Restoring the Waiapu Catchment
Mike explaining to the Hon Nathan Guy that, while reforestation has been effective in ‘shutting-down’ numerous other gullies within this forested area, the Tarndale and Mangatu gullies were already too large and unstable at the time of planting. (Photo courtesy Stuart Taylor from MPI)
The Waiapu Catchment is one of the most degraded catchments in New Zealand largely because little effort has been made to date to deal with the erosion/sedimentation problems. MPI have taken on board research by Mike Marden and his colleagues who showed that gullies are the most significant of the sources of sediment in this catchment. They plan to focus on gully restoration using funding available until 2021 and administered through the 'East Coast Forestry Project'. Mike says that restoration of gullies is the quickest and cheapest means of reducing the erosion/sedimentation issue, minimising the risk of flooding downstream and damage to infrastructure and will eventually pay dividends in improving water quality.
The Minister of Forestry, the Hon Nathan Guy, visited Gisborne on April 23rd principally to sign an agreement between Gisborne District Council (GDC), Te Runanganui O Ngāti Porou (TRONPNui) and the Ministry of Primary Industries (MPI). The focus of the agreement is to restore the environmental integrity, economic profitability, cultural revitalisation and social prosperity of the Waiapu catchment. The programme is being governed by the three partners and managed by MPI.
The partners aim to remediate erosion-prone land identified in the Combined Regional and District Plan as Land Overlay 3A (LO3A) using funding available through the East Coast Forestry Project (ECFP) and the Sustainable Hill Country Programme. The ‘Plan’ requires all 3A land throughout the District to have been established with an effective tree cover by 2021.
There are approximately 60 000 hectares of land eligible for ECFP funding across the Gisborne District, of which approximately 25 000 hectares is in the Waiapu catchment. Much of the target land includes gullies — the highest sediment yielding erosion process — contributing about 50% of the 35 million tons of suspended sediment discharged to the ocean annually by the Waiapu River. Treatment options available for the restoration of gullies include pole planting (willow), natural reversion to an indigenous cover, establishment of plantation forest species and/or, specialty native timber species.
In the past, exotic reforestation has proven to be the most effective means of restoring stability to large areas of unstable land and in particular gullies. Two of the largest gullies in Mangatu Forest — the Tarndale and Mangatu Gullies — were planted in exotic pines in the 1960s. Although reforestation in this instance has not been successful in stabilising these features, the establishment of Mangatu Forest has resulted in a ~50% reduction in the rate of sediment generation over the duration of a rotation of exotic forest (~28-years).
Conversely, if Mangatu Forest had not been planted, gullies would have undoubtedly enlarged and sediment generation rates would likely have doubled. The message is that early intervention pays dividends and the earlier the remaining untreated gullies within Waiapu catchment are treated the quicker the desired outcomes for this catchment will be achievable and realised.