·P'fo,J E Ct Report
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- ;~7' re~~~ .\ 212?S~ WESTERIPORT BAY ENViRONMENTAL STUDY \.'.:.'6»:':. i L':. ..... _..-e-- . J 1113 - 1974 ... ·P'FO,j e ct Report . :,. ~ - "" .....- ~-;":~ 4.1. 4 MJRPHOLOGY AND SEDIMENT DISTRIBurION, WES'IERNPORI' BAY M.A.H. MARSDEN AND C.W. MALLE'IT .. .. ..... ... .....: . ............. :-:-:.:-:-:<-:<", ...... ........ ". ........ "":':-::::::::." . :":> .. -: ." .':::::::::::::\::\:y\{.:> .:·::>:H~>+<~)/)::::· r . , .' .~ j ;~ :" . , MIMISlAY FOR CCIlISUIVATlON '~~ . J' • '.. ', VICTO&lIA 55l. 304 099452 'MOR P1UJECl' REPORI' 4.1. 4 MJRPHOr.cx;y AND SEDIMENT DISTRIEUTION, ~VES'IERNPORI' BAY M.A.H. MARSDEN AND C.W. MALLETT REPORT 'ID THE WESTERNPORT BAY ENVIRONMENTAL STUDY Prepared by M.A.H. MARSDEN. AND C.W. MALLE'IT GEDr.a::;y DEPARIMENT, UNIVERSITY OF MELBOURNE DECEMBER, 1974 [~MSM 0002606 4 .] 551. 30 4- 0'1~ lotS"2- MOte.. ~ 222..'8S- NOTE The investigation reported herein was carried out with the support of the Westernport Bay Environmental Study, Ministry for Conservation, Victoria, Australia. This report is one of the various project reports received by the Study from participants. The contents do not necessarily represent the official view of the Study. Copies are available from: Environmental Studies Program Mini~try for Conservation 240 Victoria Parade East Melbourne 3002 Australia --- ----------=, TABLE OF CONTENTS PREFACE page 1· 1. I NTRODUCT I ON 2 2. METHODS 4 2.1 :. INTERPRETATION OF }~ORPHOLOGY 2.2 SEDIMENT SAMPLING AND ANALYSIS 2.21 Sampling 2.22 Size Analysis and Data Computation 3. MORPHOLOGY OF WESTERN PORT BAY 6 3.1 OUTLINE OF THE HAJOR NORPHOLOGICAL SYSTEMS 3.2 DESCRIPTION OF l-!ORPHOLOGICAL UNITS 3.21 Tidal Channel Systems 3.22 Embayment Plains 3.23 Intertidal Flats and Banks 3.24 Inshore Marginal Sandy Zone 3.25 Offsbore Banks and Shoals 3.26 Mangrove Zone and Salt Marsh Zo.ne 4. SEDIMENT DISTRIBUTION IN WESTERN PORT BAY 22 4.1 DISCUSSION OF SEDlllENT ANALYSIS DATA 4.11 Distribution of Grain Sizes 4.12 Mud Distribution 4.13' Factors Controlling So~ting 4.14 Inter-relationships of Sediment Parameters ·5. CONCLUSION 30 6. ACKNOWLEDGEMENTS 32 7. REFERENCES 33 APPENDIX 1 APPENDIX 2 " ). LIST OF FIGURES AND MAPS Page Figure 1. Major Marine Physiographi~ Features of lvestern Port Bay. 8' Figure 2. Textural composition of Western Port Bay sediment samples .• 23 Figure 3. Binary scatter plot - Mean grain size against sorting. 27 Figure 4. Binary scatter plot - Hean grain size against skewness·, 28 Figure 5. Binary scatter plot - Sorting against skewness .29 Map 1 A to D. Morphology of Western Port Bay - Preliminary Edition, 1:25,000. Map 2. Sand-silt Mean Size distribution, 1:50,000. Map 3. Sorting distribution, 1:50,000. Map 4. Mud distribution, 1:50,000. Map 5. Sediment type distribution, 1:50,000. ... !i 1. PREFACE This project was started in 1971 by the Geology Department, University of Melbourne, under the supervision of M.A.H. Marsden, and with the permission of Professor J.F. Lovering. In December 1973, a grant f~om the l~esternport 3ay Environmental Study enabled the project to be en1aOrged and linked with zoobenthos study of the Fisheries and Wildlife Department. 2. 1. I NTRODUCT I ON The complexi ty of the dynamics, s.ediment movement and morphology of the Western Port system is highlighted in this report. In this regard Western Port differs greatly from Port Phillip and from many embayments and estuaries elsewhere. Lying in a mildly tectonic region, l-les '.ern Port is a drowned e:!Ibayme:nt, in which present-day sedimentation processes are still responding to a progressive series of changesiu sealevel, which, about 5000 years ~go, reac~ed a height of about 1 to 2m above the present·day level. The general rise, . followed by relative f"ll of sealevel has imposed some of the interacting morphological and sedimentation characteristics of Western Port Bay, either external but marginal such as the entrenched river and terrace systems, stranded beach ridges, spits and other associated sand bodies, and raised rock platforms, or internal characteristics, particularly controlling the evolution of the submarine topography and the provision of sources of relic sediment for re-working within the bay. Among the unusual factors controlling the behaviour of Western' Port are: the complexity of its morphology, principally controlled by distribution of bedrock rather than by sediment deposition, as is normally found in more typical estuaries and barrier- lagoon-tidal flat systems. As a result, the behaviour of Western Port Bay is dominated by its elongate, linear channel systems which open through a cliffed coastline to the high wave-energy zone of Bass Strait, complicated further by the smaller, and younger(?) Eastern Entrance, the lack of fresh water and sediment input from the hinterland, except from the Bass River, and the consequent absence of any 'significant mechanism for seaward sediment transport. Marked changes in sedimentation patterns are now occurring locally, however, as a result of the recent drainage ':If the Koo-wee-rup Swamp, and probably of forest cleariv2, the major role of tidal currents in transporting sediment, in the absence of any "salt-wedge" sediment drive. Large areal differences in tidal range and velocities, complex differences in ebb and flood movement paths, and tidal velocity asymmetry, control mally of the sedimentation patterns, the orientation of the Western Entrance and major arms of the bay in relation to the domina-r.t directie;ns of wind- generated wave movements (essentially between 1800 and 3600 ), and the availability of a range of different seqiment types and sizes, both terrigenous sedi~ent and biogenic carbonate, and the occurrence of significant net landyard tranaport and derosition fnto the bClY, with th,' concen~:ration of the m;oljor tfdal flat arc3,,' 1n th .. ~ hC3d of the bay. , 3. The resulting range of sedimentation and 'morphological complexity is evidenced, by the number of units d':lineat'ed and described herein, and also by their variability. The \ range of variability broadly embraces:- gross variation between neighbouring units, illustrated by the contrasts between the lagoon-sand bank complex of The Nits Inlet and Cowes Bank, the clay-dominant Churchill Tidal Flats, awd the sandy sediment influx area of the Bass Delta. rapid variation within individual units, illustrated by the, close juxtaposition in the Hastings Bigbt,Tidal Flat of overwash sands from the channel, of eroded shell beds, of grassed fine-grairied tidal flat sediments, and of Inshore Marginal Sandy Zone sediments backed by mangrove and salt marsh sediments. gross variation in sediment type between units having similar morphology and dynamics, illustrated by the 'contrast between the mud-dominated Tooradin Tidal Flats and the morphologically similar sandy Post Office Tidal Flats. Generalisations regarding the sedimentation behaviour of Western Port Bay' will need to be based on appreciation of the complex relationships existing in the system. 4. :2. METP-ODS. 2 .. 1 INTERPRETATION OF HORPHOLOGY This was based essentially on interpretation of colour aerial photographs, particularly for the northern half of the bay. The main source of photographs was the lYesternport Project 1106 flown in 1973-1974 at an approximate, but variable scale 0: 1:15,000. The basic ground experience used as the reference for extrapolation of the photo-interpretation was provided by detailcj mapping projects ir. the southern part of the bay, carried out by the School of Geology from 1970~1973 (Bass River delta area - Power (1971), Tickell (1971); Rhyll/Newhaven area - Brennan (1972), Gray (1972); Cowes/The Nits area - l.falter (1973), Walker (1973). Elsewhere, a limited number of spot ground checks 'has been carried out. The 'photo-interpretations, which were uncontrolled, were compiled at 1:15,000, then reduced and fitted to the controlled 1:25,000 base map sheets of the R.A.S.C. a~d Department of Crown Lands and Survey Oiap 1, A to D). Data from the ground surveys was also incorporated. Earlier interpreta tion ·work by Jenkin (1962), particul· arly on selected coastal Kreas was valuable, as were contributions by Hills (1942) and Keble (1950). Because of the close relationship between morphology and sediment type, and particularly as these also reflect the complexity of the controlling factors in Western Port Bay, it was planned that selection of sediment sample 10'cations would be based on this morphological interpretation. However, unfortuna·te delays in the supply of photographs necessitated completion of the sampling progra~ before the interpretation of morphology was available. A further difficulty arose because much of the photography was conducted at relatively high tide, which significantly reduced the amount of observable detail over much of the area. The criteria used to delineate morphological units included: .(a) surfa;,;E: textures of sediments such as drainage patterns and patterns of relief differ~nce; (b) shapes and contrasting elevations of sediment bodies; and (c) vf'.get_1tio:1 CU"/F.r, "::lo::!J uniformity of type and relative density. The deeper tidal channels were identified by absence of light return, which unfortunately also preve:.ted differentiation of units, witl~ the exceptio n of 'the shoal areas. 2.2 SEDIMENT Sfu~LING fu~ fu~ALYSIS 2.21 SAMPLING Samples were collected either by taking the top 3 cm (approximately) of sediment from Smith-McIntyre grab samples collected by the Fisheries and ~ildlife Depart ment from H.V. Ca?itella (sampl"es from No.252 through No.44l, and from No.47L through No.496), or using a small van Veen grab, yielding a sample approxi~ately 5 cm deep, collected by the Geology Depirtment, Unive~5ity of Xelbourne, from the R.V.William BucklanJ. The bulk of the s~mples were collected fro~ late September 1973 to March 1974, but ~om~ selected samples fro~ the 1971-1973 Geology Departm~'lt studies were also incorpo~3teJ. 5. The sites of samples collected by the Fisheries and tVildlife Department were selected by them as part of their zoobenthos survey, either by random selection or at intersections of the 1 km grid.