Soil erosion and sediment control in fire affected plantations

The potential for soil erosion after a bushfire can be severe due to the destruction of ground cover and the litter layer. Shrubs, forbs, grasses, trees, and the litter layer intercept rainfall and slow surface runoff after rainfall events. Fire can destroy this soil protection.

General observations in Fire affected Plantations

• Areas that present the greatest risk in terms of available ground cover are those areas cleared felled prior to the fire or those areas recently planted. These areas tend to have little/ nil groundcover.
• Areas harvested post fire can have some availability of ground cover/ slash. Even in areas that were subject to higher fire intensities and completely blackened can have a degree of slash remaining post-harvest. In areas where trees were browned, needle fall can create a reasonable ground cover.
• Standing browned trees still afford some protection to the soil and would intercept any potential rainfall and provide some ground protection.
• Stream bank erosion potential is extreme with little or no vegetation within flow lines to reduce stream flow velocity or to prevent the collapse of the dry and exposed soils.
• Road drainage needs to be critically reviewed under the current groundcover conditions. Once stable roads now have the potential to fail due to the absence of stable verges and surrounds. Roads and tracks established during the fire need to be drained to prevent the concentration of water and erosion potential.

Recommendations

1. It is critical that effective road and track drainage is established and maintained to ensure that increased overland flow onto roads and tracks is dispersed without undue concentration of flow or increased velocity. Due to the lack of ground cover the number and placement of drainage structures may need to exceed that required by the PRA code. The aim of all road drainage is to prevent the concentration of flow at any one point. Multiple drainage structures spread the flow and reduce the likelihood of concentrated flows and erosion.
2. Outlets of mitre drains and culverts that had spilt into normally stable drainage lines will require regular inspection and may require additional works such as jute mesh or rockworks to prevent scouring.
   • Due to the fire intensity and the loss of groundcover (and possible seed source) in all of the major flow lines it is recommended that seed be broadcast into these areas in an attempt to establish a quick groundcover. Refer to local grass species.
3. Whilst some existing river buffers may be planted to commercial species it is recommended that due to the sensitivity of the flow lines these areas should not be harvested at this stage. Harvest of these areas should be deferred until a reasonable groundcover has re-established within the stream. When harvesting does occur every effort should be made to minimise ground disturbance of the bank and bed of the stream. No planting / ground disturbance should occur within the 20m Buffer Zone of a River as stated in Cl. 17 (1) of the PRA Code and no Line cultivation within Drainage Line buffers should occur until an effective ground cover is established as stated in Cl. 17 (2) (c)
4. Red basalt soils on clear fell areas may be raked to form contour windrows and ripped in preparation for planting. Site preparation on granitic soils should ideally occur after some establishment of ground cover especially on steeper gradients and concave slopes etc.

Given the general characteristics of fire affected sites it is inevitable that significant soil movement may occur especially during any high intensity rainfall event.

Further information in regards to the Plantations and Reafforestation Act 1999 and Code, or by contacting one of our team.