The risk of erosion resulting from roads and tracks also depends on their location
in the landscape. Figure 14.5 shows the different ways that roads and tracks
may be located in relation to the topography. The arrows represent the direction
of runoff in the landscape. Tracks and roads that take the shortest route often
have the most erosion problems as they commonly cross slopes on a diagonal
to maintain their straight alignment, and so divert and concentrate runoff.
When choosing an alignment for a track it is important to bear in mind that if
the alignment requires any clearing of native vegetation all requirements under
the Vegetation Management Act 1999 must be followed before commencing the
work.
Roads and tracks on ridges
Ridges (e.g. section A–B in Figure 14.5) are an excellent location for roads and
tracks. Ridge slopes are not as steep as adjacent slopes and runoff naturally
drains away from ridges. However, whoa boys will still be required where the
ridge line slopes for extended distances, particularly on the toe, or if the road
diverts along a spur. Roads on ridges are excellent vantage points from which to
inspect a property because a large area of land can be seen from them.
Roads and tracks on the contour
Along the contour (e.g. B–C in Figure 14.5) is also a good alignment for roads
and tracks. Whoa-boys are not required for roads and tracks that follow the
contour, and maintenance costs are reduced because runoff is not accumulated.
Surveying for contoured roads need not be highly accurate as minor deviations
from the contour can allow wheel ruts formed in wet conditions to drain more
quickly after rain. Roads and tracks on the contour have the added benefit of
enabling access to, and inspection of, the middle of a paddock.
Roads and tracks directly up and down slope
A road running directly up and down the slope (such as D–E in Figure 14.5)
usually has a lower risk of erosion than one running diagonally across the slope
(such as X–Y in Figure 14.5). A road up and down the slope may be steeper and
hence potentially more difficult to traverse, particularly when wet, but it will
not intercept overland flow and hence has fewer drainage issues. Whoa-boys
will usually be required on roads running directly up and down slope to manage
runoff flowing down the road and to avoid scouring and gully formation.
Roads and tracks diagonal to the slope
Tracks along fence lines will often run diagonal to the slope (e.g. X–Y in Figure
14.5). Roads on such an alignment are at high risk of erosion because they divert
and concentrate runoff from a large ‘catchment’. A diagonal road will intercept
overland flows and redirect runoff down the road or table drain. This can deprive
the land on the low side of the road of useful runoff. To overcome erosion
problems, whoa-boys or culverts must be installed at regular intervals, along
with spur drains to safely disperse runoff.
Roads and watercourses
Where formed roads cross drainage lines or creeks (e.g. point F in Figure 14.5), an
invert, floodway, causeway, culvert or bridge is required. Inverts are constructed
by removing the soil in the crossing and replacing it with heavy gravel or
concrete. A sheet of geofabric laid under the gravel will ensure that the gravel
remains separated from the underlying soil. This is important to extend the
effective life of the invert. If a culvert is being installed, the diameter of the pipes
should be matched to the expected runoff (see Chapter 3 for estimating rate of
runoff). The expected runoff will be related to the area drained by the culvert
and the pattern of rainfall in the area. Culverts are susceptible to blockage from
siltation as well as the growth of grass and weeds and it is important that they
are maintained regularly.
Selecting the location at which a watercourse is to be crossed requires careful
thought. The correct choice of a crossing location will reduce construction and
maintenance costs as well as minimise any adverse impacts of the crossing on
the watercourse. When siting a crossing the following should be considered:
• Avoid locating crossings where the banks are steep. Steep banks are an
erosion risk and require considerable excavation to ensure that approaches to
the crossing are gently sloping enough to be easily accessible.
• Whoa-boys should be installed on the approaches into the drainage line to
avoid erosion of the road or track.
• Minimise disturbance of soil and vegetation by crossing drainage lines at right
angles and avoiding sites where clearing of vegetation is required.