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The Choice is not just between Flood Levees and Flood
Levies (email sent 31/1/11)
Steven Molino,
Molino Stewart
Re:
‘Levees
a better long-term solution than levies’, The Australian, 31/1/11
Your article suggested that
flood-plain management (eg by constructing flood levees) would be preferable to
periodically facing large flood damage restoration costs (and perhaps having to
increase taxes to do so). Your article also noted that Queensland has done
relatively little in flood-plain management.
I would like to suggest that
another issue related to flood management in Queensland that needs attention could
be that, for reasons suggested below, the 100 year flood level that has been
the basis for approving property development adjacent to the river in Brisbane
and Ipswich is too low. Moreover, there are now serious difficulties in
ensuring that governments act on reliable advice.
I
would be interested in your response to my speculations.
John Craig
Outline of Concerns about
100 Year Flood Level Assumptions for Brisbane
The 100 year Brisbane River
flood (which is used as the basis for defining flood levels in approving
property development) is apparently 6,000 cubic metres / second corresponding to
a flood height on the Port Office / City gauge of 3.3m, estimates that seem to
be based on
Review of Brisbane River Flood Study, 2003 (see p21).
As I understand it also:
- There has been controversy about
whether this is appropriate (eg see Thomas H.,
Incorrect measure put homes in danger zone, The Australian,
21/1/11). The latter pointed to differences between the 2003 review report and
a 1999 report (Brisbane
River Flood Study). The latter had suggested that the 1 in 100 year flood
that should be assumed as the basis for property development should be 8,600
cubic metres / second, which would be over 1 metre higher at the Port Office /
City gauge (p10), and up to about 3m higher than the presently assumed 1 in
100 year flood level further upstream;
- Estimating appropriate flood
heights for development approval is complex. For example, changes to the
Brisbane River (eg removal of a sand bar and dredging at the river mouth; and
construction of Somerset and Wivenhoe Dams with flood storage capacity) would
reduce the levels associated with historical floods. Also there are different
methods for estimating floods (eg those based on observed historical flood
levels from which flow volumes can be imputed; or those based on modelling the
effect of expected rainfall to calculate possible flood volumes, and then
using this to estimate flood levels);
- Observed flood events in the 19th
century appear significantly greater than those over the past century – noting
that the
1893 and 1841 floods reportedly reached over 8m on the Port Office / City
gauge and the associated river flows (after allowing for removal of sand bar
and dredging) may have been over 14,000 cub metres / second (Brisbane
River Flood Study, 1999, p3). Two such flood events in 200 years suggests
that the 1 in 100 year flood (if the effect of dams is ignored) might be
something over 14,000 cubic metres / second. This can be compared with the
1974 (pre-Wivenhoe Dam) flood involved a peak flow of about 9,500 cubic metres
/ second (associated with a flood height on the Port Office / City gauge of
5.45m), and the corresponding 2011 (post Wivenhoe Dam) figures of perhaps
9,000 cubic metres / sec (associated with peak a Port Office / City gauge
height of 4.45m);
Note Added Later:
"Greg McMahon, a consultant who was the chief flood expert in
Queensland's local government department until the early 1990s,
says attitudes to dam safety became compromised during the late
1980s when "political skulduggery" led to an effective lowering of
standards. He explains that when Wivenhoe was first conceived in
the 1970s, Australian design standards for large dams made of
earth and rock required them to be able to contain the flood
created by the largest possible rain event in the catchment, the
"probable maximum precipitation". But soon after Wivenhoe was
completed in 1984, meteorologists realised they had significantly
underestimated the size of this epic deluge." [1]
- The dams on the Brisbane River
(which provide flood storages for 50% of the catchments feeding into the
Brisbane River) seem to be officially assumed to reduce 100 year peak floods
by 50% (Review
of Brisbane River Flood Study, 2003, p18) – ie from a likely 12,000 cubic
metres / second to 6,000 cubic metres / second. However, in 2011 the flood
almost overwhelmed Wivenhoe Dam’s capacity to cope with the event without
damage to the Dam itself [see photograph below]. It is noted that the dam had been at full supply
level prior to the flood and that the 1999
Brisbane River Flood Study had suggested that that this would be normal
for dams that are about to encounter major floods. Moreover in both 1974 and
2011, the dominant influence on peak flows at Ipswich and Brisbane apparently
came from the Bremer River – rather than from the catchments which gained
flood protection from Wivenhoe and Somerset Dams;
Wivenhoe Dam with not much to spare - January 2011
![](wivenhoeFlood.jpg)
- The levels officially assumed to
be appropriate for a 1 in 100 year flood may be much too low, because they
seem to discount events in the 19th century. The 1999
Brisbane River Flood Study was based on historic flood levels (and
suggested allowing for 8,600 cubic metres / second floods). However the 2003
Review of Brisbane River Flood Study (which is the basis for official
6,000 cubic metre / second flood estimates) appears to derive flood levels
primarily from modelling the effect of an assumed distribution of rainfall
events. The problem with this is that rainfall data is likely to be biased
towards 20th century events (because earlier rainfall record
keeping was much less reliable and extensive). The assumed rainfall events are
thus potentially inadequate (ie don’t allow sufficiently for extreme rainfall
events). Extreme floods (such as those in 1841 and 1893) were presumably
associated with equally extreme rainfall events but these may not have been
adequately included in official rainfall records. The review report noted that
flood estimates based on rainfall data were lower than those based on
historical flood data (p14), but seemed none-the-less to use assumed rainfall
distributions as the basis for calculating 1 in 100 year flood events. Though
this allowed analysts to perform all sorts of sophisticated computer
simulations, if the basic data was wrong those simulations would be a case of
‘garbage in, garbage out’. Finally it can be noted that, if the 1 in 100 year
flood volume is actually significantly greater than is being officially
assumed, then the flood storage in Wivenhoe and Somerset Dams could be
overwhelmed, so that the hoped-for 50% reduction in 1 in 100 year flood
volumes might not be achieved.
Notes added later:
Wivenhoe Flood Storage Capacity
The flood storage capacity of
Wivenhoe Dam was reduced by about 20% (ie by 250,000 megalitres to 1.2m
megalitres) in order to improve its safety. Dam operators had been warned that
the dam could be over-topped by an extreme rainfall event with a return period
of 5000 years. The upgrade to reduce this risk involved building three 'fuse
plugs' to collapse and release water if levels reached 75.7m (below the dams 77m
at which the dam's 1.45m megalitre flood storage capacity was initially estimated). Flood
capacity is further reduced because operators seek to keep water levels below
75m [1]
Effect of Extreme Rainfall Events on Wivenhoe Dam (email
sent 5/2/11)
Hedley Thomas
Re:
Cheap Wivenhoe Dam upgrade reduced its capacity, The Australian,
5/2/11
Your article suggested that an upgrade to protect Wivenhoe
Dam was designed to provide safety protection against a 1 in 5000 year rainfall
event. However there seems to be some reason to suspect that the assumptions
about rainfall events in the Wivenhoe catchment may be somewhat too low (see
The Choice is not just between Flood Levees and Flood Levies).
Rainfall estimates seem to produce flood projections that are lower than those
based on historical flood records, perhaps because the former are biased towards
data collected in the 20th century when conditions were relatively
mild (ie not characterised by more extreme flood events).
Also it is noted that the 2011 flood (anything but a 1 in
5000 year event) seemed to give Wivenhoe Dam’s operators a challenge in
maintaining the integrity of dam. The probably implication is that the full
supply level for Wivenhoe Dam may not be able to be safely assumed in future to
be RL 67 – but may need to be several metres lower. If so, this would
significantly reduce the Dam’s
already-suspect capacity to provide reliable water supplies for SE
Queensland.
John Craig
Related notes:
- Blacked out section of
Wivenhoe Dan operating manual showed that its storage capacity may have been
reduced to 0.996m megalitres - as operators start releasing water when
level reaches 74.5 to protect fuse plugs [1]
- 0.29m megalitres are to be released (in February
2011) from Wivenhoe Dam because of the risk of further heavy rain. This
will reduce its originally planned 1.15m megalitre water storage capacity
[1].
[This tends to confirm the Hedley Thomas'
suggestion that the flood storage that
can be provided by Wivenhoe Dam (and thus its ability to safely provide
water supplies to SE Queensland) has been reduced significantly by the
installation of safety measures to guard against a 5000 year flood event.
However this reduction may not be sufficient if the rainfall data that is
the basis of estimating such an event is inaccurate (see
above)
Historical Flood Data in 19th
and 20th Centuries
The diagram reproduced below appeared in Main A.,
'Premiums set to rise, naturally', The Australian Wealth, 9/2/11
![](BrisbaneFloodPeaks.JPG)
By way of background I note
that (though a civil engineer by initial qualification) I have only an amateur
interest in hydrology – based on: (a) abandoning a canoe trip down the Brisbane
River as the 1974 flood rose, and later 'helping' dispose
of beer in a flooded riverside club; (b) informally ‘calling on the army’ to help in
the 1974 clean-up (as the off-sider to the Coordinator General who then headed
the State Disaster Control Organisation); (c) living at Wivenhoe Dam site while
working for SMEC on the Wivenhoe Power Station; and (d) attempting to identify
the structural causes of manifest ineptitude in Queensland’s water supply
management (see
Structural Incompetence and SE Queensland's Water Crisis).
In relation to the latter it
is further noted that there are increasing difficulties in ensuring that
reliable advice is given to governments. For example: (a) for the last 2-3
decades governments have clearly favoured getting advice from tame experts who
tell then what they want to hear – eg by politicisation of public services (see
Decay of Australian Public Administration); (b) increased private sector
control over the provision of public goods and services has left governments
more exposed to commercially self-interested advice; and (c) there seem to be
instances (in transport planning) where conflicts of interest have compromised
what should be purely technical advice (see
A Conflict of Interest?). Moreover there are reasons to suspect that seeking
advice from sources likely to justify low assumptions about 100 year flood
levels might be politically advantageous (eg eg in terms of: (a) the effect on
the value of assets held by property developers with political connections; and
(b) the effect on ‘climate change’ agendas, if weather events in the past were
seen to be more extreme than those that are now occurring).
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Economy for a New Climate |
Economy for a New Climate (email
sent 2/2/11)
Rob Burgess
BusinessSpectator
Re:
The climate for a new economy, BusinessSpectator, 2/2/11
Your article suggested, partly on the basis of comments by
Ian Goodwin (Macquarie University), that climate change requires Australia to
develop a new style of economy. However, the issue is more complex than this
For example the estimated storm surge associated with the
Category 5 Cyclone Yasi is currently about 7 m (at 5PM on Wednesday 2 February,
up from 2m on Tuesday night and 4m on Wednesday morning). However the Bathurst
Bay storm surge associated with the Category 5
Cyclone Mahina in 1899 was reportedly 13m. The 1899 surge may have been
higher at Bathurst Bay than in adjacent coastal areas because of funnelling
effects. However there seems little doubt that Cyclone Yasi is not unprecedented
in its intensity and effects. The perception that current weather events are
unusually extreme seems, at least in part, to reflect a loss of memory about
events in the 19th century, because conditions in the 20th
century were relatively mild. The same may apply to flooding, as floods in the
Brisbane River in 1841 and 1893 seem to have been much worse than those in
recent decades – see
The Choice is not just between Flood Levees and Flood Levies
[and added diagram
in particular].
[Note added later: It was reported that Cyclone
Yasi (which slowed and thus did not cross the coast at high tide as had been
feared) actually produced a 2m storm surge [1]
]
John Craig
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