Soakage structures
We’ve covered some broadscale approaches that make a big difference to reduce the impact of flooding in the landscape. However, there are smaller-scale strategies that property owners can implement.
Below are several soakage structures used to manage stormwater and runoff. As a general principle, we want to soak rainfall and run-off into the ground as much as possible, while allowing for their inevitable overflow in a big storm. When they overflow, we want the water to harmlessly flow over the land, ideally as a shallow, spread out, slow flow – rather than a concentrated, fast, erosive flow – before it reaches a waterway.
The company I work for, Greenbridge, has a demonstration site with most of these soakage structures in place. The neighbour’s lifestyle block on the land above sends a phenomenal volume of runoff onto the site, filling up our huge soakage basin located near the boundary after just a couple of hours of heavy rain. It then starts to overflow, gently, and spread out. This allows tens of thousands of litres of water to soak into the ground, slows down the water, and prevents erosion.
The soakage structures include:
• Soak pits: These are often 600mm diameter by 5m-deep holes, drilled into the ground, with a concrete lid placed over the top. The exact design and construction can vary from region to region (local councils usually have standard designs), and the soak pit design needs to respond to the soil type, so you’ll want to get expert advice. They are used to soak in run-off from hard surfaces such as roofs, driveways, and carparks. • Soakage basins: These are open basins, excavated into the earth, which capture concentrated run-off, soaking it in. They can be any size or depth – it depends on how much space you have available, and your willingness to dig (or whether you have access to a digger).
• Soakage swales on contour: These are shallow ditches, dug along the contour, with the excavated earth forming a soft bund on the downslope side. Swale ditches should be re-seeded and the bunds planted with stabilising shrubs and trees asap. These have been used in permaculture-style pastoral farms for capturing rainfall run-off from paddocks and soaking it in, but it’s
widely understood now that you are much better off switching to regenerative farming practices to reduce any run-off in the first place. We use soakage swales on contour to capture run-off from hard surfaces such as carparks. It’s essential that the swale base is level (on contour), otherwise you are digging a drainage channel which will speed up water flow.
• Rain gardens: These are basically shallow soakage basins, planted with shrubs and trees, that can handle both wet and dry conditions, that are integrated into your landscaping.
In the floodplain
It’s not necessarily a bad thing to live, grow crops, or farm in a floodplain. Through good planning and design, as well as appropriate management of the whole catchment, risks can be mitigated, but you need to be realistic about it.
One of the best ways to protect your property and others in a floodplain is to start or join a management group for your catchment. As a group, you can explore the big picture, and learn about land use and management that will regenerate landscapes and then sustain them for generations.
You can support each other, and also access grants and subsidies to reforest steeper slopes (permanent forest, not clear-cut forestry), and change your grazing management to reduce erosion and increase rainfall infiltration.
These sorts of actions can dramatically decrease the impacts of high rainfall in your catchment, by reducing flooding, and by reducing debris and silt in the floodwaters.
Your regional council or NZ Landcare can connect you to a local group: www. landcare.org.nz/resource/catchmentgroups/
If you’re looking to implement strategies on your property, however, there are a number of things you can do.
Firstly, if you can, site your house and other high-value infrastructure appropriately, either out of the floodplain, or raised on poles, above the floodwaters.
As we’ve seen in the heart-wrenching footage of the recent floods, debris and silt can be very destructive. But in some cases, we can design to capture the debris and silt in certain parts of your property, where they do little damage.
Wide bands of tough multi-stemmed shrubs and trees can trap debris and also slow the floodwaters, causing them to drop silt where it does little damage. Then, slowmoving and clear flood waters may rise and fall over the rest of your property, doing the minimum of damage. Consider grey willow, black willow, suckering types of cornus (dogwood), flax, swamp coprosma, kanono, karamu, and tauhinu.
I’ve used this approach in the design of a lifestyle block tucked into a bend of the Mimi River in North Taranaki. Critical infrastructure such as the house and access are sited on higher ground, or in the house’s case, on poles. A wide band of multi-stemmed shrubs and trees (sieve-like vegetation) will curve around the orchard, vege garden and house to capture flood debris and silt, slowing floodwaters. The design hasn’t yet been implemented, but this is an example of how floodprone sites can be improved. I’ve seen images of a flood-prone site overseas in which the garden was surrounded by a permeable rock wall, which acted to slow floodwaters, and prevent debris from entering the garden. The owners reported that it worked brilliantly – while the garden was still flooded, once the floodwater receded, the garden quickly recovered as it wasn’t
debris.• damaged or covered in KAMA BURWELL is an ecological landscape designer and engineer at Greenbridge, in Taranaki. She designs lifestyle blocks, farms, edible landscapes, and wastewater systems. Kama draws on her experience as an engineer, alongside her understanding of permaculture, terraquaculture, and ecological processes to deliver design solutions that heal landscapes, while building community resilience and connection.