Stormwater infrastructure investment
Rotorua Lakes Council is responsible for providing key services to support the daily life and general wellbeing of the Rotorua community. The infrastructure assets in Rotorua (transport network and three waters networks) required to support these key services are worth approximately $1.446 billion. It is important that Council continues to invest in the city’s infrastructure to ensure that the assets remain in the appropriate condition to support the services that meet the needs of the community now, and to prepare for future as the population and demand grows.
What is Stormwater?
Stormwater runoff or ‘stormwater’ is the water that ends up on our roofs, driveways, roads, gardens and all ground spaces when it rains. That water flows from land, into gutters, drains, creeks or streams and eventually ends up in waterways such as our local lakes.
What is a stormwater system?
A stormwater system is a network of pipes, open drains and overland flow paths that carries stormwater from land to nearby waterways. In nature, trees, plants, grass and soil create a system that helps to absorb some of the rain and filters it slowly into the ground. It becomes groundwater which eventually makes its way to nearby waterways. Creeks, valleys and wetlands are also parts of a natural stormwater system.
When land is developed, and hard surfaces such as carparks, driveways and buildings cover the earth, they prevent the water from being absorbed into the ground and instead it runs off over the land. To capture that run off, a system of gutters, drains and pipes is used to channel the water safely into the natural waterways. The system can be entirely man-made or it can be a mix of man-made and natural stormwater channels.
In Rotorua there is 248km of urban reticulated pipelines, 153km of open drains, three urban areas serviced by stormwater ((Ngongotahā, city and eastern suburbs) and the Reporoa land drainage scheme.
Why do we need to invest in our stormwater infrastructure?
Our city is growing – Rotorua is home to approximately 76,200 people and our population is projected to reach approximately 90,800 by 2051. Like much of New Zealand we are also facing a housing crisis and Rotorua needs more homes for our growing population. The city is already seeing new urban areas (greenfield development) along with future intensification of existing residential areas. This means we will need safe, stable and reliable infrastructure to support that development and to protect existing properties.
There are also a number of other future demands that we need to consider:
- National legislation and policy changes – better environmental compliance standards are emerging which means requirements for additional monitoring of systems or enhanced treatment of stormwater before it is discharged into our freshwaters environment
- Embedding cultural values into the way we manage our infrastructure – the partnership with Te Arawa is embedded in the way Council works. Major infrastructure projects require significant upfront input from mana whenua to ensure cultural considerations are understood and provided for, alongside other relevant factors. Council’s aim is to collectively agree about how our new infrastructure is constructed to ensure our growth is sustainable and we protect the values and taonga that make our district special.
- Climate change and environmental sustainability – Council is preparing for the impacts of climate change on the city’s infrastructure assets. New Zealand is already experiencing impacts such as prolonged droughts, higher temperatures, and increased frequency and intensity of high rainfall events. In terms of stormwater, climate change requires improved system capacity and safety standards.
- Resilience of critical infrastructure – Council needs to ensure that infrastructure will be resilient and that the community can continue to be serviced in the event of emergencies e.g. earthquakes, floods. We need to use different materials, techniques and system duplication as much as possible so if damage occurs, there are alternatives to continue the services even at reduced capacity.
- Lake water quality – Stormwater is a significant contributor of litter and other urban contaminants into waterways. This includes chemicals and metals from things like washing cars, roads debris and roofs. As part of the Stormwater Catchment Management Plan Council must consider water quality treatment opportunities where practical to reduce stormwater contamination.
What is Council currently doing to improve stormwater infrastructure?
Through the 2021-2031 Long-term Plan, and the supporting Infrastructure Strategy, Council has identified that some $150 million of capital investment in Stormwater infrastructure is required to address our historical risks and the new housing objectives. Council must invest, either directly or through government partnerships, in bulk stormwater infrastructure to enable land development and to cater for increased scale and severity of rain events due to the impacts of climate change. To do this, Council has developed and is implementing a Stormwater Master Plan to ensure that it is making cost effective investment decisions, and to ensure that infrastructure is installed at the right location, at the right size and at the right time, and is addressing both community and environmental wellbeing.
The initial locations identified through the Stormwater Masterplan are the western and eastern parts of Rotorua. This is where most of the housing development is expected to occur in the next few years. The objectives of the Masterplan are to:
- Mitigate existing flooding issues.
- Mitigate the expected additional runoff as a result of new urban areas.
- Prepare capacity for the impacts of climate change.
- Mitigate the expected runoff as a result of intensification of current residential areas.
Council has also submitted a Comprehensive Stormwater Resource Consent to the Bay of Plenty Regional Council. This Resource Consent will help guide the way stormwater is managed across the whole Rotorua urban area in terms of quantity and quality.
Why is stormwater in Rotorua a challenge?
Rotorua is a caldera (an old crater of a very large volcano). The shape of the land is like a shallow bowl. There is a lake in the middle, a large flat area around the lake and surrounding hills that make up the edges of the bowl shape. When it rains, water falls on the hills around the city and it naturally makes its way down to the lake.
Historically the stormwater system was built to service the small township of Rotorua. When it rained and water fell on the hills, a lot of it would have been soaked up by open land before it hit the residential areas closer to the lake. As the city has grown, and developments have extended to higher ground, the amount and speed of surface stormwater flowing to the lake (rather than being soaked up by open ground) has increased.
The amount of housing, as well as the type of housing that we typically see in new developments (smaller sections with large concrete areas e.g driveways and patios), combined with more frequent heavy rain events, means the current stormwater system is reaching its capacity for the average rain events, and in recent years significant and extreme rain events have resulted in pockets of flooding in low-lying areas of the city.
Why can’t we just upgrade the current system?
Where able, Council has been increasing the capacity of the stormwater system to enable residential development and to improve flood risk control. Some examples of this work is the King Street stormwater upgrade which has involved contractors installing new, larger pipes underground in the street and at Sheaf Park. Work has also been carried out at the top of Clayton Road to increase stormwater capacity in that area and on Vaughan Road to compensate for housing development However, in areas where housing is denser there is less room to be able to increase the size of the stormwater piped system, and increasing capacity isn’t always the best fix. Increasing the amount of water that is able to flow through a pipe could result in negative effects on natural waterways further downstream such as flooding, erosion and disturbance of the lake bed. Most of the stormwater network also relies on gravity to carry the water to the nearest stream or lake, this is often why you’ll find stormwater channels (pipes, drains) in low-lying areas.
How will Stormwater be managed to support new developments?
The Bay of Plenty Regional Council introduced a requirement to produce a stormwater management plan for new developments that overall reduce stormwater runoff from new developments to 80% of the pre-development runoff including the effects of climate change. Hence the need for onsite stormwater detention for a period of time before it is released into the downstream stormwater network. In effect, these requirements aim to ensure that additional runoff as a result of new developments does not overwhelm the system causing risk to downstream properties.
This means for new residential developments the creation of storage areas to retain rain water during wet weather events. Large ponding areas are constructed to hold the water like a storage tank. The water then either evaporates, soaks into the ground or filters through a small outlet pipes into the stormwater network and eventually makes its way into the lake. By holding the water back and storing it, it helps to reduce the adverse impact of large volumes of water inundating the stormwater network and flooding lower lying areas. Think of it like a funnel filling a bottle of water.
On smaller scales, property owners are encouraged to think about their own onsite drainage such as soak holes, landscaping for rainwater retention and rainwater tanks.
In Rotorua, the chemistry of the stormwater that flows into the lake is not a significant concern compared with established environmental guidelines. The biggest concern is rubbish and general litter that makes its way to the lake via the stormwater network. Stormwater also carries harmful pollution arising from animal and bird faeces and nutrients from decomposing organic material. It is important that as a community we work together to keep stormwater as clean as possible while we protect private and public properties.
Contaminants that can come from stormwater are:
- Zinc (from roads and roofing)
- Copper (from roads and roofing)
- Suspended solids (rubbish and litter)
- E. coli (from human and animal sources)
- Fertilisers and other garden products (Phosphorous and Nitrogen)
Each of these contaminants may cause harm to stream and lake aquatic life, risk to human health from contact during recreational use of the water and growth of nuisance aquatic plants and algae.
How do we stop contaminants getting into the lake?
Our roads act as a secondary flow path for stormwater that cannot be accommodated by the piped network. Our roads store rain water (that’s why sometimes there is road surface ponding) until the pipes are freed up to convey water to watercourses and streams. Each cesspit (drain that you see in the road gutter) has a catch chamber at the bottom. When then water flows in from the road, the heavy sediment such as gravel, sands, debris sinks to the bottom and settles while the water flows on into the stormwater system. There are also a number of gross contaminant traps (nets on the end of a stormwater drain that catches litter, leaves etc) where stormwater enters the lake. These cesspits are then emptied through specialist suction equipment removing all the settled material for safe disposal in landfills.
Other ways to help reduce contaminants entering the lake are:
- Retention ponds – contaminants settle at the bottom of the pond rather than going into the stormwater network.
- Wetlands – naturally filter contaminants from water. Nutrients are also helpful in this instance as they can help the wetlands grow.
- Open drains – grass and other low lying foliage helps to filter some contaminants out of stormwater, exposure to UV light (sunlight) is natural bacteria treatment and any rubbish that may have been carried to the lake is easily collected.
The community is also encouraged to help reduce contaminants getting into our waterways:
- Always wash vehicles over grass areas
- Reduce litter – place it in the waste bins
- Sweep up leaves and grass clippings from around your property and place them in the compost or green waste
- Pick up after your pet to reduce faecal matter entering our waterways
- Never let paint, oil, thinners, insecticides or other contaminants go down the stormwater drain.