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Flood Hazard Q and A

What is flood hazard information?

Flood hazard maps show which areas of the city might be flooded in an extreme rainfall event, and to what extent.

It is part of normal Council business to know the effects of natural hazards that could impact our community. Flood hazard maps for Tauranga City have been evolving since the early 1990s. The information is used for building consents, subdivisions and infrastructure planning.

A lot has changed in recent years and we need our records to be as accurate as possible, so Council is in the process of updating flood hazard maps for the whole city.

What is a 100 year rainfall event?

An extreme rainfall event is an unusual scenario, something that doesn’t happen very often. It is a dump of rain that you could reasonably expect to experience at least once in your lifetime.

The flood hazard map is modelled for a 100 year rainfall event. The term ‘year event’can be confusing because such an event could actually occur more than once every 100 years. Technically, a 100 year event means there is a 1% chance that it might happen in any given year.

Tauranga City Council has chosen to model flood hazard maps for the 100 year event because it will allow us to consider building consent applications under the Building Act 2004 and subdivision consent applications under the Resource Management Act 1991.

We have not modelled the effects of less extreme events on specific properties. Although we model the 100 year event and show it on the property file, this does not mean that flooding in lesser events will not occur.

How are the flood hazard maps prepared?

The area of land being studied is called a catchment. When rain hits the ground it all drains down hill to a particular stream or outlet. The stormwater catchment is the defined area of land that contains and collects all of that water.

To plot the potential flood hazard we build a computer model of the area that we wish to study. The flood hazard map predicts what happens to the water in the catchment under extreme rainfall conditions. We then use computer software to simulate different rainfall intensity on that catchment.

The basis of the flood hazard information is a system called LIDAR (Light Detection and Ranging) which measures the ground levels of the land using laser pulses. This generates an accurate contour map which we place into the stormwater computer model. LIDAR is very reputable technology used by most councils in New Zealand.

Our first job is to work out which criteria will be entered into the computer model. The sorts of things that need to be considered include:

  • How hard is it raining?
  • How long has it rained for?
  • What is the contour of the ground?
  • Where will rain water soak into the ground (e.g. grass)?
  • Where will rain water flow over hard surfaces (e.g. roofs, concrete)?
  • How long will it take for rain water to flow from one part of the catchment to another?
  • What stormwater systems are already in place?

Technology today is able to take all of these variables into account. (We do not include physical obstructions to flows, such buildings, walls and fences in the model.)  Once the criteria have been set we can run the flood hazard model. There is a huge amount of information to be computed so it takes longer than real time for the software to process each model. We run the model to calculate how, when and where the rainwater flows. The result tells us which parts of the catchment are likely to be covered by water, and to what depth.

The above process uses a synthetic rainfall event or design storm.  Essentially a design storm is a mathematical representation of a rainfall event that reflects conditions in a given area (as outlined above).  The design storm we use is a 100 year event taking into account high tide, duration of rainfall and a range of other factors.  The advantage of using the synthetic event is that is appropriate for determining both peak runoff rate and runoff volume and it significantly reduces the complexity of stormwater analysis.  Such a “perfect” event will rarely occur in nature but we require such a design storm to be constructed as it gives us a consistent model for determining how existing infrastructure operates under certain conditions and therefore how new infrastructure will need to be sized and designed.

When the model is complete it gets loaded into our GIS (Geographical Information System) mapping system for further analysis and processing. The information needs to be tidied up to the point where it can be presented as an accurate map. Once this is done the final map is peer reviewed. The flood hazard map is now valid and active.
 

How is the flood map information presented?

TCC models a 100 year flood event, based upon existing development (hard surfaces/buildings) within each catchment. We have chosen to separate the flooding into four different depths as shown below as of the 12 May 2015. Flooding that is less than 100mm is not shown.

  • 0.1m – 0.25m 
  • 0.25 - 0.5m
  • 0.5 - 1m
  • Greater than 1m

View the flood maps online

Overland flow paths

As well as modelling for depth, we also model the speed and direction of flood water. This gives us important information about where flood waters flow during a heavy rain event. This ‘overland flow path’ information is not shown on the general flood hazard map but it is viewable on the Council mapping website and is publicly available along with the rest of Council records.

There are some areas where overland flow paths have been identified despite there being no flood hazard. This is because we do not show flooding under 100mm on the flood hazard maps. Overland flow path modelling indicates that water will travel through these areas in an extreme rain event, but it will not collect and pond above 100mm. 

‘Safety to Persons’ Focussed Level of Service

Through the 2015-25 Long Term Plan process, Council adopted an approach to flood (stormwater) risk management (intense rainfall events) primarily for implementation in older more established areas of the city.  

Stormwater level of service

This approach provides primarily for a ‘safety to persons’ focussed level of service which results in the highest priority being placed on areas having the highest number of at-risk properties having a habitable floor within 8 metres of where flood flows exceed a depth x velocity threshold in residential and rural residential zoned private property. The next highest priority is placed on those areas having the highest number of non-residential/non-rural residential private properties.

Through our models we can identify those properties where this depth x velocity threshold is exceeded and where Council should being focussing our efforts.

Does flood hazard information go on my property file?

Yes, the updated information has been stored on the files relating your property and on our GIS mapping system. Flood hazard information for every property has always been publicly available on request. It is noted in Land Information Memorandum (LIM) reports and used when reviewing building and resource consents.

We are required by several acts of Parliament to hold this information and to make it publicly available. As custodians of this information we will always provide the most up to date information that we have available about your property.


Last Reviewed: 28/03/2017