Monitoring erosion and surface water
impacts of coal seam gas access tracks
April 2015
Introduction
Coal seam gas (CSG) development is being
undertaken on large areas of Queensland’s
agricultural land. Along with the installation
of gas wells and pipes is the establishment of
thousands of kilometres of access tracks that
enable machinery to access the network of CSG
infrastructure. Previous studies have shown that
unpaved roadways are a significant source of
erosion and sediment loss in catchments around
the world. Monitoring and managing such an
extensive road network is difficult. However,
this study has shown that modern aerial
photogrammetric techniques combined with
detailed hydrological modelling can pinpoint the
locations of high erosion risk. Ongoing surveys
should improve management by highlighting
areas of soil loss, such as rills and gullies, and
showing changes in water flow caused by such
as extensive road network.
Roadways and erosion
Studies from around the world have shown that
roadways are a major source of sediment into
waterways. Commonly, over 40% of the sediment can
be shown to have its origin in unpaved rural roads even
though these roads only make up about 1% of the total
area of a catchment (Table 1). With CSG development,
the intensity of roadways in agricultural land will
increase significantly and there is a risk that erosion
losses will also increase. Standard engineering methods
for mitigating erosion threats are available if the
location of problem areas can be identified. However,
the scale of the CSG and hydrological systems are so
large that monitoring for threat development using
traditional methods is difficult.
Figure 1. Development of an erosion rill between a CSG access
track and a cultivated field near Chinchilla, Qld
Table 1. Data on roads as % of catchment and % of sediment source.
Country
% of Area
% of Sediment
China
1
42.3
Indonesia
5
40
Brazil
1.5 to 2
28-69
Less than 5
23-30
Australia (forest)
2.4
18-39
Australia (farm)
1
41-52
USA
www.gisera.org.au
[email protected]
Aerial Photogrammetry
Aerial photography has been used for many decades
to monitor land use and to generate the contour
maps of the ground surface. Modern high precision
digital photography and computing techniques
allow these procedures to be completed with high
spatial resolution over larger areas than ever before.
A series of overlapping photographs are taken
over the study area so that any single point on
the ground surface can be identified in 3 separate
images (Figure 2). The position of that point is then
triangulated in much the same way that the human
brain perceives distance based on the views from our
two eyes. The demonstration in Figure 2 has applied
this technique for every 20 cm over an area of over
1300 km2. This provides an estimate for over 32 billion
locations. These are then brought together to create
a single visual image of the area and a 3-dimensional
representation of the ground surface. Testing of
this surface indicates that the predicted elevation of
each point is accurate to within a few centimetres.
This is approaching the accuracy of systems used
by surveyors.
Figure 2. A point “A” in an agricultural field is identified in three
overlapping images. If the position of the aircraft is known for
locations 1,2 and 3, the position of A can be calculated. Ground
surface points within wooded areas (e.g. Point B) may need to be
inferred from other nearby visible points if the view is obscured
by foliage.
Mapping of water flows
Once the ground surface is calculated and mapped,
water flow models can be applied to the soil surface
to show where water is likely to flow. This is achieved
by following water flow paths down gradients from
every point in the 3D soil surface. Also, an estimate
of the area of water draining to each point can be
calculated as water flows are followed down through
the catchment.
Figure 3. An illustration of a 3D ground elevation model with
water flow lines imposed onto the soil surface. Note the presence
of farm contour banks, gullies and depressions. Flow lines are
coloured according to upstream catchment area. The vertical
dimension has been exaggerated for clarity.
Step 1. Visual Image from aerial survey
Step 3. Ground elevation model (trees and buildings removed)
Step 2. Digital surface model including trees and buildings
Step 4. Soil surface water flow model derived from Step 3
Figure 4. The four main steps in deriving the water flow model. 1) Aerial survey conducted using digit photogrammetry.
2) A digital surface model (DSM) is constructed by triangulating the elevation of each pixel in the image. Note that contour banks and
well pads are prominent in the image. The surface also includes surface features such as vegetation and buildings. 3) Surface features
are removed and the ground surface beneath them is interpolated. Note that surface depressions (e.g. “Gilgai”) are now revealed from
beneath the trees. 4) The ground surface elevation is used to calculate water flows according to small scale topographical variation and
features such as gullies, contour banks, drains and roadways.
How can this
be used?
Information on the
location and catchment
area of water flows can be
used by land holders and
CSG staff during planning
for CSG infrastructure
placement. Furthermore,
repeated surveys can show
changes in water flow
or soil surface elevation
which may indicate
diversion of water flows,
soil loss or build-up of
sediment within the survey
area. The likely source
of any sediment buildup can be identified by
following the predicted
water flow paths to that
location. Concerns by
land holders regarding
surface water flows can
be better communicated
through the use of a water
flow map.
a
b
c
Figure 5. Predicted water flows
a) around a raised gravel pad
including rill formation, b)
across a farm track, and c)
across a CSG well pad. Arrows
show position of photo.
www.gisera.org.au
[email protected]
B&M | 15-00104