Spatial and seasonal distribution of intertidal macrobenthos with their biomass and functional feeding guilds in the Naf River estuary,Bangladesh

The Naf River estuary is one of the most productive ecological ecosystems in Bangladesh providing an important feeding area for fishes and other aquatic animals. However, detailed information on macrobenthic communities is rarely available in this area. Our study focused on the seasonal and spatial variability of macrobenthic community structure with their biomass and functional guilds. In total, fortyseven taxa were identified under nine major groups and seven functional feeding guilds. Among macrobenthic taxa, Polychaeta was dominant in both seasons and all over the estuary contributing 60% of total benthos.The mean abundance of macrobenthos was higher during the pre-monsoon(2 972±1 994 inds./m~2) than the monsoon(1 572±361 inds./m~2) being maximum at mid-estuary region and minimum at upper and lower estuary regions. Of the diversity indices, density(P=0.01, P=0.003), the number of taxa(P=0.000 3,P=0.000 9) and Margalefs Index(P=0.000 4, P=0.001) of macrobenthos were significantly different among stations and seasons. Shannon-Weiner index(P=0.009, P=0.12) and Pielou's evenness index(P=0.03,P=0.14) had significant differences among stations but not among seasons. Cluster analysis suggested that distribution of macrofauna is strongly influenced by seasonality. Among the feeding types, carnivorous were found to be dominant at the upper and mid-estuarine regions, and herbivorous were at lower estuarine regions. Carnivorous species were dominant in both seasons. Omnivores have shown to be the most abundant feeding type in terms of biomass though herbivores were highest during monsoon.     

Spatial and seasonal distribution of intertidal macrobenthos with their biomass and functional feeding guilds in the Naf River estuary,Bangladesh

The Naf River estuary is one of the most productive ecological ecosystems in Bangladesh providing an important feeding area for fishes and other aquatic animals.However,detailed information on macrobenthic communities is rarely available in this area.Our study focused on the seasonal and spatial variability of macrobenthic community structure with their biomass and functional guilds.In total,fortyseven taxa were identified under nine major groups and seven functional feeding guilds.Among macrobenthic taxa,Polychaeta was dominant in both seasons and all over the estuary contributing 60%of total benthos.The mean abundance of macrobenthos was higher during the pre-monsoon(2 972±1 994 inds./m^2)than the monsoon(1 572±361 inds./m^2)being maximum at mid-estuary region and minimum at upper and lower estuary regions.Of the diversity indices,density(P=0.01,P=0.003),the number of taxa(P=0.000 3,P=0.000 9)and Margalef′s Index(P=0.000 4,P=0.001)of macrobenthos were significantly different among stations and seasons.Shannon-Weiner index(P=0.009,P=0.12)and Pielou’s evenness index(P=0.03,P=0.14)had significant differences among stations but not among seasons.Cluster analysis suggested that distribution of macrofauna is strongly influenced by seasonality.Among the feeding types,carnivorous were found to be dominant at the upper and mid-estuarine regions,and herbivorous were at lower estuarine regions.Carnivorous species were dominant in both seasons.Omnivores have shown to be the most abundant feeding type in terms of biomass though herbivores were highest during monsoon.     

Combined use of dimensional analysis and modern experimental design methodologies in hydrodynamics experiments

In this paper, a combined use of dimensional analysis (DA) and modern statistical design of experiment (DOE) methodologies is proposed for a hydrodynamics experiment where there are a large number of variables. While DA is well-known, DOE is still unfamiliar to most ocean engineers although it has been shown to be useful in many engineering and non-engineering applications. To introduce and illustrate the method, a study concerning the thrust of a propeller is considered. Fourteen variables are involved in the problem and after dimensional analysis this reduces to 11 dimensionless parameters. Then, a two-level fractional factorial design was used to screen out parameters that do not significantly contribute to explaining the dependent dimensionless parameter. With the remaining five statistically significant dimensionless parameters, various response surface methodologies (RSM) were used to obtain a functional relationship between the dependent dimensionless thrust coefficient, and the five dimensionless parameters. The final model was found to be of reasonable accuracy when tested against results not used to develop the model. The methodologies presented in the paper can be similarly applied to systems with a large number of control variables to systematically derive approximate mathematical models to predict the responses of the system economically and accurately.     

Feasibility and acceptability study of rainwater use to the acute water shortage areas in Dhaka City,Bangladesh

This study explored the feasibility and acceptability of harvested rainwater in rural communities of Bangladesh as well as densely populated City like Dhaka, using simple and low-cost technology. As a part of this study, a field survey was conducted in the water-scarce Dhaka City. Four slums were selected for conducting questionnaire survey. A questionnaire was furnished to know some information such as the socio-economic condition for those slum dwellers and family information, housing condition, sanitation condition, health condition, economic condition, existing water supply condition, knowledge about rainwater, willingness to accept rainwater, etc. Two hundred people from different representative groups were selected randomly. Yield after spillage and yield before spillage models were developed to know the actual rainwater availability and storage conditions, which were used to justify the effective tank size. Cost-benefit analysis and feasibility analysis were performed using the survey results and the research findings. A sensitivity analysis was performed to check the important parameters toward the implementation of the system as well. The results showed that cost was the most sensitive parameter (48.1%), the second highest sensitive parameter was roof area (25.9%) and the lowest sensitive parameter was demand (2.2%). The study showed that the low-cost rainwater harvesting technique was feasible and acceptable to the slum dwellers as the only potential alternative source of safe drinking water.     

The tectonic stress field and deformation pattern of northeast India,the Bengal basin and the Indo-Burma Ranges: A numerical approach

We performed numerical simulations to determine the contemporary maximum horizontal compressive stress (σ_Hmax) in the northeast India region, the Bengal basin (Bangladesh), and the adjoining Indo-Burma Ranges, with different boundary conditions. The regional tectonic stress was simulated using the finite element method (FEM) under the plane stress condition. Most of the study areas show NE–SW regional stress orientation, which is consistent with other stress indicators, such as earthquake focal mechanism solutions. The E–W trending Dauki fault, which separates the Shillong plateau to the north from the Bengal basin to the south, plays a major role in the stress distribution and regional deformation. This fault alone accommodates ～25% of the regional surface displacement rate of the study area. The fault pattern of the study area was also simulated using rheological parameters and the Mohr–Coulomb failure criterion. The simulated results reproduce the observed tectonic state of the area, including a strike-slip regime along the Dauki fault, in the southwestern part of the Bengal basin, and in the Tripura fold belt areas. The modeling indicates that the Brahmaputra valley to the north of the Shillong plateau and to the south of the Himalayan frontal thrust exhibits thrust/reverse faulting with a strike-slip component, and in the Indo-Burma Ranges, strike-slip faulting is predominant with a reverse fault component.     

Impacts of pollution on coastal and marine ecosystems including coastal and marine fisheries and approach for management: a review and synthesis

The history of aquatic environmental pollution goes back to the very beginning of the history of human civilization. However, aquatic pollution did not receive much attention until a threshold level was reached with adverse consequences on the ecosystems and organisms. Aquatic pollution has become a global concern, but even so, most developing nations are still producing huge pollution loads and the trends are expected to increase. Knowledge of the pollution sources and impacts on ecosystems is important not only for a better understanding on the ecosystem responses to pollutants but also to formulate prevention measures. Many of the sources of aquatic pollutions are generally well known and huge effort has been devoted to the issue. However, new concepts and ideas on environmental pollution are emerging (e.g., biological pollution) with a corresponding need for an update of the knowledge. The present paper attempts to provide an easy-to-follow depiction on the various forms of aquatic pollutions and their impacts on the ecosystem and organisms.     

Development of a Nonlinear Model for Soft Rock and its Applications

A phenomenological model has been developed for soft rock based on the results of a series of triaxial compression (TC) tests conducted on Kobe sandstone with a very high precision measurement. From the analysis and interpretation of the test results, it has been found that small strain Young’s modulus (E^e) was a function of the major principal stress. E^e for elastic strains of soft rock was assumed to be cross-anisotropic. A damage function has been used to derive the appropriate elastic Young’s modulus when subjected to shear loading. As the basic stress–strain relation, the relationship between the tangent modulus and the shear stress level was used. The differential form of which was subsequently integrated by a 4th order Runge–Kutta solver to obtain the stress–strain relation. The model of soft rock is based on an isotropic strain hardening elasto-plastic framework which takes into account the pressure sensitivity, cross-anisotropy, degradation of Young’s modulus with the degree of mobilized shear stress and the nonlinearity of the shear stress-shear strain relationship. Although the model was developed from the analysis of the TC tests results of Kobe sandstone, it was also applied to the other types of soft rock or stiff geomaterials. Plate loading tests were conducted at a level of 61 m below the ground level at the bottom of a large excavated shaft at four locations. Finally, the model was used to simulate the plate loading test results successfully. This model was successfully calibrated with Akashi sandstone and applied in the simulation for the settlement of Akashi-Kaikyo Bridge piers. The simulations were carried out for both drained and undrained condition by changing the Poisson’s ratio. The layering information beneath the foundations were used in the FEM simulation. The use of very accurate Young’s modulus from the field shear wave velocity test was the key to the successful simulation of the settlement under bridge pier foundations.     

A Method for Derivation of Compression Equation and Value of Degradation Exponent for Structured Soils

A method to derive a general equation for compression of structured soils is presented. It is shown that the general equation leads to the equation of compression for structured soils as proposed by Liu and Carter (Geotechnique 49(4):43–57, 1999). Using the compression characteristics of the structured and remolded soil, an equation is developed to determine a unique value for the structure degradation exponent term of the Liu-Carter equation. This equation is used to obtain the value of the structure degradation exponent of the Liu-Carter equation from the compression behavior of undisturbed and remolded Mexico City clay. The value of the exponent is used in the Liu-Carter equation to predict the compression behaviour of the clay. Excellent agreement is observed between predictions and experimental data.     

Clay mineralogy of the pleistocene soil horizon in Barind Tract, Bangladesh

Pleistocene red soil horizons were exposed in different areas of the Barind Tract in north-west Bangladesh. X-ray diffractions of twenty seven samples from different depths of these soil horizons revealed that the soil horizons consisted of kaolinite, illite and chrysotile with significant amount of opal-CT. Samples from Maddhapara, Bogra, and Nachole contain kaolinite, illite, quartz and opal-CT, and the samples from Kantabari contain chrysotile instead of kaolinite. Clay mineral compositions of different soil horizons indicated two different types of clay assemblages, viz. (a) illitekaolinite and (b) illite-chrysotile. In the village of Kantabari, illite-chrysotile clay mineral assemblage indicate that soil horizons were formed under low temperatures with alkaline and reducing conditions. However, other soil horizons of illite-kaolinite clay mineral assemblage indicate that soils were possibly formed under humid, temperate and welldrained conditions. These two soil horizons were formed under different geochemical, geomorphological and climatic conditions from different parent materials. Scanning Electron Microscopy photographs showing the presence of glass shards and no opal-A were found using XRD, suggesting that the opal-A might not be a precursor to opal-CT in the red soil horizon of the study area. This opal-CT along with the general lack of fossils and presence of glass shards was indicative of a volcanogenic rather than biogenic origin for the Opal-CT in the study area, and X-ray fluorescence data reveals higher percentages of silica which is comparable to the Toba Ash of Toba Caldera, Indonesia of about 75,000 B.P.