Wave induced sediment transport on intertidal flats in a fetch limited environment, Pine Harbour Marina, Auckland
Hull, J. M. (1996). Wave induced sediment transport on intertidal flats in a fetch limited environment, Pine Harbour Marina, Auckland (Thesis, Master of Science (MSc)). University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/10667
Permanent Research Commons link: http://hdl.handle.net/10289/10667
This study investigates the nature of sediments and the role of tides, currents, waves and winds in driving sediment transport on the intertidal flats and shallow subtidal areas in the vicinity of Pine Harbour Marina which is situated in a shallow embayment adjoining Tamaki Strait in Hauraki Gulf, northeast North Island, New Zealand. The results from these investigations along with the study of the marina approach channel and dredge spoil dump site are used to examine the causes of sediment infilling the approach channel to the marina. This work was undertaken in support of two resource consents (W9205 and W9258) granted to Pine Harbour Marina by the Auckland Regional Council to undertake maintenance dredging to excavate 4,500 m3 of sediment from the navigation approach channel between June and October 1994.The sediments of the intertidal and shallow subtidal regions in the vicinity of Pine Harbour Marina were characterised and sediment mixing depths and transport patterns investigated with sedimentation rods, tracer studies, beach surveys and hydrographic surveys. The forcing processes of tide, wave, current and wind in the embayment were also investigated with hydrodynamic monitoring and a wave generation model for fetch limited environments was used to hind cast wave conditions. The data was collected, in conjunction with a monitoring program of the physical impacts of the 1994 dredging, to identify sediment transport pathways and to obtain quantitative sediment volume changes over the intertidal and subtidal channel areas.Pine Harbour Marina is situated in a fetch limited low wave energy environment, dominated by local wind generated waves. Wave generation model WGEN3DD for limited fetches, was set up to hind cast wave conditions from wind data recorded at the site. The wave climate is characterised by wave heights (Hₛ) between 0.1 m to 0.4 m, and wave periods (Tₛ) between 1.0 and 2.0 s. Large wave events are rare, but wave heights (Hₛ) up to 0.5 m and periods (Tₛ) of 2.5 s are generated by wind speeds greater than 10 m/s from the northwest where fetch is greatest.Wind driven circulation is important over the shallow intertidal region. On a day-to-day basis suspended sediment is transported north driven by the prevailing south-westerly winds. However large infrequent storms from the north have a greater impact on sediment transport over short time periods where significant volumes of sediment are entrained beneath high energy waves. Greater sediment transport occurs over the intertidal region than the subtidal region because sediment threshold velocities are more frequently exceeded in the shallower water depths. Thus sediment deposited in the approach channel to Pine Harbour Marina is infrequently entrained as wave orbital velocities are attenuated before they reach the bed in the deeper water.A sediment tracer experiment was undertaken using artificial fluorescent particles to identify the pattern of fine grained sediment dispersion over the intertidal flats. The dominant sediment transport direction detected was towards the southeast during a westerly storm, and a significant concentration of tracer particles accumulated in the approach channel. Sediment depth of disturbance rods on the intertidal flats indicate that sediment mixing depths were generally less than 1.5 cm due to shallow disturbance created the movement of small (h = 1.0-1.5 cm) wave ripples. Depth of sediment disturbance increased to between 2.0 and 3.0 cm associated with storm periods. The maximum sediment mixing depth recorded by sedimentation rods was 6.2 cm. Similar mixing depths of 2.5 cm were recorded duringmixing depth experiments involving short cores through buried dyed sediment and vertical mixing of fluorescent tracer particles. Fluorescent tracer was mixed to a maximum depth of 17 cm, which was not attributed to wave action but rather to bioturbation.Six monthly beach surveys indicated 2.5 cm of accretion occurred over the mid tide region on the northern intertidal flats. This volume of sediment accretion accounts for approximately half of the dredgings material dumped on the intertidal flats. The remaining dumped sediment was resuspended by wave action and either transported back into the approach channel to Pine Harbour Marina, or removed from the area in suspension.Within 9 months of dredging of the approach channel, an average of 0.5 m of sediment deposition occurred along the northern side of the landward 700 m of approach channel. This was estimated to be approximately 5,250 m³ ± 2,100 m³ of sediment infilling the channel. The increased rate of sediment infilling the approach channel is attributed to natural sediment accumulation combined with the rapid transportation of some dredgings sediment back into the channel.Initially dredgings sediment appeared to consolidate in situ, but became dispersed as a shallow layer extending up to 100 m over the intertidal flats north of the approach channel. Over the course of the investigation the dredge mounds were observed to be eroded by small drainage channels which became incised into the silty sediment at various locations transporting fine sediment from the dredge mound and intertidal flats into the approach channel. Some 12 months after dredging, the dredge mounds were dissipated and only remnant blocks of gravel to boulder sized fragments of bed rock were left.This investigation has shown that a significant amount of dredge spoil deposited on the northern intertidal flats during the 1994 dredging operation became transported back into the approach channel within 9 months of the completion of the dredging works. An alternate disposal site for future maintenance dredging works in the approach channel at a further distance from the channel edge would reduce the likelihood of "recycling" of dredged material. Possible alternate disposal options for management of dredge spoil from future maintenance dredging could include: sidecasting disposal further from the channel; offshore disposal site - adjacent or distant; contained disposal; or landfill.
University of Waikato
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