The
nature, timing and contribution of “uncommon events” (SEQWater description)
to our water supply required to examine Mr Newton’s application of “sporadic
high rainfall events”.
SEQWater advice to the
Courier Mail of the 12th February 2007 defined two types of rainfall
that 
provide the major source of our water supply. (Att 3).
Summer rainfall
(Nature)
Rainfalls in the four
months, December to March, provide the high volume rain necessary to produce
flow. The accompanying data confirms that those 4 months receive slightly in
excess of 50% in the catchments of the
Upper
Brisbane
River
(Wivenhoe),
Stanley
River
(
Somerset
) and Mary River (TCD). This applies to all rainfall recording stations in those
catchments. (Att 4,5,6)
Uncommon events
(Nature)
They are Monsoons,
Cyclones and Large Rainfall depressions. They can occur in any season. SEQWater
also defined the rainfall requirements in the catchments to fill the Dams. They
have the power to fill the Dams from very low levels to overflow in a few days
as witnessed in the 1.3 million ML Fairburn Dam recently. They generally cover
the whole of SEQ.
Summer rainfall
(Timing, stability and contribution)
Timing
and Stability of the Summer
Rainfall is evidenced by the attached rainfall data from all 
catchments in 6
year lots to aid visual assessment. It also assists in gauging the last six
years considered by the Hydrologists to be “the worst drought on record”. It
shows remarkable stability over 6 year periods with the Wivenhoe recording 99.1%
of the long term average and the
Somerset
91.4%. (Att 4,5,6 above)
They also show that they
rectify their average every 3 to 4 years. We witnessed this in year 2004 with a
small drop of 2.2% in dam levels without restrictions and most recently a well
above average rainfall to take our dams to 38% in February 2008.
Page
3 of 12
Contribution
by summer months to our water supply is gauged by the attached (Att 8) Dam level
chart of the Wivenhoe Dam prepared by SEQWater. The
Somerset
chart is
also available if required. They were both supplied by the Author of
the relevant article to the Courier Mail but not printed.
It clearly shows that
summer rainfall by itself has been significantly inadequate almost from the
commissioning of the Dam in 1986. It required the assistance of 2.06 times the
volume of the Wivenhoe/Somerset from “uncommon events”. That calculates to
3,182,700ML assistance. This is no estimate as the Dam levels permit the
calculation. (Att 9)
“Uncommon events”
(Timing, stability, and contribution)
Timing
and stability of “uncommon
events” since 1840 are best viewed in the attached (Att 10) 
schedule. They
have been derived from Bureau of Meteorology flood graph to 1900 and SEQWater
requirements from that date. It was necessary to drop the Wivenhoe catchment
requirement by 50mm to get them to register in unison with the
Somerset
catchment.
They occur at an average
of 3.5 years in the 60 years to 1900, 8.3 years in the first 50 years of the 20th
Century and 3.8 years of the last 50 years of the 20th Century. They
pay little heed to the months in which they occur. We also see that they do not
have a comparable influence, by volume of water supplied, on statistical
averages as 400mm in a few days, an “uncommon event”, is not the same as 4
months of 100mm.
The
contribution is, as
indicated by SEQWater in the Courier Mail article, quite massive in a few days
as compared to summer rains contribution. The attached (Att 8 above) Dam level graph
of the 
Wivenhoe places us in a sure position to calculate the contribution of
these “uncommon event”. The
Somerset
graph shows a similar pattern. The calculation is attached (Att 11). It shows a
proven contribution for the 14 years 1988 to 2001 of 3,182,000ML (227,285
annualised) with an estimated additional overflow of 2,193,900ML (156,707
annualised).
The floods of the 19th
Century record that the 1841, 1844, 1889 and 1893 were equal to and
significantly higher than the 1974 event. Four recording stations in the
catchments will permit us to draw conclusions as to their severity later in this
document. The recordings in the Wivenhoe/Somerset did not include any large
event of an equal volume to those years.
Page
4 of 12
Defined
and observable difficulties presented by “uncommon events” to the
Hydrologists and the consequence of the use of these figures by Mr Newton.
The HYNF (Historical
Yield No Failure) graph of the 112 years for the Wivenhoe/Somerset system
The attached graph dam
levels and the Wivenhoe/Somerset actual levels show a discrepancy of a magnitude
of well over 1 year’s supply of water (Att 12, 13, 14). It was published in
the June 2006 SEQ Regional Water Strategy. The graph showed dam levels at 10% in
June 2006 when dam levels were at 30% in the Wivenhoe and 33% in the
Somerset
. The Dam levels never dropped below 18% at any time. (Attached are the
comparisons). The Qld Water Commission has since distanced itself from that
early edition of SEQ Regional water Strategy (March 2008), nevertheless it was
part of the basis of the Grid System including the TCD.
Dam failure evident,
without “uncommon events”, frequently and as far back as 1992
“Uncommon
events” played a significant role in confusing the Hydrologists for the life
of the Wivenhoe Dam commissioned in 1986. The April 1988 and April 1989 events
filled the dam from scratch to overflow followed by another
4 events that refilled the dam from various levels, 3 to overflow (Att
8).
Mr Newton has concluded
from his supporting evidence that “sporadic high rainfall events” are
unreliable. Our position is that they are reliable with sufficient storage and
procedures put in place to take advantage of their bounty.
From
Mr Newton’s standpoint, the question should be asked “If uncommon events are
considered unreliable, then why did the Hydrologists permit the absolute
certainty of dam failure in the Wivenhoe/Somerset System if reliance was placed
on them?” It does not take a Hydrologist to observe from as far back as 1992
that Dam Failure would have occurred many times in the life of the Wivenhoe Dam
without the addition of “uncommon events” to our water supply. It
was not until year 2005 that any action was taken and blamed on the following
section.
The “worst drought in
100 years” with subsequent additions such as “its official and worst on
record”.
We have seen from the
attachments that the summer rainfall has been comparable to the long term
average. The attachments were compiled from the rainfall stations in the
catchments.
What is missing is the
occurrence of an “uncommon event” since February 2001. We are 7 years into a
gap which is quite normal as evidenced by the attached chart of uncommon events
(Att 10).
The attached graph (Att
15) of six year rainfall periods divided by 6 for an 
annual average shows two
distinct divisions. Those on the left hand side of the overall average are six
year periods without an “uncommon even”. It includes the years 2001
to 2006 with 693mm. Those on the right of the overall average contain
“uncommon events”.
Those without uncommon
events average annually 693mm to 759mm and cover 48 years of the 20th
Century. With only 66mm, or less than 10%, separating them, almost one half of
the 20th Century should have been in the category of “lowest on
record”. But that is not how “decile” maps work with the
Page
5 of 12
resultant statistical
aberration of “worst drought”.
For example, if there were
10 recordings of 790mm to 800mm, 790mm would appear as the lowest on record and
800mm as the highest on record. In my view, the use of “decile” maps
misrepresents the position. “Percentage” maps show quite an accurate
assessment of the position.
As nothing was done until
year 2005, it leaves no option other than to conclude that the Hydrologists have
not understood the operations of “uncommon events”. This is reinforced by
the previous “dam failure” section.
The “worst drought”
was used as one of the justifications of the TCD in the EIS report of SKM/QWI.
SEQWater
Yield graph of 2001
The
attached (Att 16) graph of the annual yield from the Wivenhoe/Somerset system
shows that 
446,900ML was available.
I have read all of the
Annual reports and publications of that Organization formed as a Corporation in
2000 to manage the Wivenhoe,
Somerset
and North Pine Dams. I have formed the view that they are a professional
outfit.
The Annual reports record
year after year “below average” or “well below 
average” rainfall. When
considered in the light of the yield statement on their website which reads
“Yield: currently under consideration” (Att 17) it seems apparent that the
“uncommon events” were taken into account on the year 2001 yield graph and
the absence of “uncommon events” for the 6 years to 2007 again not
recognized by the Hydrologists.
Change
to stochastic approach
The Hydrologists have
signalled a change to a “stochastic approach” instead of the HYNF method.
The cynics describe this approach as “best guess under the circumstances”.
This approach was contained in the Water Commission Strategy recently released
in March 2008.
Departure from the normal
method indicates lack of understanding of the influence of “uncommon
events”.
Attempted justification of the Traveston
by comparison with the Wivenhoe catchment, the
Upper
Brisbane
River
.
It is not generally
realised that the Wivenhoe Dam is the main storage facility of the
Stanley
River
. Its water passes through the
Somerset
into the Wivenhoe. Bureau rainfall data shows that the
Stanley
’s catchment is 1/3 the size of the spread out catchment (Att 20) of the
Wivenhoe but is 3 times more efficient (Att 18, 19). It receives 30.8%
more rainfall (Att 17) than the
Upper
Brisbane
River
, which is the catchment of the Wivenhoe, and that converts to a larger volume
of water.
The
Stanley
River
contribution to the Wivenhoe/Somerset system calculates to 65% and this was
confirmed by our most recent events. Without releases, the volume of the water
collected in the period 4th January 2008 to 14th February
2008 in the Wivenhoe and Somerset Dams were respectively 78,199ML
Page
6 of 12
and
147,595NL. This is exactly 65% contribution of the Stanley River/Somerset. Using
that percentage and the yield supplied by SEQWater in its 2001 Environmental
report of 446,500ML outlined above (Att 16), the Stanley River provides
290,000ML annually.
The Traveston stages 1
and 2 will provide 110,000ML annually. The catchments are the same size 
with the
Borumba catchment excluded. The Traveston receives 10.2% more rainfall than the
Stanley
River
in exactly the same pattern (Att 23). Not surprising as they are adjacent
coastal catchments.
I
have attached the summer rainfall comparison of both catchments (Att 22) and the
information 
from
the rainfall stations of the Stanley River (Somerset), The Mary River to
Traveston Crossing and one or two beyond plus the Upper Brisbane River
(Wivenhoe) (Att 4, 5, 6 ).
Mr Newton compares the
TCD with the inefficient and spread out catchment of the Wivehoe/Upper Brisbane
River. He is correct in saying that the TCD receives 55% more rainfall. My
calculations from all rainfall stations show 50.2%.
As one can see, it is quite different when a comparison with the TCD
sister coastal catchment of the Stanley River/Somerset, our main water supply,
is made.
The comparison of the TCD
with the Wivenhoe/Upper Brisbane River catchment was used by 
SKM/QWI on page 2
of their EIS Report (Att 34 lines 11 & 12) as the main
justification of the EIS now being assessed by you and now used by Mr
Newton. The “worst drought in 100 years” and other such justifications were
used by the SKM/QWI report and, in my view, do not stand up to scrutiny.
The rainfall in the Mary
Valley will operate in almost exactly the same way as the Stanley River
catchment and this will be useful in assessing the TCD worth under conditions
where “uncommon events” are absent for long periods. The comparisons follow
in a later section.
All of this points to the
TCD being a very inefficient Dam. The
decision to proceed with it is followed by a more unfortunate decision to
consign the Borumba to the scrap heap for a mere 40,000ML. The footings
position, lost in the process, will forever deny the natural amphitheatre of
Borumba of its full potential.
Plan
Description
Mr Newton’s response
excises the Wivenhoe/Somerset Component which he says was not examined. He then
compares the Borumba Dam expanded and drawing water from Weirs in the
Mary
Valley
, with the TCD. The comparison was made in both water resources and costs
including Weirs in the TCD catchment which were never part of our submissions to
the present EIS process. Further examination follows in the costing section.
The TCD plan is not clear
in its incorporation of the Borumba Dam. Engineers GHD on page 535 of their
report state that the current yield from the Borumba is 21,800ML of high
priority and 10,200ML of medium priority water. This makes a total annual yield
of 32,000ML from the current dam of 46,000ML capacity.
Page
7 of 12
Is this 32,000ML part of
stage 1 yield of the TCD and included in the 70,000ML of that stage? This would
produce a net effect of additional water in stage 1 of 38,000ML (70,000ML –
32,000ML). On the other hand if it is not included, then the expected yield from
the Borumba Dam would be 72,000ML being the current yield of 32,000ML plus the
additional water of 40,000ML for stage 2 being the Borumba Dam expanded to their
proposed capacity of 350,000ML.
This 72,000ML approaches
our 91,000ML for a 2,000,000ML dam based on DNRW stream flows 1964 to 2002 and
confirmed by comparison of the rainfall and yield from the North Pine Dam ( See
submission).
The Borumba Dam expanded to 2,000,000ML and it’s multiple
purposes
Again
Mr Newton does not deny the existence of the 91,000ML calculation (Att 24, 25)
nor does he 
deny that the dam wall can be raised to 2,000,000ML capacity at
Elevation above sea level 230 metres or beyond as evidenced by engineers GHD. He
relies on the comparison of the expanded Borumba, including Weirs in the
Mary
Valley
, with the Traveston. That is not our plan nor our
submissions.
The
expanded Borumba Dam has three purposes: (1) A storage dam for the
Wivenhoe/Somerset and the Grid (2) A storage dam for its own catchment and (3) a
flood mitigation dam for its own catchment and flood waters transferred from the
Wivenhoe.
We have seen from the
evidence of the Wivenhoe that “uncommon events” supply vast quantities of
water to our dams and to counter the at times lengthy gaps, extensive storage
and proper management is required. The Borumba Dam expanded to 2,000,000ML or
more is that natural storage in addition to delivering
a yield of 91,000ML. It would be the largest Dam in SEQ. The Traveston
has no such facility.
We have seen that one
further “uncommon event” on top of the summer rains and the Grid ex
Traveston will give us full dams. The Grid and summer rains combined are now in
excess of our requirements on average. We have many years to correctly solve
this situation without the urgency attributed to it via “Emergency
Legislation” which included the Traveston Dam. The addition of the Grid has
relieved the pressure. The Premier made that
very point on TV in the last week when announcing the completion of the
pipe-work between the Wivenhoe and Hinze Dams. The
only possible loss would be in that period before storage when flood water from
succeeding “uncommon events” would be lost to our proposals.
Moreton
Water Resource Plan and the changed dynamics
Mr.
Newton does not deny the existence of the
80,000ML from the Wivenhoe/Somerset identified via workings of engineers GHD.
However, he seeks to exclude it, as it was from the 2007 Hydrology Report by
Gilbert and Associates that he highlights, as it “breaches the guidelines of
the Moreton Water Resource Plan”.
Editing
note of which the Coordinator General is aware. This extract from a
letter to the Minister makes it perfectly clear that the advice offered to
Gilbert and associates in preparation of their report did not tally with this
extract from the Moreton Water Plan 2007. The Act not only provides for the
extraction and later return but also provides for rewriting the Act if the occasion
arose.
Start of extract from
letter to the Minister
An overview of the mechanics of our
proposal to observe conformity with the Act
It was necessary to deal with the mechanics
of the proposal in order to see that
The Act provides
Part
3
Section
11 General outcomes
(page 9)
(1) Each of the following is a general outcome for
surface water in the plan
area-
(a) to provide for additional water to be taken from the
plan area for future water entitlements;
PART 10
(pages 57/58)
Section 97
Amending or replacing plan
The Minister must consider amending this plan or preparing a new plan to
replace this plan if the Minister is satisfied-
(a) in relation to the outcomes mentioned in part 3-
(i) water entitlements in the plan area are not
sufficient to meet water needs sourced from the plan area having regard to-
(A) the extent to which water is being taken under water
entitlements; and
(B) the efficiency of present, and expected future, water
use; and
(C) emerging requirements for additional water; and
(D) alternative water sources including, for example,
recycled water and water savings from improved efficiency of water use; and
(E) the likely time frame in which additional water will
be required; and
(ii) there are economically viable and ecologically
sustainable uses for additional water; or
(b) the plan is inconsistent with the SEQ regional plan.
It appears clear that our proposal fits
within the Legislation particularly Part 3 (1) and Part 10 (C) and (D) in
particular.
Any inconsistence, if any exists, with the SEQ
Regional Plan would come under the Minister’s jurisdiction. The Act, as it is
drafted, has anticipated a changing situation.
Perhaps the
Moreton Plan (2007) legislation should be amended and extended (and renamed if
necessary) to include an enlarged Borumba Dam, to facilitate the completion of a
true ring-road style grid to better service SE Queensland.
If there is argument with our reasoning,
then we say this:
Our proposal is the connection of the Grid
from Wivenhoe to the expanded storage of, and access to the raised yield of, the
Borumba Dam. This will enable us to retain water now lost due to the
inadequacies of the present system to cope with the retention of excess flood
water. It is to be returned to the system when required. In addition there is
access to further yield from the Borumba. The possibility of connecting the
final stage from the Borumba to the Northern Interconnector will complete the
grid with connections to all grid related dams
in SEQ for storage purposes. The expanded storage of the Borumba will equate the
entire capacities of all dams in SEQ and will be available to all dams
in the grid system.
All of this adds to the availability of
additional water in the Moreton system and for that mater all of SEQ. It makes
sense and is in line with your efforts to find new water.
End of extract
Page
8 of 12
Mr Lucas is to be
congratulated as on the 25th July 2008 he announced that “never
again would we face severe water restrictions”. This statement recognizes that
the Dynamics of the Moreton Region have significantly changed with the
introduction of the Grid System ex the Traveston. We have been saying this
for some time in all our representations and submissions.
The problem of
insufficient water supplies that we have been living with for the last three
years now reverses. The summer rains plus the Grid contribution
ex the Traveston will see the levels slowly rise over a number of years
even with the relaxation to 170 litres a day per person and a growing
population. As we have seen Bureau records show that summer rains rectify their
average in every 3 to 4 year period. The re-appearance of “uncommon events”
will now produce an unwelcome situation as, unlike summer rains, they supply
vast quantities of water and require an equally vast storage capacity.
With
a full dam and a reduced flood compartment by “dam safety”,
recently introduced, an 1841, 1844 or 1893 type flood, which is
predicted, will produce a grave flooding
situation in SEQ without another major storage facility. Four rainfall
stations data in the catchment and contained in my EIS on page 22 confirm this
situation. Of particular interest is rainfall station No 40110 in the
Somerset
catchment, our main source of water, which recorded
416mm in January 1974 and 1,422mm in February 1893.
The weight of the
forgoing evidence indicates to me that the Moreton Water Resource Plan, born in
the time of miss-understanding of “uncommon events” known then as “the
worst drought in 100 years”, needs to
be revisited. The containment of surplus flood water from “uncommon
events” for later use in the Borumba dam would show a different position and
provide greater control over the health of the
Lower
Brisbane
River
. The EPA concern about the health of
Moreton
Bay
and its seagrass under flood conditions would also be dealt with.
An
examination of our alternative proposal compared to the TCD proposal under
“uncommon events” and further examination of long periods without them
Yield and construction
times
Our
proposal provides for
171,000ML yield including the current yield from the Borumba Dam. The
construction is estimated to take 5 years at a cost examined in the next
section of $1,895 million.
The
TCD plan provides for
multiple stages.
Stage
1 provides 70,000ML and is due for completion in
2011 at a cost of $1,700 million. This date may well be extended. The EIS
(Environmental impact study) is apparently limited to this section with the
expansion of the Traveston in stage 3 subject to a separate EIS.
Stage
2 is the raising of the Borumba Dam wall to a
capacity of 350,000ML and an additional yield of 40,000ML. The cost is
approximately $500 million. It is due for completion in 2025
Page 9 of 12
Stage 3
is the second stage of the dam will expand the Dam to provide an additional
40,000ML. It’s conclusion date is vague as are the costs. My understanding is
that addition work on the dam is required and the main highway relocated at a
cost that has been suggested at about of $1,000 million. The completion date is
ephemeral but appears to be between 2035 and 2050.
Operation of both
proposals and the influence of “uncommon events”
The
attached chart (Att 10) compiled from Bureau records as defined by SEQWater show
three 
distinct periods. The 60 years from 1840 to 1900 was “wet” with some of the events being very severe. The 50 year period
of 1901 to 1950 was correspondingly “dry”
and the remaining 50 years 1951 to 2000 which was “medium”. The average gap between events in years is 3.8 for the
wet period, 8.3 for the dry period and 3.8 for the medium period.
Current conditions
The
last event was in February 2001. We are currently 7 years into a natural gap.
Operations in periods
of gaps of 3.5 years (wet) and 3.8 (medium) and 8.3 (dry)
The
TCD position is assisted by an
article in the Courier Mail dated 27/10/2007 (Att 27) by 
environmental scientist
Doctor Peter Wylie. I have recently contacted Doctor Wylie (Att 28) and his
position has not changed. He has offered a prudent yield calculation below the
70,000ML yield expected.
The importance of the
article is that while the TCD filled three times in the 5 year period, it
also failed twice. This is not
surprising as summer rainfall, on which the QWI depends, does not fall evenly
but it does rectify to average on a 3 to 4 year basis as we have seen in the
last 7 years. What is also relevant is that there is no “uncommon events”
in that period. With gaps of 3.5 to 3.8 years it is reasonable to expect
that the two failures would not occur.
So the position of the
first stage of the TCD is:-
One “uncommon event”
in the construction period will fill the Wivenhoe/Somerset Dams with little need
to reduce the levels from full for many years.
If an “uncommon
event” delays until after construction, the Wivenhoe/Somerset will be full and
the Borumba dam will retain the overflow. The volume will depend on the
magnitude of the event. The position will be full dams at Wivenhoe/Somerset and
the start of the Borumba retention. The Borumba retention will increase by the
transfer from the Wivenhoe/Somerset of 80,000ML surplus flood water in future
events or 12 years on an annualised basis.
Forward
projections show that both
growth and current requirements have been mitigated by the introduction of the
Grid ex Traveston. The 19 year gap is easily surpassed by the actions of the
Grid and the Surplus water from the Wivenhoe/Somerset retained for better
control of these events.
Cost attributed to
both proposals by Mr Newton
Our proposal
The Engineering report
that Mr Newton refers to is by JWP. It included pipeline and associated costs 
for Weirs in the
Mary
Valley
amounting to $1.6 billion (Att 29). Weirs and their
requirements was never part
Page
11 of 12
of our joint plan
and they certainly do not figure in our proposal.
It was initially part of Mr McMah’s plan, however he withdrew it well before
the date on the Hydrology report. Our costs for the dam 
and associated saddle
dams are based on this JWP engineering report (Att 30). Other engineering
assistance was used in the costing of the 60klm pipeline as an extension of the
Grid from Wivenhoe to Borumba and reverse. Together they total $1,892 million.
The Traveston proposal
Mr Newton states that the
Traveston Dam cost is $1.6 billion for stage 1 of 70,000ML. Stage 2 involves
relocation of the main road, an additional cost said to be in the region of
$1,000 million. Add to that the estimated cost of raising the Borumba Dam wall
to their proposed level of $500 million and we arrive at a comparable
cost of $3,100 million.
Further
items mentioned
They do not apply to
our proposals as what we propose is “business as usual” in the
Mary
Valley
.
More
detail
All of this
information relating to our plan is contained in the joint EIS of
McMah/Hodgkinson and therefore would have been reviewed by the QWI. The Index
page of my EIS is attached (Att 31). You will see that I listed all the
available water, the dam and construction costs, engineering costs and costs
associated with the pipeline. In addition I have pointed out the fundamental
flaws in the Hydrology report and the SKM/QWI justification of the Traveston in
their EIS.
I became interested in
our water problem when I thought it would be a good idea to examine “the worst
drought in 100 years” with the added phrase of “it official, the lowest on
record”. The Bureau of Meteorology website 3 year maps agreed but a click on
the percentage map for the same rainfall for the same period revealed that the
rainfall had been 80% of the long term average 1961 to 1990. The Bureau agreed
by email ( Att 7) that this was correct. I had them prepare the six year map for
the same result.
Page
12 of 12
Matters
Political
We
have no political affiliations or other interests other than as citizens of SEQ.
We appreciate that it is a difficult task to change course. There are few points
to consider. I have been writing to my local member the Member for
Griffith
for a little over one year as I considered the project to be of interest to
both Federal and State Governments. With the changed Dynamics of the Grid System
ex the TCD, without available storage, there are times when the refining water
plants will have to be shut down. In addition a significant volume of water will
be lost on the return of uncommon events.
The Traveston stages 1
and 2 would be placed on hold, and upon the completion of the Borumba Dam
expansion and inter-basin transfer pipeline and pumping systems becoming
operative, can be released at a later stage to return the
Mary
Valley
to its natural state.
The funding should be a
joint project both State and Federal and a credit to both Governments and all
concerned.
We do not seek
recognition of any kind, simply an independent “without fear or favour”
review on the basis of “if there is something in it, then a detailed
examination be made”.
I wish you well in your
deliberations.
………………..
J.V.Hodgkinson F.C.A
Chartered Accountant.
PO Box
41
Stones Corner 4120
February 2007. Output from the
Government IQQM computer model, covering the 111 years 1890 to 2000, has proven Mr Drury correct. In essence 300mm of
rain in the catchments in a few days is a flood and, in the right circumstances,
can fill the dams from scratch. On the other hand 3 months of 100mm a month is a
comparative
trickle. These facts can be buried in official overall rainfall statistics the
use of which in dam performance can and did throw up statistical aberrations.
records from 1841 reveal that
they occur on average every 3.7 years with most below that average. Therefore
those above can extend well above that average such as the recent period of 2001
to 2007.
