,- '. WESTERIPORT BAY ENVIRONMENTAL STUDY 1113 - 1914 Project eport

4.3.2

THE GroMJRPHOLOGY OF BARRALLIER ISLAND

, E.C.F. BIRD

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iSU.. ISTRVfOB CllBUiYATIOI YIClnlA p~ REPORT

4.3.2

THE GEX:M)RPHOLOGY OF BARRALLIER ISLAND

E.C.F. BIRD

REPORT 'ID THE WESTERNPORT BAY ENVIRONMENTAL S'ruDY

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Prepared by E.C.F. BIRD GEDGRAPHY DEPARTMENT, UNIVERSITY OF MELBOURNE

DECEMBER, 1974

, NOTE

TRe investigation reported herein was carried out with the support of the Westernport Bay Environmental Study, Ministry for Conservation, , 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 Progr.am Ministry for Conservation 240 Victoria Parade East Melbourne 3002 Australia THE GEOfvl0RPHOLOGY OF BliREULLIER ISLAtID

E. C. F. Bird

Barrallier Island is a small depositional island off the north-west coast of French.Island in Westernport Bay. It consists of ridges of . ferruginous gravel, partly surmounted by sand, built up near the outer edge of extensive mudflats exposed at low tide. At high spring tides it is almost completely submerged, with only a small area of ~andy terrain bearing"~A~t~r=ip~l~ex= scrub, tussocks of Stipa, and associated salt marsh plants standing above calm water level. During storm surges it is washed over by waves. The main sand and gravel ridge is a cuspate" structure shaped by wave action arriving from deep water areas to the north-east (depths of up to 20 metres) and along the channel approaching the island from the south-east (depths of up to" 12.5 metres). Exposure to the dominant waves generated by the prevailing south-westerly winds is reduced partly by the extensive mudflats in this direction (which stand up to 1.5 metres abov~ chart datum) so that even at high spring tides (3 metres above chart datum) the water is shallow and wave transmission impeded, and partly by the shelter of , which reduces the effective fetch for south-westerly waves to little more than a kilometre during the highest tides. These relatively sheltered conditions on the western shore of Barrallier Island have permitted the developm~nt of a small stand of mangroves (Avicennia marina) close to mean high tide level. Some of these are rooted in coarse sand and gravel; others in s~ndy mud. The mangroves receive some protection also from a low ridge of ferruginous gravel, on which Hemichroa pentandra is growing, and in 1974 mangrove seedlings were prolific on the intervening area of sandy mud. The configuration of Barrallier Island has

- '1 - changed during recent decades, for the pattern of landforms and vegetation shown on the 1939 air photographs had changed by 1970, and has since been further modified. There are relics of earlier gravel ridges running out from the northern sqore which represent the terminations of successive ridges built and lengthened irregularly as the island prograded eastwards. This eastward growth is apparently linked with an eastward migration of the mouth of the adjacent tidal channel, which curves o~t from the north shore of French Island. Comparison of Cox's chart (1865) with the present topography shows that this channel mouth has moved eastwards during the past century, leaving on its vIes tern margin a sandy depositional area, which· is the source of the sandy material added to Barra­ llier Island in this interval. The ferruginous gravel is derived from the Tertiary formations (Baxter Sandst~nes) which outcrop on the north-west of French Island and also form a hummocky surface partly concealed by"mudflats in the intertidal zone. There are several areas of gravel, washed up and concentrated by wave action, in the inter­ tidal zone north, south and east of the deep area named Bagge Harbour on the nautical charts, but Barrallier Islaqd is the only such accumulation that has built up to a sufficient level to sustain mangrove, marsh and dune vegetation. On the eastern side of the gravel ridge that runs south from Barrallier Island there are eroding and sand-strewn relics of marshland soils. This ridge evidently formed farther eastward, and a salt marsh had developed on its western side before storm waves drove it westward, across the marshland. The sandy area that now lies east of Barrallier Island will probably be a continuing source of sand for deposition on and around the island. The closest analogy to Barrallier Island in

- 2 - Victoria is , in the southern part of Bay (Bird 1973). In its dynamic configuration, Barral~ier Island, like Mud Islands, has many features in common with the depositional islands (termed cays) that form on coralline re~f foundations off the north Queensland coast. Because of its dynamic physiography, its associated vegetation, and its.interest for ornithologists, Barrallier Isla~d should be added to the list of Sites of Scientific Interest on the Victo~ian coast prepared by the Town and C,ountry Planning Board ea;r:-lier this year.

References BIRD, E.C.F. 1973 Physiographic changes at ~1ud Islands, Port Phillip Bay, Victorian Nat., 90 (6}, 157-65.

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BARRALLIER ISLAND ... ------_ ------MANGROVES In

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Barrallier Island (arrowed) off the north-~est coast !of French Island. /. " ~;,. .:..:. . .;..~..:- ; .. - ...... , ..~~ ;-ir· .

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- - Barrallier I~land 'viewed from 'the north

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Barrallier Island viewed from the west

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