Swash saturation: an assessment of available models

An extensive previously published (Hughes et al. Mar Geol 355, 88–97, 2014) field data set representing the full range of micro-tidal beach states (reflective, intermediate and dissipative) is used to investigate swash saturation. Two models that predict the behavior of saturated swash are tested: one driven by standing waves and the other driven by bores. Despite being based on entirely different premises, they predict similar trends in the limiting (saturated) swash height with respect to dependency on frequency and beach gradient. For a given frequency and beach gradient, however, the bore-driven model predicts a larger saturated swash height by a factor 2.5. Both models broadly predict the general behavior of swash saturation evident in the data, but neither model is accurate in detail. While swash saturation in the short-wave frequency band is common on some beach types, it does not always occur across all beach types. Further work is required on wave reflection/breaking and the role of wave-wave and wave-swash interactions to determine limiting swash heights on natural beaches.     

Decadal behaviour of a washover fan,Skallingen Denmark

In this study, the decadal evolution of a washover fan on the west coast of Denmark is examined from its initial generation in 1990 until 2015. Since its inception, the bare and flat washover fan surface has recovered and accreted slowly due to re-activation by overwash during surges and due to aeolian activity and dune formation, stimulated by vegetation growth. The volume of sand on the washover has increased steadily at an average rate of about 23 m³/yr per unit length of shoreline, and a total of 175,000 m³ of sand is now deposited on the fan, while at the same time the shoreline has receded by some 250 m. The evolution can be divided into three stages: 1) An initiation phase when storm surge levels and energetic wave conditions caused a breach in the foredunes and overwash processes formed a washover fan with a relatively low elevation above mean sea level; 2) An initial recovery phase during which waves supplied sand to the fan during frequent overwash activity and winds transported this sand into marginal dunes surrounding the fan; and 3) A later recovery phase when the surface of the fan had accreted to a level where vegetation could survive and trap sediment into new foredune growth across the fan. The rate of accretion has been overall linear but scales with neither annual overwash frequency, nor with aeolian transport potential. Instead, the linear accretion is more closely related to the steady onshore migration of nearshore bars that weld to the beach and provide a sand supply for transfer to the fan. The fan evolution demonstrates the importance of washover fans in preserving barrier resilience during transgressional phases caused by increasing mean sea level. © 2019 John Wiley & Sons, Ltd.     

On the choice of reference orbit,canonical variables,and perturbation method in satellite theory

The author's second-order artificial satellite theory (Aksnes, 1970) is reviewed and compared with that of Kozai (1962). These theories differ in that the former makes use of: (1) an intermediate orbit, being a rotating ellipse instead of a fixed ellipse, (2) Hill variables instead of Delaunay variables, and (3) Hori's perturbation method in Lie series rather than Von Zeipel's method in Taylor series.It is demonstrated that because of these differences, the former theory enjoys a greater simplicity and compactness, it is non-singular at zero eccentricity, and the process of deriving the perturbations is considerably simplified (Aksnes, 1972). For example, the number of second-order short-period terms due to the planet's oblateness (J ₂) is reduced by a factor of about three (Hori, 1970). The intermediate orbit and Hori's perturbation method contribute about equally to this reduction.Presented at the Conference on Celestial Mechanics, Oberwolfach, Germany, August 27–September 2, 1972.     

Orbit improvement from satellite imaging data obtainable from outer planet missions

Equations of motion are established for a dynamical system in which a spacecraft flies close to and interacts with an outer planet and one or more of its satellites. For the computation of the state and mass partials needed in a simultaneous orbit correction ofn interacting bodies, a notably compact set of variational equations is derived. The above system of differential equations is integrated numerically on a computer.Spacecraft-satellite direction measurements accurate to ±10 were simulated along three representative trajectories (Mariner/Jupiter/Saturn 1977 missions) approaching Io, Titan, and Iapetus to within 41 000, 13 000, and 7 000 km, respectively. For example, from measurements distributed evenly at half-day intervals over a 60-day arc centered on encounter, but none so close that the satellite would fill more than 0.5° in the sky, the orbit of the satelliteand that of the spacraft can be estimated to about 100 km. In addition, the mass of the satellite is obtainable to 2.6% for Io, 1.4% for Titan, and 9% for Iapetus. If only measurements up to 3 days before satellite encounter are included, the orbit of the satelliteor that of the spacecraft can be estimated to about 300 km, all information on mass being lost.The paper concludes with a brief discussion of the need for future work on the orbits of the satellites of the outer planets.     

Thermal and haline variability over the central Bering Sea shelf: Seasonal and interannual perspectives

We examine multi-year conductivity-temperature-depth (CTD) data to better understand temperature and salinity variability over the central Bering Sea shelf. Particular consideration is given to observations made annually from 2002 to 2007 between August and October, although other seasons and years are also considered. Vertical and horizontal correlation maps show that near-surface and near-bottom salinity anomalies tend to fluctuate in phase across the central shelf, but that temperature anomalies are vertically coherent only in the weakly or unstratified inner-shelf waters. We formulate heat content (HC) and freshwater content (FWC) budgets based on the CTD observations, direct estimates of external fluxes (surface heat fluxes, ice melt, precipitation (P), evaporation (E) and river discharge), and indirect estimates of advective contributions. Ice melt, P−E, river discharge, and along-isobath advection are sufficient to account for the mean spring-to-fall increase in FWC, while summer surface heat fluxes are primarily responsible for the mean seasonal increase in HC, although interannual variability in the HC at the end of summer appears related to variability in the along-isobath advection during the summer months. On the other hand, FWC anomalies at the end of summer are significantly correlated with the mean wind direction and cross-isobath Ekman transport averaged over the previous winter. Consistent with the latter finding, salinities exhibit a weak but significant inverse correlation between the coastal and mid-shelf waters. The cross-shelf transport likely has significant effect on nutrient fluxes and other processes important to the functioning of the shelf ecosystem. Both the summer and winter advection fields appear to result from the seasonal mean position and strength of the Aleutian Low. We find that interannual thermal and haline variability over the central Bering Sea shelf are largely uncoupled.     

Metallographic cooling rates of group IIF iron meteorites

Abstract— Metallographic cooling rates have been calculated for all five members of the iron meteorites group IIF using two different techniques. We have determined cooling rates of ～5 °C/Ma based on Ni profiles through the taenite rim enclosing kamacite spindles. Ni profiles through the kamacite phase are less precise cooling rate indicators, but suggest a cooling rate of ～1 °C/Ma within an order of magnitude at lower temperatures (360–400 °C). Based on the kamacite bandwidth and the Ni profiles through the taenite, we estimate that the kamacite nucleated 130–200 °C below the temperature predicted from the phase diagram. The size of and the distance between the large kamacite spindles is found to be consistent with the thermal history that we have determined on the basis of Ni profiles in kamacite and taenite. We find that previously published kamacite bandwidth cooling rates for the five group IIF members are most likely in error because of the presence of large schreibersite spindles in some kamacite spindles and because undercooling of kamacite was ignored. Contrary to previous workers we find that the metallographic cooling rates are consistent with cooling in a common core.     

Statistical Analysis of Landslide Events in Central America and their Run-out Distance

Statistical analyses of landslide deposits from similar areas provide information on dynamics and rheology, and are the basis for empirical relationships for the prediction of future events. In Central America landslides represent an important threat in both volcanic and non-volcanic areas. Data, mainly from 348 landslides in Nicaragua, and 19 in other Central American countries have been analyzed to describe landslide characteristics and to search for possible correlations and empirical relationships. The mobility of a landslide, expressed as the ratio between height of fall (H) and run-out distance (L) as a function of the volume and height of fall; and the relationship between the height of fall and run-out distance were studied for rock falls, slides, debris flows and debris avalanches. The data show differences in run-out distance and landslide mobility among different types of landslides and between debris flows in volcanic and non-volcanic areas. The new Central American data add to and seem consistent with data published from other regions. Studies combining field observations and empirical relationships with laboratory studies and numerical simulations will help in the development of more reliable empirical equations for the prediction of landslide run-out, with applications to hazard zonation and design of optimal risk mitigation measures.     

Observations of offshore bar decay: Sediment budgets and the role of lower shoreface processes

Long-term, net offshore bar migration is a common occurrence on many multiple-barred beaches. The first stage of the process involves the generation of a longshore bar close to the shoreline that oscillates about a mean position for some time, followed by a stage of net offshore migration across the upper shoreface, and finally a stage of decaying bar form through loss of sediment volume at the outer boundary of the upper shoreface. The phenomenon has been previously documented in the Netherlands, the USA, the Canadian Great Lakes, and in New Zealand, but our present understanding of the morphodynamic processes and sediment transport pathways involved in bar decay is limited. In this paper, long-term, net offshore bar migration is investigated at Vejers Beach, located on the North Sea coast of Denmark where offshore bar migration rates are of the order of 45–55 m a⁻¹. A wave height transformation model confirmed that the decay of the outer bar results in increased wave heights and undertow speeds at the more landward bar potentially causing this bar to speed up its offshore migration. The causes for outer bar decay were investigated through field measurements of sediment transport at the decaying bar and at a position further seaward on the lower shoreface. The measurements showed that a cross-shore transport convergence exists between the bar and the lower shoreface and that the loss of sediment involved in bar decay is associated with a longshore directed transport by non-surf zone processes. At Vejers, and possibly elsewhere, the net offshore migration of bars and the subsequent loss of sand during bar decay is an important part of the beach and shoreface sediment budget.     

Field observations of nearshore bar formation

The formation of an inner nearshore bar was observed during a high‐energy event at the sandy beach of Vejers, Denmark. The bar accreted in situ during surf zone conditions and the growth of the bar was associated with the development of a trough landward of the bar. Measurements of hydrodynamics and sediment fluxes were obtained from electromagnetic current meters and optical backscatter sensors. These process measurements showed that a divergence in sediment transport occurred at the location of the developing trough, and observed gradients in cross‐shore net sediment flux were consistent with the morphological development. The main cause for the flux gradients were cross‐shore gradients in offshore‐directed mean current (undertow) speed which depended upon local relative wave height and local bed slope. Copyright © 2007 John Wiley & Sons, Ltd.     

Intrinsic biodegradation of toluene coupled to the microbial reduction of ferric iron: laboratory column experiments

Intrinsic biodegradation of toluene coupled with the microbial reduction of ferric iron (Fe(III)) as the terminal electron acceptor was studied by using laboratory column experiments under continuous flow conditions. Columns were packed with contaminated aquifer sediment and N₂-purged groundwater taken from the western part of the Gardermoen aquifer. The columns were operated anaerobically at 8 °C (in-situ temperature). Chloride was initially used to characterize flow properties of the columns. Intrinsic biodegradation of toluene, including abiotic loss and biological loss, was estimated by comparing breakthrough curves of toluene for live columns and sterilized control columns based on mass balance in steady-state conditions. The column experiments were run at two different flow velocities. The estimated average intrinsic rate was -0.73 and -0.53 mM day^-1 for pore-water velocities of 1.75 and 2.68 cm h^-1, respectively, corresponding to -0.27 and -0.22 mM day^-1 in biological loss rate. The results indicate that intrinsic biodegradation of toluene could be used as an efficient remediation approach for contaminated groundwater at the Gardermoen fire-fighting training site.