The report An assessment of the impact of riparian revegetation on stream erosion during floods in Victoria by Alluvium, commissioned by the Department of Sustainability and Environment and the North East Catchment Management Authority in 2011, investigated the impacts of stream erosion or channel change during floods in Victoria.
The report showed that:
- Flood related channel change causes major damage to public and private infrastructure and river health. The cost of flood related river restoration works has amounted to over $80 million (net present value) over the past 20 years (approximately). The cost of repairs to public infrastructure (such as roads and bridges) has been an order of magnitude greater than the cost of river repair works;
- The absence of high quality, structurally diverse native riparian vegetation increases the occurrence, extent and scale of flood related channel change and associated flood related recovery costs in Victoria;
- High quality, structurally diverse native riparian vegetation, established through revegetation programs such as those undertaken by Victoria’s catchment management authorities, can be effective at reducing the occurrence, extent and scale of flood related channel change;
- Native riparian vegetation must be of high quality and contain diverse structural elements comprising instream, bank and floodplain vegetation including sedges, reeds, ground covers, shrubs and trees suitable to address the range of erosion mechanisms present in stream systems.
Paired site assessments revealed a strong inverse relationship between erosion and the presence of high quality, structurally diverse native vegetation, established from a revegetation program. The results of the paired site assessment support the proposal that riparian vegetation established through a revegetation program can be the controlling factor that limits flood-related channel change.
At the paired sites, there was negligible channel change at sites that had been the subject of revegetation programs resulting in the establishment of high quality, structurally diverse native riparian vegetation. However, high rates of flood-related channel change were found at sites that did not have high quality, structurally diverse native riparian vegetation.
The characteristics of riparian vegetation in streams that are resistant to flood related channel change include:
- Vegetation corridor structural diversity: the vegetation should be structurally diverse (contain instream vegetation, bank ground cover, shrub species and trees). This diversity will take many years (potentially 10 to 15 years) to establish, dependent upon local conditions;
- Vegetation corridor width: native vegetation riparian corridors need to be an appropriate width to ensure they are ecologically and physically functional. The scientific literature is clear that the appropriate width varies across sites and for the specific function that needs to be achieved. Other studies have demonstrated that sites with well-developed riparian widths of vegetation (generally 10 to 30 metres wide on each side) were effective in providing for resistant and resilient streams;
- Vegetation corridor longitudinal connectivity: Native vegetation riparian corridors should be continuous (or largely continuous).
For more information about the impacts of riparian vegetation on channel change in rivers, see the attached report.