Author : J. V. Hodgkinson F. C. A. Chartered
Accountant : Aug 2006 to November 2013
The principal thrust of this
This is my review based on official
statistics and documents. It is done in conjunction with Ron McMah, grazier
of Imbil and Trevor Herse, retired of the Gold Coast
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.
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 :-
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.
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.
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.
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
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.
Esk Post Office