Amir Etemad-Shahidi

Wave data assimilation using a hybrid approach in the Persian Gulf

The main goal of this study is to develop an efficient approach for the assimilation of the hindcasted wave parameters in the Persian Gulf. Hence, the third generation SWAN model was employed for wave modeling forced by the 6-h ECMWF wind data with a resolution of 0.5°. In situ wave measurements at two stations were utilized to evaluate the assimilation approaches. It was found that since the model errors are not the same for wave height and period, adaptation of model parameter does not result in simultaneous and comprehensive improvement of them. Therefore, an approach based on the error prediction and updating of output variables was employed to modify wave height and period. In this approach, artificial neural networks (ANNs) were used to estimate the deviations between the simulated and measured wave parameters. The results showed that updating of output variables leads to significant improvement in a wide range of the predicted wave characteristics. It was revealed that the best input parameters for error prediction networks are mean wind speed, mean wind direction, wind duration, and the wave parameters. In addition, combination of the ANN estimated error with numerically modeled wave parameters leads to further improvement in the predicted wave parameters in contrast to direct estimation of the parameters by ANN.

Classification and Regression Trees Approach for Predicting Current-Induced Scour Depth Under Pipelines

Reliable prediction of scour depth is important in engineering analysis concerned with pipeline stability. The aim of this study is to develop an accurate formula for prediction of the current-induced scour depth under pipelines. Previous experimental data are collected and used as a database by which to study the effect of different parameters on the scour depth. Decision tree and nonlinear regression approaches are used to develop engineering design formulae for estimation of the current induced scour depth in both live bed and clear water conditions. It is demonstrated that the proposed formulas are more accurate than previous ones in predicting the scour depth in all conditions. Probabilistic formulas are also presented for different levels of risk, aimed at safe and economic design of submerged pipelines.

Classification of the Caspian Sea coastal waters based on trophic index and numerical analysis

In order to characterize the trophic state of the southern coastal waters of the Caspian Sea, trophic index (TRIX) as well as numerical analysis using cluster and discriminant analysis were employed in this study. Chemical and biological parameters (NO3, NO2, NH4, PT, DO, and Chla) used in this study were collected seasonally from summer 1999 to spring 2000. A new trophic index developed by modification of TRIX indicated mesotrophic to eutrophic conditions for the Caspian Sea. Numerical analysis revealed three groups of the study area and it was found that the used methods are in good agreement. Both of them predicted poor to moderate conditions in the western part of the study area and the numerical classification predicted trophic conditions in the study area. However, TRIX was found to be a more accurate and suitable method. It performs more conservatively than the numerical classification and characterized lower classes of water quality for the stations in central and eastern parts of the study area.

Anatomy of turbulence in a narrow and weakly stratified estuary

A weakly stratified straight channel in the Swan River estuary was examined using a CTD probe and a microstructure profiler capable of measuring two components of velocity, temperature and conductivity. In addition, a pair of acoustic Doppler current profilers were deployed at the bottom to record profiles of the horizontal velocity. The results indicated that the mean flow was barotropic with little shear. The turbulent overturns were mostly active and viscous forces were negligible in their energetics. Although the stratification was too weak to influence the mean motion, it was found that the turbulence was influenced by buoyancy, and a collapse of the vertical anisotropy was documented with decreasing turbulent Froude number, Frt. Using the results from laboratory and numerical studies, we infer that the mixing efficiencies associated with these turbulent events ranged between 0 and 0.3, with a mean value of 0.07.

Climate change impact on wave energy in the Persian Gulf

Excessive usage of fossil fuels and high emission of greenhouse gases have increased the earth’s temperature, and consequently have changed the patterns of natural phenomena such as wind speed, wave height, etc. Renewable energy resources are ideal alternatives to reduce the negative effects of increasing greenhouse gases emission and climate change. However, these energy sources are also sensitive to changing climate. In this study, the effect of climate change on wave energy in the Persian Gulf is investigated. For this purpose, future wind data obtained from CGCM3.1 model were downscaled using a hybrid approach and modification factors were computed based on local wind data (ECMWF) and applied to control and future CGCM3.1 wind data. Downscaled wind data was used to generate the wave characteristics in the future based on A2, B1, and A1B scenarios, while ECMWF wind field was used to generate the wave characteristics in the control period. The results of these two 30-yearly wave modelings using SWAN model showed that the average wave power changes slightly in the future. Assessment of wave power spatial distribution showed that the reduction of the average wave power is more in the middle parts of the Persian Gulf. Investigation of wave power distribution in two coastal stations (Boushehr and Assalouyeh ports) indicated that the annual wave energy will decrease in both stations while the wave power distribution for different intervals of significant wave height and peak period will also change in Assalouyeh according to all scenarios.

Evaluation of ECMWF wind data for wave hindcast in Chabahar zone

ABSTRACT Saket, A., Etemad-Shahidi, A., Moeini, M.H., 2013. Evaluation of ECMWF wind data for wave hindcast in Chabahar zone Wind waves are the most important environmental forces acting on the marine structures. Due to the incompleteness of measured wave parameters, wave prediction plays a key role in the design of coastal and offshore structures. Nowadays, numerical wind wave models are widely used for wave hindcast and forecast. Since wind is the most important forcing term in the numerical wind wave model, the selection of appropriate wind source is a vital step in the wave modeling. In the present study; two wind sources i.e. the measured synoptic and the ECMWF (European Center for Medium Range Weather Forecasts) data, were evaluated for wave simulation near the Chabahar zone. To simulate wave parameters the third generation spectral SWAN model was utilized and the results were compared with those of in situ measurements in a depth of about 17 m. The whitecapping dissipation coefficient and bottom friction factor were used for calibration of the model. The sensitivity analysis showed that other physical parameters have no specific effect on the wave characteristics. Calibration of whitecapping dissipation rate led to the overestimation of high waves. Therefore, a combination of whitecapping dissipation and bottom friction factors was used to calibrate the model. It was found that the SWAN model forced by ECMWF wind data predicted the south-west and west waves successfully while underestimated the east, south-east and south waves. This was mainly due to well prediction of south-west and west wind and underestimation of wind from the east to the south by the ECMWF model. In addition, it was revealed that synoptic wind data can be used as an appropriate wind source for wave hindcasting at the studied area.

How Does the Driver’s Perception Reaction Time Affect the Performances of Crash Surrogate Measures?

With the merit on representing traffic conflict through examining the crash mechanism and causality proactively, crash surrogate measures have long been proposed and applied to evaluate the traffic safety. However, the driver’s Perception-Reaction Time (PRT), an important variable in crash mechanism, has not been considered widely into surrogate measures. In this regard, it is important to know how the PRT affects the performances of surrogate indicators. To this end, three widely used surrogate measures are firstly modified by involving the PRT into their crash mechanisms. Then, in order to examine the difference caused by the PRT, a comparative study is carried out on a freeway section of the Pacific Motorway, Australia. This result suggests that the surrogate indicators’ performances in representing rear-end crash risks are improved with the incorporating of the PRT for the investigated section.

An alternative data driven approach for prediction of thermal discharge initial dilution using tee diffusers

Mixing of heated water discharged from outfalls is an efficient and effective method of waste disposal in coastal areas. Discharging the heated water with large quantities of mass flux generally requires multi-port diffusers. In recent years, using numerical models to predict the plume behavior has received attention from many researchers, who are interested in design of outfalls. This study reports the development and application of an artificial neural network model for prediction of initial dilution of multi-port tee diffusers. Several networks with different structures were trained and tested using error back propagation algorithm. Statistical error measures showed that a three layer network with 9 neurons in the hidden layer is skillful in prediction of initial dilution and the outputs are in good agreement (R = 0.97) with experimental results. Furthermore, the sensitivity analyses showed that the width of the equivalent slot of the diffuser is the most important parameter in the estimation of initial dilution.

Assessment of CGCM 3.1 wind field in the Persian Gulf

ABSTRACT Kamranzad, B., Etemad-Shahidi, A., Chegini, V. and Hadadpour, S., 2013. Assessment of CGCM 3.1 wind field in the Persian Gulf Increasing of the greenhouse gases emission causes the climate changes. Therefore, usage of the marine renewable energy resources such as wind and wave energies has been increased during the last decades. Climate variability can change the wind and consequently wave patterns and the available energy amounts. Therefore, assessment of the potential effect of climate change on the wind regime is important. In this paper, wind characteristics obtained from a global climate model (CGCM 3.1) is used for assessing the effects of climate change on the wind regime in the Persian Gulf. CGCM 3.1 results were compared quantitatively with those of ECMWF in the Persian Gulf and the results showed that in comparison to ECMWF, CGCM 3.1 wind speeds are mostly underestimated for both average and maximum values. Dominant wind directions represented by two wind fields are nearly similar except for a point located in Strait of Hormuz. Variations of the annual wind speed in this area were also evaluated until 2100 and time series of annual averages of the wind speeds and statistical analysis indicate a decreasing trend in all points in the domain. Variation of wind speed frequency based on CGCM 3.1 winds was also assessed for different intervals and the results showed that the variations of wind speed frequencies are marginal. Although there is a insignificant reduction in the average wind speed, this can lead to large variations in the wind and wave energies and also extreme values of wave characteristics.

Estimation of Transverse Mixing Coefficient in Straight and Meandering Streams

Transverse mixing coefficient (TMC) is one of the key factors in the modelling of lateral dispersion of pollutants. Several researchers have attempted to estimate this coefficient using various models. However, robust equations that can accurately estimate lateral mixing in both straight and meandering streams are still required. In this study, novel formulae were developed using the hydraulic and geometric parameters of rivers. The multiple linear regression (MLR), genetic programming based symbolic regression (GPSR) and dimensionless parameters were used for this purpose. Two extensive data sets including data from straight channels/streams and meandering ones were employed to develop the formulae. The main advantage of the developed formula for meandering streams is proper consideration of the effects of aspect ratio, friction, and sinuosity. The formulae performances were then compared quantitatively with those of existing ones using accuracy metrics such as RMSE (Root Mean Square Error). The results illustrated that the proposed formulae outperform others in terms of accuracy and can be used for estimating TMC in straight and meandering streams. In addition, the comparison of MLR and GPSR models showed that the latter is marginally more accurate than MLR specially in meandering streams. However, the MLR models presented a more justifiable relationship between the TMC and governing dimensionless parameters. The main advantages of the presented formulae are that they are more accurate than previous models, can be used in both meandering and straight streams; and can be easily implemented in numerical models to estimate the pollutant concentration and mixing length.