Why does it happen?
Flooding is a big issue in Elwood. As simply as possible, here’s why it occurs.
Elwood is low lying land, and was largely swampland at the foot of what is now known as the Elster Creek Catchment. Swamps are by their nature exceptionally porous, and in past times the swamp received the run off from the catchment.
Rainfall and stormwater from the entire Elster Creek catchment, an area of 40 sq kilometres (mainly located in the Cities of Bayside and Glen Eira, and partly in the City of Kingston), now flows out into the bay from two exit points, one being Elwood Canal, the other being the Head St diversion outfall.
The construction of Elwood Canal redirected Elster Creek and enabled swampland to be drained. What had been swampland was then sold and developed in stages in the early part of the 20th century. In the early days of development of Elwood, people may have put their faith in the engineering marvel of the canal, and not realised that the low lying land would still be susceptible to flooding.
In November 1934 widespread flooding occurred after a massive storm over Port Phillip Bay. Articles started appearing in The Argus in 1935 about a proposal for a diversion of Elster Creek. The Head St diversion was completed in 1957 (built following the first major flood post-WWII in 1955), which means that since the diversion was built, Elwood floods less frequently. The Bureau of Meteorology has an excellent website which gives high and low tide data for each day, and for previous decades as well.
Elwood Canal, one “exit” of the Elster Creek catchment, is the only part of Elster Creek that is located in the City of Port Phillip. It is two kilometres in distance from John Monash’s bridge on St Kilda Street, under which the creek flows to Port Phillip Bay.
The second “exit” is via an underground drain junction at New Street which redirects about 60% of the normal flow of stormwater to discharge into the Bay at Head Street.
Flash flooding is the category of flooding that occurs after a heavy rainfall event, often in urban catchments. One characteristic of flash flooding is that it is difficult to predict as the timeframe is short and there is little advance warning that flooding will occur (unlike riverine flooding where oncoming waters are expected days in advance of arrival).
Flash flooding can occur after heavy rainfall in the Elster Creek Catchment, especially around high tide, when there is nowhere for the stormwater to go as the outlet from the canal into the Bay is already blocked by the high tide. Have a look for yourself next time you are at the outlet of the canal, or at the Head Street outfall at high tide. The time between high and low tide is approximately 6 hours, so each day at different times, the high tide/low tide cycle occurs twice. A lesser degree of flooding, or no flooding at all, would be expected if the rainfall occurred around low tide, when the stormwater from the catchment and the drainage system could run into the canal and out into the Bay unimpeded.
During a flash flooding event, the canal fills up and can no longer receive the flow of stormwater and rainfall. When this happens, the excess water finds its own level around the canal, and around the drainage inlets leading to it, as the drains are already full of the water which can’t discharge into the canal. This then leads to a build up of water around drain inlets, in streets, and unfortunately through houses in lower lying areas of Elwood. It is only when the tide lowers that water from the canal is able to issue into the bay, and the build up of water around drainage inlets is then able, in its turn, to issue into the canal. Other flooding occurs in Elwood around the Byron Street Main Drain for the same reason – the drain can’t empty into the canal. The Byron Street Main Drain, a Melbourne Water asset, carries stormwater downhill to the canal from the Caulfield area of the catchment. Elwood can also be susceptible to overland flows – i.e. the excess rainfall that cannot enter the drainage system, and flows downhill along streets in the higher parts of the catchment to lower lying land.
Melbourne Water is the Catchment Management Authority, responsible for the water element of the catchment.
Local governments are only responsible for the land adjacent to the water.
Melbourne Water has engaged a consultant to study the entire Elster Creek Catchment and this work is expected to be finalised in 2012/2013.
When this work is completed and made publicly available, it is likely to contain recommendations for flood alleviation in the catchment. Any recommendations would then need to be considered for their technical and financial feasibility.
Any works would involve many partners, local governments, Melbourne Water and other state government agencies and department.
It is also likely that any proposed work would need to be linked to studies of expected sea level rise in Port Phillip Bay.
Please note, this is my summary of readings, observations and my work on the issue of flooding in Elwood during my four years as councillor for Point Ormond ward – I do not claim to be a water engineer, historian or scientist!
What have I been doing?
In my role as councillor, after the floods of 4th February 2011, I brought the facts of the Elster Creek Catchment to the attention of the senior management team at the City of Port Phillip, as I realised that they may not be aware of these facts (as previous flooding had been in 1989).
I stressed that the nature of flooding in Elwood was qualitatively different from flooding in other parts of the city.
This information, and my suggestion that liaison with the other councils in the catchment was important, led to the General Manager of Infrastructure setting up a professional interest network with his peers throughout the catchment.
I have also made sure in any interactions with Melbourne Water that they understand that Council is well aware of the vulnerability of Elwood in regard to both catchment flooding and possible impacts of sea level rise. I have also alerted the Association of Bayside Municipalities (ABM – all local governments around Port Phillip Bay) to the vulnerability of Elwood, and encouraged them to have confidence in pursuing support for their work at higher levels of government due to the significance of Port Phillip Bay being the world’s largest “inland” sea, and home to a major capital city.
Elwood was a case study in the Port Phillip Bay Coastal Adaptation Pathways study funded by the Australian Government and carried out in partnership with Cities of Kingston and Melbourne, Mornington Peninsula Shire, Melbourne Water, DSE, DPCD and the Federal Department of Climate Change.
The City of Port Phillip is taking a lead role in equipping the ABM to try to secure funding from the federal Government for a comprehensive Coastal Hazard Vulnerability Assessment for Port Phillip bay. While Councillor Janet Bolitho has been the ABM representative for many years, I have been the alternative representative and attended many meetings of the ABM during my term as councillor.
~ Jane Touzeau
Candidate for re-election, Point Ormond ward