smiths lake planning study volume 1: text - Great Lakes Council

SOILS AND GEOTECHNICAL 6
Results are presented on the following figures:
• Terrain Classification (Figure 2-1) showing the inferred distribution of terrain units,
• Slope Stability and Acid Sulphate Soil Zonation Plans (Figure 2-2) assessing the risk of
slope instability and acid sulphate soil occurrence, and
• Test bores and sample locations together with regional geological boundaries (Figure 2-3).
2.3 Physical Characteristics
2.3.1 Topography
The topography across the study area is variable and comprises:
• Moderate to steep hillside areas with elevations up to 220m AHD. Surface slopes are
generally concave in profile with steep upper hillside slopes up to 50% usually decreasing to
10% to 25% across footslope areas. The hillside areas occur as a series of north-west /
south-east striking ridgelines separated by broad drainage paths. The alignment of the
ridgelines coincides with the regional bedding strike of the geological units. Geological
structure appears to have had a strong influence on the region’s topography. These areas are
characterised by rock outcrop and shallow residual soils that have weathered in place from
the underlying rock.
• Broad gently sloping drainage paths along the valley floor areas between the ridgelines
(Wamwarra, Tarbuck, Jacks and Wallis Creeks). The drainage paths follow the same
general alignment (north-west, south-east) as the ridgelines. These areas are characterised
by deep soil profiles comprising soils that have weathered in place, been washed off
surrounding hillside areas or accumulated as alluvium along the drainage paths.
• Low lying areas with surface elevation less than 5m AHD that occur around the margin of
Smiths Lake and the southern edge of Wallis Lake and are subject to estuarine influence.
Past fluctuation in lake and sea level has resulted in periodic inundation of these areas with
the deposition of sandy, muddy and organic estuarine sediment.
• Coastal barrier sands and ridges in the Sandbar area. The coastal and wind blown (aeolian)
sand deposits have formed sand dunes up to 47m AHD in elevation. Most of the sand dunes
have gentle to moderate surface slopes of less than 25%, however there are some steep lee
slopes with gradients in excess of 50%.
2.3.2 Drainage
Smiths Lake is a shallow, saline coastal lagoon that is protected from the ocean by a sand
barrier that is occasionally opened to the sea by floods.
Catchments in the study area discharges to Smiths Lake via Wamwarra and Tarbuck Creeks,
with the larger catchment area of Ducks and Wallis Creek discharging to Wallis Lake. There
are several creeks, but no major rivers draining into the lake.
SMITHS_LAKE_PLANNING_STUDY.DOC
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