Effectiveness of Riparian Margins

In:
2017 Forest Growers Conference
Report No:
AC2017-06
Report Date:
October 18, 2017
Author(s):
Dr Chris Philllips: Landcare

Effectiveness of riparian margins for trapping sediment in steepland plantations

Conference presenter: Dr Chris Phillips, Landcare Research

Steep forest sites are highly vulnerable to landslides and erosion in the first years following harvest, with associated risks of sediment and debris reaching watercourses at the bottom of harvested slopes.

To date, there has been little research into ways of alleviating these risks. The standard technique used by forest managers is to leave a buffer of standing vegetation between the harvest area and any streams. Based on observations that buffers are not always effective in stopping sediment reaching streams in post-harvest sites, this project aimed to test the hypothesis that:

‘slope steepness and form are the primary drivers of connectivity between sediment sources and streams: therefore, the presence of riparian buffers for removing or filtering sediment within plantations is secondary.’

Key research findings

The research team assessed streams within three steep, recently harvested plantations, first using GIS analysis and then by field survey. The three sites studied were as follows:

Paroa Forest Whangapoua Forest Tairua Forest
V. steep, deeply dissected Moderate to steep Moderate to steep
Shallow to skeletal soils Deep soils Deep soils
Bedrock controls slope and stream profile Bedrock controls slight Bedrock controls moderate
Established buffers Established buffers Minimal buffers
No storm Moderate storm 2016 Very large storm 2017

 

The key findings from the analysis of the three sites were:

  • More water reaches the bare landscape following harvest, which leads to the landscape’s response
  • Channel scour, riparian (stream-edge) failure, hillslope landslides and debris flows are the key erosion processes
  • A flat slope near the stream edge is a key determinant of sediment-trapping ability: this is regardless of whether or not there is a vegetative buffer
  • The effectiveness of riparian buffers in mitigating sediment flows increases as slope decreases, but large storms resulting in landslides, debris flows and floods, override this.

What next?

Forest managers need to know what can and what can’t be managed: the researchers concluded that there is still much to be learnt about the design of buffers to minimise sediment flows to streams in different landscapes and with different geology. More research is needed: the issue of post-harvest debris and sedimentation is definitely a risk to the forest industry’s public reputation.

Presentation below.

Document:
06 Riparian Margins for trapping sediment_C Phillips (1.50 Mb)
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