Wrays Site

Erosion On-ground Works Case
Study – May 2008
Wrays Site
Project Officer: Shari McConachy and Ralph Cotter,
Department of Primary Industries
Case Study for Erosion and Landslides
1.
Site Description
1.1
Site I.D.
1.2
Site address
Lance and Marjory Wray, “Clipswood Park”, Gluepot Road, Morrisons
1.3
Brief site description and general overview
Large active gully formed on a valley leading to Eclipse Creek, a tributary to the
Moorabool River
1.4
Map datum/ Map projection/ Zone 2004 Aerial photo
1.5
Easting 778492
1.6
Northing 5813536
1.7
Municipality Moorabool Shire
1.8
CCMA landscape zone Moorabool
1.9
Previous ID
1.10
Previous Data Source
Case Study for Erosion and Landslides
2.
Hazard Description
2.1
Soil degradation type Erosion
2.2
Soil degradation sub-class Gully and tunnel erosion, some sheet
and rill erosion
2.3
Description of hazard present on site or threatening site from
above or below Sedimentation of the Eclipse Creek below the gully
site and eventual sedimentation of the Moorabool River. Also loss of
marginal agricultural land.
2.4
Dimensions of Hazard (width, length and depth if appropriate)
2.5
Extent of Hazard (spatial area and volume if appropriate) The
area treated for the hazard covers 10ha as this includes the area
around the gully which has been fenced off and revegetated.
2.6
Magnitude of hazard (travel distance or rate of occurrence)
2.7
List previous reports or studies relevant to this site N/A
2.8
Custodian of previous reports and studies
Case Study for Erosion and Landslides
3.
The Event Has Already Occurred
3.1
Date of first occurrence The gully has been active since Wray’s
bought the property in 1951.
3.2
Date of most recent re-activation or acceleration The gully has
always been actively eroding up until time of earthworks
3.3
Actual or postulated trigger event including magnitude and
duration
Was most likely triggered by clearing of the highly dispersive clay soils and
rabbit infestations which allowed water to concentrating in the subsoil and
begin tunnelling.
3.4
Frequency of Trigger Event if known Highly dispersive clays are
found across the region due to the parent material of sedimentary
soils. The clearing of the land was completed in one go during the
1950’s but the rabbits continue to be an issue although not in as high
numbers as they were historically.
3.5
What damage or impact occurred? Sedimentation Eclipse Creek
would occur after any rainfall event and loss of low value agricultural
land has occured. While no injury has resulted from the erosion it did
have the potential to occur because of the steep sided and deep
gully.
3.6
Was there a risk of injury or loss of life?
Risk of injury or loss of life would be possible if the landscape was unknown as the gully
was over 8m deep in places with verticle sides.
3.7
How important was it?
The sedimentation of Eclipse Creek and eventually the Moorabool River makes this of
high importance. It is a low risk and therefore moderate importance of the risk of injury or
loss of life.
3.8
What asset classes were impacted?
Water quality – by sedimentation
Ag. land
Case Study for Erosion and Landslides
3.9
What asset sub classes were impacted?
3.10
What are the asset values?
3.11
How severely were assets impacted?
The Eclipse Creek would be highly effected from the sediment lost from this site and
deposited in the river. Whether all this material would reach the Moorabool River is
questionable as the Eclipse Creek system has several ponds before it joins the
Moorabool and these may act as a sediment trap. However, when the creek was freely
flowing I believe sediment would reach the Moorabool and reduce it’s water quality.
3.12
Estimated cost of impact (including qualitative and quantitative
costs for loss of asset, investigations, remedial works, cultural,
business and environment)
$20,000 was put towards the earthworks on the gully ($18,000 funding and $2,000
landholder contributions) which involved gully battering and ripping then compaction of
tunnels.
Further funding was provided through salinity funding (Ralph Cotter) to fence and
revegetate the site.
The owners have put at least a further $2000 in materials such as haybales, rabbit proof
fencing etc into the site.
This does not take into account anyone’s time, including the owners.
Case Study for Erosion and Landslides
4.
Remediation Has Already Been Undertaken
4.1
What remediation option was used? Gully battering and tunnel
ripping
4.2
How was the site initially assessed? Visually by DPI staff Ralph
Cotter and Shari McConachy. Works design was completed by the
contractors, Darryl and Scott Ferry
4.3
How was the remediation designed and by Who? As above
4.4
Did it require specialist equipment or subcontractors? As above,
contractors local to the area and very familiar with this sort of erosion
completed the works.
4.5
How effective has the remediation been? Currently very effective.
While some small channels began on the bare ground after a heavy
rainfall even the landholder place haybales around to slow the flow
and catch the sediment. These seem to have been effective.
One area has opened up a new tunnel, this will be monitored for any movement.
4.6
How was the effectiveness judged? Visually
4.7
Would other treatments worked here? Sediment traps may be
prevented the sediment reaching the waterways but would not have
stopped the erosion. Some for of earthworks was always required to
stop further eroding.
4.8
Was it early intervention or reactive? Very much reactive
4.9
What was the cost of remediation (including design,
construction and implementation)? $20,000 total for earthworks
4.10
How was the remediation funded? $18,000 through DPI funds,
$2,000 landholder
Case Study for Erosion and Landslides
5.
Ongoing Review and Monitoring
Requirements
5.1
What is the likely ongoing monitoring and review strategy?
5.2
What is the nature of future monitoring and maintenance?
5.3
What are the likely costs of monitoring and maintenance?
Case Study for Erosion and Landslides
Photos
Photo 1: prior to works, 2007, facing south down gully with tree for scale.
Photo 2: Spring 2007, note same tree for scale. Rip lines for tree planting can be seen.
Case Study for Erosion and Landslides
Photos
Photo 3
Photo 4
Works completed on the main gully, another smaller gully still to go.
Case Study for Erosion and Landslides
Photos
Photo 5: January 2008, Ralph Cotter inspects the tunnels that have opened up down one
of the small arms of a gully. Also showing the hay bales Lance’s put in place to slow
surface water flow.
Case Study for Erosion and Landslides
Photo 6
Lance Wray, in front of his complete gully works. January 2008
Case Study for Erosion and Landslides
Photos
Photo 7
Aerial photo of gully erosion site flowing directly into Eclipse Creek.
Case Study for Erosion and Landslides