Giinagay ngujawiny. Hello everyone i'd like to acknowledge that we're filming today on the traditional lands of the
Gumbaynggirr people and pay my respects to elders past present and emerging and
extend that respect to all aboriginal and Torres Strait Islander people watching this video today
g'day my name's Jane Eals i'm one of the team leaders for the local land services
riverbank rehabilitation project which
is jointly funded by the new south wales
and Australian governments under the
disaster recovery funding arrangements
the project is to build resilience
of flood-damaged riverbanks for future
flood events today we're in gamma
reserve on the mid-north coast of new
south wales with the beautiful world
creek which flows into the Nambucca
river behind us if you own riverfront
land no doubt some of you have seen
quite dramatic changes with this year's
floods
you might be concerned about the erosion
you've seen or the land you've lost
today we'd like to provide some insight
to key concepts of river behaviour and
the factors that influence how water
flows and how it moves riverbank
material along the way so there's some
key concepts to understand about river
behaviour
essentially they're all about stream
power and energy
the first one is pretty simple and that
is fast first slow the faster water
flows the more power and energy it has
to carry material with it as it goes
so the size of that material might vary
from small fine silt and sand through to
larger river gravel and rocks or even
tree trunks the faster the water flows
the more power and energy it has to
carry larger material this concept of
fast verse slow applies to the three
other concepts that we'll touch on and
it might help you if you picture water
flowing down a gutter when it rains
so the steeper the gutter the faster the
water will flow
and the the longer the steeper and
longer the gutter is the faster the
water flows down the gutter and the same
concept applies in a river
the next one is
straight versus curved so
water flows faster down a straight
gutter than it does if it's trying to
flow around a bend
if you've ever played slot cars as a kid
you'll know you can go faster down the
straights than you can around the bends
and again the same thing applies in a
river
and the last one is
a smooth versed rough
so water will flow faster down a smooth
gutter than when it hits some rough
spots that might have leaves piled up or
grass growing in it and you'll notice
that that's where the water slows down
and and dirt starts to accumulate there
so that roughness is a critical concept
for erosion management and in a river
setting that roughness is provided by
things like ground cover
a root matrix from trees and shrubs of
different heights
your course
of course bank and bed material
and angular rocks so don't worry about
the fine print this diagram is just to
show you the concept of a balancing
scale where the river is in a state of
equilibrium when the bank material on
the left is stable and strong enough to
stay in place with the power of the
river flow on the right if there was
good ground cover some angular rocks and
some woody debris on the bank to
increase the roughness the water will
slow down lose some of its power and
deposit some of the material it is
carrying deposition or aggregation will
occur which is the opposite to erosion
another key concept for river condition
and management is the importance of
riparian vegetation the riparian zone
includes the river banks and adjoining
land the image on the left shows a
riparian zone in good condition
you can see good vegetation cover with
deep roots for bank stability to prevent
erosion
the trees also provide shade
cool rivers are more resilient to
impacts after floods and provide a
better environment for aquatic life
cycles
you can see large woody debris from
fallen trees for habitat bank stability
and pool depth you can also see grasses
that filter sediment and other
pollutants as the runoff flows into the
river the image on the right shows that
without vegetation and large woody
debris the channel becomes wider and
shallower banks erode more easily and
there's less habitat
water temperature increases and fewer
aquatic species can breed and live in
the water if there's no grass or short
grass that allows pollutants to flow
directly into the river the next image
shows the wider the riparian zone is the
more benefits it has you can see that
stabilization is achieved at a width of
15 meters and flood control at 25 meters
although that's a generalization it is a
good guide to best practice the key
concepts we've just covered are a good
foundation for understanding river
behaviour and erosion management
in the next video we'll look at how
these apply to different types of
erosion and their causes in this photo
you can see that the trees at the bottom
of the bank have been pushed over with
the power of the flow and the entire
surface of the bank has been sheared off
or scraped away this type of erosion is
called scouring it can happen wherever
the power of the water exceeds the
stability of the bank but it's more
dominant on outside bins where the banks
are longer and wider and the water has
to travel further it builds up more
momentum and speed than on the inside
bend the water is also deeper on the
outside bend so all these factors
combine to create a corkscrew movement
called helical flow which increases the
power on the outside bend and
exacerbates the shearing or scraping
effect of water on the riverbank this
photo shows erosion at the toe or base
of the bank which is often caused by the
power of wind and wave wash lapping
against it it also shows a crack above
the toe of the bank caused by a change
in bank material for example
coarse material and clay in tow and a
finer more dispersive material above it
which is eroding away with the power of
higher faster flows i can't see any rock
at the top of the bank so it will
continue to erode further back and
eventually we'll undercut the material
above it depending on the conditions and
materials the top part of the bank will
then either slip down feet first
slumping
or topple overhead first called slothing
the other factor that can cause slumping
is when the bank materials are saturated
after being inundated with water and
there's a drawdown effect as the force
of gravity pulls or sucks the water down
and pulls the bank material with it this
can happen periodically with flooding or
daily with tides this last photo shows a
severe case of gullying which is often
caused by overland flow runoff the
waterfall feature in the middle finds a
weak spot in the area of impact where it
lands and continues to erode and move
upstream this is called a nick point or
head cut as it retreats the banks
collapse in on both sides stream bed
lowering is similar to gullying but it
happens in the bed of a waterway
it's something to be aware of and to
look out for
it's quite common in areas that have a
history of dredging because the level of
the riverbed has changed and is trying
to find a new equilibrium now you know
the common types of erosion and what
causes them the next video outlines a
range of best practice actions you can
do now if you notice vegetation debris
that's squashing native regrowth on the
riverbank and it's small enough to move
by hand you can lift it up and move it
aside otherwise it's best practice to
let sleeping logs lie
large woody debris provides fish habitat
and can increase bank stability and
reduce erosion to address gullying you
can divert or slow overland runoff with
hay bales and star pickets coil logs or
sandbags these simple techniques are
effective for small scale gullying but
for larger scale gullying or bed erosion
hybrid solutions of hard engineering and
re-vegetation will be required so you
should seek guidance from a river
practitioner keep stock away from the
riverbank and out of the adjoining
riparian zone they may eat weeds but
they also spread them manage stock
access by installing fencing and
offstream watering or at least limit
access to one point and stabilize it
after a flood it's easy to be fooled
that the slate's been washed clean and
all the weeds have gone away beware of
the seeds lurking below the flood mud
it's important to keep an eye out for
emerging weeds and manage them as soon
as you see them poking through target
priority weeds such as vines camphor
laurel seedlings tropical soda apple and
thorny weeds install monitoring pegs one
to five meters away from the erosion
edge and take photos to track erosion
over time it's important to remember
that aggregation and erosion are natural
river processes when the riparian zone
is wide enough and provides a buffer on
each side it's natural for the banks of
most rivers to move laterally from side
to side constantly adjusting over time
as long as there's no bed erosion
occurring some erosion sites will find a
state of equally brim in a river setting
that state of equilibrium is not static
rivers are dynamic natural systems that
change over time the first question
river practitioners always ask
themselves is what would happen if we
did nothing in practice that's not very
accurate so i've changed it to bolster
it and let it be
by bolster i mean the things we've just
covered manage stock access and do weed
control
in the right conditions sometimes these
actions are all it takes for the
riverbank vegetation to regenerate and
for the erosion to stabilize
you'll probably need a river
practitioner to assess your site and
help you make this decision if there's
no mature native trees nearby to provide
seeds and you can't see many native
seedlings starting to grow it's good to
re-vegetate
scatter natives grass seed over bare
ground and plan for planting a mix of
local native shrubs and trees of
different heights in spring this graph
shows rehabilitation effort on the left
and resilience on the right
it shows that as the rehabilitation
matures
effort reduces and resilience increases
for more details about all these actions
there's lots of guidance resources
designed to help you to help yourself
here's a good one for the richmond
catchment and if you search our local
land services website ask your local
council or landcare group you should be
able to find some for your local area
now you understand the key concepts of
river behaviour common types of erosion
and actions you can do yourself
we'd like to give you some insight into
best practice erosion techniques that
require hybrid solutions a combination
of hard engineering and regeneration
approvals are required for this type of
works and you should be guided with
designs by a river practitioner there
are over 50 river styles in new south
wales based on geomorphology traits such
as slope bed and bank geology location
in the landscape and shape
typically they start with a gorge
upstream and transition to a valley in
the middle and flood plains downstream
it's best to work with natural river
processes and changes over time by using
river styles as a template for choosing
and designing river management options
what works in one style won't work in
another for example wood may be
inappropriate in steep upper sections
due to high flow power and rock may be
unnecessary expense in lower sections
some river styles are more likely to fix
themselves and find an equilibrium over
time so some management options could be
a waste of effort and resources in those
river styles historically we've utilised
a lot of rock to provide erosion control
however
rock doesn't necessarily provide all the
all the habitat requirements that
aquatic life actually requires in that
we've moved towards more of a composite
design where rock is complemented by
logs snags different types of woody
structures to actually provide those
interstitial gaps and complexity in the
environment that aquatic life require
both to escape heavy flows but also for
breeding and spawning requirements large
woody debris from fallen trees provides
fish habitat and can increase bank
stability and reduce erosion
in the previous video we explained its
best practice to let sleeping logs lie
sometimes fallen logs can cause erosion
through associated bank collapse and
directing flow into the bank
it's usually only a problem with one
large individual tree that's not
supported by surrounding vegetation
rather than removing the wood it's
preferable to modify it by lopping
realigning or moving it fallen logs can
be useful in protecting the toe of the
bank if they're repositioned at an angle
of 40 degrees or less to the eroding
bank with the root ball upstream
this provides armouring against the flow
and encourages water to move to the
centre of the channel
approval will be required we'll touch up
more on that soon re-vegetation around
the fallen tree and surrounding bank is
crucial to ensure long-term resilience
in the previous video we touched on some
small scale options to divert or slow
runoff for managing gullying
the larger scale gullying and stream bet
erosion you'll need more sophisticated
techniques
a common solution for bed erosion is to
install a rock chute at the waterfall
point or head cut the key design
features are rock spans the width of the
channel and has a gradual change in
slope and ties into the existing bed
level without a sudden jump in height
this structure helps to dissipate energy
and allow energy to be consumed as it
passes over the rock chute and then
passed gently into the new bed level
below this photo shows the site
conditions before we started the project
it shows fine silt material on the
vertical face of the bank which is a
recipe for erosion salt requires a very
gentle bank angle to rest upon
another erosive force is the suction
characteristic of water passing over the
face of the bank the only way to prevent
this from mobilizing the bank material
is by increasing the roughness of that
bank face with logs and rocks the next
photo shows the two techniques we used
one we placed rock to provide new bank
material on the on the bank face
we used a mix of sizes to minimise
opportunities for material to be sucked
out as water passes over but also
provide a mixture of interstitial gap
environments we also added large snags
for habitat and complexity for
commercial and recreational fish species
logs also deflect the flow and help
prevent erosion by preventing energy
from hitting the bank face during flow
or flood events
logistically can be difficult and costly
to source large woody debris or timber
for use in river ester iron works if you
have to use rock revetment a few design
features can be used to increase their
effectiveness and river habitat value
the usual flat shape of a rock revetment
wall doesn't provide shelter or habitat
for the array of fish species in the
estuary this corrugated design allows
velocity shadows to protect fish from
flows
allowing them to hide from extreme
events
in the next photo we also add complexity
at the toe of the rock revetment with
reef balls and large woody debris this
breaks up the velocity of flow at the
toe and that allows smaller fish species
to occupy the environment and creates a
greater range of habitat features within
the bank profile corrugation along the
toe and horizontally along the crest
allows tidal inundation to occur within
the riparian zone
and promotes mangrove growth and more
salt tolerant species such as salt marsh
this approach is generally not more
expensive than rock but the outcome is
far more beneficial
the site is actually now recognised as a
valuable recreational fishing point
because of the structure and variety of
habitat that we've provided within the
back face
this corrugated log fillet design that
we utilized in immigrant creek
essentially deflects wave and boat wash
but creates a velocity shadow behind the
structure that allows mangroves to
recruit and colonize
one of the additional benefits of this
site is that we're also increasing the
deposition of material along what was
formerly an eroding bank face
this photo show over time
from july 2020 to now
how mangroves have increased in size and
abundance and essentially created a new
riparian forest depending on what you
want to do and where and who owns the
land approvals and permits may be
required from either council new south
wales water dpi fisheries or crown lands
the bed and part of the banks are often
owned by crown land
as a general rule anything that involves
machinery
in-stream works disturbing soil on banks
changing native vegetation or placing
fill will require approval local land
services can help to guide you through
the process for advice and support to
manage riverbank erosion on your
property contact your local land
services office by phoning 1300 795 299
or send an email to riverrehabproject@lls.nsw.gov.au
send your contact details property
address and photos of the erosion site
and we'll use that information to start
doing an impact assessment
we'll call you to do an initial desktop
assessment and if necessary we may do a
site visit before providing management
recommendations
if you're able to fund works yourself we
can provide guidance and referrals to
contractors
otherwise the best thing to do is get in
touch with us so we can start doing
impact assessments provide advice on
best practice and keep you updated if
funding opportunities become available
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