magmas (e.g. rhyolites to SiO₂-saturated basalts) show a negative pressure dependence of viscosity to depths where they originate in the lower crust or upper mantle. In contrast, the most depolymerized, naturally-occurring melts, such as strongly SiO₂-undersaturated basalts and picrites, may exhibit a viscosity minimum. The viscosity of these melts may be sufficiently high at depths within the upper mantle to inhibit their segregation, rise and eventual eruption at the surface.     

Passive seismic control of cable-stayed bridges with damped resonant appendages

A study is conducted to investigate the effectiveness of attaching to cable-stayed bridges resonant appendages with a relatively small mass and a high damping ratio as a means to reduce their response to earthquake excitations. The study is based on a previously developed formulation that shows that the use of these appendages may increase the inherent damping of building structures and, as a result, may reduce their response to seismic disturbances. It includes numerical and experimental tests that are conducted to assess the validity of such a formulation for the case of cable-stayed bridges and the extent to which such appendages can reduce their seismic response. In the numerical study, an actual cable-stayed bridge is modelled with finite elements and analysed with and without the proposed appendages under different earthquake ground motions. Appendages with damping ratios of 10, 15, 20 and 30 per cent and masses that, respectively, represent 0.67, 1.5, 2.7 and 6.0 per cent of the total mass of the bridge are considered. In the experimental test, a 3.7 m long cable-stayed bridge and an appendage consisting of a small mass, a small spring and a small viscous damper are built and the bridge tested, without and with the appendage, on a pair of shaking tables which are set to reproduce ground acceleration records from past earthquakes. The damping ratio of the appendage in this test is 32 per cent and its mass represents about 8 per cent of the total mass of the bridge model. In the numerical test, it is found that the appendages reduce the longitudinal response of the bridge deck up to 88 per cent. Similarly, in the experimental test it is found that the appendage reduces the longitudinal bridge deck response by about 41 per cent. From these studies, it is concluded that the suggested appendages may indeed be effective in reducing the seismic response of cable-stayed bridges as they are for building structures.     

High latitude river runoff in a doubled CO2 climate

A global atmospheric model is used to calculate the monthly river flow for nine of the world's major high latitude rivers for the present climate and for a doubled CO₂ climate. The model has a horizontal resolution of 4° × 5°, but the model's runoff from each grid box is quartered and added to the appropriate river drainage basin on a 2° × 2.5° resolution. A routing scheme is used to move runoff from a grid box to its neighboring downstream grid box and ultimately to the mouth of the river. In a model simulation in which atmospheric carbon dioxide is doubled, mean annual precipitation and river flow increase for all of these rivers, increased outflow at the river mouths begins earlier in the spring, and the maximum outflow occurs approximately one month sooner due to an earlier snow melt season. In the doubled CO₂ climate, snow mass decreases for the Yukon and Mackenzie rivers in North America and for rivers in northwestern Asia, but snow mass increases for rivers in northeastern Asia.     

Radiocarbon and δ13C Values Related to Ground-Water Recharge and Mixing

Ground-water levels in the Upper Floridan aquifer beneath the southeastern coast of South Carolina have undergone pumpage-induced declines approaching 20 ft below sea level at the southern end of Hilton Head Island. This scenario suggests the potential exists for the inducement of recharge to the Upper Floridan aquifer across the island, which could affect the quality of water being pumped by wells. However, low radiocarbon concentrations in ground-water samples (0.5 to 1.4 ± 0.1 PMC) indicate that most of the water is relict ground water reflecting prepumpage ground-water flow conditions in the Upper Floridan aquifer. The isotopic data indicate long residence times and water-chemistry evolution more characteristic of ground-water recharge occurring farther inland prior to the commencement of pumpage in the late 1800s. Radiocarbon concentrations (as Percent Modern Carbon) and stable carbon isotope ratios (as δ¹³C in dissolved inorganic carbon) determined during this study and reported in other studies on and around Hilton Head Island varied in a systematic manner. Heavier δ¹³C values (–2.8 to –1.6 per mil) in ground water beneath southern Hilton Head Island reflect ground-water discharge from prepumpage flowpaths originating over 100 miles away, hence a depletion in radiocarbon concentration with corrected ground-water ages no younger than 16,000 yrs BP. In contrast, lighter δ¹³C values (–13.9 to –8.67 per mil) beneath the northern part of the island indicate recent recharge as a result of water-level declines, and recharge in areas off the island that have not changed as a result of pumpage (evidenced by enrichment in radiocarbon with corrected ground-water ages no older than 4,000 yrs BP). This suggests that the δ¹³C composition of ground water in the Upper Floridan aquifer is a useful indicator of mixing between ground waters from different sources, and can be used to delineate recharge-discharge patterns. This approach may be applicable to other aquifers of highly evolved ground-water chemistry in regional carbonate aquifer systems that may be receiving recent recharge. Moreover, this approach could prove useful in delineating the contribution of recent water being captured by pumped wells as part of wellhead protection programs designed to assess aquifer vulnerability from surficial contaminant sources.     

A Method for Estimating Field-Scale Mass Transfer Rate Parameters and Assessing Aquifer Cleanup Times

A general method for estimating ground-water solute mass transfer rate parameters from field test data is presented. The method entails matching solute concentration and hydraulic head data collected during the recovery phase of a pumping test through application of a simulation-regression technique. Estimation of hydraulic conductivity and mass transfer rate parameter values is performed by fitting model simulations to the data. Parameter estimates are utilized to assess cleanup times for pump-and-treat aquifer remediation scenarios. Uncertainty in the cleanup time estimate is evaluated using statistical information obtained with the parameter estimation technique. Application of the method is demonstrated using a hypothetical ground-water flow and solute transport system. Simulations of field testing, parameter estimation, and remedial time frames are performed to evaluate the usefulness of the method. Sets of random noise that signify potential field and laboratory measurement errors are combined with the hypothetical data to provide rigorous testing of the method. Field tests are simulated using ranges of values for data noise, the mass transfer rate parameters, the test pumping rates, and the duration of recovery monitoring to evaluate their respective influence on parameter and cleanup time estimates. The demonstration indicates the method is capable of yielding accurate estimates of the solute mass transfer rate parameters. When the parameter values for the hypothetical system are well estimated, cleanup time predictions are shown to be more accurate than when calculated using the local equilibrium assumption.