| || || Estuarine sediments -- Fiji -- Suva Lagoon|
| || || Transportation of fine suspended sediments in the Suva lagoon using numerical modeling|
Author: Singh, Awnesh Muni
Institution: University of the South Pacific.
Subject: Sediment transport -- Fiji -- Suva Lagoon, Estuarine sediments -- Fiji -- Suva Lagoon, Marine sediments -- Fiji -- Suva Lagoon
Call No.: Pac TC 175 .2 .S56 2007
Copyright:Under 10% of this thesis may be copied without the authors written permission
Abstract: The city of Suva is a densely populated area, which is home to nearly a quarter of the population of the Fiji Islands thereby placing a lot of anthropogenic pressure on its lagoon. The Suva Lagoon has been subject to substantial sediment inputs generated by erosion and human activities. Freshwater input into the lagoon comes predominantly from the Rewa River, the largest fluvial system in the country. The high sedimentary load from the Rewa River, especially during the wet-warm season, has a strong impact on the water properties in the lagoon. The salinity, temperature and turbidity in the Suva Lagoon are some important parameters for water quality which are continuously changing with the seasons. They are also efficient indicators of variations in the lagoon and can transform the marine ecosystem. This project involved gathering of river discharge and rainfall data, collection of wind and field data for the Suva Lagoon. Analysis of the results shows that the variations of CTD and turbidity measurements are dependent predominantly on the river discharge, while the dominant wind regime is the southeast trade winds. The mean monthly wind speed ranges from 4.4 − 5.6 m s-1 during the dry-cool season and 3.3 − 4.5 m s-1 during the wet-warm season. The discharge from the Rewa River is highly dependent on rainfall. A linear relationship with a correlation coefficient of 0.82 between the mean monthly river discharge and total monthly rainfall was established. The salinity range at the surface in the Suva Lagoon during the wet-warm season is between 23.6 – 34.4 psu and between 28.7 – 34.5 psu during the dry-cool season. The bottom salinity shows little horizontal variation. The field results also show that the temperature variations in the lagoon are due to the warm freshwater input from the Rewa River during both seasons. The surface temperature in the lagoon is lower in the dry-cool season (between 24.5 − 25.4 °C) compared to the wet-warm season (between 28.9 − 30.2 °C). The surface turbidity remains almost the same during both seasons with the Rewa Delta (2.6 FTU) and Laucala Bay (1.9 FTU) showing slightly higher turbidity than in Suva Harbour (0.8 FTU) and Nasese Channel (0.6 FTU). A MARS3D hydrodynamic-sediment transport coupled model (built by IFREMER) was applied to the Suva Lagoon with the aim of simulating the water properties in the lagoon. The model is able to simulate the effects of bathymetry, bottom friction, turbulence, river runoff, tides and wind forcing on the salinity and suspended sediment concentration in the Suva Lagoon for the completeness of the model. Model parameters to simulate suspended sediment concentration in the lagoon were estimated x from the model trial runs. It was found that using the critical shear stress cr τ = 0.020 N m-2 and the erosion rate coefficient c ke = 6.5 × 10-5 g m-2 s-1 showed good agreement with the percentage of fine sediments found at the bottom under the combined effect of the tide and wind. Using these values, the model was verified for salinity and turbidity distribution and the results showed good agreement between field data and model predictions. Increasing the model resolution from σ = 10 levels to σ = 23 levels shows a better representation of the vertical salinity profile, especially on the surface layer. The verified model was used to simulate the effect of the presence of the sandbank in the Rewa Delta on the surface salinity distribution in the lagoon. Results showed that the sandbank is an important factor in determining the surface salinity variation in Laucala Bay. During high discharge rates, the surface salinity in Laucala Bay is in the range of 25.0 – 29.0 psu in the presence of the sandbank. Without the sandbank, model simulations show that the surface salinity range in the bay is between 28.0 – 32.0 psu. The presence of the sandbank in the Rewa Delta has the effect of channeling more freshwater from the Rewa River into Laucala Bay (through the Vunidawa River) than when there is no sandbank. This lowers the surface salinity in Laucala Bay significantly. Model runs with different Rewa River runoffs were performed to observe the effect of river discharge on suspended sediment concentration in the Suva Lagoon. It was found that during low discharges (80 m3 s-1) from the Rewa River, the suspended sediment concentration in Laucala Bay is around 7 FTU at the Vunidawa River mouth. This high value is due to the resuspension of bottom sediments due to the tidal and wind effects. There is almost no input of suspended sediments from the Rewa River discharge as the sediments settle along the river channel due to the low river currents. A similar situation is observed for a discharge of 200 m3 s-1, however, the SSC is more than 12 FTU at the Vunidawa River mouth. It is during high discharge rates from the Rewa River (400 m3 s-1) that the effects of river discharge on suspended sediment concentration can be clearly seen. The concentration of suspended sediments at the mouth of the Vunidawa River is more than 18 FTU. This high amount is due to the introduction of the sediments from the freshwater discharge of the Rewa River. The suspended sediments in Laucala Bay are transported further towards the passages by the tides and the wind. The discharge from the Rewa River is the most influential factor in determining the suspended sediment concentration in the lagoon. However, river discharge is directly related to rainfall, thus it can be said that rainfall is the deciding influence in the transport of fine suspended sediments in the Suva Lagoon.