Ways of reducing the damage caused by forest debris in storms are being investigated by the Bay of Plenty Regional Council and the forestry industry.

August 2010 floods in the Eastern Bay of Plenty.

This follows three major storms in the last two years in the region, which affected the Whakatane River, Ohiwa and Nukuhou, Waimana, and Opotiki areas.

'The first storm left the harvested upper catchments relatively unaffected and did not cause much forest debris movement or mass movement erosion,” says the council's manager land resources Simon Stokes.

'The landscape was still saturated when the second storm arrived. High intensity rainfall, combined with the saturated landscape, created significant issues of forest debris movement and mass movement and fluvial erosion,” says Simon.

The percentage of forest area eroded was about five percent of the total forest area. The storm was localised and cut a path about three kilometres wide, says Simon.

The most noticeable damage was where forest debris backed up against bridges, culverts, farm fences, heavily wooded native bush or was deposited in streams and their floodplains. Forest debris that did flow through was deposited on paddocks, tracks, roadways, estuaries and on beaches.

Structures affected were largely within forests. An earlier storm removed two walkway bridges and helped to flood and damage several properties in the urban area.

Forestry companies and Regional Council staff assisted with the clean-up. Forest industry, Regional Council staff and other technical experts discussed current and future forest practices to reduce or eliminate the movement of forest debris.

Opotiki and Whakatane District Councils, local community and the regional council analysed how to manage the debris issue and recommend how to reduce or avoid future forest debris issues at harvested sites.

Regional council staff are educating the regional forestry sector on high-risk erosion classes of land to enable improved harvest and replanting planning, ultimately removing those slopes and site locations from a harvestable plantation forest land use.

Forest companies are to implement four key on site harvest management practices – removing slash from at-risk sites, burning where practical, poisoning standing trees and establishing slash racks on alluvial plains or planted stands, providing another form of alluvial plain buffer.

The debris management strategy was presented at last week's regional council operations monitoring and regulation committee meeting.

The Bay of Plenty has the second highest amount of plantation pine forestry of any region – 50 per cent of the productive land. Forestry and wood processing, contribute about 10 per cent of the regional Gross Domestic Product annually (about $1 billion).

In the next 10 years, the log harvest in the region is forecast to increase.

'Most of the 287,000 hectares of exotic forestry is located in catchments east of the Waitahanui River catchment (84 per cent), with most in the Rangitaiki River catchment,” says Simon.

'Economically, forestry and wood processing provides 19 per cent of the Eastern Districts' GDP.”

The vulnerability of the region's forestry estate to storm events is approximately 25,700 hectares (nine per cent) and the most vulnerable landscape is in the Waitahanui, Whakatane, Ohiwa, Waioeka and Motu/East Coast river catchments.

Ninety per cent of the exotic forest estate also has identified high risk soil types vulnerable to erosion.

'While forest debris management may appear to be an increasing problem we need to see this in context of the wider benefits that forestry brings to the region both in terms of the economic returns and benefits in reducing erosion on our steeper and more unstable hill country,” says Simon.