Managing outback floodplains

About outback floodplains

Outback creek and river floodplains are vital for both agricultural production and biodiversity. These areas often stay green longer after rain and support rich plant and animal life. They should be managed as part of the whole catchment, because extra water flow and heavy grazing can lead to faster erosion and land damage.

How healthy floodplains work

A healthy floodplain is regularly flooded from flows exiting the main creek/river channel as a result of moderate to large rain events. This happens because the channel has limited capacity to hold all the flow. This causes water to exit at natural points resulting in calm flows which spread away from the main channel. 

This water is held on the floodplain, filling all the billabongs, pans and swamps. The whole floodplain remains waterlogged long after the flow event has ended. 

Natural sandy levee banks act like small dams. They hold water on the floodplain and stop it from draining too quickly back into the channel. These levee banks are base levels that hold up the ponding of the floodplain and are often subtle sediment sills.

The primary base level along a stretch of floodplain is the main channel base. Rock bars in the channel base can also help by keeping the channel base high enough for floodwaters to spread out regularly. This ensures that even moderate rain events result in flows out onto the floodplain.

Unhealthy floodplains and their causes 

Channel expansion 

Floodplains can become unhealthy when creek or river channels deepen or widen.  This can happen when poor groundcover in the catchment allows more runoff to flow into the channel. The increased water flow erodes the channel, causing less water to reach the floodplain.

These unhealthy floodplains usually get short, strong bursts of water from the channel, and only when there’s heavy rain

When moderate rain no longer floods the floodplain, the area can dry out, reducing plant cover and soil health.

Channels can also expand following the erosion of a levee bank or breaching of a natural rock bar.

Erosion of natural levee banks drains floodplains

Loss of natural level banks and rock bars can cause the floodplain to drain.

If a natural rock bar is breached, the channel’s base level is lowered, which starts the erosion process.

Livestock pads and vehicle tracks can break down natural sandy levee banks, which act like small dam walls.

Once these levee banks wear away, water from the floodplain can now escape through the breach. This “unplugs” the floodplain and triggers the formation of gully heads and erosion gullies.

Diverted flows have a fast exit route from the floodplain. This results in water rushing into the main channel and out of the system.

The area of healthy floodplain shrinks in size as erosion expands and takes away flows. The smaller area becomes more vulnerable to overgrazing and further degradation.

Excessive deposition from upslope catchments 

Heavy erosion in the upper catchment can wash large amounts of sediment onto the floodplain. This build-up of sediment changes how the floodplain works. When too much sediment settles, the original channel can rise higher than the surrounding floodplain.

The built-up sediment acts as a bank, which diverts flows from their original course. These diverted flows can dry out some parts of the floodplain and send water to areas that rarely flooded before. This can sometimes create new erosion gullies.

You can often see lines of dead river gums on floodplains where excessive sediment has diverted the natural flow of water. These trees have died because they no longer receive enough water to survive.

Floodplain repair in a catchment context 

Floodplain repair takes a systems approach. Landholders should plan, restore and manage floodplains together within the wider catchment. Some key principles include:

1. Maintain groundcover within the catchment.
2. Ensure flows are calm coming onto floodplains from the surrounding catchment.
3. Reinstate natural levee banks.
4. Plug unnatural and eroded re-entry points into the main channel. This will help to stop the draining of the floodplain into the main channel.
5. Encourage flows within the channel to exit and spread over the floodplain.
6. Stabilise and regenerate active gully heads.
7. Restore flows to dehydrated floodplain areas that have been cut off by a build-up of sediment.

Ongoing floodplain management

Long-term success depends on addressing the root causes of floodplain degradation. Before starting any repair work on floodplains focus on addressing the root causes.

The primary causes are: 

• Poor management of groundcover in the surrounding catchment.
• Poor management of groundcover on the floodplain, reducing the health of perennial species. Resting floodplain areas during floods or waterlogging helps these plants recover. This helps to maintain the health of perennial species.
• Poorly located infrastructure, such as water points, that focus livestock impacts in sensitive areas.
• Access tracks and fences that change surface flows and take water away from the floodplain. 

Healthy floodplains support sustainable agricultural production and ecological biodiversity. Ongoing management should encourage the key ecological processes that underpin healthy floodplains.

Managing farm track erosion

This video shows how you can use a diversion bank to minimise erosion along farm tracks on floodplains. 

videocam 5:42

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Video transcript

Addressing farm track erosion in Western NSW

My name is Paul Theakston

I’m the rangeland rehabilitation officer, based out of Cobar

Rangeland rehabilitation is all about reestablishing

natural patterns and processes in the landscape

and fixing soil erosion

So in this sort of landscape

what you generally have is, when it rains water flows off

in a sheet,

in a broad sheet, shallow sheet, very low energy

What we're looking at here is track erosion

so any sort of disturbance to that sheet flow,

concentrates that water in a narrow spot

t energizes it, and it causes soil erosion

If the road's eroded,

we sometimes need to put in ‘whoa boys’

that is an actual diversion bank

that would take the water off the road

and then spread it out onto the natural contour

My name is Jason Jackson

I run JAJ logistics, that's my off-farm business

so with our earthmoving equipment, we've been doing a lot of work

with LLS to do with erosion rehabilitation.

Best first step is to hop off the machine

just get on the ground, have a quick

look at what's been marked out

so you know exactly what's there

get a plan in your mind

how you’re going to start.

In order to establish

what the natural flow path

is, you need to know what the contour line is

so if you can imagine two points that are the same level

water will be flowing

perpendicular or 90 degrees to those two points

Once I know where the water should be flowing

and we look at the track

and we can see the track’s

diverted the water in a completely different direction

what we do, we

go across the track to the other side of the track

and go up anywhere from 5 to 10cm

in vertical height and put another point there

So we've got three points

now, we've got

the two points of the contour

and across the track

we've got another point anywhere from 5 to 10cm higher

than those two contour points

If there's vegetation on the site

you might need to push the vegetation

to the far end of the borrow pit

If the ground is hard, and you've got rippers

you may need to use rippers to loosen the soil up

If not, you can use the corner of the blade

So the first pass with the grader

will be to back down the borrow pit

so you're facing the roadway

Get your blade on a sharp angle

away from the edge of the level sill,

and take a series of passes that sort of take out,

like fillets of the ground,

and move that material up to the edge of the roadway

Don't go on the roadway with sort ‘v’ bits

you want to stockpile the dirt on the edge of the roadway

so that might take a series of passes

always pulling the dirt away from the level sill

then you'll back back down, spin

your blade back around the other way,

and then move

the stockpiled material

up and over the roadway to form the

whoa boy

starting from the close end

and pushing the dirt further out to the end of the bank

The temptation is to just push the dirt

like with a straight blade like a bulldozer

would, up and onto the roadway

Graders don't work that way very well because the blade’s

not big enough

so you have to work the dirt left and right,

with the blade from side to side as you push it up

Once you start spreading the stockpiled material,

you're not cutting into the roadway

in any way, shape or form

you're actually just spreading

the material that you've already removed

from the borrow pit and stockpile

so the roadway never gets cut at all

so it's only just spreading the stockpiled material

And that's how we're forming the bank or the whoa boy

and the water will come down the track

hit that whoa boy or that diversion bank

turn and go into the excavated pit,

and then it'll fill up and then flow out

on that contour line that we've talked about

Energy has been dissipated

and it's going in its natural flow direction

The advantage of this system is

that you're not carting dirt or gravel from a long way

you're actually just excavating it right next to the track,

and you're just pushing it onto the track

you can do that with a road grader

you can do it with a front end loader

Any machine that can move dirt, you can use that

to do this system

If you're making a whoa boy

with a front end loader,

what we've found out works

best is if you drive into the borrow

pit from the close

end to the borrow pit, and take dirt out

the way you normally do it with a loader

by scooping and carrying the dirt back to the road

Remove any vegetation if there's vegetation in the road

and then start extracting the dirt

Always stay away from the level edge for a start

like, from your level sill

and always work

back to the level sill as you get closer to completion

Start at the high side of the bank and work back to the roadway

the last couple of buckets for the bank

will actually close off your access to the borrow pit

so that's why you always start at the high end

and work back

Yeah, and then make sure you've got enough dirt

also shovelled up on the actual roadway section

so that you've got enough to back blade back and form

your ramps to make it nice and trafficable and

make everyone happy.

Once you've constructed these whoa boys across your tracks

as you're driving along them,

yeah, you will be slowing down, definitely, but

that's the price

you've got to pay to just slow down the water,

de-energise it

and spread it out across the landscape to grow more grass

If any landholder wants to learn how to survey

these whoa boys in

and construct them, yeah, they can contact

the Local Land Services