Sustainable shellfish aquaculture in Saldanha Bay,South Africa

The carrying capacity for bivalve shellfish culture in Saldanha Bay, South Africa, was analysed through the application of the well-tested EcoWin ecological model, in order to simulate key ecosystem variables. The model was set up using: (i) oceanographic and water-quality data collected from Saldanha Bay, and (ii) culture-practice information provided by local shellfish farmers. EcoWin successfully reproduced key ecological processes, simulating an annual mean phytoplankton biomass of 7.5 µg Chl a l^–1 and an annual harvested shellfish biomass of about 3 000 tonnes (t) y^–1, in good agreement with reported yield. The maximum annual carrying capacity of Small Bay was estimated as 20 000 t live weight (LW) of oysters Crassostrea gigas, or alternatively 5 100 t LW of mussels Mytilus galloprovincialis, and for Big Bay as 100 000 t LW of oysters. Two production scenarios were investigated for Small Bay: a production of 4 000 t LW y^–1 of mussels, and the most profitable scenario for oysters of 19 700 t LW y^–1. The main conclusions of this work are: (i) in 2015–2016, both Small Bay and Big Bay were below their maximum production capacity; (ii) the current production of shellfish potentially removes 85% of the human nitrogen inputs; (iii) a maximum-production scenario in both Big Bay and Small Bay would result in phytoplankton depletion in the farmed area; (iv) increasing the production intensity in Big Bay would probably impact the existing cultures in Small Bay; and (v) the production in Small Bay could be increased, resulting in higher income for farmers.     

Empirical correlations of soil parameters based on Cone Penetration Tests (CPT) for Greek soils

This study presents the results of Cone Penetration Tests (CPT) and estimated correlations with the results of other in-situ tests (standard penetration test, cross-hole, seismocone) and laboratory tests (classification and undrained shear strength tests). These tests cover nearly all types of penetrable soils in Greece and, together with the formulation of empirical correlations, complement and improve on previous studies due to the significant number of CPT measurements.     

An answer to an important controversy and the need for caution when using simple models to predict inelastic earthquake response of buildings with torsion

This paper presents evidence that the extension of conclusions based on the widely used simplified, one story, eccentric systems of the shear‐beam type, to actual, nonsymmetric buildings and consequent assessments of the pertinent code provisions, can be quite erroneous, unless special care is taken to match the basic properties of the simplified models to those of the real buildings. The evidence comes from comparisons of results obtained using three variants of simplified models, with results from the inelastic dynamic response of three‐ and five‐story eccentric buildings computed with detailed MDOF systems, where the members are idealized with the well‐known plastic hinge model. In addition, a convincing answer is provided on a pertinent hanging controversy: For frame‐type buildings, designed in accordance with the dynamic provisions of modern codes (such as EC8 or IBC2000), which allow reduced shears at the stiff edge due to torsion, the frames at the flexible sides are the critical elements in terms of ductility demands. Copyright © 2009 John Wiley & Sons, Ltd.     

Local member response in offshore structures under earthquake excitations

It is pointed out that local vibrations of certain members in earthquake excited offshore structures can induce substantial stresses that may often dominate the design of these members. Such vibrations are not accounted for in dynamic analyses of structural models with masses lumped at the joints. A practical, easy to implement solution to this problem is recommended and results from an actual structure, subjected to a three-component real earthquake, are presented demonstrating the importance of local inertia effects.     

An investigation of earthquake induced pounding between adjacent buildings

The response of adjacent buildings in city blocks to several strong earthquakes is analysed, taking into account the mutual collisions, or pounding, resulting from insufficient or non-existing separation distances. The buildings are idealized as lumped-mass, shear beam type, multi-degree-of-freedom (MDOF) systems with bilinear force-deformation characteristics and with bases supported on translational and rocking spring-dashpots. Collisions between adjacent masses can occur at any level and are simulated by means of viscoelastic impact elements. Using five real earthquake motions the effects of the following factors are investigated: building configuration and relative size, seismic separation distance and impact element properties. It is found that pounding can cause high overstresses, mainly when the colliding buildings have significantly different heights, periods or masses. This suggests a possibility for introducing a set of conditions into the codes, combined with some special measures, as an alternative to the seismic separation requirement.     

Spatial distribution of upstream magnetospheric ≥50 keV ions

We present for the first time a statistical study of 50 keV ion events of a magnetospheric origin upstream from Earths bow shock. The statistical analysis of the 50–220 keV ion events observed by the IMP-8 spacecraft shows: (1) a dawn-dusk asymmetry in ion distributions, with most events and lower intensities upstream from the quasi-parallel pre-dawn side (4 LT-6 LT) of the bow shock, (2) highest ion fluxes upstream from the nose/dusk side of the bow shock under an almost radial interplanetary magnetic field (IMF) configuration, and (3) a positive correlation of the ion intensities with the solar wind speed and the index of geomagnetic index Kp, with an average solar wind speed as high as 620 km s^–1 and values of the index Kp 2. The statistical results are consistent with (1) preferential leakage of 50 keV magnetospheric ions from the dusk magnetopause, (2) nearly scatter free motion of 50 keV ions within the magnetosheath, and (3) final escape of magnetospheric ions from the quasi-parallel dawn side of the bow shock. An additional statistical analysis of higher energy (290–500 keV) upstream ion events also shows a dawn-dusk asymmetry in the occurrence frequency of these events, with the occurrence frequency ranging between 16%-34% in the upstream region.     

Saldanha Bay,South Africa I: the use of ocean colour remote sensing to assess phytoplankton biomass

The efficacy of ocean colour remote sensing in assessing the variability of phytoplankton biomass within Saldanha Bay is examined. Satellite estimates of chlorophyll a (Chl a) were obtained using the maximum peak-height (MPH) algorithm on full-resolution (300?m) data from the Medium Resolution Imaging Spectrometer (MERIS). Subsurface Chl a maxima often occur within Saldanha Bay below the mean detection depth of the satellite (1.5?m) during periods of thermal stratification. Consequently, the MPH product was poorly correlated to in situ data from 4?m depth (r² and average relative percentage difference [RPD] of 0.094 and 53% respectively); however, the coefficient of determination was much improved if limited to in situ data collected under conditions of mixing (r² and RPD of 0.869 and 89%, respectively). Composites of monthly MPH Chl a data reveal mean concentrations consistent with in situ seasonal trends of phytoplankton biomass, confirming the capability of the MPH algorithm to qualitatively resolve surface Chl a distribution within the bay.     

Study on the onsets of solar energetic electron events

Fine time resolution observations of the angular distributions of the intensities of energetic electrons (220 ≤ E _e ≤ 500 keV) by the IMP-7 and 8 spacecraft during the onsets of solar electron events and the technique of mapping the solar wind to the solar corona have been incorporated in this work in order to obtain the large-angle scattering distance of these particles under different configurations of the large scale structure of the interplanetary medium. It is found that in the presence of stream-stream interaction regions with compressed magnetic fields beyong 1 AU, the large-angle scattering is determined by the distance along the streamlines from the spacecraft to their intersection by a faster solar wind stream. In cases of diverging magnetic fields the estimated large-angle scattering distance exceeds 1 AU.     

Seasonal and sub-seasonal oxygen and nutrient fluctuations in an embayment of an eastern boundary upwelling system: St Helena Bay

Seasonal, sub-seasonal and spatial fluctuations in bottom dissolved oxygen (DO) were examined in St Helena Bay, South Africa’s largest and most productive embayment, between November 2013 and November 2014. Alongshore bay characteristics were assessed through comparison of variables along the 50-m depth contour. A mean coefficient of variation of 0.35 provided a measure of the relative variability of near-bottom DO concentrations along this contour. Consistently lower DO concentrations in the southern region of the bay in summer and autumn are attributed to enhanced retention. Across-shelf transects captured the seasonal development of hypoxia in relation to the distribution of phytoplankton biomass. Exceptional dinoflagellate blooms form extensive subsurface thin layers preceding the autumn DO minima in the south of the bay, prior to winter ventilation of the bottom waters. The seasonal decline in DO concentrations in the bottom waters was marked by sub-seasonal events of hypoxia, and ultimately anoxia linked to episodic deposition of organic matter, as indicated by increases in bottom chlorophyll-a concentrations. Seasonal changes in bottom water macronutrient concentrations followed trends in apparent oxygen utilisation (AOU), both of which mirrored DO concentrations. In the south of the bay, nitrogen loss through denitrification/anammox in suboxic waters was indicated by a dissolved inorganic N deficit in the bottom waters, which was most pronounced in autumn.     

Effect of soil stiffness in the attenuation of Rayleigh-wave motions from field measurements

The amplitude of vertical ground surface vibrations generated by impact tests on the ground surface was measured at various radial distances from the point of impact at locations of Greece. The results of measurements were analyzed in the frequency domain (in the range from 0–100 Hz) and the attenuation characteristics of soil materials were studied in terms of a frequency-independent attenuation coefficient, a₀, of the empirical Bornitz equation. The aim of the study was to investigate the effect of soil stiffness (expressed by the value of low-amplitude shear wave velocity of soil, V_SO) on the value of attenuation coefficient, a₀. Values of V_SO for the tested soils were estimated by applying the methodology of Spectral Analysis of Surface Waves (SASW) technique and utilizing the surface vibration data. An empirical relationship between a₀ and V_SO1 (V_SO1 is the representative value of V_SO for the soil profile up to a depth of one wavelength) was established for values of V_SO1 ranging from 140 to 1000 m/s. A similar relationship in terms of the low-amplitude shear modulus of soil, G_O1, was also established by converting the V_SO1 values to G_O1 values. The experimental results were compared to values reported in the literature for comparable soil types and frequencies of vibration and a reasonable agreement was found to exist. The proposed empirical relationship can be utilized in many practical applications of soil dynamics requiring the knowledge of the attenuation rate of Rayleigh waves with distance in various types of soils.