EIS 21390
ABO1 9611
Marine aggregate proposal Vol 1 - n - 14 :L £ OS 21390 I 552 UN EOMARINE
15 53 >7 ci2 P0 R TJA'______ 6Ao te14524Hd 40 31 jJ 4 4 CO 75 S G < Gpfl(210sec 3P 25 M
IL
28
27 rT (Co ' I Lr 14 FS 37 46 ç5BnBuCkler 31 35 /8 54
23 14 SX oee B 32 43 GS
IT$âii1ggregate 63
72
%ps ': 3jo2Ch 3 -.:: : o102/ 7/ flY/)a 72 '2 B 0 T A N Y4' 26 / ! .35 46 •. 46 2 BAY '27 62 73
74
54
ISO - i Baily 77 / L 7/ ZIj J2 4 ,rOr/ulI / :BATE04BAY ,g /7 )23 3 34 73 45OSborn S Sh S 77 f73 S S Sh '1;5 11 JjiIA i. ) 3 e 7,9 49 22 C 77 63 - 72 73 r 7 54 /L200L •/./ /
- 64 1 73 lncrasuzg (ibOut .5½'(,n,uallv Mag. Varn. 12Th5'E. (/964) 22 14 Jtily....1993 S
M 73
I
F GEOMARINE I I I GEOMARINE P/L ACN 003 280 046 Consulting Coastal, Estuarine and Port Engineers I 81 Enmore Road, Enmore NSW 2042 ph:(02) 565 1377 fax:(02)565 1570
I LAWSON & TRELOAR PtyLtd ACN 001 882873 Coastal, Ocean and Port Consulting Engineers I 24/177-199 Pacific Highway, North Sydney NSW 2060 ph:(02) 922-2288 fax. (02) 922 1195
a A It Ri oil I Proposal 00fto
H Coastal rocesses I I Volume 1 of 3 I Prepared for Metromix Pty Limited
I GEOMARINE Report Number 50-002-RiO July, 1993 Published 1993 by WaveLength. Tress, a division of GEOMARINE P/L 81 Enmore Road ENMORE NSW 2042
Copyright © GEOMARINE P/L 1993 All intellectual property and copyright reserved.
Apart from any fair dealing for the purpose of private study, research, criticism or review, as permitted under the Copyright Act, no part of this report may be reproduced by any process without written permission.
This documentation was prepared for the purpose and exclu- sive use of METROMIX Pty Limited to accompany a Mining Lease application to the Department of Mineral Resources, NSW for approval to extract marine aggregate over areas described in the report and is not to be used for any other purpose or by any other persons or corporation.
GEOMARINE P/L and LAWSON & TRELOAR Pty Ltd accept no responsibility for any loss or damage suffered howsoever arising to any third party, person or corporation who may use or rely on this report.
No reliance should be placed on any data for any purpose other than for the purpose of this application for a Mining Lease.
National Library of Australia
Marine Aggregate Proposal Coastal Processes / A.F. Nielsen, D.B. Lord & P.D. Treloar.
ISBN 0 646 09863 2 LAWSON & TRELOAR GEOMARINE
I P Li Foreword
I The studies reported herein were undertaken by GEOMARINE and LAWSON & TRELOAR on behalf of METROMIX to provide I advice to R W Corkery & Co for the preparation of an Environmental Impact Statement to the requirements of the Director of the Department of Planning; those requirements I are detailed in Appendix I. Pursuant to the Environmental Planning and Assessment I Regulation 1980 this report addresses, inter alia, the Director's requirements for the following specific matters:
I • studies of the seabed and coastline conditions, prevailing wave and current climates, seabed bathymetry, to define implications for beach stability; 1 formulation of proposals for on-going environmental monitoring of effects on physical resources, processes I and pollution potential; identification of the likely effects of destabilisation or scouring of shipwrecks and the determination of I appropriate buffer zones;
. assessment and modelling of likely impacts on coastal
I geomorphology including sediment deposition, wave action and beach erosion. In respect of these studies Government consultation was un- I dertaken with the Public Works Department, Sydney Water Board, Department of Minerals and Energy, Maritime Services I Board and the National Parks and Wildlife Service.
A F Nielsen GEOMARINE I 1 D B Lord 9 GEOMARINE
I
P D Treloar I LAWSON & TRELOAR
I Forewort1 GEOMARINE LAWSON & TRELOAR
Summary
This report documents coastal processes studies relating to a proposal to extract marine aggregate from two areas on the inner continental shelf adjacent to Sydney. The northern area, extending offshore and across the ocean entrance to Botany Bay, lies adjacent to recreational beaches and the scientific marine research area at Cape Banks which has international significance as do the wetlands within Botany Bay. The south- ern area, offshore from the Royal National Park, lies adjacent to the popular recreational beaches at Marley and Wattamolla. Several shipwrecks are located adjacent to the proposed extrac- tion areas and are sites for recreational diving. The areas are fished commercially and are popular for recreational fishing.
For these studies there existed an extensive field data set unparallelled for any investigation of this type anywhere in the world: The seabed north of Port Hacking Point, encompassing the Cape Banks application area, had been mapped thoroughly and precisely by the Public Works Department, NSW with sounder, side scan sonar and seismic methods and extensive surface sediment sampling. Detailed sediment coring had been undertaken also nearby by the Department of Mineral Resources. Additional sampling and coring within the proposed extraction area were undertaken for this project.
The Providential Head area was mapped for this study with sounder and seismic surveys, and detailed surface sediment sampling and coring. The hydrodynamic data included continuous measurements of near seabed wave-induced and oceanic currents on the inner shelf off Sydney at sites in water depths of 24m (some 2 years), 60m and 80m (6 months), field observations of sand transport mechanisms at those sites, some 18 years of deepwater wave measurements offshore of Botany Bay, 12 months measurements of near surface and near bed oceanic currents at the Ocean Reference Station offshore at Bondi (and subsequent confirmation of these statistics with an additional 2 years of data) and long term wind data.
These data were augmented with site specific current and wave measurements. The wave and current climates were defined using these measured data and numerical modelling techniques to extend these data bases.
60 Summanj
LAWSON & TRELOAR GEOMARINE I n Li Detailed field descriptions of the adjacent beaches were made I and assessments of their variability were undertaken from detailed photogrammetric surveys produced from recent and historical vertical aerial photography. I Sediment transport assessments were made using field data and the latest computational techniques.
A review of the international marine dredging and aggregate extraction experience, studies and regulations exemplified the I various effects that could arise from seabed extraction and indicated that in all areas studied where extraction is permit- I ted it could be undertaken safely beyond 30m water depths. The criterion adopted for the design of the extraction plans was I that there should be no measurable change to the existing shoreline environment; specifically, the unconsolidated shore- lines comprising the beaches, the entrances to the estuaries I and the rocky reef communities. No change was defined as where calculated changes were an order of magnitude smaller than the equivalent natural variations as well as not being I measureable in the field using standard techniques. This led to the development of constraints on extraction that were I adopted in the design of the proposed extraction plans. Specif- ically, these constraints were that extraction be limited to: I a 5m below present seabed levels; a beyond the 35m isobath off beaches; and I a beyond the 25m isobath off the cuffed coastline. The analyses demonstrated that extraction as proposed would I not change the shoreline wave climates or sand transporting processes or result in erosion or realignment of adjacent beaches. The rates of sand transport in the areas proposed for I extraction were assessed to be very small. While there would be some slight changes to these transport rates in the extracted areas such changes would be negligible and insignificant. That I the time over which the extracted depressions would fill in from the surrounding seabed areas was calculated to be millennia indicated that, for all intents and purposes, the extracted I depressions would remain stable.
Programmes for necessary additional studies, management 1 practices and monitoring procedures have been outlined.
I Summaiy NO GEOMARINE LAWSON & TRELOAR
Contents Volume 1 I Foreword (i) Summary (ii) Preface and Acknowledgments (ix) I Units and Abbreviations (x) 1 Introduction 1 1 1.1 Description of Proposal ...... 1 1.2 Study Aims and Objectives ...... 3 1.3 Study Methods, Consultation and Reporting ...... 3 1.3.1 Case Studies Review ...... 3 1.3.2 Definition of the Existing Environment ...... 3 1.3.3 Constraints Assessment ...... 4 1.3.4 Assessment of Impacts ...... 4 1.3.5 Consultation ...... 4 1.3.6 Reporting ...... 5 1.4 Investigation Team ...... 5 1.4.1 Introduction ...... 5 1.4.2GEOMARINE ...... 5 1.4.3 LAWSON & TRELOAR ...... 6 1.4.4 HYDROGRAPHIC SURVEYS ...... 6 1.4.5 SOUTHERN AERIAL SURVEYS ...... 6 2 Case Studies 7 2.1 Introduction ...... 7 2.2 Projects ...... 9 2.2.1 Botany Bay, N.S.W...... 9 2.2.2 Kirra Beach, Qid. ...... 9 2.2.3 Kochi Coast, Japan ...... 9 2.2.4 Genkai Sea, Japan ...... 9 2.2.5 United Kingdom ...... 10 2.2.6 USA ...... 10 2.3 Analytical and Field Studies ...... 10 2.3.1 Introduction ...... 10 2.3.2 United Kingdom ...... 11 2.3.3 France ...... 11 2.3.4 Japan ...... 11 2.3.5 The Netherlands ...... 11 2.3.6 New Zealand ...... 11 2.4 Regulations ...... 11 2.5Summary ...... 12
64 Contents LAWSON & TRELOAR GEOMARINE I I 1 3 Physiographic Setting 13 3.1 Introduction ...... 13 3.1.1 Objectives ...... 13 1 3.1.2 Methods ...... 13 3.2 Geomorphology ...... 14 I 3.2.1 Inner Continental Shelf ...... 14 3.2.2 Coastline and Beaches ...... 14 3.2.3 Summary ...... 19 ' 20 3.3 Coastal Meteorological Processes ...... 3.3.1 Winds ...... 20 3.3.2 Waves ...... 20 I 3.3.3 Currents ...... 23 4 Sediment Transport 25 4.1 Introduction ...... 25 4.2 Nearshore Sediment Transport ...... 27 1 4.2.1 Introduction ...... 27 4.2.2 Field Data ...... 28 4.2.3 Analytical and Laboratory Studies ...... 31 1 4.3 Inner Shelf Sediment Transport ...... 32 4.3.1 Introduction ...... 32 4.3.2 Regional Sediment Transport ...... 32 I 4.3.3 Analytical Assessment at Cape Banks ...... 35 4.3.4 Analytical Assessment at Providential Head ...... 38 4.4 Summary ...... 39
5 Constraints Assessment 41 5.1 Introduction ...... 41 5.2 Generalised Criteria for the Design of I Extraction Configurations ...... 41 5.3 Coastal Process Considerations ...... 42 1 5.4 Sediment Transport Considerations ...... 46 5.4.1 Nearshore Sediment Transport ...... 46 5.4.2 Shelf Sediment Transport ...... 47 I 5.5 Generalised Constraints for the Design of I Extraction Configurations ...... 47
I Cotitenta GEOMARINE LAWSON & TRELOAR
6 Impacts of Extraction 49 6.1 Extraction Proposal ...... 49 6.2 Inner Shelf ...... 49 6.2.1 General ...... 49 6.2.2 Cape Banks ...... 52 6.2.3 Providential Head ...... 52 6.2.4 Shipwrecks ...... 52 6.3 Coastline ...... 53 6.3.1 Wave Propagation ...... 53 6.3.2 Beaches ...... 53 6.3.3 Rocky Shores ...... 56 7 Further Studies 57 7.1 Introduction ...... 57 7.2 Pre-extraction Investigations ...... 57 7.3 Management Practices ...... 58 7.3.1 Mapping ...... 58 7.3.2 Currents ...... 58 7.4 Monitoring Programme - Basic Elements ...... 58 7.4.1 Aims ...... 58 7.4.2 Regular Aerial Photograph Coverage ...... 58 7.4.3 Offshore Surveys ...... 59 7.4.4 Sediment Sampling ...... 60 7.4.5 Current Data ...... 60 7.4.6 Wave Data ...... 60 8 Conclusions and Recommendations 61 8.1 Preamble ...... 61 8.2 Conclusions ...... 61 8.3 Recommendations ...... 62 8.3.1 Constraints ...... 62 8.3.2 Further Studies and Monitoring ...... 63
(vi) Conterrt,c LAWSON & TRELOAR GEOMARINE
I Appendices
Volume 2
Appendix I Specification Appendix II Case Studies Appendix III Physiography I Appendix W Hydrographic Surveys Appendix V Photogrammetric Survey
Volume 3
Appendix VI Wave Regime Appendix VII Current Regime I Appendix VIII Nearshore Sediment Transport Appendix IX Shelf Sediment Transport Appendix X Impacts on Shipwrecks Appendix XI Glossary [
I
I
I
Coiuettts (vii) I GEOMARINE LAWSON & TRELOAR
Figures
1.1 Location Diagram Showing Study Region & Proposed Extraction Areas ...... 2 3.1 The Locations of Sand Bodies on the Sydney Shelf ...... 15 3.2 Beaches in the Cape Banks Study Region ...... 17 3.3 Beaches in the Providential Head Study Region ...... 18 3.4 Average Annual Wind Rose for Sydney ...... 21 3.5 18 Years Significant Wave Height Exceedance Probability Botany Bay ...... 22 3.6 Bondi Current Roses 65m Water Depth; 24:11:90 to 2 1:04:93 ...... 24 4.1 Definition Diagram of Terms Describing the Coastal Area ...... 26 4.2 Measured Beach and Nearshore Seabed Fluctuations ...... 29 4.3 Sediment Units of the Inner Continental Shelf of New South Wales ...... 30 4.4 Sandwaves in Providential Head Study Region ...... 34 4.5 Averaged Annual Rates of Alongshore Sand Transport on the Inner Shelf of the Study Region ... 36 4.6 Averaged Annual Rates of Cross-Shore Sand Transport on the Inner Shelf of the Study Region ... 37 5.1 Maximum Shoreline Changes on a Long Sandy Beach caused by 5m Deep Holes Extracted in Various Depths ...... 44 5.2 Alongshore Extent of Edge Effects from Holes Extracted to the 25m Isobath ...... 45 6.1 Cape Banks Proposed Extraction Area ...... 50 6.2 Providential Head Proposed Extraction Area .... 51 6.3 Locations of Nearshore Wave Climate Computations ...... 54
Tables
2.1 Criteria for Extraction in Various Countries .....12 6.1 Predicted Changes to Wave Climates from Proposed Extraction ...... 55
(viii) Coute,it,c LAWSON & TRELOAR GEOMARINE
Preface
The detail of the analyses and the bulk of the reporting re- quired that the study be presented in three volumes:
o Volume 1 Report Elviarine ..9qgregate Proposal - Coastal Processes 0 Volume 2 - Appendices I to V I - Specficati.on II— Case Studies III - TlIysiograpfuj 11/ - 5-[ydrograpfth Surveys V Pliotogrammetric Survey I a Volume 3 Appendices VI to XI VI— Wave Xcgime I VII Cun"ent R.cgime VIII Wearslwre Seuiment Transport IX - S/le(fSedinierit Transport Impacts onS/uipwrecks Giossary
Additional to page numbers, for ease of reference the page footers have been annotated with the appropriate report or appendix title. The page headers indicate which consultant in the main undertook the work. Figures in the appendices are at the end of each chapter. The references are given as the last chapter of each appendix. Acknowledgments
The Company provided for a peer review. This was undertaken by Associate Professor D N Foster, University of New South Wales (retired), who made many constructive suggestions and gave much helpful advice.
We gratefully acknowledge the provision of data by the Public Works Department, N.S.W., Sydney Water Board, Maritime Services Board, Department of Mineral Resources, Macquarie University and the Bureau of Meteorology.
Preface (i;) GEOMARINE LAWSON & TRELOAR
Units
cm ...... centimetres k ...... thousand kg ...... kilograms km ...... kilometre 1 ...... litre M ...... Million m ...... metres m2 ...... square metres m3 ...... cubic metres mg ...... milligrams mm ...... millimetres nm ...... nautical mile s ...... seconds t...... tonne y ...... year °(TN) ...... degrees (True North)
Abbreviations*
inter alia ...... among other things LS.L.W ...... Indian Springs Low Water datum V:H .....Vertical:Horizontal (slope measurement) > ...... greater than ...... less than +1 ...... plus or minus / (year, m etc) ...... per (year, metre etc) % ...... per cent a ...... statistical: standard deviation of a sample
* others defined where used
(i) Wilts GEOMARINE
1 Introduction
1.1 Description of Proposal
METROMIX Pty Limited (METROMIX; formerly READY MIXED INDUSTRIES Pty Limited a company owned I equally by PIONEER (NSW) Pty Limited and CSR Invest- ments Pty Limited) proposes to extract marine aggregate from two sites offshore from Cape Banks and Providential I Head south of Sydney (Figure 1.1) and discharge it at a terminal in Port Jackson for processing and distribution to the I fine sand markets in Sydney. The aggregate would be used in making concrete. In brief the proposal comprises: • extraction of three grades of marine aggregate within I the Cape Banks extraction area; . extraction of two grades of marine aggregate within I the Providential Head extraction area; and . the unloading, processing, loading and dispatch of I products from a terminal in Port Jackson. Extraction of marine aggregate would be undertaken with the extraction vessel skimming, with each pass, an average thick- ness of 0.2m off the sea floor over the extraction areas. The maximum depth of extraction proposed is 5m below the exist- ing sea floor. Marine aggregate extracted through the suction head would be pumped into the hold of the vessel in the form of a slurry (about 80-90% sea water and 10-20% aggregate). Excess slurry water containing some fines would be discharged near the seabed. The loading operation would take approxi- mately 2 hours 20 minutes and some 170 to 450 trips per year are envisaged. It is likely that extraction would be concentrated in water depths of between 25m to 35m at Providential Head, extending later to 55m to 60m at both Providential Head and Cape Banks. Assuming that both areas become operational, these operations could continue for some 40 to 50 years.
94athze iqgregue fProposat - Coastal Processes 1 GEOMARINE
it
JA1X
S?)
r V1N#A
A€ VA5EY
P6 / ROO6LE MA6A 60TN(
M6TOA'&LE
WL.58R ai BOTANY BAY
KLMNELL
WOPBORA
or air
aRosEa P
I
* Shlai-eck Locatan
39 Botht Coot th1d - P p Major Rood
. Proposed Extraction Boui&ry
2 0 2 4 6 8 Din
GEOLOGICAL AM) ENVtiGE&€NTAL CLTANTS PRO.ECT No 06 6th Fobsoy. 662 —i Coastal Processes Figure Location Diagram Showing Study Region & 1.1 Proposed Extraction Areas
2 I7vlathje Jggregate Proposal - Coasta(fProcesses I GEOMARINE I 1 1.2 Study Aims and Objectives The objective of these studies was to provide a detailed assess- I ment of the impacts of the aggregate extraction proposals on the coastal processes and the marine environment.
The studies were designed to achieve this objective by: reviewing case studies of extraction projects undertaken previously within Australia and overseas and reviewing overseas studies and regulations regarding extraction; defining the existing environment of the physical coastal processes throughout the study region including beach dynamics and sediment transport; defining constraints on extraction procedures to minimise the impacts on the coast that the extraction configurations could have; assessing the impacts of the extraction proposals on these coastal processes and, hence, defining any changes to the existing environment which could be expected should extraction proceed; and providing guidelines for monitoring the extraction operations to verify the predicted impacts.
1.3 Study Methods, Consultation and Reporting
1.3.1 Case Studies Review A review of marine dredging and aggregate extraction practice was undertaken with a view to identifying those projects where adverse impacts had resulted and to define the causes for those adverse impacts. The overseas experience had to be related to local conditions by the comparison of oceanographic conditions (waves and currents) pertaining to each site. The review was undertaken also with a view to documenting studies and reg- ulations that are applied overseas to dredging and extraction projects.
1.3.2 Definition of the Existing Environment The definition of the existing environment required: detailed descriptions of the physiography of the sites including bathymetry, sediments and geomorphology;
Mathie Aggregate Proposal - Coastal Processes 3 GEOMARINE
assessment of the historical changes to the physiography especially of the beaches; definition of the coastal processes including winds, waves, currents and sediment transport; assessment of the variability of coastal processes; and definition of the interaction between coastal processes and the environment.
1.3.3 Constraints Assessment A constraints assessment was undertaken to assist in the design of the proposed extraction plans with a view to: defining criteria for impact assessment; recommending measures to minimise impacts; recommending maximum extraction depths; recommending extraction areas; designing the extraction configurations; and designing monitoring programmes.
1.3.4 Assessment of Impacts The assessment of the likely impacts on the physical environ- ment by the extraction proposals was undertaken on the basis of the detailed understanding of the existing physical environ- ment, the experience of extraction proposals undertaken else- where and detailed analyses of changes to bathymetry and, hence, wave refraction patterns, current climates and sedi- ment properties. Generally, computer simulation models were verified with field data.
1.3.5 Consultation Throughout the course of the studies meetings were held in consultation with the relevant Government Departments and Instrumentalities. Details of the Government's requirements for the coastal process studies are in Appendix I - Specficcztion.
Aspects of public consultation include: o The presentation of technical papers documenting various results of the studies to conferences of the Institution of Engineers, Australia including: - the 10th and 11th Australasian Conferences on Coastal and Ocean Engineering; and
4 ulvlarine 4ggregate Proposal- Coastal Processes GEOMARINE
- 5th Australasian Ports and Harbours I Conference. a The presentation of the studies to seminars of The I Institution of Engineers, Australia and The Association of Consulting Engineers, Australia. a The presentation of the studies to the 1992 Local I Government Coastal Management Conference. a Presentations and discussions at forums of invited I senior academics as arranged by METROMIX. a The preparation of notes and responses to queries I from questions and issues raised at the various public forums arranged by METROMIX. I 1.3.6 Reporting The report has been structured so that the bulk of the data and the details of the studies are documented in fall in appendices. 1 The body of the report (Volume 1) summarises the studies and draws together their inter-relationship as appropriate.
1.4 Investigation Team
1 1.4.1 Introduction ' The studies were undertaken jointly by GEOMARINE and LAWSON & TRELOAR with specialist consultants HYDRO- GRAPHIC SURVEYS and SOUTHERN AERIAL SUR- VEYS undertaking the surveys as outlined in the following.
1.4.2 GEOMARINE The coastal process investigations and preparation of the de- I tailed report were undertaken mainly by GEOMARINE in con- junction with LAWSON & TRELOAR. GEOMARINE undertook the literature review and assessment of offshore I aggregate extraction experience world wide, the interpretation of the bathymetric and photogrammetric surveys, assessment of the geomorphology and stability of the sandy shorelines of I the study region from historical records, specified and directed the wave refraction studies and undertook the detailed model- I ling and assessment of the sediment transport processes re- lated to the beaches, shipwrecks and the seabed extraction proposals. These studies were synthesised to provide a detailed I understanding of the existing environment, to recommend guidelines for extraction, to assess the impacts of the extraction I proposed and to recommend a monitoring programme.
gvlarine aggregate Troposal - Coastal Trocesses 5 GEOMARINE
1.4.3 LAWSON & TRELOAR LAWSON & TRELOAR undertook the definition of the wave and current climates over the proposed extraction areas. This required data collection on local waves and currents. A meter designed to record waves and currents was deployed on the seabed at Providential Head for a period of eight weeks. Results from this meter were calibrated with simultaneous records from a similar current meter that had been deployed for an extended period offshore from Bondi for the Water Board as well as with simultaneous records from the long term Waveri- der buoy deployed offshore from Botany Bay by the Maritime Services Board. The record from the longer term current meter was used to synthesise a long term current climate for the study region using a computer model. In conjunction with GEOMAR- INE wave transformations were modelled and verified by field data in a wave refraction study which was used to define the wave climates at selected locations throughout the study region
1.4.4 HYDROGRAPHIC SURVEYS HYDROGRAPHIC SURVEYS undertook a nearshore bathy- metric survey including surface sediment sampling between Port Hacking Head and Garie. Subsequently, they undertook a detailed offshore bathymetric survey of the Providential Head proposed extraction area. This survey included some seismic profiling. A third survey was undertaken to define more precisely some large bedforms found within the study region during the second survey and to undertake additional shore-normal profiling. The three surveys were combined in a single bathymetric chart of the Providential Head proposed extraction area.
1.4.5 SOUTHERN AERIAL SURVEYS SOUTHERN AERIAL SURVEYS undertook the photogram- metric surveys for the sandy beach embayments within the study region. This involved the detailed plotting of both plani- metric and elevation information (where possible) from suit- able dates of available photography at each location.
Data capture comprised detailed large-scale plotting of plan information and detailed beach profiling for each date of pho- tography analysed. These profiles provided information on sand volume changes over the sandy beach areas.
6 !7v[arine Aggregate Troposat - Coastal Trocesse GEOMARINE
2 Case Studies I 2.1 Introduction
In the following discussion dredging refers to the winning of large volumes of sediment over relatively short time periods I for purposes such as beach nourishment, navigation and the like whereas extraction involves the winning of marine aggre- gate at much lower rates but over much longer time periods.
The extraction of marine aggregate in water depths ranging from less than lOm to greater than 35m has been conducted for many years in Japan, the United Kingdom (UK), and France. Current production of marine aggregate is dominated by Japan I and the UK (which exports to France, Holland and Belgium) which, together, account for 85% of global output. The marine sand and gravel industry in France has long been established, although it has remained relatively small. In the Netherlands there is some extraction from estuaries to win sand but, until very recently, there were no Dutch offshore aggregate opera- tions. Since the 1970s, however, offshore dredging to supply sand for beach and dune reconstruction has been undertaken as part of the coastal defence strategy to combat natural erosion of the Dutch beaches. Recently, offshore dredging for beach nourishment has been undertaken also in Queensland, I on behalf of the Gold Coast City Council, and marine dredging for fill has been undertaken for the development of Brisbane Airport. In New South Wales, dredging for port works and fill I has been undertaken for many years in Botany Bay as well as for navigation in other ports.
1 Marine dredging and aggregate extraction offshore may affect the coastline in the following ways: if too close to the shore it may create a depression such that beach sediment is transported offshore (known as drawdown) into the extracted area;
I7vfathze Jggregate TroposalT - Coa.ta[ fProc&cses Pi GEOMARINE
an offshore bank may protect the coastline, scattering or absorbing some of the wave energy, and the removal of such a barrier may result in beach erosion; the locally increased depths may alter the angle of incidence of waves and distribution of wave energy approaching the adjacent beaches thereby resulting in erosion and accretion; and the removal of offshore sediment may deprive the coast of a natural source of sediment and may create a trap for sand transported by waves and currents and littoral drift. Further, dredging or extraction can affect the seabed by: changing local currents thereby causing changes to sediment movement patterns; and changing the characteristics of the surflcial seabed sediments, thereby altering sediment transport rates.
Appendix II - Ca.ce Stiu[i&c provides a review of a variety of relevant projects and studies carried out to assess the physical effects of marine dredging and aggregate extraction on the coastal environment. Ten reviews have been included; three within Botany Bay, New South Wales and one at Kirra Beach, Queensland, two in Japan, two in the UK and two beach nourishment projects in the United States of America (USA). Included also are three reviews of overseas studies of impacts of dredging and aggregate extraction comprising field and laboratory studies, analytical studies and case studies under- taken by Hydraulics Research Ltd (UK) and the French Central Laboratory of Hydraulics, an analytical study of dredging off the coast of the Netherlands and a summary of the studies undertaken for the Draft Auckland Sand Management Plan (New Zealand). The relevant regulations in the various countries studied are presented also.
There have been other major dredging projects, undertaken for port works, which have resulted in significant and adverse shoreline impacts and not all dredging projects undertaken in other areas of Australia and elsewhere are purported to be presented herein. The case studies herein were chosen to exemplify the possible effects of marine dredging and aggre- gate extraction.
8 !lvlarine .4ggregate Troposal - Coastal Trocesses
I GEOMARINE I 1 2.2 Projects ' 2.2.1 Botany Bay, N.S.W. In Botany Bay, since the early 1950s, dredging in excess of some 50Mm3 has been carried out in shallow water, generally less than lOm depth. The dredging has affected wave heights I within sections of the Bay by changing wave refraction pat-S terns. This has resulted in either wave reflection or changes to the wave heights along the shorelines. In some areas there I have also been changes to the obliquity at which waves arrive at the shoreline and this has changed the patterns of littoral drift transport. The dredging has led to shoreline realignment I with erosion in some areas and accretion in others.
While the entrance dredging was undertaken in an area ex- I posed to full ocean wave energy there has been little evidence of infill from either littoral drift or beach drawdown over the 5 I years of survey records. There is no evidence of any long term nett sand gain or loss from the Bay caused by dredging.
I 2.2.2 Kirra Beach, Qid. The Kirra Beach dredging was carried out on the open coast in water depths seaward of the 18m isobath and out to the 28m I isobath for beach sand nourishment. The wave climate in this region is similar to that of the Sydney coast but the rate of longshore drift is large compared with the Sydney area. Little I information is available to assess the impacts of the dredging. However, as a direct result of the nourishment, beach accretion I has taken place and no erosion attributable to the dredging is apparent. This, however, is not unexpected because the dredg- ing was planned to be parallel to the seabed contours.
2.2.3 Kochi Coast, Japan On the Kochi coast, Japan, where the wave climate is lower I than that of the Sydney coast, extraction was carried out in shallow water depths (-5 to 8m) very close to shore (hOrn offshore). The shoreline retreated 50m behind the extracted I hole and the offshore contours shifted landward. Following hole infihling with littoral drift and river sediments the shore- I line accreted and the offshore contours moved seaward. 2.2.4 Genkai Sea, Japan In the Genkai Sea, Japan, which has a similar wave climate to that of Sydney, marine sand and gravel were extracted from depths of 15 to 20m. While a degree of shoreline erosion and accretion was taking place prior to extraction, it appears that
- Màthie f4ggregate Proposal - Coastal Processes 9 GEOMARINE
further shoreline erosion was caused by extraction. The hydro- graphic surveys indicated that significant drawdown" oc- curred in some areas resulting in the transport of sediment from the beachllandward side into the extracted depressions. Considerable infiling of experimental extracted depressions with sand from the surrounding bed occurred at depths of2lm. However, these holes were adjacent to the entrance of a river that was supplying very large quantities of sand to the coast. In water depths of 35m to 40m the holes retained their shapes.
2.2.5 United Kingdom In the UK studies of extraction of the Solent Bank, Pot Bank and Prince Consort region were inconclusive because of the complexities arising from a combination of interactions of tidal currents and waves, foreshore protection works and natural processes which did not appear to be satisfactorily separated into their respective effects. Extraction on the shallow Solent Bank (-.lOm water depth) appears to have affected the adjacent Solent estuary shores but the degree to which changes have occurred could not be ascertained. Elsewhere in the UK there have been no adverse effects of extraction licensed since 1950.
2.2.6 USA While the USA does not have a marine aggregate extraction industry, offshore dredging is undertaken in many areas for beach nourishment as well as for port and harbour works. For the nourishment of Revere Beach, Massachusetts dredging was carried out in water depths of approximately 2m some 50m to 200m offshore and demonstrated that the borrow site holes filled in rapidly. At Redondo Beach, California, where the beach was nourished with dredgings from between the 9m and 20m isobaths, the nourished beach remained stable and the dredged depression filled in with sand moving onshore from deeper water. In both cases there were no adverse impacts apparent on the beaches because they had been nourished with the dredged sand. However, it is noted that the wave climates in both areas are low relative to that of the Sydney coast.
2.3 Analytical and Field Studies
2.3.1 Introduction Studies of the effects of dredging and marine aggregate extrac- tion which have been undertaken overseas and which have synthesised field data, laboratory experiments and analytical assessments are summarised in the following.
10 ilvfarine Aggregate Proposal - Coastal Processes 1 GEOMARINE I 2.3.2 United Kingdom I Analytical studies of wave refraction undertaken by Hydrau- lies Research concluded that for the North Sea and English Channel coasts the effects of wave refraction point to a 1 nearshore depth limit for extraction of 18m. Field studies undertaken by Hydraulics Research conducted to determine the mobility of shingle at various depths at Worthing, England, I found that beyond the 18m isobath movement would be negli- gible at all times. i 2.3.3 France Detailed field, laboratory and theoretical studies undertaken I by French government authorities showed that in the Gulf of Gascony sand transport under wave action was practically non-existent in 30m water depth and extraction could be un- I dertaken safely beyond 2 im water depth. 2.3.4 Japan 1 Theoretical and small scale laboratory studies of wave refrac- tion on an infinitely long beach concluded there would be no effects from extraction beyond the 40m isobath and little effect at 30m. While the study exaggerated the effects of extraction, ' being based on simplified wave conditions, it demonstrated that the effects of extraction on wave action diminish rapidly with increasing water depth. Field studies indicated that ex- traction could be undertaken safely off beaches in 35m depth.
2.3.5 The Netherlands Theoretical studies comprising wave refraction and sand trans- I port assessments undertaken by the Dutch Laboratories for the coast of Holland concluded that there was little sand transport in 16m water depth and offshore dredging can be undertaken I safely beyond the 20m isobath. 1 2.3.6 New Zealand Only about 80,000 tonnes of marine aggregate is extracted annually in New Zealand. Various oceanographic and sedi- I ment studies support the recommendation of the Draft Auck- land Sand Management Plan to limit extraction to beyond 25m.
2.4 Regulations
Regulations overseas require, inter alia, various minimum operating depths and distances from the shore (Table 2.1). No rationale for the distances could be found; they do not appear to be justified on coastal engineering grounds.
£1vfaritwiggregate Proposal- Coastal Processes 11 GEOMARINE
Table 2.1 Criteria for Extraction in Various Countries
Distance to Minimum Averaqe Average Location Shore (min. Water Depth Wave Height' Wave Period2 N.S.W. NA NA 1.5m Gs Japan i GenkaiSea 1km 30m 1.7m 7s Seto Sea 1km 20m NA NA UK 600m - 18m-22m im 5sto8s France 3nm 20m 0.8m NA Nothor!ands NA 20m 1.5m 6s New Zealand NA 25m 1 m to 2m 5s to 7s NA: Not Available 1. Significant wave height 2. Zero-crossing wave period
2.5 Summary
Marine aggregate extraction has been undertaken overseas for many years. The industry has been monitored in Japan, the UK and France and detailed field studies, laboratory studies and theoretical analyses have been undertaken.
While being cognisant of the differing effects of various wave climates and sediment characteristics, the case studies have shown that: a Dredging undertaken in water depths less than 15m may change the shoreline rates of littoral drift and patterns of erosion and accretion (e.g. Botany Bay). a Beach erosion from drawdown may occur in areas where extraction is undertaken in water depths of 22m or less (Genkai Sea, Japan). However, this conclusion was based on the results of an experimental hole dredged offshore from a river supplying large volumes of sediment to the coast. a Extraction in shallow waters ( 12 7vfarine 2ggregate fProposa[- Coa.ctatTrocesses GEOMARINE 3 Physiographic Setting 3.1 Introduction 3.1.1 Objectives The beaches of the study region undergo large natural fluctu- I ations in response to the variations in winds, waves and cur- rents. A detailed description of the physiography of the study region and its beaches is in Appendix III TIlysiagrapluj and was I prepared to: . define the variability of the existing environment with I which the predicted impacts of the proposals could be compared; and I • provide detailed descriptions and surveys as a basis for monitoring programmes with which the measurement of any future change could be compared. 3.1.2 Methods A physical description of the seabed has been provided in the I form of a detailed hydrographic survey (Appendix IV - J-[yIro- graphic Surveys). This was prepared both from surveys undertaken specifically for this project and using pre-existing survey data. I The survey information was augmented by seismic reflection profiling and sampling of both surface and sub-surface sedi- I The onshore features were mapped in the field and from avail- I able vertical aerial photography (Appendix V Plwtogrammetrie Survey). This description incorporated detailed studies of certain ' areas which were previously prepared for other projects. The stability of the sandy beach embayments was defined through precise mapping and analysis of data obtained from available historical mapping quality photography. The physical processes of winds, waves and currents which 1 shape the seabed and the shoreline have been examined in Marine Aggregate Proposal - Coastal Processes 13 GEOMARINE detail and a climate has been defined for the study region. This included both the assessment of long term average conditions, annual variability and the assessment of severe events. Waves (Appendix Vi Wave Regirne) and currents (Appendix VII Curreiit1 3.2 Geomorphology 3.2.1 Inner Continental Shelf The continental shelf is narrow and the inner continental shelf is steep; water depths in excess of 80m lie within 3km of the coast. The shelf surface seaward of 80m is relatively flat. There are two distinct sand bodies on the Sydney shelf in water depths ranging from 25m to 75m (Figure 3.1). One large sand body is found across and adjacent to the estuary entrances of Port Hacking-Bate Bay and Botany Bay in the south. A smaller sand body is found at the ocean entrance to Port Jackson. Generally, the sand on the sand bodies is typically fawn-grey, fine to medium-grained and moderately sorted quartzose ma- rine sand with a highly variable shell content (10% to 60%). Reefs off Maroubra and Bondi extend to 70m water depth between the sand bodies and between these reefs the inner shelf sediment varies from fine-grained, grey coloured sand with up to 20% mud and some 40% shell to medium to coarse- grained, orange coloured sand with 40% shell. The lower parts of the major estuaries of Port Jackson, Botany Bay and Port Hacking-Bate Bay are marine dominated and contain large flood-tide deltas composed of marine sand. 3.2.2 Coastline and Beaches The Sydney coast is characterised by bedrock cliffs and head- lands interspersed with small pocket beaches and narrow, drowned-valley re-entrants infihled with Quaternary sedi- ments. Water depths around the headlands and along the cliffed sections of coastline are such that the beaches are compartmented with little or no alongshore littoral sand sup- ply or loss. Any sand transport alongshore through the study region would only occur as a result of wave stirring combined 14 gvtarine 2ggregate Troposal - CoastalTrocesses I GEOMARINE 1 I FGT 121 SYDWY Cgai I 5 - Jf7 I 05 VOctL6F05A — JA2EY JN SYUWY I MAAFXXVLLE RANDWCK 104GWORD I I I WTAW I WTRMtL€ §6 BOTANY I BAY tUNELL 1 §6lO1A §6 I air 6UEY T ?44,G I B01hnefrlc Contr (hterd • 0-0 20 LMOLA IdoJor Rood I 2 0 2 4 6 8 Oki I GEOLCGCAL NsD EN per4.ENTAL CLTN4TS PROECT No §6 I 62. Fth.aiy. 1992 Coastal Processes I Figure The Locations of Sand Bodies 3.1 I on the Sydney Shelf I 9tfarine i4ggregate Troposa( - Coastat !Processes 15 GEOMARNE with longshore currents in deeper water as there is no trans- port of littoral drift by breaking waves at the shoreline from one embayment to another. The beaches of the Port Jackson to Bate Bay coast (Figure 3.2), typically, are narrow and generally comprise relatively thick and uniform beach sand deposits capped by dunes. Old head- land dunes occur in many places along the coastal cliffs. Bondi Beach is backed by a very large but, what is today, stabilised dune field. Tamarama Beach, Bronte, Clovelly Beach, Gordons Bay, Lurline Bay and Long Bay, however, comprise relatively thin beach deposits abutting bedrock. Stud- ies of both Bondi Beach and Coogee Beach show both embay- ments to be essentially stable, closed sediment compartments. Surveys of Maroubra Beach show little long term change with the water line now being at about the same location as it was in 1930. Foredune re-stabilisation at the southern end has resulted in the foredune now being located further seaward. It is likely, therefore, that it would be more susceptible to erosion during storms in future than has been the case in the past. Within Botany Bay, Lady Robinsons Beach and Silver Beach preserve a record of coastal accretion over the past six thousand years in the form of a succession of beach ridge deposits and a massive, but very old, dune field extends from the northern margin of Botany Bay to Sydney Harbour. The beaches within Botany Bay have undergone considerable changes in recent times that can be attributed, in part, to works undertaken within the Bay, in particular dredging and reclamation, and to subsequent rectification and beach improvement works. For example, to arrest erosion of Silver Beach an extensive groyne field has been constructed which has been supplemented with sand nourishment. Yarra Bay and Frenchmans Bay have been affected by the construction of the Port Botany revetment and have been modified by rectification works also. Kurnell Isthmus, between Bate Bay and Botany Bay, corn- prises a massive dune deposit that has been modified exten- sively by sand extraction. While studies show that there has been some sand loss from aeolian transport landward, revege- tation of the foredunes has now prevented this and recession of the shoreline is likely to have been stopped. No offshore sand losses or supplies have been identified. 16 Marine aggregate Proposal - Coastal Processes GEOMARINE 1/;' f\•. J , 52 II I / "I3ondi B. amaraffi It Coogee L Luhne 10 Note nway even 1O93 Maroubra B. / o n port work ( 1 / 1 2, are ot shown \J. 2 ,s d 14 _-L dy2 Robiisois1 .25 Is' 307 2 •••2; a3 I Long Pay (Malabar) '18 B(ij) 2 s .I.7 II c/so Au,. 'ssYara/ B - - • ,.-1 "P 2 2, - ,. J/(.fr 07k.) ,'' ' 1 0 .11 •0 I 2 Frencbman 1 2, 2 ••. 5, 3 3y' 0 / 17 fS 'SilVer 04 I 12 41 U 11 i B I yCoC / S 5 I •5 17 / - - 51) 4. "f IS C—I flli Ui 1Cs.pk.I 01.1 l2'I 2 __' s 22.2 it; ; 35 45 . 35 011 Cronullao I ) 7> I 8 sb I 441 Coastal Processes Figure Beaches in the Cape Banks 3.2 Study Region LMaritte aggregate Proposal Coastal Processes 17 GEOMARINE -74,• •Y d' iTaren A Weeney / > \ : thcote(j'' .•: p L/ °°Ly;lJlbb2 B. 55 M ..,. 34 ,5iS• ra 8each a, P0/rn br 8ur111119 M 55 BulgoBurn Pims 34 43 ii 29 i -Ue11 Ho14 29 Coastal Processes Figure Beaches in the Providential 3.3 Head Study Region 18 7vfarine Aggregate Proposal - Coastal Processes I GEOMARINE I The beaches from Port Hacking to Burning Palms (Figure 3.3) I comprise small embayments and pocket beaches formed within slight indentations or valleys cut through the cliffs by small I creeks draining to the coastline over geological time. The more exposed ocean beaches, such as Marley Beach, un- dergo large fluctuations resulting from storms with measured 1 changes in subaerial sand volume of up to 200m31m (cubic metres of sand per metre length of beach above mean sea level) and lateral shoreline movements of up to 20m. Over the period I of photographic record the large sand dunes behind Marley Beach have moved landward. At the same time, the beach face I and foredune have prograded seaward. This is not a result of the supply of additional sand to the beach but has resulted from redistribution of the existing sand reserves and a general I lowering of the deflation area behind the fore dune. Sand trans- ported landward is returned to prograde the beach face by the I creek discharging at the northern end of the beach. Little Marley Beach and Wattamolla Beach are deeply embayed and are protected further by extensive offshore reefs. I They are less responsive than Marley Beach to storm events with typical subaerial fluctuations being less than 50 to I 100m3Im. Little Marley Beach has undergone no significant change over the period of analysis. At Wattamolla Beach, the ' beach face and dune system have moved landward under wind action into the lagoon. This has been measured as an apparent recession of the shoreline. However, there is no change in the volume of sand in the beach and lagoon system and, as at I Marley Beach, it is likely that the changes observed are a part of a sediment re-circulation system between the lagoon, the beach face and the dunes with little or no nett change over time. Thebeaches at Garie and Era are pocket beaches. Subaerial beach fluctuations from storms have been measured at 120m3/m. Historical subaerial beach fluctuations at Burning Palms have been small and less than 40m3/m. 3.2.3 Summary The shoreline of the study region comprises a high cuffed I coastline with small sandy beaches contained within indenta- tions and drainage channels within the bedrock. Water depths around headlands and along the cliffs are such that the beaches are compartmented with no alongshore littoral sand supply or loss. i gv(arine 2ggregate Tmposat- Coastal Trocesses 19 GEOMARINE Individual beach compartments exhibit fluctuations as a result of wave activity and reworking caused by winds and creek flows; they were found to be, essentially, stable over the period of historical analysis. The changes observed lay within the range of natural fluctuations expected to result from the ambi- ent weather conditions and human interference. 3.3 Coastal Meteorological Processes 3.3.1 Winds The N.S.W. coast experiences a complex wind regime (Figure 3.4). Offshore westerlies dominate in winter and diurnal sea breezes and land breezes dominate in summer; the latter occur from early spring to early autumn. Storm winds tend to ap- proach the coast from the south to southeast being generated by low pressure systems off the east coast or as southerly busters associated with the eastward migration of fronts. The cliffed coast of the study region may have a considerable influence on local wind speed and orientation. East to south- east winds approaching the coast are likely to be steered towards the northeast at the shoreline by the high cliffs. Data for the correlation of wind-driven currents measured at Provi- dential Head were obtained from a meteorological station operated by Macquarie University at La Perouse. 3.3.2 Waves The Sydney coast experiences a high-energy wave regime. Waves from the southeasterly quarter are more frequent in winter whereas northeasterly waves tend to be more common in summer. The largest waves approach from the southeasterly quarter. Long term (>20 years) data are available from a deep water Waverider buoy located offshore of Botany Bay and operated by the Maritime Services Board. Analysis of 18 years of these data shows that the significant wave height exceeds 1.5m some 50% of the time (Figure 3.5). The significant wave height may peak at about lOm during very severe storms. While measured wave spectra may exhibit multiple peaks reflecting the co-existence of sea and swell, the average wave period associated with the significant wave height is 8s (ap- proximately) and the average zero crossing period is some Gs. However, longer periods can be associated with severe storms and distant cyclones have generated swell with periods of 18s. 20 Marine 2ggregate fProposa[ - Coastal Troc&cses GEOMARINE SYDNEY CaIrn 21.1% (3.4% in each limb w 16.2 E 11.3 S 10.4 SCALE OF PERCENTAGE OF OCCURRENCE AND SCALE OF WIND SPEED (KNOTS) Direction of wind flow Reference: "Coastal Evolution and Coastal Erosion in New South Wales " Report prepared for the Coastal Council of New South Wales Chapman, D.M., Geary, M., Roy, P.S. & B.C. Thom. 1982 Coastal Processes Average Annual Figure Wind Rose 3.4 for Sydney Mathze aggregate Troposa[ - Coastal fProeesses 21 GEOMARINE Coastal Processes Figure 18 Years Significant Wave Height Exceedance Probability 3.5 Botany Bay 22 9vlarine 2ggregate Proposal * CoastilfProcesses GEOMARINE A range of wave characteristics was measured in the field at Providential Head allowing the verification of computer mod- elling techniques to transfer long term Waverider buoy statis- tics accurately over the study region. These techniques were developed originally by advisors to the dredging regulatory authorities in the UK and have been updated and modified to suit local conditions. They have significant advantages over physical models in that: the numerical model allows precise assessments of very small changes not possible with physical models; random sea states comprising broad spectra of wave frequencies, directions and heights can be tested; various seabed configurations can be economically tested once the model is established. The wave climate and analyses used in the study are defined in Appendix VI - Wave Regime. 3.3.3 Currents Currents in the ocean are caused by several physical processes I that vary over time and with location and can be tidal, wind- driven, coastally trapped waves (CTW), large scale oceanic currents such as the East Australian Current (EAC) and its I associated eddies, internal waves and currents induced by surface waves. I The oscillatory currents (tides, CTW, surface and internal waves) exhibit a wide range of periods from a few seconds to many days. Quasi-steady currents (wind-driven, EAC) may 1 persist for weeks (EAC). The currents can exhibit different structures over the water column; wind-driven currents and [1 the EAC are much stronger at the surface whereas CTWs are more uniform over depth. The simultaneous occurrence of these current structures may result from a single meteorolog- I ical event; for example, a CTW may be induced by the passage of a strong front across Bass Strait which may induce also wave-induced and wind-driven currents coincident with the Li passage of the CTW offshore of Sydney. A considerable measured data base on Sydney shelf currents I at various water depths was available (e.g. see Figure 3.6) and it was possible to generate a statistical current climate over the I study region using numerical modelling techniques calibrated with in-situ field data obtained in the proposed extraction area at Providential Head. The current climate over the study I region is defined in Appendix VII - Curretit Rcgime. I Mathie .fggregate Troposat- Coastal Trocesses 23 GEOMARINE N 11 000.4 0-20 4 04-30 0-0-00 06! 0 I 0' SPEED (/o) 3 10 20 30 40 50 60 f N 11 00-02 0.2 - 0 4 0406 0-6 - 06 06-1-0 q0' C SPEED (m/) 0 10 20 30 40 50 60 Coastal Processes Figure Bondi Current Roses 65m Water Depth; 24:11:90 to 21:04:93; 3.6 Top:15m Below Surface; Bottom: 15m Above Bed 24 I71athie aggregate Proposat - Coastat Processes I GEOMARINE I 4 Sediment Transport I 4.1 Introduction In the study of coastal sediment transport it is convenient to define zones relating to the principal hydrodynamic forces influencing the transport (see Figure 4.1). Here we define the nearshore zone as that pertaining to beach face processes and includes the surf zone, where sediment motion is dominated by I currents generated by wave shoaling and breaking, and the region offshore to where sediment transport is dominated by I nearshore currents, including rips, and the hydrodynamic forces related to the asymmetry of wave motion. I Beyond the nearshore zone, where beach face processes are dominant, we define the inner shelf zone where the asymmetry of wave motion is negligible and wave motion acts simply as a I stirring parameter moving sediment back and forth; the main transporting agents being ocean currents generated by a vari- ety of mechanisms. Additional to these general zone definitions I is the estuary inlet zone where tidal current velocities may I dominate sediment motion. While it is convenient to undertake the studies required as being specific within each zone (and as is almost universally - I presented in texts and references on coastal sediment trans- port), it is recognised that in nature there are no distinct borders to these sediment transport zones and that there would I be significant variations in the extent of these zones from time to time as the coastal processes of winds, waves and currents vary. Nevertheless, given the different nature of the sand I transporting mechanisms within each of the zones as defined, the sediment transport in the study region has been assessed I in relation to these zones. Sediment transport at the estuary entrances is considered in the inner shelf zone as defined by I the water depth. 94farine 2ggregate fProposa(- Coastal Trocesses 25 CD Coastalarea P — Coost Beach or shore Neorshore zone (defines area of n.arshor. currents) owe Backshore Foreshore Inshore or shoreface Offshore ,- ( extends throuçh breaker zone - Bluff Surf Zone or - escarpment $•rms Breakers Beach scorp f Kh water IeL Crest of berm - Ordlnpry w wot r level Plunçe point Bottom Reference: "Shore Protection Manual" Coastal Engineering Research Centre US Army Corps of Engineers 1984 ------I GEOMARINE I 4.2 Nearshore Sediment Transport 4.2.1 Introduction Often the beach is perceived to be the sandy area between the water line and the dunes. The overall beach system, however, may extend from the hind dunes some several hundred metres I landward of the water line to some several kilometres offshore in water depths typically of some twenty metres. I The beach comprises unconsolidated sands which can be mobilised under certain meteorological conditions. The dy- namic nature of beaches is witnessed during storms when I waves remove the sand from the beach face and the beach berm, and in severe cases from the dunes as well, and transport it by a combination oflongshore and rip currents beyond the breaker I zone, where it is deposited in the deeper waters as sand bars. As the offshore sand bars build up, the waves break further I offshore until, eventually, sufficient wave energy is dissipated in the surf zone to prevent any further beach erosion. This I erosion process may take place over several days to months. Ocean swell following storms replaces the sand from the off- shore bars onto the beach face where onshore winds move it back onto the frontal dune. This beach building phase typically may span many months to several years. The processes causing this onshore sand transport result from the asymmetry of wave I action as swell waves propagate into shoaling waters. As waves move into shallow water their wavelength decreases, the wave height increases and the wave profile becomes asymmetrical. I At the seabed the back and forth motion of the water under wave action becomes stronger in the onshore direction but for I shorter durations whereas the offshore velocities decrease but last longer. Further, a mass transport of water is induced near the bed in the onshore direction resulting from changes to the I near-bed orbital motion of the water. On flat beds this asym- metry of wave motion caused by shoaling waves would tend to I result in sediment transport onshore. The reason that beaches exist, therefore, lies in the shoaling I action of waves and the transport of sand onshore against the coastline. There would be a limit to the amount of sand that can build up against the shore and the increase in beach slope I caused by onshore sand transport would be balanced, inter alia, by gravitational forces; this being the basic concept of the so I called equilibrium profile. !Marine 2,regate Tivposa( - CoaswfTrocesse.c 27 GEOMARINE I There is a considerable body of data available from various sources which allows an accurate definition of the limits of beach movement to be made and this is presented in detail in I Appendix VIII - A[cars1iore Sediment 'Tran.sport. The following sum- marises the assessment of the extent of subaqueous beach fluctuations in the study region and, in particular, the seaward limit beyond which any extraction would not result in beach drawdown or loss of sand from the beach. This assessment is based on a number of independent studies both within the study region, in nearby locations, at several other locations along the Australian eastern seaboard and in several areas overseas. The data sources and assessment comprise: direct measurements of changes to beach profiles by I survey methods including the use of seabed stakes; variations in sedimentological and biological data reflecting hydrodynamic boundaries; sediment tracer studies; beach slope measurements; rip measurements; and analytical and laboratory studies. I I 4.2.2 Field Data A considerable body of field data comprising actual measure- ments of seabed fluctuations using sounder surveys taken prior to and following storms and seabed stake measurements over extended study periods both on the Australian eastern sea- board and overseas show little beach profile fluctuations be- yond the 15m isobath (Figure 4.2). Extensive studies at many sites along the New South Wales coast have identified two distinctive sediment units on the innermost part of the continental shelf; Nearshore Sand (Inner and Outer) occupies the shoreface and somewhat coarser Inner Shelf Sand (previously referred to as Shelf Plain Relict Sand) occurs further seaward (Figure 4.3). In general terms, the two sediment units correspond to those parts of the seabed consid- ered to be active (the beach) and palimpsest (inner shelf). The sedimentological data show consistently distinct changes in the characteristics of the sediments with water depth. These changes included changes in grain size, sorting, carbonate content and colour. 28 !7(athze Aggregate Troposat - CoastalTrocesses I GEOMARINE 40 Legend + Stockton B4t (P.W.D. 1977) gal Palm Beach Stakes (P.W.D. 82309) 0 Gosiord Beaches (Higys & Nittim 1988) 3.0 * Gold Coast Beaches (Chopmon & Smith 983) <) Gold Coast Beaches (MacDonald et oL 1973) E A Danish North Sea Coast (Mongor et al. 984) 2.0 + +0 Envelope of actual recorded chcnyes in seabed levels on vorous beaches 0 + 0 V \' - * • 0 0 - '4 I I 0 5 10 15 20 25 30 Water Depth (m) Coastal Processes Measured Beach and Nearshore Figure Seabed Fluctuations 4.2 (see Appendix VIII - WcarsfwreSeIimeitTransport for references) Ma,ine qNregate Proposal - Coastal Processes 29 GEOMARINE Reference: "Coastal Evolution and Coastal Erosion in New South Wales". Report prepared for the Coastal Council of New South Wales. Chapman, D.M., M. Geary, P.S. Roy & B.C. Thom. 1982. RidlO -J U) 0 0 0 (N (0 LLO -.Jz UJ < IU) 00 Ui 0 0 z U) Coastal Processes Figure Sediment Units of the Inner Continental Shelf 4.3 of New South Wales 30 9vlathu 2ggregate Proposal . Coa.ctat Processes I GEOMARINE iii I The depths at which changes have been detected from the Gold Coast, Queensland to the south coast at Shoalhaven and on the northeast coast of New Zealand show a remarkable consis- tency. While there are variations along beaches, with marked changes occurring in shallower depths on beaches where there I is some protection from the offshore wave climate, generally it has been shown that for beaches exposed fully to the offshore wave climate, distinct sedlimenthlogical differences are found consistently at water depths of about lOm to 15m (generally), defining the boundary between the Inner and Outer Nears ho re I Sands and 18m to 26m (generally), defining the boundary of the Inner Shelf Sands. At Maroubra Beach and Malabar Beach near Cape Banks the sedimentology changes at about 27m U water depth. Off Marley Beach the sediment grain size changes at about 22m water depth. I Surveyed profiles normal to the beaches of the study region at Providential Head indicated a consistent geomorphologic dis- continuity at about 25-28m water depth, seaward of which the 1-1 seabed slope became very flat. This suggested that the I nearshore zone does not extend beyond the 28m isobath. Studies of rips that occur during severe storms, carrying sand offshore, showed that these sediment plumes at Narrabeen I Beach (March 1976), Palm Beach (June 1976), Curl Curl Beach (May 1951) and Wamberal Beach (June 1974) did not extend I offshore beyond the 19m isobath. The French Central Laboratory of Hydraulics has con- ducted various field studies on the possible movements of I sediments on the continental shelf in respect of the extraction of marine aggregates in the Gulf of Gasgony and has concluded I that the displacement of sand under swell less than 7m wave height is practically non-existent in depths of 30m and very weak in depths of 20m. In 15m depth, however, sediment movement became appreciable and increased very quickly ap- proaching the coast. I 4.2.3 Analytical and Laboratory Studies Considerable analytical research verified by laboratory and, in some cases, field studies has been undertaken in the area of onshore/offshore sand transport; that is, beach response to I storms and the subsequent beach recovery. I I7(athze Aggregate fProposat - CoastatTrocesses 31 GEOMARINE The maximum depth limit of offshore sand transport on the beaches of the study region during extreme storms (such as those that occurred in 1974) was calculated to be 26m. The calculations of onshore sand transport under low swell waves showed that the potential for onshore sand transport falls rapidly beyond lOm water depth and beyond 25m water depth it is virtually non-existent, being some two orders of magnitude lower than that at lOm water depth. These results compared well with those for the Dutch North Sea coast where the potential rate of onshore sand transport there was calcu- lated to fall very rapidly with increasing depth to being virtu- ally non-existent at 16m water depth. 4.3 Inner Shelf Sediment Transport 4.3.1 Introduction Sediments on the seabed can be agitated and transported under the action of waves and currents. On the inner continen- tal shelf where depths become shallow (<80m say) waves can be important agents in sediment transport. Transport can be effected under the back and forth wave motion, placing the sediment into suspension, with nett transport occurring in the presence of a superimposed steady current. Notwithstanding the enhanced potential for sand transport under wave action, from time to time quasi-steady currents on the inner shelf off Sydney can be strong enough alone to transport sand. Such currents can result from the occasional excursion onto the shelf of the East Australia Current, or may be Coastally Trapped Waves or Internal Waves. In respect of sediment transport on the inner shelf there may be an inter-dependence in the occurrence of waves and cur- rents. For example, during severe storms the strong winds that generate large seas may be the agent also for determining the current structure directing sand transport on the shelf. The sand transport regime on the Sydney shelf is described in detail in Appendix IX - SIietfSe&ment 'Transport. 4.3.2 Regional Sediment Transport The broader understanding of sand movement on a regional scale encompassing the inner continental shelf off Sydney and its adjacent estuaries has been obtained from the analysis of 32 9vlathie i.ggregate Troposal - CoastatTrocesses GEOMARINE F~ data and reporting given in studies carried out by various government agencies and universities and from field studies for this project. This provided the context within which sedi- ment transport over the study region was evaluated. Sandwaves were evident in 25m to 60m water depths surveyed in the Providential Head study region (Figure 4.4). They occur I also at the entrance to Botany Bay and have been found in deeper water off Bondi. Dimensions of the 56 sandwaves mea- I sured varied but lengths reached 1,000m (450m average) with heights up to 5m (1.6m average). The sandwaves surveyed appeared as transverse sandwaves with their crests approximately perpendicular to the southerly current direction. Here, the East Australian Current causes a strong but intermittent and predominantly southerly flow. This is reflected in the asymmetrical shape on most of the sandwaves observed. Seismic records from the Providential Head study region (presented in Appendix IV) are characterised by steep southward-dipping events immediately below the seabed reflection, interpreted as indicating south- ward mobility. Sandwaves do not indicate strong seabed currents or large rates of sand transport; they are a reflection of the turbulent flow structure which, for vertical eddies, is restricted by the water depth to which their scale is related. Current speeds need only exceed the threshold speed for sand transport occasionally so that these bedforms can be established. The sizes of the sandwaves infers sand volumes of several hundred cubic me- tres, indicating that the sandwave structures are not changing with fluctuations in the current. Computed bedload sand trans- port rates indicate rates of sandwave translation in the order of centimetres per year. That the strong currents are intermittent results in the sandwaves being moribund for much of the time, allowing the colonisation of the seabed surface by shelly species. It allows also some degradation of the sandwave form. The slow passage of sandwaves over shelly seabed deposits could produce the shelly layers found in the cores. Examples of sandwaves shown in Figure 4.4 indicate that the extent of reworking of the seabed surface can be in the order of metres, albeit at the very low rates indicated. The depth of reworking, however, could be variable along any isobath. 1'sIaiine Mqrgate fPivposa( - Coastal 33 GEOMARINE - PT HACKPOW 9 6' 54 Legend BEACH - - Asrretrlcd and direction of lee okle \ cresl of syrretrical bedfom 8225000 6225000 29 Ufl6 BEACH - PRO V0294T00}€AD - U*TTMCL*&04 45 VAflNLA 4) 2 top -6220000 6220000 OJAO 4 41 40 - 8 TN - OJACJ HE 15 - AO1 500 0 - eoo Do 2000 2oo 20000 17 - 36 6215000 I I I I I I I I - 5,7 Coastal Processes Figure Sandwaves in A Providential Head Study Region 34 IMathte Aggregate Proposal - Coastat Processes GEOMARINE I Wave-generated ripples are found on the seabed of the inner I shelf off Sydney to depths of 60m (approximately). These cause reworking of the seabed surface to depths of 0.2m (approxi- mately) over much shorter time frames; on a day-to-day basis I in 25m water depth and for some 25% of the time in 60m. Based on 6 months of continuous field data on near-bed cur- I rents and waves at several seabed locations on the inner shelf offshore of Sydney, the gross sediment transport over that period was calculated to have been directed both northerly and I southerly, with a small nett bias to the south. The rates of sand transport calculated were low but it was considered that higher I rates to the north could occur during storms. There is no conclusive evidence of any significant sediment I transport along the continental shelf and into the estuaries of Port Hacking and Bate Bay and it is considered that sediment I infeed from the shelf into the estuaries here is negligible. There is no conclusive evidence of either any significant trans- port of sediment into or out of the entrance at Botany Bay. S Detailed field and model studies of tidal circulation within Botany Bay indicated that the tidal velocities across the en- I trance to the Bay were closely balanced and monitoring of the channels dredged at the entrance to the Bay (Appendix II - Case Stu4Ties), albeit over only a short interval from 1981 to 1986, I has not indicated any significant infilling at the entrance to the Bay either from seaward or landward. I 4.3.3 Analytical Assessment at Cape Banks Computations of sediment transport were undertaken at a suite of sites over the proposed extraction area and covering a I range of water depths from 45m to 65m. The computations indicated generally that the rates of longshore sediment trans- I port in the proposed extraction area are virtually negligible; the annual gross transport rates being in the order of it/rn ( I The cross-shore (shore-normal) transport rates were calculated to be lower than the alongshore rates and were close to being evenly distributed both offshore and onshore (Figure 4.6). I However, these computations did not take account of the slope of the inner shelf which would reduce further the small nett I onshore rate as calculated. 7vfarine A®ate !Propcsa[ - Coa.cta(Troeesses 35 GEOMARINE 36 ±7vlaiine Aggregate Troposat - Coastal Processes I I GEOMARINE I 5 - I I 4 - C) 03 I cI I C) I I 04 1 Cd I I I I I I I 20 Water Depth (m - I.S.L.W.) I Coastal Processes I Figure Averaged Annual Rates of Cross-Shore Sand Transport on the 4.6 Inner Shelf of the Study Region Marine aggregate Proposal - Coastal Processes 37 GEOMARINE The annual gross rate of sand transport calculated at a point located in the centre of the entrance to Botany Bay was 22.4t/m with a nett rate of 1.2tJm calculated to be directed into the Bay. This rate is in the order of that determined from limited field measurements (see Appendix IX - Slietfsediment 'Thin.cport). The analysis of a storm similar to that of the May 1974 storm, while undertaken on hindcast wave and current data, indi- cated that rates of transport during storms can be an order of magnitude higher than the annual synoptic transport rates. At 55m water depth the gross transport rate for the storm was calculated to be 1.1tJm. However, based on the calculated recurrence interval of the storm, this transport equates to 1.3% of the gross annual longshore rate of sand transport. The equivalent annual rates for the 1978 storms were found to be negligible in these water depths. 4.3.4 Analytical Assessment at Providential Head Computations of sediment transport were undertaken over a suite of sites from 25m to 55m water depth over the proposed extraction area. The computations indicated generally that the rates of sediment transport on the inner shelf are low, partic- ularly beyond the 35m isobath where they become virtually negligible; there the annual gross transport rates were calcu- lated to be in the order of 1 t/m (Figure 4.5). For the annual statistics, the synoptic picture showed that transport parallel to the shoreline is evenly distributed both southerly and northerly. However, it is noted that in the shallower depths the sand transport potentials indicated from the calculations may not be realised due to the occurrence of rock reef inshore of the 25m isobath. Further, the nett rates calculated were negligible. The cross-shore (shore-normal) transport rates were calculated to be lower than the alongshore rates (Figure 4.6). However, the calculations did not take account of the gravitational effect of the nearshore slope which would have resulted in a slight overstatement of the onshore rate. Nevertheless, the transport rates decreased rapidly with depth and were calculated to be negligible beyond the 35m isobath. The analysis of the extreme storm events indicated that the rates of sand transport increase markedly during storms. The analysis of the May 1974 storm, while undertaken on hindcast wave and current data, indicated that rates of transport during 38 9(arine Aggregate Troposal. Coastal Tro c-eases I GEOMARINE I storms can be an order of magnitude higher than the annual I synoptic transport rates. When storm recurrence statistics were applied to these high rates of sand transport it was shown that severe storms account for a significant proportion of the I transport rates as determined on an annual basis. For example, the transport that may occur during an event such as the May 1974 storm could account for some 5% to 15% of the gross annual longshore rate of sand transport; the higher proportions applying to the shallower regions. 1 4.4 Sunimary The synthesis of field data, laboratory data and analytical I studies of sand transport in the nearshore zone presented a coherent and consistent assessment of the limits of subaqueous beach fluctuations. Three typical water depths were consis- I tently determined: . 12m (-i-I- 4m) - the depth to the outer face of the surf I zone bars representing the subaqueous limit of the active beach face on an annual basis; . 22m (+1- 4m) - the absolute limit of offshore sand I transport under extreme storm events; and 30m (approximately) - the limit of significant reworking and transport of beach sized sand onshore under wave action alone on a horizontal bed. I In the study region at Cape Banks the beach sands at Maroubra Beach and Malabar were mapped to extend to 27m water I depth. In the study region at Providential Head the sedimen- tology indicated that the nearshore zone of Marley Beach extends approximately to 22m water depth. Shore-normal I bathymetric profiles offshore from the beaches and from the cliffed coast in the Providential Head region show an inflection point at about 25m-28m water depth, beyond which the seabed I is very flat for some considerable distance offshore. This point - coincides with the calculated offshore limit of significant on- 1 shore sand transport under wave action alone. The assessment of sand transport on the inner shelf zone, I described in relation to previous studies and defined by de- tailed computations based on extensive field data, has mdi- cated that sand transport occurs over the range of water depths I found in the proposed extraction areas. ±7fathte aggregate Proposat - Coasta(Trocesse$ 39 GEOMARINE Sand transport is effected principally under the combined actions of waves and oceanic currents. There are various cur- rent structures, resulting from the various coastal processes, having characteristic signatures of sand transport on the sea- bed. The classic signature resulting from sand transport under the action of the low frequency, strong but intermittent, south- erly East Australia Current is found in sandwave features over various parts of the study region. Local promontories affect this current run by causing separation of the current from the coast at various locations, thereby resulting in variations in current speeds and directions on the inner shelf and, hence, variations in the distribution of sandwave features. These features attest to a depth of reworking of the seabed in the order of metres over thousands of years. Sand transport occurs also in the shore- normal direction mainly as a result of internal wave structures. Wave-generated seabed ripples have been observed also on the inner shelf of the study region, being ubiquitous in 25m water depth but becoming rare in some GOm water depth. These features attest to an enhanced sand transport capacity under wave action in the shallower waters of the proposed extraction areas where the bed is reworked to 0.2m on a day to day basis. The frequency of bed reworking by wave-generated ripples reduces with depth and over a six month period of time lapse camera data no wave-generated ripples were observed in 80m water depth. The sand transport analysis showed that the rates of sand transport on the inner shelf are very low beyond 25m water depth (gross rate less than 20m3/mlyear approximately) and are negligible beyond 35m water depth (gross rate 2m3/mlyear approximately). The rates are variable in time with relatively larger amounts of sand being transported during storms. The nett rates of sand transport calculated were negligible. 40 9vlathze aggregate Troposaf - Coastat Trocesse.c I GEOMARINE I I I 5 Constraints Assessment I 5.1 Introduction The site-specific understanding of coastal processes and sedi- ment transport, developed for the proposed extraction areas, I led to the definition of various criteria, considerations and constraints that were adopted, with those from other specialists' studies, for the design of the extraction plans pro- I posed. 5.2 Generalised Criteria for the Design of Extraction 1 Configurations The following criteria relating to coastal processes were I adopted in determining the boundaries of the extraction areas: a Extraction should not have a measurable effect on the wave climate of beaches or entrances to the adjacent estuaries. Changes induced by extraction should be: - an order of magnitude smaller than the natural I annual average variations experienced on beaches and across estuary entrances; and - less than that which was possible to detect or I measure in the field. a The extraction should take place beyond the area I that experiences the onshore/offshore beach fluctuation and beyond the limit of significant I wave-induced onshore sand transport. a Extraction should not take place within the nearshore beach sand zone along the cuffed coastline I if it exists. flvIarine iggregate fEroposaf - Coasta(!Processes 41 GEOMARINE Extraction should not affect the wave climate of the reef communities. Extraction should not disturb the stability of or otherwise harm shipwrecks in the region. 5.3 Coastal Process Considerations It was considered that extraction should not alter the general coastal processes of winds, waves and currents; particularly at the shoreline. Obviously, seabed extraction would have no effect on the wind climate. While extraction in estuaries can alter tidal currents and river flows significantly, extraction of sediment to 5m on the inner continental shelf over the proposed extraction areas would have only a localised effect on oceanic currents in that there would be a slight lowering of current speeds near to the bed where extraction occurs; there would be no effect, however, on the overall current structures on the shelf. There is, however, potential for extraction to alter shore- line wave climates. When a wave moves into shallow water it slows down and, if it is travelling obliquely to the bottom contours (isobaths), this would result in changes to the direction of wave travel and the distribution of energy along the wave. This process is known as wave refraction. Unless carried out in accordance with established procedures extraction has the potential to change the wave refraction patterns and, consequently, the wave conditions and beach stability at the shoreline. If extraction were undertaken along the isobaths such that the alignment of the seabed was not altered it can be shown, theoretically, that there would be no change to the nearshore wave refraction patterns irrespective of at what depth the extraction had been undertaken. If, however, extraction were undertaken across the isobaths then, unless the extraction took place in relatively deep water, there would be some change to the nearshore wave refraction pat- tern. It is clear that at the extremities of any extraction config- uration the latter cannot be avoided and there could be edge effects. These effects are local variations in the nearshore wave climate and can cause recession and progradation of un- consolidated shorelines; particularly on long sandy beaches. 42 ±'v1arine Aggregate Troposat - CoasatfProcesses I GEOMARINE I The adverse impacts from edge effects could be eliminated by: I undertaking extraction in sufficiently deep water so that there would be no change to the refraction patterns irrespective of the shape of the extracted I configuration; or ensuring that the extracted boundaries are sufficiently up coast or down coast from beaches so that any impacts from edge effects are limited to rocky 1 shorelines. Figure 5.1 compiles results of various different refraction I studies undertaken on the edge effects from various dredged configurations on erosion and accretion along a long sandy beach (Appendix VI - Wave Rcgime). The results showed that I the effects of extraction on beach erosion and accretion dimin- ish rapidly with increasing water depth. The studies showed also that when no account is taken of the range of wave I directions occurring in natural wave trains (studies in Japan) the changes calculated at the shoreline are exaggerated when 1 compared with those that assume a directional spread in the natural wave energy spectra (this study). When appropriate parameters describing the variability in directional spread I found in natural wave spectra are used, edge effects on beaches would be virtually eliminated if extraction were limited to 5m below existing seabed levels and were undertaken in water I depths exceeding 35m. Figure 5.2 shows the theoretical result in the calculation of I variations in the direction of mean annual wave energy flux along the shoreline (effectively the variation in orientation of I the beach face) that may be caused by Sm holes in 25m water depth. Maximum variations approach 10 but beyond 1.5km from the edge of the modelled depressions the change is less I than an order of magnitude smaller than the natural annual variation of the Sydney wave climate. While extraction in 25m water depth showed slight effects on erosion and accretion on I long sandy beaches, for extraction of 5m as proposed offshore of the rocky coastlines, the edge effects would still be extremely small and would be limited to 1.5km up coast or down coast I from the edge of the extracted configurations (Figure 5.2). These considerations were used to define the extraction config- uration in the vicinity of beaches to ensure that there would be I no change to the nearshore wave conditions. I Marine 2ggregate fProposal - Coastal Trocesses 43 GEOMARNE 44 9vlaiine Jqgregate Troposat - Coasta(Trocesse.c I ------ Centreline 2° - rD 2km alongshore x 5km rectangle LJcf— 5m deep to 25m isobath lOm deep to 25m isobath 1° ''-f-- 2km diameter circle, 5m deep to 25m isobath C - 0 0 o bZ 1 S z Extent 3f Hole 2° - ___ 2,500 11 000 0 1,000 2,500 Distance from Centreline (m) C, m 0 zm GEOMARINE 5.4 Sediment Transport Considerations 5.4.1 Nearshore Sediment Transport Beaches experience significant fluctuations in response to changing meteorological conditions. For the beaches in the study region these changes are reflected predominantly in direct onshore and offshore sand movement. Offshore transport within the surf zone of a beach and onshore transport seaward of the surf zone results in a continuous interchange of sediment between the two zones; the boundary generally being defined by the presence of a sand bar. As the prevailing wave conditions change the location of the bar (or bars) moves inshore or offshore; the maximum offshore move- ment occurring during severe storms. However, countering this is the large increase in the onshore-directed hydrodynamic forces seaward of the surf zone with increasing wave height. The deposition of sediment offshore during storms, therefore, does not occur to any great distance seaward of the surf zone beyond the bars or beyond the offshore limit of rip currents. It was considered that any extraction proposed should not interfere with the natural cross-shore (onshore/offshore) sand transporting processes of the beaches. For the beaches of the study region the absolute limit of offshore sand transport during severe storms was determined to be 27m and the prac- tical limit of onshore sand transport under wave action alone was determined to be 28m (Appendix VIII - ears1wre Sediment Tnsport). On this basis alone extraction beyond 30m (say) with suitable batters would have no immediate effect on the beach processes and would not cause beach drawdown. In respect of batters, underwater slopes in fine sand subject to wave action and steepening (as would result from extraction) experience changes in stress and variations in pore water pressure within the sediment. The consequent reduction in effective frictional resistance may cause slope instability. Sta- ble design slopes in sand underwater are often 1:6 (V:H). However, this applies to harbours and estuaries which experi- ence low wave climates. For the inner shelf off Sydney, consid- ering the high energy wave climate, stable slopes are more likely to be in the order of 1:15. We recommend a stable design slope of 1:20, which is commensurate with the steeper natural stable slopes found in the proposed extraction areas. 46 9(athie Aggregate fProposaf Coasw.(Trocesses GEOMARINE 5.4.2 Shelf Sediment Transport I When considering proposals for sand extraction generally, it is recognised that extraction configurations on the shelf could be altered in time by natural sand transporting processes. The 1 side batters of extraction configurations could flatten out as a result of the gross sediment transport processes. The centreline positions of the edges of extracted shapes could translate as a I result of the nett sediment transport processes. In the longer term, the texture of the surficial sediments in extracted areas I could become coarser as natural armouring of the seabed occurs, thereby altering sediment transport rates. I It is possible that, generally, changes to the shapes and loca- tions of extraction configurations could alter wave refraction patterns and, hence, there may be a potential to alter beach alignments. Being mindful of longer term sand transporting processes, inshore limits of extraction need to be set at suffi- cient depth so as to not interfere with the natural sand trans- I porting processes of the beaches. On the shelf the sand transporting processes could reduce buffers left around ship- wrecks and reefs. 5.5 Generalised Constraints for the Design of Extraction Configurations The coastal engineering criteria established for the design of the proposed extraction configurations, in conjunction with criteria from other specialised studies, led to the following generalised constraints: the nearshore depth limit for extraction off the rocky cuffed coast be the 25m isobath; the alongshore extent of extraction to the 25m isobath should not encroach within 1.5km of the end of a beach; the inshore limit of extraction directly off beaches be the 35m isobath; extraction depth be limited to 5m below the natural surface; allowance be made for initial batter slopes around the extraction configurations to develop to 1:20 (V:H); suitable buffers be left around shipwrecks (250m to 300m) and from reefs (250m). 9e1athie 2iqregiue fPmpo.ca( - CoastatfProcesses 47 GEOMARINE Within these constraints it was considered that it would be possible to undertake any extraction configuration within the proposed extraction areas without any measurable impact on the shorelines of the study region. I I I 1 r] 1 I Li I 48 A(atine f.ggregate Tmposa(- CoastatTroce.cses I GEOMARINE 6 Impacts of Extraction 6.1 Extraction Proposal The proposed extraction areas for Cape Banks and Providential Head are presented in Figures 6.1 and 6.2 respectively. While the areas were designed such that any quantity of sediment (to 5m maximum depth) over any planform configuration within those areas could be extracted without measurable impact on the beaches and shoreline, the assessment of impacts was prepared on the basis that the maximum of 5m of sediment was extracted from the seabed over the whole of the areas indicated. This was seen as the worst case. Any intermediate level of extraction from these areas would have lesser impacts. 6.2 Inner Shelf 6.2.1 General Extraction would have a localised effect on currents within the extraction areas. However, there would be no change to the general current structure in the study region nor would there be any change to the tidal currents at the entrances to Botany Bay and Port Hacking. Within the extraction areas the cur- rents near the seabed would be reduced in speed slightly. While the extraction proposed would reduce wave and current actions at the seabed where extraction occurs, because the I sediments at depth are, generally, finer than those at the surface, there would be little effect, initially, on the rates of shelf sediment transport following extraction. In the longer I term, however, the winnowing of the finer fractions in the sediments within the extracted areas and the transport into the extracted areas of the coarser sediments from without I would result in a reduction of sand transport rates over the extracted areas. This would result in a very slow infihling of the I extracted areas by the flattening out of the batter slopes. 7v1arine Aggregate Tropo.ca.C. Coastal Trocesses 49 GEOMARINE BOTANY BAY HENRY hEAD I\ISCRPTKN PNT /1 30 .- I CAPE SOLAMER u WONIORA * 20 E0.tRe 7 dy o 5*.dI Lwv- B&AIWI*t Conlo, k bdo. ISLW. In 0 On 25 Qe W EhqI 045o. thOne MID) B"dwY of Ronon. R09000d for Eo0onlon I' "a. IIILDA .0.nloI L000IOn 40 be Do Ond SonJI PI-opo.d Prood 75 0.5 0 05 10 15 2o, RWC[JKERY a co. PTY. LIMITED 1±1 1EOLAL N'O ENo1O10,11TAI. c1010.LTA1ITS PIDLECT N. 156 Lh I2 Coastal Processes Figure Cape Banks 6.1 Proposed Extraction Area 50 MTarine 4.ggregate Proposal - Coastal Processes GEOMARINE iH E of PoQ 1 ~Ww Rody 'o 25 2 Fd oe AuDi 0 Loo, to bo O.t.'* toopo Poc..d RW.UJKERY & CD. PTY. LIMITED I±J (15CLOAL MC BW15CSIENTAL Cou'GLTwrS P151.EET N. 156 15 Coastal Processes Figure Providential Head Proposed Extraction Area 6.2 9vlarine 9ggregate Proposal - Coastal Processes 51 GEOMARINE 6.2.2 Cape Banks Given that the rates of sand transport that were assessed for the Cape Banks extraction area were very low the effects of extraction would be very slow to occur. For the differential rates of transport computed, the tops of the batter slopes would translate at very low rates calculated to be O.lmIy. The cen- trelines of the batter slopes would translate at even lower rates calculated to be 0.025mJy. There would be no effect on the sand transporting processes at the entrance to Botany Bay or at the adjacent beaches. The analysis indicated that the dredged depression would remain stable for millennia. 6.2.3 Providential Head The rates of sand transport calculated over the proposed ex- traction area at Providential Head indicated that the extracted configuration would remain stable for millennia. For example, it would take over 1,500 years for the top of the batter slope in 25m water depth to extend to a position offshore of Marley Beach. Even by this time there would still be no effect on the beach. Further, there would be no effects from changing refrac- tion patterns on Marley Beach (or any other beach adjacent to the proposed extraction area) as the rates of movement of the centrelines of the batter slopes would be very much lower. In respect of the cross-shore (onshore/offshore) sand transport- ing processes, offshore of Marley Beach where extraction to the 35m isobath is proposed, the top inshore edge of the extracted depression would move shoreward as the batter slope flattens out (given that the bed becomes armoured with coarser sedi- ment). Such a process would continue until the slope of the batter coincides with the natural slope off the beach. The time required for this to occur was calculated to be in excess of some 3,000 years at which time the slope would be stabilised. That the point of intersection of the flattening slope would coincide with the limit of offshore sand transport at Marley Beach (after a period of 3,000 years) indicated that extraction to Sm at the 35m isobath would have no effect on beach drawdown even over these time scales. Potential drawdown along the cuffed coast- line is limited by the extent of rock reef along the 25m isobath. 6.2.4 Shipwrecks Because of the depths of the shipwrecks in the immediate vicinity of the proposed extraction areas (the ss Woniora and the ss Tuggerah) and the adoption of suitable buffers around these wrecks, there would be virtually no possibility of extrac- tion disturbing the stability of these wrecks. 52 9vlarine .ggregate fProposal- Coastal Trocesses I GEOMARINE I 1 6.3 Coastline 6.3.1 Wave Propagation I A summary of the results of the wave climate studies (Appen- dix VI- 'Wave 2 a The proposed extraction plans have been designed so I that any perturbations to the long term nearshore wave climate that may be occasioned by extraction I would be an order of magnitude smaller than the natural variations in the average wave climate that are experienced annually on the sandy shorelines of I the study region and, as such, would not be discernible nor would they be able to be measured; that is, the extraction plans proposed would have no I measurable effect on the long term wave climates of the beaches. 1 0 The proposed extraction plans would cause no measurable change to the effects that storms may have on the beaches of the study region. 1 a The changes that the extraction may cause to the shoreline wave energy along the rocky shore would be far smaller than the natural fluctuations of wave I energy experienced and would not be discernible or measurable. I a The proposed extraction plans would have no discernible effect on the wave climate across the entrances to Botany Bay or Bate Bay-Port Hacking I and, hence, to the nearshore wave climates of ' beaches within Botany Bay, Bate Bay and Port Hacking. That the extraction configurations would remain stable en- sures that there would be no change to the long term wave I climates of the beaches induced by extraction. 1 6.3.2 Beaches The beaches landward of and adjacent to the proposed areas of aggregate extraction are compartmented pocket beaches with I no transport of littoral drift occuring along the shoreline be- tween the compartments. In the main, the beaches are sepa- rated from each other by sheer cliffs with headlands and rock I reef extending offshore. Along the cuffed section of coastline 9'Iarine iTggregate Tivposat. Coastal fProcesses 53 GEOMARINE Coastal Processes Figure Locations for Nearshore Wave Climate Computations 6.3 (Table 6.1 refers) 54 17v(athie 2ggregate Proposal - Coastal Processes I GEOMARINE I I Table 6.1 Predicted Changes to Wave Climates from Proposed Extraction No. Natural Bed Extraction Plan 1 I (Figure Locations ON HN UE 0E Or HE HE -HN 6.3) (1) I (°iN) (°) (iii) IN) (°) (m) (iii) I Cape Banks Proposed Extraction Area I I I I Maroubra BeachN 117.8 3.99 5.98 117.77 0.01 5.97 -0.01 I 2 Maroubra Beach S 89.97 4.80 4.15 89.97 0.00 4.15 0.00 3 Malabar Beach: 116.53 1.35 1.70 116.56 0.03 1.70 0.00 4 Tupia Head Bay 123.27 4.22 3.91 123.16 -0.11 3.89 -0.02 I 5 Cape Banks Sm 162.79 6.18 5.62 162.84 0.05 5.61 -0.01 6 BotanyBayEntranceN. 146.29 4.41 4.13 146.30 -0.01 4.13 0.00 I .7 Botany Bay Entrance C 138.52 3.80 4.08 138.58 0.06 4.07 -0.01 8 Botany Bay Entrance S 130.37 3.22 3.93 130.37 0.00 3.93 0.00 I 9 Wanda Beach N 136.21 2.66 5.15 136.21 0.00 5.15 0.00 10 Elouera Beach S 115.46 2.03 4.29 115.45 -0.01 4.29 0.00 I 11 i. . . Port Hacking . 85.54 4.36 2.34 85.17 -0.07 2.35 0.01 ProvIdenti1Head Proposed Extraction Area I I I I .12 •.: .. . . JjbbOii .. 123.80 16.14 4.75 124.12 0.32 4.77 0.02 1 13 Cobblers 12867 280 485 12858 009 481 004 .14 MarleyN 147.13 2.96 5.56 147.01 -0.12 5.60 0.04 P MarleyS...... 125.61 2.07 4.46 125.54 -0.07 4.48 0.02 : 16 Inner Wattamolla 98.21 3.04 5.68 98.21 0.00 5.68 0.00 r 17...... OuterWattamolla 138.16 3.78 5.21 138.11 -0.05 5.21 0.00 18 .. Curracurrang 119.02 2.49 4.82 119.01 -0.01 4.81 -0.01 I 9...... Eagle Rock; 146.93 3.14 5.61 146.97 0.04 5.67 0.06 20 Garie 145.04 3.09 5.52 144.73 0.31 5.52 0.00 21 Garie S 137.19 2.96 5.64 137.09 -0.10 5.66 0.02 1--] 22 Era 134.64 2.28 5.06 134.59 -0.05 5.06 0.00 23 Brning.... Palms N 148.40 2.70 5.14 148.40 0.00 5.14 0.00 I 24 Burning Palms S 140.25 2.91 5.34 140.23 -0.02 5.34 0.00 25 Hell Hole . 115.80 1.42 4.16 115.81 0.01 4.16 0.00 1 where: ON, OE are the directions of nearshore wave energy flux for the natural (N) and extracted (E) bathymetries as determined from annual wave statistics; I AO is the natural variation in direction of nearshore wave energy flux as mea- sured at each location; and HN, HE are the natural (N) wave heights and those following extraction (E) ex- I ceeded for 24 hours per year. I Matine aggregate fPmpo.calT - Coasta[Pracesses 55 GEOMARINE offshore of Cape Banks, the rock reef extends to water depths well in excess of 25m. Between Marley Head and Jibbon Bombora the inshore reef is continuous to depths of about 25m and extending out to approximately 30m depth seaward of Jibbon Bombora. Under virtually all conditions there is no surf zone seaward of these cliffs and, consequently, no alongshore current generated by breaking waves. The beaches have been stable over the forty year period analysed, with changes re- corded being attributable to either ambient weather conditions or human intervention. The dominant sand transport processes at the beaches are movements of sand offshore under a system of rips and currents associated with storm wave conditions and onshore under low swell. The studies undertaken show that the offshore limit for this transport does not extend beyond a depth of 27m seaward of the beaches under extreme storm conditions. The sand extraction proposed would have no impact on these beach processes. The proposed extraction areas are well sea- ward of the littoral zone and are outside the depth of onshore/ offshore transport under extreme storm events. Changes to the wave climate at the shoreline resulting from the propagation of waves across the proposed extraction area would be negligi- ble and would be smaller than an order of magnitude less than the average changes that occur naturally on the beaches on an annual basis in response to changing weather conditions (Table 6.1). There would be, virtually, no changes to the beaches as a result of extraction. It should be anticipated that the beaches in the study region would undergo large fluctuations in response to future storm events. Further, with a scenario of a rising sea level as a result of a Greenhouse warming, there could be an increased propen- sity for all the beaches to be eroded more severely and more frequently during storm events. This erosion would in no way be exacerbated by the aggregate extraction proposed. 6.3.3 Rocky Shores Table 6.1 shows that there would be no measurable changes to wave heights or directions along the rocky shorelines as a result of the extraction proposed. With 250m buffers left off the reef edge, extraction offshore would not affect the sand levels at the toe of the reef for several hundred years, at which time there would begin a slow lowering of the sand levels against the reef. 56 9v(athie Aggregate Tmposa(. Coasta(Troce.cses 1 GEOMARINE I 7 Further Studies I 7.1 Introduction The studies to date have been sufficient for the design of the extraction configurations and the assessment of the impacts of I the extraction proposals. Should extraction proceed, further studies would be required I to establish suitable management practices and monitoring i programmes. 1 7.2 Pre-extraction Investigations Prior to any aggregate extraction proceeding, detailed surveys I of the seabed comprising sounder and side scan sonar records should be undertaken in the areas proposed for extraction. These surveys should provide total coverage of the seabed in I the proposed extraction areas and be designed, specifically, to: . identify shipwrecks or parts of shipwrecks which may I be affected by extraction or interfere with extraction operations; • identify isolated discarded ballast or wreckage which 1 may interfere with the extraction operations; . delineate the boundaries of bedrock reef outcrops I adjacent to the extraction areas; . provide accurate baseline hydrosurvey data for the I later assessment of bathymetric changes resulting from extraction; and I . map the bedforms and textural changes of the seabed. 9e(aiine 2ggregate Proposal - Coastal Processes 57 GEOMARINE 7.3 Management Practices 7.3.1 Mapping Each extraction pass should be mapped in real time and the mine plan updated accordingly to ensure that extraction is limited to the configurations designed. 7.3.2 Currents Real time information on currents and waves is available in oceanographic stations established by the Government to mon- itor the Sydney ocean outfalls. It would be desirable to access these data and to establish an automated real time capability to predict oceanographic conditions in the area. 7.4 Monitoring Programme - Basic Elements 7.4.1 Aims Should extraction proceed we recommend an ongoing monitor- ing programme be devised which aims to: verify that extraction is undertaken in accordance with the approved mine plan and the conditions of development approval; monitor the coastal processes and their impacts to verify predictions presented within the E.I.S.; provide an early warning of any unforeseen problems or changes that may be attributed to the works as they progress; and refine and improve the extraction technique and operational procedures so as to further minimise any impacts should that be warranted. The basic elements of a suitable monitoring programme are outlined in the following. 7.4.2 Regular Aerial Photograph Coverage Accurate mapping of beaches can be effected with photogram- metric techniques and these can be used to monitor closely changes to beaches over time. Baseline photograimnetric data dating back to the late 1940s and against which future changes could be mapped has already been obtained on the beaches in the study region for the E.I.S. 58 ±Mathie 2ggregate fProposa[- CoastTroc.esses GEOMARINE I While it has been assessed that none of the beaches in the study region would be affected by extraction, it could be argued by I others that beaches immediately adjacent to the proposed extraction areas from Maroubra to Silver Beach could be af- I fected directly by extraction from Cape Banks as could Marley Beach, Little Marley Beach and Wattamolla Beach by extrac- tion from Providential Head area. While these beaches should be monitored, data should be obtained on other beaches also to serve as controls and, in this regard, Bondi Beach, Bate Bay, Jibbon Beach and Garie Beach provide a range of suitable I control beaches with various exposures to the wave climate. A suitable photogrammetric programme would comprise map- I ping from vertical aerial photography at a suitable scale ob- tained on a routine basis at the same time of year at low tide, and in the morning covering: • Bondi Beach, Maroubra Beach, Malabar Beach, Little I Bay, Congwong Bay and Silver Beach in the Cape Banks area; I Bate Bay and Jibbon Beach between the two proposed extraction areas; and I . Marley Beach, Little Marley Beach, Wattamolla Beach and Garie Beach in the Providential Head area. Additional photography should be obtained following severe ' storms, Data obtained from the photography should be in a similar form to that of the photogrammetric surveys under- taken and assessed in this study. 1 7.4.3 Offshore Surveys While it is understood that the Company would undertake real time track plot surveys with each dredge pass and maintain a I current updated plan of extraction as it progressed, precise sounder surveys on a fixed survey grid should be repeated at suitable time intervals and compared to show the progress of I extraction and changes to the depression and surrounding areas over the longer term. The survey should extend beyond the alongshore extent of the proposed extraction areas to in- I clude the larger bedform features that may lie adjacent to the areas. I (arine aggregate Tmpasa[- CoastofTrocesses 59 I GEOMARINE I I I I 7.4.4 Sediment Sampling From time to time graded samples should be assessed in I respect of natural sand transport processes. 7.4.5 Current Data Real time current data could be made available to the project I from the Ocean Reference Station and could be used (in lieu of the deployment of other instruments) for the prediction of I oceanographic conditions. 7.4.6 Wave Data I Real time wave data could be made available to the project from the Ocean Reference Station or from the Waverider buoys at present deployed and would be relevant to management deci- I sion making in respect of determining specific extraction passes as well as in conjunction with the longer term monitor- 1 ing of the beaches. I 1 1 1 60 MaiinzqMregate Trvposa( - Coastal Trocesses GEOMARINE I U I 1 8 Conclusions and I Recommendations 8.1 Preamble I The studies of extraction have shown that shoreline effects are dependent upon the depth of extraction and the water depth at which extraction occurs. The shoreline effects reduce dramat- I ically and markedly with increasing water depth. The interna- tional experience is that extraction is commonly approved and undertaken in depths beyond the 18 to 25m isobath and it 1 indicates universally that extraction can be undertaken safely beyond the 30m isobath. I The data set available and obtained for the study of this proposal was as extensive, if not more extensive, than any data I set that has been available for the study of such a proposal anywhere in the world. The data and analyses were sufficient to enable confident assessments of the water depths at which I extraction offshore of the Sydney coast could be undertaken safely; that is, without any measurable physical impacts. I ' 8.2 Conclusions The extraction of marine aggregate as proposed from two sites offshore from Cape Banks and Providential Head would result in minimal impact on the coastal processes of the region pro- I vided that the constraints on extraction as recommended are I observed. Extraction would not alter the nearshore wave climates or current patterns. Consequently, there would be no measurable I impact on the adjacent sandy beach areas or on the cliffed coastlines. I I Marine aggregate Troposal. Coastal Trocesses 61 GEOMARINE [ I In the proposed extraction areas the rates of sediment trans- port are very low. Extraction would cause a lowering of the seabed by up to 5m. This would expose slightly fmer sediments at the seabed to wave and current action and, initially, the rates of sand transport would not alter much. I However, with time, the finer sediments would be transported away selectively by the ambient currents to settle in deeper I waters and there would be an armouring of the exposed sand to a grain size similar to that which exists at present. The rates I of transport within the extracted areas, therefore, would slowly decrease. As a result, the side slopes of the depressions would gradually flatten out and the centrelines of the slopes would begin to translate very slowly in the directions of the small nett sediment transport rates. That these processes would take place very slowly and over thousands of years indicated that the extracted depressions, essentially, would be very stable and would not affect the beaches or the shorelines. I 8.3 Recommendations 8.3.1 Constraints I The following constraints in the development of suitable ex- traction plans have been recommended: the nearshore depth limit for extraction off the rocky cuffed coast be limited to the 25m isobath; I the alongshore extent of extraction to the 25m isobath should not encroach within 1.5km of the end of any I beach; the inshore limit of extraction directly off beaches be 1 the 35m extraction depth be limited to Sm below the natural surface; and I provision be made to allow initial batter slopes around the extraction configurations to slump to 1:20 (V:H). I 62 Ilv(arine Aggregate Troposal- Coastal Trocesses I GEOMARINE I I 8.3.2 Further Studies and Monitoring Should extraction proceed it is recommended that: I specific surveys be designed and carried out to: - identify the precise locations of shipwrecks and I other obstacles to extraction operations; - delineate reef boundaries; I - provide an accurate baseline survey for the later assessment of bathymetric changes; and the details of a monitoring plan be determined to I include: - regular aerial photography of mapping quality; I - precise bathymetric surveys; - sediment sampling; and I - current and wave data acquisition and I interpretation. I I I I I I I I I I !Maiine .ggregzte Pivposa( - Coastal Trocesses 63