Flooded with Ideas

SERIES 33 | Episode 31

Devastating floods in Queensland and NSW left many asking how we can better prepare ourselves in the face of a climate that's becoming more extreme and unpredictable. This extends to where we choose to live, how we build our homes, and also how we garden.

Sam Bowstead is an architect who's built his practice on helping people design buildings that are more resilient to floods. "We've been doing it since 2011, which was a bad flood. In the 10 years we've seen what we do work in a major disaster, and been able to prove the approaches have worked".

The House

When buying a home in Brisbane with his two siblings, Sam decided to "put my money where my mouth is". He bought a classic Queenslander in Wilston, an area with a higher flood risk, and compounded by the fact they're adjacent to a creek.

The 2022 floods saw water to a depth of 2 metres go through the lower part of the house, garden and property The home was "95% fine" and Sam and his siblings were able to move back in the next day, with power restored within a week. This was thanks largely to the renovations Sam had made to the house explicitly to prevent flood damage. "When you can get back in quickly, it changes how it affects you. To be able to practically clear it, make it safe and liveable".

The changes Sam made to the house were straightforward:

• The entire house was raised by 1.5 metres, to sit the living space well above the predicted 2 metre level of severe floodwaters
• Downstairs was retained as a utility space only, holding a garage and a laundry
• A polished concrete floor was laid downstairs, for ease of cleaning silt and floodwater
• "Single skin" walls (with no cavities) were installed downstairs, made out of water-resistant marine ply. Other materials used were selected for water resistance.

"The idea is you want to let the water go through…You want to make it so it can be washed out, is water resistant and is designed with potential flood heights for your area in mind". Local councils can provide flood mapping information, so you know what you're up against. "You're looking for the flood height for the 1% event….to embrace the understanding of risk and know what you're dealing with. They used to say 1 in 100 years but it's not 1 in 100 years anymore".

The Garden

Of course, a home isn't just a house-it's a garden as well. The property holds a front and rear garden. The front garden was cleared to make room for the renovations, and the rear contains established plantings from the previous owner (mostly palms).

The flood not only impacted the house, it impacted the garden. Spearheading the gardening efforts at the property is Peter, Sam's brother.

After they were able to get back onto the property, Peter turned his attention to garden triage. The steps he took were simple, but were able to get the garden back on track

Many of the mature plants at the rear of the property survived the deluge, largely because they were tall enough to hold foliage above the water level. However, silt deposited on leaves below the waterline can block sunlight, preventing them from growing. As soon as he could, Peter washed this silt off with clean water from a pressure washer, to allow the plants to continue to photosynthesize.

Waterlogged soils can become acidic. To remedy this, Peter applies a handful of lime per square metre on the surface. Compost is also applied to help restart the life in the circle.

Before restarting any gardening, Peter sent samples of the soil off to be tested for contaminants. Flood waters can contain chemical and biological contaminants (like sewerage), and so by testing Peter was sure there wasn't anything nasty brought in by the water still lurking in the soil.

Since the flood Peter has noticed the topsoil has changed and has become far more sandy due to deposits from the floodwater. Digging down to a depth of 400mm reveals a waterlogged, sloppy clay that is still retaining a large amount of water. Peter says engineers classify this as "problem soil" for its inability to hold loads, so any future plantings will need to have non-invasive root systems and not grow into large trees, to avoid them toppling over in the unpredictable soil.

The force and velocity of floods mean that any exposed top soil is stripped away into stormwater systems, exposing roots and destabilising plants. Prior to the floods Peter had laid down turf, and when he was able to return to the property noticed it was successful in stabilising the soil and preventing runoff. The small edge area that wasn't covered in turf has suffered from soil erosion. Here the lesson is to cover bare soil to slow the flow of water and prevent runoff. If you've got time turf or groundcover plants will do the job and bind the soil with their roots. If you have less time use fast-growing cover crops-or if you're in a pinch, mulch will do the same job.

Jerry's Advice

Since the floods Peter has put in a few things he thinks will do well in the garden.

They're deliberately avoiding edibles because of concerns about ongoing soil contamination appearing over the future.

Both Peter and Sam are keen to get Jerry's advice on what else would be good to put in their garden, with a mind to flood resilience. "We're amateurs but are grateful for any help" says Peter. Jerry has brought a long a few plants he thinks may work in the conditions, including:

• Brisbane lily Proiphys cunninghamii. These large-leaved lillies occur in lightly shaded areas of the rainforest or in open forest bordering rainforest regions in the Brisbane area down to northern NSW. They grow from an underground bulb, which makes them perhaps adapted to flood events; if the top foliage is sheared off by strong waters they're set to regenerate from the well-anchored bulb
• Another bulb is one of Jerry's favourites. Crinum pendunculatum var asiaticum is native to SE Qld and up into Asia, and has what could be the largest bulb on the planet. But don't get it confused with the regular Crinums you see around; this is a super-sized relative! Enormous white flowers and long flesh leaves to 1.5 metres in length leave an impression, and the natural habitat of swamps and flooding gullies mean this heavyweight knows how to hang on in rough seas
• Banksia robur, the Swamp Banksia, occurs in swamps along the coast and nearby plateaus of New South Wales and Queensland. It's an open tree to large shrub to 2 metres, with large stiff leaves that are very tough. As the name suggests it thrives in damp, poorly drained conditions and will tolerate seasonal flooding.
• Even though they don't want edbiles, Jerry's brought along some lemongrass. Even when it's not used for eating it's a worthwhile ornamental plant, with fragrant clumps of foliage that love living in waterlogged conditions. If they decide they want to start using it in the kitchen, it's easy enough to break off a rhizome and start growing it safely in a pot.

Floods are sudden, dangerous and frightening. With climate change they are likely to become more common, so it's important we think about how we can continue to live, and garden, in the areas prone to flooding. In the fallout, being able to return to something as everyday as working in the garden can bring a much-needed sense of normalcy to a ruptured world.

Sam says "it's beautiful living here, we're on the park and it's near the creek. 99 times out of 100 it's super beautiful. For that 1 time you can be prepared when you come up against it."

* Check before planting: this may be an environmental weed in your area

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