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Thorpdale (Tp)

A study of the land in the Catchment of Gippsland Lakes - Vol 2 - land system Thorpdale- geoArea: 109 sq. km (0.6%)

Areas of Tertiary basalt which have been subjected to extensive landslide activity, are mapped as the Thorpdale land system. The cause of the landslides is not clear but tectonic activity could have contributed. Most landslide debris has now been removed from the valleys by the streams but irregular debris deposits and landslide scars are characteristic of the slopes. Most occurrences are near Thorpdale where the volcanics have suffered considerable tectonic shock during uplift. Clearing of forests and irrigation from sub-basaltic aquifers has activated many slopes prone to landslides.

The soils have developed on remnants of old, deep profiles which formed in a period with a warm, humid climate. They are very strongly weathered, leached and tend to fix phosphate in unavailable forms. Due to the high iron content in basalt and the weathering of most minerals to clay, the soil is a reddish brown, strongly-aggregated, granular or fine blocky clay. This fine, stable structure produces a high degree of friability and the soils are considered suitable for intensive cropping. The fine structure also promotes high permeability which results in lower rates of sheet erosion than those of other soils on similar gradients.

The original vegetation, probably dominated by an open forest II or III of E. cypellocarpa, E. obliqua, E. radiata and E. viminalis, has been almost entirely cleared.
A study of the land in the Catchment of Gippsland Lakes - Vol 2 - land system Thorpdale- image
Numerous small landslips occur on the moderate slopes

CLIMATE
Rainfall, mean (mm)
Temperature, mean (C)
Seasonal growth limitations

Annual 800 - 1400; lowest January or February (40 - 70), highest August or October (90 - 120)

Annual 12 - 14; lowest July (8 - 10), highest February (19 - 21)
Temperature <10C (av.): May - September
Rainfall < potential evapotranspiration: December – February
GEOLOGY
Age, lithology

Tertiary basalts (Older Volcanics); weathered
PHYSIOGRAPHY
Landscape
Elevation range (m)
Relative relief (m)
Drainage pattern
Drainage density (km/km2)

Hills and slopes with extensive landslide activity

100 - 440
40 - 220
Dendritic
1.2
PRESENT LAND USECleared: grazing of dairy and beef cattle and fat lambs on improved pastures; some cropping, particularly potatoes; apiculture

A study of the land in the Catchment of Gippsland Lakes - Vol 2 - land system Thorpdale- csA study of the land in the Catchment of Gippsland Lakes - Vol 2 - land system Thorpdale- graph

LAND COMPONENT
Percentage of land system
Diagnostic features
1
75
Steeper slopes with landslide
scars and debris
2
15
Gentle stable crest slopes
3
2
Swamps impounded behind landslide
debris
4
8
Drainage depressions,
often permanently wet due to seepage
PHYSIOGRAPHY
Slope %, typical and (range)
Slope shape
20 - 25, (5 - 50)
Straight or concave
10, (0 - 15)
Convex
<1, (0 - 2)
Concave
<5, (0 - 10)
Concave
SOIL
Parent material
Weathered basalt with landslip deposits on steeper slopes
Description
Dark reddish brown to black loam to clay loam topsoil grading into friable reddish brown clay loam to silty clay subsoil. Mainly deep in component 1, moderately deep in component 2
Black organic loam grading into
mottled, sometimes stony grey clay
at depth; inundated or with
shallow water table
Locally derived alluvium
Mottled dark greyish brown loam
to clay loam topsoil over mottled
lighter greyish brown similarly
textured subsoil; shallow water
table
Classification
Krasnozems
Gn3.11, Gn4.11, Um6.33
Humic Gleys
Um5.52, Gn3.92
Humic Gleys
Um6.12, Gn -
Surface texture
Loam to clay loam
Loam
Loam to clay loam
Surface consistence
Slightly hard when dry, friable when moist
Slightly plastic when wet
Friable when moist
Depth (m)
>2.0
1.2 - 2.0
>2.0
>2.0
Nutrient status
Low to moderate
Low to moderate
Moderate
Moderate
Available soil water capacity
Low to moderate
Low to moderate
High
Moderate to high
Perviousness to water
Rapid
Rapid
Rapid
Rapid
Drainage
Good
Good
Very poor
Very poor to poor
Exposed stone (%)
0
0
0
0
Sampled profile number
30
-
-
-
NATIVE VEGETATION
Structure of vegetation and
characteristic species of
dominant stratum
(+ Predominant species)
Open forest II, III:
One or more of E. cypellocarpa, E. obliqua, E. radiata, E. viminalis usually predominant; E. ovata occasionally associated. (Vegetation is similar to Neerim land system, components 1, 2)
Probably open forest II, III:
E. ovata, or sedgeland
Probably open forest II, III:
E. ovata+ or E. viminalis+
Original vegetation difficult to determine due to clearing

    Disturbance
    Affected process and trend
    Primary resultant deterioration
    Casual activities
    Primary off-site process
    Form
    Susceptibility of components
    Incidence with components
    Alteration of vegetation:


    — reduction in leaf area, rooting depth and/or perenniality

    — reduction in density of tree roots
    Reduced transpiration,
    resulting in:

    a) increased deep
    percolation and leaching



    b) increased regolith
    wetness


    Decreased root-binding



    Nutrient loss




    Landslip and soil creep


    Landslip and soil creep



    1,2; moderate




    1; moderate
    2; low

    1; moderate
    2; low



    Not determined




    Common



    Common



    Removal of trees




    Accelerated by major disturbance of native vegetation

    Accelerated by major disturbance of native vegetation



    Increased movement of water to groundwater; increased base-flow of streams

    Increased sediment load

    Increased sediment load

    Increased exposure of surface soil
    Increased overland flow and soil detachment
    Sheet and rill erosion
    1; low - moderate
    Common: particularly on cleared, steeper slopes
    Overgrazing, road and dam building and other earth-moving activities, rabbit burrowing, trafficking by stock and vehicles.
    Increased flash flows and sediment load.
    Increased physical pressure on soil
    Increased compaction

    With

    Reduced infiltration
    Structure decline



    Sheet and rill erosion
    1,2; moderate
    3,4; high


    1; low - moderate
    Uncommon



    Common: particularly on cleared, steep slopes
    Increased trafficking overgrazing, export of organic matter

    As for sheet and rill erosion above
    -



    Increased flash flows
    Increased soil disruption
    Increased soil break-up
    Gully erosion
    1; moderate
    2; low
    Uncommon: locally severe on steep slopes if water is channelled over cultivated or disturbed land.
    As for sheet and rill erosion above
    Increased sediment load
    Comments: Some landslips in the area would have occurred prior to European settlement but recent landslips have resulted from clearing, logging and major earthworks on steep slopes
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