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IC14_NZ

Hydrodynamics, Sediment Transport Modelling and Applications on the Tairua Estuary, New Zealand

Research Background
The tidal inlet at Tairua is rapidly infilling due to sedimentation within the harbour, fed principally from both the Tairua River and onshore drift from the inner shelf. Several factors have been suggested as contributing to the high sedimentation rates within the estuary, including: episodic storm events in the steep land catchment debauching large slugs of sediment to the lower harbour; accelerated erosion from by forest harvesting; subdivision developments within the catchment; potential accelerated catchment erosion from climatic change effects; and long term persistent onshore creep of shelf sands. These combine to reduce tidal prism, and hence the sediment flushing capability of the lower harbour and inlet. To date the Tairua tidal inlet and estuary has not been modeled with a well calibrated hydrodynamic and sediment transport model, and there is generally a paucity of hard field data for the estuary.

Aims and Objectives
Of high importance and concern for environmental management is the need to understand the features of the Tairua Estuary:
The physical oceanography: harmonic analyses of the tidal elevation and current speed; semi-diurnal or diurnal tides dominance; tidal range; flood/ebb tidal asymmetry; tidal propagations; max./min. Current speed calculation and comparison with model calculated results; residual current speed during spring tides and neap tides; and river freshwater input and tidal exchanges.
The sediment transport: the rate of sedimentation in the lower harbour and reduction in tidal prism; the sediment transport pathways at the inlet, and relating to sand circulation in the lower harbour and to the adjacent beaches; the sediment transport pathways in the upper harbour associated with the movement and deposition of median sand.
The potential impacts of natural hazards (wave effects, river floods, relative sea level rise, and the harbour seiching and harbor development) on the physical oceanography and the sediment transport.
And the oil spill trajectory study for forecasting the locations of the spilled oil and fast responding to protect the oil cleaning-up and coastal protections.

Research Statement
The research involved two substantial field data collection of tidal currents, river flow, suspended sediment concentrations, water salinity and temperature as a basis to establish a fully calibrated and verified hydrodynamic and sediment transport numerical model of the Tairua Estuary on the East Coromandel Coast: including simulating tidal currents, river inflows, and wave interaction; and from that undertaking sediment transport modeling. Three different resolution models (50 m, 20 m and 10 m) were calibrated against the field observations of water levels and tidal currents.
On the basis of the field data and the calibrated hydrodynamic models, the hydrodynamic and sediment transport characteristics are studied; the salinity and temperature structure was simulated using a 3-dimensional hydrodynamic model; and the surficial sediment characteristics predicted for comparison with observations.
An important purpose of this research is to apply the hydrodynamic and sediment transport models to assess the potential impacts of the harbor/catchment developments and natural hazard analyses.
Current Stage of Work
Field deployments: two field works were carried out during August 2010 and July 2011 respectively to collect the water surface elevation, current speed and direction, temperature, salinity and turbidity data for hydrodynamic and sediment characteristics analyses and numerical model calibrations.
Numerical model development: three nested models of increasing resolution (50m, 20m and 10m grid size) using depth-averaged hydrodynamic models (MIKE 21 HD and 3DD Flow) were developed and calibrated with the field collected surface elevation and current speed data. A depth-averaged non-cohesive sediment transport model (MIKE 21 ST) was also implemented to study the sediment transport characteristics of the Tairua Estuary. Also, a three-dimensional flexible mesh hydrodynamic model (MIKE 3 FM) was used to investigate the forcing mechanisms of a harbor seiching event that happened during the field work in August, 2010, and to simulate the salinity and temperature structure of the lower Tairua Estuary.
Review of the hydrodynamic and sediment transport features of the Tairua Estuary, in order to identify the distribution of surficial sediments, sediment transport pathways and zones of accretion and deposition.
Case studies: a series of scenarios were modeled to examine the hydrodynamics features of Tairua Estuary including: the tidal elevations, tidal currents and river fresh water input, and the non-cohesive sediment transport characteristics including the sediment threshold current speed, bed-load transport and total load transport to understand the sediment erosion, transportation and deposition rate and locations within the Tairua Estuary. The numerical models were then applied to four case studies to assess the impacts of future changes on the estuary, including: the impacts on the sediment transport due to relative sea level rise: harbor seiching; marina construction and related channel dredging: and also the oil spill trajectory forecasting.
Achieved Results
Three nested (50 m, 20 m and 10 m) 2-dimensional and 3-dimensional hydrodynamic and sediment transport models are developed and calibrated using the software of MIKE and 3DD.
The Tairua Estuary is a micro-tidal estuary. The relatively small variation of the water depth during the neap tides, the tidal wave undergoes more distortion during the spring tides than during the neap tides. The Tairua Estuary is dominated by semi-diurnal tides at the lower reach of the estuary with a significant diurnal effect at the river mouth. Based on the tidal propagation analysis, more water can accumulate in the upper part of the Tairua Estuary during the spring tidal cycle. It causes an increase of salinity during spring tides than during neaps. It can also result in net sediment movement to landwards during spring tides and net seaward sediment movement during neap tides. The maximum current speed could reach 1.0 - 0.1.4 m/s at the locations of the tidal inlet, and 0.4 – 0.6 m/s at the Pepe Stream and Grahams Stream entrance and the northern corner of the Paku Hill close to the Tairua Beach. The minimum current speed is about 0 - 0.1 m/s along the main channel with flood tides. At the Tairua estuary, the average high spring tide is 1.0 m, the average low spring tide is -0.82, therefore the average tidal range is 1.82 m during the spring tides; the average high neap tide is 0.6 m, the average low neap tide is -0.53 m, and the average tidal range is 1.13 m during the neap tides. The water surface areas is calculated using the number of the grids covered by water multiplied by the area per grid (50 m2) as 6.41 km2 at spring high water, 3.62 km2 at neap high water. Therefore the average water surface area is 5.02 km2. As a result, the tidal prism at springs and neaps could be calculated as 9.12×106 m3 during spring tides and 5.67×106 m3 during neap tides. The field recorded and model calculated mean current speeds are 0.606 m/s at the tidal inlet. As a result, the average water discharge from the estuary to the ocean is 131.37 m3s-1 through a 430 m2 mean throat area. Therefore, the Tairua Estuary could be flushed after 19.29 hours during spring tides. This conclusion is consistent with the saline mixing research by Bell (1994): in Tairua Estuary 82% of incoming water was new ocean water and that harbor flushing took 1.3 tidal cycles to replace the entire tidal prism volume.
Case Study I estimated the combined impacts of potential sea level rise and river discharge on the hydrodynamics and sediment transport of Tairua estuary. A general trend of increased tidal range was forecasted with rising sea level. The residual current speed in the estuary is currently dominated by the ebb tidal flow. Under the extreme scenario of a 1.2 m rise in sea level, the flood tidal flow would likely dominate the estuary. At present, the residual sediment at the tidal inlet is generally transported seaward and tends to deposit on the outer side of the tidal inlet where it forms the ebb tidal delta. However, as sea level increases, the model results predict greater sediment transport into the estuary and subsequent deposition at the flood tidal flat area. This research also indicated that saltwater intrusion was more likely to occur with sea level rise. An area of future research could be to conduct 3D modelling of changes to the salinity gradient.
Case Study II calculated the period of the water level oscillations of the lower Tairua Estuary that happened on 4 August 2010 as 20.9 minutes using spectrum analysis. This result matches the theoretical response calculated by Marian’s equation and prove these fluctuations were generated by a seiche event. To reveal the forcing mechanisms of the seiche event, the field data and meteorology data, including air pressure, precipitation, flood discharge from the river mouth, turbidity and wind, were analyzed and the calibrated 20 m MIKE 21 HD model was used to study the impacts of the river flood discharge and wind conditions. The results show that the flood discharge from the river mouth and the wind combined to force the seiche event, but the wind driven fluctuations were larger than those due to the river flood discharge. A MIKE 3FM-ST (Flexible Mesh non-cohesive Sediment Transport) model was developed based on the calibrated MIKE 21 HD model to compute the vertical profile of the salinity and density. The model simulation results and the field meteorological data indicate that a sudden drop in air pressure resulted in a high-speed, intense, meso-scale storm that produced heavy precipitation. The intense rainfall it resulted in a sediment-laden flood from the Tairua River that discharged into Tairua Estuary. As a result, density stratification occurred in the lower estuary, and flow instability developed in the pycnocline, forming the seiche event. Further work needs to be done to determine the mode and limits of the seiche event within the estuary, and to characterize the impacts of the shelf wave that travels along the coast on the seiche event.
Case Study III. The POL3DD oil spill trajectory model was used. According to the simulation results, oil discharged on a water surface immediately starts to increase its surface area. In calm water this leads to a continuous decrease in the oil slick thickness in a circular pattern, which finally reaches a minimum thickness of 0.1 to 0.01 mm. Random motions induced by waves, wind and tidal currents translate the elements of the slick relative to each other and to the centre of mass of the slick. Tidal processes have a great influence on the variation in the shape and size of the oil slick, producing stretching and contracting periods as velocities fluctuate. Generally the oil slick extends in the wind direction and increases with time in proportion to the wind speed, while the lateral spreading of the slick is mainly due to gravity spreading. Under natural conditions, oil spreading will not stop when the terminal thickness is reached. The oil slick will tend to break up into patches and small fragments due to wave action and current shears, and these patches or fragments are spread due to oceanic turbulence. The area of oil spreading is reduced when waves are present, and the rate of reduction is larger for lighter oil (eg. petrol). In this study, the air temperature does not influence the oil spreading and evaporation obviously. It might be because of the short simulation time.
Case Study IV. The lower reach of the Tairua Estuary is a dynamic system, affected by tidal exchange and river flows. There are no significant changes of the flood/ebb tidal flow, current speed and residual currents on the intertidal flat after proposed marina construction (including associated structures and realigned Grahams channel). However, the flood flow from the access channel travelling between the breakwater and the piling of the marina will be increased slightly (about 0.05m/s). Before marina development, during peak ebb tides, some of the sediments carried by the Grahams Stream are distributed to the lower Tairua Estuary. A portion of the sediment also deposits at the south-eastern corner of Paku Bay because of the sheltering effects of the Paku Spit. After the marina construction, the Grahams Stream channel is re-aligned, aiming to reduce the fine sediment deposition due to the combined shielding effects of the Paku Spit and Marina. The current speed at the south-eastern corner will increase due to the channel realignment. However, finer sediment will deposit close to the Paku Beach and the area between the previous and realigned channels. As a result, the realigned Grahams channel will not be stable and can be expected to migrate. The marina access channel is dredged deeper and wider between the bank of the Paku hill and the flood tidal delta. It results in slower flood and ebb flow along the deeper access channel. Further, the channel dredging changes the topography of the flood tidal delta and results in the changes of the hydrodynamics and sedimentations to the flood tidal delta and as a result the tidal inlet system. More sediment will be flushed from the flood tidal delta to the ocean outside and deposit at the ebb tidal shoal; a portion of sediment taken from the Pauanui Beach re-enters the tidal inlet with the flood tides and is deposited on the left side of the main estuary channel where the Pauanui Wharf is located; and more sediment will be transported from the access channel to the intertidal flat. Inside the marina will be dominated by the flood flow. It means that the marina will be eventually be filled with sediment, most likely to deposit at the end of the marina. The increased current speed across the front of the marina entrance by the breakwater will generate a turbulent flow at the front of the marina entrance and impede the water exiting the marina during ebb tides, as a result, there will be less water circulation and lower water quality. All the model calculated results reveal that the potential changes in the dimensions of the tidal inlet and tidal delta system due to the proposed marina redevelopment will not be significant compared with the 22,000 tonne/year catchment sediment yield (Mead et al., 2005) and 5.3×106 m3 spring tidal prism.

Members

Proponents:Dr. Willem de LangeUniversity of Waikato
Prof. Dr. Karin Bryan
:PD Dr. Christian WinterUniversity of Bremen
PhD Candidate:Cathy Zhi Liu University of Waikato

Publications

Published (Peer-reviewed)
Liu, C., W. de Lange, C. Winter and K. Bryan, (online), Forcing mechanisms of a seiching event at Tairua Estuary, New Zealand, in: Coasts and Ports 2011 : Diverse and Developing: Proceedings of the 20th Australasian Coastal and Ocean Engineering Conference and the 13th Australasian Port and Harbour Conference. Barton, A.C.T.: Engineers Australia, 2011: 416-421

Submitted
Liu, C, W. De Lange, V. Pickett, C. Winter and K. Bryan (subm.), 3DD Hydrodynamics Numerical Model Development of Tairua Estuary. Technical Report to Waikato Regional Council, New Zealand.

Miscellaneous

Research stay at the University of Bremen: 01.10.2011 - 31.12.2011