Naenae Channel Naturalisation
“Isn’t that just a drain?”
“Isn’t that just a drain?”
This child’s observation of the Waiwhetū stream as it flows in straight concrete-lined channels in the Lower Hutt suburb of Naenae is very unfortunate, but all too real. For some young people they are disconnected from the stream because it has been so heavily modified.
Waiwhetū stream was straightened in Naenae as early as the 1940s, to make room
for market gardens. In the mid Twentieth Century ‘hard engineering’ was seen as an
acceptable way to control waterways in urban areas. Close to 2.3 km of concrete
lining was carried out in the early 1980s to contain the Waiwhetū stream. The work
was carried out by many people on employment schemes set up by the Department
of Labour.
Since the 1990s, the science has shifted away from such drastic engineering
approaches and favoured softer (nature-based solutions) working with nature instead of against it and restoring the stream’s ecology.
Concreting channels can destroy a
stream’s ecology and may also make the flooding problem worse downstream by
allowing the faster transfer of floodwaters to lower reaches. Streams naturally want
to inundate their adjacent floodplains and erode streambanks, so today planners and
engineers work with and around the stream’s natural energy.
Channel naturalisation is one approach with this new way of thinking and involves restoring a modified stream to a more natural state. Removal of the concrete lining is often too expensive and disruptive and there may not be enough space to restore the natural stream meander patterns. An alternative, inexpensive strategy is to infill the low-flow channel with gravel and some bigger stones to create pools and riffles without exacerbating the flood risk. Planting of the edges of the low-flow channel with a variety of suitable plants also softens the hard edges of the channel as well as creating shade for fish and other aquatic life.
The advantages of channel naturalisation:
-
Mitigation and reduction in ‘flash flooding’ and higher flood peaks, especially downstream as vegetation enhances infiltration, temporary water storage and groundwater recharge.
-
Vegetation helps to bind the soil thus reducing sediment load and erosion in a flood.
-
Increased biodiversity and improved water quality as the water is oxygenated
-
more. Water temperature is also more stable in the natural state.
-
The planting of overhanging grasses (such as Carex secta) provide shade for
-
fish and other aquatic species.
-
The waterway starts to look more like a natural stream – with benefits such as educating people about streams, improved aesthetics and creating more inviting natural green spaces. Overseas research has shown less graffiti and vandalism occurs when channels have been restored to a more natural state.
-
People have more pride in their environment with more naturalised channels and this enhances people’s well-being and has been shown to improve real estate values as well.
The goal of channel naturalisation
As the concrete-lined channel system is in the upper reaches of the Waiwhetū Catchment, the volume of storm runoff is unlikely to be high enough to overtop the
wide banks – most of the flooding that affects housing and infrastructure occurs further downstream. The flood hazard downstream during a heavy rainfall event will
be exacerbated slightly by the faster movement of the floodwaters through the concrete-lined channel system.
The goal of channel naturalisation is to improve the ecological health of the stream, enhance biodiversity and restore natural hydrological processes. There are many
examples overseas of other naturalisation efforts and in New Zealand, sections of the Avon River in Christchurch (after the earthquakes) have been carefully restored
to a more natural state.
In summary, channel naturalisation does require careful planning and management to ensure that it balances ecological restoration with the protection of human
communities and infrastructure from flooding. The naturalisation happening now in sections of the stream in Naenae is not true channel naturalisation (whereby the
concrete is removed), but it is a cheap and effective solution one step towards true naturalisation.
Friends of Waiwhetū Stream, in conjunction with Naenae Nature Trust and with support from Hutt City Council and Greater Wellington Regional Council, are proud to be involved in this project.
You can read more about this work from Chapter 6 of our book