ClarenceValley Independent,
26 August 2020:
As
the new highway nears completion across the Lower Clarence, fears
have been raised about what happens during a flood, due to the
highway apparently acting as a dam at various locations.
The
areas around Ferry Park to Shark Creek (where rising floodwater first
breaks the river’s banks) and Chatsworth Island (where long
sections of the highway “act like a dam wall”) are locations
where floodwater behaviour will be affected.
And,
with the Bureau of Meteorology announcing that there is a 70 per cent
chance of La Nina developing (“roughly three times the average
likelihood”), which “typically increases the chance of above
average rainfall across much of Australia during spring”, the flood
modelling in Pacific Complete’s Hydrological Mitigation Report,Glenugie to Devils Pulpit could be put to the test sooner, rather
than later.
In
general terms, according to a letter from Transport NSW, in answer to
an enquiry by Chatsworth Island resident Shane Williams, changes to
flood behaviour across the Clarence River regional catchment are as
follows:
- A minor increase in flood level to the west of the highway;
- A minor decrease in flood level to the east;
- A minor increase in flood duration to the west and also to the east in some areas; and,
- No perceptible change in flood velocity or direction.
The
mitigation report states in the overall flood impact assessment that
resultant increases in flood levels across the floodplain
are“considered minor and generally meet the limits set by the flood
management objectives”.
On
flood duration, the report states that, overall, any changes in
duration “meet the limits set by the flood management objectives
(less than five percent increase)”, however, at “some small
localised areas … between Maclean and Iluka Road … the flood
duration is predicted to be affected by more than five percent”.
“Under
existing conditions, most of the land within the Clarence River
regional floodplain is flooded for more than 72 hours for the 20, 50
and 100-year ARI (average recurrence interval) events,” the report
states.
“For
the 5-year ARI event, areas around the fringe of the floodplain are
flooded for a range of durations from less than six hours up to 72
hours.”
The
report states that “there are small increases in flood level
between five millimetres to 15 millimetres in the Clarence River main
channel” and that “there are no impacts to the township of
Maclean in the 5 and 20-year ARI flood events, as the Maclean levee
is not overtopped”.
While
the levee “is overtopped under both existing and future conditions
for the 50-year ARI event … and a large part of Maclean is
flooded”, the report notes that “further analysis” of the
50-year ARI event is incomplete and that “mitigation measures are
under investigation”.
At
Harwood “a minor increase in flood levels upstream due to the
bridge piers” is an expectation……
Read
the full article here.
BACKGROUND
No comments:
Post a Comment