Updated reading October 2010 on Uncommon
events
May 2009
The following information is left as it was in August 2007 being the
date of this website except for the following additions.
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There have been refinements made in the meantime to further
illustrate the importance of these events and also the fact that they are not so
"uncommon".
These refinements are "frequency of Uncommon events"
(second picture); Updating of the Wivehoe dam level graph (First picture with
wide border) and; Analysis of the May low pressure system (third picture) and;
Years to fill the dams by the various sources available (fourth picture). Together they provide concrete evidence that "uncommon events" are our
main water supply and a great volume of flood water currently goes over the dam
walls.
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The website as it was in August 2007
The importance of uncommon events was articulated by 
SEQWater's
Dam Manager in the Courier Mail 10/02/2007. The article contained several
matters of importance and they are listed in the body of the "Home"
page. The article is reproduced for examination.
From this we perceive that there are two types of rainfall
in the catchments. They are the normal rain that we receive year after year and
"uncommon events". The uncommon events can fill the dams in a
few days whereas normal summer rainfall takes around 9 years to fill.
One of the principal objectives of this section is to
examine the frequency of these events.
The matters in that article relating to uncommon events are
the following :-
Summer rainfall
This aspect has been examined in detail in the "Summer
V
Non Sum" button. All rainfall stations are detailed and confirm the
50 percent for the life of the Bureau of Meteorology records. As explained in that
section, it makes it convenient in our examination to select the Summer months. We will now examine
the Summer rains for both dams from 1910 to 2006. Prior to 1910 the Somerset was
reduced to one rainfall station and might have proved unreliable back past that
date.
Somerset
dam.
Wivenhoe
dam.
Both of these graphs are based on the same gradients and are
visually comparable. The "thumb nails" show the rainfall variances at
a glance.
High Rainfall requirements.
The article lists specific requirements of rainfall. They are
300mm to 350mm in the Wivenhoe and 350mm to 400mm in the Somerset. It is
required over a period of just a few days.
The following are listings for both dams over the period 1898
to 2006. They are for a full month. Therefore the month must be equal to, or
more than, the uncommon event requirement.
As required in the article, the rainfalls listed are for the
whole of the catchments.
Frequency
In the Twentieth Century the frequency of these "uncommon
events" were : 1988-1999, 5 in 12 years : 1971-1974, 4 in a tight group :
1955 and 1956 : 1927 and 1928 : 1910.
Conclusion to this point
Five major groups in the Twentieth Century devolves to 20
years apart. There are many other minor events we will consider later in this
web site when assessing the feasibility of harnessing their surplus water.
A detailed analysis of the Rainfall Stations listed
in the article together with the Bureau of Meteorology data for their full period
will follow at the end of this section. You will then be in an ideal position to
compare the last 21 years 1986 to 2006 with the remainder of the 108 years 1898 to 1985.
Five uncommon events including 1992 and 1999
These events are listed in the Courier Mail article. They are best
viewed
in the attached graph. They are accompanied by two events that occurred in April
of 1988 and 1989, both on the border of Non-Summer rainfall. An event in May
1996 is also included. It appeared in "Flood history" prepared
by the Bureau of Meteorology and viewed elsewhere in this web site.
The specific mention of of 1992 and 1999 clearly indicates the
amount of water lost over the spillway in these two events as well as the others
with the 1996 event being the exception.
An analysis of all the base data of all the rainfall stations
in the Wivenhoe and Somerset Dams now follows. The base data from which it is
drawn is in the "Rain 1986 to 2006" button.
SEQWater has pinpointed the events in 1992 and 1999. 
There
are three such other events, two of which cover all the catchments and one that
partially covers the catchment. The years since February 2001 are added for
observation. For those who require the base data of all rainfall stations that
make up this summary they are contained in the "Rain 1986 to 2006"
button.
The single month requirements of 350mm in the Somerset and
300mm in the Wivenhoe are more than adequately covered in all five events.
We see in this period 1986, being the start of Wivenhoe, to
February 2001 at least 5 of these rare events. They filled the Dams from
Scratch in the Wivenhoe's case to flood mode in 1999 and February 2001. This was
done in the face of the steady but high population growth as outlined above. To
do this requires surplus water well beyond our needs for 12 years.
Detailed frequency of events of the past and possible
future occurrences.
I
have listed out all of the major and relatively minor events of the past 96
years. They may give the reader some sense of the likelihood of events occurring.
I have made an attempt for guidance, however it is up to the reader to make
his/her own assessment.
The events of the Twentieth Century are quite different to the
events of the latter part of the Nineteenth Century. A view of it follows.
Latter part of the Nineteenth Century
This gauge heights of floods registered at Brisbane City are a
guide to floods in the 
Catchments
of the Somerset and Wivenhoe Dams. They do not necessarily cover all the floods
that occurred in the catchments and vice versa. They do , however, give us a
picture of what may occur in the future.
A more detailed examination of the "uncommon events"
of both of these centuries is conducted when consideration is given later in
this web site to harnessing the surplus water from these events.
FINAL CONCLUSION
The importance of Uncommon Events in filling Dams has been
adequately demonstrated and SEQWater statement is confirmed.
We have touched on the variations between centuries and
seen the infrequent occurrence in the Twentieth Century. We have not as yet
examined the minor variations of these major events. This will be concluded when
the feasibility of harnessing of these events is considered later in this web
site.
Individual Rainfall Stations highlighted by SEQWater
article in Courier Mail 10/02/2007
These particular Rainfall Stations were highlighted by 
the
SEQWater operations Manager in an article in the Courier Mail on the 10th
February 2007.
I have summarised the "uncommon events" of these
three Stations and inserted the Bureau of Meteorology data so that you may
make your own assessment. The data goes back to 1893 in some cases. I have added
the "Somerset Dam Rainfall Station" for balance.
Crows Nest 
Somerset
Esk Post Office
Kilcoy
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.
Mr Drury also provided dam level charts
which were not published by the Courier Mail. They provided clear evidence of
the importance of "uncommon events" and the inability of the
"Summer wet season" to provide sufficient water from as far back as
1992. 
