5 July 2012
Tamahere Stormwater Study Released

The Waikato District Council has released a study into the drainage characteristics of the Tamahere Country Living Zone which shows parts of the area may experience a buildup of surface water in the event of a period of extreme rainfall.

The study by engineering firm GHD combines climate and topographical data with local surveys and a computer model to predict which areas would be subject to surface water ponding in the event of a one-in-100 year level of rainfall (or expressed another way, a one per cent chance that such a rainfall event would occur every year). Maps that show these areas are available on the council’s website.

“This study is based on the best information currently available, however it is important to note that it is a forecast; there is no guarantee that it is exactly what will occur in a one-in-100 year storm,” said Waikato District Council general manager of Water and Facilities Richard Bax.

The aerial mapping method used in the study is known as LiDAR (Light Detection And Ranging), which was obtained in late 2007 and early 2008. Given the amount of development that has occurred in Tamahere since this time, the council has decided to commission new LiDAR data to obtain a more up-to-date picture of the potential for ponding in the Tamahere Country Living Zone.

Therefore houses that have been built in the last four and a half years will not be shown until the updated maps are available, expected to be in about six months. The council has published the current maps because they represent the best assessment of the potential for ponding in the area based on the available information.

For many Tamahere residents the study formalises what is common local knowledge: that parts of Tamahere with high ground-water level and poor-draining soil are prone to surface water ponding during periods of extremely heavy rain.

“Drainage issues have always been an important consideration when planning for development in the Tamahere area, and this study does not change that. It simply gives us more detailed information with which to make planning decisions,” Mr Bax said.

The study is not expected to affect future development in the Tamahere Country Living Zone, however in some cases steps may need to be taken to mitigate the potential effects of water ponding before a building or resource consent is granted.

“The important thing to remember is that this does not change the likelihood of such an extreme weather event hitting the area,” said Waikato District Councillor Allan Morse, Chair of the Water and Facilities Committee. “What this gives us is the ability to be better prepared.”

Letters have been sent to the ratepayers of the approximately 920 properties within the Tamahere Country Living Zone, directing them to http://www.waikatodc.govt.nz/Tamahere-storm-water.aspx for further information. Residents can also ring 0800 492 452 with queries,
and the council will be hosting sessions in the Tamahere Community Centre on 10 July and 17 July from 7pm to 8.30pm with council engineers available to answer questions.

The council has funding set aside in its Long Term Plan to invest in stormwater infrastructure in Tamahere, and the information provided by this study means funding will be spent where it is needed most, Mr Bax said.

The study was prepared as part of the Tamahere Structure Plan, which is a blueprint for managing expansion in fast-growing areas.

Further background information is available in the Frequently Asked Questions section below or from the Waikato District Council’s website.

ENDS
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Tamahere stormwater drainage
Frequently Asked Questions for residents

1. Why did council get these maps prepared?
The council commissioned these maps as a part of the structure plan for the Tamahere Country Living Zone.
A structure plan is a blueprint for expansion which looks at an area and takes into account proposed housing development, infrastructure, open spaces and any other issues that might impact on planning in the future.
Tamahere is one of the fastest growing parts of the Waikato district, and the council developed a structure plan to ensure this growth is conducted in a managed and sustainable fashion.
Whenever we develop a structure plan, we need to produce an accompanying catchment management plan, in order to comply with our stormwater discharge consents. To assist in the Tamahere Catchment Management Plan, we engaged environmental engineering firm GHD to provide an indication of what could happen in the event of a period of extreme rainfall in the area.

2. What does a catchment management plan tell us?
Tamahere has a history of surface water ponding during heavy rain, which many long-term residents will know from experience. Parts of the Tamahere area are prone to ponding because of the flat topography, high ground-water level and poor-draining soil. A catchment management plan will help us to improve our knowledge of the impact of extremely heavy rain in the area, so we can take this into account in our future decisions.

3. How should I interpret this information – what do the maps show?
The maps show the expected depth of surface water ponding from rainfall in the event of a one-in-100 year weather event allowing for climate change.

4. What is a one in 100 year weather event?
A one-in-100 year weather event is equivalent to the worst storm in 100 years, also known as a 1 per cent annual exceedance probability (AEP). Another way to express this is that there is a 1 per cent chance of such a storm occurring every year. The last one-in-100 year event was in 1958, and there was a one-in-50 year event (or a 2 per cent AEP) in 1998. For the purposes of this study, the one-in-100 year event is based on rainfall of 149.2 millimetres in 24 hours, or if adjusted for climate change assumptions, 174.3 millimetres of rain in 24 hours.

5. How many properties are affected?
There are approximately 920 properties within the Tamahere Country Living Zone, although not all of these properties will be affected. According to the available data, about 630 properties, or 69 per cent of the Country Living Zone, could expect to have 5 per cent or more of the site subject to ponding. Many properties include gullies, drains and other low-lying features within their boundary and these areas are likely to be subject to a degree of ponding.

6. I can’t see my house on the maps – Why not?
The maps are based on an aerial contour mapping survey commissioned by the Waikato Regional Council and conducted in late 2007 and early 2008. There have been about 120 houses built since the original mapping was done, which in many cases includes earthworks and drainage.
Because of this additional development, the council is in the process of obtaining new aerial contour mapping data, which will be used to produce more up-to-date maps. This process is expected to take about six months. In the meantime, land that has had houses built since the aerial survey was flown is flagged on the maps, and contains a reference to note that it will be shown on the updated version.

7. How different will the updated version of the maps look from this one?
This depends on how much has changed in the area around your property since the first contour information was collected in early 2008. As much of Tamahere is quite flat, this may include changes on nearby properties. In the majority of cases we would expect that drainage has improved since 2008, but this may not always be the case.

8. Can I talk to the council about the maps?
If you have any questions, queries or feedback about the maps we would like to talk to you. Also, if you are aware of significant changes to the contour of your land, or if you believe the maps do not accurately reflect the drainage on or near your property, please contact us.

9. Is this “new” information?
The fact that Tamahere is prone to surface water ponding during heavy rain is not new information to most residents in the area and consideration of ponding and drainage issues have always been an important factor in planning decisions. For example in parts of the Tamahere Country Living Zone with known ponding issues, houses have been required to be built 300 millimetres above the ground, as opposed to the usual 150 millimetres. In some cases additional drainage work has needed to be undertaken to mitigate the effects of water ponding.
This is, however, the first time that the council has obtained information with this level of detail for the Tamahere Country Living Zone.

10. If this is based on a computer model and engineers assumptions, how accurate is it likely to be?
All reasonable efforts have been made to ensure the GHD study is as accurate as possible based on the information and technology available. However there is no guarantee this is exactly what will occur in a one-in-100 year weather event.

11. Will the map be recorded against my property?
The council is required to make available all relevant data it holds concerning properties within the district. In the case of the Tamahere Country Living Zone, the following note will be recorded against any Land Information Memorandum (LIM) prepared for properties within the area.
“The property is located in the Tamahere Stormwater Catchment Management Area. Surface water ponding may occur on some properties during high rainfall events and in some cases mitigation measures may be required when developing the land. Maps are available for reference at the Council’s head office or on the website.”

12. What will it mean for property values in Tamahere?
The council cannot make any comment as to what the affect on property values will be. We have been bound first to obtain this data, and once we have it, to make it available to our ratepayers. We are likewise legally required to make it available for the consideration of anybody looking to buy a property in the area.

13. Will it affect future development in the area?
We do not expect that this will affect future development in the Tamahere Country Living Zone. Previously when development in the area was being planned, information on potential ponding issues was obtained from the drainage committee, which relied primarily on the knowledge of long-term local residents. The GHD study allows us to improve our decision making process by providing a wider range of information to assess the implications for drainage from planned expansion in the area, and plan accordingly. What this means in practice is that additional work may be needed to mitigate the risk of surface water ponding as a result of this information. Stormwater issues have always been an important consideration in planning in Tamahere, and this has not changed.

14. What will I have to do as a result of this information and the Catchment Management Plan?
If in the future you plan to carry out work that could affect how stormwater flows across your property (such as contouring your land, or building a retaining wall) we recommend you contact the council first. Also, if you want to do work that requires a building or resource consent, then the person completing the application will need to show in the application that they have considered the information in the catchment management plan.
15. What is the council doing with this information?
The council has set aside funding over the next ten years for storm water improvement works on council infrastructure in the Tamahere area as development occurs. The information will be used in future planning and investment decisions and as a factor in consent applications to try and mitigate the effect of surface water ponding.

16. Where did the raw data for this study come from?
The study used rainfall data from the National Institute of Water and Atmospheric Research (NIWA). Ground elevations were obtained by LIDAR (Light Detection and Ranging) technology and soil data provided by the Waikato Regional Council. Ground surveys were also completed and the input of some local residents sought. This created a model of what was likely to happen in the event of extreme rainfall.

17. Does this mean the stormwater system is inadequate?
The council plans to provide drainage that can cope with a one-in-ten year event. For larger events such as the one shown in the study, overland flow paths are required to carry the excess water away. The maps will help us to address any issues that may exist within the overland flow path network.

18. Do you have this information for the remainder of the district?
We have broad data available for the remainder of the district. A similar detailed study to this one has been completed for Te Kauwhata, and one is currently being undertaken for Raglan. The cost of obtaining this data is very high, and this means we intend to only acquire it for other parts of the district when undertaking structure plans. The studies for both Tamahere and Raglan cost about $100,000 each, while Te Kauwhata cost $65,000.

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