PATTERNS AND RATES OF SEDIMENTATION WITHIN PORIRUA HARBOUR
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PATTERNS AND RATES
OF SEDIMENTATION
WITHIN PORIRUA
HARBOUR
Report prepared for Porirua City Council
JULY 2009
C.R. 2009/1
200Rangitane Road
RD1, Kerikeri 0294
New Zealand
Telephone (64) 09 401 6493
Mobile 021 150 0754
Facsimile (64) 09 401 6463
Email jgibbcmc@ihug.co.nzPATTERNS AND RATES OF
SEDIMENTATION
WITHIN PORIRUA
HARBOUR
by
Jeremy G Gibb, PhD, BSc (Hons), TIPENZ
Managing Director
Coastal Management Consultancy Limited, Kerikeri, New
Zealand
and
Gregory J Cox, IHO Cat A
Managing Director
Discovery Marine Limited, Tauranga, New Zealand
DISCLAIMER
Coastal Management Consultancy Limited & Discovery Marine Limited (the Service
Providers) shall have no liability;
i. to any person other than the client to whom the Service Providers’ report is
addressed; nor,
ii. in the event that the Service Providers’ report is used for any purpose other
than the specific purpose stated in the report.
© Jeremy G Gibb & Gregory J Cox 2009
All rights reserved. This work is entitled to the full protection given by the Copyright Act 1994 to the authors. No part of this work covered by the authors'
copyright may be reproduced or copied in any form or by any means (graphic, electronic or mechanical, including photocopying, recording, recording
taping, or information retrieval systems) without the written permission of the authors. It is accepted that the client is able to copy any report in its
entirety for internal purposes and distribution to its consultants.
ISBN 978-1-877548-00-0 (print)
ISBN 978-1-877548-01-7 (online)
COASTAL MANAGEMENT CONSULTANTS LTD IIPatterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
EXECUTIVE SUMMARY
In May 2009, CMCL and DML were commissioned by PCC (acronyms attached) to determine the pattern
and rate of sedimentation on the Porirua Harbour area seafloor over the last 160 years. The study was
exclusively based on a comparison of hydrographic surveys made in 1849, 1950, 1965-67, 1974, 1991 and
2009. Previous work on sedimentation rates, tectonics of the area, sea level trends since the first survey by
HMS ‘Acheron’ in 1849, set the context. Compared to the 2009 survey by SMB ‘Discovery’, past
hydrographic surveys were limited to a greater or lesser degree by their coverage and accuracy, an
important factor that we took into account.
Over approximately the last 9,500 years, both the Onepoto Arm and Pauatahanui Inlet of Porirua Harbour
have progressively shoaled from the deposition of sand and mud at a net average rate of 1.0-1.5mm/year,
with relatively short-term rates ranging from 0.5-11.7mm/year over this period. The steady infilling of the
arms of Porirua Harbour has occurred in the context of rising global sea-levels at 10-15mm/year up to
about 7,300 years ago with relative stability over the last 7,300 years. Since 1849, GMSL has risen some
210mm of which about 152mm has occurred since 1931 at 1.95mm/year.
The tectonically active Ohariu Fault bisects the Harbour and on the upthrown side W of the Fault the land
has risen at about 0.5m/1,000 years tapering to about 0.2m/1,000 years at Karehana Bay. In contrast, the
land on the downthrown side E of Ohariu Fault has remained relatively stable. During both the 1848
Marlborough Earthquake (Magnitude 7.4-7.5) and the 1855 Wairarapa Earthquake (Magnitude 8.0-8.2),
there was no detectable coseismic uplift or down drop of the Porirua Harbour area and the faults that bound
and dissect the area did not rupture. There has been no detectable interseismic deformation after these
events so that the area has remained tectonically stable over the last 160 years.
During the period of human occupation involving the clearing of native forest and development of the
surrounding land, all previous studies reveal that rates of sedimentation have progressively accelerated
with time. Our measurements show that from 1974-2009, net average deposition rates have increased to
5.7mm/year (13,500-14,000m3/year) in the Onepoto Arm and 9.1mm/year (42,000-43,000m3/year) in
Pauatahanui Inlet. Since 1974, the tidal prism has reduced by 1.7% in the Onepoto Arm and by 8.7% in
the Pauatahanui Inlet.
Allowing for uncertainties, at current deposition rates Pauatahanui Inlet will have ceased to exist over the
next 145-195 years (A.D. 2155-2205) and the Onepoto Arm over the next 290-390 years (A.D. 2300-
2400). Although both marine and terrestrial sources supply the sand and mud to Porirua Harbour, the
stream catchments draining into both arms appear to be the dominant source. It is recommended that
PCC, after due consideration of the findings of this study:
1. Adopt Action Plans that effectively reduce the current net average rates of deposition of sand and
mud of 5-10mm/year within both the Pauatahanui Inlet and Onepoto Arm of Porirua Harbour, to
the geologic rate of 1.0-2.0mm/year, to preserve both arms of the Harbour as estuaries.
2. Consolidate and enhance the re-vegetation and silt-trap programmes within the catchments
draining into Porirua Harbour to permanently reduce the volume of terrestrial-derived sediment
entering the Harbour.
3. Where marine-derived sand may be extracted from time to time from both the ebb and flood tide
deltas, and throat area around Mana Marina, the first priority use for this sand should be for
replenishment of depleted updrift recreational beaches such as Plimmerton Beach, coupled with the
construction of appropriate retention structures to both retain and prevent the sand from being
washed back into the Harbour.
COASTAL MANAGEMENT CONSULTANTS LTD IIIPatterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
ACRONYMS USED IN THIS REPORT
Local & Central Government Agencies, Companies and Boating Clubs
CMCL Coastal Management Consultancy NIWA National Institute of Water & Atmospheric
Ltd Research
CSIRO Commonwealth Science & Industrial PBC Plimmerton Boating Club
Research Organisation
DML Discovery Marine Ltd PCC Porirua City Council
DoC Department of Conservation RNZN Royal New Zealand Navy
GWRC Greater Wellington Regional Council RRL Rafter Radiocarbon Laboratory
HMS Her Majesty’s Ship SMB Survey Motor Boat
IGNS Institute of Geological & Nuclear LINZ Land Information New Zealand
Sciences
MCC Mana Cruising Club
Sea & Tide Levels
PMT Postglacial Marine Transgression MWHS Mean High Water Springs
GMSL Global Mean Sea Level MHWN Mean High Water Neaps
MSL Mean Sea Level MLWN Mean Low Water Neaps
SLR Sea Level Rise MLWS Mean Low Water Springs
HAT Highest Astronomical Tide CD Chart Datum
LAT Lowest Astronomical Tide
Surveying
GIS Geographic Information System NZMG NZ Map Grid
DTM Digital Terrain Model RTK Real Time Kinematic
DGPS Differential Global Positioning SEB Sounding Error Budget
System
NZTM New Zealand Traverse Mercator WVD Wellington Vertical MSL Datum 1953
True (T) Compass Directions
N North @ 0000/3600 T S South @ 1800 T
NE Northeast @ 0450 T SW Southwest @ 2250 T
E East @ 0900 T W West @ 2700 T
SE Southeast @ 1350 T NW Northwest @ 3150 T
Note: The wind blows FROM these directions and tidal streams & ocean currents flow TO these directions.
COASTAL MANAGEMENT CONSULTANTS LTD IVPatterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
TABLE OF CONTENTS
1 INTRODUCTION .......................................................................................................................................................................................... 1
2 CONCEPTUAL FRAMEWORK ................................................................................................................................................................. 2
3 METHODS ........................................................................................................................................................................................................ 2
3.1 DESKTOP ANALYSIS ........................................................................................................................................................ 3
3.2 CONSULTATION.................................................................................................................................................................. 3
3.3 HYDROGRAPHIC SURVEYING..................................................................................................................................... 3
4 FACTS FOUND............................................................................................................................................................................................... 6
4.1 TECTONIC DEFORMATION........................................................................................................................................... 6
4.1.1 Active Faults ......................................................................................................................................................................... 6
4.1.2 Uplift Rates............................................................................................................................................................................ 8
4.1.3 Major Earthquakes............................................................................................................................................................ 9
4.1.4 Coseismic Versus Interseismic Deformation ...................................................................................................10
4.2 SEA-LEVEL TRENDS ........................................................................................................................................................10
4.3 TIDES......................................................................................................................................................................................12
4.3.1 Tidal Streams .....................................................................................................................................................................15
4.4 SEAFLOOR............................................................................................................................................................................16
4.4.1 Sediment Sources............................................................................................................................................................18
4.5 SEDIMENTATION RATES AND PATTERNS..........................................................................................................20
4.5.1 Previous Work ...................................................................................................................................................................20
4.5.2 Tidal Prism Trends ..........................................................................................................................................................22
4.5.3 Porirua Harbour Approaches.....................................................................................................................................23
4.5.4 Entrance Bar.......................................................................................................................................................................26
4.5.5 Throat.....................................................................................................................................................................................27
4.5.6 Onepoto Arm ......................................................................................................................................................................29
4.5.7 Pauatahanui Inlet............................................................................................................................................................31
5 FORECAST INFILLING ...........................................................................................................................................................................34
6 SUMMARY .....................................................................................................................................................................................................35
7 CONCLUSIONS ...........................................................................................................................................................................................36
8 RECOMMENDATIONS .............................................................................................................................................................................37
9 ACKNOWLEDGEMENTS..........................................................................................................................................................................37
10 REFERENCES ...............................................................................................................................................................................................38
APPENDICES
APPENDIX A: Tables of Data…………………………………………………………………………………………………………………...-1- to -5-
APPENDIX B: Historical Erosion and Deposition Rates 1849-2009………………………………………………………….-1- to -13-.
FIGURES
• Figure 1: Map showing the location and extent of Porirua Harbour including place names and Mana Island. 1
• Figure 2: Map showing the location of the Pukerua, Ohariu and Moonshine Faults that dissect the Porirua Harbour area after
Stevens (1974), Healy (1980), Begg & Mazengarb (1996), & Heron et al. (1998).....................7
• Figure 3: A global mean sea-level (GMSL) curve 1870-2007 clearly showing an accelerating rise in MSL from about 42mm
(1870-1930) to about 148mm (1930-2007) over the last 137 years. (Provided courtesy of Dr J.A. Church, CSIRO Marine &
Atmospheric Research, Hobart, Tasmania)..............................................................................................12
COASTAL MANAGEMENT CONSULTANTS LTD VPatterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
• Figure 4: Diagram illustrating tidal terms (Adopted from LINZ 2009). ........................................13
• Figure 5: Time curves for a flood tide wave moving from seaward into Porirua Harbour based on tidal measurements at the
4 tide gauge sites shown. The curves are relative to the site at Mana Cruising Club................15
• Figure 6: Map showing the bathymetry of Porirua Harbour derived from the 2009 survey by DML. 17
• Figure 7: Sketch map of Pauatahanui Inlet showing the location of profiles 1-9 across the intertidal flats monitored by
Pickrill (1979); two deep cores, (#1 & 2) by Mildenhall (1979) and 9 shallow cores (BRN, BAS4, etc) by Swales et al. (2005).
20
• Figure 8: Chart of the approaches to Porirua Harbour and entrance bar showing the location of the representative areas of
seafloor used to assess sedimentation rates (1967-2009) and locations of named transects with sites for comparison of spot
soundings (1849-2009)...................................................................................................................................25
• Figure 9: Chart showing the sedimentation pattern in the Throat area of Porirua Harbour from 1974-2009. 28
• Figure 10: Chart showing the sedimentation pattern within the Onepoto Arm from 1974-2009. 30
• Figure 11: Chart showing the sedimentation pattern in the Pauatahanui Inlet from 1974-2009 33
• Figure 12: The Approaches to Porirua Harbour from Karehana Bay. Photo by JG Gibb 13 December 2004. 37
TABLES
• Table 1: Example Sounding Error Budget for the inshore area of Porirua Harbour (Mana, Onepoto Arm, Pauatahanui Inlet)
prepared by DML (Mana Tide Gauge Reduced Data – for inshore areas)........................................4
• Table 2: Tectonic uplift or down drop rates for the Porirua Harbour area calculated from selected data from Table A-2,
Appendix A. Eustatic sea-level is for the New Zealand region after Gibb (1986) and is metres above the 1975-1985 average
sea-level. 8
• Table 3: Porirua Harbour tide levels derived from tide gauges during the 2009 Survey. All levels are in relation to CD where
the gauge zero was set at 2.55m below LINZ Mark C1K1 at MCC. Manual tide readings by DML during the course of the
survey confirmed that gauge readings were accurate to ±0.01m...................................................13
• Table 4: Sediment deposition rates in millimetres per year (mm/yr) over the last 9,267 Calendar years BP (1950) within
Pauatahanui Inlet based on radiocarbon dated marine silt layers (Cores 1 & 2) and shell in 4 cores. All levels given are
normalised to MSL Datum using the 2009 bathymetry. Rates were calculated by dividing the amount of sediment
accumulation by the time interval between Calibrated Ages. ............................................................20
• Table 5: Sedimentation rates in Pauatahanui Inlet, determined in millimetres per year (mm/year) by NIWA (Swales et al.
2005) from 0.4m-long cores at 9 sites (Figure 7) sampled from 27-29 April 2004 for 3 periods spanning human occupation of
the Inlet area over the last 150 years........................................................................................................22
• Table 6: Tidal prism calculations in cubic metres for both the Onepoto Arm and Pauatahanui Inlet with an uncertainty value
of ±3%. Tidal data are from the 2009 survey. Volumes of seawater were calculated between the surveyed seabed in 1974
and 2009 and the levels of MHWS and MLWS above CD. Tidal prisms were determined by subtracting MLWS volumes from
MHWS volumes.22
• Table 7: Net Rates of deposition (+) or erosion (-) of the seabed within the Porirua Harbour area. Data derived from Table
A-3, Appendix A, Columns B, D, G, H & I. Average uncertainty values of ±3% apply to the 1974 & 2009 DTMs and ±5-10%
to the 1967 & 1991 DTMs. .............................................................................................................................24
• Table 8: Indicative projection [Column ( E )] for the infilling of the arms of Porirua Harbour determined by dividing Column (
B ) by Column ( D ) and allowing an uncertainty value of approximately 15%. Columns ( A ) & ( B ) were determined from
the 2009 survey and Column ( C ) from Table 7, representing net deposition from 1974-2009. Column ( F ) allows for the
uncertainty value of approximately 15% for Column ( E ). ................................................................35
COASTAL MANAGEMENT CONSULTANTS LTD VIPATTERNS & RATES OF SEDIMENTATION WITHIN
PORIRUA HARBOUR
by
Jeremy G Gibb and Gregory J Cox
1. INTRODUCTION
In May 2009 Coastal Management Consultancy Ltd (CMCL) and Discovery Marine Ltd (DML) were
jointly commissioned by Porirua City Council (PCC) to analyse and report on historical seabed
changes in the Porirua Harbour area based largely on a comparative study of hydrographic surveys
made in 1849, 1950, 1965-67, 1974, 1991 and 2009. PCC requested that the results of the
comparative study be placed in the context of earlier studies and compared with earlier results. This
study builds on the work of MetOcean Solutions Ltd (MetOcean 2009) for PCC involving the digitizing
and georeferencing of the 1849-1991 historic charts. Their analysis was limited in that the 2009
hydrographic survey by SMB ‘Discovery’ was not available at the time. A full description of the 2009
survey is provided in a separate report by DML (2009). Note, that although there are historical
differences of opinion regarding place names in the Porirua Harbour area, we have adopted those
currently favoured by PCC (Keith Calder, pers. comm. July 2009). A list of acronyms used in this
study is provided with the Executive Summary. The study area is shown in Figure 1.
• Figure 1: Map showing the location and extent of Porirua Harbour including place names and Mana Island.
1Patterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
2. CONCEPTUAL FRAMEWORK
The main purpose of this study is to determine patterns and rates of sedimentation within Porirua
Harbour over the last 160 years (1849-2009). The method of comparing earlier soundings of the
seabed with a precise survey made in 2009 (DML 2009) is, however, not without inherent problems
(MetOcean 2009). The problems arise from the combination of deficiencies in the historic data and
physical processes which unless understood can give rise to unreliable and misleading patterns and
rates of sedimentation. To resolve potential problems, we have adopted the following conceptual
framework.
i. Over time, the seafloor of Porirua Harbour may remain either static, shallow from deposition of
sediment, or deepen from erosion of sediment. Relative to a common stable vertical datum,
change in elevation of the Harbour seafloor can be quantified by comparing soundings and
levels of the Porirua Harbour area that were surveyed at discrete time intervals (e.g.
1974-2009).
ii. Unreliable results can arise from the effects of historic trends in sea-level and/or tectonic
deformation of the land surface. For a trend of sea-level rise (SLR) deepening of the seabed
may be detected over time which is not the result of erosion. Conversely, a fall in sea-level
may result in a shoaling of the seabed over time which is not the result of deposition of
sediment.
iii. Tectonic uplift or down drop of the land surface may occur which can equally result in the same
problems as trends in sea-level. Such deformation may either be coseismic, aseismic or
interseismic. Coseismic movements of the order of decimeters or metres are instantaneous and
are directly associated with significant earthquakes and ruptures along active faults. In
response to earthquake shaking, such events may also cause a relative deepening of the
seabed in thick sequences of waterlogged unconsolidated sediments from compaction and
water loss.
iv. The southern North Island is located on the plate boundary between the Australian and Pacific
Plates. The interface between these two lithospheric plates dips W underneath the Wairarapa
and Wellington regions. Interseismic elastic deformation of the crust occurs due to strain
accumulation on this plate boundary in between large plate interface-rupturing earthquakes.
Interseismic uplift of the crust may result in an apparent shoaling that is not the result of
deposition of sediment and interseismic subsidence of the crust may result in an apparent
deepening which is not a consequence of erosion.
v. Finally, vertical and horizontal errors are inherent in the various survey methods adopted over
the last 160 years. With the passage of time and improvements in precision of survey
techniques, there is a progressive increase in the reliability of surveys from 1849 to 2009.
3. METHODS
Data were gathered for this project from a combination of desktop research, computer analysis of
hydrographic surveys, and hydrographic surveying in February to April 2009 (DML 2009) of the
Porirua Harbour area. For this project DML carried out the hydrographic survey and computer
analysis of historical charts and CMCL the desktop research, interpretation of results, and production
of the report.
COASTAL MANAGEMENT CONSULTANTS LTD 2Patterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
3.1 DESKTOP ANALYSIS
As a first step, 16 relevant published and unpublished reports were identified from a literature
review of Porirua Harbour and its catchment (Blaschke et al. 2009) and supplied to CMCL. As the
study progressed the list grew to more than 40 reports which are alphabetically listed in Section 9
(References) of this report. References cited in the text are by the author’s name and date of
publication.
3.2 CONSULTATION
During the course of research, specialist staff were consulted at LINZ and CSIRO on historic
sea-level change and on tectonics and sedimentation rates in geologic time at IGNS and the Rafter
Radiocarbon Laboratory. Specialists that contributed are acknowledged in Section 8
(Acknowledgements) of this report. Where appropriate, tables of data (see Appendix A) and
sections of the report were reviewed by the specialists and the draft report by the Porirua Harbour
Science Group. This report is the final version of those reviews.
3.3 HYDROGRAPHIC SURVEYING
i. A tide gauge network was established before the start of the survey. This network comprised
four automatic tide gauges which were installed by Greater Wellington Regional Council
(GWRC) in consultation with PCC and DML. Three gauges were of a temporary nature, whilst
the primary gauge located within Mana Marina is a permanent device with data being logged
by GWRC via a telemetry link. This gauge was levelled to the nearby LINZ survey Mark C1K1,
being a known height above Chart Datum (CD). The height value of this mark was derived
from historic RNZN surveys.
ii. Data from all four gauge sites were provided by GWRC and was analysed using Sea Level and
Information System (SLIM’s) software which is a tidal software package endorsed and used by
LINZ. From the analysis, a series of tide levels at each site was derived which has enabled a
co-tidal model to be developed for Porirua Harbour. However, for the purposes of comparing
the latest survey results with historic data, only tide readings from the Mana Marina tide gauge
have been used for the reduction of raw depths for tide. Whilst this ‘single point’ tide reduction
method has created vertical errors in the 2009 data, particularly at distance from Mana due to
tidal constriction, our research indicates that all historic surveys have been reduced using a
single location tide station centrally located at Mana. Thus, for comparisons to be as accurate
as possible, the same methodology has been used for data reduction.
iii. The final accuracy of soundings for any survey can only be determined with some degree of
certainty by inspection of cross-lines or overlapping depths within the same survey dataset. A
lack of dense overlapping data makes accuracy assessments very difficult. Unfortunately, this
is the case with all the Porirua Harbour historic data sets due to the scale of sounding sheets
and lack of availability of raw data. However, an element of confidence can be derived by
comparing two separate surveys over flat seabed areas. Consistent agreement (or consistent
discrepancy) provides an element of assurance that surveys have been internally well
controlled and may therefore provide worthwhile information.
iv. Repeatability is the key factor and unfortunately, the lack of regular surveys undertaken to
similar standards and density has made the task of comparing historic datasets very difficult.
Taking account of typical survey methods used at the time of the early survey (e.g. 1849),
extensive research and recent knowledge of the local tidal regime, the estimated errors for
each survey have been listed in Table A-1, Appendix A.
COASTAL MANAGEMENT CONSULTANTS LTD 3Patterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
v. Much effort has gone into ascertaining and confirming the vertical origin for each survey.
However, whilst vertical datum is of utmost importance, it should also be remembered that
there are many other sources of error that must be considered. For the 2009 survey, the
estimated accuracy of soundings was calculated via a Sounding Error Budget (SEB), taking into
consideration all sources of error. Table 1 provides an example of the SEB for the approaches
to Porirua and the inner harbour areas produced at the 95% (2-sigma) confidence level.
• Table 1: Example Sounding Error Budget for the inshore area of Porirua Harbour (Mana, Onepoto Arm, Pauatahanui
Inlet) prepared by DML (Mana Tide Gauge Reduced Data – for inshore areas).
Source of Error Depth Depth Note Depth Depth Depth
Independent Dependent 2m 5m 10m
Error Error
Vessel Draught Setting 0.01 A 0.01 0.01 0.01
Variation of Vessel Draught 0.00 B 0.00 0.00 0.00
Vessel Settlement & Squat 0.03 C 0.03 0.03 0.03
Echo Sounder Instrument Accuracy 0.01 ±0.20% d D 0.01 0.02 0.03
Roll Error 0.000 d E 0.00 0.00 0.00
Heave Error 0.01 F 0.01 0.01 0.01
Sound Velocity Measurement 0.0007 d G 0.00 0.00 0.01
SV Spatial Variation 0.0006 d H 0.00 0.00 0.01
SV Temporal Variation 0.0025 d I 0.01 0.01 0.02
Tide Readings 0.01 J 0.01 0.01 0.01
Application of Tides (no co-tidal) 0.08 K 0.08 0.08 0.08
Combined Total √a 2 + b2 + c2 …. 0.088 0.090 0.095
Notes:
A Set by daily bar check
B Minimal – due to nil significant changes in fuel state during period of each survey
C Minor squat in shallow water – minimised by operating at slow speeds
D Manufacturer rated accuracy
E No vessel roll encountered
F Nil significant heave effects experienced inside the Mana ‘throat’ region
G SV determined by daily bar check and verified with SVP
H Sounding kept to small distinct survey areas each day. Negligible fresh water effects
I Surveys undertaken during high water periods – minimal time delays from SV observations
J Accuracy of tide gauge readings as proven via pole/gauge comparison
K Worst case accuracy of co-tidal model for maximum distance from tidal site
vi. The current chart of Porirua Harbour (NZ4623) is derived from a number of historic RNZN
surveys. However, the latest edition of this chart, published in 2000 contains depth data
derived from an RNZN survey of 1967 and PCC surveys of 1991. The specific coverage areas
are indicated on the source data diagram. The RNZN surveys were undertaken to CD at Mana
(details of which have been confirmed) and the 1991 survey was undertaken by a land survey
company for PCC with Mean Sea Level (MSL) as the reference datum. Initially, it was believed
that a MSL-CD adjustment of 0.80m was undertaken to incorporate this data into the chart.
However, further advice from LINZ has confirmed that an adjustment of 1.0m was used, being
COASTAL MANAGEMENT CONSULTANTS LTD 4Patterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
the 0.8m MSL/CD offset plus a 0.2m ‘safety margin’. It should be noted however, that this shift
is of no consequence with respect to survey comparisons, as it is a nautical charting issue only.
vii. We know that MSL is affected by topography, particularly in confined waters and bays due to
constriction. Whilst Wellington Vertical Datum 1953 (WVD) is a fixed geodetic datum, a review
of tidal data gathered at all sites confirms that MSL varies throughout the project area due to
tidal restriction - as expected. Furthermore, the Porirua Harbour tidal regime is rather complex
and will of course be a major contributor to sounding error for all surveys undertaken in the
past. The magnitude of error will largely depend on the state of tide at which the data were
gathered. DML’s digital depth analysis has shown that depth error attributed to tides can be in
the order of 0.26m or more within the upper reaches of Pauatahanui and Onepoto Arms.
viii. As well as the vessel positioning accuracy at the time of the survey, the conversion and/or
transformation of older surveys to modern datum and grid can also incorporate errors - such as
distortion in old sheets, errors in digitizing and also software errors due to outdated
transformation parameters between relative datum’s. However, the latter is a minor concern
since most GIS and survey packages use transformation packages that have been rigorously
tested.
ix. Whilst positional errors of less than 10m for example may not create issues when comparing
depths over flat seabed areas, problems do arise when comparing data sets over rugged
terrain or steeply sided channels or near-shore slopes. For example, an error in position of
2.0m or more in a channel environment can manifest itself as a vertical depth error of metres.
Hence, positional errors must be considered when inspecting datasets over shallow inshore
areas.
x. A key factor in these comparisons has been to ensure that historic data (acquired on various
reference surfaces and origins) has been adjusted correctly. The recent study carried out by
MetOcean Solutions Ltd (MetOcean 2009) for PCC involved the ‘normalising’ of all historic data
to a common datum. MetOcean digitised soundings from historic sounding fairsheets and then
converted data to the New Zealand Map Grid (NZMG) with depths vertically adjusted to CD
(Mana). Excel spreadsheets containing xyz data for each historical survey were received by
DML via PCC.
xi. Adjustment computations (vertical shifts) within the spreadsheets were checked for
correctness. This was achieved by DML reviewing hard copies of historic sounding plans and
reports held within the LINZ data repository at Upper Hutt. Discussions were also held with
LINZ staff as well as surveyors involved in previous surveys. The MetOcean report (MetOcean
2009) was also reviewed.
xii. Since the 2009 hydrographic and topographic surveys by DML have been undertaken in terms
of New Zealand Transverse Mercator (NZTM) projection, the historic data (digitized and
converted to NZMG by MetOcean) have been further converted by DML to NZTM via a standard
7- parameter datum transformation.
xiii. As far as we could ascertain, the adjustments to historic data undertaken by MetOcean (2009)
appear to be correct. The only issue that came to light pertained to the 1974 and 1991 surveys
where the sounding plans refer to depths being reduced to MSL - using survey mark ‘BM14’ as
origin, being 4.837m above MSL. In fact, this BM (correctly known as L14) was upgraded in a
geodetic levelling network in 1958 and is a first order vertical survey mark in terms of WVD.
Therefore, the 1974 and 1991 surveys have in fact been referenced to WVD and not MSL.
From our own geodetic observations undertaken at the beginning of the survey, we found that
WVD is approximately 0.05m above MSL at Mana (0.85m above CD). However, a block
COASTAL MANAGEMENT CONSULTANTS LTD 5Patterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
adjustment of 5cm cannot be made to the 1974 and 1991 data as the WVD-MSL relationship is
not fixed over the entire project area. The datum offset was therefore taken into account when
deriving volume calculations.
xiv. All historic data was imported into Terramodel software as individual layers and inspected
manually. Various combinations of layers were interrogated for data overlaps to determine
depth differences. Due to a large range in depth differences and sporadic nature of the seabed
coverage between surveys, tangible results could not be gleaned from any data sets older than
1974, although the 1950 survey which includes a portion of Pauatahanui inlet does provide
some worthwhile data with respect to ascertaining general trends.
xv. For the arms of Porirua Harbour, DTM grids could only be derived from the 1974, 1991 and
2009 surveys. A series of 1:2000 A1 size plans were generated from the 1974-2009 and 1991-
2009 survey comparisons to illustrate the magnitude of depth differences. Inspection of these
plans clearly shows better agreement across flat seabed areas, but large discrepancies within
the channels. This is mainly due to positioning errors and sparse sounding density in the
historic data, such that sporadic lines of sounding have not adequately delineated the true
shape and depth of some of the key channels.
xvi. Spot depth comparisons between combinations of older surveys and the 2009 data indicate
large depth differences in overlapping data. This is due to varying depth and position errors
from each survey and accurate assessments as to seabed trends have not been possible for all
harbour areas. The fact that past surveys have not been undertaken at regular intervals and
have been conducted using different technologies means that an element of caution must be
exercised when delivering findings on sediment trends. Establishing rates of sedimentation
based on dubious survey data and where other supporting physical or actual evidence is not
available could result in dubious results.
4 FACTS FOUND
The following are the facts that we found from the combination of previous research and survey and
the hydrographic survey in 2009.
4.1 TECTONIC DEFORMATION
1. The proximate cause of tectonic deformation of the Wellington and Wairarapa regions is the
convergence of the Australian and Pacific lithospheric plates at about 40mm/year where the
former to the W is being obliquely underthrust from the E by the Pacific Plate, the interface
reaching about 30km beneath Porirua Harbour (Begg & Johnston 2000; Heron et al. 1998)
4.1.1 Active Faults
2. Within the region, most of the strike-slip component of plate motion is taken up by faults of
the North Island Dextral Fault Belt. The Porirua Harbour area is bounded by 3 active fault
lines, ruptures along which largely drive tectonic deformation of the area. All three faults are
dextral strike-slip faults with the upthrown side to the W and the downthrown side to the E.
3. The active faults are the Pukerua Fault which strikes 0350True and intersects Hongoeka Bay
passing up through the Pukerua Corridor; the Ohariu Fault which strikes 0200True through
Porirua Harbour, the entrance to Pauatahanui Inlet, passing up through the Kakaho Valley,
and the Moonshine Fault which strikes 0550True at Judgeford passing along the Moonshine
Road (Healy 1980). Figure 2 shows the location of the 3 faults.
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• Figure 2: Map showing the location of the Pukerua, Ohariu and Moonshine Faults that dissect the
Porirua Harbour area after Stevens (1974), Healy (1980), Begg & Mazengarb (1996), & Heron et al.
(1998).
4. The last movement on the Pukerua Fault occurred more than 1,200 years ago. For a
Magnitude 7.6±0.3 earthquake triggered by a single-event fault displacement of 2.3-4.0m a
recurrence interval of 2,500-5,000 years has been estimated (Begg & Mazengarb 1996).
5. Relative to the Ohariu and Pukerua Faults the Moonshine Fault may not be as active as
most of the fault features are rounded and eroded. There is some evidence for displacement
during the Last Glacial period about 20,000 years ago (Begg & Mazengarb 1996).
6. The Ohariu Fault is one of the major active dextral strike-slip faults in the Wellington
Region, the last movement occurring 1,070-1,130 years ago during which the average
horizontal surface displacement was estimated to be 3.7m and the estimated earthquake
Magnitude M 7.1-7.5. A recurrence interval of 1,530-4,830 years was determined for similar
magnitude events along this fault (Heron et al. 1998).
7. Taupo Swamp just N of Plimmerton and about 2km NW of the Ohariu Fault has been
tectonically uplifted by a series of surface rupture earthquakes associated with movements
on the Fault about 700-1,300, 2,000-2,600 and 2,800-3,900 years ago (Cochran et al.
2007).
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8. Along the Ngatitoa Domain foreshore there is a stranded gravel beach about 1.0m above
the present-day forming feature. The difference in crest heights is consistent with the
tectonic uplift of the Taupo Swamp-Plimmerton Beach area W of the Ohariu Fault (Gibb
1993). At Camborne, there is a sequence of 6 undated beach ridges which increase
progressively in height inland to about 2.7m above the present-day ridge suggesting uplift
(McFadgen 2007).
9. Further W at Karehana Bay, there was a transition from an estuarine environment to a
peaty freshwater swamp about 3,356-2,947 calendar years ago (Table A-2, Appendix A).
The age of transition coincides with the earliest rupture recorded in the Taupo Swamp that
was associated with movements on the Ohariu Fault and may have resulted in a small
amount of uplift in this area. Equally, eustatic sea-level was slightly higher at that time and
has fallen about 0.2m since (Gibb 1986).
4.1.2 Uplift Rates
10. Estimated uplift rates are provided for the Porirua Harbour area in Table 2. Rates are
determined by comparing the formation height of a radiocarbon dated paleosea-level marker
with an estimated sea-level that existed when the marker was laid down. For Table 2, eleven
of the most reliable dated markers are used which were carefully selected from the 26 dated
markers listed in Table A-2, Appendix A.
11. For the upthrown side to the W of Ohariu Fault, including the Plimmerton-Mana coast, a net
average tectonic uplift rate of about 0.5m/1,000 years is determined here tapering to about
0.2m/1,000 years at Karehana Bay (Table 2). Evidence of Holocene uplift along the coastline
at Whitireia Park and the W shores of the Onepoto Arm (Adkin 1921; Eiby 1990; Walton
2002; McFadgen 2007) suggests a similar uplift rate.
12. More than 80% of Pauatahanui Inlet is located on the downthrown side (E) of Ohariu Fault.
For the Inlet, Gibb (1986) calculated an uplift rate of 0.3±0.04m/1,000 years from 8
radiocarbon dated paleosea-level markers spanning a period from about 9,300 to 3,000
years ago. One of these dates was from the Taupo Swamp, another from Motukaraka Point
and the rest from 2 cores in the central mud basin of the Inlet (Table A-2, Appendix A).
13. For this area, new data gathered since Gibb (1986) from shoreline sites around the Inlet
including Pauatahanui Stream valley, Ration Point, Motukaraka Point and the Kakaho Stream
valley generally indicate very low rates of uplift (Table 2). As the rates are all within the
uncertainty limits of both the formation heights and eustatic sea-levels (Table 2) we interpret
the data to indicate relative tectonic stability to very low uplift of the Porirua Harbour area E
of the Ohariu Fault over the last 7,500 years.
• Table 2: Tectonic uplift or down drop rates for the Porirua Harbour area calculated from selected data from Table A-2,
Appendix A. Eustatic sea-level is for the New Zealand region after Gibb (1986) and is metres above the 1975-1985
average sea-level.
Mid Range Uplift (+) or
14C Dated Depositional Formation Calibrated Eustatic Down drop (-)
Number Location Sample Environment Height Age Sea Level Rate Tectonics
(cal. years (m/1,000
(m) BP) (m) years)
NZ 7379 Karehana Bay Shell Lower tidal Flat 0.96 3152±205 0.2±0.5 0.24 Uplift
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NZ 4866 Taupo Swamp S Shell Tidal Flat 1.9±0.8 4224±178 0.3±1.0 0.45 Uplift
Taupo Swamp N (TS 97-
WK 8095 1) Organic Mud Upper tidal Flat 1.75 2540±210 -0.2±0.5 0.77 Uplift
Taupo Swamp N (TS 98- Organic
WK 8353 2) Sand Upper tidal Flat 1.90 3150±700 0.2±0.5 0.54 Uplift
NZ 7387 Kakaho Stream W Shell Tidal Flat 0.0 to 0.3 5457±183 -0.3±1.0 0.05 Stable
NZ 7393 Kakaho Stream W Shell Tidal Flat 0.6 3122±209 0.1±0.5 0.16 Stable
NZ 7421 Kakaho Stream W Shell Tidal Flat 0.6 3410±215 0.5±1.0 0.03 Stable
NZ 3118 Motukaraka Point W Shell Upper tidal Flat 1.24±1.0 7113±118 0.0±1.0 0.17 Stable
NZA 29687 Ration Point (Core RPA) Shell Tidal Flat -1.4 7094±126 -0.1±1.0 -0.18 Stable
NZ 7381 Pauatahanui Stream Shell Tidal Flat -0.15 7498±192 -0.5±1.0 0.05 Stable
NZ 7383 Pauatahanui Stream Shell Tidal Flat -0.66 7588±196 -1.0±1.0 0.04 Stable
14. We consider the 11 dated marine silt layers in the 2 cores from the central mud basin to be
unreliable on the grounds that they are not paleosea-level markers and if they were, they
would indicate a confusion of tectonic uplift rates up to 0.56m/1,000 years and down drop
rates up to -2.08m/1,000 years within the same cores (Table A-2, Appendix A).
15. Having considered the available evidence at this point in time, we are of the opinion that
the Porirua Harbour area W of the Ohariu Fault is undergoing coseismic tectonic uplift at
about 0.5m/1,000 years tapering to about 0.2m/1,000 years at Karehana Bay. In contrast,
the arms of the Harbour E of the Fault appear to be either tectonically stable or subject to
very low tectonic uplift.
4.1.3 Major Earthquakes
16. Major ruptures on the largest active faults dissecting the Wellington region give rise to
equally major earthquakes. Such events are accompanied by coseismic uplift or down drop of
the foreshore and seabed. Furthermore, the possibility exists that between such major
events interseismic deformation of the land surface may occur.
17. Since 1840, four moderate to large earthquakes have occurred on 16 October 1848, 23
January 1855, 24 June 1942 and 2 August 1942 (Begg & Mazengarb 1996). In 1848, rupture
along the Awatere Fault in Marlborough produced a magnitude M 7.4-7.5 earthquake. In
1855, rupture along the Wairarapa Fault produced the well documented magnitude M
8.0-8.2 Wairarapa Earthquake (Grapes & Downes 1997; Begg & Johnston 2000).
18. The 1855 Wairarapa Earthquake caused the Wellington region to tilt W, with coseismic
uplift of the order of 6m near Cape Turakirae and up to 13.5m horizontal movement along
the W Wairarapa Fault (Begg & Johnston 2000). Uplift associated with W tilting generally
tapered from several metres along the fault, to 2.1m along the eastern shores of Wellington
Harbour, to 1.5m in the Wellington City area, tapering to zero at Cape Terawhiti (Stevens
1974).
19. In the Porirua Harbour area, perception of coseismic uplift during the 1848 and 1855
events is controversial. During the 1848 event “the ground shook for 3 days at Paremata”.
During the 1855 event, “parts of the Porirua Harbour were as dry beds”. The seabed in the
Onepoto Arm “was lifted to such an extent that the tidal flow at the harbour entrance was
reduced and the original shoreline at Parramatta Point was gradually lost” (Kay 1996).
20. Adkin (1921) interpreted uniform uplift of 0.9m of the Porirua Harbour coast from the 1855
event, but Eiby (1990) refuted this claim suggesting zero uplift of the coast. Others reported
COASTAL MANAGEMENT CONSULTANTS LTD 9Patterns & Rates of Sedimentation within Porirua Harbour
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differential subsidence of the foreshore and seabed in response to compaction of soft
sediment whilst still others reported uplift of 0.3-0.9m of the tidal flats in the upper reaches
of Pauatahanui Inlet (Grapes & Downes 1997).
21. According to a report by a local registered surveyor, Mr K.E. Wynne, there is no conclusive
survey evidence of “raised beaches or rock areas” around Porirua Harbour that can be
attributed to the 1855 event (Wynne 1981). Healy (1980) in the comprehensive
multidisciplinary ‘PEP’ scientific study of Pauatahanui Inlet surmised that the “Pauatahanui
region was neither uplifted or downwarped” during the event.
4.1.4 Coseismic Versus Interseismic Deformation
22. Whilst there is good evidence for coseismic uplift of the Porirua Harbour area W of the
Ohariu Fault, recent research by IGNS indicates no evidence of coseismic uplift or down drop
E of the Fault (Cochran et al. 2007; Wilson et al. in prep 2009). Furthermore, the available
geologic evidence suggests that coseismic uplift and W tilting of the Wellington region during
the 1855 event did not extend to Porirua Harbour.
23. In addition, there is no known reliable evidence of either uplift or down drop since the
1855 event. However, interseismic recovery of any 1855 uplift or subsidence is not expected
because it was an upper plate fault earthquake, rather than a plate interface event (Wilson &
Berryman, pers. comm. IGNS, June 2009).
24. Having considered the above evidence, we adopt zero vertical tectonic deformation of the
entire Porirua Harbour foreshore and seabed over the 160-year period of hydrographic
survey (1849-2009) utilized in this study.
4.2 SEA-LEVEL TRENDS
25. Sea-level rise (SLR) is caused by the combination of both thermal expansion of ocean
waters as they warm plus an increase in ocean mass from meltwater from land-based
sources of ice such as valley glaciers and ice caps, and the Greenland and Antarctic ice
sheets. Global warming is the proximate cause of both factors (Church et al. 2008). Global
cooling results in the reverse.
26. At the peak of the Last Glaciation about 20,000 years ago, eustatic (global) sea-level
around New Zealand stood at about 130-135m below present-day sea-level (Gibb 1980). In
the Porirua Harbour area, the shoreline at that time lay about 2km W of Mana Island.
27. With the onset of global warming of some 4-50C the Last Glaciation ice sheets disintegrated
and eustatic sea-level rose on average at about 10mm/year (1.0m per century) (Gibb 1986;
Church et al. 2008 ) with peak rates of about 50mm/year (5.0m per century) (Rohling et al.
2007).
28. The global rise is widely known as the Postglacial Marine Transgression (PMT) and was
punctuated by a number of stillstands. During the latter part of the PMT, two stillstands
occurred about 10,500-9,500 and 8500-8000 years ago at about -24.0±2.9m and
9.0±2.8m, below present-day sea-level, respectively. Both stillstands were followed by rapid
marine transgressions of about 1.5m per century (15mm/year) (Gibb 1986).
29. In New Zealand the PMT culminated at the present sea-level about 7,300±100 calendar
years ago (cal. Years B.P.). During the last 7,300 years, eustatic fluctuations on the order of
a few decimeters have occurred with a regression minimum of about -0.4±1.0m at about
COASTAL MANAGEMENT CONSULTANTS LTD 10Patterns & Rates of Sedimentation within Porirua Harbour
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5,300 years ago and a transgression maximum of about 0.5±1.0m at about 3600 years ago
(Gibb 1986). There has been little net change in eustatic sea-level from 2,000 years ago until
the start of the 19th century (Gibb 1986: Church et al. 2008). The period of relative sea-level
stability is known as the Present Interglacial.
30. The Present Interglacial contrasts with the Last Interglacial when global sea-level stood
about 3.0±0.3m higher than modern sea-level about 124 to 119,000 years ago around New
Zealand (Gibb 1986). During the Last Interglacial global mean surface temperatures were at
least 20C warmer than present and relatively ‘short-term’ rates of SLR averaged 1.6±1.0m
per century (16mm/year). A 1.6m per century SLR would correspond to the disappearance
of an ice sheet the size of Greenland (Rohling et al. 2007).
31. Figure 3 shows that from 1870 to about 1930 global mean sea-level (GMSL) rose at about
0.70mm/year, accelerating to about 1.95mm/year from 1930 to 2007, averaging
1.4mm/year over the 137-year period (Church et al. 2008). These scientists noted that there
were significant regional variations in the rate of SLR and that the rate of rise is not uniform
around the globe.
32. The most recent analysis of tidal records for New Zealand (Hannah 2004) revealed that
regional sea-level rose on average at 1.61±0.24mm/year last century, with a rise at
1.78mm/year being recorded at Wellington from 1891-2001, the closest port to Porirua
Harbour with the longest tidal records. Hannah disclosed a linear trend finding no evidence of
an acceleration in the rate of SLR last century.
33. For New Zealand an analysis of combined tidal data from Auckland, Wellington and
Lyttelton, showed no significant SLR trend until 1931, with an increase to 1.9±0.1mm/year
after then (Gibb 1991), showing excellent agreement with the global trend established by
Church et al. (2009). Using linear regression, Gibb established a net rate of 1.6±0.1mm/year
showing excellent agreement with Hannah (2004).
34. The slightly higher rate of SLR for Wellington above the New Zealand average is thought to
be the result of interseismic subduction for which there is no evidence at Porirua (Wilson &
Berryman, IGNS, pers. comm., June 2009). As there is no long-term tidal record for Porirua
Harbour to derive a trend, we must infer a rate from elsewhere for this area.
COASTAL MANAGEMENT CONSULTANTS LTD 11Patterns & Rates of Sedimentation within Porirua Harbour
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• Figure 3: A global mean sea-level (GMSL) curve 1870-2007 clearly showing an accelerating rise in MSL from about
42mm (1870-1930) to about 148mm (1930-2007) over the last 137 years. (Provided courtesy of Dr J.A. Church, CSIRO
Marine & Atmospheric Research, Hobart, Tasmania).
35. Having considered the available evidence, we adopt rates of SLR of 0.7mm/year
(1849-1931) and 1.95mm/year (1931-2009) from Figure 3 for the Porirua Harbour area for
the study of historic sedimentation from 1849 to 2009.
4.3 TIDES
36. The periodic rise and fall of sea-level, known as the tide, is caused by the gravitational
interactions of the Moon and Sun on the oceans of Planet Earth. While gravity provides the
driving force, the rotation of the Earth, the size and shape of the ocean basins and local
coastal circumstances ultimately determine the magnitude and frequency of the tide at a
particular place (LINZ 2009).
37. Around the 18,000km-long New Zealand coastline, the tidal regime is semi-diurnal. This
means that on most days 2 high and 2 low tides will occur at any given location including the
Porirua Harbour area (LINZ 2009).
38. Standard tidal terms used in this study are defined in the New Zealand Nautical Almanac
(LINZ 2009) and shown on Figure 4. Highest and lowest astronomical tide (HAT & LAT) are
COASTAL MANAGEMENT CONSULTANTS LTD 12Patterns & Rates of Sedimentation within Porirua Harbour
Consultancy Report (CR 2009/1) prepared for Porirua City Council
the highest and lowest tidal levels which can be predicted to occur under average
meteorological conditions over 18 years (LINZ 2009). Modern CDs are set at the
approximate level of LAT (Figure 4).
• Figure 4: Diagram illustrating tidal terms (Adopted from LINZ 2009).
39. Table 3 shows that high tide arrives first at the jetty at Plimmerton Boating Club (PBC) and
20 minutes later at Mana Cruising Club (MCC). About 45-50 minutes later high tide reaches
the inland extent of both Pauatahanui Inlet and the Onepoto Arm (Figure 5; Table 3).
• Table 3: Porirua Harbour tide levels derived from tide gauges during the 2009 Survey. All levels are in relation to CD
where the gauge zero was set at 2.55m below LINZ Mark C1K1 at MCC. Manual tide readings by DML during the course
of the survey confirmed that gauge readings were accurate to ±0.01m.
GAUGE SITE Mean Time Differences Mean Spring, Neap and Sea Level Heights (metres)
HW LW MHWS MHWN MLWN MLWS MSL
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MCC 0000 0000 1.769 1.170 1.033 0.434 1.101
PBC -0020 -0017 1.693 1.142 0.918 0.366 1.030
Onepoto Arm +0020 +0034 1.722 1.183 0.968 0.429 1.075
Pauatahanui Inlet +0022 +0032 1.728 1.190 0.948 0.410 1.069
Highest Astronomical Tide (HAT) Lowest Astronomical Tide (LAT)
MCC 1.848 0.248
PBC 1.863 0.273
Onepoto Arm 1.954 0.345
Pauatahanui Inlet 1.954 0.330
40. Relative to Mana Marina there is negligible difference (0.97-0.99) in tidal range ratios at all
4 tide stations (Figure 5) during typical spring tidal periods and average meteorological
conditions. Across all 4 sites the Spring Tide range is 1.293-1.335m and the Neap Tide
Range is 0.137-0.224m (Table 3).
41. Highest astronomical tide (HAT) ranges from 0.079m above MHWS at Mana Marina up to
0.232m at the head of Onepoto Arm. In contrast, lowest astronomical tide (LAT) ranges from
0.08m below MLWS at the head of Pauatahanui Inlet up to 0.186m at Mana Marina (Table
3).
42. During severe storms from the W-NW quadrant the combination of wind setup and the
inverted barometer effect associated with such storms can create a pronounced increase in
sea-level known as a storm tide. Such phenomena are known to flood low-lying areas such
as Grays Road from time to time for several hours at high water.
43. During a severe storm on 11-13 September 1976 that produced 11-13m swells and
sustained NW winds of 50 knots in the western Approaches to Cook Strait, a storm tide of
0.72m above normal High Water was observed in Pauatahanui Inlet (Gibb 1978). These
conditions were generated by a Mid-Latitude Depression with a central pressure of 970hpa
and recurrence interval of some 30-50 years.
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