Thorpdale (Tp)

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

Area: 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 <10°C (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 USE	Cleared: 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- cs

A 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
Disturbance	Affected process and trend	Form	Susceptibility of components	Incidence with components	Casual activities	Primary off-site process
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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