Correction for effects of temperature and pressure on sound speed in shallow seafloor sediments

Abstract

Three types of sediments were selected to measure their sound speed under changing temperature and pressure conditions in laboratory. The effects of temperature and pressure on sound speed in sediments and their trends were analyzed. The results showed that, with increasing temperature and pressure, the sound speed exhibits an increasing trend in all selected sediments. For each sample, the ratio of the sound speed in sediments to that in seawater almost remained unchanged at different pressures and temperatures, with a maximum fluctuation of 1.09% for temperature dependence and 0.68% pressure dependence. Combining the analysis of experimental results and sound speed correction procedure given by Hamilton, specific correction formulas of sound speed for temperature and pressure were presented. The laboratory-measured sound speed in the experiment and the sound speed obtained in the South Yellow Sea were corrected to reduce the effects of temperature and pressure using the correction formulas. The results show that the correction formulas with constant sound speed ratio are effective for correcting the sound speed measurement errors caused by changes in temperature and pressure. As a further consideration, the effects of the fluctuation of sound speed ratio on sound speed correction were analyzed.     

New formulae for capacity energy-based assessment of liquefaction triggering

Abstract

Liquefaction of loose saturated soil deposits is a hazardous type of ground failure occurring under earthquake excitations. Therefore, an accurate estimation of liquefaction potential is extremely important in geotechnical engineering. In the current study, a new model is proposed which estimates the level of strain energy needed for liquefaction initiation. A compiled database containing cyclic tests gathered from previously published works was used to develop new models to predict liquefaction potential. M5′ algorithm was used to find the best correlation between parameters. It was shown that not only the derived formulas are acceptably accurate but also they feature a very simple structure in comparison with available formulas in the literature. The proposed equations are accurate, physically sound and uncomplicated. Furthermore, safety factors were given for different levels of risk, which can be useful for engineering practice. In addition, the influence of different predictors on the liquefaction potential was evaluated and also the significance of input variables was assessed via sensitivity analysis. Finally, a new model was introduced for preliminary estimation of liquefaction potential.     

Stability of rubble-mound breakwater using H50 wave height parameter

The prediction of rubble mound breakwaters' stability is one of the most important issues in coastal and maritime engineering. The stability of breakwaters strongly depends on the wave height. Therefore, selection of an appropriate wave height parameter is very vital in the prediction of stability number. In this study, H₅₀, the average of the 50 highest waves that reach the breakwater in its useful life, was used to predict the stability of the armor layer. First, H₅₀ was used instead of the significant wave height in the most recent stability formulas. It was found that this modification yields more accurate results. Then, for further improvement of the results, two formulas were developed using model tree.To develop the new formulas, two experimental data sets of irregular waves were used. Results indicated that the proposed formulas are more accurate than the previous ones for the prediction of the stability parameter. Finally, the proposed formulas were applied to regular waves and a wide range of damage levels and it was seen that the developed formulas are applicable in these cases as well.     

Discrete-element numerical modelling method for studying mechanical response of methane-hydrate-bearing specimens

Abstract

It is important to understand the deformation mechanism of methane-hydrate specimens (MHSs) to avoid deforming the seabed during methane-hydrate production. For this purpose, discrete-element method (DEM) modelling is advantageous because it requires much less cost and effort compared to artificial specimens and in situ sediments. In this study, a method for generating DEM numerical simulation models of MHSs is proposed to study the deformation mechanism of MHSs. First, numerical models that consider the saturation of methane hydrate (S_MH), following which the bi-axial compression of these models is simulated. The mechanisms controlling the shear strength of MHSs are verified and modified by investigating the stress–strain response behavior, crack-development process, and evolution of the void change rate (E_change) of MHSs. The increases of the peak strength and secant elastic modulus of the MHS with the increment of confining pressure follow parabolic relationships. Under different loading rates, the peak strength tends to increase parabolically with the increment of loading rate, while the relationship between the secant elastic modulus and loading rate is linear. Based on the testing results, empirical formulas of peak stress and elastic modulus are proposed for different confining-pressure and strain-rate conditions.     

Prediction of consolidation parameter using multiple regression analysis

Since existing empirical formulas have been proposed without clear verification of normality, they involve very high uncertainty in their design parameters. Therefore, in the present study, marine clay regions located in river estuary regions with deep soft layers were selected as study subject regions, the normality of those soil investigation data that were relatively highly reliable was verified, simple regression analyses of the data were conducted to propose prediction formulas, and the results from the prediction formulas were compared with the results from existing empirical formulas. Multiple regression analyses were conducted and the results indicated that regression models composed of G_S, e_o, LL were statistically significant in explaining compression indexes in both regions. It can be seen that the structures of the models presented in the present study changed depending on soil properties. In particular, because of diverse features of soil, G_S had negative effects on C_C in the case of Pusan while having positive effects on C_C in the case of Kwangyang. In addition, the results of tests of equality between regional models rejected the null hypothesis indicating that parameters were statistically significantly different between the two regions.     

Experimental study on the coefficient of lateral pressure at rest for calcareous soils

Abstract

The coefficient of lateral pressure at rest (k₀) is a key parameter required to determine the horizontal stress distribution on subsoils. In this paper, k₀ was measured from consolidation test for calcareous and siliceous sand, and the effect of water contents on k₀ of calcareous sand was also discussed. Comparison of k₀ values measured and calculated based on empirical formulas were carried on. Results indicated that (1) the reduction rate of k₀ for calcareous sand with an increase in relative density was less than that of siliceous sand. The k₀ at low water content was generally smaller than that of siliceous sand, and it was similar to that of siliceous sand for high-water content; (2) the k₀ of calcareous sand increased with increasing initial water content initially, and then decreasing. However, the magnitude of variation was small. So the impact of water content on the k₀ of calcareous sand was limited; (3) the formulas proposed by Abdelhamid and Krizek (1976 Abdelhamid, M. S., and R. J. Krizek. 1976. At-Rest Lateral Earth Pressure of Consolidating Clay. Journal of the Geotechnical Engineering Division 102 (7): 721–738. [Google Scholar]) for estimation of the k₀ was highly applicable to calcareous fine sand.

• Highlights

• The k₀ of calcareous fine sand was investigated.

• The k₀ of calcareous and siliceous sand were compared.

• Shape parameters of sand particles for calcareous and siliceous sand were analyzed.

• Effects of related density and initial water content on the k₀ were discussed, respectively.

• The applicability of various empirical formulas for calculating k₀ of calcareous fine sand was evaluated.

     

Comparison of turbulence schemes for prediction of wave-induced near-bed sediment suspension above a plane bed

Based on a wave bottom boundary layer model and a sediment advection-diffusion model, seven turbulence schemes are compared regarding their performances in prediction of near-bed sediment suspension beneath waves above a plane bed. These turbulence algorithms include six empirical eddy viscosity schemes and one standard two-equation k-ε model. In particular, different combinations of typical empirical formulas for the eddy viscosity profile and for the wave friction factor are examined. Numerical results are compared with four laboratory data sets, consisting of one wave boundary layer hydrodynamics experiment and three sediment suspension experiments under linear waves and the Stokes second-order waves. It is shown that predictions of near-bed sediment suspension are very sensitive to the choices of the empirical formulas in turbulence schemes. Simple empirical turbulence schemes are possible to perform equally well as the two-equation k-ε model. Among the empirical schemes, the turbulence scheme, combining the exponential formula for eddy viscosity and Swart formula for wave friction factor, is the most accurate. It maintains the simplicity and yields identically good predictions as the k-ε model does in terms of the wave-averaged sediment concentration.     

Large-scale seafloor stability evaluation of the northern continental slope of South China Sea

Abstract

With the continuous expansion of energy demand, the deep-water continental slope in the northern South China Sea has become one of the significant offshore oil and gas exploration regions. The frequent occurrence of marine geological hazards in this region, especially submarine landslides, can cause serious damage to engineering facilities. However, there have been few studies on the stability of the northern continental slope of the South China Sea; these studies mainly focused on a specific submarine slope or small-range evaluation, resulting in a lack of large-scale and quantitative understanding. Hence, considering the variation in the physical and mechanical properties of marine soils with depth, formulas for calculating the safety factor of submarine slopes by an infinite sliding model are established, and the factors affecting slope stability such as soil properties, slope gradient and horizontal seismic action are systematically investigated. Using GIS techniques, the terrain slope gradients and a historical seismic database of the northern South China Sea are obtained. Combined with soil mechanical parameters, a regional stability evaluation of the northern continental slope is carried out. Furthermore, the distribution of risk zones is given. On the whole, under strong seismic action, large-scale submarine slope instability occurs and must be highly considered when assessing risk. This achievement is of great significance to engineering sites, route selection and engineering risk assessment.     

来源于印度洋深海区的烟曲霉次生代谢产物的研究

为了研究烟曲霉次生代谢产物,我们采用硅胶柱层析、sephadex LH-20凝胶柱层析、半制备液相色谱等分离方法对来源于印度洋深海区的烟曲霉(Aspergillus fumigatus)的乙醇提取液进行系统的次生代谢产物分离,根据所分离得到的化合物的理化性质和波谱数据确定化合物的结构。共分离鉴定了15个化合物,其中11个二酮哌嗪类生物碱(diketopiperazine),4个喹唑啉类衍生物(fumiquinazoline),分别为:fumitremorgin B(1),13-oxofumitremorgin B(2),fumitremorgin C(3),12-hydroxyfumitremorgin(13),3-hydroxyfumiquinazoline A(14)和fumiquinazoline E(15)。分离得到的15个化合物,丰富了烟曲霉次生代谢产物的类型,为烟曲霉的开发利用提供了化学研究基础。     

Analytically derived wind-wave directional spectrum part 2. Characteristics,comparison and verification of spectrum

The dependence of the angular spreading on frequency and wind-wave growth status is discussed in great detail for the proposed spectrum. The calculated angular spreading agrees with the measurements of Donelanet al. but is slightly broader. Explanation is given to the appearance of the narrowest spreading at a frequency slightly smaller than that of the wind-wave frequency spectrum peak as found by these authors. There is also basic agreement between the calculated spreading and the formulas of Mitsuyasuet al. and Hasselmannet al. for the specific wind-wave status on which these empirical formulas are based, though the former is narrower. The wind-wave frequency spectrum obtained by integrating the proposed directional spectrum with respect to direction agrees with the JONSWAP spectrum and that derived by the authors previously. The proposed spectrum is preliminarily verified with field data obtained by optical method.Project supported by the National Natural Science Foundation of China.     

Siphonostomatoid copepods infecting Squalus acutipinnis Regan, 1908 off South Africa

Squalus acutipinnis (previously included in S. megalops) individuals caught off the east, south and west coasts of South Africa were examined for siphonostomatoid copepod infection. Collected siphonostomatoids were fixed and preserved in 70% ethanol and studied under microscopes using both transmitted and incidental light. The copepods were identified and the host-parasite associations estimated by calculating their prevalence, mean intensity and mean abundance. The siphonostomatoids represented four families and six species, and included new host records for Nemesis sp., Lernaeopoda sp. and Eudactylina acanthii, and a new geographical record for E. acanthii. Squalus acutipinnis individuals were infected by relatively low loads of siphonostomatoids. The most commonly found species was Achtheinus pinguis, the only species found off all three coasts. There was no difference in the prevalence of A. pinguis in sharks sampled off the three coasts but the highest mean intensity was found on the East Coast followed by the West Coast. The overall prevalence of A. pinguis was 33.6%, and was an order of magnitude higher than that of the other siphonostomatoids recorded. At such low levels of infection it will thus be necessary to examine a large number of hosts to determine the biodiversity of siphonostomatoids and geographical differences in prevalence in South African waters.     

Systematic errors in estimation of insolation by empirical formulas

Systematic errors in the estimation of surface, insolation,Q, by two popular empirical formulas are investigated statistically by using coincident measurements of the global solar radiation and the total cloud cover at JMA observatories over Japan. The results show that Reed’s (1977) widely-accepted formula remarkably overestimatesQ under overcast conditions. The overestimation is particularly evident in the summer months. The formula also overestimatesQ in cloud-free conditions, which may be due to an overestimation of the clear-sky transmittance by Seckel and Beaudry, (1973). By contrast, Kondo and Miura’s (1985) formula underestimatesQ under overcast conditions, whereas it slightly overestimates in partially cloudy conditions. It is shown that these systematic errors can explain some, of the published differences between the estimation ofQ by the two formulas. The users of these formulas should be careful since these cloudiness-dependent errors can contaminate not only the absolute values but also the temporal anomalies or the spatial variability of the insolation, predicted by them. In particular, it can be serious in regions of dense cloud cover such as the northern North Pacific, the northern North Atlantic and the Southern Ocean. It is also shown that the ratio ofQ to the insolation at the top of the atmosphere,Q _TOA, takes on a range of values, particularly under dense cloud cover. This implies an inherent difficulty in estimation ofQ by a simple empirical formula utilizing only readily-available observables such as cloudiness.     

Equivalent void ratio controlling the mechanical properties of cementitious material-clay mixtures with high water content

Abstract

This research develops a parameter defined as the equivalent void ratio, e?_st, which is able to accurately describe the dependence of the mechanical properties of cementitious material-clay mixtures on the influencing parameters, i.e., the mixing proportion, curing time, and initial state of the mixture, for different types of cementitious materials based on the results of unconfined compression, oedometer, and triaxial tests. Besides Portland cement, cementitious materials, such as fly ash and rice husk ash, are considered supplementary cement with different levels of efficiency. This equivalent cementitious material concept is then adapted for parameter development in conjunction with the effective void ratio proposed from our previous study. The developed single parameter, e?_st, can assess the mechanical properties of cementitious material-clay mixtures with different types of cementitious materials and under different test conditions.     

Comparison of marine insolation estimating methods in the adriatic sea

We compare insolation results calculated from two well-known empirical formulas (Seckel and Beaudry’s SB73 formula and the original Smithsonian (SMS) formula) and a radiative transfer model using input data predicted from meteorological weather-forecast models, and review the accuracy of each method. Comparison of annual mean daily irradiance values for clear-sky conditions between the two formulas shows that, relative to the SMS, the SB73 underestimates spring values by 9 W m^-2 in the northern Adriatic Sea, although overall there is a good agreement between the annual results calculated with the two formulas. We also elucidate the effect on SMS of changing the ‘Sun-Earth distance factor (f)’, a parameter which is commonly assumed to be constant in the oceanographic context. Results show that the mean daily solar radiation for clear-sky conditions in the northern Adriatic Sea can be reduced as much as 12 W m^-2 during summer due to a decrease in thef value. Lastly, surface irradiance values calculated from a simple radiative transfer model (GM02) for clear-sky conditions are compared to those from SB73 and SMS. Comparison within situ data in the northern Adriatic Sea shows that the GM02 estimate gives more realistic surface irradiance values than SMS, particularly during summer. Additionally, irradiance values calculated by GM02 using the buoy meteorological fields and ECMWF (The European Centre for Medium Range Weather Forecasts) meteorological data show the suitability of the ECMWF data usage. Through tests of GM02 sensitivity to key regional meteorological factors, we explore the main factors contributing significantly to a reduction in summertime solar irradiance in the Adriatic Sea.     

Wave run-up on bermed coastal structures

Reliable estimation of wave run-up is required for the effective and efficient design of coastal structures when flooding or wave overtopping volumes are an important consideration in the design process. In this study, a unified formula for the wave run-up on bermed structures has been developed using collected and existing data. As data on berm breakwaters was highly limited, physical model tests were conducted and the run-up was measured. Conventional governing parameters and influencing factors were then used to predict the dimensionless run-up level with 2% exceedance probability. The developed formula includes the effect of water depth which is required in understanding the influence of sea level rise and consequent changes of wave height to water depth ratio on the future hydraulic performance of the structures. The accuracy measures such as RMSE and Bias indicated that the developed formula is more accurate than the existing formulas. Additionally, the new formula was validated using field measurements and its superiority was observed when compared to the existing prediction formulas. Finally, the new design formula incorporating the partial safety factor was introduced as a design tool for engineers.     

A new predictive formula for inception of regular wave breaking

Efforts are made to enhance the predictive formula for the inception of wave breaking. To achieve success, the existing formulas are extensively reviewed. They are categorized into four types, i.e., the McCowan type, the Miche type, the Goda type and the Munk type. The inherent relations among the different types are then exploited. The differences among each formula within a group are also discussed. Four representative formulas from the different types are chosen to compare with the measured data for a total number of 1193 cases reported in literatures. It is shown that Goda's and Ostendorf and Madsen's formulas are advantageous in general among the selected ones. Goda's formula, however, is found to be inaccurate as the beach slope becomes steeper than 1/10. Ostendorf and Madsen's formula is fairly good even for cases of very steep slopes, but its accuracy for the cases of ordinary slopes is not as good as Goda's. A new predictive formula for the inception of wave breaking is proposed. The unique index, defined by ψ′_b = (1.21 − 3.30λ_b)(1.48 − 0.54γ_b)ψ_b, where ψ_b = gH_b/C_b², H_b is the breaking wave height, C_b is the breaking wave celerity, λ_b is the breaking wave steepness, γ_b is the relative breaking wave height, and g is the gravity acceleration, is introduced. The incipient condition of wave breaking is then given by ψ′_b = 0.69. This formula is a significant improvement to the existing ones in terms of the accuracy. In addition, it is a local relation. Further verification shows that the proposed formula performs similarly well when applied to the field and to the waves over permeable bed.     

Estimation of the wave-related ripple characteristics and induced bed shear stress

A large data set on ripples was collected and examined. A set of new formulas for the prediction of the ripple characteristics is proposed with an emphasis on the disappearance of the ripples. The ripple wavelength was observed to be proportional to the bottom wave excursion but also to be a function of the grain-related Shields parameter and wave period parameter introduced by Mogridge et al. (1994). The ripple steepness was found to be nearly constant for orbital ripples, and with a sharp decrease for suborbital ripples. Two empirical functions are added including the effects of the critical Shields parameters (inception of transport and inception of sheet flow), i.e. giving the boundaries for the ripple existence's domain. The proposed formulas yield better prediction capabilities compared to the previously published formulas, especially when ripples are washed out. The effect of the ripple characteristics on the roughness height and the calculation of the bed shear stress is also discussed. It appeared that the bed shear stress calculation is more sensitive to the empirical coefficient a_r introduced in the estimation of the ripple-induced roughness height or to the limits of existence of the ripples than the ripple characteristics themselves.     

The MV Rena shipwreck: time-critical scientific response and environmental legacies

ABSTRACT

The sinking of the MV Rena on Astrolabe Reef (Otaiti) in the Bay of Plenty, New Zealand, resulted in the release of oil and ship debris, including dangerous goods carried as cargo. Two key questions of concern to the public and environmental managers were posed immediately: what was the impact of the Rena oil spill and how long would it take for the marine environment to recover? The research that began immediately after the ship grounded provided answers, as documented in this special issue.     

A new expression for the production rate of sea water droplets on the sea surface

A new set of empirical formulas for the production rate and the number concentration of sea-water droplets on the sea surface are proposed, synthesizing past observation data of sea-salt particles in the sea and water droplets in wind-wave tanks. A new levelz _c is introduced as the effective wind-sea surface where seawater droplets are produced. The new formulas are expressed in linear functions in logarithmic scales ofu*²/v _p , a parameter to describe overall conditions of airsea boundary processes, whereu _* is the friction velocity of air,v the kinematic viscosity of air and _p the peak angular frequency of wind-wave part of wave spectra. A model of coexistence of spray droplets and suspended particles near the sea surface is proposed. As for the independent parameter, a comparison between the uses ofu*²/v _p and ofu _* ³ which was the traditional way of parameterization excluding wave measure, shows that the advantage of usingu*²/v _p is statistically significant with a confidence limit 89% in F-test.     

Detection of nitrogenase in individual cells of a natural population of <Emphasis Type="Italic">trichodesmium</Emphasis> using immunocytochemical methods for fluorescent cells

The intercellular distribution of nitrogenase was detected in a natural population of Trichodesmium spp. collected from Shitaba Bay, Uwajima, Ehime Prefecture, Japan. Polyclonal antibodies raised against the Fe-protein and the MoFe-protein (α-subunit) of nitrogenase were used as probes. Visualization of the antibody binding to the nitrogenase was performed using horseradish peroxidase-conjugated secondary antibody and the chromogenic substrate, 3,3′-diaminobenzidine tetrahydrochloride, to avoid the masking effects caused by the bright autofluorescence emitted by the Trichodesmium cells. Nitrogenase proteins were detected in 66 to 81% of the trichomes. More than 77% of the cells were immunostained in individual nitrogenase-containing trichome. The data show natural populations of Trichodesmium spp. do not developed heterocyst-equivalent cells.