No 6
WAT E R C O U R S E S A N D E A R T H W O R K S
HOW IT WAS
CAUSE AND EFFECT
One hundred and fifty years ago the watercourses of
the Mount Lofty Ranges would have looked very
different from their appearance today. Typically, the
watercourses were well vegetated with trees and
shrubs plus reeds, rushes, sedges and grasses.
Erosion associated with watercourses is most often a
response to human interference. Often, the linkage
between cause and effect is not readily apparent or
the two may be hundreds of metres apart. A sound
understanding of the causes and processes of erosion
in watercourses is essential. If in doubt, always seek
professional advice!
The onset of European settlement has initiated most
of the erosion along watercourses in the Mount Lofty
Ranges. Watercourses are now deepening and
widening in response to the altered conditions.
A common sight now is that of sparsely vegetated,
deeply incised watercourses with unstable beds and
eroding banks.
These changes have resulted in reduced water quality,
degraded wildlife habitat, loss of productive
agricultural land, reduced recreation values plus
threats to roads, bridges and other capital assets.
The following information will give you a guide to
issues involved in undertaking erosion control works
on your watercourse. Whilst the main focus of this
brochure is ‘engineering’ solutions to erosion
problems, these should in no way be taken as a
substitute for good watercourse management
practices such as stock exclusion and revegetation
with native plants. Revegetation is an integral
component of any stream rehabilitation project.
The design limits of erosion control techniques should
never be exceeded by applying them to conditions for
which they were not designed.
BEFORE YOU START
Before embarking on some in-stream works, you
should ensure that:
• professional advice has been sought
• careful consideration has been given to the benefits
and costs of the project
• council approval has been granted
• experienced contractors are employed to do the
work
• the works to be undertaken are not contrary to
your Catchment Water Management Board’s
Comprehensive Catchment Water Management
Plan (where applicable).
COMMON REASONS FOR FAILURE
Over the years, many erosion control works have
been undertaken by landholders, only to fail for one of
the following reasons:
• treatment of symptoms rather than causes
• underestimation of power of flood waters
• inadequate design and the use of inferior materials
in the construction of erosion control works.
BENEFIT - COST ANALYSIS AND RISK
MANAGEMENT
All erosion problems can be ‘fixed’ but for a price.
Consideration should be given to the cost or
implication of ‘doing nothing’ as compared to the cost
of intervention and the benefits that might accrue.
Such an assessment will invariably involve subjective
value judgements.
The cost of stream works will vary depending on the
nature of the problem and the solution used to fix it.
As a general rule, the higher the flood capacity rating,
the greater the cost. For example, a structure
designed to survive a 1-in-100 year flood event will
be larger and more robust, and thus cost substantially
more, than one designed for a 1-in-5 year flood event.
The vast majority of erosion problems can be
redressed through fencing the site to restrict stock
access and replanting local indigenous plants (ie
reeds, rushes, sedges, grasses, shrubs and trees). Well
vegetated watercourses are much more resistant to
erosion. The vegetation reduces the velocity of flow
and physically protects the banks from erosion.
LEGAL CONSTRAINTS
Under the Water Resources Act 1997, there are legal
constraints upon you as either the owner or occupier
of land, namely:
• an obligation to maintain in good condition any
watercourse situated on or adjoining your land (S. 14)
Environment Protection Agency
Government of South Australia
W A T E R C O U R S E S
• a duty to take reasonable steps to prevent damage
to the bed and banks of a watercourse and the
ecosystems that depend upon it (S. 17)
You should not undertake any activity that is contrary
to a Catchment Water Management Plan or water
Allocation Plan. You should not not:
- construct, erect or enlarge a dam, wall or other
structure that will collect or divert water flowing in
a watercourse
- construct, erect or place any building or structure
in a watercourse or its floodplain
- drain or discharge water into a watercourse
- deposit or place an object or solid material in a
watercourse or obstruct a watercourse in any other
manner
- deposit or place an object or solid material on the
floodplain of a watercourse to control flooding from
the watercourse
- destroy vegetation growing in a water course or its
floodplain
- excavate or remove rock, sand or soil of a
watercourse or its floodplain.
Refer to back page for your Board’s contact details.
A N D
E A R T H W O R K S
However, floodplains are an integral component of a
river system. They act as a safety valve, storing and
releasing flood waters.
The effect of channelisation is to shorten the length of
the channel and increase the gradient and flow
velocity. (Figure 1)
Consequently, a new episode of erosion is initiated.
Channelisation also interferes with the meander
pattern of a watercourse. This generally results in
increased bank erosion. Channelisation will result in a
complex set of responses (ie bed and bank erosion)
that will affect upstream and downstream landholders.
Ford Construction
Poor culvert construction is one of the most common
causes of erosion in watercourses in the Mount Lofty
Ranges. Problems typically encountered include:
undermining and outflanking of the structure, the
development of a large scour hole downstream of
culvert plus severe damage or loss of structure during
flood. (Plate 1 and Figure 2)
Under section 25 of the Environment Protection Act
1993 you should not undertake any activity which
pollutes, or might pollute, the environment unless you
take all reasonable and practicable measures to
prevent or minimise any resulting environmental harm.
TRAPS FOR NEW PLAYERS
Channelisation
Watercourses are
often channelised in
order to ‘get the
water away faster’,
so as to avoid
flooding or increase
the area of land for
agricultural, urban or
industrial purposes.
Figure 1. Stream erosion
caused by channel
straightening
Plate 1.Erosion at a ford
A
Erosion
Cu
Bank erosion
Structure
tof f
B
Erosion
Flow
Natura
A
l slope
New
Drop
slop
e
B
Distance of channels between A and B
Upstream erosion caused
by steeper slope
Downstream deposition
of eroded sediment
Figure 2.
Erosion at a
ford (adapted
from DLWC
Riverwise
Advisory
Notes)
A well-designed culvert should have the following
features:
• located on a straight and stable part of the river
meander
• secured to the underlying bedrock or footings sunk
>2.0 m into the stream bed
• a shallow profile (ie <50 cm) with small pipe(s) to
take the low flows
• both ends ‘keyed-in’ approximately 3–5 m into both
banks. (Figure 2)
W A T E R C O U R S E S
A well-designed low profile design should only be
impassible for short periods of time (2–3 hours)
during the peak of the flood flow.
Levee Banks
Levee banks are often constructed to confine flood
flows within the stream channel, so as to minimise
flooding. Unfortunately, the confining of flood flows
can lead to an increase in bed and bank erosion.
Furthermore, if the levee is breached, it can result in a
life threatening inundation of the area and the
occurrence of severe erosion at the site of the breach.
Desnagging/Removal of Trees
Large woody debris (eg branches, limbs, whole trees)
is an essential component of a healthy watercourse.
Some of the debris found in the River Torrens is more
than 2500 years old, while debris up to 8000 years
old has been found in the Inman River.
In the past it has been commonly assumed that any
large woody debris within a watercourse would direct
stream flow onto the banks, thus causing increased
erosion, or exacerbating flooding problems. (Figure 3)
Bank erosion
Obstruction
A N D
E A R T H W O R K S
Large logs in the channel will only have a measurable
influence on flooding if they occupy >10% of the
cross- sectional area of the channel. Large woody
debris is essential habitat for native fish and other
animals that live in the watercourse.
Bank Battering
The technique of bank battering is often employed to
alleviate bank erosion, increase the flow capacity, or
to make the banks of a watercourse more
aesthetically appealing.
On its own, it is rarely a sound technique for erosion
control. Where banks are eroding, it is essential to
determine if the erosion is a response to any recent or
active bed deepening.
For many sites, arguably the most cost effective
solution to bank erosion is to revegetate the site with
local native plants (refer to Water Wise Fact Sheet
No 2 Revegetating Watercourses). In doing so, it is
essential to stabilise the toe of the bank.
Battered banks (Plate 2) are extremely vulnerable to
stream erosion while vegetation is being established.
The bank should be ‘laid back’ to an angle of 1–on–3
or less (the less the better).
Eddy
Flow
Flow directed
at bank
Flow accelerates
through constriction
Bank erosion
Figure 3. Erosion caused by fallen trees (adapted from DLWC
Riverwise Advisory Notes)
Do not remove the snag. Just drag it back against the
bank. As a general rule of thumb, snags dragged back
to an angle of <40o have little effect in diverting water
flow onto the banks. Refer to Figure 4.
Plate 2.Bank battering
Rock Rip-rap
This technique is used to protect the bank on the
outside of the meander bend from erosion. The rock,
which needs to be hard and angular in shape, is
placed at the base of the bank. The size of the rock
needs to be determined in relation to the velocity of
stream flow.
Always seek professional advice on rock size and
placement. If the rock size is too small it will be
washed away in a flood.
Conversely, if the rock size is too large, it will not
provide adequate protection and may exacerbate
bank erosion. The technique is suitable for most
situations, except where the rock would block a large
part of the channel.
Water Wise No 6
W A T E R C O U R S E S
The rock is generally dumped from a truck directly
over the bank or on to the top of the bank and then
pushed over. It is essential that the rock is then placed
into position by an excavator.
The rock is generally placed to a height of 1 m above
the low flow water level, with a grade of approximately
1:2.5 (Figure 5). Once the rock has been placed, the
site should be immediately revegetated with native
plants.
A N D
E A R T H W O R K S
FURTHER INFORMATION AND HELP
• Mount Lofty Ranges Catchment Program Upper Level, cnr Mann & Walker Sts
Mt Barker SA 5251
ph: 8391 7500, fax: (08) 8391 7524
• Torrens Catchment Water Management Board
4 Greenhill Road
Wayville SA 5034
ph: (08) 8271 9190, fax: (08) 8271 9585
CROSS SECTION VIEW
Plant trees
and shrubs
B
Eroded bank
A
Ro
1 metre above
low flow water level
ck
Lo
low
wf
cha
nne
• River Murray Catchment Water Management Board
PO Box 1374
Berri SA 5343
ph: (08) 8582 4477, fax: (08) 8582 4488
l
Bar
• Northern Adelaide and Barossa Catchment Water Management Board
1st Floor, 59 Commercial Road
Salisbury SA 5108
ph: (08) 8285 2033, fax: (08) 8285 2133
PLAN VIEW
Ba
nk
Flo
See cross section
w
• Onkaparinga Catchment Water Management Board
The Hub Civic Centre
Aberfoyle Park SA 5159
ph: (08) 8374 6012, fax: (08) 8271 6274
A
• Environment Protection Agency
Mount Lofty Ranges Watershed
Protection Office
85 Mount Barker Road
Stirling SA 5152
Telephone (08) 8139 9900
Facsimile (08) 8139 9901
Bar
Bank
Eroded bank
Rock
Information in this brochure has been adapted from:
B
Figure 5. Rock rip-rap (adapted from DLWC Riverwise Advisory
Notes)
Farm Dams
In the Mount Lofty Ranges watershed, any landholder
who is intending to erect, construct or enlarge a dam,
wall or other work that collects, impedes or directs the
flow of water in a watercourse is required to obtain a
permit.
The construction of farm dams should be in a manner
which protects the rights of the downstream user, the
need to provide part of Adelaide’s water supply and
the ecology of the watercourse. For further
information, refer to Water Wise Fact Sheet No. 5
Farm Dams, and your local council. • A Guide to Erosion Control Structures for Small
Watercourses in the Mount Lofty Ranges,
Jason Carter and Ed Collingham
• Department of Land and Water Conservation,
Riverwise Advisory Notes
• Land and Water Resources Research and
Development Corporation (LWRRDC), Riparian
Management Fact Sheet No. 2: Streambank
Stability.