Managing erosion after bushfire

Soil erosion after a bushfire can impact your farm. Small-scale erosion control can help to limit further soil loss and aid recovery.

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Video transcript

Farm-scale erosion control

the land holders will expect to see
more erosion after bushfire events
obviously the ground cover's gone
when rain falls there's nothing to
capture the rain
and the bushfire actually makes the
ground more hydrophobic so it repels
water
and that causes increases in real
erosion into gullion erosion
and also major impacts of sediment into
streams
the main drivers of erosion in the
landscape are obviously
slope intensity of rainfall events
following so
very intensive rainfall events will
cause more erosion than
less intense and ground cover obviously
so when we've got
beared out soils after bushfires the
intensity of erosion and the damage
caused can be much higher there are some
things that landholders can do to
address problems themselves with wood
and timber on properties and realignment
of
timber and small-scale erosion works
themselves there are some things that
are too big
at too large scale for really than to
effectively
change and they need to look at change
land management around those sites
knowing when to get involved and when to
intervene and when not really important
so really
if it's large catchments and steep
catchments where you can't physically
get in
to work on things like sediment traps
and
and construct control structures it's
very hard to
really become effective however if
you've got smaller catchments that
you know in measuring maybe a hectare or
two and where you can get access
uh they're the type of circumstances
that you could actually
try to make a change on ground and do
some
do some work after a fire when
particularly when properties have lost
fences
and had dramatic impacts on farm dams
it's really important to look at the
planning for the
the re fencing and protecting catchment
assets
protecting your gullies and your dams
with where your fencing goes
and also some of the interventions that
can be done because of the erosion
impacts happening
and look in some situations too you may
be better off
waiting until things settle down you
could get
repeated sort of sediment slugs coming
down your gullies and valleys which can
fill up dams repeatedly
understanding when to actually hold off
a bit and
and not waste your time in intervening
is also important yeah it's really
important to
prioritize your efforts in some cases it
might be important just to let things go
but in other cases where you've got high
priority
water sources like dams or
also assets like crossings or culverts
or tracks might be better to actually
think about
interventions for those type of cases
it all depends on the pasture before the
fire but if you've
had annual pastures or annual weeds it's
likely to be like that
after a fire whereas if you've got
perennial pastures
you're likely to see some good recovery
afterwards
existing areas sensitive
to erosion are likely to get worse under
post-fire conditions with increased
runoff so
anything you can do to address them is
always a good idea before the fire
there is a lot of information out there
through the different departments and
agencies working with land holders to
support them into the future so
and we learn after every fire so we're
increasing our knowledge about
what's the best intervention to take so
in summary really the important things
are about prioritizing where you put
your work
and when to intervene and also consider
the farm planning aspects of recovery
and the importance really of ground
cover
you

Common types of erosion after bushfire

These types of water erosion may increase in risk after bushfire: 

• gully erosion
• sheet erosion
• streambed and streambank erosion
• existing erosion.

Burnt areas are more erosion prone due to lost tree canopy and groundcover (living vegetation and dry matter). The chemistry and structure of the soil may also have changed in these areas and could be hydrophobic (unable to absorb water).

Farm-scale erosion control after bushfire

The key to minimising erosion after a bushfire is to retain or re-establish groundcover.

Ways to do this include:

• destocking burnt areas
• installing sediment traps in water flowlines or across steep slopes
• hand broadcasting seed
• avoiding further disturbance
• monitoring groundcover and plant regeneration
• seeking expert advice if gully erosion is activated/re-activated. 

Impacts to infrastructure

Dams, roads and fences can be impacted by organic matter, ash and debris washed from the fireground.

High levels of run-off can overwhelm pipes under roads, causing overtopping which damages surfaces and erodes road material.

Organic matter, ash and debris can wash over unburnt paddocks into farm dams at some distance away from the fireground and higher rates of runoff can damage fencing by trapping debris.

Broader landscape impacts

Organic matter, ash and debris flowing off properties can impact:

• the environmental values of waterways
• animal and plant habitat
• pollute drinking water catchments.

The water quality of major waterways can be severely impacted a long way from firegrounds.

Increased run-off and large debris can also cause further erosion downstream.

Erosion control techniques

These are some of the low cost and relatively low labour options for small-scale control structures:

• coir logs
• silt fence
• jute mesh
• seedlings
• straw bales
• revegetation
• cover crops
• rock
• large woody debris. 

Tubestock or seedlings can also be planted into coir logs. 

Place burnt tree trunks or large branches across slopes to trap sediment and debris.

Use hardwood stakes on steep slopes to secure them in place. 

Rushes and sedges may be useful in moist areas including species like:

• Carex
• Juncus
• Lomandra.

How to prioritise erosion control works 

The key factors for determining where to focus efforts and what structures to design include:

• soil type and steepness of slope
• fire intensity and groundcover condition
• catchment size above structure location
• size of flowline or order of creek
• proximity to dams, wetlands and waterways.

Soil type, slope and fire intensity can change the level of erosion risk. You can use this information to help prioritise erosion control works on your property. 

Fire intensity depends on many factors, including:

• fuel loads
• temperature
• wind speed and direction.

This table shows how the steepness of a slope and the intensity of a fire, can change the level of erosion risk.

This table shows how the steepness of a slope and the soil type can change a farm's level of erosion risk.

• All numbers above are calculated as tonnes/hectare (t/ha).
• 1200t/ha is equivalent to a loss of 12mm of soil.
• Dispersive (sodic - or that which contains sodium) soils and other inherent limitations may increase the risk of erosion.
• Where the risk of erosion is high-extreme, avoid undertaking risky practices such as ripping, clearing or even just tracking machinery.

Where to place structures in the landscape

Important things to consider when you are placing structures include:

• rock and large woody debris from on-site can also be used in head cuts or holes
• slowing water will help to retain sediment
• seeding or planting can improve stability but take care with selection of the species to avoid introducing exotic species to fire affected areas unless already present
• an option for seeding is to broadcast sterile cover crops such as rye corn to re-establish groundcover quickly on small erosion-prone areas
• the catchment size and likely flow volumes should come into play to design suitable road drainage when constructing new roads
• roll over banks constructed across slopes can carry run-off but must spill into thick groundcover to avoid erosion occurring off the road
• there are modified fencing designs available to use in flood-prone areas, for example drop-down fencing and using plain wire rather than hinge joint.

Groundcover management

The speed of pasture recovery will depend on the rainfall patterns in the months following fire. Exposed soil may make the land more vulnerable to wind erosion and invasion of weeds.

To help predict pasture recovery after fire, consider watering small test areas several times to see what regrows. A poor response may indicate that new pasture may need to be sown, but it is best to wait for adequate rainfall before making any decisions.

If the pasture was highly weedy and/or degraded prior to the fire, consider sowing annual forages to help provide groundcover, such as: 

• oats
• wheat
• grazing brassicas. 

This will also assist with weed control prior to sowing a new pasture in future.

The heat of the fire will impact the ability of the soil to recover. Different levels of burns include:

• a cool-moderate burn where most dead plant material is burnt, some seed, perennial grasses and clovers survive. Usually, a small residue of unburnt pasture remains.
• a hot burn where all dead plant material, many seeds, young and weaker perennial grasses destroyed. Topsoil usually appears charred and bare.
• a very hot burn where soil is virtually sterilised. All plant material and seed are destroyed as the fire burnt into the top organic matter layer of the soil. Usually under tree cover.

Grazing practices after bushfire

It can take some time for pastures to return to pre-fire (and pre-drought) condition.

Reducing stocking rates or using stock containment areas can enable pastures to rest and start to recover through a growing and seed-setting season.

This will allow the seed to set to replenish the seed bank and allow the plant to re-establish its root reserves, both of which will result in faster pasture recovery.

Pasture recovery after bushfire

The post fire period may be a good opportunity to control persistent weeds and rejuvenate pastures through a combination of: 

• strategic spraying
• application of fertiliser or ameliorants
• re-sowing in perennial improved pastures. 

Agronomic advice should be sought to help make the most appropriate decisions for your landscape context and type of farming enterprise. 

If your pastures were largely native grasses prior to burning, they will recover given time- often much more efficiently then improved exotic pastures. 

If you want to change your native pasture system to an exotic, improved pasture, it may involve getting permission for native vegetation clearing under the Native Vegetation Code. 

Local Land Services can provide advice on your specific circumstances.

Monitoring

Monitoring your property for weed invasion, pest animal activity and worsening erosion is especially important after bushfires.

Monitoring for weeds

Early identification and control before seed-set are the most important steps in weed control and eradication.

Perennial weeds with deep root systems are likely to survive bushfire and regrow, however their seed bank may be depleted. Weeds are some of the first plants to recover and are often prominent after bushfires, including: 

• flatweed
• docks
• sorrel
• onion grass. 

These weeds provide a short-term groundcover but may need to be controlled in time.

In bushland contexts, weeds suited to disturbance may regrow quickly in the following areas:

• on forest margins
• in waterways
• along tracks
• on containment lines. 

The post-fire period may provide an opportunity to reduce weed infestations in these areas. If identified at the seedling stage, many weed species may be hand pulled. Other bush regeneration techniques may also be used.

The post-fire period may also be a good time to establish competitive pasture, like phalaris, cocksfoot and sub clovers in pastures with high infestations of difficult perennial weed grasses like Chilean needlegrass, serrated tussock and African lovegrass.

The type of land will determine if cultivation or no till methods of seed broadcasting are appropriate, and which species may establish effectively.

Be vigilant about monitoring areas where stock feed has been introduced, and earthmoving machinery has been operating. 

Monitoring for pest animals

Controlling pest animal species (both herbivores and carnivores) is a crucial post-fire management activity. 

From an erosion control perspective, grazing species including rabbits, pigs and deer may destroy regenerating groundcovers, compact or disturb the soil further and may place additional pressure on erosion control structures.

Local Land Services provides advice and assistance in controlling pest species, developing vertebrate pest management plans and cooperative management programs. Local Land Services can:

• coordinate group pest animal control programs
• provide vertebrate pest management training
• provide baits to control pest animals.

Monitoring erosion control structures

It is important to see if erosion control structures are trapping sediment and groundcover is re-establishing. 

Keeping a photo record of these sites is helpful. 

If safe to visit, look at how the structures are performing during periods of heavy rainfall. Ask yourself the following questions:

• are the structures working in slowing the speed of the flow?
• are they secure or being dislodged or undercut by heavy flow?
• are structures needed in other areas?