Davies-Colley & Healy, 1978) . ..... areas and tidal flats along the harbour (Davis and Healy 1993). The sediment ...... Moreover, I would like to thank Greg Cox,.
MASTER SML SCIENCES DE LA MER ET DU LITTORAL MENTION
Marine Geosciences SPECIALITY
Sediment dynamics – Backscatter data
Gwenaël TRIVIDIC Gwenaël TRIVIDIC Impact of the dredging in Tauranga Harbour Bathymétrie participative : méthode et traitements pour son usage par les services hydrographiques Internship Report Master 2 Years 2016-2017 Host Organization: Coastal Marine Group University of Waikato New Zealand Academic Advisor: Pascal LE ROY Training Supervisor: Willem DE LANGE Ce stage a bénéficié du soutien à la mobilité internationale du LabexMER, sous la forme d'une aide de l'état gérée par l'Agence Nationale de la Recherche au titre du programme « Investissements d'avenir » portant la référence ANR-10-LABX-19-01.
1
Table of Contents I.
Abstract ..................................................................................................................................................... 3
II.
Acronym correspondence ......................................................................................................................... 4
III.
List of Figures ......................................................................................................................................... 5
IV.
Introduction ........................................................................................................................................... 6
V.
General overview of Tauranga Harbour .................................................................................................... 8 1.
Study area .............................................................................................................................................. 8
2.
History of Tauranga Harbour ............................................................................................................... 10
3.
Geological settings and generalised stratigraphy of the Tauranga region .......................................... 11
4.
Hydrodynamical context ..................................................................................................................... 14 4.1.
Sedimentology ............................................................................................................................. 14
4.2.
Waves .......................................................................................................................................... 17
4.3.
Currents ....................................................................................................................................... 18
4.4.
Salinity ......................................................................................................................................... 19
4.5.
Temperature ................................................................................................................................ 20
4.6.
Retention times ........................................................................................................................... 21
VI. Instrument deployment and sediment analysis ......................................................................................... 21 1.
Deployment ......................................................................................................................................... 21
2.
Analysis of sediment grain size ............................................................................................................ 24 2.1. Samples and methodology ............................................................................................................... 24 2.2. Results .............................................................................................................................................. 24
VII.
Characterizing the seafloor using backscatter data ............................................................................ 28
1.
Utilisation ............................................................................................................................................ 28
2.
Ways of collecting ................................................................................................................................ 29
3.
Influence of the seafloor properties .................................................................................................... 30
4.
Data and methodology ........................................................................................................................ 32
5.
Comparison between before and after the dredging ......................................................................... 33
VIII.
Perturbation due to the dredging ....................................................................................................... 39
1.
Previous studies ................................................................................................................................... 39
2.
Interpretation of the results post 2015-2016 dredging ...................................................................... 41
3.
Accuracy and uncertainty concerning this study................................................................................. 44
4.
Comparison with another similar areas .............................................................................................. 45
IX.
Conclusion ........................................................................................................................................... 46
X.
References ............................................................................................................................................... 48
XI.
Appendices .......................................................................................................................................... 51
2
I.
Abstract
Tauranga Harbour is the largest export port in New Zealand. For this reason, maintenance dredging and capital dredging are regularly undertaken in order to maintain the navigational channels and allow larger ships to enter the Port. The impact of the 20152016 dredging episode is studied in order to obtain a better comprehension of this impact on the hydrodynamic and sediment patterns within the harbour. A field deployment was realized in order to collect physical and sedimentological data as current velocities or grain size data. Moreover, backscatter data are used. These data are compared to existing pre dredging data in order to obtain the differences between pre and post dredging. Backscatter and sediment grain size results permits us to obtain first results concerning the dredging influence. However, a new hydrodynamical model has to be realised in order to obtain an accurate outlook of the impact of the dredging into Tauranga Harbour.
Le port de Tauranga est le plus important port de Nouvelle-Zélande en termes d’export de marchandises. A ce titre, plusieurs épisodes de dragages ont eu lieu afin d’entretenir et d’améliorer la profondeur des chenaux de navigation. L’impact du dragage effectué en 2015-2016 est donc étudié afin de déterminer son influence sur l’hydrodynamique et sur les mouvements sédimentaires à l’intérieur du port. Suite à une campagne d’instrumentation du port, des données granulométriques ainsi que de vitesse de courants sont utilisées et comparées aux données disponibles ayant été collectées avant le dragage. De plus, des données backscatter provenant de sondeurs multifaisceaux pre et post dragage sont comparées afin d’établir les différences relatives au dragage. Les résultats des données backscatter ainsi que les comparaisons de la taille des grains sédimentaires et leur mise en relation avec les vitesses de courants permettent d’obtenir une première indication concernant les changements liés à l’épisode de dragages. Ces premiers résultats nécessitent cependant d’attendre l’étude de l’ensemble des paramètres hydrodynamiques et sédimentaires ainsi que la réalisation d’un modèle hydrodynamique afin de mesurer précisément l’impact du dragage sur le port de Tauranga.
3
II.
Acronym correspondence
CMG: Coastal Marine Group DML: Discovery Marine Ltd ETD: Ebb Tidal Delta FTD: Flood Tidal Delta GPS: Global Positioning System JWP: Jade Wesser Port MBES: Multi Beam Echo Sounder NIWA: National Institute of Water and Atmospheric research NE: North East NNE: North North East NW: North West PSU: Practical Salinity Unit SE: South East
4
III.
List of Figures
Figure 1 : Tauranga Harbour, situated in North Island of New Zealand (Source: Brannigan, 2009) ......................... 8 Figure 2: Map of the harbour situated in the southern bassin detailing channel names and geographic features (following De Lange, 1988, Mathew, 1997)............................................................................................................... 9 Figure 3: Schematic drawings of the 8 hydrodynamic classes of estuary showing their dominant morphometry and oceanographic properties (Source: Hume et al., 2007).................................................................................... 10 Figure 4: Geological settings of the Tauranga basin (Source: Briggs et al., 1996) .................................................. 12 Figure 5: Stratigraphy of the Tauranga Region (Source: Briggs et al., 1996) ........................................................... 13 Figure 6: Surficial settings of the Flood Tide Delta (down) and the Ebb Tide Delta (up) in 2007. (Source: Brannigan & Environment Bay of Plenty, 2009) ....................................................................................................................... 16 Figure 7: Wave rose for Tauranga Entrance - Frequency of occurrence of various classes and directions (Source: Davies-Colley & Healy, 1978) ................................................................................................................................... 17 Figure 8: Pathway of the two main eddies modelled during the ebbtide. The circles denote 20-minute intervals. (Source: Spiers, 2009) ............................................................................................................................................. 19 Figure 9: Depth-averaged salinity and temperature with and without wind conditions for 14 days in summer (Source: Tay et al. 2013) .......................................................................................................................................... 20 Figure 10: Depth-averaged residence times for 14 days in summer (Source: Tay et al., 2013) .............................. 21 Figure 11: Instruments positions outside the harbour ............................................................................................ 22 Figure 12: Instruments positions outside the harbour ............................................................................................ 23 Figure 13: Sediment grain size D50 outside the harbour ........................................................................................ 25 Figure 14: Current velocities and orientation outside the harbour (Source: Thomas Saillour) .............................. 25 Figure 15: Sediment grain size D50 inside the harbour ........................................................................................... 26 Figure 16: Current velocities inside the harbour (Source Thomas Saillour) ............................................................ 27 Figure 17: Bottom acoustic scattering mechanisms, including refraction and scattering at the water-bottom interface and attenuation and scattering in the sediment. (Jackson et al., 1986) .................................................. 29 Figure 18: Description of the Side-scan sonar system (Source: USGS) .................................................................... 29 Figure 19: Example of a correlation between backscatter data, mean grain size and bathymetry (Goff et al., 2000) ................................................................................................................................................................................. 31 Figure 20: Backscatter data pre-dredging ............................................................................................................... 33 Figure 21: Backscatter data post-dredging .............................................................................................................. 34 Figure 22: Backscatter data post-dredging superposed to a DEM - Stella Passage ................................................ 34 Figure 23: Backscatter data post-dredging superposed to a DEM - Cutter Channel ............................................... 35 Figure 24: Results of the pixel by pixel backscatter data comparison - the red squares coresponds to areas of interest were the silty sediment was determined through analysis of cores ........................................................ 36 Figure 25-A: Predicted sediment textural analysis deduced from cores – Stella Passage and Maunganui Roads . 37 Figure 25-B: Predicted sediment textural analysis deduced from cores - Cutter Channel ..................................... 38 Figure 25 - C: Predicted sediment textural analysis deduced from cores – Entrance Channel ............................... 38 Figure 26: Bathymetry of the Tauranga Entrance compiled from historical survey data (Source: Brannigan, 2009) ................................................................................................................................................................................. 40 Figure 27: Relative changes in meters in the bathymetry after 2015-2016 capital dredging (Source: Thomas Saillour) .................................................................................................................................................................... 42 Figure 28: Medium grain size D50 in 2013 (Source: Ramli, 2016) ........................................................................... 43 Figure 29: Overview of the process in order to study the impact of the dredging ................................................. 47
Table 1: Typical sonofacies corresponding to backscatter data .............................................................................. 31 Table 2: Characteristics of the 2016 backscatter survey ......................................................................................... 32
5
IV.
Introduction
Tauranga Harbour is the largest export port in New Zealand, and has welcoming 1482 vessels in 2016. Forestry, kiwifruit and dairy exports account for almost 80% of exports, at the destination of Japan, China, South Korea, South East Asia, Australia or the Pacific Islands. Importation have also an important part in the development of the Port’s business. These include petroleum, fertilizer, coal, dry and liquid bulk, as well as a range of other products. Imports of containerized cargo have grown rapidly over the past eight years (Port of Tauranga, 2013). To ensure the security for the vessels operating into Tauranga Harbour, maintenance dredging and capital dredging are regularly undertaken in order to maintain the navigational channels and allow larger ships to enter the Port. 1. General Overview – Aim of the internship This internship is part of a PhD project concerning the effect of the dredging episodes into Tauranga Harbour. From October 2015 to August 2016, capital dredging has been undertaken in the harbour in order to deepen and widen the shipping channels. Previous research in Tauranga Harbour (De Lange et al., 2015) conducted by the University of Waikato were used to predict the likely effects on the hydrodynamics and sediment transport within the harbour. Major morphological changes affecting many area of the harbour has been highlighted. The most important change since the channel formation appears to have been a switch from bar bypassing to tidal bypassing of longshore sediment. This has resulted in an increase in the volume of sediment transported by tidal currents within the tidal inlet system. Moreover, bathymetric surveys has been realized by the Port of Tauranga before and after the dredging episode. Backscatter data has been extracted from these bathymetric surveys and can be used in order to characterize the sea floor. Hence, backscatter data could determine the sediment changes on the seafloor before and after dredging episode. 2. Internship progress As this internship is part of a PhD project concerning the hydrodynamic in Tauranga Harbour, the first part was to quantify the hydrodynamic and morphological changes caused by the dredging episode. A field campaign of 35 days has been realized in order to obtain a better 6
understanding of the sediment pathways. The second part of this internship is to utilize backscatter data to characterize the composition of the sea-floor. Different backscatters data will be used with the objective to determine the difference before and after the capital dredging. 3. Presentation of the Coastal Marine Group The Coastal Marine Group (CMG) is part of the School of Science, and also the University of Waikato's Environmental Research Institute. It is a multidisciplinary group of researchers from across the University, with research activities focused on estuarine, surf-zone and inner shelf environments. The Group have staff based at the University of Waikato Hamilton campus and at the Coastal Marine Field Station in Tauranga. It is composed by a dozen of researchers supported by technicians, specialized in survey work, data analysis and interpretation and shallow water mapping. An overview of the Coastal Marine Group research activities are expressed bellow:
Estuarine and Coastal Ecology
Numerical modelling of coastal dynamics
Sediment movement and sediment stability in coastal environments
Coastal waves, mixing, turbulence and currents
Morphology, seabed and shoreline mapping through optical and acoustic techniques and inlet stability
Coastal hazards
The CMG has strong links with NIWA (the New Zealand National Institute of Water and Atmospheric Research), Regional Councils, and port companies for example. In particular, the Coastal Marine Group is collaborating with the Port of Tauranga to investigate dredging influences.
My Job during the internship During this internship, I have taken part in the field campaign. I have spent 5 days on a vessel in order to install the instruments and position them into the harbour in collaboration with the divers. On the boat, my role was to set up the instruments on the frames, and to attach the anchors and the weights to avoid to the frames moving during the deployment. Instruments were put into the water on the measurement point 7
previously defined and located by a GPS point. I have also participated in a routine inspection of the instruments, one week after the instruments deployment. This general inspection was to ensure that the instruments were still in the same place and in the same position. Moreover, some batteries for the pingers, the device that serve to transmit the signal, were changed during this time. New Sediment traps were set on the frames and sediment samples were collected next to the instruments. Finally, at the end of the deployment period, I have taken part in the instruments removal and I have disassembled the instruments and the anchors from the frames. I have also cleaned and downloaded the data from the instrument. Moreover, I have also undertaken all the sediment grain size analysis, using a sediment lasersizer in order to obtain different results, presented later in this report. Finally, I have used and compared backscatter data in order to highlight the differences between before and after the capital dredging episode.
V.
General overview of Tauranga Harbour
1. Study area The Port of Tauranga is situated in the Bay of Plenty on the east coast of New Zealand’s
North
Tauranga
Harbour
Island is
(Figure a
1).
mesotidal
estuarine lagoon (851 km2), separated into two main sub-basins by intertidal flats. The intertidal area of the estuarine lagoon is mainly composed of marine sand. The Wairoa River is the main river draining into the Tauranga basin, and flows along the boundary between the Whakamarama Plateau and the Mamaku Plateau. The discharge of freshwater into the Tauranga Harbour is very small 8
Figure 1 : Tauranga Harbour, situated in North Island of New Zealand (Source: Brannigan, 2009)
compared to the tidal flow (Davies-Colley 1976), so the harbour is a tidally-dominated system. A Holocene barrier island and two Holocene tombolos enclose the harbour. The Holocene barrier island, called Matakana Island, is a 24 km long sand barrier island and the two tombolos, Mt Maunganui in the Tauranga southern basin (Figure 2) and Bowentown in the Katikati northern basin, connect the mainland to rhyolite domes. Bowentown and Mt Manganui tombolos were formed by a progradational dune ridge system, composed by fixed and moving dune sands, and formed during the Holocene, since the maximum of the post-glacial marine transgression, 6.5-4 Ky ago. Nowadays, the two basins are connected at high tide, but can be considered independent, as there is a limited exchange of water between the two basins (De Lange, 1988). Tauranga Harbour is the New Zealand’s largest export port in terms of total cargo volume, and
the
commercial
port
development is centred on the southern basin. It is a shallow estuary, with a narrow entrance and classified
Panepane Point
as a Category E estuary - Tidal lagoon
or
barrier
enclosed
lagoon
–
based
on
the
classification of Hume et al. (2007) (Figure 3). This area is well
mixed
with
a
strong
influence of wind, inducing the resuspension of a homogenous sandy sediment by wave. Two different areas of sand shoals or banks in vicinity of the harbour entrance,
Flood
Tide
Delta
(FTD) and Ebb Tide Delta (ETD)
Figure 2: Map of the harbour situated in the southern bassin detailing channel names and geographic features (following De Lange, 1988, Mathew, 1997)
9
can be distinguished for Tauranga Harbour, consistent with a Category E estuary. The FTD is situated in the inner harbour while the ETD flanks the offshore area of the entrance channel. The average depth at low tide in the harbour is 3 m, with 8.2 km of navigation channels in the southern sub-basin. The tides in Tauranga Harbour are semidiurnal and have an average tidal range of 1.62 m for spring tide and 1.24 m for neap tide. The Harbour has two tidal inlets, one at each end of Matakana Island. The more important inlet for navigation is the south-eastern end bounded by the rocky headland of Mt. Maunganui (Mauao), where it is also the entrance to the Port of Tauranga (Figure 2) (Cussioli et al, 2015). The
southern
inlet,
dominated by the tide, has a maximum depth of 34 m and a minimum width of 500 m, with the ebb-tidal delta (Matakana Banks) extending up to 3.5 Km seaward from the
throat
of
the
(Mathew,
1997).
entrance
channel
Tauranga
harbour
inlet The of was
regularly dredged in order to Figure 3: Schematic drawings of the 8 hydrodynamic classes of estuary showing their dominant morphometry and oceanographic properties (Source: Hume et al., 2007)
maintain at 14.1m depth relative
to
chart
datum
(Spiers, 2009), but it was recently deepened to a minimum depth of 14.5 m within the harbour and 15.8 m outside the harbour during the capital dredging programme of 20152016. No maintenance dredging has occurred since the deepening, although it is scheduled for 2018.
2. History of Tauranga Harbour Although plans to establish a port started in 1873, the Port of Tauranga became operational in 1919 after the construction of the first berths at Tauranga and a pier at Mt Maunganui, followed by the construction of the Railway Wharf at Tauranga in 1927. In 10
1950, the high production of logs, pulp and paper from the central North Island resulted in a rapidly expanding export trade, particularly to Japan and Korea. A 372 m long wharf was constructed at Mt Maunganui to handle these exports (Port of Tauranga, 2013; De Lange, 1988). As the vessels visiting Tauranga became larger, dredging of shipping channels became necessary. Dredging inside the harbour started in 1965 and the main entrance channel through the ebb tidal delta was dredged for the first time in 1968. The channels were widened and deepened during a major capital dredging programme in 1992. To maintain channel depths adequate for navigation, maintenance dredging has been carried out approximately every two years since 1992, and in the lead up to the most recent capital dredging, the Port undertook annual maintenance dredging in some areas (Cussioli et al, 2015). Dredging was mainly focused within the Stella Passage, the southern end of the main shipping channel and main turning basin for vessels at the Port of Tauranga (Jorat et al., 2016). During an appeal to the Environment Court relating to the recent capital dredging, concerns were raised about the possible destabilisation of the ebb tidal delta as a consequence of dredging, as well as suggestions that previous dredging activities had adversely affected adjacent shorelines (De Lange, 1988). This resulted in ongoing research to monitor the impacts of the capital dredging and identify possible mitigation measures that could be used if there are adverse impacts.
3. Geological settings and generalised stratigraphy of the Tauranga region Tauranga Harbour is situated in a highly dynamic area dominated by tectonism and young volcanoclastic sediments. The oldest material that can be observed are a series of volcanic domes and cones aged between 2.18 and 2.95 Ma. These protrude through the Whakamarama Plateau, which forms the near-surface basement of the area, at depths of between 50 and 150 m (Figure 4). Overlying and surrounding the Whakarama plateau is the Waiteariki Ignimbrite formation, which forms a sloping plateau dipping between 3º and 5º to the NE, from the Kaimai Ranges into the Tauranga Basin. (Jorat et al., 2016). Very little faulting has been identified in the Tauranga basin since the deposition of the Waiteariki ignimbrite, and the only faults that 11
have been mapped are two faults situated in the Papamoa Ranges, based on an alignment of rhyolite domes, striking NNE at 30º (Briggs et al., 1996). However, geophysical data suggest that the two sub-basins of Tauranga Harbour may be two
overly
volcanic may
caldera
be
Recent
separate and
subsiding.
research
also
indicates that faulting around the middle of the harbour may be associated with uplift of the
central
harbour
area around Omokoroa (Christophers, Podrumac, 2016).
2015; Figure 4: Geological settings of the Tauranga basin (Source: Briggs et al., 1996)
The Matua Subgroup, with ages between 2 Ma and 50 Ka, consists of fluvial pumiceous silts, fluviatile sands and gravels, lacustrine silts and estuarine sands, intercalated with volcanic deposits (Figure 5). The Pahoia Tephras are a unit of reworked rhyolitic volcanoclastic materials, from approximately 2.18 to 0.35 Ma (Briggs et al., 1996). Other ignimbrites, intercalated with the Matua Subgroup can be observed, such as the Te Puna Ignimbrite, characterised by a presence of hornblende and a high quantity of pumice-rich, or the Te Ranga Ignimbrite, a variably pumice-poor to pumice-rich, with a low quantity of crystal and a sandy textured ignimbrite (Jorat et al., 2016). Above the Matua Subgroup, there is a thick tephra cover including the Hamilton Ashes, dated from 0.35 to 0.10 Ma (Briggs et al., 1996). Clay-rich rhyolitic tephra deposits from the Central North Island containing paleosols representing breaks in volcanic deposition compose this unit (Lowe et al., 2001).
12
The
upper
unit
in
the
stratigraphy of the Tauranga region (Figure 5) is composed by Holocene
sediments
alluvium
and tephras, including a distinct sandy marker horizon of coarse volcanic ash associated with the Rotoehu ash of ~50 Ka together with modern soil horizons (Jorat et al., 2016). During the early Holocene, fluvial and alluvial sediments
and
pumiceous
ignimbrites covered the area with at least tens of meters of relief (Davis and Healy, 1993) and the Holocene shoreline was probably 5 or 6 km offshore of its present location. The
shoreline
transgressed
during a period of sea level
Figure 5: Stratigraphy of the Tauranga Region (Source: Briggs et al., 1996)
augmentation, and waves eroded the coast, including both the fluvial and fan accumulations of volcanoclastic sediments and the estuarine mud (Jorat et al., 2016). As an effect of this transgression, it is possible to observe shelly sand deposits along ancient shorelines, probably showing a wave energy and storm frequency pattern. This pattern can be assimilated to nowadays processes. (Davis and Healy, 1993). Holocene and late Pleistocene river and stream alluvium deposits have formed terraces oriented NE or NNE and mainly composed by silts, sands, clays, gravels and carbonaceous material. Many of these terraces are crossed by shallow valleys, and are ended seaward by low cliffs or steep slopes, between 0.5 m to 80 m above the sea level (Briggs et al., 1996).
13
The offshore of the Tauranga area is characterised by a deeper highly faulted unit, with non-deformed sedimentary sequences on its’ top. Briggs et al. (1996) hypothesised that there may be a series of NNE normal faults, which occur in the basement. However, these faults are covered by the Waiteariki Ignimbrite formation and by younger pyroclastics and sediments (Figure 4). These faults could have controlled the localisation of volcanic vents and the NNE direction of many rivers of the region. The large number of hot springs in the Tauranga area is also related to and controlled by the NNE fault system (Briggs et al., 1996).
4. Hydrodynamical context 4.1.
Sedimentology
A wide range of volcanoclastic sediments, including primary ignimbrites and tephras, associated with fluviatile reworked sediments composes the sediment stratigraphy in the coastal area of Tauranga Harbour. These sediments are deposited in a complex arrangement showing a significant vertical and lateral variability (Jorat et al., 2016). Tauranga Harbour can be interpreted as a drowned valley complex with numerous swampy areas and tidal flats along the harbour (Davis and Healy 1993). The sediment facies within the Tauranga Harbour can be divided in nine differents sedimentological facies listed below:
-
Shell lag
-
Poorly developed mega ripples
-
Very shelly sands
-
Clean sands
-
Rock outcrop
-
Shelly sands
-
Gravel or boulders
-
Silty sands
-
Strongly developed mega ripples
The strongly bedformed areas correspond to active sediment pathways, while the shell lag areas are associated with strong currents and scouring (Brannigan, 2009). The sediment with Tauranga Harbour is mostly sand-sized with a small gravel fraction of mollusc shells, shell fragments, and a very small proportion of pumice and rhyolite fragments. However, the proportion of gravel-sized sediment increases in areas with 14
strong flows. The proportion of shell in the surficial sediment in the inlet area is about 30%, and this quantity can reach 90% in the deeper channels. In contrast, the proportion of mud (silt and clay) in the harbour is generally not important in the Holocene sediments, being around 1% (Davies-Colley and Healy, 1978). The proportion of mud is higher in the underlying Pleistocene sediments, which were exposed to chemical weathering during previous glacial periods when sea level was around 100 m lower. During the 1992 capital dredging, some of this muddy sediment caused a milky white discoloration of the harbour waters. This caused a public outcry and opposition to further dredging. Therefore, a focus of recent research was on mapping the extent of muddy Pleistocene sediments that would be disturbed by capital dredging so that the discoloration could be prevented or minimised (Moon et al, 2013; de Lange et al, 2014; Jorat et al, 2016). This work was successful, with no complaints about discoloration being made during the recent capital dredging. The sediment of the harbour have a low amount of organic matter, comprising around 0.5% and mostly contained in the fine mud fraction. Concerning the chemical composition of the acid-insoluble fraction of the sediment, it is composed of about 50-55% volcanic glass, 25% sodic plagioclase, and 20% quartz with lower amount of heavy minerals, probably hornblende, magnetite and hypersthene (Davies-Colley and Healy, 1978). The FTD (Figure 6) area is mainly composed by medium sand, surrounded to the east and west by finer sand with a minor shell coverage. An area of very fine sand without shell occurs in the Stella Passage (Boulay, 2012; Brannigan, 2009). The northern part of the FTD closest to the harbour entrance appears to be covered by a high percentage of shells, while the centre and the southern part is mainly characterised by a low amount of shell. The peripheral channels are composed of medium sand interspersed with fine sand, with varying levels of shell coverage decreasing away from the tidal inlet (Brannigan, 2009). The main part of ETD is covered by fine sand without shells, except for the swash platform which consists of medium sand with a high amount of shells. The ebb channel is covered by medium sand with a significant quantity of shell, whereas an area with medium sand and a lower amount of shell can be observed in the west part near the ebb channel.
15
On the western side of the ETD near the terminal lobe, large rippled surface of medium sand, measuring approximately 0.25 km longshore x 1 km cross-shore and orientated perpendicular to the shore can be observed. Other smaller medium sand features, with an approximately size of 75 m longshore and 250 m cross-shore are situated on both sides of the ebb channel (Brannigan, 2009).
Large Rippled surfaces
Medium sand features
Figure 6: Surficial settings of the Flood Tide Delta (down) and the Ebb Tide Delta (up) in 2007. (Source: Brannigan & Environment Bay of Plenty, 2009)
The source of the harbour sediment could be from the wave erosion of the Bay of Plenty continental shelf or from littoral drift along the Bay of Plenty beaches. The original marine 16
sediment has a modal grain size ranging between a fine to a medium sand. However, the tidal currents in the vicinity of the Tauranga harbour entrance could have reworked the original sediment and changed its textural character (Davies-Colley and Healy, 1978). Further away from the entrance, erosion of coastal cliffs contributes most of the terrestrial sediment (Podrumac, 2016).
4.2.
Waves
An analysis of wave conditions shows that calm conditions associated with waves
