Part 2: Better understanding of flood risk is needed

Managing stormwater systems to reduce the risk of flooding

In this Part, we discuss the three councils':

Incomplete assessment of flood risk

To date, the three councils have an incomplete understanding of the flood risk in their districts. This limits the three councils' ability to make informed decisions about whether to reduce flood risk and by what methods.

Flood risk is determined by combining the likelihood of a flood and the potential consequences (or effects) of that flood. The severity of a flood's consequences depends on how many people and assets are exposed to the flood and how vulnerable those people and assets are to the flood.

Weather systems, land forms, watercourses, people, development, and economic activity all influence flood risk. If the expected size or frequency of a flood increases, or the potential effects of a flood increase, so too does the risk.

There are different sources of flood risk. These include rivers and streams after heavy rainfall, ponds that form in urban areas as a result of heavy rainfall, and inundation by groundwater or high sea levels.

We expected councils to have enough information about their flood risk to take a structured long-term planning approach to reduce the risk and effects of flooding, including through using a stormwater system.

The three councils have used hydraulic modelling3 for some assessments in response to specific events or regulatory pressure, such as to comply with resource consents.

In 2011/12, Dunedin City Council completed 11 integrated catchment management plans.4 Of these, 10 were in response to Otago Regional Council's consent requirements for discharging stormwater into Otago Harbour, and one was for Mosgiel.5

As part of these integrated catchment management plans, Dunedin City Council modelled each catchment to understand their current and future flood risks and the capacity of the stormwater system. However, except for Mosgiel, the plans did not cover inland areas, had limited information about the effect of groundwater on flood risk, and generally did not consider the flood risk from watercourses.

Both Porirua City Council and Thames-Coromandel District Council have assessed flood risks in specific areas in response to flooding issues. For example, Wellington Water has modelled flooding in Porirua's central business district to help develop options to reduce the risk and effects of flooding.

The three councils use other sources of information, such as complaints from the public, field visits by staff, and post-event reviews, to confirm the flood risks that the modelling has identified.

The three councils also use these sources of information to identify flood risks for areas that have not been modelled. For example, Wellington Water has processes and procedures to identify the cause of any flooding issues and any actions that need to be taken. In some instances, this includes asking people to fill out questionnaires about flooding.

These sources of information are unlikely to provide a complete picture and are available only after flooding occurs. For example, staff members from Dunedin City Council told us that not everyone in an area will inform the Council that their house has been flooded. This means that the Council would need to talk informally with people in the area to get a more accurate picture and better define the problem.

Porirua City Council and Thames-Coromandel District Council are working towards improving their information about flood risks. Thames-Coromandel District Council is planning further modelling to identify their flood risks and assess the capacity of the stormwater system for the main towns in the district.

Porirua City Council is in the process of finalising stormwater models and hazard maps with Wellington Water. The Council will use this information in its district plan and to identify priorities for future stormwater work.

To get a broader understanding of their flood risks, councils also need to work with regional councils. We talk further about this in paragraphs 5.41-5.45.

This incomplete view of the flood risks is not unique to the three councils. A paper by an associate at Beca Limited for Water New Zealand's 2016 Stormwater Conference used case studies to show that council funding becomes available for stormwater and flood mitigation works in the years immediately after local flood events, often for a limited time.

This approach also risks councils focusing on reducing the effects of the most recent event rather than considering all possible events and effects throughout a city or district.

We were told that most councils have not determined their flood risk other than through the experience of actual floods. This is consistent with the findings from the 2008 Ministry for the Environment report Meeting the Challenges of Future Flooding in New Zealand, which is about flood risk management more generally. The report stated:

Current flood risk management strategies on the whole tend to mitigate known hazards rather than address actual risks across a region. That is, there is a focus on large rivers that have flooded in the past with a known flood history, rather than where flooding could occur in an area. While this approach has been sufficient in the past, it is unlikely to be a sustainable approach in the future with climate change increasing the risk of flooding.6

Having a fuller understanding of their flood risks is critical for councils to take a structured long-term planning approach to reduce the risk and effects of flooding, including through managing stormwater systems. As Wellington Water stated in its Three Waters Strategy – Wellington Metropolitan Region:

Underpinning our future investment strategy is the need to better understand the limitations of our networks so that improvements can be planned on the basis of risk and the achievement of service standards. Hydraulic modelling is an important tool for understanding the "capacity" of our pipe networks. Although complex and costly to develop they not only assess the potential impacts of flooding but determine the effectiveness of proposed mitigation options.7
Recommendation 1
We recommend that councils understand the current and likely future flood risks in their district or city sufficiently to take a proactive approach to reduce the risk and effects of flooding.

Information about climate change and land use change is needed to understand flood risk

The three councils have acknowledged the need to plan and design for the effects of climate change and land-use changes, including urbanisation. For the flood risk assessments based on modelling, the three councils considered and used information about climate change and land use.

However, to make better decisions that incorporate the effects of climate change and land use changes, the three councils need to address the limitations of their flood assessments described in paragraphs 2.2-2.20. Understanding their future flood risk will help councils ensure that stormwater assets are appropriately designed to maintain their effectiveness in reducing the effects of flooding.

Improving councils' understanding of current and future flood risks would also enable them to answer people's questions about the flood risk to their home, now and in the future.

Climate change is expected to increase existing risks and create new risks. Climate change will affect flood risk through:

  • projected changes in rainfall, with some parts of the country becoming wetter and other parts becoming drier, depending on the season and interactions with existing natural processes such as El Niño climate cycles;
  • a likely increase in extreme rainfall in most areas, meaning an extreme rainfall event that is currently considered a one-in-50-year event will become more frequent;
  • an increase in sea levels that decreases the effectiveness of stormwater systems in draining the rainfall away, because of the backflow of water into pipes and watercourses; and
  • an increase in sea levels that increases the level of groundwater in coastal areas, because it leads to more infiltration into stormwater pipes and reduces their capacity.8

Increased urbanisation, such as new housing areas and increased density of housing in existing areas, results in more impervious surfaces. This can result in larger rainfall run-off that needs to drain away through the stormwater system.

As a consequence, flooding in New Zealand is likely to become more intense and more frequent. We expected the three councils to consider the effects of climate change and urbanisation to understand their future flood risk and ensure that their stormwater systems maintain their effectiveness in reducing the risk of flooding.

Councils need to consider changes in flood risk during the life of flood mitigation assets to ensure that the assets maintain their effectiveness in reducing the risk and effects of flooding. This is important because flood mitigation infrastructure, including stormwater assets, can last a long time. For example, the estimated useful life for a stormwater concrete pipe is about 80 years.

The three councils are doing some work to consider the change in flood risk. For example, in its Three Waters Strategy – Wellington Metropolitan Region, Wellington Water highlights the effect that climate change and the increase in impervious surfaces because of urbanisation will have on the management of stormwater systems.

Dunedin City Council and Thames-Coromandel District Council base their climate change projections on guidance from the Ministry for the Environment and update the projections when new information became available.

Porirua City Council uses a report commissioned by Greater Wellington Regional Council on the effects of climate change in the Wellington region.

The three councils have also done, or are planning, further work about the effects of climate change.

In 2010, Dunedin City Council commissioned its own report on the expected effects of climate change. The Council recognises that it needs to update this report.

Thames-Coromandel District Council is planning further work on investigating coastal hazards in the district.

Wellington Water has changed the climate change projections it uses to ensure that they are consistent with the projections that Greater Wellington Regional Council uses.

For the flood risk assessments based on modelling, the three councils considered and used information about climate change and land use. For example, as part of the hydraulic modelling done for Dunedin City Council's integrated catchment management plans, the consultant modelled 14 scenarios representing different land use, rainfall, climate change, and tide combinations.

The three councils also incorporate the effects of climate change and land use changes when designing stormwater infrastructure and in their financial forecasts. For example, adding a factor for an increase in rainfall when designing the capacity of a stormwater pipe.

Incorporating information about these changes helps the three councils to understand their future flood risk and to make decisions about the need for current and future flood mitigation work.

Questions to consider
For councils:
How extensive is your understanding of your current and future flood risk? What more do you need to know?
For people to ask their councillor:
How often is my house likely to be flooded by stormwater?

3: Hydraulic modelling uses a mathematical model to develop an understanding of flood risk and the performance of the stormwater system. It can also be used to develop options for reducing flood risk and to measure their effectiveness.

4: An integrated catchment management plan takes a holistic approach to managing the inputs and outputs of a stormwater catchment. Because stormwater travels from roads and roofs to streams, rivers, and seas without treatment, contaminants in stormwater contribute to pollution of the receiving environment. An integrated approach means taking a range of factors into consideration – activity and urban development in the catchment, the state of the stormwater and wastewater networks, levels of contamination, flooding – when planning how to make improvements.

5: Dunedin City Council published an updated integrated catchment management plan in 2017. This combined 10 of the 11 original plans, and contains updated stormwater management information as necessary. The update did not contain any new information about Dunedin City Council's flood risk.

6: Page 25.

7: Page 31.

8: For more information about climate change, see the Ministry for the Environment's website The Deep South national science challenge website also provides useful information about the implications of climate change (see