Elster Creek Catchment Area

The image below represents the approximate catchment area of Elster Creek/Canal.
It includes a close-up inset showing the catchment area around Elsternwick Park.

Relevant extracts from South East Water "2009/10 to 2012/13 Water Plan" Report (Nov 2008):

• "Three catchments (Elster Creek, Elster Creek South and Mile Creek) have been identified as currently not meeting EPA requirements to contain a 1 in 5 year wet weather event. $10.1m has been scheduled to address these issues..."
• "The Elster Creek and Elster Creek South catchments are the most critical catchments in that they are modelled as containing close to the one in one year storm only."

Reference Link: 2009/10 to 2012/13 Water Plan

PDF Version of Elster Creek Catchment Area

Elsternwick Park South - Contour Map & Flood Zone Area

The image below shows the contour-map for Elsternwick Park (South) and near surrounds.
It includes the Flood-Zone (shaded) as suggested by Melbourne Water.
The contour-map shows:

• the flood-zone area (shaded light-blue) suggested by Melbourne Water.  It includes the residential area bordered by Murphy & New Streets (northern section of that area),
• Elster Creek/Canal which receives water flow from the 'catchment area',
• some underground 'street drain-pipes' including the major 'Diversion-Drain' which runs diagonally (to the South-West) across Elsternwick Park South,
• the most significant 'low area' (3 metre contour-line) of Elsternwick Park South (ie. excluding the lake)
• the location of the pipe-drain which is designed to drain water from the 'lowest area' of the eastern area of the park, under New Street, to the 520mm diameter pipe-drain underneath the New Street service-road.  The location of this pipe-drain is approximately opposite No. 520 New Street.

Basic Design of Elsternwick Park Retardation Basin

The image below aims to explain the design of Elsternwick Park 'Retardation Basin' which was designed in the late 1990's by Melbourne Water.

Retardation Basin:
Works were undertaken in 1998 by Melbourne Water within Elsternwick Park area to provide flood attenuation capacity. The entire park area has been configured to function as a 100 year flood retardation basin which becomes engaged when the capacity of the Head Street diversion-drain (under Elsternwick Park South) is exceeded. A series of box culverts crosses beneath Bent Ave link the north and south portions of the Elsternwick Park retarding basin during 'full engagement'. The retarding basin is subsequently drained via the choked outlet (~30 m3/s) at the downstream end of Elster Creek and a grated opening into the Head Street drain immediately south of the existing Elsternwick Lake.

View of the 'Choking-Outlet', in the Golf Course, viewed from the east...

A Lot of Rain Fell from the Sky - 4th February 2011

The image below represents a view of what probably happened on the evening of 4th Feb 2011 during the excessive rain-event.

Summary of Events:

• When the Head Street Diversion-Drain exceeded it's 'flow capacity', the 'retardation facility' of the park was triggered.
• The majority of the water flow was 'directed' down the natural course of Elster Creek, through the Golf Course, and through the 'choking-outlet' near St Kilda Street.
• When the total water-flow exceeded the 'limit' of the choking-outlet, the 'excess water' flowed back into the Golf Course and ultimately into the southern area of Elsternwick Park, through the culverts under Bent Avenue.
• It should be noted that the lowest area of Elsternwick Park South (apart from the lake) is located on the eastern side of the park, approximately opposite No. 520 New Street.  Located in this region of the park is a pipe-drain which is apparently designed to drain water from this area, under New Street, to the 520mm diameter drain-pipe under the New Street service-road.
• The evening of the rain-event, a large volume of water also flowed from the south, along the New Street service-road ie. from the vicinity of Head Street.

• From eye-witness accounts, water levels rose rapidly within residential properties inside the 'flood-prone' zone (indicated in the image by the magenta-coloured area). Properties outside this zone were also affected.
• Water levels, in residential properties and Elsternwick Park South, remained very close to their maximum level until just after 11 pm on February 4th.
• Between 11pm and 11:30pm water levels rapidly decreased... "It was like someone had pulled the plug out of a bath!"
• No-one could discover why water levels fell rapidly after 11pm that night!  People are still trying to find out "why?"   ...see below for a possible explanation.

Important Factors to Consider:

1. The Pipe-Drain under New Street from Elsternwick Park South to the pipe-drain in the New Street service-road (approximately opposite No. 520 New Street):
It should be noted that Pipe-Drain under New Street is designed to drain water from Elsternwick Park South to the pipe-drain in the New Street service-road.  Hence, when there is a 'significant level' of water in Elsternwick Park South a LOT of pressure is put on the ability of the service-road pipe-drain to cope with draining the service road, because it is also draining the Park!
2. The Choking-Outlet, on Elster Creek, in the Golf Course near St Kilda Street:
Considering how water levels in the Elsternwick Park and surrounds remained fairly constant for many hours until just after 11pm... and then rapidly decreased in less than 30 minutes, it is a distinct possibility that the choking-outlet (near St Kilda Street) was 'blocked' with debris until about 11pm?  If the 'blockage' was cleared around this time, then it would allow about 30 cubic meters per second to flow from Elsternwick Park and surrounds down the natural course of Elster Creek (through Elwood).  This flow rate would explain the speed at which water levels fell after 11pm.

Limit Water Damage in the Future - Possible Solutions?

Two possible ways to limit water damage to residential properties along New Street (opposite Elsternwick Park South) if the Elster Creek catchment area encounters a future excessive rain-event are:

1. Close off the Pipe-Drain from Elsternwick Park South under New Street to the service-road pipe-drain.  This will remove 'pressure' on the 'draining ability' of the New Street service-road when there are abnormal rain events which cause Elsternwick Park South to be inundated with water. (see Image 1. below)
2. Construct a 'custom designed' vertical 'Filter Structure' to prevent the choking-outlet from becoming blocked with 'debris'.  This 'possible solution' is based on the probability that the choking-outlet actually did become 'blocked' on 4th Feb 2011.  (see Image 2. below)

Image 1 (...Solution 1):

Image 2 (...Solution 2)