Dynamic modelling of coastal lagoon opening processes

Abstract

The coastline of south-eastern Australia contains a concentration of coastal lagoons which cyclically open and close to the ocean. Ocean waves act to build a barrier in front of the lagoon, isolating the lagoon from the ocean until such time as the barrier is breached, typically in response to catchment rainfall and barrier overtopping. In total, around 70 of these systems exist in New South Wales, with the majority south of Sydney. Similar systems also exist worldwide in places such as South Africa, California, New Zealand, Vietnam and Portugal to name several. A common management measure in New South Wales involves artificial breaching of the barrier when water levels in the lagoon become high enough to cause flooding concerns in surrounding low lying residential areas. With an expected rise in sea level, the viability of this management strategy is likely to decline. It is desirable to gain a better understand of entrance dynamics, including the breakout process. Numerical morphological models represent one way of improving that level of understanding. This thesis asserts that the coastal lagoon breaching process differs in key ways from the breaching of anthropogenic structures such as embankment dams, levee’s and sea dikes. While the modelling of anthropogenic structures has received significant attention in the past, the breaching of coastal lagoons has been subject to comparatively less examination. By appreciating the differences, numerical morphological models can be better employed to represent the coastal lagoon breaching process. Perhaps foremost is the flatter slope and typically longer breach channel length of a coastal lagoon. While anthropogenic structures typically have relatively steep slopes (say ~ 1 in 5 to 1 in 3), the slope of the wave built barrier fronting coastal lagoons in New South Wales is commonly 1 in 10 or flatter on the ocean side, and significantly flatter again on the landward side. As the breach channel develops, it flattens even further and previous research indicates that the system settles into a highly efficient flow, characterised by near critical conditions (Froude No. ~ 1.0) including the development of standing wave bed forms and weak hydraulic jumps. The time for full breaching is typically longer O (~10hrs) than for anthropogenic structures O (~1hr) which also tend to have a more catastrophic impact when they breach. The increased time for coastal lagoon breaching also has important implications, for example, in the prediction of times required to relieve the effects of catchment flooding. Breaching is also significantly controlled by the rate at which the breach channel widens, and hence the geotechnical mechanisms controlling collapse of the side walls. Field and laboratory observations have repeatedly confirmed that the side walls of a sandy channel are near vertical during the breach. This presents some interesting challenges for numerical modelling. The research included the collection of a comprehensive set of field data during an artificial breach at Tabourie Lake (South Coast, New South Wales) in February, 2008. That data set has been combined with a pre-existing, but less comprehensive data set from Wamberal Lagoon (Central Coast, New South Wales) in September, 1993. These data have provided a basis for the development, testing and calibration of a two dimensional fixed grid numerical morphodynamic model, built around a preexisting hydrodynamic model that is characteristic of the types of models presently applied in engineering practice for the assessment of flooding. Following development of the morphodynamic code, particular effort has been placed into improving the representation of: -Bank collapse dynamics, noting that the near vertical slopes and discrete shallow slipping failures which cause the channel to widen occur on a sub-grid scale; and -Sediment transport and morphodynamics of the channel base, utilising available research on the transport characteristics and roughnesses of shallow and transcritical flows. Various strategies have been tested with reference to the available field data sets, and recommendations regarding those strategies are made. An important pre-cursor to modelling the impact of sea-level rise on flooding involves prediction of the barrier elevation prior to breaching. This greatly affects the peak water levels that result from catchment flooding. A framework for the assessment of barrier elevations that could occur as a result of sea level rise is also developed and presented. Finally, a number of ‘what-if’ scenarios are presented to put application of the improved model into context, including the impact of assumptions regarding barrier elevation changes due to sea level rise.