Western Port Bay - River Basin Management Society
Western Port Bay (Mangrove Swamps): GDEs and Hydrology
LOCATION, GEOLOGY AND TOPOGRAPHY
Western Port (WP) Bay is a large, semi-enclosed embayment (680 km²) on an exposed coastline, comprising extensive intertidal mudflats and two islands (Phillip Island and French Island). The tidal flats are cut by deep channels,
with several catchments draining into the Bay. All these components make for a complex water circulation system within WP Bay. Directly north of WP Bay is the WP Sunklands, which is a low-lying plain that was formerly
swampland and includes the Koo Wee Rup (KWR) swamp. The sites location and topography is presented in Figure 1. Figure 2 illustrates the local ecohydrogeology, topography and key ecological features of a cross section
running from French Island to the WP/ KWR swamp. The location of the WP Bay Groundwater Dependent Ecosystems (GDEs) in the broader landscape (Phillip Island to uplifted basement rock to the north) is shown in Figure 3.
Figure 4 illustrates the key GDE sites along the WP Bay’s northern coastline.
Ecosystem Type: The majority of the WP Bay coastline is fringed by mangroves and saltmarshes, along with extensive seagrass meadows on the intertidal mudflats and below low tide level. The area forms part of the WP RAMSAR
Land use: Over the past 200 years, WP Bay has undergone significant changes as a result of land use e.g. vegetation clearing within the catchment, draining of the KWR swamp and agriculture, industrial and residential area growth.
Of most significance was the draining of the KWR swamp in the 1890s for conversion in to agricultural land. Originally there was no natural drainage systems from the KWR swamp, and in effect no direct water or sediment
movement into WP. As a result of the draining, excess water is channelled through surface drains and in to the Bay. Consequently, these channels increase the sediment sources for WP Bay.
Values: WP Bay was listed as a RAMSAR site in 1982, providing habitat for a number of species listed in the JAMBA and CAMBA agreement. WP Bay is also within the UNESCO Mornington Peninsula and the WP Biosphere Reserve and
a number of Marine National Parks (including French Island MNP) that were established in 2002.
Geology/Geomorphology: The tertiary aged WP groundwater (GW) basin is structurally controlled sedimentary basin. The main features controlling the basin structure are the Tyabb Fault, which lies on the eastern side of the
Mornington Peninsula, and the Heath Hill Fault further to the east (Figure 2). The western side of the WP GW basin coincides with the Clyde Monocline-Tyabb Fault system, and to the north the basin wedges out against uplifted
basement rocks. The major lowland area in the WP catchment, the KWR swamp was formed during the Paleocene and lies between the Tyabb fault in the west and the Heath Hill fault in the east. The basin is composed of a
sequence of Tertiary and Quaternary sediments overlying a Devonian granite and Silurian to Devonian mudstone and sandstone bedrock. The Tertiary aged sediments consist of the Childers Formation overlain by the Older Volcanics,
which are consecutively overlain by the WP Group, composed of three main units: the Yallock, Sherwood and Baxter Formations, comprised of sandstones and sands. The WP Group consists of fine to coarse sands and gravels with
minor limestone and carbonaceous clay. The entire sequence is in excess of 250m thick in some areas. The Quaternary sediments consist of clay, shoe-string sands and dune sands with up to 2m of peat occurs in the former swamp
areas around Koo Wee Rup. The WP Sunkland was formerly a major river drainage system, however it was inundated together with Port Phillip by the rising sea in the Holocene period; the WP sunkland now forms an extensive tidal
bay; WP Bay.
Hydrology/ Hydrogeology: The catchment (3433 km2) is drained by 2232 km of rivers and creeks, the Bunyip, Tarago, Bass and Lang Lang
B Rivers and the Cardinia and Yallock Creeks. Originally ephemeral the drainage lines are now permanent as a result of the channels being
excavated to drain the KWR Swamp. Prior to European development artesian groundwater pressures existed across the WP Sunklands. It is
likely that the drainage network is draining the shallow groundwater system.
The WP Group makes up the main aquifer systems at the site and is generally considered as a single aquifer system as there is hydraulic connection between each formation. However, the basaltic clay of
the Older Volcanics is considered to form a semi-confining to confining layer between the WP sequence and the underlying Older Volcanics/Childers formations. The water table resides within the
quaternary deposits with the natural GW gradient in the WP basin is radial, from the basin edges towards WP Bay (i.e. generally southward – from recharge areas to WP Bay). Little information is available
around the GW flow beneath the WP Bay to French Island; however hydrographs on both the coastline and French Island suggests that GW flow direction between the bay is a northerly direction from
French Island to the coastline. The main zone of groundwater recharge is in the upper catchment reaches (the northern section where the more permeable sediments outcrop). One of the main recharge
zones for the WP Group of aquifers occurs in the Cranbourne area where recharge is controlled by direct infiltration into small areas of outcropping Tertiary aquifer (Baxter Formation), along the eastern
and western basin boundaries, and infiltration via Quaternary sediments in the northeast along the Bunyip and Tarago Rivers. GW levels vary depending on seasonal conditions and the extent of pumping.
In the irrigation season, GW levels typically drop by 2.0m to 5.0m in high usage areas, but generally recover rapidly in wetter months. The WP Group aquifer provides for more than 80% of the total
extraction from the basin and since the 1970’s, seasonal cones of depression have developed each year as a result of GW pumping. The cone of depression centred on the KWR Swamp area has reversed
GW gradients across much of the basin, with GW flow towards this area (i.e. in an easterly direction) which remained below sea level for a number of years. It is probable that the cone of depression in
the KWR Swamp area caused by pumping has caused the fresh water / salt water interface to move inland.
Ecosystem Services: While it is likely that shallow GW exist across the western pot bay and at the terrestrial and coastal interface, there is no direct evidence of how GW and the coastal environments
interact. Little is known as to the dependency of mangrove, salt marshes and seagrass communities to GW and given the uncertainty in the movement of GW beneath WP Bay; it is difficult to conclude on
the nature of connection. Research into the interaction of mangroves and groundwater suggest the relationship is very subtle and is associated with root zone dynamics rather than GW providing a source of
water for transpiration. GW can act a regulator of salts and nutrients within the root zone of mangroves , and contributes to the physical integrity of sediments around mangrove rooting systems.
Irrespective of existing research, in regards to the WP mangroves, additional work is required to describe the nature of GW and mangrove interaction. GW-surface water interaction and GDEs would have
existed across the KWR Swamp area prior to surface drainage and GW extraction. Remnant vegetation (e.g. swamp she-oaks and melaleucas) are still likely to be GW fed, however this relationship has
likely altered due to the surface drainage and groundwater extraction.
Threats: The tertiary aquifers of the WP Basin were exploited in the 1960s and 1970s to meet irrigation and stock demands. This led to the GW levels being up to 10 meters below sea level and a high risk of
salt water intrusion. Since then, severe restrictions on GW extraction has been in place and the establishment of the KWR Water Supply Protection Area to manage this threat, however seasonal cones of
depressions are still evident as a result of GW pumping in the area and therefore there is potential for salt water intrusion to occur. Salt water intrusion can occur when the pressure level in the aquifer drops below sea level, establishing a pressure gradient between the aquifer and the sea.
Suspended sediments are the most important aspect of water quality in the area, and as a result of draining KWR swamp significant sediment movement now occurs towards the Bay through the drainage channels, increasing mud deposition and contributing to the historical seagrass dieback.
Sensitivity: Prior to landscape drainage the swamp she-oaks and melaleucas
vegetation across the WP Sunklands would have had a high level of interaction
with GW, subsequent drainage and GW extraction lowering the water table must
have some degree of impact on theses GDEs, therefore, while not quantified
theses species will have some degree of sensitivity to any further decline in GW
levels. The sensitivity of seagrass to changes in GW levels remains poorly
understood, however the measured impact from increased sedimentation
indicates seagrass is sensitive to changes from terrestrial processes and therefore
likely to include decline in GW levels. Evidence indicates that the mangroves are
migrating inland due to changes in hydrology and increasing salinity of surface
water, therefore appearing not very sensitive to the present GW level changes.
Monitoring: In order to predict or draw conclusions on the sensitivity of
mangroves and the tidal environmental there is a need to establish the physical
relationship with GW. To achieved this, the installation of GW bores within WP
Bay or where possible within the intertidal mangrove zones would be required.
Alternatively, soil cores from the intertidal zone could be analysing for evidence
of groundwater movement within the root zone of mangroves providing some
measure of GW interaction. Regional flow dynamics between the WP Sunklands,
the tidal environment and French Island remain poorly understood. Therefore
determining the implication of GW extraction/ pumping on the freshwater/
saltwater interface are difficult to predict.
CHANGING LANDSCAPES AND GEOMORPHOLOGY
Key information sources: 1.Adi Susilio2004.Groundwater Flow in Arid Tropics Tidal Wetlands
and Estuaries. James Cook University. 2.Rogers, K.2004. Mangrove and Saltmarsh Surface
Elevation Dynamics to Environmental Variables in Southeastern Australia. University of
Wollongong. 3.Melbourne Water. 2011.Understanding the Western Port Environment: A summary of Current Knowledge and Priorities for Future Research. 4.SKM .2012. Casey Growth Area impact Assessment: Groundwater and GDE investigation. 5. B. R Thompson.1974.The Geology and Hydrogeology of Western Pork Sunklands. 6.R. Lakey and
Tickell. 1981.Geological Survey Report No.69–Explanatory Notes on the Western Port Groundwater Basin