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Floodplain Management Planning

http://www.gw.govt.nz/floodplain-management-planning

Floodplain Management Planning

Updated 25 August 2015 3:16pm

Greater Wellington's Flood Protection group works with communities to manage flood risk from the region’s rivers and streams. 

Our approach is to understand the processes affecting a river/stream and its floodplain within a wider catchment, and to provide a co-ordinated response through our floodplain management plans (in partnership with the community) to reduce the impact of flooding. 

What is floodplain management planning?

Floodplain management planning is a process that results in a long-term strategy for managing flood risk, helping improve the security and quality of life for present and future generations living on a floodplain.  It better prepares communities for coping with a flood when it occurs, and aims to ensure that any future development considers flood risk.  A floodplain management plan (FMP) emphasises the need to ‘keep the river away from people’ rather than ‘keep people away from the river’.

The outcome of a floodplain management plan process is a document that guides how the floodplain and catchment should be managed to:

  • Minimise risks to life, health and safety
  • Reduce severity of flood damage
  • Promote sustainable use of flood and erosion prone land
  • Promote sustainable development of the wider catchment
  • Use planning and community preparedness to ensure sustainable land use
  • Identify options to manage the flood risk

What is the GWRC policy regarding property development in flood risk zones?

Our policy is that new development is avoided in areas of flood risk.  Where development does take place in these areas (including infill development), we recommend that 'residual risk', or risk that is 'greater than the design period' event is recognised.  

Definitions of terms used

Flood 'size' and frequency
Flood Protection Engineers in New Zealand and around the world use measurements to describe floods such as a 1-in-5 year event, 1-in-50 year, 1-in-100 event and so on. The larger the number the bigger the flood - but the less likely it is to happen, based on probablity supported by historic flood records.  A 1-in-100 year flood means that a very large flood is statistically likely to happen once in every 100 years, in everyday terms there is a 1% chance of such a flood happening in any given year. 

Return period 
The  'return period' is the expected frequency of a flood. A 100-year 'return period flood' is an event that is predicted to happen on average once in 100 years or there is a 1% chance of it happening in any given year.

Design Period
A 'design period event' - is the return period to which flood risk management measures are designed for.

Residual risk
This is the risk that a flood larger than that for which the flood protection measures are designed for occurs.

How we measure floods
We describe floods as a 1-in-5 year, 1-in-50 year, 1-in-100 event and
so on. The larger the number the bigger the flood but the less likely
it is to happen, based on records of past floods. But don’t be misled
into thinking that a 100 year “return” period flood can only happen
once in a hundred years – several big floods could happen in quick
succession.

During development of a floodplain management plan, the 100-year return period flood event may be used as the design period event for developing measures used to manage the flood risk.  In some areas like in the Hutt River catchment a larger design period event has been used (1 in 440-year return period).