Hydrodynamic and sediment transport numerical modelling and applications at Tairua Estuary, New Zealand

摘要

Tairua Estuary is a partially mixed estuary located on the east coast of the Coromandel Peninsula, North Island, New Zealand. Like many estuaries worldwide, Tairua Estuary is experiencing rapid sedimentation, which is causing a range of environmental and management issues. This study was undertaken to develop a combined hydrodynamic and sediment transport numerical model as a tool for improving management of sedimentation issues within Tairua Estuary.Two field campaigns were undertaken in July 2010 and June 2011 to obtain calibration and verification datasets for two suites of numerical models – DHI MIKE and ASR 3DD – that were both used for hydrodynamic and sediment transport simulations to allow comparisons of the model suites. Additional models were used to simulate additional processes such as oil dispersal.Observations and numerical modelling showed the tidal wave was distorted as it propagated into the Tairua Estuary, becoming increasingly asymmetrical with distance from the entrance. The tidal wave underwent more distortion during spring tides than during neap tides, resulting in the influx of more oceanic water in the upper Tairua Estuary during a spring tidal cycle, causing a greater increase of salinity. The changing tidal distortion with tidal range also resulted in spring tides being flood dominated and neap tides being ebb dominated. The fate of fine sediments introduced into the lower estuary was, therefore, dependant on the state of the spring-neap cycle, with spring tides favouring deposition in the upper estuary, and neap tides favouring export to the continental shelf.The effects of tidal behaviour in the estuary were modified by river discharge, with increasing discharge resulting in increased ebb dominance and export of sediment from the estuary. Further, as river discharge increased, the estuary became more stratified, particularly during periods of low tidal velocities. However, areas of Tairua Estuary with tidal current velocities higher than 0.5 m s-1, mostly around the tidal inlet, remained partially mixed, even when the river discharge reached a peak value of 200 m3 s-1.During flood events, the upper part of estuary becomes highly stratified due to the large increase in freshwater discharge. Observations and numerical modelling showed that instabilities can develop in the resulting pycnocline in response to wind forcing and fluctuations in flood discharge, and these propagate as forced seiches within the estuary. The seiches interact with the turbid floodwaters and the underlying salt wedge to influence the locations where fine sediment is deposited within the estuary, with enhanced deposition at the nodes of the seiches.Sediment transport modelling indicated that suspended sediment from the river and sediment eroded from the estuary bed, primarily is transported seaward along the main channel of the estuary and through the northern side of the tidal inlet. Subsequently, coarser suspended sediment tends to deposit on the terminal lobe of the ebb tidal delta due to lower current speeds. Meanwhile, sediment suspended along Pauanui Beach by wave action enters the estuary along the southern side of the tidal inlet, and this sediment is mostly deposited on the flood tidal delta. The interaction of the sediment transport entering the estuary and exiting the estuary forms a large eddy over the ebb tidal delta, which acts as a sediment deposition-centre. The model results were consistent with the field observations.The numerical models were calibrated against one field dataset and verified against the second using a variety of statistical measures for the goodness of fit. The results were characterised as excellent for elevation changes over most of the estuary, apart from the elevation for the Tairua River channel around the limit of tidal influence. The calibration and verification of current velocities resulted in a range of results from reasonable to excellent, depending on the numerical grid resolution and the complexity of the local bathymetry. The finest 20 m grids produced the best results, with some minor problems with the current velocity directions along the seaward boundary of the models. Overall the MIKE software suite was easier to use and ran faster, but the 3DD suite produced better results for shallow areas with narrow channels in the upper estuary. Once calibrated and verified, the models were used to simulate a range of scenarios requested by the Waikato Regional Council. These included assessing the impact of potential sea level rise, development including channel realignment and marina construction, and the effects of oil spills within the estuary.As sea level rises, the estuary is predicted to become increasingly flood dominated, which would result in greater sediment transport into the estuary from the Pauanui Beach system, and hence, subsequent deposition on the intertidal flats. Effectively, sea level rise would reverse the normal sequence of estuarine evolution, turning the clock back towards a more youthful estuary. It is also likely that saltwater intrusion was more frequent with increased sea level. However, the influx of sediment would also compensate for some of the sea level rise, and reduce the tendency for flood dominance.