Flood Hazard Information Questions and Answers

Why is Council getting new flood information?
What is a Land Information Memorandum (LIM)? Does this information need to appear on a LIM?  What would the LIM say?
How is this information being used in the Proposed District Plan?
What is the difference between 'hazard' and 'risk'?
I am on a hill or a long way from the Waikato River, how can my land possibly be affected by flooding?
How does this affect property values?
How does this affect insurance?
What is an 'extreme rainfall event' (1 in 100 year flood event)?
Why is Council using a 1 in 100 year flood event?
What is involved in modelling a 1 in 100 year flood event?
How do I get more detailed information about the modelling process and the model outputs?
What do the different 'flood hazard areas' mean?
When was the last 1 in 100 year event? Why can’t we use this as an example?
How has the flood information been refined since its first release in April 2012?
Why is there detailed modelling for only parts of the City? Is Council going to investigate flood hazards further?
What will Council do to fix the flooding?
I’m in a proposed flood hazard area, does this mean I have to move out or raise the floor levels of existing buildings?
If only part of my property is affected what does this mean?
Will my house be flooded?
What is freeboard? When and why is it required?
Is Council going to investigate potential flood hazards further?
For which parts of the city has the flood hazard modelling been completed?
Why are we updating the flood hazard information?
Why is Council not using all the new flood hazard information?
How many properties are affected by the flood information?
Can I talk to the Council about the maps?
If the flood information is based on a computer model and engineer’s assumptions, how accurate is it likely to be?
Does this flood information mean that the stormwater system is inadequate?


 ​

1. Why is Council getting new flood information?

Council is preparing Catchment Management Plans for the city. These are required as part of the stormwater discharge consent requirements imposed by the Regional Plan. As a result, Council’s knowledge of potential flooding areas is growing and new flood mapping is being produced.

Council has used this information as part of the review of the District Plan. The  flooding information will be considered alongside new subdivisions and development of land potentially affected.

 

2. What is a Land Information Memorandum (LIM)? Does this information need to appear on a LIM? What would the LIM say?

A LIM is a report prepared by Council which summarises a range of information on a property, available at the time it is issued. A LIM is issued for a whole property (title) and cannot ‘exclude’ parts of land within that property. For example, in a cross lease situation the LIM will provide information relevant to the whole title, not just individual houses or parts of the land defined in a cross lease plan.

It is a legal requirement to provide LIMs and the types of information that must be included are set out in the Local Government Official Information and Meetings Act 1987. LIMs outline all information a council holds on a property, including permitted land use, consents, storm water and sewer drains, rating information and potential natural hazard risk including erosion and flood risks. They are most commonly requested by potential property purchasers.

As the flood hazard areas are identified in the Proposed District Plan maps the LIM will simply refer to any flood hazard area type relevant to the property for which the LIM has been requested.

back to top

3. How is this information being used in the Proposed District Plan?

The Proposed District Plan was publicly notified for submissions on 10 December 2012 and uses flood information to define flood hazard areas including objectives, policies and rules specific to each hazard area. This sets out what requirements must be met, whether resource consent is required, and what is considered to decide whether to grant or decline resource consent.

The proposed approach is that new subdivision, use and development should avoid areas subject to flood hazards if the level of risk is unacceptable. Refer to Chapter 22 and the features maps of the Proposed District Plan for details.

back to top

4. What is the difference between ‘hazard’ and ‘risk’?

‘Hazard’ and ‘risk’ are related but not the same. It is important not to confuse the two.

In this case the ‘hazard’ is flood water – in our hazard mapping we have separated the areas affected into different categories based on the depth and/or speed of the flood water at its peak. This gives us the three new flood hazard areas being used in the Proposed District Plan (Low, Medium and High Flood Hazard Areas).

‘Risk’ is how common the ‘hazard’ occurs combined with the impact it could have on people, property and the environment.

Existing development has not been assessed to determine levels of ‘risk’. This is not what the modelling work produces. The modelling allows us to identify hazards only.

Being in a Low Flood Hazard Area does not automatically mean existing development is at a low level of risk, nor that existing development within a High Flood Hazard is at a high level of risk. To work this out you need to consider the hazard and its probability of occurring against the existing development. Expert advice is recommended, things that may need looking at as part of assessing the risk to existing development could include the floor levels of buildings in relation to water depth, what the effect of water getting in could be (e.g. water in a garage or shed versus water in a house), and whether the building has been constructed to withstand the force of the flood water expected.

The Proposed District Plan uses the Low, Medium and High Flood Hazard Areas to determine whether new subdivision, new use or new development should be assessed through a consent process to identify the level of ‘risk’ and whether this is acceptable or not. If the level of ‘risk’ is unacceptable then consent for the new subdivision, new use or new development may be refused.

The flood hazard areas we have modelled are based on an extreme rainfall event occurring, on average, only once every 100 years. See What is an extreme rainfall event for more information.

back to top

     

6. I am on a hill or a long way from the Waikato River, how can my land possibly be affected by flooding?

A rising river is not the only source of potential flooding. A significant portion of the flood hazard areas are as a result of rain falling and flowing away via overland flowpaths (e.g. water flowing downhill, through channels and gullies) or ponding in localised depressions or low points. On ‘flat’ land it is often very difficult to actually see gradual or subtle changes in the contour of the land that would create low points or overland flowpaths on a property.

back to top

7. How does this affect property values?

The nature and extent of restrictions on the development potential of land and the risk to existing development will vary from site to site.

Council cannot make any comment about what the affect on property values may be. Council is required to get flood related data and make it available to the public and for the consideration of anybody looking to buy a property in the area.

Council’s new flood modelling information, communicated in letters sent to Hamilton property owners on 14 November 2012, has not been taken into account in the recent property revaluations. The new valuations, which are as at 1 September 2012, are a reflection of the sales and market evidence leading up to the valuation date. If there were any impacts to values as a result of the flood hazard information, this would be determined by sales evidence over a period of time.

back to top

8. How does this affect insurance?

Council can not advise you about the affect this flood information may have on the ability to obtain insurance or on insurance premiums. Individuals should contact their insurance provider to discuss this. There is often a clause with insurance policies requiring policy holders to advise the insurer of any new information concerning natural hazards and their property. Flood information is publicly accessible information and is available to insurance companies.

back to top

9. What is an 'extreme rainfall event' (1 in 100 year flood event)?

A 1 in 100 year flood results from an extreme rainfall event that, on average, would only occur once every 100 years. However, it is much, much more than a very rainy day. The term does not imply there will be exactly 100 years between these events. It’s more accurate to describe it as an event of a size that has a one percent chance of being exceeded in any one year.

back to top

10. Why is Council using a 1 in 100 year flood event?

The proposed Waikato Regional Policy Statement tells us that Hamilton City Council should plan for flood events of a scale that occur, on average, once every 100 year. This is reinforced by best practice in New Zealand, where more and more councils are planning to this level.

back to top

11. What is involved in modelling a 1 in 100 year flood event?

As there is insufficient historic, observational information to define the extent of land affected in a 1 in 100 year flood event Council has used a stormwater computer model to recreate one.

The output of the computer modelling process is reliant on the information put into it and the criteria used. Firstly, the city is separated into different areas called catchments. Because of the shape of the land, water that falls when it rains cannot get into another catchment.

The model is told:

  • What the shape of the land is – these ‘contours’ represent the hills, gullies, depressions in the ground, open channels etc of the city. The contour information comes from LiDAR (Light Detection and Ranging) data that was taken at the end of 2008 as part of aerial photography work. LiDAR is a system that measures ground levels of the land using laser pulses.
  • How much rainfall to drop onto the land – the volume, intensity and duration of rainfall used is what represents a 1 in 100 year event. For Hamilton this means 104mm of rain in 6 hours, adjusted for an increase of 2.08 degrees celsius by 2090 due to climate change.
  • The general surface conditions – the type of surface rain falls on affects how fast water flows over it i.e. water flows faster over paved or concreted surfaces compared with grass.
  • The capacity of public stormwater infrastructure – infrastructure such as culverts are included.
  • Ground conditions are near saturation point which means it cannot absorb much rainfall – the model removes some water due to infiltration (soaking in).

The modelling result tells us which parts of the catchment are likely to be covered by water, the flood water depth, and how fast it is likely to be flowing (although we only have depth information for the Waikato River corridor which is sourced from Waikato Regional Council).

back to top

12. How do I get more detailed information about the modelling process and the model outputs?

A detailed technical report on the modelling process is available from Council’s website – www.hamilton.co.nz/districtplan/flood

back to top

13. What do the different 'flood hazard areas' mean?

The available flood information has been split into five flood hazard areas. The differences between them reflect the nature of the information Council holds.

1. Temple View Flood Hazard Area (already known information)

These areas are susceptible to flooding associated with small-scale farm dams and secondary flow paths that are part of the Waipa Flood Prevention Scheme. The extent of this hazard area is based on a 1 in 100 year event. This information is already shown in the Operative District Plan, being included as part of the Environmental Protection Overlay (EPO).

2. Culvert Block Flood Hazard Area (already known information)

The Culvert Block Flood Hazard Area applies upstream of significant culverts along the gully system. These represent the maximum effect of a culvert becoming blocked whereby water backs up the gully until it eventually flows over the accessway or road above the culvert. This hazard area is already shown in the Operative District Plan, being included as part of the EPO.

3-5. High, Medium and Low Flood Hazard Areas (new information)

These areas have been identified from computer modelling as part of Council’s ongoing Catchment Management Plan programme. The areas have been identified on maps which have been produced by modelling and flood hazard experts.

This modelling creates a picture of what flooding may look like from an extreme rainfall event (i.e. a 1 in 100 year event). Two sets of modelling is used, one for the Waikato River corridor dealing with river flooding and another for sub-catchments in the City dealing with overland flowpaths and ponding flooding. The land affected has been divided and mapped into high, medium and low categories, according to the different flood water depths and velocities that the model shows could occur in an extreme rainfall event.

The flood hazard areas for overland flowpath and ponding flooding elsewhere in the city are defined by the following depths and velocities:


Flood hazard areas in the Waikato River corridor are defined using the following depths:

Depth and velocity (speed) are the key factors in determining the effect of flood water on people and property, this is summarised in the table below.

​Floodwater depth Floodwater velocity
(​metres per second)
Depth x velocity​ Effect on people and property​
0 to 10cm​ Any velocity​ -​ At this depth, surface water is unlikely to be a hazard to people and unlikely to cause damage to property. ​
10 to 50cm​ <1.0m/s​ -​ At this depth and velocity flood hazards are normally traversable by emergency vehicles and damage to property is minor to moderate. People can usually stand but more vulnerable people can be more significantly affected (e.g. children, elderly, injured, physically disabled). Scour/erosion of building foundations are unlikely to occur.​
50 to 100cm​ <2.0m/s ​ -​ At this depth and velocity the stability of people in water is at risk.
Damage to property can be financially significant. ​
>100cm​ >2.0m/s ​ >1​ At velocities greater than 2 metres per second the stability of buildings and their foundations can be significantly affected, as the force of the water can scour building supports. At depths greater than 1m significant damage to building and risk to life is very likely. ​

Note
The effect on property depends in part on the floor height of a building. Where the water is not flowing (i.e. ponding) a building with floor heights above the height of the flood water and an adequate freeboard is unlikely to suffer significant damage, whereas a building with floor heights below the height of the water is likely to suffer inundation damage (e.g. water and silt damage).

back to top

14. When was the last 1 in 100 year event? Why can’t we use this as an example?

The last 1 in 100 year event for Hamilton was 1958 but we cannot use this occurrence to fully define our flood hazard areas because:

  • We have incomplete records of the extent of the area affected and no clear comprehensive information about the depth or velocity of floodwater
  • Since 1958 there have been changes to:
    • stormwater infrastructure
    • topography – land contours have been changed through natural processes and land development
    • land use and the level of development – there are more buildings, more concrete, more property and more people within the city. What may have once been rural land is now urban
    • the volume and intensity of rainfall – climate change has altered the frequency and severity of large rainfall events.

As there is insufficient historic, observational information to define the extent of land affected in a 1 in 100 year flood event, Council has used a stormwater computer model to define this.

back to top

15. How has the flood information been refined since its first release in April 2012?

  • All ‘indicative’ flood hazard areas have been removed from the Proposed District Plan maps.
  • Only ‘confirmed’ high, medium and low flood hazard areas have been included in the Proposed District Plan maps.
  • The depth and speed factors used to define the high, medium and low flood hazard areas have been changed. This means that shallow flooding (less than 10cm) is no longer used to define flood hazard areas and large areas of what used to be medium flood hazard areas are now within the low flood hazard area.
  • New modelling data for the Waikato River is now available and has been used to identify medium and high flood hazard areas in the Waikato River corridor.
  • Minor isolated clusters of flood hazard area have been rationalised or removed.
  • In response to submissions on the Proposed District Plan, and following detailed assessment, the Flood Hazard Area was removed from several properties. Generally, those changes can be attributed to works completed on the properties that altered the contour of the land, or additional modelling undertaken on specific gullies. The latter also resulted in changes in the nature and extent of Flood Hazard Areas. No additional properties were affected and the Flood (e.g. High Flood Hazard Areas changed to Medium or Low) Hazard Area was removed from some properties as a result of these refinements.  

back to top

16. Why is there detailed modelling for only parts of the City? Is Council going to investigate flood hazards further?

Council has a programme for completing detailed modelling for other catchments in the city over the next 10 years. Those catchments of the City that were ‘first up’ were prioritised because initial screening work indicated those catchments had the highest number of potentially affected properties.

Council will continue to develop its understanding of how flooding may affect the City and will keep property owners advised as further work is completed.

back to top

17. What will Council do to fix the flooding?

Council will take the flood hazard information into account when managing its assets and planning future stormwater improvements. Decisions on the priority and timing of stormwater upgrade spending are taken via the Annual Plan and 10-Year Plan processes.

By using the information in the Proposed District Plan, Council will manage new subdivision, use and development of land to make sure flood risk is not made worse.

back to top

18. I’m in a proposed flood hazard area, does this mean I have to move out or raise the floor levels of existing buildings?

No. The Proposed District Plan only affects new subdivision, use and development (such as new buildings and extensions to existing buildings).

back to top

19. If only part of my property is affected what does this mean?

The Proposed District Plan would require the floors of new buildings and extensions to existing buildings to be a minimum height above the floodwater level (minimum freeboard). Other controls in the Proposed District Plan apply only within the actual part of the land within the flood hazard area.

back to top

20. Will my house be flooded?

Whether flood water gets into a building depends on how high off the ground the floor levels are compared with the depth of the surrounding flood water. Council has incomplete records on the floor levels of buildings in the City. The high, medium and low flood hazard areas represent different depth ranges. More specific depth data is available from Council: phone 07 838 6699 or email districtplan@hcc.govt.nz.

back to top

21. What is freeboard? When and why is it required?

This is the vertical distance between the calculated top level of flood water and the underside of the floor of a building. Minimum freeboards are used to ensure that buildings remain protected from flood water ‘breaking’ on the building (think of how water ‘breaks’ or ‘climbs’ up an obstacle placed in flowing water) or from waves created by the turbulence caused as water passes around or over obstacles, or the wake of people or vehicles travelling through water.

Where any part of a property is within a flood hazard area then all new buildings and extensions to existing buildings must comply with the minimum freeboard.

 

back to top

22. Is Council going to investigate potential flood hazards further?

Yes. Detailed flood information is available for only part of the city. Council will continue to develop its understanding of how flooding may affect other parts of the city and will keep property owners advised as additional work is completed. It is expected to take about 10 years to complete the work for all the catchments of the City.

back to top

23. For which parts of the city has the flood hazard modelling been completed?

​The areas of the city for which flood hazard modelling has been completed are shown on the Flood Hazard GIS Viewer, which was updated in July 2014. (In order to see the "Detail Flood Hazard Boundary", you many need to turn off the "2012 Aerial Photos" using the "Layers" menu in the bar near the top of your computer screen). They are also shown on Figure ES1 on page (iii) of the Flood Hazard Report (AECOM, 29 October,2012).

back to top

24. Why are we updating the flood hazard information?

Council is preparing Catchment Management Plans for the City as part of stormwater discharge consent requirements. This is improving Council’s knowledge about potential flooding and new flood mapping is becoming available. Council was also reviewing its current District Plan. Council has a statutory duty to control the effects of subdivision, land use and development for the purpose of avoiding or mitigating natural hazards. This information is needed to identify areas potentially affected by flood hazards and to allow Council to fulfil its duties.

back to top

25. Why is Council not using all the new flood hazard information?

Only a portion of the city has been modelled to a sufficient level of detail to justify using it in the Proposed District Plan or to require it to be included in a LIM. Shallow flooding (less than 10cm deep) is not included within the Proposed District Plan or used for LIMs.

back to top

26.How many properties are affected by the flood information?

Many properties include gullies, drains and other low-lying features within their boundary and these areas are likely to have some flooding.

There are about 6,700 properties that have some low, medium and/or high flood hazard area on them. Some of these properties were already affected by flood hazard information relating to the Waikato River and the effect of a culvert blocking.

There are about 1,300 properties that are affected by the Culvert Block Flood Hazard Area.

There are about 60 properties that are affected by the Temple View Flood Hazard Area.

(see What do the different flood hazard areas mean?)

back to top

27.Can I talk to the Council about the maps?

If you have any questions or queries about the flood information we would like to talk to you.  Call one of the following numbers:

  • Regarding resource consents: Duty Planner                              838 6719

  • Regarding building consents: Building Call Centre                   838 6677

  • Other enquiries: City Planning                                                     838 6478

back to top

28. If the flood information is based on a computer model and engineer’s assumptions, how accurate is it likely to be?

All reasonable efforts have been made to ensure the modelling work is as accurate as possible, based on the information and technology available. However there is no guarantee this is exactly what would occur in a 1 in 100 year flood event. The information is sufficient and reliable for use in the Proposed District Plan and inclusion in a LIM.

back to top

29. Does this flood information mean that the stormwater system is inadequate?

The level of service for the stormwater system has altered over time. From the 1980s the piped network has been expected to be designed to cater for the following sized rainfall events.

  • 1 in 2 year event in residential areas
  • 1 in 5 year event in industrial areas
  • 1 in 10 year event in commercial areas

Current requirements for new subdivision design mean, when the piped network cannot cope, roads become overland flowpaths to keep private property as free from flooding as possible.

 

Page reviewed: 14 Jul 2014 12:59pm