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Morphology of the Te Tumu Cut Under the Potential Re-diversion of the Kaituna River

Following the diversion of the Kaituna River from Maketū Estuary, out to sea at Te Tumu in 1956, the local community has continually voiced concerns over the estuary’s increased sedimentation rates and decreasing ecological health. These concerns led to the partial re-opening of Fords Cut in 1996. However, this has only resulted in a slight improvement in water quality, and no measurable reduction in sedimentation. The Bay of Plenty Regional Council is currently investigating a number of different re-diversion options to partially or fully restore the flow of the Kaituna River into Maketū Estuary, with the aim of restoring the estuary’s health. However, a consequence of re-diversion is the possible reduction in discharge through the inlet at Te Tumu. Significantly reducing the discharge would likely cause a large amount of sedimentation in, and around the cut, which could ultimately lead to the closure of the channel. In order to predict the geomorphic response to the proposed options of lowering the Ford’s Cut Culverts to RL -1.6 m (Option J), or reopening Papahikahawai Channel (Option N) a coupled 2-dimensional, wave, hydrodynamic and sediment transport model was developed. Despite being unable to calibrate the hydrodynamic model due to the highly variable nature of the ADCP data collected, the coupled model was entirely calibrated through the sediment transport model. Comparisons between five transects extracted from a survey conducted in 2008 and from the model results of a simulation run over the same time period, showed predicted bed levels to match on average within 0.35 m. Bed levels inside the river were predicted almost exactly, however, the error increased over the ebb delta, although, the same overall shape was still recreated. Through simulations of average and low river flows, and average and twice the average wave height conditions the morphological response of the Te Tumu Cut to various potential re-diversion options were found. Simulations of Option J showed that the majority of increased water entering the estuary through Ford’s Cut occurred during the incoming tide. This therefore increased the flood tidal volume and sediment transport capacity entering the cut, and caused the ebb channel to become shorter by approximately 20 metres. Similarly, reopening Papahikahawai Channel with one-way flapgated culverts caused an increase in the flood tidal volume entering the Cut. However, unlike Option J there was no corresponding increase in sedimentation around the river mouth. This was attributed to dredging required to install the flapgates, resulting in a loss of sediment from the system and the inlet not yet reaching a new equilibrium. When Papahikahawai Channel is reopened without flapgated culverts both the peak flood tidal discharge and peak ebb discharge increased. The increased ebb flows resulted in a small increase in size of the ebb channel through the delta, with the increase in size varying between the different river flow and wave condition simulations. The effect of adding parallel twin jetty structures to the Te Tumu Cut was also assessed at a preliminary level. Model simulations showed jetties spaced either 45 or 35 metres apart would aid in increasing the navigability through the river mouth. The 35 metre spaced jetties produced a more uniform and deeper channel compared to the 45 metre spaced option. However, further investigation is required to determine the ideal jetty configuration and long term effects on the surrounding coastlines.
Type of thesis
Mawer, J. C. (2012). Morphology of the Te Tumu Cut Under the Potential Re-diversion of the Kaituna River (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/6511
University of Waikato
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