Stormwater modelling exercise

As a result of Council’s planning to assess the capacity and capability of our three waters* infrastructure, a modelling exercise was undertaken in Putāruru.

Part of this assessment looked specifically at stormwater capacity and capability, including identifying flood hazard areas.

The flood hazard information shows which properties are less likely or more likely to be affected during extreme rainfall events.

This information allows Council to better understand how flooding from extreme rainfall affects each area in Putāruru. The map updates flood hazard information that Council has previously held about Putāruru. Using more accurate technology, the potential floodable area is shown to have changed since our records were last updated.

* three waters refers to stormwater, water supply and wastewater

Frequently Asked Questions - flood hazard modelling

Why update flood hazard information?

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

Updating our flood hazard maps ensures Council is in a better position to plan for stormwater management.

It is part of normal Council business to know the effects of natural hazards that could impact our community. This enables Council and our community to better prepare for high rainfall events. Flood hazard maps for South Waikato District have been evolving since the late 1990s. The information is used for building consents, planning new subdivisions and improving our infrastructure planning.

What is a 100-year rainfall event?

An extreme rainfall event doesn’t happen very often.

The flood hazard map is modelled for a 100-year rainfall event. The term ‘100-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.

Council has modelled 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.

The rainfall event used to develop the 100-year flood uses a High Intensity Rainfall Database from NIWA* to create a set of 100-year events of differing durations, from 10 minutes to 24 hours.

*NIWA – National Institute of Water and Atmospheric Research

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 downhill to a particular stream or outlet. The stormwater catchment is the defined area of land that contains and collects all 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 the ground surface which has been measured using a system called LIDAR (Light Detection and Ranging). This measures the ground levels of the land using laser pulses and 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 we enter into the computer model. The sorts of questions we consider 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 (eg, grass)?
• Where will rain water flow over hard surfaces (eg, roofs, roads)?
• How long will it take for rain water to flow from one part of the catchment to another?
• What stormwater systems (ie, pipes and ponds) are already in place?

Technology is able to take all of these variables into account. Once the criteria are 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.

When the model is complete it gets loaded into our GIS (Geographical Information System) mapping system to display the potential flood hazard information. The updated flood hazard map is now available online.

What is Council doing about flood management?

Council’s strategy to flood management is to:

• Provide comprehensive flood information and technical advice to aid stormwater management.
• 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 undertaken via the Annual Plan and Long Term Plan processes.
• Ensure that flood emergency plans consider the appropriate responses for a 100-year flood.
• Provide for main building height assessments (please register interest).

You can view more information on Council’s website – www.southwaikato.govt.nz. Click on Maps - Online Mapping Tool.

Does this flood hazard information go on my property file?

Yes, the updated information has been stored on the files relating to 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 (LIMs) reports.

Council is 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.

Will this affect my insurance?

Council cannot advise you about any effect that this information might have on insurance. In this matter you may wish to contact your insurance company.

Frequently Asked Questions - Building Height Assessment

When will the assessments be done?

During July and August

Which company is doing the work?

Nicholson Survey

What does the work entail?

Likely the surveyor will GPS one of the existing manholes from the modelling survey in the vicinity of the house, then GPS a second point just outside the house. Using those two points as a reference, we would then survey the level of the house relative to the flood hazard layer.

How long is the onsite assessment?

Each assessment is likely to take no more than 30 minutes. Appointments will be scheduled at 45 minute intervals.

Does the property owner get the results?

Yes

Are the results included on the property file and LIM?

Yes, this information will be on the property file. We are still working through the process as to how this information will be added to the LIMs.

Are the assessments going to be done in streets/groups?

Yes, to keep costs down Council will try to coordinate streets to be done on the same day.

Do I need to be available on site during the assessment?

Ideally yes. If you have dogs, definitely yes. Or if you don’t have dogs, you will need to give permission for the surveyor to access the property in your absence. Or liaise with a neighbour. Or an alternative contact.

What is the cost of the assessment?

Each assessment will cost $80 if we can get them grouped together well to cut down costs. This is at Council’s cost, not the property owners.

FAQs raised from affected property owners

Will this affect my property valuation? Will Council be revaluing properties now? Will this affect my rates?

Properties are revalued for rating purposes every three years. Flood hazard information has been recorded on more than 100 Putāruru properties for many years. Rating values used by South Waikato District Council are based on actual property sales so a material change in property prices paid will have an impact on rating valuations.

Stormwater rates are set annually based on rating valuations. General rates are charged partly on rating valuation and partly on a per property basis. So this assessment won’t affect the amount of rates on each property until after the next valuation, ie 202-23 rating year. Valuation done in 2021.

Will this affect my insurance?

Council cannot advise on this. You are advised to contact your insurer.

Why did you only do a desktop exercise?

This modelling was undertaken by an industry leader Watershed Engineering Ltd, using current best practice and based on the Waikato Regional Infrastructure Technical Specification for stormwater, which requires allowance for climate change forecasts.

Desktop modelling is really the only effective way of assessing the impact of a theoretical 100-year flood event. It requires aerial mapping to get detailed contours, stormwater asset data to detail stormwater infrastructure and a complex model to show the impact of a range of rainfall events. This is not something that can be done by visually assessing the land and properties.

Will the building floor height assessment together with the flood hazard information mean a more complete picture?

The flood hazard data coupled with the building floor height assessment will provide a more accurate assessment of the building platform level which can then be compared to the predicted flood hazard water level. This would advise whether the existing habitable area within the building is above or below the flood hazard area and by what amount.

How many people have registered for building floor height assessments to date?

As at Thursday 13 June, 10.30am, 42 people have registered for building floor height assessments to date. If you haven’t done so and would like one, please register with Lisa or Kerry today.

What is Council doing about its own drainage? Should people call Council to report flooding issues during high rainfall events?

Yes, please do report flooding issues to Council. Members of the public can assist by cleaning stormwater grates and removing obstacles within stormwater flow paths which can, if not cleared, increase stormwater flooding upstream. The more data we collect the better we can plan and prepare for future development and future budget for operational activities. There are several known open drain issues that need work. Budget has been included in our Annual Plan for the coming year (2019-20) to undertake preliminary design work and to assist in the estimation of required infrastructure upgrades. This work will feed into the Long Term Plan capital works upgrade budgets.

Why are some areas not showing as flooded when they have flooded in the past and are known problem areas, eg, Galway Crescent?

The scope of study was focussed on the stormwater infrastructure in the ground so some small areas of the Putāruru township on the fringes of town were not assessed during this exercise. There is no updated data for these small pockets of land. Council is however aware of flooding in some areas; and will ensure these are included in future assessments.

What is Council going to do about the water flooding across the road on SH1 by the engineering firm/Putāruru College turnoff?

Council works closely with the New Zealand Transport Agency on issues such as these; however this area is primary under the authority of NZTA. We will continue to work with NZTA to look at remedies for this particular spot due to the frequency of flooding in that area and the seriousness being SH1 and for the two businesses and school in the area. Council passes any issues raised by the public or staff on to the NZTA for their action.

What is Council doing about the Oraka stream itself, clearing trees and debris from the water?

The Oraka stream and the stop banks associated with the Oraka Stream are the responsibility of the Waikato Regional Council (WRC). This includes being responsible for trees and debris within the stream. We liaise closely with WRC during flood and other incidents. Council takes service requests regarding the Oraka Stream and passes them on to WRC. As a result of this modelling and this query that came up, Council staff will get in touch with WRC to see what data they have on their files regarding flood modelling along the Oraka Stream and its stop banks.

What is Council going to do about wastewater manholes that overflow during heavy rain events?

Some ingress from stormwater and high ground water levels can infiltrate into wastewater pipes and wastewater manholes. This inflow is continually monitored by Council and repairs are undertaken to reduce this inflow. Another way that stormwater enters the wastewater system is by home owners having connected their stormwater downpipes into the wastewater system. Council has an inspection program to identify these properties.

Following this modelling assessment, all of Council’s pump stations and pump station storage areas are being assessed. Upgrades to the pump station and increased pump station storage areas will be prioritised and constructed over the next number of years.

The reason we don’t want stormwater in the wastewater system, is that the wastewater collection and treatment system is designed for wastewater plus a small amount of stormwater inflow through the pipes. It would be far too expensive to design a wastewater plant that had the capacity to collect and treat all wastewater and stormwater.

Can we have other maps for other areas in Putāruru?

Of course, we can help with that or you can look for these online yourself too. Council’s Development Engineer Andrew Pascoe will demonstrate our Online Map feature a little later in this session, and is happy to help further by phone.

Why just Putāruru? What about other towns in the district?

We have simply started in Putāruru, but three water assessments will be carried out in all towns in due course; as part of Council’s normal planning processes.

Can I have my property removed from the flood hazard layer?

We suggest that you have your building floor height assessment done. This will complement the 100-year flood hazard information and could indicate that your building is well above the 100-year flood level. This ensures there is a relationship between your main dwelling floor height and the flood levels is on the property records.

We will re-run the model again as development requires or at least every five years. This involves having the LIDAR contour data re-flown and surveyed.

If property owners do stormwater mitigation work on their property, we suggest you share that with Council to have put on your property records so that this detail also appears on your LIM. We suggest you ensure that any mitigation work is done by a suitably qualified person.

Why exclude under 100mm (10cm)?

We excluded water under 100mm on the maps because that depth is clearly very low to insignificant and would likely not affect any main building.

If water is coming from my neighbour’s property and flowing on to my property, what do I do?

Liaise with your neighbour in the first instance to see if you can come to a mutually agreeable solution. Council is not involved in this process because stormwater will naturally flow from one property to the next.

Can I divert stormwater off my roof into my drains (sewerage)?

No. The wastewater system is not a stormwater system. It is not designed to cope with the significant volumes of stormwater generated following a rainfall event (refer to comment above).

Under what legislation does Council have to put this information on property files?

• LGOIMA – Local Government Official Information and Meetings Act 1987
  • information identifying each (if any) special feature or characteristic of the land concerned, including but not limited to potential erosion, avulsion, falling debris, subsidence, slippage, alluvion, or inundation, or likely presence of hazardous contaminants, being a feature or characteristic that—
• Resource Management Act
• Local Government Act

Section 44A (2) The matters which shall be included in that memorandum [LIM] are:

• is known to the territorial authority; but

Section 7

In achieving the purpose of this Act, all persons exercising functions and powers under it, in relation to managing the use, development, and protection of natural and physical resources, shall have particular regard to—

• the effects of climate change:
  • to be better informed of their duties and of the functions, powers, and duties of the local authority; and
  • to participate effectively under this Act.
• records of natural hazards to the extent that the local authority considers appropriate for the effective discharge of its functions; and

And Section 35

(3) Every local authority shall keep reasonably available at its principal office, information which is relevant to the administration of policy statements and plans, the monitoring of resource consents, and current issues relating to the environment of the area, to enable the public—

(4) Every local authority shall keep reasonably available at each of the offices in its region or district such of the information referred to in subsection (3) as relates to that part of the region or district.

(5) The information to be kept by a local authority under subsection (3) shall include—

• Principals
• Sustainability
• The concepts of sustainable development under the Local Government Act 2002, and sustainable management of an area’s natural and physical resources under the Resource Management Act 1991, imply the ongoing ability of communities and people to respond and adapt to change in a way that avoids or limits adverse consequences. Since 2004, the Resource Management Act has included a requirement that people making decisions in terms of the Act must have particular regard to the effects of climate change.
• The reasonably foreseeable needs of future generations
• This means taking into account the interests of future communities, and the direct and indirect costs that future generations may bear, as a result of decisions made in the present. The concept is found in key sections of the Local Government Act 2002 (section 14) and the Resource Management Act 1991 (section 5), and is the fundamental basis for international, national, regional and local responses to climate change.

In summary: The Local Government Act 2002 requires us to consider the cost to future generations when making decisions. Climate change is an accepted future state for planning. Therefore, the impact of climate change on infrastructure should be considered during our decision making process.

Some additional details regarding stormwater in Putāruru

• Most of the stormwater pipes in Putāruru were installed in the 50s and 60s. There were no models in those days so the engineers of the day would have only looked at the immediate catchment, in designing the pipes.
• Any new residential developments will have to effectively manage stormwater so as not to increase the volume of stormwater leaving the development.
• Climate change is likely going to increase the intensity of storms.
• The desk top model is a computer model that uses the ground profile determined by LIDAR. LIDAR is produced by a plane flying over the town bouncing laser beams off the ground to determine the ground profile. This can have inaccuracies depending on what the laser has bounced off. That is why a ground survey is more accurate but much more expensive and time consuming.
• Council will use the model to determine stormwater improvements for the next LTP, which will reduce stormwater flooding.