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Living with Floods
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- Living with floods
- What is flooding?
- Who is responsible for the planning & management of flood prone land?
- Who is responsible for emergency management and response during a flood event?
- History of Flooding in Gosford
- Why do drains overflow?
- Why do I need to report a flood event to Council?
- Why does Council need to address flooding?
- What is Council doing to manage the flooding problem?
- Is my house affected by floods?
- How do I obtain more flood information for my property?
- Is my house protected by flood mitigation measures?
- Am I liable for any work I do in a drainage easement, flood area or watercourse?
- Reporting a stormwater drainage issue on private land
- Flood Insurance issues
Living with Floods
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Over thousands of years Gosford’s local landscape (the wetlands, creeks, floodplains and estuaries) has been formed by natural processes which include flooding and/or elevated ocean levels.
Flooding is generally a rare event and it is not always possible to predict when, where, or how big the next flood will be. However, Council can estimate the likelihood of different sizes of floods and their consequences to plan in advance. |
Floods can bring considerable environmental and consequently, human benefit. Groundwater is replenished, wetlands recharged, alluvial soils renewed and flood debris provides habitat and food for many species. For the floodplains to be both healthy and productive, periodic flooding is essential.
However, there are also negative consequences to flooding in the Gosford floodplains. This includes potential risk to life and catastrophic financial losses through damage to buildings and/or the building contents, public infrastructure, utility services and the environment. Flooding also has the potential to disrupt essential services and cause human trauma. This can be especially true in many of Gosford’s suburbs where flooding can occur very quickly and with little or no warning.
Facts associated with flooding:
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Fast moving water can knock down a person
- Moving water of little more than 300mm can float and wash away an average vehicle
- Approximately 50% of deaths are caused by people trying to escape from a flood affected area via vehicle or on foot.
There are no quick fix solutions to the flooding issues in Gosford. The complete elimination of flood risk on the floodplains of Gosford is an impossible task. It would inevitably entail the construction of massive drainage systems through existing urbanised areas which would be both cost-prohibitive and practically impossible. It is therefore necessary to find other ways to manage and live with the flood risks in Gosford.
What is Flooding
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Flooding is defined as a relatively high water flow that overtops natural or artificial banks in any part of a stream, river, estuary, lake or dam. Flooding also includes local overland flow paths outside the defined drainage reserve and/or coastal inundation resulting from:
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The dynamics and process used to determine a tsunami event varies greatly from that used in determining flood risk. Flood risk is based on statistical data and a combination of hydrologic and hydraulic processes (and storm surge where applicable). Therefore, tsunamis have been excluded from the definition of flooding.
The most common forms of flooding in the Gosford region are flash flooding, within creeks, rivers and waterbodies, and overland flow due to intense rainfall within the catchment. Tidal action, such as King Tide, and storm surge, generated by storms off the east coast of Australia, are also very common causes of flooding in Gosford Local Government Area estuaries and beach areas
Who is responsible for the planning and management of flood prone land?
Councils are generally the responsible authority for the planning and management of land subject to varying degrees of flood risk. They do this by undertaking flood related studies and plans and then setting development controls and / or undertaking works to alleviate the potential of future risks to life and damage to property.
Development controls are reinforced through the preparation of LEPs, DCPs and Policies and Procedures. In preparing the LEPs and DCPs, Councils are required to liaise and adhere to various policies and directives released by State Government Departments e.g. Department of Planning and Department of Environment and Heritage.
Who is responsible for emergency management and response during a flood event?
The SES is the combat agency for emergency response planning for a flood. They do this by preparing DISPLANS (or Local Flood Plan) with input from Council’s flood studies and plans.
The SES is the lead agency for emergency response during a flood, visit SES website for more information.
History of flooding in Gosford
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Since Australia's early settlement, Gosford has been gradually increasing its urbanised areas. Settlers and freed convicts began moving into the Gosford region during the 1830's as Sydney's colony expanded and sought good farming land and building materials. Gosford provided timber and lime to the colony. Fishing villages sprang up around Brisbane Water and small farms and orchards flourished along the rich alluvial floodplains. Ship building around Brisbane Water began in the mid 1800's due to the availability of timber and flourished until the advent of the main northern railway line in the late 1800's. |
Gosford provided timber and lime to the colony. Fishing villages sprang up around Brisbane Water and small farms and orchards flourished along the rich alluvial floodplains. Ship building around Brisbane Water began in the mid 1800's due to the availability of timber and flourished until the advent of the main northern railway line in the late 1800's.
With the completion of the railway line, Sydneysiders began to spend their vacations in the region and many holiday cottages sprang up in low lying areas around Brisbane Water and the coastal beaches and lagoons to the east. More people were encouraged to retire to the region or take up small farm holdings during the early 1900s through the sale of crown lands for minimal sums, particularly to the north of Gosford.
With the completion of the railway line, Sydneysiders began to spend their vacations in the region and many holiday cottages sprang up in low lying areas around Brisbane Water and the coastal beaches and lagoons to the east. More people were encouraged to retire to the region or take up small farm holdings during the early 1900s through the sale of crown lands for minimal sums, particularly to the north of Gosford.
Urban development has placed considerable pressure on existing drainage systems and natural waterways. As the land was developed, much of the permeable surface was replaced by impermeable surfaces such as rooftops, asphalt roads and concrete footpaths and driveways. These surfaces significantly reduced the quantity of water able to infiltrate into the soil and consequently increases the quantity of stormwater run-off being generated.
Increased development in some catchments also reduced the area of land available for water to travel toward the receiving waters. This has lead to natural overland stormwater flow paths being cut off by development further down the catchment. As a result the water tends to pond in areas where it previously did not.
These problems have been further compounded by the fact that, historically, when there was little understanding of the drainage pattern and ecology of the area, many of the natural stormwater detention areas, such as wetlands, had been filled in.
The history of settlement and subsequent development has left Council with some very serious flooding and drainage issues. During the late eighties and early nineties urbanised areas in Gosford experienced serious flooding problems.
Why do drains overflow?
For economic and environmental reasons, stormwater pipelines have a limited capacity. Most pipelines in Gosford were designed for a 10% AEP or 1 in 10 year storm event.
A 10% AEP is the probability (i.e. a 10% chance) that a particular rainfall event will be equalled or exceeded in any one year. It should be noted that this does not mean that a 1:10 year storm will only occur once every 10 years.
Storms of greater intensity than the 10% AEP produce overflow which then proceeds down secondary flowpaths or lies in depressions.
These secondary systems may be natural or manmade. They may consist of roadways, drainage easements, and swales, retarding basins, wetlands etc or a combination of these items.
Drains also overflow due to blockages in the drainage system by debris which accumulates either before or during a storm event.
Why do I need to report a flood event to Council?
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One of Gosford City Council’s core responsibilities is to ensure that the health and safety of the community and the local environment is maintained as well as is practicable. In order to fulfil this task, Council generally relies on information that it obtains from the general public.
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If you have experienced consistent minor flooding on or near your property which has originated from a local stormwater drain, natural stream or estuary, please make sure it has been reported to Council. This information is important to help identify new or repeated areas of flooding, assist with determining flood heights for future flood events, and identifying areas for priority investigation and potential mitigation works.
Flooding issues can be reported by contacting Gosford City Council
Why does Council need to address flooding?
Gosford City Council has a ‘duty of care’ to manage lands subject to flooding within the Local Government Area.
Under the NSW Government’s Flood Prone Land Policy, local government has responsibility for managing flood liable land. Council is responsible for formulating and implementing Floodplain Risk Management Plans in accordance with the Policy. The Policy aims to reduce private and public losses resulting from flooding. The Policy encourages the development of:
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solutions to existing flood problems in developed areas, and
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strategies for ensuring that new development is compatible with any identified flood hazards and does not create additional problems in existing developed areas.
Gosford City Council, with the assistance of the Floodplain Risk Management Committee, is responsible for this task in each drainage catchment.
What is Council doing to manage the flooding problem?
Investigation of flood reports
One of Council’s core responsibilities is to ensure the health and safety of all residents and the local environment. In order to fulfil this responsibility, Council generally relies on information obtained from the general public.
Council is committed to investigating all reports of flooding. Your information is valuable and will assist Council to understand the events that caused the problem. If you have experienced consistent flooding on or near your property which has originated from a local stormwater drain, natural stream or estuary, these flooding issues can be reported by contacting Gosford City Council.
Prioritising Work
Problems which present a serious risk to life, property and the environment are carried out first as it is not possible to predict when the next major storm will occur.
Some flooding issues are simple and can generally be carried out by Council staff specially trained to carry out such work in a relatively short timeframe.
However, some flooding issues are complex and require detailed flood studies to be carried out by specialist Council staff and Consultants in this field of expertise. Computer models are generally used (using data supplied by local residents and rainfall and water level recordings from previous flood events) to determine which properties may be affected by various flood events and what flood mitigation measures will achieve the best results. This process is conducted using the NSW Government Floodplain Development Manual – the management of flood liable land (April 2005).
NB:It should be noted that local drainage is only intended to manage lesser and more frequent rainfall events. Local drainage problems can occur anywhere and are not included in Council’s floodplain management planning.
For further information please refer to the NSW Floodplain Development Manual
Works Program
Once the more complex problems have been investigated and solutions identified and prioritised, a Floodplain Risk Management Plan is formulated and implemented. Some large systems may take many years to complete due to the large overall cost, whilst smaller high priority problems can be fixed with little expense.
Where possible Council includes identified works in its annual capital works programmes. Council currently spends in the order of $4 million per annum on flood mitigation and drainage augmentation works. This funding can also be boosted by State and Federal Government grants for flood related projects.
As part of the works programme, Council has augmented existing drainage systems where possible, constructed swales and detention systems on public land, widened channels and removed thick exotic vegetation from creek beds and banks so that the risk of flooding on properties is reduced. Council has also been successful in attracting some grant funding from the State and Federal Governments to undertake this type of work.
Creek Maintenance
Council currently maintains fifteen high priority creeks which have been identified via various Floodplain Risk Management Plans. The maintenance of these creeks is undertaken in accordance with site specific Maintenance Management Plans which have been prepared for each creek system. Inspections of the creeks are carried out at six to twelve monthly intervals with additional inspections carried out following major storm events.
The core objective of Council’s creek maintenance activities is to maintain adopted Flood Planning Levels thereby limiting the impact of flooding on surrounding developments. This is generally achieved via creek stabilisation or repair works and through the management of sediment, debris and vegetation build up.
Creek maintenance activities are regulated by a number of government departments and with the exception of emergency situations, works cannot be undertaken without the appropriate licences in place.
Specific creek maintenance activities include:
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Typha control - this plant can congest waterways, alter water flows and may increase the risk of flooding in urban areas.
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Tree removal – removal of non native weed species from the bedways of creeks eg Willow trees form thickets which divert water outside the main watercourse or channel, causing flooding and erosion where the creek banks are vulnerable.
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Sediment Removal – where blockages occur adjacent to culverts.
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Noxious Weed Control – in stream woody weeds and smaller aquatic weeds
Creek maintenance activities do not include removal of the following unless under exceptional circumstances and with the appropriate licences in place:
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Large woody debris - consists of trees or shrubs that have fallen or been washed into rivers and streams. Large woody debris is a significant ecological and structural component of streams and rivers.
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Native riparian vegetation - vegetation on land that adjoins, directly influences or is influenced by a body of water. Riparian vegetation is found alongside creeks and rivers, areas around lakes, wetlands and on river floodplains.
The removal of large woody debris and native riparian vegetation are listed as a Key Threatening Process under the Fisheries Management Act. A Threat Abatement Plan has been developed which outlines management strategies for address this problem. For further information please refer to the following links from Department of Primary Industries
Data Collection
Whilst undertaking investigations and studies on the flood risks in the Gosford Local Government Area (LGA), Council has also been collecting extensive data. The data includes:
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acquiring aerial photos of the entire LGA. These enable Council to measure areas of rooves, roads, trees and parks etc which can be used for computer calculations of how much water will soak in to the soil and how much will run off the land under different severities of rainfall storm events.
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Lidar land mapping which gives Council greater accuracy with ground levels across the flood affected areas. This allows computer calculations of potential floods to be confidently estimated for small to very large floods.
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historic flood observations and surveyed flood levels to calibrate (confirm the accuracy of) the computer models against real observed floods and measured rainfall.
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mapping Council’s stormwater infrastructure assets such as pits, box culverts, headwalls, channels and pipelines.
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collecting surveyed floor levels and building types of potentially flood affected properties.
Mapping
As part of the management strategy, Council also develops flood maps. These maps show the public whether there are any identified flooding issues affecting the property of interest. The maps can be found on Council's GEMS mapping constraints website or alternately can be viewed at Councils main office at 49 Mann Street Gosford.
These maps are continuously updated as new studies are completed or reviewed for various catchment areas.
These maps mainly show the extent of the 1% AEP (1 in 100 year) flood event which is generally used for development control.
Is my house affected by floods?
Gosford City is surrounded by many waterways which in the past or sometime in the future will exceed the capacity of their channels or basins and flood adjoining land. Council is undertaking flood related studies to determine to what extent these waterways will flood so as to protect existing and restrict future development in the problem areas.
From these studies Council is producing flood maps to assist in identifying the extent and characteristics of each individual floodplain. When a study is completed it is placed on Council's GEMS mapping constraints website which is mainly used for development control and is updated on the website for community information. It should be noted however, that these maps are not detailed enough to provide flood levels for individual properties and this should be sought from Council as detailed below.
How do I obtain flood information for my property?
It is not Council's general practice to provide flood information about a property over the phone. Flood information is normally supplied in writing after paying a fee for a Flood Level - Minimum Floor Level Enquiry.
Council does have some flood maps available on Council's GEMS mapping constraints website which give an indication of areas that will most likely be affected by various flood events. However, these maps are not detailed enough to provide flood levels for individual properties.
Should you require more detailed information from that supplied on the maps and in relation to a Development Application, you may need apply to Council for Flood Level - Minimum Floor Level Enquiry.
NOTE: The insurance industry uses its own estimates for flood risk assessment and its own definitions for flooding. These may differ when compared with Council’s information. Council has not supplied its flood information to private insurance companies.
Is my house protected by flood mitigation measures?
Many areas in Gosford have flood mitigation measures in place. These may include a levee, a floodgate, a detention basin or larger drains. These measures will reduce the frequency of flooding and the length of time the stormwater remains on flood liable properties. However these measures can fail in severe and prolonged storm events. Therefore you should always be prepared for a major flood if your house is located on identified flood liable land or within close proximity to a creek, water course, water body or depression, etc.
To find out whether your property is within an area identified as being affected by flooding, please refer to Council's GEMS mapping constraints website. For more detailed information you can refer to Council’s list of current flood related studies.
If your property is affected by flooding, you should also refer to the SES Community Safety brochures for information on preparedness and safety before, during and after a flood event.
Am I liable for any work I do in a drainage easement, flood area or watercourse?
In short, yes! A solution to solving your problem may adversely affect your neighbour.
You should not construct any works that divert extra stormwater flows onto your neighbour’s property. You could be held liable by your neighbour for any damage as a result of such action.
A common problem in built-up residential areas is the construction of boundary fences and gates which impede natural overland flow paths. You should be careful when making these types of improvements to your property that you choose construction materials which will not dam or divert overland flood waters.
Likewise, placing fill on flood liable land may cause an increase in floodwaters upstream and downstream. Placing fill around your house and blocking or diverting pipes may also lead to serious flooding problems either to yourself or your neighbours. Many properties which were once flood free can be made flood liable because of these types of “improvement” works. You should always check with Council before placing any fill material on or near an existing pipeline, overland flow path or watercourse.
Similarly, do not attempt to change the location of a natural watercourse. There is a good reason why nature has placed the watercourse in that location and by relocating it you could cause damage to both your own and other people’s properties and also cause detrimental affects to flora and fauna.
Before undertaking any type of work near a drainage easement, flood prone area or watercourse please check with Council to seek advice on whether the work is permissible and whether a development application will be required.
Flood Insurance Issues
Any property, which is identified as being flood prone and that attracts development controls, will be encoded on the property’s 149 planning certificate message and formally notified by Council.
However, some insurance companies are also identifying flood prone land as a result of undertaking their own flood studies, analysis and flood mapping exercises, or using data from studies conducted by Local Government Councils. This information is being used to assess the hazard & risk and to then set premiums for flood insurance by individual insurance companies.
From enquiries made of a number of insurance companies by Council, it would appear that the method of setting policies and premiums is very much done on an individual company basis. This leads to some difference with premiums and flood liability across many properties under different insurance companies.
You should therefore make your own enquiries about how your insurance company classifies flooding (and stormwater), and how this relates to your insurance provisions and premiums.