Flood Hazard Information Questions and Answers

Abbreviati​on or termAbbreviations in fullDefinition
Catchment The catchment for any location in the city is the area of higher land from which fallen rains drains to that location. 
Council Hamilton City Council
Comprehensive Stormwater Discharge Consent This is a resource consent the Waikato Regional Council has granted Hamilton City Council. It authorises, subject to conditions, the discharge of urban stormwater to the Waikato and Waipaa Rivers and various lakes at multiple locations.
Design flood level The surface level of flood water in a 100-year event, measured as metres above sea level using the Moturiki datum.
100-year flood event A flood that is caused by extreme rainfall that on average, would occur once every 100 years. Such an event has a 1% probability of occurring in any year.
FAQsFrequently Asked Questions 
Hazard A hazard is a source or a situation with the potential for harm in terms of human injury or ill-health, damage to property, damage to the environment, or a combination of these.
Risk 

A risk is the chance of something happening that will have a negative effect. The level of risk reflects:

  • the likelihood of the unwanted event
  • the potential consequences of the unwanted event.
Freeboard Freeboard is the vertical distance between the modelled top level of flood water and the underside of a concrete floor slab, or the underside of a floor joist.
Floodviewer A Hamilton City Council viewer displaying Hamilton flood information.
GISGeographical Information SystemA computer system capable of storing, analysing and mapping data.
ICMPIntegrated Catchment Management PlanA plan to integrate the management of land use and the three waters (water supply, stormwater and wastewater) within a catchment.
LiDARLight Detection and RangingAerial surveying method used to create ground surface maps, such as contour maps, and digital terrain models.
LIMLand Information MemorandumA LIM is a report prepared by the Council that summarises information on a property available at the time the LIM is issued. The Local Government Official Information and Meetings Act 1987 requires LIMs to be provided on request and specifies the types of information that must be included in them. They are most commonly requested by potential property purchasers
Moturiki Datum The Moturiki Datum 1953 is a vertical reference point used in land surveying in the Waikato and elsewhere. It is a fixed survey datum set by Land Information NZ. It is based on the mean level of the sea observed at Moturiki Island, just off Mount Maunganui Beach, between February 1949 and December 1952.
RMAResource Management Act 1991New Zealand's primary legislation for managing air, soil, freshwater and coastal marine areas
Waikato River Corridor
 This is the main stem of the Waikato River through Hamilton City.  During the 100-year flood, flood levels in the lower reaches of tributaries to the Waikato River will be affected by the Waikato River flood levels.  For the extent of this flooding, see "100-year Waikato River Flood Area" in Question 1.​

Types of flood information and how to access it

1.          What flood information is available for Hamilton?

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

3.          Why is flood modelling only available for parts of the city?

4.          Will Council do flood modelling for the rest of the city?

5.          What flood hazard area, if any, applies to my property – high, medium, or low?

6.          Can I talk to someone at Council about the flood information?

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What the flood information means

7.          What does Floodviewer tell me?

8.          What use is the 100-year flood information?

9.          Will my house be flooded?

10.      My house is in a flood area. Does this mean I have to move out or raise the floor level?

11.      What does it mean for building on my property if only part of it is in a flood hazard area?

12.      If Floodviewer shows a property is not affected by flooding, is the property free of any flooding risk?

13.      What is the earthworks layer in Floodviewer showing?

14.      What does the flood information mean for resource consents under the Resource Management Act?

15.      What does it mean if my property is not in a flood hazard area on a District Plan planning map, but is in a flood area on Floodviewer?

16.      What does it mean if flood hazard areas for my property shown on a District Plan planning map are          different from those shown on Floodviewer?

17.      What does the flood information mean for building consents under the Building Act?

18.      Does the flood information affect insurance?

19.      Will the flood information affect property values?

20.      Will flood information appear on a LIM?

21.      What will a LIM say about flooding on a property?

22.      Will the flood information affect my rates?

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Why Council gets and publishes flood information

23.      Why does Council map flooding?

24.      Why is Council getting new flood information?

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The process Council follows to get and publish flood information

25.      Where does the flood information shown on Floodviewer come from?

26.      Will the flooding shown in Floodviewer for my property change in the future?

27.      Why doesn't Council include new flood hazard maps in the District Plan as soon as they are available?

28.      What can I do if I disagree with the flood maps?

29.      Will the flood hazard maps in the District Plan be updated?

30.      How will I know when changes are made to Floodviewer?

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Technical matters

31.      My property is on a hill or a long way from the Waikato River, how can flooding possibly affect it?

32.      What is a 100-year flood event?

33.      When did a 100-year flood event last occur in Hamilton

34.      Why is Council using a 100-year flood event?

35.      What do the different flood hazard areas mean?

36.      Why does the edge of a flooding area on a map sometimes look pixelated, with squares or triangles, including over buildings?

37.      Is all flooding in Floodviewer the same accuracy, and can it be used in consent applications for development?

38.      On what climate change assumptions are the flood models based?

39.      How do I know which climate change assumption applies to the flood mapping in which I am interested? 

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What can be done about flooding

40.      Is the stormwater system inadequate?

41.      What will Council do to fix flooding?

42.      What can I do to reduce the effects of flooding on my property?

43.      What is freeboard?

44.      Why is freeboard required?

45.      When is freeboard required?

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Types of floo​​d information and how to access it​​​​

​​​1. What flood information is available for Hamilton?

The following table describes the flood information available and where it can be viewed.

Type of flood informationDescriptionIs it included on the Operative District Plan (2017) planning maps?How you can see or get the information
Earthworks AreaSignificant earthworks have occurred that mean that the mapped flooding within this area is no longer reliable. The flooding may be remodelled in the future to update the flood information.No
On Floodviewer
100-year Flood HazardThis layer shows the flood hazard that is predicted to occur on average once in 100 years. This hazard has a 1% probability of occurring in any year. Any flood water shallower than 10cm is not shown because it is not considered a significant hazard. Flood hazards are shown as low, medium, or high – see Question 35 for more information.  "Yes" for some areas, "no" for others.  To determine which, you will need to compare the 100-year flood information on Floodviewer for your area of interest with the flood hazard mapping in the District Plan. The next column to the right shows how to see or get that information.  See also Question 27.
  • All available 100-year flood information (including that in the District Plan) is shown on Floodviewer.
  • Flood hazard mapping included in the District Plan can be viewed on the Features Map on the left-hand side of the District Plan GIS Map Viewer – click here.
Detailed 100-year flood hazard data for individual properties

For each 2m x 2m area of a property the following information for a 100-year flood is available:

  • Flood hazard categories (low, medium and high)
  • Maximum flood depth
  • Maximum flood velocity
  • Design flood level.

On request Council can supply these for areas where flood modelling has been completed.

No

These maps may be requested by:

100-year Flood Extent

This layer shows the full extent of flooding likely to occur on average once in 100 years. This flooding has a 1% probability of occurring in any year and includes flooding less than 10cm deep.  Any changes to ground levels resulting from earthworks or development that have occurred since 2008 are not reflected in this layer.

Currently this information is only available from the Mangakootukutuku and Te Awa o Katapaki flood models for parts of those catchments.   

The 100-year flood extent from the Mangakootukutuku flood model is based on ground levels that existed in 2008 and accounts for extra stormwater that may come from predicted future developments. 

The 100-year flood extent from the Te Awa o Katapaki flood model is based on ground levels that existed in 2008 and accounts for extra stormwater that comes from development that existed in 2008. 

No
On Floodviewer
100-year Waikato River Flood Area

This information is provided by Waikato Regional Council to show the extent of Waikato River flooding likely to occur on average once in 100 years.

Only the depth of flood water is used to determine the flood hazard. See Question 35.

Yes
  • On Floodviewer
  • On the Features Map on the left-hand side of the District Plan GIS Map Viewer – click here; or
  • On the Waikato Regional Hazards Portal – click here – on the "River Flooding" tab – see "Waipa and Waikato 1% AEP flood extent"
Design flood levels for the Waikato RiverThe locations of surveyed cross-sections and their identification numbers and a form for requesting the design flood level at a cross-section.No
On the Waikato Regional Hazards Portal – click here – on the "River flooding" tab
Karapiro dam breakThe predicted extent and depth of flooding if Karapiro Dam were to breakNo
On the Waikato Regional Hazards Portal– click here – on the "Karapiro dam break" tab
Drainage scheme boundariesWaikato Regional Council's drainage scheme boundariesNo
On the Waikato Regional Hazards Portal– click here – on the "Flood management" tab
Temple View Flood Hazard AreaThese 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 100-year event. Yes
On the Features Map on the left-hand side of the District Plan GIS Map Viewer – click here.
Culvert Block Flood Hazard Area
This applies upstream of significant culverts in the gully system. It represents the maximum effect of a culvert becoming blocked and water backing up the gully until it eventually flows over the accessway or road above the culvert. Yes
On the Features Map on the left-hand side of the District Plan GIS Map Viewer – click here
Flood hazard maps included in the District Plan

Flood hazard mapping included in the District Plan is based on computer modelling.  In the near term, no development is expected to change it. 

In some areas, the flood hazard mapping included in the District Plan has been superseded by more recent flood modelling that has yet to be included in the District Plan. In all instances, the best available information will be that shown on Floodviewer.

To determine whether the flood hazard mapping for your area of interest has been superseded, you will need to compare the two sets of flood hazard information or contact Council at 838 6699.

The boundary of the area modelled is shown as a dashed line labelled "Flood Hazard Subcatchment Boundary" on the District Plan planning maps.

Yes
  • On Floodviewer
  • On the Features Map on the left-hand side of the District Plan GIS Map Viewer – click here.
  • Detailed flood hazard maps for individual lots showing the design flood level and maximum flood depth and velocity on each 2m by 2m grid are available on request from the Council: phone 07 838 6618 or email districtplan@hcc.govt.nz.
Ground contours from LIDAR dataLiDAR stands for "Light Detection and Ranging". It is an aerial surveying method that measures distance to the ground by illuminating the ground with laser light and measuring the reflected light with a sensor. LiDAR works in a similar way to radar and sonar, but uses light waves from a laser, instead of radio or sound waves. LiDAR data is used to create ground surface maps, such as contour maps, and digital terrain models that are used for flood modelling. No
  • Ground contours can be viewed on the Council's My Property Viewer – click here. Click to accept the terms and conditions, then click "OK". Click on the "Layers" icon in the top right of the viewer, then click in the "Contours" square to view the contours.
  • The ground contour data is available from the Council on request. A time-based charge may apply. Email your request to: gis@hcc.govt.nz.

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2. For which parts of the city has flood m​odelling been completed?

Suitably detailed flood modelling has been completed for the unshaded areas shown on Floodviewer, when none of the layers on the Layer List are ticked. However, the flood modelling will not be reliable for properties that lie within an Earthworks Area.

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3. Why is flood ​​​modelling only available for parts of the city?

Flood modelling is expensive. The Council prioritised the catchments for modelling. Those catchments that initial screening indicated had the highest number of flood-affected properties were modelled first. Catchments proposed for large scale development were the next priority.

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4. Will Council do fl​​​ood modelling for the rest of the city?

Yes, Council continues to model flooding for the rest of the city as part of its ongoing Integrated Catchment Management Plan programme.

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5. What flood hazard area, if any, applies to my property – high, medium or low?

Different coloured shading overlying your property on Floodviewer will indicate any flood hazard area on the property.  The legend on Floodviewer explains what each colour means. Question 35 explains the difference between the flood hazard areas.

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6. Can I talk to ​​someone at Council about the flood information?

Yes. Call one of the following during normal business hours:

  • Regarding flood data: 07 838 6699
  • Regarding resource consents: Duty Planner via Customer Services: 07 838 6699
  • Regarding building consents: Building Call Centre: 07 838 6677
  • Other enquiries: City Planning: 07 838 6478
  • Customer Services: 07 838 6699

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What floo​​d in​​formation means

7. What does Floo​​dviewer tell me?

Floodviewer shows whether properties within modelled areas are affected by flooding in a 100-year flood.

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8. What use is​​ 100-year flood information?

Flood information shows people whether their property is likely to flood during a 100-year flood. These floods have a 1% probability of occurring any year.

The information shows whether this is low, medium or high flood hazard and enables you to prepare an emergency plan. The flood information can also inform what you do on your property. For example, owners of properties with a flood hazard showing should consider where flood water will rise and flow on the property particularly if intending to develop. People must avoid making flooding worse on any other properties by blocking or diverting flood water. For example, this could happen by constructing a raised garden, garden shed or play house in a flood area. They should also take care, when altering the level of their property that they do not inadvertently direct water where it is not wanted, for example, towards the door of a building.

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9. Will my hou​​​se be flooded?

Flood maps can look like they show flooding on top of the buildings. In reality, the water may go under, through or around the building.  Whether flood water gets into a building depends on how high off the ground the floors are compared with the depth of the surrounding flood water.  Council is unable to provide floor levels of existing buildings but can supply detailed maps showing maximum flood depth and velocity and design flood level for individual properties in a 100-year flood for flood-modelled areas. These maps may be requested by:

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10. My house is in a​​ flood area. Does this mean I have to move out or raise its floor level?

You may wish to understand the flood risk to your property, and you may wish to make modifications to lessen the risk.  Council does not undertake risk assessment. Flood hazard rules in the Operative District Plan (2017) apply to new subdivision, use and development and guide the development of new buildings and extensions to existing buildings but it doesn't mean you need to modify your home.

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11. What doe​s it mean for building on my property if only part of it is in a flood hazard area?

If any part of your property is shown in a flood hazard area in the Operative District Plan (2017), then floors of any new buildings and extensions to existing buildings on that property are required to be a minimum height above the floodwater level to achieve the minimum freeboard. ​

However, if Floodviewer updates the flood hazard area on your property from that shown in the Operative District Plan, or if your property is in a flood hazard area not identified in the Operative District Plan (2017), then contact Customer Services on 07 838 6699 who will direct you to someone who can advise you of any relevant requirements for new buildings and extensions to existing buildings on that property.

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12. If Floodviewer​​ shows a property is not affected by flooding, is the property free of any flooding risk?

Floodviewer shows best available flood information, but this does not mean there is no risk of flooding for the following reasons:

  • Some areas are yet to be modelled, so we don't have data available for them; these are greyed out in Floodviewer.
  • Because modelling relies on data inputs and assumption, the model may not predict the full extent of flooding.
  • Flood hazard mapping is for a 100-year event. A larger flood event could flood a larger area and/or to a greater depth than shown on the flood hazard maps.

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13. What is th​​e earthworks layer in Floodviewer showing?

The earthworks layer shows areas that have been affected by significant, consented earthworks. Within this area, the flood hazard mapping is no longer reliable because land contours and, as a result, flooding may have changed. The earthworks layer will be updated at least annually.

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14. What d​​​​oes the flood information mean for resource consents under the Resource Management Act?

The Operative District Plan (2017) identifies flood hazard areas and has objectives, policies and rules specific to each area. These provisions set out the requirements to be met, whether resource consent is required, and the criteria to be used to assess whether to grant or decline resource consent. When making decisions on resource consents, the Council will also consider the flood information displayed on Floodviewer if the District Plan provides the discretion for this to happen. New subdivision, land 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 Operative District Plan (2017) for details. 

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15. Wha​​t does it mean if my property is not in a flood hazard area on a District Plan planning map, but is in a flood hazard area on Floodviewer?

The flood hazard information displayed on Floodviewer is the best available information. Whether a resource consent is required for any proposed activity on such a property depends on the requirements of the District Plan and whether the District Plan provides discretion for consideration of Floodviewer information with respect to the proposed activity. Best available information, regardless of whether it is shown in the District Plan, should be considered whenever planning development. For example, it is expected to be used as part of the Building Consent process.

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16. What d​​​oes it mean if flood hazard areas for my property shown on a District Plan planning map are different from those shown on Floodviewer?

The flood hazard information displayed on Floodviewer is the best available information. Whether a resource consent is required for any proposed activity on such a property depends on the requirements of the District Plan and whether the District Plan provides discretion for consideration of Floodviewer information with respect to the proposed activity. 

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17. What d​​oes the flood information mean for building consents under the Building Act?

The Council may consider the flood information displayed on Floodviewer when making decisions on building consents. The Building Act requires any building consent application for a site that has a flood hazard to demonstrate that the proposed buildings and site work will protect the outfalls of drainage systems and safeguard people from injury or illness, and other property from damage, caused by surface water. This includes demonstrating, amongst other things, that:

  • The disposal of surface water collected or concentrated by buildings or sitework will avoid damage or nuisance to other property in a 10-year event (an event having a 10% probability of occuring any year); and
  • Surface water resulting from a 50-year event (an event having 2% probability of occuring any year) will not enter any housing, communal residential or communal non-residential buildings.
  • Adequate provision has been or will be made to protect the land, building work or other property from the likely flood hazard (considered as a 100-year event, i.e., an event having 1% probability of occuring any year).

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18. Does th​​e flood information affect insurance?

Council cannot advise on how flood information affects insurance cover. If you have questions we recommend discussing this with an insurance provider. Insurance policies often require policy holders to advise the insurer of any new information about natural hazards on an insured property. The public, including insurance companies, can access Floodviewer.

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19. Will the f​​​lood information affect property values?

Council is required to produce the best available flood information and make it available to the public on request, including anybody considering buying a property. Property valuations reflect sales evidence up to the valuation date. Over time, property sales evidence will reveal any effect the flood hazard information has on property values. If you are concerned about the potential impact of flood hazard information on property value, then you could seek advice from a registered valuer.

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20. Will flo​​od information appear on a LIM?

Yes. A LIM outlines all information a council holds on a property, including permitted land use, consents, stormwater and sewer drains, rating information and potential natural hazards including erosion and flood hazards.

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21. What w​​​ill a LIM say about flooding on a property?

Where the property that is the subject of the LIM lies within a flood hazard area identified in the Operative District Plan, the LIM will simply state what types of flood hazard are present, that is, whether it is high, medium or low. Where the subject property lies in a flood area identified on Floodviewer, additional details of the flooding will be included in the LIM.

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22. Will the floo​d information affect my rates?

The flood information will not directly affect your rates. Any works that Council undertakes to address flooding (see Question 39) would normally be determined through the 10-Year Plan process and funded from either rates or a combination of rates and development contributions.

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Why Coun​cil prepares and publishes flo​​od hazard information

23. Wh​​y does Council map flooding?

The Resource Management Act 1991 makes the Council responsible for controlling any actual or potential effects of the use, development or protection of land to avoid or mitigate natural hazards. In addition, the Council's Comprehensive Stormwater Discharge Consent requires the Council to avoid as far as practicable and otherwise minimise adverse flooding of land, property and stormwater receiving water bodies. Mapping flood hazards helps the Council fulfil these requirements.

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24. Why is Co​​​uncil getting new flood information?

The Council is getting new flood hazard maps to achieve flood hazard mapping coverage of the entire city for the reasons set out in the answer to Question 23. The Council is preparing an Integrated Catchment Management Plan (ICMP) for each catchment in the City to comply with the Council's comprehensive stormwater discharge consent. New flood hazard maps are becoming available as each ICMP is developed. 

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The proces​s Council follows to get and publish flood inform​​ation

25. Where ​​does the flood information shown on Floodviewer come from?

Detailed information about the computer-based flood models used to generate the flood information that is shown in the District Plan, which is also shown in Floodviewer, can be obtained from Council's website at www.hamilton.co.nz/districtplan/flood. All other flood modelling shown in Floodviewer follows Council's Stormwater Modelling Methodology and has been assessed as suitably accurate. There are different purposes for creating a model, for example, for assessment of growth effects, or flood hazard mapping. See Question 34 for more details.

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26. Will the​​​​ flooding shown in Floodviewer for my property change in the future?

Flooding shown in Floodviewer is the best available information. Models will be created or updated from time to time. Updates may include changes to modelling methods, the inclusion of new infrastructure (e.g. new pipes), new or updated inputs and assumptions (e.g. how much allowance is made for climate change), and changes to landforms (e.g. from earthworks that may change the way water flows). This means that flooding shown on properties may change.  See Question 13.

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27. Why do​esn't Council include new flood hazard maps in the District Plan as soon as they are available?

New flood hazard maps may cover areas where future development is likely to change the flood hazard and existing developed areas where future development anticipated during the life of the District Plan is unlikely to significantly change the flood hazard.

The Council will include in the District Plan only flood maps that are unlikely to be changed by development anticipated during the life to the Plan. Such maps would be included in the District Plan by means of a plan change or plan review, which is a lengthy and expensive process. Until such a plan change is proposed, new flood hazard maps must remain outside the District Plan.

However, flood hazard modelling information, regardless of whether future development will change it, is useful to people planning or managing development. Therefore, it is appropriate to make this information available to the public by means other than the District Plan, that is, via Floodviewer.

If new flood hazard maps overlap areas for which flood hazard mapping is already included in the District Plan, the new maps will represent the best available information. 

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28. What ca​​​n I do if I disagree with the flood maps?

Write to Council stating what it is that you disagree with, and why, and Council will consider your feedback. Such feedback should be sent to: city.development@hcc.govt.nz for Council to consider and respond.

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29. Will the flood​ hazard maps in the District Plan be updated?

Yes, but this will be done infrequently because of cost. See Question 27.

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30. How ​​​will I know when changes are made to Floodviewer?

Council will inform the owners of properties for which, and when, new flood data becomes available that indicates a significant change to the flood hazard status of those properties. This is unlikely to occur more frequently than once a year.

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Technical matt​e​​rs​

31. My pro​​perty is on a hill or a long way from the Waikato River, how can flooding possibly affect it?

A rising river is not the only source of potential flooding, it can also be caused by pipe network blockage or overloading. Most flood hazard areas result from rain flowing downhill overland, through channels, waterways and gullies, or ponding in localised depressions or low points.

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32. What is​​​ a 100-year flood event?

A flood event that is caused by extreme rainfall that on average, would occur once every 100 years. Such an event has a 1% probability of occurring in any year.

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33. When did a 100-year flood event last occur in Hamilton?

The last wide-spread 100-year flood event in Hamilton occurred in 1958.  The Waikato River was last at a 100-year flood level in 1998. Waikato River flooding is affected by rainfall upstream of Hamilton. Flooding in Hamilton, other than in the Waikato River Corridor, is affected by rainfall directly on the city. A 100-year flood event has a 1 % probability of occurring any year.

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34. Why is Co​​​uncil using a 100-year flood event?

The Waikato Regional Policy Statement (20 May 2016) requires this. See its Implementation method 13.2.6 (a). Understanding and managing risks associated with 100-year flood events are recognised as current best practice.

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35. What do ​​the different flood hazard areas mean?

There are three categories of flood hazard - high, medium and low. The following figure shows how the modelled maximum flood water depths and speeds resulting from overland flow and ponding determine the hazard category applying at any location, except within the Waikato River corridor.

Figure 35.1: Flood hazard areas, except within the Waikato River Corridor

Flood hazard areas in the Waikato River corridor are defined using the following figure. 

Flood hazard areas in the Waikato River corridor are defined using the following figure.

Figure 35.2: Flood hazard areas in the Waikato River Corridor

 Floodwater depth and speed affect people and property as summarised in the following table.​

Floodwater depth and speed affect people and property as summarised in the following table.​

What the flood hazard categories mean for people and property

Flood Hazard CategoryFloodwater depthFloodwater speed
(metres per second)
Depth x speedEffect on people and property
(None)0 to 10cmAny velocity likely to occur in a flood-Surface water is unlikely to be a hazard to people and unlikely to cause damage to property.
Low10 to 50cm< 1.0m/s-Emergency vehicles will usually be able to drive through the flood water. Damage to property will be minor to moderate. Damage will be more severe if the flood level is above floor level. Scour or erosion of building foundations is unlikely to occur. People can usually stand up in and wade through the flood water, but more vulnerable people (e.g., children, elderly, injured and physically disabled) may not be able to do this.
Medium50 to 100cm< 2.0m/s <1People may not be able to stand up. Damage to property can be financially significant.
High>100cm> 2.0m/s >1Flood water can scour building foundations, resulting in buildings becoming unstable. Significant damage to buildings and risk to life are very likely.

Notes: During a flood, buildings with floors that have adequate freeboard (see Glossary) above floodwater are unlikely to suffer significant flood damage, whereas buildings with floors below the floodwater level are likely to suffer damage from water and silt.

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. The level of risk associated with the flooding of an existing development or land use depends upon how often the flooding would occur and its consequences. It is recommended you consider seeking expert stormwater engineering / risk assessment advice, if you require an assessment of this risk. Such an assessment should consider the floor levels of buildings in relation to the modelled flood water depth, the effects of water getting in to the building, whether the building has been constructed to withstand the force of the flood water expected, and whether it will be safe for people to get into or out of the building during a flood.

The 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. ​

Floodviewer uses the same Low, Medium and High flood hazard categories. Flood hazard information from Floodviewer, which is not in the District Plan, can be considered by Council as part of determining whether to grant, or impose conditions on, a subdivision consent.

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36. Why does the edge of a flooding area on a map sometimes look pixelated, with squares or triangles, including over b​​uildings?

This is a result of the modelling process, which calculates the flood depth and speed within each 2m by 2m area. Sometimes, the modelling includes a process that smooths the edge of the flood area. Flood maps can look like they show flooding on top of the buildings. In reality, the water may go under, through or around the building.

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37. Is all flooding in Floodviewer the same accuracy, and can it be used in consent applications for develop​​​ment?

The flood data on Floodviewer is best available information. Council's flood data is based on models that vary in age and levels of accuracy. This means some flood information can be used directly, while other information requires interpretation by a suitably qualified person to ensure its limitations are understood and the information is correctly applied. For example, whether an old model is accurate enough to set the height of a habitable floor in a new house. Council's flood modelling information has been assigned an information level which relates to the current assessment of each model's level of accuracy. There are five information level categories generally based on Waikato Regional Council's classifications. Floodviewer shows model data that has been assessed as level 1, 2 or 3. Information level 5 is not considered accurate enough to be displayed in Floodviewer. Level 4 is general information which may be included later. Anyone can request data that informs the flood maps (the maximum depth and velocity and design flood level) and the information level associated with that data by:

Information Levels shown in Floodviewer

Information ​level Level Description
1​
Property Scale
  • Information is suitable for determining floor levels and/or Building and Resource Consent applications.
  • Input data is accurate/best available and consistent with current guidelines.
  • Modelling process is detailed and robust.
2
Local Scale
  • A suitably qualified person will need to assess whether the information is suitable for determining floor levels and /or Building and Resource Consent applications.
  • Input data is accurate but not consistent with current guidelines.
  • Modelling process is detailed and robust.
3 Regional Scale
  • Information is generally not suitable for determining floor levels and /or Building and Resource Consent applications.
  • A suitably qualified person will need to assess for which purposes the information can be used.
  • Could be used for high-level constraints mapping, emergency management or future mitigation scenarios.
  • Input data is not consistent with current guidelines and contains minor errors that impact model accuracy, but the information has some value.
  • Modelling process is robust.

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38. On what climate change assumptions are the flood models based? 

Each flood model used to produce the information shown on Floodviewer is based on one of three climate change assumptions described below.  There are three different climate change assumptions because the flood models have been developed at different times and the understanding of climate change and its effects on flooding, and the recommended approach to modelling these effects, have changed over time.

  1. No allowance for climate change. (This applies to the 100-year Waikato River Flood model only).

    Information (metadata) about the 100-year Waikato River Flood Area model can be requested from Waikato Regional Council via an on-line request for service at: https://bps.waikatoregion.govt.nz/online-services/new/RequestForService/step/1

  2. Increase of 2.1 degrees Celsius average to 2090 relative to 1980 - 1999

    In 2007, the Intergovernmental Panel on Climate Change (IPCC) issued its Fourth Assessment Report on Climate Change. In 2008, the National Institute of Water & Atmospheric Research Limited (NIWA) predicted climate change scenarios for Hamilton using the IPCC's mid-range emission scenario. NIWA predicted for Hamilton an increase of 2.08 degrees Celsius in the annual average temperature by 2090, relative to the baseline climate between 1980 and 1999, and the associated changes to the extreme rainfall intensity. The latter predictions were also based on advice issued by the Ministry for the Environment in 2004. The 2.08 degrees Celsius increase was rounded up to 2.1 degrees Celsius in Policy 4.1.13 of the Waikato Regional Policy Statement (Waikato Regional Council, 2016).

  3. Increase of 3.1 degrees Celsius average to the period 2081 – 2100 (RCP8.5) relative to 1986 - 2005

    From 2019 onward, a precautionary approach has been adopted for new flood modelling by using the rainfall intensities associated with Representative Concentration Pathway (RCP) 8.5. This pathway represents the highest greenhouse gas concentrations of the 4 climate change pathways presented in Climate Change Projections for New Zealand: Atmosphere Projections Based on Simulations from the IPCC Fifth Assessment 2nd Edition, which NIWA prepared in 2018 for the Ministry for the Environment. The climate predictions presented in the report were based on global climate model simulations from the IPCC's Fifth Assessment Report on Climate Change (2013) and a detailed New Zealand regional climate model run on the NIWA supercomputer.  For the RCP8.5 climate change pathway, NIWA modelled an increase of 3.1 degrees Celsius in the average annual temperature of Waikato in 2090 (2081-2100) compared with the 1986-2005 base.

39. How do I know which climate change assumption applies to the flood mapping in which I am interested? 

When you left click on a blue or green flooding layer in Floodviewer, a pop-up box will appear with information about that layer, including the climate change assumption that applies to it. 

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What ca​​​n be don​​​e about flooding

40. Is the stormwat​er system inadequate?

No. Whenever stormwater runoff exceeds the drainage capacity of the stormwater network, ponding and/or overland flow is expected. It would be extremely expensive to provide a drainage network that avoids all ponding and overland flow. The Council first set design levels of service for the stormwater network in Hamilton in the 1980s but has since amended them. As at 2018, different design levels of service applied to existing and new development. Within existing development, Council plans to maintain the following levels of service for the stormwater system: 2-year event for residential, 5-year event for commercial, and 10-year event for industrial (page 123 of the 2021-2051 Infrastructure Strategy). However, the Regional Infrastructure Technical Specifications (the RITS) requires all new primary stormwater systems (primary stormwater systems may include (but are not limited to) wetlands, ponds, lakes, rain gardens, swales and filters, pipelines, inlet/outlet structures and soakage areas (RITS, updated May 2018, p.15) to be designed for the 10-year rainfall event for residential, industrial and commercial areas.

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41. What will Council do to fix the flooding or make sure it doesn't get worse?

Council will consider flood information when managing its assets and planning future stormwater network improvements. These improvements could include, e.g., increasing culvert or stormwater pipeline sizes, or constructing stormwater detention facilities. Council may also require developers to install on-lot stormwater detention devices in new developments. Decisions on the priority and timing of stormwater upgrades are made via Council's Annual Plan and 10-Year Plan processes.​

When processing resource consents for new subdivision, development and use of land, Council will consider the flood information in the District Plan and on Floodviewer to make sure flood hazard or risk is not made worse.

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42. What c​an I do to reduce the effects of flooding on my property?

We encourage you to seek professional advice (e.g. from a stormwater engineer) about ways you can reduce the risk of flood damage to your property. Reducing the risk of flooding on your property must not make the flooding of any other property worse.

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43. What i​​s freeboard?

Freeboard is the vertical distance between the modelled top level of flood water and the underside of a concrete floor slab, or the underside of a floor joist.

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44. Why is f​​reeboard required?

Minimum freeboard provides a safety factor to reduce the risk a building will be flooded in a 100-year event. The freeboard allows for some uncertainty in the predicted flood level, the energy of flowing water (think of how water "breaks" or "climbs" up an obstacle placed in flowing water), waves created by the turbulence caused as water passes around or over obstacles, and the wake of people or vehicles moving through flood water. The freeboard may also provide a margin of safety should the flood be greater than the 100-year event.

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45. When is freeboard required?

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.

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Page reviewed: 06 Dec 2021 12:23pm