Porirua Harbour Patterns and Rates of Sedimentation Report

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 [email protected]

COASTAL MANAGEMENT CONSULTANTS LTD II

PATTERNS 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)

Patterns & Rates of Sedimentation within Porirua Harbour Consultancy Report (CR 2009/1) prepared for Porirua City Council

COASTAL MANAGEMENT CONSULTANTS LTD III

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 IV

ACRONYMS USED IN THIS REPORT

Local & Central Government Agencies, Companies and Boating Clubs

CMCL Coastal Management Consultancy

Ltd

NIWA National Institute of Water & Atmospheric

Research

CSIRO Commonwealth Science & Industrial

Research Organisation

PBC Plimmerton Boating Club

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

Sciences

LINZ Land Information New Zealand

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

System

SEB Sounding Error Budget

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 V

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 VI

• 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

1

PATTERNS & 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.


COASTAL MANAGEMENT CONSULTANTS LTD 2

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.



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.



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 Independent

Error

Depth Dependent

Error

Note Depth 2m

Depth 5m

Depth 10m

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



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



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.



• 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).



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.

14C Number Location

Dated Sample

Depositional Environment

Formation Height

Mid Range Calibrated

Age Eustatic

Sea Level

Uplift (+) or Down drop (-)

Rate Tectonics

(m) (cal. years

BP) (m) (m/1,000

years)

NZ 7379 Karehana Bay Shell Lower tidal Flat 0.96 3152±205 0.2±0.5 0.24 Uplift



NZ 4866 Taupo Swamp S Shell Tidal Flat 1.9±0.8 4224±178 0.3±1.0 0.45 Uplift

WK 8095 Taupo Swamp N (TS 97-

1) Organic Mud Upper tidal Flat 1.75 2540±210 -0.2±0.5 0.77 Uplift

WK 8353 Taupo Swamp N (TS 98-

2) Organic

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



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



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.



• 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



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



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.



• 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.

4.3.1 Tidal Streams

44. The configuration of the coastline and the seafloor topography directly affects the pattern

and rate of flow of tidal streams (currents) in and out of Porirua Harbour. The strongest flows

are experienced along the centerline of the Harbour throat where velocities up to

1.76m/second (3.42 knots) have been measured during Spring tides and 0.74m/second

(1.43 knots) during Neaps (Wynne 1981).



45. At the N end of the throat just inside the entrance bar, maximum ebb flow velocities of

0.61-0.7m/second (1.19-1.36 knots) were measured (Wynne 1981). The flow velocities in

the Harbour throat are more than capable of transporting sand and mud both into and out of

Porirua Harbour.

46. During flooding tides, dye tests in March & April 1980, revealed that about 60% of the tide

entered Pauatahanui Inlet and 40% into the Onepoto Arm. During ebbing tides, the relatively

stronger flow from Pauatahanui Inlet deflected the flow from the Onepoto Arm against the

western shore of the entrance channel (Wynne 1981).

47. Our measurements in 2009 revealed a total area of the arms of Porirua Harbour below

MHWS of 714.04ha, comprised of about 469.97ha (Pauatahanui Inlet) and 244.07ha

(Onepoto Arm). As the rise and fall of the tide affects both arms, it follows that about 66% of

the tidal streams entering and leaving the Harbour are controlled by Pauatahanui Inlet and

34% by the Onepoto Arm which is in close agreement with the dye tests in 1980.

48. Little is known about tidal streams within the Onepoto Arm. Observations during the 2009

survey suggest that there are insignificant topographic deflections during both ebb and flood

tides and that flows across the flood-tide delta are fairly uniform (DML 2009). It is highly

likely that there is either a clockwise or anticlockwise low velocity circulation within the

central basin of this arm.

49. In contrast, a mobile bed hydraulic model of Pauatahanui Inlet constructed in 1977

revealed significant topographic deflections during both ebb and flood tides. Although most of

the ebb-tide flow was concentrated through the main channel, significant streams occurred

across the flood-tide delta with higher velocities through the 2 shallow channels bisecting the

delta (Bewick 1977).

50. Although high flood tide velocities were mostly concentrated in the main channel, the

Harbour topography contributed to a clockwise circulation in Browns Bay producing a back

eddy along Moorehouse Point. In the muddy basins to the N and E of the flood-tide delta and

spit, there were anticlockwise low velocity circulations (Bewick 1977).

51. For the approaches to Porirua Harbour, the RNZN measured the direction and velocity of

tidal streams in about 16m CD depth. Flood tides were observed to generally flow NNE-NW

at about 0.26-0.57m/second (0.5-1.1 knots) during Springs and 0.15-0.31m/second

(0.3-0.6 knots) during Neaps. Ebb tides generally flowed SW at similar velocities (RNZN

Chart NZ4632).

52. The relatively higher tidal velocity areas of Porirua Harbour, appear to be associated with

coarser ‘bedload’ sediments (sand, gravel, shell banks, etc.). In contrast, the relatively lower

tidal velocity areas such as the central basins of both arms of the Harbour appear to be

associated with finer ‘suspended load’ sediments such as silt and clay (mud).

4.4 SEAFLOOR

53. Figure 6 highlights the important bathymetric features of the seafloor of Porirua Harbour.

Sampling and particle size analyses have been made in the past of seafloor sediments by a

number of workers including McDougall (1976); Wynne (1981); Lewis (1988), & Swales et

al. (2005).



• Figure 6: Map showing the bathymetry of Porirua Harbour derived from the 2009 survey by DML.

54. Approaching the Harbour from seaward, the seafloor shallows uniformly from about

15-18m CD between Te Rewarewa Point to the N and Te Paokapo Point to the S to about

10m CD by Tokaapapa Reef (Figure 6). RNZN Chart NZ4632 shows that the seafloor in this

area is composed of Fine Sand & Broken Shell (F.S.bk.Sh).

55. Tokaapapa Reef (Grandfather Rocks) have a drying height of 0.3-0.9m above MHWS

(NZ4632) acting as a natural offshore breakwater reducing wave energy in its lee resulting in

the formation of a shore-connected sub-tidal tombolo to the E (Figure 6). A line intersecting

the apex of the tombolo with the reef indicates that the dominant wave approach is from

about 2800True (WNW).



56. The sand tombolo anchors the N end of the entrance bar, an ebb-tide delta defined by the

2m CD depth contour (Figure 6) to seaward. Ebb-tide discharges via the Harbour throat

together with the sheltering effect of the Whitireia coast anchor the ebb-tide delta at its S

end, the throat petering out near Goat Point.

57. The seafloor landward of Tokaapapa Reef including the entrance bar is composed of a

uniform Very Well Sorted Very Fine Sand (V.f.S). Jet probing of the bar revealed a minimum

thickness of V.f.S of at least 4m, thinning to about 2.4m over rock closer to shore (Wynne

1981). Wynne found no mud in the sand samples analysed.

58. The sand coarsens slightly moving inshore to Well Sorted Fine Sand along Karehana Bay

and Plimmerton Beach. Between these two beaches the sand overlying the rock platforms

and within small pocket beaches is Well Sorted Medium to Fine Sand (Med.F.S) (Lewis

1988). Lewis found no mud in the sand samples analysed.

59. In 2009 a maximum depth of 18.7m CD was recorded in the throat channel adjacent to the

northern breakwater of Mana Marina (Figure 6). The floor of the throat right through to

Moorehouse Point (Pauatahanui Inlet) is armoured with a mix of very coarse gravel and

shells (Irwin 1976; Healy 1980). Prior to excavation for Mana Marina, the tidal flat was

composed of Moderately Well Sorted Fine Sand (Lewis 1988).

60. Within both the Onepoto Arm and Pauatahanui Inlet, the throat channels peter out into

flood-tide deltas defined to the S and E, respectively, by the 0.75m CD depth contour (Figure

6). The deltas exhibit drying banks composed principally of Fine Sand (F.S) (Healy 1980)

capped in places by mobile banks of dead estuarine shells (Irwin 1976).

61. Although the flood-tide delta is reasonably well defined in the Onepoto Arm, it is rather

complex in Pauatahanui Inlet (Figure 6). Within the Inlet the delta takes the form of a

generally sub-tidal spit that is breached in 2 places by shallow N-S trending channels through

which the flood tide flows.

62. In a qualitative study of vertical aerial surveys, Irwin (1976) found that the position and

shape of the banks and main channel within Pauatahanui Inlet had “remained very stable”

over a 31 year period (1942-1973). The only exception was the sub-tidal spit off Moorhouse

Point (Figure 6) which had “become a narrow tail”.

63. Beyond the flood-tide deltas are the central relatively deeper basins of both arms of Porirua

Harbour (Figure 5) with maximum depths in 2009 of 1.68m CD in the Onepoto Arm and

1.32m CD in Pauatahanui Inlet. Within both basins the seafloor is composed of mud with less

than 10% sand (Healy 1980).

64. The tidal flats bordering the Inlet shorelines are generally composed of Poorly Sorted

Muddy Sands (M.S). At the top of the flats the narrow shorelines are composed of either

rock, gravel, dead shell or Well Sorted Sand (Irwin 1976; Pickrill 1979).

4.4.1 Sediment Sources

65. The similarity in colour, particle size and composition of the sand comprising the ebb and

flood-tide deltas of Porirua Harbour with the sediment comprising the seafloor out to about

20m CD depth in the approaches to the Harbour strongly suggests the seafloor to be an

important source area for the deltaic sand.



66. Under low, long period swell conditions coupled with flooding tides, sand on the seafloor is

moved shoreward by mass transport to weld on to the Plimmerton beaches as low, flat bars.

Under WNW wave conditions the beach sand is then transported S into the Harbour entrance

by wave generated longshore currents coupled with flooding tides (Lewis 1988; Gibb 1993).

67. The 315m-long by 30-50m-wide dune barrier complex at the N end of Ngatitoa Domain is

also a source area. From 1900-1960 the dune line advanced at 0.3m/year from accretion of

sand supplied from the seafloor via a wide stable sand beach (Wynne 1981; Gibb 1993).

Historical photographs confirm the presence of a wide sand beach prior to 1960 (Calder

2009).

68. From 1960-1979 the dune line trend reversed to erosion at -0.32 to -1.16m/year (Wynne

1981). Sand eroded from the dune complex has been transported into Porirua Harbour

where in more recent years it has accumulated at the Mana Marina site since dredging

commenced in 1963 (Gibb 1993).

69. Based on timing and observations, the most likely causes for the trend reversal from

duneline advance to retreat are thought to be firstly, the reclamation of 1.71ha of seabed at

Goat Point by NZ Railways in the 1950s, and secondly, dredging and removal of sand at the

Marina site since 1963 disrupting the natural tidal circulation of sand to and from the

Ngatitoa Beach (Wynne 1981; Stirling 1983; Gibb 1993).

70. Sand within the seafloor source area is not being naturally replenished and is regarded as a

finite non-renewable resource (Gibb 1993). Many thousands of years ago the sand was

originally supplied to the area at a lower sea-level from the Kapiti Coast S littoral drift system

that bypassed Pukerua Bay S to Plimmerton.

71. When the rising sea-level reached its present level about 7,300 years ago, the sand supply

was effectively terminated and the flow of sand directed offshore at Pukerua Bay to be lost

down one of the arms of Cook Strait canyon near Mana Island (Gibb 1993).

72. In 2006-07, the formation of an artificial gravel beach by PCC adjacent to the 315m-long

Ngatitoa dune complex has significantly reduced erosion rates, with erosion of sand only

occurring during severe WNW wave storms capable of overtopping the gravel beach crest

(PCC, pers. comm. Aug 2009). As a consequence sand from this relatively small supply

source area to the Harbour has also significantly diminished.

73. No mud is present in the marine sands of the outer seafloor and entrance bar so that the

muddy basins in both areas of the Harbour are not derived from this source. The most

obvious sources of mud are the streams draining into both the Onepoto Arm and

Pauatahanui Inlet.

74. The relatively larger streams have well developed bird’s-foot deltas at their mouths which

dissect the tidal flats. From the stream mouths the coarser sediments are spread a limited

distance laterally by wave action to form the narrow beaches (Pickrill 1979).

75. Although the streams discharging into the Harbour deliver some sand to the nearby

intertidal flats, we have found no evidence to indicate that stream-derived sand crosses

either the muddy basins or moves laterally around the intertidal flats to nourish the

predominantly sandy flood-tide deltas.

76. On the basis of available evidence we are of the opinion that the dominant source

supplying sand to both the ebb-tide and flood-tide deltas is the seafloor W of the Harbour



entrance. In contrast, the dominant source supplying mud to the central basins and mud,

sand and gravel to the intertidal flats beyond the flood-tide deltas are the streams draining

into both the Onepoto Arm and Pauatahanui Inlet.

4.5 SEDIMENTATION RATES AND PATTERNS

77. Sedimentation rates from previous research are summarized in Tables 4 & 5. Our research

reveals that all previous work has concentrated on Pauatahanui Inlet to the exclusion of the

Onepoto Arm.

4.5.1 Previous Work

78. In April 1976, 2 deep cores were recovered from the central mud basin of Pauatahanui

Inlet (Figure 7), Core 1 penetrating 12.83m of soft muddy sediments and Core 2, 7.55m of

similar sediments before striking a bed of ancient stream gravels deposited during the Last

Glacial Period when eustatic sea-level was relatively lower (Mildenhall 1979; Healy 1980).

• 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).

• 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.

CORE Calibrated Age Depth below MSL Sedimentation Rate Net rate Data Source

(Cal.years B.P.) (m) (mm/year) (mm/year)

1 9267±248 14.38

(R26/696100) 8732±295 12.05 4.4

8925±442 10.85 11.7

7973±192 9.45 1.6



8017±291 9.45

5669±233 8.25 0.5

3810±256 6.35 1.0

1131±160 2.35 1.5

Seabed 2009 1.55 0.7 1.4 Mildenhall 1979

2 8414±224 7.85

(R26/701100) 6029±254 4.83 1.3

2108±209 1.61 0.8

Seabed 2009 0.3 0.6 0.9 Mildenhall 1979

BAS 4 1934±31 4.17

(R26/690103) Seabed 2009 1.85 1.2 1.2 NIWA 2005

BRN 2021±31 2.05

(R27/681097) Seabed 2009 0.65 0.7 0.7 NIWA 2005

NET AVERAGE SEDIMENT DEPOSITION RATE 1.1 Pauatahanui Inlet

79. A total of 11 radiocarbon dated “marine silt” layers in the 2 cores allow sedimentation rates

to be calculated (Table 4). In Core 1, 12.83m of silt have accumulated over the last

9267±248 calendar years BP at a net rate of 1.4mm/year. In Core 2, 7.55m of silt have

accumulated over the last 8414±224 cal. years BP at a net rate of 0.9mm/year.

80. In both cores, sedimentation rates have ranged from 0.5mm/year up to 11.7mm/year

(Table 4). In relative terms, the higher rate is associated with a shorter time frame of a few

hundred years and the lower rates with timeframes of a few thousand years (Table 4).

81. In 1978, 2 short cores were recovered in the central mud basin between the Kakaho

Stream mouth and Moorehouse Point and sedimentation rates calculated using radionuclide

methods. Mean deposition rates were found to increase after 1967-69 from

1.3±0.2kg/m2/year up to 8.1±0.8kg/m2/year (Whitehead et al. 1998). Adopting an average

wet density of 2.60g/cm3 for greywacke-derived F.S. after Gibb (1977), indicates deposition

rates increasing from 0.5mm/year (pre-1968) to 3.1mm/year after 1968.

82. Cores BAS4 and BRN in Table 4 were recovered by NIWA in April 2004 (Figure 7) and 2

radiocarbon dates obtained from estuarine shells (Swales et al. 2005). In BAS4 a net

sedimentation rate of 1.2mm/year was recorded over the last 2,000 years or so and in BRN

a net rate of 0.7mm/year over the same period (Table 4).

83. Table 4 shows that over the entire seafloor of Pauatahanui Inlet the net average sediment

deposition rate over the last few thousand years was 1.1mm/year, ranging from 0.7mm/year

(BRN) up to 1.4mm/year (Core #1) (Table 4).

84. Table 5 provides sedimentation rates for post 1850, post 1950 and post 1985 from 9 x

0.4m-long cores sampled from Pauatahanui Inlet in April 2004. The rates were determined

by Swales et al. (2005) from a combination of radioisotope and pollen time zones within each

core.



• 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.

PERIOD Deposition Rates (mm/year)

BRN DUK PAT HRK KAH BAS1 BAS2 BAS3 BAS4

Post 1850 2.7 1.7 2.8 3.2-3.7 2.0-2.5 1.5-1.7 3.4 1.0 1.9

Post 1950 5.9 2.6-3.0 3.8 3.2 3.6-4.1 2.4-3.0 3.5-3.6 1.8-2.3 2.3-2.8

Post 1985 2.9 1.6 5.5 10.0 5.0 5.0 2.4 3.9 5.3

Average Range

Post 1850 2.32mm/year 1.0-3.7mm/year



85. For the post-1850 period, the average rate was 2.32mm/year ranging from 1.0mm/year

(BAS3) up to 3.7mm/year (HRK). For the post-1950 period, the average rate increased to

3.38mm/year, ranging from 1.8mm/year (BAS3) up to 5.9mm/year (BRN). For the post

1985 period, the average rate increased yet again to 4.62mm/year, ranging from

1.6mm/year (DUK) up to 10.0mm/year (HRK) (Table 5).

86. Sedimentation rates in the very short term were measured at 9 sites (Figure 7) on the

intertidal flats of Pauatahanui Inlet over a 15-month period starting 15 March 1976. Although

a net trend of deposition at 2.9mm/year was recorded, the net rate ranged from maximum

erosion of -64mm/year and maximum deposition at 47mm/year on various profiles (Pickrill

1979).

87. Pickrill (1979) observed that that the surface of the intertidal flats was “very stable” with

monthly changes generally not exceeding ±20mm. Of the suspended sediment (mud)

introduced to the Inlet from streams, Pickrill inferred that “more than two-thirds is flushed

through to the open sea”.

88. In summary, in geological time sedimentation rates in Pauatahanui Inlet have averaged

around 1mm/year, ranging up to about 12mm/year over shorter time frames in localized

areas. Over the last 150 years, rates of sedimentation have progressively increased from

about 2.32mm/year up to an average of about 4.6mm/year since 1985. In the very

short-term, maximum erosion of -64mm/year and maximum deposition at 47mm/year was

recorded at selected sites.

4.5.2 Tidal Prism Trends

89. To determine trends we have measured the tidal prisms for both the Onepoto Arm and

Pauatahanui Inlet by subtracting the volume of seawater between the seafloor and MLWS

from the volume at MHWS. The two most reliable hydrographic surveys made in 1974 and

2009 that have the most extensive coverage were compared to determine the general trends

(Table 6). The other surveys made in 1849, 1950, 1965-67 and 1991 either had inadequate

coverage or were unreliable.

• 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.

SURVEY

YEAR TIDAL DATA TIDAL PRISM NET REDUCTION

1974-2009 RATE

(m3) (m3) (m3/year)

ONEPOTO ARM MHWS MLWS

1.44m 0.15m

Water

Volumes 1974 5,331,375 2,448,884 2,882,491

(m3)

2009 4,709,139 1,876,291 2,832,848 -49,643 -1,418 1.70%

PAUATAHANUI INLET MHWS MLWS

1.45m 0.14m

Water

Volumes 1974 8,994,177 3,579,071 5,415,105

(m3)

2009 7,269,692 2,327,352 4,942,340 -472,765 -13,508 8.70%

TOTAL PORIRUA HARBOUR 1974 8,297,597

2009 7,775,188 -522,408 -14,926 6.30%

SUMMARY VOLUMES 1974 Onepoto 34.74% 2009 Onepoto 36.43%

Pauatahanui 65.26% Pauatahanui 63.57%

90. Over the last 35 years (1974-2009), the tidal prism of Porirua Harbour has reduced in

volume by 6.3%. Of this reduction, 8.7% has occurred in Pauatahanui Inlet and 1.7% in the

Onepoto Arm (Table 6).

91. In 1974, 65.26% of the spring tidal flow was into Pauatahanui Inlet and 34.74% into the

Onepoto Arm. By 2009, the balance had changed to 63.57% into Pauatahanui Inlet and

36.43% into the Onepoto Arm (Table 6).

92. The progressive reduction in volume of the tidal prism is consistent with sediment

deposition recorded on the intertidal flats (Pickrill 1978; Healy 1980; Swales et al. 2005).

Note that the reduction in tidal prism is not related to sedimentation of the seafloor below

MLWS such as in the channels and central mud basins.

4.5.3 Porirua Harbour Approaches

93. The approaches to Porirua Harbour is generally the area of seafloor seaward of Tokaapapa

Reef and the 7m depth contour. Tentative rates of sedimentation for this area were

determined by firstly, comparing DTMs (1967 & 2009) of a representative area of seafloor of

about 236.4ha and secondly, by comparing spot soundings surveyed in 1849 and 2009 along

2 transects at 8-11 sites (Figure 8) in about 16-7m water depth.

94. For the period 1967-2009, a net average rate of erosion of the 236.4ha seafloor of

-24.4mm/year was determined, equating to a loss of some 57,700m3/year of Fine Sand

(Table 7).



• 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.

95. For the period 1849-2009, we recorded an average erosion rate of -1.5mm/year along the

transect N of the Reef (Figure 8), ranging from maximum erosion at -8.3mm/year (Site 5)

up to maximum deposition at 4.6mm/year (Site 1) over the last 160 years. For the transect

S of the Reef, we recorded an average deposition rate of 3.2mm/year, ranging from

maximum erosion at -4.6mm/year (Site 1) up to maximum deposition at 13.3mm/year (Site

11) over the last 160 years (1849-2009) (Figure 8).

96. The large conflicting range in net rates (-23.8mm/year erosion to 3.2mm/year deposition)

indicates that both the trends and rates should be viewed with caution as they may reflect

unknown inherent errors in the surveys, notwithstanding the fact that we have applied

corrections that also allow for the effects of SLR. The fact that the approaches seafloor has

been identified as a non-renewable source area of Fine Sand for the Porirua Harbour strongly

suggests a long-term trend of erosion of the order of a few millimeters per year.

Area Survey Interval

Average Seabed Area

Volume Change

Net Erosion (-) or Deposition (+)

Net Rates of Deposition (+) or Erosion (-)

(m2) (m3) (m) (mm/year) (m3/year)

Approaches 1967-2009 2,363,870 -2,423,960 -1.02542 -24.4 -57,713

Entrance Bar 1967-2009 931,154 -51,222 -0.05501 -1.3 -1,220

Harbour Throat 1974-2009 389,271 380,244 0.97681 27.9 10,864

Onepoto Arm 1974-1991 2,190,521 225,843 0.10310 6.1 13,285

1991-2009 2,187,560 209,525 0.09578 5.3 11,640

1974-2009 2,333,032 463,877 0.19883 5.7 13,254

Pauatahanui Inlet 1974-1991 3,941,748 1,388,087 0.35215 20.7 81,652

1991-2009 3,901,667 -249,160 -0.06386 -3.6 -13,842

1974-2009 4,515,750 1,431,719 0.31705 9.1 40,906



• 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).



4.5.4 Entrance Bar

97. The entrance bar is the ebb-tide delta of sand that ‘plugs’ the entrance to Porirua Harbour

(Figure 8). Tentative rates of sedimentation for the bar were firstly determined by comparing

DTMs (1967 & 2009) of a representative 93.1ha area of seafloor and secondly, by comparing

spot soundings along 3 transects at 5-6 sites (Figure 8) surveyed in 1849, 1950 and 2009 in

about 6-1m CD water depth.

98. For the period 1967-2009, a net average rate of erosion of the 93.1ha seafloor of

-1.3mm/year was determined, equating to a loss of some 1,220m3/year of Fine Sand (Table

7).

99. Along the Karehana Bay transect (Figure 8), that is slightly N of the bar, we recorded a

very low average rate of erosion of -0.4mm/year from 1849-2009, ranging from maximum

erosion of -3.4mm/year (Site 2) up to maximum deposition at 3.3mm/year (Site 1) over the

last 160 years.

100. Along the Plimmerton Beach transect, a very low average rate of erosion of -0.2mm/year was recorded for the period 1849-1950, ranging from maximum erosion of -5.1mm/year

(Site 3) up to maximum deposition at 5.5mm/year (Site 2) over the 101-year survey period.

101. In contrast, for the period 1950-2009, there was a reversal to an average rate of deposition of 1.8mm/year along this transect, ranging from maximum erosion of

-9.2mm/year (Site 4) up to maximum deposition at 6.0mm/year (Site 5) over the 59-year

survey period.

102. For the entire 160-year period (1849-2009), a very low average rate of erosion of -0.1mm/year was recorded along the Plimmerton Beach transect, ranging from maximum

erosion of -4.7mm/year (Site 4) up to maximum deposition of 4.8mm/year (Site 2).

103. The Goat Point Leads transect (Figure 8) crosses the bar at the point where vessels navigate to and from Porirua Harbour. Relative to CD, we recorded minimum bar depths of

1.02m (1849), 1.32m (1950), 0.43m (1967), and 0.94m (2009). Allowing for unknown

seasonal variability of depths, minimum bar depths have remained remarkably constant over

the last 160 years with a slight tendency toward shoaling.

104. For the period 1849-1950, an average rate of deposition of 11.2mm/year was recorded on the Leads transect, ranging from maximum erosion of -6.6mm/year (Site 3), up to

maximum deposition of 32.6mm/year (Site 2) over the 101-year survey period. For the

period 1950-2009, the average rate of deposition dropped to 3.1mm/year, ranging from

maximum erosion of -6.5mm/year (Site 4) up to maximum deposition of 18.1mm/year (Site

6) over the 59-year survey period.

105. For the entire 160-year period (1849-2009), an average rate of deposition of 8.9mm/year was recorded along the Goat Point Leads transect, ranging from maximum erosion of

-4.3mm/year (Site 3) up to maximum deposition of 20.5mm/year (Site 2).

106. With a reduction in tidal prism of 6.3% of Porirua Harbour from 1974-2009, we would expect a small increase in the volume of the entrance bar over this period in proportion to

the progressive unknown drop in ebb-tide velocities. Averaging the net rates over the 3

transects and 93.1ha of seafloor provides an overall average rate of deposition of Fine Sand

on the entrance bar of 1-2mm/year. We believe this value to be a reasonable best estimate

of both trend and rate.



4.5.5 Throat

107. The throat area of Porirua Harbour covers the main channel of the Harbour including Mana Marina, and where the channel bifurcates into Pauatahanui Inlet and the Onepoto Arm. For

the throat, DTMs were compared from the 2 most reliable surveys made in 1974 and 2009 to

determine the pattern and rate of sedimentation (Figure 9).

108. The 1974 survey was undertaken prior to construction of Mana Marina (1980s), hence the sedimentation patterns in the vicinity of the marina are greatly affected by this man-made

structure.

109. Table 7 shows that over the last 35 years (1974-2009) there was a net trend of deposition at 27.9mm/year, equating to a gain of some 10,860m3/year of Fine Sand over an area of

seafloor of 38.9ha.

110. Figure 9 shows the pattern of sedimentation since 1974. Most rapid deposition has occurred along the seaward side of the W Mana Marina breakwater and to a lesser extent

along the S breakwater and within the Marina basin entrance. Some of this deposition may

have been affected by dumping of spoil from the Marina (PCC, pers. comm. July 2009).

111. Whilst most of the seafloor of the throat has tended to deepen from erosion since 1974, especially where it leads into Pauatahanui Inlet, the E side of the channel leading into the

Onepoto Arm has shoaled by about a metre or so. The deepening in the Marina basin is the

result of dredging in the 1980s to form Mana Marina.

112. Areas of erosion depicted within the main N-S channel are not entirely due to a general deepening, as the seabed coverage of the 1974 survey was not of sufficient density in places

to accurately determine the deepest part of the channel.

113. Prior to the establishment of Mana Cruising Club (MCC) in 1959 and the commencement of dredging in front of the Club in 1963 coupled with construction of Mana Marina in the 1980s,

the intertidal bank dredged for the Marina was a natural deposition area for Fine Sand.

During flooding tides a low velocity anticlockwise gyre formed over the bank reversing to a

clockwise gyre during ebbing tide (Stirling 1983), promoting sand deposition during both

phases of the tide.

114. From 1963-1980 about 20,000-70,000m3 of Fine Sand accumulated in the basin left by

dredging at rates of about 1,200-4,100m3/year. The W breakwater of the Marina was

constructed along the axis of an actively S growing sand spit across the dredged basin (Gibb

1993). Currently, less than 1,000m3 of sediment are dredged from the Marina area each

year (PCC, pers. comm. July 2009).

115. Figure 9 shows the same pattern of sand accumulation is occurring today as occurred during dredging from 1963-1980. According to Wynne (1981) the bulk of this sand is

probably derived from the coast and seafloor outside the Harbour entrance, including the

eroding dunes of Ngatitoa Domain. The sand is transported to the site by flood tide flows

through the throat which lose velocity near the breakwaters promoting deposition. We

concur with the views of Mr Wynne.



• Figure 9: Chart showing the sedimentation pattern in the Throat area of Porirua Harbour from 1974-2009.



116. Whilst most of the sand entering the Harbour is deposited both within Mana Marina and against both breakwaters, Figure 9 also shows that a portion of this sand is being

transported into both the Onepoto Arm and Pauatahanui Inlet, thus contributing to the

continued growth of the flood-tide deltas.

4.5.6 Onepoto Arm

117. For the study of both sedimentation patterns and rates within the Onepoto Arm, DTMs were compared for 1974 and 2009 covering almost all of the Arm. In addition, rates were

determined along 3 transects at 4-6 sites carefully selected to include comparative soundings

from the 1974, 1991 and 2009 surveys. Within Appendix B, the transects are shown on

Figure B-1 and rates for sites in Table B-3.

118. Table 7 and Figure 10 summarise sedimentation patterns and rates from 1974-2009. For the 35-year period, a net average rate of deposition over the 233.3ha seafloor was recorded

at 5.7mm/year, equating to a gain of some 13,250m3/year of mud and sand. Over the

35-year period (1974-2009) net rates of deposition have remained reasonably constant,

averaging 6.1mm/year from 1974-1991 and 5.3mm/year from 1991-2009 (Table 7).

119. Along the N transect at the entrance to the Onepoto Arm (Figure B-1), an average deposition rate of 1.9mm/year was recorded from 1974-1991, ranging from maximum

erosion of -3.9mm/year (Sites 1 & 5) up to maximum deposition at 13.7mm/year (Site 2).

From 1991-2009, average deposition rates increased to 5.4mm/year, ranging from

maximum erosion of -40.8mm/year (Site 4) up to maximum deposition at 46.4mm/year

(Site 1) (Table B-3, Appendix B).

120. For the entire 36-year period (1974-2009), the average deposition rate along the northern transect was 2.2mm/year, ranging from maximum erosion (Site 4) of -11.9mm/year up to

maximum deposition (Site 3) of 11.3mm/year (Table B-3, Appendix B).

121. Along the Mid transect at the S end of the flood-tide delta (Figure B-1), an average deposition rate of 3.4mm/year was recorded from 1974-1991, ranging from maximum

erosion of -3.9mm/year (Site 2), up to maximum deposition of 13.7mm/year (Site 1). From

1991-2009, average deposition rates increased to 4.2mm/year, ranging from maximum

erosion of -5.3mm/year (Site 3), up to maximum deposition at 18.6mm/year (Site 2) (Table

B-3, Appendix B).

122. For the entire 35-year period (1974-2009), the average deposition rate along the Mid transect was 2.3mm/year, ranging from maximum erosion of -1.1mm/year (Site 3) up to

maximum deposition at 4.9mm/year (Site 1) (Table B-3, Appendix B).

123. Along the S transect across the central mud basin (Figure B-1), an average deposition rate of 7.8mm/year was recorded from 1974-1991, ranging from maximum erosion at

-3.9mm/year (Site 1) up to maximum deposition at 13.7mm/year (Sites 3 & 4). From

1991-2009 there was a reversal to an average erosion rate of -2.2mm/year, ranging from

maximum erosion of -6.4mm/year (Site 4) up to maximum deposition at 3.1mm/year (Site

2) (Table B-3, Appendix B).



• Figure 10: Chart showing the sedimentation pattern within the Onepoto Arm from 1974-2009.

124. For the entire 35-year period (1974-2009), the average deposition rate along the S transect was 1.6mm/year, ranging from -2.8mm/year maximum erosion (Site 1) up to

3.9mm/year maximum deposition (Site 3) (Table B-3, Appendix B).

125. The pattern of sedimentation in the Onepoto Arm from 1974-2009 reveals that the greatest rates of sedimentation (20-30mm/year) occur mostly on the flood-tide delta and to a lesser



degree at the mouth of the Porirua Stream (Figure 10). As there are significant areas of

erosion on the delta, it is highly likely that the eroded sand is being redistributed by tidal

currents to accumulate in the areas shown.

126. An erosion trend is evident around much of the shoreline (Figure 10) and this may be associated with wave reflection off the armoured railway embankment and general retreat of

the Whitireia Park shoreline in response to SLR.

127. Unfortunately, neither the 1991 or 1974 surveys completely covered the intertidal part of the Porirua Stream delta so that the very low rates of sedimentation here are indicative only.

Notwithstanding, it appears that bedload discharged from this stream during floods is

deposited along the E side of the Inlet. Suspended load from Porirua Stream is likely to

mostly be accumulating within the central mud basin at about 5mm/year (Figure 10).

128. In summary, since 1974 most if not all of the 244.07ha Onepoto Arm has silted up at 5.7mm/year on average from about 13,500-14,000m3/year of sand and mud with annual

rates ranging from maximum erosion of -40.8mm/year up to maximum deposition at

46.4mm/year at various sites.

4.5.7 Pauatahanui Inlet

129. For the study of both sedimentation patterns and rates with Pauatahanui Inlet, DTMs were compared for 1974 and 2009 that covered almost the entire Inlet. In addition, rates were

determined along 5 transects at 5 to 9 sites carefully selected to include comparative

soundings from the 1950, 1974 and 2009 surveys. Within Appendix B, the transects are

shown on Figure B-2 and rates for sites in Table B-4.

130. Table 7 and Figure 11 summarise sedimentation patterns and rates from 1974-2009. For the 35-year period, a net average rate of deposition over the 451.6ha seafloor was recorded

at 9.1mm/year, equating to a gain of some 40,900m3/year of mud and sand (Table 7).

131. Over the last 35 years (1974-2009), Table 7 shows net rates have varied from 20.7mm/year deposition from 1974-1991 to -3.6mm/year erosion from 1991-2009. These

rates should be treated with caution as we believe they reflect unknown inherent errors in

the 1991 survey. As the geologic trend has been consistent deposition (Table 4) within

Pauatahanui Inlet, it is highly unlikely that there has been a widespread reversal to erosion

from 1991-2009.

132. Along the Mana transect that crosses the western part of the flood-tide delta (Figure B-2, Appendix B), an average rate of erosion of -7.0mm/year was recorded for the period

1950-1974, ranging from maximum erosion of -18.9mm/year (Sites 3 & 4) up to maximum

deposition at 15.3mm/year (Site 1). For the period 1974-2009, the average erosion rate

reduced to -2.6mm/year, ranging from maximum erosion of -11.8mm/year (Site 2) up to

maximum deposition at 2.2mm/year (Site 4) (Table B-4, Appendix B).

133. For the entire 59-year period (1950-2009), an average rate of erosion of -4.4mm/year was recorded along the Mana transect, ranging from maximum erosion of -7.2mm/year (Site 2)

up to maximum deposition at 3.0mm/year (Site 1). Net erosion is a departure from the

overall trend of deposition and may reflect the fact that a large part of this transect lies along

shallow channels or that there has been a significant reduction of sand transported to this

area since dredging commenced at MCC in 1963.



134. Along the Camborne transect that mostly runs along the crest of the flood-tide deltaic spit (Figure B-2, Appendix B), an average deposition rate of 0.6mm/year was recorded for the

period 1950-1974, ranging from maximum erosion of -6.4mm/year (Site 3) up to maximum

deposition at 6.1mm/year (Sites 1 & 5). For the period 1974-2009, the average deposition

rate reduced to 0.1mm/year, ranging from maximum erosion of -11.2mm/year (Site 2) up to

maximum deposition at 10.5mm/year (Site 5) (Table B-4, Appendix B). Once again the

reduction in deposition rates may reflect the effects of dredging at MCC since 1963.

135. For the entire 59-year period, the average deposition rate on the Camborne transect was 0.3mm/year, ranging from maximum erosion of -5.9mm/year (Site 2) up to maximum

deposition at 8.7mm/year (Site 5). The very low average deposition rate from 1950-2009

compared to relatively high contrasting rates at each of the 9 sites may suggest no new

sediment supply but reworking of sediments along the crest of the spit.

136. Along the Moorehouse Point transect that crosses the western part of the central mud basin (Figure B-2, Appendix B), a very high average deposition rate of 21.0mm/year was recorded

for the period 1950-1974, ranging from maximum erosion of -10.1mm/year (Site 2) up to

maximum deposition at 109.0mm/year (Site 5) on the edge of the main channel. For the

period 1974-2009, there was a reversal to an average erosion rate of -4.4mm/year along

this transect, ranging from maximum erosion of -29.5mm/year (Site 5) up to maximum

deposition at 14.8mm/year (Site 2) (Table B-4, Appendix B).

137. For the entire 59-year period, a relatively high deposition rate of 8.0mm/year was recorded along the Moorehouse Point transect, ranging from maximum erosion of -3.1mm/year (Site

4) up to maximum deposition at 26.9mm/year (Site 5) (Table B-4, Appendix B).

138. Along the Browns Bay transect that parallels the S shore of the Inlet (Figure B-2, Appendix B), an average erosion rate of -1.9mm/year was recorded for the period 1950-1974, ranging

from maximum erosion of -10.5mm/year (Site 1) up to 6.1mm/year maximum deposition

(Site 3). For the period 1974-2009, there was a reversal to an average erosion rate of

-4.3mm/year, ranging from maximum erosion at -10.3mm/year (Site 3) up to maximum

deposition at 12.8mm/year (Site 5) (Table B-4, Appendix B).

139. For the entire 59-year period, an average deposition rate of 1.8mm/year was recorded along the Browns Bay transect, ranging from maximum erosion of -3.6mm/year (Site 3) up

to maximum deposition at 4.7mm/year (Site 2) (Table B-4, Appendix B).

140. Along the Motukaraka Point transect that runs N-S through the central mud basin (Figure B-2, Appendix B), an average erosion rate of -3.3mm/year was recorded for the period

1950-1974, ranging from maximum erosion of -26.8mm/year (Site 5) up to maximum

deposition at 11.1mm/year (Site 4). For the period 1974-2009, there was a reversal to an

average deposition rate of 2.9mm/year, ranging from maximum erosion at -0.1mm/year

(Site 5) up to maximum deposition at -5.9mm/year (Site 1) (Table B-4, Appendix B).

141. For the entire 59-year period, an average deposition rate of 0.4mm/year was recorded along the Motukaraka Point transect, ranging from maximum erosion at -10.9mm/year (Site

5) up to maximum deposition at 6.2mm/year (Site 1) (Table B-4, Appendix B).


33

• Figure 11: Chart showing the sedimentation pattern in the Pauatahanui Inlet from 1974-2009


34

142. Of the 5 transects (Figure B-2, Appendix B), 4 recorded deposition over the last 59 years (1950-2009) at average net rates of 0.4-8.0mm/year and one transect erosion at an

average net rate of -4.4mm/year. Maximum short-term variability for both erosion and

deposition generally exceeded 20mm/year at specific sites.

143. Figure 11 reveals that most of the seafloor of Pauatahanui Inlet has shoaled from sedimentation over the last 35 years (1974-2009) at varying amounts ranging from less

than 0.2m (≤5mm/year) to greater than 1.0m (≥30mm/year). The few areas not silting up

include the main channel around Golden Gate Peninsula, a very small area of the central

mud basin, and localized areas of intertidal flats especially W of the Kakaho Stream, between

Duck Creek and the Pauatahanui Stream, and channel areas dissecting the flood-tide delta

(Figure 11).

144. The greatest areas of deposition lie in the western and eastern parts of the central mud basin (Figure 11). As no mud enters Pauatahanui Inlet from the open sea, the accumulating

mud must be directly supplied from streams discharging into the Inlet such as the

Pauatahanui, Ration Point, Horokiri, Duck and Kakaho streams. Maximum deposition is

occurring where tidal flows are relatively weak and gently rotating gyres and counter

currents are set-up by ebb and flood tides.

145. Other areas undergoing relatively rapid sedimentation from fluvial supply include the intertidal deltaic areas at the mouths of the Pauatahanui and Horokiri streams and to a lesser

degree, Kakaho Stream. Browns Bay appears to be a natural sink for sediments along with

the distal tip of the flood tide formed spit around Moorehouse Point and the S edge of the

main channel.

146. Whilst Browns Bay is known to have received a considerable locally derived deposition of mud in the mid 1970s (Swales et al. 2005), the Bay also appears to be a sediment trap for

other sources outside the Bay. In contrast, spit growth of the flood-tide delta is more likely

related to both Fine Sand being transported to the area from the open sea by flooding tides

coupled with reworking of sediments along the spit.

147. Although sedimentation of the intertidal flood-tide delta to the W of Mana is occurring the rates are less than those in the central mud basin (Figure 11) perhaps largely as a

consequence of dredging for Mana Marina basin. If we accept that the dominant supply

source of sediment for the flood-tide delta is from outside Pauatahanui Inlet, then the

sedimentation pattern in Figure 11 clearly implies that streams discharging into the Inlet are

the dominant supply source.

148. In summary, since 1974 almost all of the 469.9ha Pauatahanui Inlet has silted up at 9.1mm/year on average from about 41,000-42,000m3/year of sand and mud with annual

rates ranging from maximum erosion of -29.5mm/year up to maximum deposition at

109.0mm/year at various sites.

5 FORECAST INFILLING

149. Based on data from this study, a best estimate is provided in Table 8 for the complete infilling of both Pauatahanui Inlet and the Onepoto Arm to the 2009 level of MHWS. As there

are many uncertainties in factors such as future rates of SLR, sedimentation and tectonics

over the next few centuries, we have adopted a tentative uncertainty value of approximately

15% for this forecast.



• 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 ).

( A ) ( B ) ( C ) ( D ) ( E ) ( F )

PORIRUA HARBOUR

Full Area @ MHWS

Volume @ MHWS

Net Deposition Net Rate Projection

Forecast Infilling from 2009

(m2) (m3) (m) (m3/year) (years) (years)

Pauatahanui Inlet 4,699,693 7,269,692 0.31705 42,573 145-195 A.D. 2155 - 2205

Onepoto Arm 2,440,699 4,709,139 0.19883 13,865 290-390 A.D. 2300 - 2400

150. Table 8 shows that based on quantified trends over the last 35 years (1974-2009) it is highly likely that Pauatahanui Inlet will cease to be an estuary within the next 145-195 years

(A.D. 2155-2205) and the Onepoto Arm within the next 290-390 years (A.D. 2300-2400). A

steady transition from tidal estuaries to brackish swamps, similar to the outcome of evolution

of the Taupo Swamp, is the most likely outcome.

6 SUMMARY

151. At the level of MHWS surveyed in 2009 the total surface area of the arms of Porirua Harbour determined by DTM was 714.04ha, made up of the 244.07ha Onepoto Arm and

469.97ha Pauatahanui Inlet. The 2009 shoreline length at MHWS was 9,028.30m (9.03km)

around the Onepoto Arm and 13,241.49m (13.24km) around Pauatahanui Inlet.

152. West of the Ohariu Fault, Porirua Harbour and its approaches are subject to 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 vertically

tectonically stable or subject to very low, almost undetectable, tectonic uplift.

153. Coseismic uplift W of the Ohariu Fault is associated with surface rupture earthquakes on the Fault the most recent occurring about 700-1,300, 2,000-2,600 and 2,950-3,360 years

ago. The recurrence interval for severe earthquakes of Magnitude M 7.1-7.5. is estimated to

be 1,530-4,830 years.

154. During both the Magnitude M 8.0-8.2 Wairarapa Earthquake of 23 January 1855 and M 7.4-7.5 Marlborough Earthquake of 16 October 1848, the Pukerua, Ohariu and Moonshine

Faults that dissect the Porirua Harbour area did not rupture and no measurable vertical

deformation of the Harbour occurred, particularly during the 1855 event.

155. Although there has been little change in both global (eustatic) sea-level and regional sea-level around New Zealand from 2,000 years ago until the start of the 19th century, GMSL

rose at about 0.70mm/year from 1870-1931, accelerating to about 1.95mm/year from

1931-2007, showing excellent agreement with the New Zealand trend.

156. Notwithstanding historical SLR, the seafloor of the approaches to Porirua Harbour, composed of Fine Sand and Broken Shell, appears to be eroding at an unknown rate of the

order of a few millimeters per year and is supplying sand to Porirua Harbour that is mostly



being deposited within the throat and on both the ebb-tide delta and flood-tide deltas, thus

contributing to the progressive reduction in tidal prism.

157. Although there has been little change in minimum depths along the Goat Point Leads at the entrance to Porirua Harbour between 1849 (1.02m CD) and 2009 (0.94m CD), an overall

average rate of deposition of Fine Sand is inferred to have occurred on the ebb-tide delta

over the last 160 years (1849-2009) at about 1-2mm/year.

158. Within the Harbour throat, a net average rate of deposition of Fine Sand of 27.1mm/year has occurred from 1974-2009, a large proportion of which is being trapped against the

breakwaters and entrance to Mana Marina since construction in the 1980s.

159. In consideration of both tectonic deformation coupled with SLR, a net average rate from 1974-2009 of deposition of sand and mud at 9.1mm/year (41,000-42,000m3/year) has

occurred in Pauatahanui Inlet and 5.7mm/year (13,500-14,000m3/year) within the Onepoto

Arm. At localized sites sedimentation rates typically exceeded ±40mm/year.

160. The tidal prism has reduced in volume by 8.7% in Pauatahanui Inlet and 1.7% in the Onepoto Arm from 1974-2009 as a direct consequence of sedimentation of intertidal areas.

Although this trend is consistent with the overall net infilling of Porirua Harbour, it does not

take into account the deposition that has occurred below the 2009 level of MLWS over the

last 35 years.

161. Based on sedimentation trends over the last 35 years (1974-2009) over the entire seafloors of both arms of Porirua Harbour, it is highly likely that both Pauatahanui Inlet and

the Onepoto Arm will change from tidal estuaries to brackish swamps within 145-195 years

(A.D. 2155-2205), and 290-390 years from now (A.D. 2300-2400), respectively.

162. Within both arms of the Harbour the dominant supply sources contributing to increasing sedimentation rates are discharges of both bedload and suspended load from streams such

as the Porirua Stream within the Onepoto Arm, and the Kakaho, Horokiri, Ration,

Pauatahanui, Duck and Browns streams within Pauatahanui Inlet.

7 CONCLUSIONS

163. During the evolution of the arms of Porirua Harbour from stream valleys at a lower sea-level some 10,000 years ago to the tidal estuaries of today, persistent deposition of

sediment has occurred at about 1mm/year. Since human occupation and development of the

catchments draining into the Harbour, sedimentation rates have increased to 5-10 times

above the geologic rate.

164. Unless human intervention takes place immediately to reduce average sedimentation rates from the current 5.7-9.1mm/year down to the geologic rate of 1.0-1.5mm/year, both the

Onepoto Arm and Pauatahanui Inlet will inevitably finish up as brackish swamps.

165. At current sedimentation rates we estimate Pauatahanui Inlet will cease to function as an estuary within the next 145-195 years (A.D. 2155-2205) and the Onepoto Arm within the

next 290-390 years (A.D. 2300-2400).



• Figure 12: The Approaches to Porirua Harbour from Karehana Bay. Photo by JG Gibb 13 December 2004.

8 RECOMMENDATIONS

It is recommended that PCC after due consideration of this report:

166. 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.

167. 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.

168. 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.

9 ACKNOWLEDGEMENTS

Keith Calder, Porirua Harbour Strategy Coordinator, Strategy & Policy Group, Porirua City Council,

had the vision to commission and support this project.



The Porirua Harbour Science Group, provided helpful comments on the original draft.

Drs Kate Wilson, Geoscientist, Ursula Cochran, Paleoecologist, & Kelvin Berryman, Principal Scientist,

IGNS, kindly provided useful input on tectonics and reviewed relevant sections of our report.

Dr John A. Church, Principal Scientist, CSIRO Marine & Atmospheric Research, Hobart, Tasmania, for

permission to use his latest information on global sea-level rise.

Dawn Chambers, Rafter Radiocarbon laboratory, Gracefield, kindly reviewed and updated all the

Carbon 14 dates.

Glen Rowe, Senior Tidal Office, LINZ, provided supporting advice on the tides and sea-level trends.

Lastly our wives, Sally Cox and Anne Gibb fully supported our work and produced the report.

10 REFERENCES

The following published and unpublished reports were reviewed for this study.

Adkin, G.L., 1921: Porirua Harbour; A study of its shoreline and physiographic features.

Transaction and Proceedings of the New Zealand Institute 53:144-156pp

Begg, J. & Mazengarb C., 1996: The Geology of the Wellington Area. Lower Hutt, New Zealand.

Institute of Geological and Nuclear Sciences Ltd 1:50,000. Geological Map 22. 1 Sheet +

128p.

Begg, J. & Johnston, M., 2000: The Geology of the Wellington Area. Lower Hutt, New Zealand.

Institute of Geological and Nuclear Sciences Ltd 1:250,000. Geological Map 10. 1 Sheet

+ 64p.

Bewick, D.J., 1977: Hydraulic model studies of Pauatahanui Inlet. Prediction of physical changes

caused by reclamation and dredging, Ministry of Works and Development, Central

Laboratories Report No. 3-77/5. December 1977. 25p + Appendices

Blaschke, P., Woods, J. & Forsyth, F., 2009: The Porirua Harbour and its Catchment: A literature

summary and review. Unpublished Consultancy Report prepared for Porirua City Council,

March 2009. 122p.

Calder, K., 2009: Historical Images of Porirua Harbour. Unpublished internal compilation for

Porirua City Council.

Church, J.A., N.J. White, T. Aarup, W.S. Wilson, P.L. Woodworth, C.M. Domingues, J.R. Hunter &

K. Lambeck 2008:, Understanding global sea levels: past, present and future.

Sustainability Science, 3(1):9-22.

Cochran, U.A., 2000: Paleoenvironmental analysis of uplifted coastal lake and wetland

sequences in Wellington Region. Wellington, New Zealand, School of Earth Sciences,

Victoria University of Wellington, Report to the Earthquake Commission Research

Foundation Project No 991324.



Cochran, U.A., 2002: Detection of large Holocene Earthquakes in the sedimentary Record of

Wellington, New Zealand, using Diatom Analysis. Unpublished PhD Thesis, Victoria

University of Wellington, Wellington.

Cochran, U., Hannah, M., Harper, M., van Dissen, R., Barryman, K. & Begg, J., 2007: Detection

of large, Holocene earthquakes using diatom analysis of coastal sedimentary sequences,

Wellington, New Zealand. Quaternary Science Reviews 26 (2007), 1129-1147pp.

DML 2009: Porirua Harbour Survey. Report of Survey. Unpublished Survey Report prepared for

Porirua City Council. June 2009. 31p.

Eiby, G., 1990: Changes to Porirua Harbour in about 1855: historical tradition and geological

evidence. Journal of the Royal Society of New Zealand 20(2) 233-248pp.

Gibb, J.G. 1977: Sediment data for beaches and rivers, North Island, New Zealand. Victoria

University of Wellington, Geology Department publication No.7, August 1977: 24p.

Gibb, J.G. 1978: The problem of coastal erosion along the "Golden Coast", western Wellington,

New Zealand. Water and Soil Technical Publication 10: 20p.

Gibb, J.G. 1980: Late Quaternary shoreline movements in New Zealand. Unpublished PhD

thesis lodged in the library, Victoria University of Wellington, Wellington. 217p.

Gibb, J.G., 1986: A New Zealand regional Holocene eustatic sea-level curve and its application to

determination of vertical tectonic movements. A contribution to IGCP-Project 200.

Proceedings of the International Symposium on Recent Crustal Movements of the Pacific

Region. Wellington, New Zealand. 9-14 February 1984. Royal Society of New Zealand

Bulletin 24: 377-395pp.

Gibb, J.G., 1991: Implications of Greenhouse-Induced Sea-Level Rise for Coastal Management.

Proceedings of the 10th Australasian Conference on Coastal and Ocean Engineering,

Auckland, New Zealand, 2-6 December 1991. 8p.

Gibb, J.G., 1993: A Strategic Plan to solve the problems of Coastal Erosion at Ngatitoa Domain

and Deposition of Sand near Mana Marina. Consultancy Report C.R. 93/1 prepared for

Porirua City Council. December 1993. 36p.

Grapes, R. & Downes, G., 2007: The 1855 Wairarapa, New Zealand, Earthquake – Analysis of

Historical Data. Bulletin of the New Zealand National Society for Earthquake Engineering,

30, 271-368pp.

Hancox, G.T., Dellow, G.D., Perrin, N.D., & McSaveney, M.J., 2005: Western Corridor

Transportation Study: Review of geological hazards affecting the proposed Coastal

Highway Upgrade and Transmission Gully Motorway route. Institute of Geological and

Nuclear Sciences Ltd for Porirua City Council, Client Report 2005/161.

Hannah, J., 1990: Analysis of Mean Sea Level Data from New Zealand for the Period 1899-1988.

Journal of Geophysical Research 95(B8): 12,399-12,405.

Hannah, J., 2004: An updated analysis of long-term sea level change in New Zealand.

Geophysical Research Letters, 31, L03307. 4p.



Hayward, B.W., Grenfell, H.R., Sabaa, A.T., 2008: Formaminiferal Evidence for Holocene History

of Pauatahanui Inlet. Unpublished Geomarine Research Report BWH 115/08, August

2008. 10p + Appendices

Healy, W.B., 1980: Pauatahanui Inlet: An environmental study. DSIR Information Series 141,

Wellington, New Zealand, Department of Scientific & Industrial Research.

Heron D., van Dissen, R., & Sawa, M., 1998: Late Quaternary movement on the Ohariu Fault,

Tongue Point to Mackays Crossing, North Island, New Zealand. New Zealand Journal of

Geology and Geophysics 41:419-440pp.

Irwin, J., 1976: Morphological stability of Pauatahanui Inlet, Porirua Harbour. New Zealand

Journal of Marine & Freshwater Research. 10(4):641-50pp.

Irwin, J., 1978: Porirua Harbour Bathymetry, New Zealand. NZ Oceanographic Institute Chart,

Department of Scientific and Industrial Research 1:10,000. Miscellaneous Series No 49.

Kay, Barbara, 1996: Anthony Wall, Settler of Porirua. The Papakowhai Story. Kerslake Billens &

Humphrey Ltd, Levin, New Zealand. 231p.

Leamy, M.L., 1958: Pleistocene shorelines at Porirua Harbour, near Wellington, New Zealand.

New Zealand Journal of Geology and Geophysics 1:95-102p.

Lewis, K.B. 1988: Preliminary Feasibility Study for Beach Replenishment at Plimmerton.

Unpublished Consultancy Report prepared for Department of Conservation, NZ

Oceanographic Institute, DSIR, Wellington: 19p. + Appendices

LINZ 2009: New Zealand Nautical Almanac 2009/10 Edition. NZ204. 260p.

McDougall, J.C., 1976: Distribution of surface sediments of Pauatahanui Inlet. NZOI

Oceanographic Field Report 7.

McFadgen, B. 2007: Hostile Shores. Auckland University Press, University of Auckland, New

Zealand. 298p.

MetOcean 2009: Historical Charts of Porirua Harbour. Unpublished Consultancy Report prepared

for Porirua City Council, March 2009. 78p.

Page, M., Heron, D., & Trustrum, N., 2004: Pauatahanui Inlet – Analysis of historical catchment

land use and land use change. Institute of Geological & Nuclear Sciences Ltd Client

Report 2004/169, December 2004 prepared for Porirua City Council. 16p + maps

Pickrill, R.A., 1979: A micro-morphological survey of intertidal estuarine surfaces in Pauatahanui

Inlet, Porirua Harbour. New Zealand Journal of Marine & Freshwater Research 13(1):59-

69.

Rohling, E.J., Grant, K., Hemleben, C.H., Siddal, M., Hoogakker, B.A.A., Bolshaw, M. & Kucera,

M., 2007: High rates of sea-level rise during the last interglacial period. Letters. Nature

Geoscience, Advance Online Publication. 5p.

Stevenson, G., & Mills, G.N., 2006: Porirua Harbour long-term baseline monitoring programme:

Sediment chemistry and benthic ecology results from the October 2005 survey.

Consultancy report prepared for Greater Wellington Regional Council. 107p.



Stirling, J.J., 1983: Mana Marina: An environmental impact assessment. Wellington, New

Zealand. Beca Carter Hollings and Ferner Ltd. 49p + Figures.

Swales, A., Bentley, S.J., McGlone, M.S., Ovenden, R., Hermanspahn, N., Budd, R., Hill, A.,

Pickmere, S., Haskey, R., Okey, M.J., 2005: Pauatahanui inlet: effects of historical

catchment land cover changes on inlet sedimentation. National Institute of Water &

Atmospheric Research Ltd Client Report HAM2004-149, April 2005 prepared for Greater

Wellington Regional Council and Porirua City Council. 135p.

Walton, A., 2002: An archaeological survey of Whitireia Park, Porirua, Wellington, New Zealand.

Department of Conservation Internal Report Series 62.

Whitehead, N., Ditchburn, R.G., McCabe, W.J., Mason, W.J., Irwin, J., Pickrill, R.A. & Fish, G.R.,

1998: Application of natural and artificial fallout radionuclides to determining

sedimentation rates in New Zealand lakes. New Zealand Journal of Marine and

Freshwater Research 32:489-503.

Williamson, B., Olsen, G., Green, M., 2005: Greater Wellington Regional Council Long-Term

Baseline Monitoring of Marine Sediments in Porirua Harbour. National Institute of Water

& Atmospheric Research Ltd Client Report HAM2004-128, Revised September 2005

prepared for Greater Wellington Regional Council. 63p.

Wilson, K., Haward, B., Cochran, U., Grenfell, H., Hemphill-Haley, E., Mildenhall, D., Hemphill-

Haley, M., Wallace, L., (Unpublished in prep. 2005): Elusive evidence for paleo-

coseismic subsidence across a highly coupled plate boundary, southern Hikurangi

margin, New Zealand. GNS Science, Lower Hutt, Geomarine Research, Auckland,

Department of Geology, CA, USA.

Wynne, K.E., 1981: Porirua Harbour Authority Harbour Investigation May 1981. Unpublished

Consulting Report prepared for the Porirua Harbour Authority. 37p.


- 1 -

APPENDIX A

Tables of Data

TABLE A-1: Estimated errors by DML for hydrographic surveys made in 1849, 1950, 1965-67, 1974, 1991 and 2009.

TABLE A-2: Radiocarbon dated potential paleosea-level and other markers from the Pauatahanui Inlet to Plimmerton area. 9282-971 calendar years Before Present (BP 1950).

TABLE A-3: Net rates of deposition (+) or erosion (-) of the seabed within the Porirua Harbour 1967-2009.

TABLE A-1: Estimated errors by DML of all hydrographic surveys made between 1849 and 2009.


COASTAL MANAGEMENT CONSULTANTS LTD - 2 -

Survey Year General Comments Estimated Horizontal Accuracy

Estimated Vertical Accuracy

1849 HMS Acheron survey, controlled by sextant (running fixes) and lead line.

Sparse data density. Soundings in fathoms reduced to 1ft below MLWS.

Probably based on short term tidal observations. Some areas of agreement

with 2009 survey within ±0.2m, but up to 5m in deeper waters due to positioning and lead-lining. This survey is of little no value due to its age

and known positioning errors. Used selectively in digital depth comparisons.

100m+

offshore 30-

50m or

greater -inshore

±1m or greater -

offshore

±0.3m or greater -

Inshore

1950 HMNZS Lachlan survey. Depths in ft reduced to BM on Paremata Road

Bridge. Believed to be close to CD. Poorly controlled survey with running

fixes, however some data within Pauatahanui Arm that is of interest.

Depths over some shoal areas in Pauatahanui indicate a shoaling of

between 0.4-0.6m from 1950-2009. Other areas indicate 0.2-0.3m

shoaling. Insufficient data to create useable DTM. Used selectively in digital

depth comparisons.

30-50m ±0.5m or greater -

offshore

±0.25m within

Pauatahanui

1965-1967 RNZN surveys. First controlled survey of approaches to Porirua and

Onepoto Arm. Soundings in fathoms reduced to RNZN BM at Mana. Level

confirmed. Positioning via Hifix 6 (offshore) and sextant and transits inshore. Depths gathered with early echo sounder. Digital comparisons

with 2009 data show large range in discrepancies. Possible heave/swell

offshore and tide errors inshore. This, together with 1:12,000 scale (sparse

data) did not allow for accurate digital depth comparisons or creation of

useable DTM. Used selectively in digital depth comparisons.

5-30m

offshore 10m

or better inshore

±0.5m - offshore

±0.35m or better -

inshore (dependant

on tide station)

1966 Large scale survey of small area in Hongoeka Bay. Probably an

investigation survey for a proposed development. Data of little value. Used

selectively in digital comparisons.

Not assessed Not assessed

1974 Truebridge, Callender & Beach surveys for PCC - encompassing Mana,

Onepoto and Pauatahanui Arms. No survey report to assess. No other

details available. Survey sheets depict good coastline detail but rather

sparse sounding coverage. However, overall area coverage has enabled a

reasonable DTM to be derived from digitized data and compared to 2009 survey.

5-10m ±0.35m or better

(dependant on

distance from tide

station).

1991 Truebridge, Callender & Beach surveys for PCC - encompassing Mana,

Onepoto and Pauatahanui Arms. No survey report to assess but details

received from surveyors. Depths were gathered by NIWA and reduced for

tide observed at Mana. Vessel positioning via Microfix. Topographic work

via Total Stn and theodolite. Survey mark network around inlets based on

MSL. Consistent sounding coverage throughout Mana and the two inlets but

no data for upper reaches. DTM derived from digitized data and compared

to 2009 survey.

5m or better

for boat work.

3m or better

for

topographic

work

±0.30m or better

(dependant on

distance from tide

station). Topo work

±0.15m or better

2009 Modern hydrographic survey undertaken by DML for PCC. Encompassing all

harbour areas and approaches with high density sounding lines (10-20m)

within the Mana Confluence and inlet areas. Greater line spacing offshore. Depths reduced to CD at Mana. Positioning via DGPS and RTK systems.

±1.5m or

better

±0.05m - 0.26m

dependant on location

& distance from Mana tide station

Patterns & Rates of Sedimentation within Porirua Harbour Consultancy Report (C.R. 2009/1) prepared for Porirua City Council

- 3 -

TABLE A-2: Radiocarbon dated potential paleosea-level markers from the Pauatahanui Inlet-Plimmerton area. All original levels given have been adjusted from MSL Wellington Vertical Datum 1953 to new formation heights using the 2009 tidal data. Radiocarbon dates were checked and updated by Dawn Chambers, Rafter Radiocarbon Laboratory in June/July 2009.

14C Number

R Number Location

Grid Reference Dated Sample

Depositional Environment

Formation Datum

Formation Height

14C Age Old T½ CRA*

Calibrated Age Years BP (95%)

Calibration Note

With respect to which standard

Eustatic Sea Level (Gibb

1986) Reference

(m)

NZ 7379 11331/1 Karehana Bay R26/661132 Shell Lower tidal Flat MLWS 0.96 2950±70 3290±78 3356-2947 1 NZ Marine shell 0.2±0.5 JG Gibb unpublished

NZ 4866 5962/2 Taupo Swamp S R26/672123 Shell Tidal Flat MSL 1.9±0.8 3780±50 4121±32 4351-4096 1 NZ Marine shell 0.3±1.0 Gibb (1986)

NZA 9275 24359 Taupo Swamp N (TS 97-1) R26/674129 Wood Estuarine MHWS -1.23 4984±56 4980±55 5502-5823** 2 0.95 N.B.S. Oxalic 0.0±1.0 Cochran et al. (2007)

WK 8095 Taupo Swamp N (TS 97-1) R26/674129 Organic Mud Upper tidal Flat MHWS 1.75 2460±80 2750-2330 3 ?? -0.2±0.5 Cochran et al. (2007)

NZA 10568 24860/4 Taupo Swamp N (TS 98-2) R26/672129 Shell Lower tidal Flat MLWS -0.11 4940±60 4940±55 5462-5128 1 0.95 N.B.S. Oxalic 0.0±1.0 Cochran et al. (2007)

WK 8353 Taupo Swamp N (TS 98-2) R26/672129 Organic Sand Upper tidal Flat MHWS 1.90 3010±230 3850-2450 3 ?? 0.2±0.5 Cochran et al. (2007)

NZ 7387 11396/3 Kakaho Stream W R26/690118 Shell Tidal Flat MSL 0.0-0.3 4740±80 5079±90 5640-5274 1 NZ Marine shell -0.3±1.0 NIWA (2005)

NZ 7393 11396/4 Kakaho Stream W R26/690118 Shell Tidal Flat MSL 0.6 2920±70 3258±78 3331-2913 1 NZ Marine shell 0.1±0.5 NIWA (2005)

NZ 7421 11396/5 Kakaho Stream W R26/690118 Shell Tidal Flat MSL 0.6 3150±70 3487±88 3623-3195 1 NZ Marine shell 0.5±1.0 NIWA (2005)

NZ 3118 4844/1 Motukaraka Point W R26/695112 Shell Upper tidal Flat MHWS 1.24±1.0 6250±60 6566±41 7230-6995 1 NZ Marine shell 0.0±1.0 McFadgen (1980)

NZA 29687 29832/3 Ration Point (Core RPA) R26/705105 Shell Tidal Flat MSL -1.4 6547±45 6547±45 7220-6968 1 0.95 N.B.S. Oxalic -0.1±1.0 Wilson et al. in press

NZA 29752 29832/4 Ration Point (Core RPA) R26/705105 Peat Swamp MSL -5.4 7521±85 7521±85 8416-8047 2 0.95 N.B.S. Oxalic -9.0±2.8 Wilson et al. in press

NZA 30261 29948/2 Ration Point (Core RPC) R26/704102 Shell Tidal Flat MLWS -1.0 7495±25 7495±25 8063-7916 1 0.95 N.B.S. Oxalic -9.0±2.8 Wilson et al. in press

NZ 4399 5430/12 Pauatahanui Inlet (Core 2) R26/701100 Marine silt Estuarine MSL -4.83 5410±130 5615±117 6282-5775 1 NZ Marine flesh 0.0±1.0 Mildenhall (1979)

NZ 4400 5430/13 Pauatahanui Inlet (Core 2) R26/701100 Marine silt Estuarine MSL -7.85 7750±110 7931±99 8638-8190 1 NZ Marine flesh -11.8±2.0 Mildenhall (1979)


NZ 4388 5430/1 Pauatahanui Inlet (Core 1) R26/696100 Marine silt Swamp MSL -14.38 8160±130 8360±115 9282-8623 1 NZ Marine flesh -19.4±2.5 Mildenhall (1979)








NZ 7381 11331/2 Pauatahanui Stream R27/711091 Shell Tidal Flat MSL -0.15 6650±95 6985±106 7689-7306 1 NZ Marine shell -0.5±1.0 JG Gibb unpublished

NZ 7383 11331/3 Pauatahanui Stream R27/711091 Shell Tidal Flat MSL -0.66 6720±95 7058±107 7784-7392 1 NZ Marine shell -1.0±1.0 JG Gibb unpublished



Notes for Table A-2:

* CRA with respect to 0.95 N.B.S. Oxalic Standard. Conventional Radiocarbon Age (CRA) years before present (1950). (years BP)

Cal Note 1 GNS Science Winscal 5.0 Marine data from Hughen et al (2004) - Regional Delta-R = -30±13. Calibrated age in calendar years (cal. years BP) (1950)

Cal Note 2 GNS Science Winscal 5.0 Southern Hemisphere Atmospheric data from McCormac et al (2004). Calibrated age in calendar years (cal.years BP) (1950)

Cal Note 3 Program & which calibration curve?

** Three possible age ranges in this period

P1 Core 1 All levels increased by 1.55m, to normalise levels to MSL WVD.

P1 Core 2 All levels increased by 0.3m, to normalise levels to MSL WVD.

TABLE A-3: Net rates of deposition (+) or erosion (-) of the seabed within the Porirua Harbour area determined by precisely comparing hydrographic surveys made between 1967 and 2009 that were nomalised to a common vertical datum. A sea-level rise (SLR) correction of 1.95mm/year was applied after Church et al. (2008). The total areas of both Onepoto Arm and Pauatahanui Inlet are determined

at the 2009 MHWS elevation. Note that for the total harbour areas, sedimentation rates were extrapolated from the 1974-2009 rates. Volumes for Area 5 - Pauatahanui based on the 1991 survey are



distorted due to incomplete survey coverage. Excluding the throat, bar and outer approaches, the areas of the inner Porirua Harbour measured at MHWS in 2009 are about 244ha (Onepoto Arm) and 470ha

(Pauatahanui Inlet), totalling about 714ha.

A B C D E F G H I J

Area Survey Interval

Survey Period

Average Seabed Area

Uncorrected Volumetric change

SLR Correction

Corrected Volume Change

Corrected Net Erosion (-) or Deposition

(+)

Net Rates of Deposition (+) or

Erosion (-) Trends

(years) (m2) (m3) (m) (m3) (m) (mm/year) (m3/year)

AREA 1

Approaches 1967-2009 42 2,363,870 -2,617,569 -0.0819 -2,423,960 -1.02542 -24.4 -57,713 Possible erosion

AREA 2

Entrance Bar 1967-2009 42 931,154 -127,481 -0.0819 -51,222 -0.05501 -1.3 -1,220 Slight erosion or static

AREA 3

Harbour Throat 1974-2009 35 389,271 406,813 -0.06825 380,244 0.97681 27.9 10,864 Deposition

AREA 4

Onepoto Arm 1974-1991 17 2,190,521 317,474 -0.03315 225,843 0.1031 6.1 13,285 Deposition

1991-2009 18 2,187,560 304,838 -0.0351 209,525 0.09578 5.3 11,640 Deposition

1974-2009 35 2,333,032 622,312 -0.06825 463,877 0.19883 5.7 13,254 Deposition

AREA 5

Pauatahanui Inlet 1974-1991 17 3,941,748 1,724,486 -0.03315 1,388,087 0.35215 20.7 81,652 Deposition

1991-2009 18 3,901,667 -442,696 -0.0351 -249,160 -0.06386 -3.6 -13,842 Unlikely erosion

1974-2009 35 4,515,750 1,724,485 -0.06825 1,431,719 0.31705 9.1 40,906 Deposition


- 1 -

APPENDIX B

Historical Erosion and Deposition Rates 1849-2009

TABLE B-1: Area 1 – Approaches to Porirua Harbour

TABLE B-2: Area 2 – Entrance Bar

TABLE B-3: Area 4 – Onepoto Arm

TABLE B-4: Area 5 – Pauatahanui Inlet

TABLE B-1: Area 1 - Approaches to Porirua Harbour. Sedimentation rates (1849-2009) of erosion (-) or deposition (+) on the seafloor of the approaches to Porirua Harbour along 2 transects (see Figure 8).



North 1849 2009 Difference SLR

Transect Depth Depth 1849-2009 Correction Sum Rate Av Rate

(m CD) (m CD) (mm) (mm) (mm) (mm/y) (mm/y)

1 16.56 16.03 -530 -210 -740 4.62

2 14.74 15.35 610 -210 400 -2.50

3 14.74 14.26 -480 -210 -690 4.31

4 11.08 12.32 1240 -210 1030 -6.44

5 9.25 10.79 1540 -210 1330 -8.31

6 9.71 9.46 -250 -210 -460 2.88

7 7.88 8.18 300 -210 90 -0.56

8 6.05 7.24 1190 -210 980 -6.13 -1.52

South 1849 2009 Difference SLR



1 14.74 15.69 950 -210 740 -4.63

2 14.74 15.05 310 -210 100 -0.63

3 14.74 14.47 -270 -210 -480 3.00

4 14.74 13.71 -1030 -210 -1240 7.75

5 12.91 12.89 -20 -210 -230 1.44

6 11.08 11.71 630 -210 420 -2.63

7 11.08 10.46 -620 -210 -830 5.19

8 9.25 9.28 30 -210 -180 1.13

9 9.25 8.86 -390 -210 -600 3.75

10 9.25 8.22 -1030 -210 -1240 7.75

11 9.25 7.34 -1910 -210 -2120 13.25 3.22 TABLE B-2: Area 2 - Entrance Bar. Sedimentation rates of erosion (-) or deposition (+) on the seafloor along 3 transects (see Figure 8) from 1849-2009.

Karehana Bay 1849 2009 Difference SLR





1 6.96 6.64 -320 -210 -530 3.31

2 4.68 5.44 760 -210 550 -3.44

3 4.22 4.46 240 -210 30 -0.19

4 2.85 3.30 450 -210 240 -1.50

5 1.92 2.14 220 -210 10 -0.06 -0.38

Plimmerton Beach 1849 1950 Difference SLR



1 5.59 5.59 0 -95 -95 0.94

2 4.22 3.76 -460 -95 -555 5.50

3 1.93 2.54 610 -95 515 -5.10

4 1.02 1.32 300 -95 205 -2.03 -0.17

5 1.32

6 0.56




1 5.59 5.45 -140 -115 -255 4.32

2 3.76 3.66 -100 -115 -215 3.64

3 2.54 2.41 -130 -115 -245 4.15

4 1.32 1.98 660 -115 545 -9.24

5 1.32 1.08 -240 -115 -355 6.02 1.78

6 0.95




1 5.59 5.45 -140 -210 -350 2.19

2 4.22 3.66 -560 -210 -770 4.81

3 1.93 2.41 480 -210 270 -1.69

4 1.02 1.98 960 -210 750 -4.69

5 1.08

6 0.56 0.95 390 -210 180 -1.13 -0.10

Goat Pt Leads 1849 1950 Difference SLR



1 8.33 5.90 -2430 -95 -2525 25.00

2 7.88 4.68 -3200 -95 -3295 32.62

3 2.39 3.15 760 -95 665 -6.58

4 1.63

5 1.48 1.32 -160 -95 -255 2.52



6 2.39 2.24 -150 -95 -245 2.43 11.20




1 5.90 5.85 -50 -115 -165 2.80

2 4.68 4.81 130 -115 15 -0.25

3 3.15 3.29 140 -115 25 -0.42

4 1.63 2.13 500 -115 385 -6.53

5 1.32 1.14 -180 -115 -295 5.00

6 2.24 1.29 -950 -115 -1065 18.05 3.11




1 8.33 5.85 -2480 -210 -2690 16.81

2 7.88 4.81 -3070 -210 -3280 20.5

3 2.39 3.29 900 -210 690 -4.31

4 2.13

5 1.48 1.14 -340 -210 -550 3.44

6 2.39 1.29 -1100 -210 -1310 8.19 8.93


- 5 -

Figure B-1: Area 4 – Plan showing 3 transects across the Onepoto Arm.



TABLE B-3: Area 4 - Onepoto Arm. Sedimentation rates of erosion (-) or deposition (+) on the seafloor of the Onepoto Arm along 3 transects (Figure B-1) from 1974-2009.




1 2.20 1.50 -700 -68 -768 13.02

2 2.60 2.25 -350 -68 -418 7.08

3 0.70 0.10 -600 -68 -668 11.32

4 0.60 1.37 770 -68 702 -11.90

5 0.40 0.75 350 -68 282 -4.78

6 -1.00 -0.84 160 -68 92 -1.56 2.20




1 2.20 2.30 100 -33 67 -3.94

2 2.60 2.40 -200 -33 -233 13.71

3 0.70 0.70 0 -33 -33 1.94

4 0.60 0.60 0 -33 -33 1.94

5 0.40 0.50 100 -33 67 -3.94

6 -1.0 -1.0 0 -33 -33 1.94 1.94




1 2.30 1.50 -800 -35 -835 46.39

2 2.40 2.25 -150 -35 -185 10.28

3 0.70 0.10 -600 -35 -635 35.28

4 0.60 1.37 770 -35 735 -40.83

5 0.50 0.75 250 -35 215 -11.94

6 -1.0 -0.8 160 -35 125 -6.94 5.37

Mid 1974 2009 Difference SLR





1 0.30 0.08 -220 -68 -288 4.88

2 0.60 0.40 -200 -68 -268 4.54

3 0.50 0.63 130 -68 62 -1.05

4 0.20 0.23 30 -68 -38 0.64 2.25




1 0.30 0.10 -200 -33 -233 13.71

2 0.60 0.70 100 -33 67 -3.94

3 0.50 0.50 0 -33 -33 1.94

4 0.20 0.20 0 -33 -33 1.94 3.41




1 0.10 0.08 -20 -35 -55 3.06

2 0.70 0.40 -300 -35 -335 18.61

3 0.50 0.63 130 -35 95 -5.28

4 0.20 0.23 30 -35 -5 0.28 4.17




1 0.50 0.63 130 -35 95 -5.28

2 1.70 1.68 -20 -35 -55 3.06

3 1.40 1.44 40 -35 5 -0.28

4 1.00 1.15 150 -35 115 -6.39 -2.22




1 0.40 0.50 100 -33 67 -3.94

2 1.80 1.70 -100 -33 -133 7.82

3 1.60 1.40 -200 -33 -233 13.71

4 1.20 1.00 -200 -33 -233 13.71 7.83




1 0.40 0.63 230 -68 162 -2.75

2 1.80 1.68 -120 -68 -188 3.19

3 1.60 1.44 -160 -68 -228 3.86

4 1.20 1.15 -50 -68 -118 2.00 1.58


- 8 -

Figure B-2: Area 5 – Plan showing 5 transects across Pauatahanui Inlet.


- 9 -

TABLE B-4: Area 5 - Pauatahanui Inlet. Sedimentation rates of erosion (-) or deposition (+) on the seafloor of the Pauatahanui Inlet along 5 transects (Figure B-2) from 1950-2009.

Mana 1950 1974 Difference SLR



1 2.54 2.22 -320 -47 -367 15.29

2 0.50 0.56 60 -47 13 -0.54

3 0.50 1.00 500 -47 453 -18.88

4 0.50 1.00 500 -47 453 -18.88

5 0.50 0.85 350 -47 303 -12.63

6 0.50 0.70 200 -47 153 -6.38 -7.00




1 2.22 2.48 260 -68 192 -5.49

2 0.56 1.04 480 -68 412 -11.77

3 1.00 1.00 0 -68 -68 1.94

4 1.00 0.99 -10 -68 -78 2.23

5 0.85 0.97 120 -68 52 -1.49

6 0.70 0.80 100 -68 32 -0.91 -2.58




1 2.54 2.48 -60 -115 -175 2.97

2 0.50 1.04 540 -115 425 -7.20

3 0.50 1.00 500 -115 385 -6.53

4 0.50 0.99 490 -115 375 -6.36

5 0.50 0.97 470 -115 355 -6.02

6 0.50 0.80 300 -115 185 -3.14 -4.38



Camborne 1950 1974 Difference SLR



1 0.10 0.00 -100 -47 -147 6.13

2 0.10 0.10 0 -47 -47 1.96

3 0.50 0.70 200 -47 153 -6.38

4 0.50 0.60 100 -47 53 -2.21

5 0.50 0.40 -100 -47 -147 6.13

6 0.10 0.20 100 -47 53 -2.21

7 0.10 0.10 0 -47 -47 1.96

8 0.10 0.10 0 -47 -47 1.96

9 0.10 0.20 100 -47 53 -2.21 0.57




1 0.00 0.12 120 -68 52 -1.49

2 0.10 0.56 460 -68 392 -11.20

3 0.70 0.68 -20 -68 -88 2.51

4 0.60 0.44 -160 -68 -228 6.51

5 0.40 0.10 -300 -68 -368 10.51

6 0.20 0.48 280 -68 212 -6.06

7 0.10 0.35 250 -68 182 -5.20

8 0.10 0.10 0 -68 -68 1.94

9 0.20 0.15 -50 -68 -118 3.37 0.10




1 0.10 0.12 20 -115 -95 1.61

2 0.10 0.56 460 -115 345 -5.85

3 0.50 0.68 180 -115 65 -1.10

4 0.50 0.44 -60 -115 -175 2.97

5 0.50 0.10 -400 -115 -515 8.73

6 0.10 0.48 380 -115 265 -4.49

7 0.10 0.35 250 -115 135 -2.29

8 0.10 0.10 0 -115 -115 1.95

9 0.10 0.15 50 -115 -65 1.10 0.29

Moorehouse Point 1950 1974 Difference SLR





1 0.10 0.20 100 -47 53 -2.21

2 0.71 1.00 290 -47 243 -10.13

3 1.02 -47 -47 1.96

4 0.10 0.00 -100 -47 -147 6.13

5 4.07 1.50 -2570 -47 -2617 109.04 20.96




1 0.20 0.04 -160 -68 -228 6.51

2 1.00 0.55 -450 -68 -518 14.80

3 0.63 -68 -68

4 0.00 0.40 400 -68 332 -9.49

5 1.50 2.60 1100 -68 1032 -29.49 -4.42




1 0.10 0.04 -60 -115 -175 2.97

2 0.71 0.55 -160 -115 -275 4.66

3 1.02 0.63 -390 -115 -505 8.56

4 0.10 0.40 300 -115 185 -3.14

5 4.07 2.60 -1470 -115 -1585 26.86 7.98

Browns Bay 1950 1974 Difference SLR



1 0.10 0.40 300 -47 253 -10.54

2 0.40 0.40 0 -47 -47 1.96



3 0.10 0.00 -100 -47 -147 6.13

4 1.32 1.30 -20 -47 -67 2.79

5 1.02 1.30 280 -47 233 -9.71 -1.87




1 0.40 0.07 -330 -68 -398 11.37

2 0.40 0.24 -160 -68 -228 6.51

3 0.00 0.43 430 -68 362 -10.34

4 1.30 1.32 20 -68 -48 1.37

5 1.30 0.92 -380 -68 -448 12.80 4.34




1 0.10 0.07 -30 -115 -145 2.46

2 0.40 0.24 -160 -115 -275 4.66

3 0.10 0.43 330 -115 215 -3.64

4 1.32 1.32 0 -115 -115 1.95

5 1.02 0.92 -100 -115 -215 3.64 1.81

Motukaraka Point 1950 1974 Difference SLR



1 0.41 0.30 -110 -47 -157 6.54

2 0.41 0.60 190 -47 143 -5.96

3 1.02 1.10 80 -47 33 -1.38

4 1.32 1.10 -220 -47 -267 11.13

5 0.41 1.10 690 -47 643 -26.79 -3.29






1 0.30 0.16 -140 -68 -208 5.94

2 0.60 0.56 -40 -68 -108 3.09

3 1.10 1.01 -90 -68 -158 4.50

4 1.10 1.13 30 -68 -38 1.09

5 1.10 1.17 70 -68 2 -0.06 2.91




1 0.41 0.16 -250 -115 -365 6.19

2 0.41 0.56 150 -115 35 -0.59

3 1.02 1.01 -10 -115 -125 2.12

4 1.32 1.13 -190 -115 -305 5.17

5 0.41 1.17 760 -115 645 -10.93 0.39

Date post:	28-Apr-2015
Category:	Documents
Upload:	paul-marlow
View:	14 times
Download:	3 times

Porirua Harbour Patterns and Rates of Sedimentation Report

Documents