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Karaaf Wetlands Estuary Mouth Assessment (2023)

Title Karaaf Wetlands Estuary Mouth Assessment (2023)

Increased stormwater flows from the urban development to the west of the Karaaf wetlands have been linked to dieback of saltmarsh and changed vegetation communities across the western section of the wetland complex. Dieback has occurred at places across the broader Karaaf, which is more likely the result of expected periods of estuary closure rather than from stormwater inflows. Many of Victoria's estuaries close from time to time as a response to natural processes and the estuary ecosystems, plants and animals are adapted to this. This review tested the assumption that keeping the Thompson Creek estuary in an open condition would improve flushing of the wetland to the west of Point Impossible Road and assist recovery of the impacted saltmarsh vegetation. The focus questions were: 1. Could opening the estuary allow more saline water mixing thereby increasing salinity in the areas of saltmarsh impacted by freshwater from stormwater inflows? 2. Could opening the estuary allow stormwater to be more readily flushed out of the wetland and thereby increase the salinity in the saltmarsh most impacted by freshwater flows? To answer these questions, we have reviewed the available information and undertaken a detailed evaluation of the way in which inflows to the wetland interact with the vegetation communities, and how the water levels in Thompsons Creek affect this interaction. Based on this review, the following outcomes are summarised: • The most westerly section of the wetland which has been most impacted by stormwater flows, is where ground levels are above 1 m AHD and this means it is above the level of normal tidal inundation. These areas would previously have been seasonally inundated claypans where evaporation processes would dominate resulting in a saline environment. • Increased stormwater inflows have increased the duration of freshwater inundation in these areas, and this has resulted in a change to the vegetation communities from more salt tolerant to more freshwater tolerant species. • Tidal exchange will not increase saline mixing and increase salinity of either the surface or groundwater in these most westerly areas of the wetland due to their elevation above typical tidal levels and the lack of direct connection to Mullet Creek. • Reducing stormwater inflows would reduce the duration of freshwater ponding in the most westerly section of the wetland. Whether this would return the system to a more saltmarsh dominated wetland is uncertain. • Lowering of the water level in the estuary by artificially opening the mouth of Thompson Creek once it has closed is unlikely to be able to be achieved for an extended period until the water level in the estuary reaches ~1.7 m AHD and wave energy is low. At this level all the Karaaf wetland west of Point Impossible Road will be inundated. The estuary may also open naturally at this level. • The duration of an estuary closure is a function of the magnitude of catchment flows. These flows are needed to "fill" the estuary to the level at which the conditions allow the mouth to open successfully. Once closed, opening the entrance of the Thompson Creek estuary either naturally or artificially to lower the estuary water levels for an extended period can typically only be achieved when water levels in the estuary reach ~1.7 m AHD or higher. So, periods of inundation of the entire wetland complex west of Point Impossible Road will occur, with the potential for die-back of more saline dependent vegetation depending on the time it takes to fill the estuary to this level. This is a natural process for intermittently open and closed estuaries like Thompson Creek and occurs when catchment flows are low and wave energy can move sand onshore. Once open and as long as it remains open, the water level in the estuary and wetland areas will drop to a level consistent with the new channel at the mouth. As the water drains out of the wetland, the most westerly section will retain some freshwater as it is not directly connected to Mullet Creek via a channel. The estuary opening alone does not flush more stormwater out of the wetland, increase the salinity of the local groundwater system, or alter the interaction between the stormwater inflows and the claypan areas at the westerly end of the wetland. Overall, the conclusion of this review is that opening of the estuary will not mitigate the effects of increased stormwater inflows on the Karaaf wetlands to the west of Point Impossible Road.

Keywords Breamlea, Karaaf, Thompsons, Thompson Creek Estuary, estuarine vegetation, estuaries, estuary, intermittently open/closed estuaries, icolls, stratification, estuary geomorphology
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