Draft Proposals Paper - Full - Victorian Environmental Assessment ...

The Overbank Flood Event
The foremost environmental threat to the floodplain
ecosystems of the Investigation area is not merely lack of
water volume as such but insufficient overbank flooding.
Currently around 70 percent of Murray–Darling inflows are
extracted, and the floodplains are likely to be among the
most severely affected parts of the natural system. Studies
suggest that river health declines steeply when more than
30 percent of inflows are extracted. Given current use
patterns, it is not surprising that floodplains are likely to
be among the most severely affected parts of the natural
system. The main intent of VEAC's environmental water
recommendations is to ensure that overbank floods are
large enough and frequent enough to avert this threat.
A program to deliver such floods could be reasonably
simple at the broadest level. The following example shows
how an overbank event might be delivered at least once
every five years, for instance.
The five year cycle would commence as the flood waters
of the previous overbank event recede. In the first two
years there would be no need for further flooding.
However, if an overbank flood occurred naturally in these
years (that is, there was much higher than average
inflows) the 'clock' would be reset and the five year cycle
would start again. If the third year of the cycle had high
inflows, but not quite high enough to generate an
overbank event, then a small amount of water could be
added to the naturally high winter-spring flows to
generate overbank flows and restart the cycle.
The fourth year would be similar to the third year, except
that the threshold of above average inflow required to
justify augmentation would be lower. If no overbank
events occur in the first four years, then whatever water is
required to generate an event would be added to any
natural flows to ensure an overbank event in the
winter–spring of the fifth year.
Any such program would also need to include variations
to cater for the varying demands of ecosystem health such
as wetlands that require more frequent inundation. More
elevated parts of the floodplain—typically found at a
greater distance from the main river channel and
vegetated with Black Box trees—would require less
frequent but larger events (that is, using more water) to
ensure floodwaters reach them.
In reality, of course, the operation of such a program
would be more complicated than this notional concept.
The complexity arises at a number of levels such as:
• cross-jurisdictional: ultimately the recommended
changes would require the support of the
Commonwealth and other state and territory
governments in the Murray–Darling Basin
• operating rules: there are many rules which govern the
operation of the River Murray system and several of
these would require modification to accommodate
VEAC’s recommendations. The Council acknowledges
that many of these problems will require considerable
work to resolve, and that it is not VEAC’s role to
develop detailed proposals for the supply and delivery of
environmental water. VEAC’s task is to provide strategic
advice relating to the protection and ecologically
sustainable management of the environment and
natural resources of public land. It is Council’s view that
there is no more important matter in this regard than
the delivery of adequate floodplain inundation to permit
the river red gum forest ecosystems to be maintained in
a healthy condition.
100 percent of estimated minimum
overbank flow requirement
- 100 percent of intended floodplain inundation
75 percent of estimated minimum
overbank flow requirement
- much less than 75 percent of intended
floodplain inundation
river channel
floodplain edge
12 River Red Gum Forests Investigation July 2007