Lithgow Floodplain Management
Lithgow Flood Study Review
What was the objective of the Flood Study Review for Lithgow?
The study objective was to review and update the hydrologic and hydraulic modelling for the Farmers Creek catchment to define the nature of both Main Stream Flooding and Major Overland Flow in the urbanised parts of Lithgow for floods up to the Probable Maximum Flood. A requirement to provide similar information in future growth areas led to the development of new hydrologic and hydraulic models for the adjacent catchment of Marrangaroo Creek.
The information presented in the report will form the basis for the preparation of the future Floodplain Risk Management Study and Plan for Lithgow. The Floodplain Risk Management Study and Plan will assess the economic impact of flooding on existing urban development, review options for flood mitigation and prepare a plan of works and measures for managing the present and future flood risk in Lithgow.
Where does Main Stream Flooding occur in Lithgow?
Main Stream Flooding in Lithgow occurs when flow surcharges the channels of Farmers Creek and Marrangaroo Creek, as well as their major tributaries. These flows may be several metres deep in the channels and relatively fast moving. Main Stream Flooding also occurs when flow surcharges the major pipelines that have been laid along the routes of natural channels as the catchments have urbanised.
Where does Major Overland Flow occur in Lithgow?
Major Overland Flow results from runoff which travels as either sheet or concentrated flow over grassed and paved surfaces in individual allotments, or along roads en-route to the trunk drainage system, or surcharges the minor pipes in the catchment headwaters and the lateral sub-catchments bordering the trunk drainage systems. Major Overland Flow is limited to areas affected by depths of flow greater than 100 millimetres.
What is the Probable Maximum Flood?
The Probable Maximum Flood is the largest flood that could conceivably occur at a particular location, usually estimated from probable maximum precipitation coupled with the worst flood producing catchment conditions. The Probable Maximum Flood defines the extent of flood prone land (i.e. the floodplain). The extent, nature and potential consequences of flooding associated with events up to and including the Probable Maximum Flood should be addressed in a floodplain risk management study.
What has been defined as flood prone land in Lithgow?
Flood prone land in Lithgow has been defined as land which is inundated to depths greater than 100 millimetres during a Probable Maximum Flood event. It comprises areas that are affected by both Main Stream Flooding and Major Overland Flow.
What is the Interim Flood Planning Level for Lithgow?
The Interim Flood Planning Level for Lithgow is a combination of flood level and freeboard which has been selected for planning purposes. Pending the completion of the future Floodplain Risk Management Study for Lithgow, it is equal to the peak 1 in 100 year flood level plus 500 millimetres freeboard. The Interim Flood Planning Level has been defined along the major overland flow paths in Lithgow, as well as in areas subject to Main Stream Flooding.
What is the Interim Flood Planning Area for Lithgow and how does it affect my property?
The Interim Flood Planning Area for Lithgow is defined as land which lies below the Interim Flood Planning Level. Properties which lie either partially or wholly within the extent of the Interim Flood Planning Area (i.e. land which lies below the Interim Flood Planning Level) will be subject to S149 flood affectation notification, and as such subject to the flood related development controls set out in Clause 7.2 of the Lithgow Local Environmental Plan, 2014.
The Interim Flood Planning Area (IFPA) for Lithgow (refer to LFSR Report Vol 2 Figure 6.16 Sheets 1-4 for extent) has been derived as part of the present investigation for areas subject to both main stream flooding and major overland flow.
What are the next steps in the process?
Council, subject to funding will undertake the preparation of a Floodplain Risk Management Study and Draft Plan for Lithgow. The Floodplain Risk Management Study will evaluate management options for the floodplain in respect of both existing and proposed development.
This will include the assessment of various:
- flood modification measures such as levees, trunk drainage upgrades and channel improvements;
- property modification measures such as the development of a grade set of prescriptive flood related development controls which would apply to future development located on flood liable land; and
- response modification measures such as improvements to flood emergency planning and weather warning systems, and the implementation of a flood awareness and education program.
A Draft Floodplain Risk Management Plan would also be prepared as part of the study, which would set out a preferred set of measures which are aimed at managing the flood risk in Lithgow. Upon adoption of the Draft Floodplain Risk Management Plan by Council, funding can then be sort from the NSW Government for its implementation.
While a date has not been set for the commencement of the Floodplain Risk Management Study and Draft Plan, its preparation would typically take between 12-18 months to complete. Input from the local community will be sort at the commencement of the study on the range of potential measures which could form part of the Draft Floodplain Risk Management Plan. The community will also be given an opportunity to comment on the Floodplain Risk Management Study and Draft Plan prior to its adoption by Council.
My property was never classified as ‘flood prone” or “flood liable” before. Now it is. Why?
The Lithgow Flood Study Review has a wider and more comprehensive scope of investigation than previous flood reports. This study includes Farmers Creek catchment and the catchment of Marrangaroo Creek to define the nature of both Main Stream Flooding and Major Overland Flow in the urbanised parts of Lithgow and future growth areas for floods up to the Probable Maximum Flood.
This is why more properties, some of which are a long distance from any natural channel, are now identified as being flood prone.
What if I disagree with the flood mapping for my area?
Council has used up-to-date modelling techniques, as well as expert assistance from consultants and the NSW State Government, to determine mapping and analysis of risk. Council believes our flood level estimates are as accurate as possible at this point in time.
If you believe that are not correct for your property, you can seek professional advice relating to your property and request Council to review.
How do I find out what the flood level is for my property?
If your property has been included in the Lithgow Flood Study Review Report, Council’s duty planners can assist you to identify the 1% AEP flood level, the interim flood planning level and area, the flood hazard and hydraulic category and other relevant data from the computer modelling data that supports the report.