Creek Stabilisation - Thirroul
The stabilisation of an eroding creek at
4 Cornock Avenue, Thirroul. This was a project with a number of challenges to be
Challenges and Solutions
more severe than commonly observed in creek reaches of this nature and context.
Creation of a
TIN from the initial survey highlighted the distinct and localised nature of
severe erosion. Analysis of the existing surface TIN identified several areas
where the erosion pattern was inconsistent (areas where the creek had proven
more resistant). Additional survey work was conducted to develop a finer terrain
model around creek features which had resisted or altered erosion patterns. The
final TIN of existing ground surfaces showed features such as boulders, trees,
bedrock outcrops and constructed weirs which had demonstrated erosion
resistance, as well as a line midway up the eroded banks which showed the
pre-erosion toe of bank levels. To minimise cost of works and risk of disturbing
unstable banks, the presence of stable ‘nodes’ and a pre-erosion bed definition
were used as a starting point around which the rest of the creek stabilisation
was designed. Analysis of the existing TIN also revealed a distinct change in
creek bed level at the upstream end of the eroded zone (a ‘headcut’),
precipitating investigation into possible incipient factors for this type of
erosion (headcut erosion of this magnitude relative to the starting creek depth
is not often associated with natural geomorphology).
Further investigation included introduction of existing utility and drainage
models into the 12d environment, which revealed that the root cause of the
problem was lowering of the creek bed at the inlet to a road culvert, which was
in turn lowered to below the grade of the surrounding terrain to pass beneath a
sewer. The lowered creek bed level was not effectively stabilised, and the
erosion headcut had moved 60m upstream over 40 years. Defining the likely root
cause of the erosion was an important step in developing a cost-effective
treatment, and in determining the likelihood that erosion would continue, which
in turn informed cost/benefit factors for intervention options.
Site access very difficult due to existing erosion, surrounding landform and
Recognition of erosion mechanisms as described above identified an ideal course
of treatment including stabilisation of the erosion headcut prior to filling the
creek back to pre-erosion levels and creating a formal drop structure at the
culvert inlet. Construction is most economical, and diverted creek flows easier
to manage, if headcut stabilisation precedes bed filling. The existing channel
section is typically 0.5m-1m wide at the bed with near vertical banks 1m-2m high
either side, then steep banks (around 1:1 to 1.5:1) grading up to a top of bank
3-5m above the creek bed. The existing creek profile did not allow plant access.
Plant access was required as far as the site of the headcut, around 60m upstream
of the culvert inlet. At no point were the bank gradients trafficable in their
existing condition, even at the culvert inlet banks (near vertical 2m-3m high).
Excavation in the vicinity of the culvert inlet was constrained by an existing
sewer buried perpendicular to the creek and immediately behind the headwall to
the culvert inlet. Excavation to widen an access path in the creek bed was
prohibited by steep, unstable banks either side. Removing material from creek
banks to reduce bank grades was constrained by the presence of existing houses
and outbuildings close to top of bank on each side (translate functions were
used to create TIN surfaces related to the ZOI for existing structures in order
to check proximity of excavations to building foundation zones). 12d Model
software was used to design a two-stage filling regime, the first stage of which
provides an access track from a cut through the creek embankment 20m upstream of
the culvert inlet to the point where the headcut is to be stabilised. 3D design,
visualisation and rapid incorporation of alignment changes and associated volume
recalculations allowed the access track to meet the following functional
Provide reliable access by incorporating width, grades and curve geometries
identified as suitable for likely construction plant with safe batter
heights and grades.
Avoid any excavations with the potential to further destabilise banks; also
avoid any excavations with potential to destabilise material within the zone
of influence of adjacent structures.
Effectively balance cut and fill including volume allowance for imported
rock material to stabilise the track surface.
Fill over existing surface to achieve required geometries with no
Maintain grade and crossfall sufficient to allow for a constant flow
diversion trench to be incorporated into the access track formation.
Minimise removal of tree branches while allowing room for plant operation.
Minimise cut and fill required to bring access road surface to final
subgrade levels for creek reconstruction (ideally, riprap should be placed
directly over the access road where possible, so access track surface levels
should be ‘finished creek levels minus riprap thickness’, but only where
possible, subject to grade and geometry requirements).
To the novice observer during
construction, this access track resembled a pile of dirt. In design, getting
this ‘pile of dirt’ to meet each of the mandatory objectives and approach the
desireable ones is a significant feat of constraint, juggling which is only
really possible with access to rapid iterative modelling and analysis. The
adopted design approach for this element would not have been cost-effective
without stacking redesign and analysis operations in 12d Model - including
template design, interface, volume analysis, and TIN analysis tools. Getting
this element of the design right is crucial to the success of the overall
concept in this instance. Without good, safe access to the creek and a
hassle-free flow diversion, the rest of the works become much more risky and
Significant trees in the vicinity of works.
Based on advice
from a riparian vegetation specialist, additional survey also captured
attributes of significant trees to be retained including location, surrounding
ground levels, likely extent of root zone, limbs potentially conflicting with
plant movement envelopes, and canopy spread. This information was included in
the 12d terrain model and used to ensure that existing ground levels in the
immediate vicinity of significant trees were preserved, and that the proposed
revegetation scheme following creek reconstruction would be compatible with
likely shading patterns from old growth trees. The position and attributes of
existing trees exerted a significant influence on the geometry of the design
creek, which in turn had an effect on the hydraulic parameters influencing
design of riprap and the drop structure. Once again, rapid iterative analysis
was instrumental in preparing a design creek geometry that ticked the hydraulic
and constructability boxes without jeopardising significant trees.
Constant flow through work site.
noted, a flow diversion was incorporated into the access track formation. The
flow diversion remained in place throughout riprap armouring works and will
eventually form the preferred path for interstitial trickle flows through the
riprap, allowing for greater control over long term erosion potential. Iterative
terrain modelling was again employed, along with TIN flow analysis and export of
design geometries to a separate hydraulic modelling package, to design the
access track formation so as to capture flows from areas disturbed during
construction and deliver these to a single point sediment control facility.
Further refinement of grades and levels with 3D design allowed development of a
sediment capture pit as an integral component of the excavation and backform
works for the drop structure without compromising safe batter excavation
parameters or exceeding plant reach distances. The sediment capture pit remains
in place as sacrificial backform and permanent backfill drainage adjacent to the
walls of the concrete drop structure.
Records from previous major storm events and observation of catchment conditions
indicated high energy flows and significant potential for blockage by medium to
large boulders and woody debris. 3D design was used to model excavations and
plant reach distances to enable design of debris control measures within the
drop structure which maximise the utility of available space within geotechnical
and plant reach constraints, to provide effective deflection of debris into an
offline storage area which can be readily accessed by the plant commonly used
for reactive debris clearing works in the wake of storm events where larger
machinery is not always readily available. The finished levels adjacent to the
drop structure included a permanent access pad for advantageous positioning of
the cleanout plant.
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Need to quantify hydraulic impacts on piped and overland flow regimes.
taken from design channel geometry were used, along with calculated interstitial
flow and roughness parameters to model design creek flows. Unacceptable
hydraulic impacts were identified, necessitating changes in the final channel
geometry. Due to the streamlined design processes and operation stacking
developed during formation design, the changes in surface profile could be
readily translated into changes in formation models. This approach encouraged
the designer to preserve functionality of the formation design without resorting
to shortcuts such as changing riprap design away from the ideal, or adding
additional depth of riprap to make final geometry changes easily. Shortcuts
taken with the riprap to meet hydraulic considerations can reduce confidence
levels in the final design and increase construction costs by using more
rockfill than necessary and/or using a riprap grading mix other than the most
economical blend of graded and ungraded rock. To model impacts on piped drainage
systems, 12d Model’s interoperability with DRAINS was used to export data for
hydraulic modelling of the piped drainage system. Terrain model data was also
exported to define geometries of overland flow routes used to assess major storm
impacts when the pipe and creek system are surcharged.