Planning for Floods
The development of towns and agricultural enterprises on floodplains has highlighted the need for improved floodplain management practices. Effective floodplain management is necessary to reduce economic losses and minimise the potential loss of life from flooding.
In 2000, the Agriculture and Resource Management Council of Australia and New Zealand published Floodplain Management in Australia - Best Practice Principles and Guidelines
for full document reference). This document states that the principal aim of floodplain management "is to reduce the effect of flooding and flood liability on individual owners and occupiers of flood-prone property, and to reduce private and public losses resulting from floods."
The Guidelines promote an integrated approach to floodplain management based upon a consideration of the total catchment, the integration of roles and responsibilities, the integration of floodplain management plans with flood emergency plans and the integration of stormwater and mainstream flood behaviour.
Total catchment considerations are necessary to avoid moving flood effects from one point to another via inappropriately designed flood amelioration measures. For example, the construction of levees may reduce the flooding on one parcel of land, but increase flooding on upstream or downstream properties.
Integration of roles and responsibilities is necessary to ensure that the various technical elements and planning tools (ie planning schemes, flood response, flood warning, flood assessment) come together to form a workable floodplain management plan.
A fundamental element in developing a floodplain management plan is to collate information about past floods so that areas prone to flooding and flood potential can be estimated.
Under various funding programs, the Department of Primary Industries, Water & Environment has guided the development of Floodplain Maps and Flood Data Books
. Floodplain Maps are detailed maps of areas inundated with various flood risks. Floodplain maps are usually restricted to urban areas where significant historic information is available, and where the economic cost of flooding is high.
Flood Data Books are a compilation of information on past flooding, mainly in rural areas, including photographs, maps of flood extent, flood profiles and a tabulation of heights reached by historic floods.
Construction in Floodplains
Construction of dwellings and other structures in floodplains should generally be avoided. However, where such development cannot be avoided there are a number of guidelines which should be followed.
Tasmanian Building Legislation
The Tasmanian Building Act 2000
and Regulations 2004
provide some guidance on developments in areas below the "designated flood level". In particular, buildings containing habitable rooms must not be built unless the floor level is at least 300 millimetres above the designated flood level for that land.1.
For the following watercourse floodplains the designated flood level is the level which has a 1% probability of being exceeded in any year (ie the 1:100 annual exceedance probability flood level). Most of these areas are covered by Floodplain Maps or Flood Data Books.
- Derwent River through New Norfolk
- Upper Reaches of the Tamar River and lower reaches of the North Esk River
- Huon River at Huonville and Mountain River
- South Esk River through Longford to the Tamar River
- Jordan River below Pontville
- Mersey River through Latrobe
- Bagdad Rivulet
- Elizabeth River through Campbell Town
- Macquarie River at Ross
- Coal River at Richmond
- Meander River at Deloraine
Where land is subject to flooding affected by the rise and fall of the tide, the designated flood level is 600 millimetres above the ordinary high water mark of the spring tide.3.
Otherwise, where land is known to be subject to flooding, the designated flood level is 600 millimetres above ground level, or the highest known flood level, whichever is higher.
Historic flood levels can often be determined using local knowledge (preferably pictures of extreme events) in conjunction with 1:25000 map sheets. Provided levels are identified with respect to some known geographical feature (such as a bridge abutment, tree, etc.) they are generally reliable. If two or more pieces of evidence support a flood level then you can be more confident of the information.
A flood levee is defined as a "dam" under the Water Management Act 1999
, for the purpose of development approval. A permit for "dam works" must be obtained under that Act before a levee is constructed, enlarged, modified or repaired.
Whether or not a habitable building is to be built is not the only consideration when building on a floodplain. In some cases, developments on flood plains can affect flood levels upstream. In other cases the development may concentrate flows and lead to more excessive damage on existing infrastructure compared with what would have occurred previously.
Where floodplain maps have been completed, hydraulic models of the area may be used to assess the effects of a development proposal. In other cases advice from an experienced hydraulic engineer may be sufficient to identify any issues which should be taken into account as part of the development.
In cases where no floodplain map or flood data book exists, it may be necessary to utilise advanced hydraulic modelling techniques to translate stream gauging data or estimated flood peaks to the point of interest. This is a job for an experienced hydraulic engineer.
Proposed development should also recognise all existing flood risks, including the increased flood effects of potential dam break floods, where dams exist in the catchment upstream of the proposed development.
Catchment Care and Floodplains
Hydraulic changes to the river can lead to higher flood potential for floods of the same size.
Rivers are natural landscape elements which are subject to aggradation (build up of material) and scouring (removal of material). Aggradation and scouring can lead to significantly different flood heights for the same flow.
Dense willow growth in the river channel can lead to significant changes in flooded area. For example, at points along the Meander River, floods of the same flow have been found to be 1 - 1 1/2 m higher when dense willow growth is present.
Flood hydrology can also be affected by major changes in land use. Deforestation can lead to increased peaks and volumes of runoff. Channelisation of waterways and wetlands may reduce the time in which an individual property is flooded, but may increase the severity of flooding downstream. Cumulative effects of levee building can also lead to significantly increased flood problems when a large flood arises.
Land managers and Local Government need to be aware of these potential hazards and incorporate effective policies, planning and management to ensure sensitive management of floodplain areas.
Further InformationFlood Data BooksFloodplain Mapping in TasmaniaDam Works Permit Guidelines
Agriculture and Resource Management Council of Australia and New ZealandFloodplain Management in Australia: Best Practice Principles and Guidelines
SCARM Report 73
ISBN 0 643 06034 0