Improving flow characterisations in complex estuary and coastal waterways using Lagrangian drifter data

By utilising new measurement technologies and advances in numerical models, especially through the use of open-source software, this project paves a way for the better understanding of estuarine hydrodynamics. Using state-of-the-art GPS-tracked drifters and an ensemble-based data assimilation approach, this research presents several novel methods to improve the accuracy of hydrodynamic models for shallow estuaries. It opens new opportunities for researchers/engineers to use Lagrangian data to better employ hydrodynamic models to understand real-time and future estuarine dynamics

Investigating the effect of intake withdrawal direction on critical submergence and strength of vortices

Free surface vortex formation phenomenon at intakes is one of the most important problems in the water withdrawal process. In the present study, the free surface vortex formation was experimentally investigated. Experiments were performed on a single intake with three common intake withdrawal directions (vertical, horizontal and with angle of 45°). One of the main objectives of present study was determination of the strength of vortices. The tangential velocity component of vortices was measured by an Acoustic Doppler Velocimeter (ADV). The results indicated that, by varying the withdrawal angle from vertical to horizontal, the strength of vortices decreased by about 31%. Based on analyzing of experimental data, an empirical relationship between the circulation number and Froude number for various submergence depths was developed for each withdrawal direction. The critical submergence for an air entraining vortex at intakes was also investigated. The results indicated that the critical submergence was considerably affected by the changing of withdrawal direction. It could be concluded that the minimum of critical submergence was occurred at horizontal direction. Based on analysis of the experimental data for each withdrawal direction, an empirical equation was also obtained, which is used to calculate the critical submergence. The results also were compared and analyzed by other researcher’s investigations and showed satisfactory agreement

Effect of dual intake direction on critical submergence and vortex strength

Intake direction is one of the most important factors which have significant effect on vortex formation at intakes. Due to the importance of withdrawal direction to vortex formation, the present study examined the effect of different intake angles on strength of vortices and critical submergence. Experiments were performed on dual and single intakes with three withdrawal directions (with angles of 0°, 45° and 90°). To calculate the strength of formed vortices, the tangential velocity component of vortices was measured using an ADV. Results showed that, by varying the intake angle from vertical to horizontal, strength of vortices decreased by 31% and 35% for single and dual intakes, respectively. An empirical relationship between the circulation number and Froude number for various submergence depths was developed for each withdrawal direction of the dual intakes. Furthermore, the critical submergence of single and dual intakes was investigated and results indicated that the critical submergence is considerably affected by changing the withdrawal directions.</p

Calibration and Assimilation in Hydrodynamic Model of a Micro-Tidal Estuary and Comparison with Lagrangian Drifter Data

Deployment of Lagrangian drifters in water systems can provide a larger spatial coverage and an additional insight into horizontal motion of particles than Eulerian techniques. This feature has provided an opportunity to assimilate Lagrangian data into hydrodynamic models to enhance their accuracies. Numerical models suffer from both systematic and random errors. Conventional data assimilation methods were designed to reduce the stochastic errors, and systematic errors can negatively affect the assimilation systems. Therefore, a calibration process, which is an effective way to reduce systematic errors and consequently biases in the numerical models, is required to be performed before implementation of data assimilation techniques. In this study, D-Flow FM, a hydrodynamic model, was set up for simulating the essential processes in a micro-tidal estuary in Queensland, Australia. To calibrate the model, bathymetry and bed roughness were selected as calibration parameters, while most studies in estuarine application have only considered the bed roughness as the calibration parameter. Evaluation of model performance in terms of correlation and root mean square error between model outputs and observations for both water level and velocity showed that calibration of bathymetry is important. Herein model outputs are validated with Lagrangian drifter velocity data for different environmental conditions. The results showed that calibration with the consideration of bed roughness and bathymetry reduced the systematic errors and increase the correlation between model outputs and Lagrangian drifter data. This is an important step prior to assimilation of Lagrangian data to reduce stochastic errors

Turbulence characteristics of a tidal embayment

There is a lack of observations of near-bed turbulence characteristics in hypersaline estuaries with inverse circulation. With the aim of providing baseline flow and turbulence characteristics of such system, a field study was carried out in Hervey Bay, a tidal embayment. The flow field from the bed to the mid-water column was measured using a 2MHz Aquadopp upward-looking acoustic Doppler current profiler (ADCP). The turbulence characteristics of the bottom boundary layer were measured using a downward-looking 5-beam Signature1000 ADCP in high resolution configuration. In order to evaluate the energy budget within the channel, we performed and present a preliminary estimate of the rate of dissipation of turbulent kinetic energy, ε, within the channel. The ε varied up to four orders of magnitude within two diurnal tidal cycles, and ranged between 10- 6 - 10-2 m2/s3 for the spring and between 10-8 - 10-5 m2/s3 for the neap tides. These estimates were within the range observed in similar tidal channels

Hydrodynamic modelling and model sensitivities to bed roughness and bathymetry offset in a micro-tidal estuary

Tidal estuaries support everyday functions for over 80% of Australia’s population living within 50 km of the coastline and thus come under immense pressure of physicochemical changes. Most studies in estuarine applications have used the bed roughness as the single calibration parameter to calibrate hydrodynamic modelling, yet errors in bathymetric data can significantly impose uncertainties into the model outputs. In this study, we evaluated the sensitivity of a hydrodynamic model of a micro-tidal estuary to both the bed roughness and bathymetry offset through comparing observed and modelled water level and velocity. Treating both bathymetry offset and bed roughness as calibration parameters, three calibration scenarios were tested to examine the impact of these parameters. To validate the model, Lagrangian drifter data as a new dataset in shallow estuaries were used. The analysis shows that model outputs are more sensitive to the variation of bathymetry offset than bed roughness. Results show that calibrating the bathymetry offset alone can significantly improve model performance. Simultaneous calibration of both parameters can provide further improvement, particularly for capturing the water level. Drifter and modelled velocities are highly correlated during flood tides, whereas the correlation is low for slack water because of wind-induced current on drifters.</p

Comparative design of plug and recirculation RO systems; thermoeconomic: Case study

The main objective of this article is to determine which arrangement between the plug and recirculation reverse osmosis (RRO) is the optimal reverse osmosis (RO) system for a specific test case. Another objective is to compare the optimal operating conditions of these two arrangements. The RO system is designed to supply a water mass flow rate of 4000 m3/day for a 20-years period from Caspian Sea's brackish water to the city of Miarkelar, Iran. A multi-objective genetic algorithm is used to determine the best design of both arrangements of RO systems. Hence, feed water pressure, recirculated water flow rate, sea water membrane types, feed water flow rate and the number of elements in each pressure vessel were optimized. The objective function were the recovery rate and the unit production cost. Considering the optimal state of each arrangement, an exergy analysis has been performed. An exergy efficiency of recirculating and plug systems of 82.6% and 92.4% were obtained respectively. The thermo-economic analysis demonstrates that RRO results in a 36% decline in investment cost, 2% increase in operation cost, 6% recovery rate increment, and 19.7% permeate salt content decrease compared to the plug RO arrangement.</p

Lagrangian data assimilation for improving model estimates of velocity fields and residual currents in a tidal estuary

Numerical models are associated with uncertainties that can be reduced through data assimilation (DA). Lower costs have driven a recent tendency to use Lagrangian instruments such as drifters and floats to obtain information about water bodies. However, difficulties emerge in their assimilation, since Lagrangian data are set out in a moving frame of reference and are not compatible with the fixed grid locations used in models to predict flow variables. We applied a pseudo-Lagrangian approach using OpenDA, an open-source DA tool to assimilate Lagrangian drifter data into an estuarine hydrodynamic model. Despite inherent challenges with using drifter datasets, the work showed that low-cost, low-resolution drifters can provide a relatively higher improvement over the Eulerian dataset due to the larger area coverage of the drifter. We showed that the assimilation of Lagrangian data obtained from GPS-tracked drifters in a tidal channel for a few hours can significantly improve modelled velocity fields (up to 30% herein). A 40% improvement in residual current direction was obtained when assimilating both Lagrangian and Eulerian data. We conclude that the best results are achieved when both Lagrangian and Eulerian datasets are assimilated into the hydrodynamic model.</p