Estuary rejuvenation in response to sea level rise: an example from Tairua Estuary, New Zealand

Under stable sea level conditions, estuaries tend to accumulate sediment resulting in changes that can be conceptualised as a progression from a juvenile system to maturity. Sea level rise results in an increase in accommodation space within the estuary that can effectively reverse the progression towards maturity. This effect was investigated for Tairua Estuary, New Zealand, using a set of MIKE 21 HD (hydrodynamic) and MIKE 21 ST (sediment transport) numerical models forced by tides, wind and fluvial discharges. A range of scenarios of sea level rise and changing fluvial inputs were simulated. The simulation results predicted a general trend of increasing tidal range with sea level rise, which was partially offset by higher fluvial discharges within high fluvial input scenarios. Under present conditions, the estuary is ebb-dominated with predominantly fluvial inputs, but with increasing sea level, the estuary becomes increasingly flood-dominant and the transition from fluvial to marine sediments within the estuary progressively moves landward. If the sediment supply increases in tandem with the accommodation space, the result would be no significant change to the estuary’s hydrodynamics and sediment transport patterns. Under present conditions, observations and modelling indicate that sediment is exported by ebb-dominant conditions, suggesting a capacity to compensate for increasing accommodation space. However, modelling indicates a sediment deficit at higher projected sea levels, resulting in sediment being scavenged from the seaward margin of the sandy barrier spit enclosing the estuary. Scavenging would increase the coastal erosion hazard for the residents of the barrier spit. This situation does not arise if the rate of increase in accommodation space remains below the annual fluvial sediment supply. Ultimately, our results show the importance of considering catchment-derived sediment supply and morphological complexity, when predicting how the changing hydrodynamics modulate the interplay between sediment stored within the estuary and the open coast, and determining the fate of the coast during sea level rise.