A two-dimensional mathematical model investigation of the hydrodynamics and sediment transport of Saldanha Bay and Langebaan Lagoon

[摘要] ENGLISH ABSTRACT: Since the construction of the causeway and the jetty during the early 1970's in Saldanha Bay, various alterations to the coastline in the area of the Langebaan Lagoon mouth were observed. These alterations include the erosion of Langebaan Beach located near the town of Langebaan. An investigation was undertaken to identify the possible impact these structures had on the hydrodynamics and sediment transport of the Saldanha Bay and Langebaan Lagoon systems, focusing on the entrance to the Langebaan Lagoon. A two-dimensional numerical model was implemented for this investigation.The outdated information available for the generation of a bathymetry, which indicated the conditions prior to the erosion of Langebaan Beach, complicated the calibration process. However, calibration of the numerical model was acceptable. Due to the bathymetry not providing an indication on the current situation at the Langebaan Lagoon mouth, the results from the numerical model were approached with caution, providing an overview of the hydrodynamics present in Saldanha Bay and Langebaan Lagoon and would be able to broaden the understanding of the impact the causeway and jetty had on the hydrodynamics and sediment transport of Saldanha Bay and Langebaan Lagoon. Results provided by the sediment transport model only provide an indication on the effect tidal variations and wind forcing have on the bay and lagoon and not realistic total sediment transport rates due to the omission of wave action during the modelling process.Results from the numerical model, based on tidal oscillations and wind forcing only, have indicated that no major impact on the hydrodynamics and sediment transport were experienced due to the construction of the causeway and the jetty. During the investigation of the impact of various extreme water level and extreme wind conditions, it has been observed that a 1 in 100 year wind velocity across the longest fetch towards Langebaan Beach resulted in the greatest velocities prior to the construction of the causeway and the jetty, and after the construction of the causeway and the jetty tidal storms, or storm surge, generated the greatest velocities and thus the most sediment transport in the main channels of the mouth of the Langebaan Lagoon.From this investigation it was recommended that future studies would require an updated survey of the area, to ensure accurate modelling of the conditions as experienced during field surveys. Further recommendations on the investigation of sediment transport were the inclusion of wave action to provide realistic results. Wave action is a fundamental part of sediment transport along the coastline.

[发布机构] Stellenbosch University