Stormwater is a hot topic! Roads, driveways, footpaths and roofs do not absorb rainwater, it flows over them. Typically this water is collected and piped away to be released untreated into streams, waterways and the sea.
This can cause lots of problems. But there are solutions.
Auckland Botanic Gardens now incorporates sustainable stormwater management in our landscape projects wherever possible. As a result, much of our stormwater is treated on-site, often by devices that use plants to treat the water.
We are happy with our results and now visitors can share in our learning. Come to the Gardens and pick up a Sustainable Water Trail brochure from the Visitor Centre to discover some of our onsite initiatives.
You can download an overview of our Water sensitive design here.
Reusing water is a great way to reduce the amounts taken from the city water supply and save on your water bill. It is also an effective way to reduce water discharge.
Water is re-used in a variety of locations at the Auckland Botanic Gardens.
Nursery irrigation run-off is collected in a 30,000 litre underground tank. It is stored here and pumped back through sprinklers to water the plants. Nutrients that have washed out of the potting mix are carried along by the water, and hence reapplied with each watering.
Huakaiwaka Visitor Centre
Rain water is collected from the Visitor Centre roof and stored in an underground tank. It is then used to run the nikau water feature at the entrance to the Visitor Centre and to flush the Visitor Centre toilets.
Potter Children's Garden
Rainwater is collected from the roof of the large shelter in the Potter Children's Garden. It is stored in a tank that is used for watering the plants, and to demonstrate water re-use in our education programmes.
Rain gardens are important in places like carparks where contaminants such as heavy metals and oil wash off cars. Our carpark is at the highest point of our site and the rain garden captures and treats this water before it flows into the stream running through our native plant collection, and then the Puhinui stream and out to the Manukau Harbour. The water pools in the rain garden, which is deep, for up to 24 hours, allowing sediment and other contaminants to settle into the soil, or be absorbed by plants. The water then slowly seeps through the special rain garden growing medium, restocking ground water, with excess ground water draining into the stream.
Spot one? Rain gardens can be seen alongside motorways and within new subdivisions where large amounts of stormwater can be treated on-site, in a beautiful way!
Living roofs act as a trap for rain. The growing medium and plants absorb water, decreasing the stormwater volume flowing off it by up to 65%, and slowing the water speed down to a trickle.
Living roofs can also insulate against heat, cold and noise and provide habitats for plants, birds and insects.
What sort of plants?
Although living roofs can treat large amounts of stormwater, they often dry out. Their growing medium is shallow, they have no ground water replenishment and being up in the air are exposed to drying winds. The plants need to be shallow rooted and tough!
Living roofs are usually created on new buildings, where support to cope with the extra weight can be built in. Some, like our edible living roof can be retro-fitted, providing that the structure is strong enough, and passes an engineering inspection.
We are trialing a variety of plants to see which ones perform well on the three living roofs here at the Auckland Botanic Gardens:
Succulent living roof
In the Potter Children's Garden is a living roof planted with a variety of succulents such as Echeveria species, Lampranthus, and Mesembryanthemum and bulbs such as Ornithogalum dubium. A colony of a native orchid (Microtis unifolia) has arrived on its own to live on this roof. Succulents cope well with the harsh environment that living roofs provide and look stunning during their winter flowering.
Native living roof
There is lots of research from overseas on exotic plants suitable for living roofs, but using New Zealand native plants on roofs is relatively new. Our native living roof, on the Potter Children's Garden toilet block, is a trial and we test many different native species hoping to find plants we can confidently recommend for NZ roofs. The roof is dry and hot which is a difficult habitat to establish plants. Coastal plants that cling onto cliffs faces and rocky outcrops seem to like it up there on the roof which is not surprising as these wild habitats also have very little soil and hot dry conditions. So far native iris (Libertia peregrinans), needle grass (Austrostipa stipoides), and Astelia banksii have done well.
We have planted a large number of native species around our lakes. These plants benefit the water quality of our lakes. How do they do this?
- Contaminants carried by stormwater are trapped by stems, leaves and roots -preventing them from entering the lakes
- Stormwater flow is slowed by the plants, reducing the risk of bank erosion
- Where large trees and shrubs are planted they provide cooling shade, discouraging algal growth in the water
- Native fish lay eggs on submerged leaves of bank side plants • Insects live among the plants, providing a food source for native fish
- Ducks are less inclined to feed amongst dense vegetation, thus decreasing the amount of defecation (and hence nutrients) in the lakes.
What's the issue with algae?
Algae multiply very quickly and are short-lived. When they die en-masse their decomposition removes vital oxygen from the water. This is harmful to water insects, fish and beneficial plants. The warmer the water, the faster algae can grow. Algae also thrive when nutrient levels are high - particularly phosphates. So keeping waterways cool, and preventing nutrients from entering them is essential.
Swales are suitable for many situations where stormwater needs to be slowed and moved to be further treated. They are commonly seen alongside car parks, roads and motorways. They can even be adapted for use at home.
The Perfect Plants
Swales can be tricky environments for plants to grow. Conditions vary from very dry to very wet, so the plants need to be hardy. A successful swale plant at the Gardens has been native oioi (Apodasmia similis). The rush-like leaves of oioi allow water to flow between them. This prevents scouring of the swale bank as the water does not have to curve around the plants to flow. The leaves also trap sediment, preventing it from flowing downstream.
A wetland swale moves water from the visitor car park, paths and nursery overflow down to the sediment forebay. It's termed a wetland swale as water seeping in from the surrounding land keeps it almost permanently wet.
A vegetated swale moves water from the living roof toilet block at the Potter Children's Garden and the surrounding gently sloped hill, through to one of the lakes.
We also have a vegetated swale moving stormwater from the Potter Children's Gardens infiltration trench overflow.
Permeable surfaces allow rain water to flow right through them, as opposed in impermeable surfaces, such as concrete and asphalt.
Gravel, sand and crushed shells are permeable surfaces often used for garden paths and driveways. In areas where harder more robust surfaces are needed, porous pavers can be useful.
These strong and durable pavers are made from small pebbles 'glued' together. Rain flows straight through them and then filters through a layer of gravel below. This slows the water, removes some sediment and other contaminants before the water is piped away.
At the Gardens, porous pavers form a courtyard outside the Potter Children’s Garden. This provides a puddle free area for children to use year round.
Stormwater tree pit
Stormwater tree pits are designed to treat large volumes of stormwater from roads and car parks.
Under light rain conditions the plants and the planting medium within the tree pit act as a biological filter to treat and slow stormwater. When heavy downpours create large volumes of stormwater the water overflows from the planted area into underground filter chambers (beneath the green mesh cover).
The filter chambers use chemical processes to remove pollutants from the water including hydrocarbons. This is especially key with car parks and roads, whose surfaces commonly collect contaminants from vehicles.
Our plant picks
We've planted pohutukawa, Metrosideros excelsa, and oioi, Apodasmia similis. These native plants are hardy enough to cope with the often dry environment of the quick draining tree pit.
Stormwater planter boxes
Two stormwater planter boxes treat stormwater from the Education Centre roof in the Potter Children’s Garden.
Stormwater planter boxes can be an attractive way to treat stormwater. At the Potter Children's Garden, water flows in to two planter boxes, fed by rain falling on the Education Centre roof. A perforated pipe disperses water across the surface of the planter, where it slowly seeps down through the soil mix and is absorbed by plant roots.
Our planter boxes are completely lined so water from a planter box won't seep into the ground and around building foundations. Any water that eventually flows out from a drainage pipe at the base of our planter boxes is directed through further treatments and finally to the wetland area.
Our plant picks
Hardy plants are best for planter boxes - we are using the native oioi, Apodasmia similis, and day lily, Hemerocallis.
The infiltration trench provides stormwater treatment. Water is held within the trench slowly absorbing into the surrounding soil. This re-stocks groundwater, which is beneficial to surrounding trees. Sediment and other contaminants carried in by the water settle to the bottom of the trench.
Our trench is 10 metres long, by one metre deep and is filled with small porous rocks. These rocks provide structural strength to prevent it collapsing or eroding. Large rocks have been placed on top to be visually appealing.
The infiltration trench is part of a 'treatment train', where a series of stormwater treatments are linked together. If the trench fills and overflows, it is treated in a vegetated swale.
The majority of the stormwater created within the Potter Children's Gardens goes through a series of treatment devices termed a treatment train. It is eventually discharged at a low lying area called a wetland.
The wetland naturally collects surface run-off and ground seepage from the surrounding lawns and gardens. Mature cabbage trees and other native plants grow along one of its boundaries.
We will plant further native plants that will thrive in the boggy, wet conditions.
Wetlands effectively treat stormwater, they trap sediment and other contaminants, and correct the water's pH and temperature levels. Wetlands also provide habitat for native insects and birds. Over long periods of time they also help store carbon.
Wetland related websites
Pipes deliver stormwater in to the Auckland Botanic Gardens from the surrounding suburb and roads. We collect and treat it when it arrives, in a sediment forebay.
The forebay also treats stormwater flowing in from the adjacent wetland swale, carrying stormwater from the Gardens' car park and nursery.
As the fast moving water slows down and pools in the sediment forebay, sediment and other suspended contaminants are able to settle to the bottom. The surface water becomes the cleanest, this water flows through an overflow pipe and into the adjacent lake.
Wetland plants around the forebay margins help to correct pH and oxygen levels. Young trees on the northern bank, once grown, will provide shade to cool the water. Dense native planting on the forebay banks filter stormwater as it flows in from the hills, removing sediment. All the plants bind the soil with their roots, reducing the risk of bank erosion, and hence reducing the amount of sediment crumbling into the forebay.
What's the fuss with sediment?
Sediment is small particles of soil, which erodes from steam banks, or washes in from gardens and poorly managed earthworks. It is harmful to waterways as it smothers stream banks, destroying fish habitats. It can also clog up waterways and interferes with light levels.
When we dug out the area to create the forebay, the diggers also contoured the surrounding slope. This will allow for easy access in the future should the forebay need to be emptied of sediment.