Flooding affects both urban and rural areas alike. Flood flows and resulting flood levels vary in response to the severity of storms affecting the particular catchment. Natural rivers and waterways have a characteristic flow capacity controlled by channel size, slope and roughness. Similarly, urban systems may also have a capacity determined by a pipe size, slope and roughness. When this basic capacity is exceeded by catchment runoff, flooding of various levels can occur. Flood flows can then spread out from the primary drainage channel or pipe with the potential to damage property and compromise personal safety.
It is not economically feasible to engineer rivers, drainage channels and pipe systems of sufficient capacity to protect against all levels of flooding. A risk-based approach is taken to investment in stormwater and floodplain management which, in turn is based on the estimated damage costs associated with the different sizes of flood.
Floodplain risk management applies to mainstream rivers and water bodies such as Wallis Lake, Wallamba River, Myall River and Port Stephens. At a different scale the principles of floodplain management also apply to significant urban waterways and small floodplains that are part of a trunk drainage system.
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Floodplain Management Process
Great Lakes Council, in conjunction with NSW NSW Office of Environment and Heritage conducts flood studies for all mainstream flooding areas (lakes and rivers) in accordance with principles and methods established in the NSW Floodplain Development Manual, 2005. Great Lakes Floodplain Management Committee provides a forum for community, Council and state authority involvement in the planning and technical oversight of the floodplain risk management process.
The floodplain management process consists of four sequential stages:
1. Flood Study: Determines actual flood behaviour and the extent of the flood problem.
2. Floodplain Risk Management Study: Evaluates management options for the floodplain in respect of existing and proposed development.
3. Floodplain Risk Management Plan: Confirms management measures and costs prior to adoption by Council.
4. Plan Implementation: Comprises project planning for the implementation of risk management options identified by their cost-effectiveness. This stage includes both structural and non-structural measures, for example cross-referencing of Local Environmental Plans to encourage compatible floodplain development.
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Flood Levels and Magnitude
Flood heights are measured in relation to a base level defined by Australian Height Datum (AHD). In open sea, Mean sea level (MSL) is approximately equal to zero (0.00m) AHD.
Floods are measured in terms of their annual recurrence interval (ARI) or annual exceedence probability (AEP). It is a means of describing how likely a flood is to occur in a given year. The probability also allows the estimation of damage costs for different size floods. Common magnitudes modelled include 5 year ARI, 20 year ARI, 50 year ARI and 100 year ARI. These flood sizes are approximately equal to 20%, 5%, 2% and 1% AEP's respectively.
For urban stormwater, flood risk is managed by designing a 2-stage system. The Minor system consists of the underground pipe network and is designed to deal with flows up to the 5 year ARI (residential networks) or the 10 year ARI (commercial areas). When floods exceed pipe capacity, excess (Major) flows are conveyed via a designated surface system of roadways, paths, drainage reserves and easements to a trunk drainage system for safe disposal.
Under Great Lakes Council's Flood Management Policy the 1% AEP (~100 year ARI) flood is adopted as the design flood standard for planning and general risk management purposes. This policy also sets the minimum habitable floor level (Flood Planning Level) consisting of the 1% AEP flood level plus a freeboard allowance of 0.50m which defines the minimum habitable floor level for a particular location.
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The projected effects of climate change, including associated sea level rise (SLR), have the potential to increase the severity of flooding and coastal erosion across Council's area. Continued global monitoring shows that the main climate change indicators are tracking towards the upper end of projections established in 1990 ("Climate Change in Australia", CSIRO, 2007). SLR, by increasing the downstream tailwater levels, can directly affect flood behaviour across the range of storm events. Similarly, increases in storm intensity due to climate change have the potential to increase peak flood flows and associated depths.
In June 2011 Great Lakes Council adopted as policy Sea Level Rise (SLR) Benchmarks (NSW Government, 2009) of 0.50m by 2060 and 0.90m by 2100 (interpolated linearly). This policy confirmed measures adopted in Council's draft policy over one year before. SLR estimates may change (+/-) with further scientific advice. A review of Council SLR benchmarks is expected to occur following IPCC 5 in 2014. A revision of Council's floodplain mapping and management measures would be expected shortly thereafter.
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The majority of flood studies, risk management studies and plans are available for download using the links listed below. They are also available for viewing at Council's Forster Office. Copies may also be purchased by contacting Council on (02) 6591 7222. In circumstances where more detailed flood study interpretation is required it may be necessary to consult a registered surveyor and/or suitably qualified engineer.
Written information relating to flooding can be purchased from Council by applying for the following Certificates:
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This certificate will provide information on the availability and standard of flooding information. If Council possesses flood information for the subject land, the certificate will give an estimate level in AHD. It is limited to providing flood information that Council has in its possession in relation to flood studies or historical information and does not take into consideration Council's planning controls or policies. A flood level certificate provides estimates of current flooding and does not deal with the impacts of sea level rise.
Prior to making an application for the Flood Level Certificate please contact Council's Customer Service Centre to confirm that Council has information available for the property in question. Council does not hold flood level information for every property. A fee of $60 is to be paid at time of application. Please allow approximately five working days for this information to be sent to you.
Section 149 of the Environmental Planning and Assessment (EP&A) Act 1979, requires that local councils, upon application, must issue a Planning Certificate providing information on the planning controls relating to land within the Council area which includes flooding information. To obtain an estimated flood level (if available) an application can be made for a combined 149(2) and/or 149(5).
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Flood Management Studies and Plans
Council has prepared a number of Flood Management Studies and Plans for download.
Because of the size of these documents some have been broken into more manageable downloadable segments.
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Last Updated: 05/11/2012