2016-07-18 General Committee Meeting - Item 3 - Attachment 1 - Flood Management Plan

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as they drain through the lake. The lakes provide a          Apart from regular ocean tides, storm surges affect




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significant amount of temporary storage for passing          water levels in Laguna Bay and flood levels in the lower




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floodwaters. Downstream of the lakes the temporary           reaches of Noosa River. Increased water levels in Laguna




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storage of floodwaters: reduce peak water levels and         Bay result in coastal waters flowing into the Noosa
discharges; slows the rate of rise of floodwaters (flood     River and its lakes, with an associated increase in water
levels typically rise for 1-2 days at Tewantin before        levels. Storm tide flooding can inundate normally dry
peaking); and assist with maintaining elevated flood         land and affect property even without the contribution
levels and discharges for a relatively long period of        of additional rainfall generated flooding in the River.
time (eg. at Tewantin, the February 1992 flood level         The magnitude of a storm surge is determined by
remained over 1.5 m AHD for 45 hours).                       a combination of factors which include: inverted
                                                             barometric pressure; wind setup; and wave setup. These
Flooding behaviour along the Noosa River, especially         affects are cumulative and are added to the normal tidal
its lower more populated reaches, is characterised           conditions at the time. The combined water level from
by relatively low flood depths with generally slow           all of these is referred to as the storm tide. Offshore
moving floodwaters which may persist for extended            storm tide gauges are used to record storm tide levels.
periods which increase inconvenience and nuisance.           The nearest gauge to Noosa River is located offshore of
Fortunately, relatively long ‘warning times’ are available   Mooloolaba.
in which downstream residents can be alerted to the
severity and timing of impending flooding and can            In a similar manner to flood studies, storm tide studies
undertake necessary actions to reduce damage to their        are undertaken to estimate the potential magnitude of
goods and possessions.                                       storm surges for a range of statistical frequencies. Whilst
                                                             generally conducted separately, results of a storm tide
With respect to the consequences of flooding, the main       study are included as a possible boundary condition in
issues along the Noosa River relate to the modest level of   the flood studies to reflect circumstances when the two
flood damage and the nuisance and inconvenience that         events (i.e. storm surge and rainfall flooding) occur at
flooding causes urban communities. As noted above,           the same time. It is a complex science to consider the
flood hazard and risk to life and limb are generally low     coincidence of storm surges and extreme rainfall events.
along the Noosa River system.
                                                             The future of flooding in Noosa is unpredictable. Severe
Normal water levels in the lower Noosa River estuary         weather events that bring flooding will occur. However,
display tidal characteristics and reflect the semi-diurnal   recent research into the world’s changing climate
nature (i.e. there are two high tides and two low tides in   suggests that over time factors such as increased ocean
every period of 24 hours 50 minutes) of water levels in      water levels and increased frequencies of severe weather
Laguna Bay. Water levels in the river will rise and fall     may result in an overall increase in the magnitude and
in response to the tide levels. These affects are greatest   frequency of significant flood events. As a community
at the mouth and progressively dissipate upstream with       with a large proportion of its residents located near the
only relatively minor tidal fluctuations evident in Lake     coast and adjacent to waterways it is imperative that
Cootharaba.                                                  these factors be considered in our long term planning
                                                             for the region.




                                                                                       Noosa Council | Flood Management Plan | Page 9