1.5 TOPOGRAPHY AND SOILS

The most distinctive topographical feature is the levee bank constructed around Herring Island's perimeter. The levee bank slopes steeply from the water's edge to an average height of 5m above low water level. The inland side of the levee also slopes steeply, but rises only 2m above the flat central area of Herring Island. The top of the levee bank is undulating and narrow (half a metre to a metre wide). A small amount of relatively flat land outside the levee bank is found in the north- east and north-west 'corners' of Herring Island. The levee bank has breaches on the eastern bank of Herring Island, and at three points on the north-west portion. The eastern breach was constructed to allow barges to deposit silt on Herring Island. Other breaches allow drainage from the low-lying area within the levee.

Studies undertaken by the BOW suggest that the levee bank surrounding Herring Island remains an effective flood protection measure. The levee is unlikely to suffer any significant erosion as a result of floodwaters due to extensive beaching and vegetative cover. The BOW have assessed the adequacy of the floodway in the vicinity of Herring Island and believe it to be adequate. A map of the area indicating designated flood levels (FI . 6) shows Herring Island to be almost entirely above such flood levels (BOW, 1981). It can be concluded that Herring Island will cause no major floodway obstruction and faces no major flood risk.

There is a large depression of topographical interest in the north-western portion of Herring Island. Although this area has been suggested as a potential wetland site (Drohan, 1987), factors such as the permeable nature of the soil, the extensive disturbance involved in artificially creating such a wetland, the presence of nearby water habitats (the river banks and the lagoon system of the Royal Botanic Gardens), and the cost involved reduce the viability of this proposal. Scattered throughout the remaining inner area are several relatively steep hummocks (2-3m) caused by silt deposition. Although these hummocks are steep and pose potential erosion/access problems, they provide landscape diversity and offer protection to the central conservation zone. With adequate protection, the mound in the northeast could offer opportunities for views over Herring Island and its surrounds.

Herring Island's soil profile consists largely of silt dredged from the Yarra River. Extensive rock beaching was used as a foundation for later silt deposition. Soil tests undertaken in 1980 indicated salinity levels were sufficiently high to prevent tree planting. More recent tests indicate a reduction in salinity levels, presumably as a result of leaching (Table 1. Refer to Figure 4 for sampling sites). The process used assessed the electrical conductivity of soil samples, rather than specific ion determinations.

Although based on a very limited number of samples, it would appear that areas of greater salt concentration tend to occur around the base of hummocks, while areas at a higher elevation generally showed less salinity hazard. Vegetation reflects these differences: low-lying areas are characterised by salt-tolerant species such as spiny rush (Juncus acutus), salt barley grass (Distichlis distichopliylla) and wallaby grass (Danthonia spp.), with very few woody species. These differences support the hypothesis that salt is gradually leaching from higher areas on Herring Island. Exceptions to this pattern are several noticeably bare patches on the northern levee, one of which indicated a particularly high salt concentration. More complete testing is necessary to determine whether these areas are merely eroding or have localised salt or other soil toxicity problems. Preliminary results indicate that a suitable planting strategy would be to commence revegetation on levee and mound areas where soil is generally of a better quality. While some salt-pan areas still exist, there has been sufficient leaching in this time to allow the establishment of many tree and shrub species.

A small area of natural soil profile occurs in the northeast corner of Herring Island and supports a small remnant grassland association. This area is easily accessible from the north - eastern shore and is particularly prone to trampling or other damage. This area is included within the Conservation Zone and will be fenced to restrict access.

Aims

To protect Herring Island from further erosion and to rehabilitate existing eroded areas on the northern and western levee banks.

To rehabilitate saline areas by progressive revegetation.

To conduct works in a manner which will minimise soil disturbance and erosion risk.

Actions

Medium	TS-1	Conduct more extensive salt testing and establish revegetation trials to determine details for the revegetation strategy.
High/ Ongoing	TS-2	Progressively revegetate with salt-tolerant species (Danthonia spp., Distichlis distichophylla, Eucalyptus camaldulensis, Myoporum insulare) in high salinity areas.
High	TS-3	Fence high access areas to prevent erosion (NE- corner, viewing mound) and provide suitable surfacing and drainage for tracks.
High/ Ongoing	TS-4	Revegetate steeper areas to lower erosion risk, particularly in levee areas where high usage is expected. Avoid excessive clearing of ground storey vegetation (including pest species) in these areas in initial stages of revegetation

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Table 1             Soil Test Results

Site No1	Description	pH	Electrical Conductivity (EC) (ds/m)	Salinity hazard*
1.	bare area, heavily compacted	6.01	2.87	high
2.	edge of bare area; some vegetation	6.01	0.98	low
3.	bare ridge area	6.01	0.48	nil
4.	bare ridge area	5.98	2.75	high
5.	edge of ridge area	5.94	0.69	low
6.	depression, N.E. corner	6.84	0.06	nil
7.	revegetated mound, S.W	6. 52	0.06	nil
8.	palm mound	5.97	1.23	moderate
9.	swampy area (central)	6.08	2.07	fairly high

*ratings from Handreck, K. & Black, N. (1984). 'Growing Media for Ornamental Plants & Turf' UNSWP
¹ Refer to Figure 4