Protecting Landscape from the Queensland and NSW Floods with Vetiver Grass

The recent floods on the east coast of Australia have been disastrous.
Over the past 18 months we've experienced our biggest flood and our biggest bush fire.
But with the flood comes erosion and sedimentation. With most ecosystems in south-east Queensland " no longer resilient to extreme weather” and with 49 million tonnes of sediment produced you'd have to say that the long term knock-on consequences of that alone is another crisis.

What's interesting about this flood, was that the major culprit wasn't the big rivers -- like the Brisbane -- but all the catchment creeks, which were in no condition to absorb the impact of the deluge.
That this has happened AGAIN -- and even worse than the last big flood -- suggests that no serious attempt has been made to protect the waterways from erosion or promote greater soil absorption through, say, by creating weirs.
Indeed, this issue is replicated right along the coastline, and as Vetiver researcher,  Paul Truong, has pointed out, the Great Barrier Reef is similarly being destroyed -- not just by sedimentation and sea temperature rise -- but through nitrate run off from farming.
Of course, there is a cheap solution....

“Desktop studies we have done shows this is probably five to 10 times more silt coming into Moreton Bay than what happened in 2011,” Ms McLellan said.
“This time – and we’re still out ground-truthing it – we’re estimating about 49 million tonnes of sediment, and of that, about 10 million tonnes is estimated to flow into Moreton Bay.”
Ms McLellan said the longer-term effects on the Port of Brisbane’s silt levels, seagrass beds, dugongs and turtles were being evaluated.
“There is going to be an impact on seagrass, but we know it will recover, as long as we don’t get another flood like this next year,” she said.
Moreton Bay Marine Park is the only place in the world where dugongs are found close to a major capital city.
Professor Stuart Bunn, director of the Australian Rivers Institute at Griffith University, said most ecosystems in south-east Queensland “were no longer resilient to extreme weather”.
“About half of the 42,000 kilometres of stream channels in south-east Queensland have banks in poor condition and are vulnerable to erosion,” Professor Bunn said.

When you consider the immense scale of the challenge,  bioengineering seems to be the only practical way forward. What I'd call waterholes are sediment retention basins that also serve to enable greater moisture absorption by the subsoil (as per the principle advocated by systems such as Natural Sequence Farming  and Keyline. Combining that with strategically placed Vetiver hedgerows and you begin to get a handle on the problem. 

Vetiver will prevent erosion and will slow run off if the Vetiver System is applied to slope management. If you do the sums, it is also the cheapest solution. 

Given our prices, that's $10 per metre and a hedge system can be planted as simply as popping a slip into a hole every 10 cm aklong a contour.

This topic has been researched in many contexts worldwide. In summing up the efficacy of Vetiver System for mitigating the impact of storm events, Richard Grimshaw writes:

i) stabilizing soil and slopes. vetiver’s root system is excellent for stabilizing soils. Because of its huge deeply penetrating root mass (particularly in the first meter) and high root tensile strength (1/6 the strength of mild steel – 75Mpa) greatly increases soil shear strength (by as much as 40%). Vetiver has the added advantage of light weight and low wind profile thus avoiding problems associated with greater stress loading on an unstable slope.

(ii) trapping sediments – evidence from many countries all concur on the effectiveness of vetiver hedges to trap sediments. Recent studies in Honduras showed that traditional slash-and-burn sites average 92 tons/ha/year of soil loss compared to 43 tons/ha/yr with crop residues and a “green mulch” cover crop compared to 0.9 tons/ha/year on sites with vetiver grass barriers and the crop residue/mulch. In Colombia soil loss was reduced from 143 tons per ha on bare land to 1.3 tons when protected by vetiver.

(iii) reducing runoff velocities – flume studies in the USA and Australia have shown vetiver hedges to be very effective at reducing total head (flow depth and velocity) of water flows. The hedge’s effectiveness at doing so increases with hedge thickness (maturity). It appears that mature hedges can be quite effective at reducing runoff velocities of flows less than 20cm in depth, moderately effective with flows up to 35 or 40 cm, and have some impact on flows up to possibly 60 to 80 cm; and

(iv) protecting hard structure/soil interface interfaces – experience has shown that vetiver hedges are excellent at protecting the often vulnerable interface between soil and hard structures. It is here that runoff is concentrated, causing soil to be scoured away. Oftentimes, this is how structures begin to be undermined, leading to the eventual failure of the structure (e.g., gabions along stream channels, bridge footings and ’wings’ of approaches, concrete drainage channels along roads, etc.).