May 2009: Further information
This is the leading paragraph of this
section to November 2008.
Pattern of Rainfall
The Mary Valley is the site of the proposed Traveston 
Dam. Its
catchment is adjacent to the Somerset Dam catchment. Its rainfall operates in
the same way as the Somerset (picture left). A
comparison of the rainfall of the two catchments is attached. In
the rare "uncommon events" the Mary Valley has a tendency to be up to
20 percent higher but the remainder are just marginally higher than the
Somerset.
The overall variation is 6% or almost the
same without "uncommon events" and in the same pattern.
This additional information is added:-
Both the Stanley and Mary Rivers have their
source in the same area of Mt Mee. They are the subject of a paper by a CSIRO
geologist who notes that it is a "geological structural anomaly" with
both Rivers initially flowing to the Coast and then turning inland and eventually flow
out to sea both North and South of their respective source. It leaves the coast line with
no interruption of Rivers from Brisbane to the Maryborough Region.
With this
common geological occurrence and with exactly the same rainfall pattern (picture
above) they lead me to believe that the probable action of the
Traveston proposal could be judged by examination of the Stanley and the
Somerset dam system.
To me the primary purpose would be to observe that it could
maintain the 70,000ML annual yield without "uncommon events" for a 14 year period
with normal summer rainfall only with the exclusion of all "uncommon events".
14 years being the longest period without an uncommon event.
Prior website resumes
Pattern of Rainfall
The Mary Valley is the site of the proposed Traveston
Dam. Its
catchment is adjacent to the Somerset Dam catchment. Its rainfall operates in
the same way as the Somerset. A
comparison of the rainfall of the two catchments is attached. In
the rare "uncommon events" the Mary Valley has a tendency to be up to
20 percent higher but the remainder are just marginally higher than the
Somerset.
River heights
There are no inflow figures as there is no Dam. However 
the
Bureau of Meteorology do have the River 
heights
at Gympie located below the Dam. They effectively replace inflow assessments. In
addition their web-site records the major floods and it is also attached. These
majors date back to 1893 and are mirrored by the "uncommon events" of
the Somerset with the exception of 1968.
You will have noticed that the Bureau labels the
"floods" 
Minor,
Moderate and Major. It also gives the River height at Gympie. It is therefore
possible to match the rainfall with the River heights.
The pattern of Rainfall in the Mary Valley is therefore
proven to be similar to the Somerset. An examination of the Wivenhoe and North
Pine elsewhere in this web-site also show a similar pattern. This is
understandable as the four catchments are adjacent.
Catchment area
The Borumba Dam operates within the Mary River catchment. It
holds 46,000ML currently with plans to 
increase
the height of the Dam wall. It releases 15,000ML into the Mary River annually.
Its catchment is 466 Square Kilometres. More on this Dam later. The total area
of the Mary Valley covered by the catchment of the proposed Traveston Dam with
the exclusion of the Borumba catchment is
approximately the same as the Somerset 1503 sq Klms.
With a degree of tolerance there appears to be the same
inflow from both catchments and they follow the same pattern. The exception is
the uncommon events.
Individual rainfall stations
An examination of the Individual rainfall stations used is 
necessary
to eliminate any statistical aberrations. They include the rain gauge at the
Borumba Dam for comparison. A glance along the bottom line indicates no
significant departure from normal. Notice that the Borumba Dam mirrors the
Mary Valley catchment average.
Two other matters of significant interest is that the four Summer Months
December to March received 54 percent of the rainfall. They also received 94.9 percent
of average summer rains without the 1974 flood aberration. The eight non-summer
months received 74.4 percent of average. Except in exceptional circumstances, the
non-summer months do not produce inflow. As in the Somerset and Wivenhoe Dams
there is no drought in the productive Summer months. The rainfall deficiencies
are in the non-productive Summer months.
These figures mirror the Somerset figures.
Long term history of the Mary Valley catchment
SEQWater has pointed out the importance of "uncommon
events" in filling large Dams. The Somerset has a backup 
Dam
in the flood mitigation Dam the Wivenhoe. It has a capacity of 1.1 million
megalitres. We have seen that in the Somerset/Wivenhoe with 4 uncommon events
between 1988 and 1999, the Dams not only filled but took care of a population
increase of 700,000 on top of the 1.8 million people present at the start of the
Wivenhoe in 1986. We have seen the draw down of these flood waters in the last 6
years and it has been misrepresented as a "drought".
We have also seen that the Wivenhoe is an inefficient Dam in
providing water. See "Wivenhoe" button.
Therefore the Proposed Traveston Dam should operate in 
the
same way as the Somerset. The short
term history does match and we can now examine the long term.
You will see that they are visually similar. The uncommon
events occur at the same time with only one or two exceptions in 95 years. As we
saw before they are up to 20% higher than the Somerset.
The conclusion is that the Mary Valley catchment will mirror
the Somerset in its operation on a long term basis. This means that the long
periods that separate uncommon events will not be covered by any Dam in the Mary
Valley. This is especially so as the Somerset has a very large flood mitigation
Dam in the Wivenhoe Dam.
