A gully-complex model for assessing gully stabilisation strategies

The North Island East Coast region is one of the areas most seriously affected by gully erosion, both in New Zealand and globally. Despite extensive efforts to mitigate erosion within the last fifty years, the overall area of actively eroding gullies has increased. In this study we present a gully model and its application for assessing the effectiveness of several afforestation scenarios to mitigate gully erosion in the East Coast region. The model uses empirical relationships to represent the major physical processes and environmental factors governing gully sediment yield: 1) vertical degradation (incision), 2) spatial extension (growth), 3) stabilisation (closure), 4) the influence of forest cover, and 5) the influence of the underlying geologic terrain. Overall the model shows a good fit and deviates only − 6% from the observed actively eroding gully area and 8% from the number of actively eroding gullies. The application of the model to assess past and future afforestation schemes indicates that 1) past afforestation efforts were effective in reducing sediment yield by approximately 33% in the Waipaoa catchment and by 16% in the Waiapu catchment from what it would have been without afforestation, 2) the most effective strategy to minimize gully erosion is to reforest all present and newly initiating gullies, which would halve the annual sediment yield by 2050, and 3) the earlier the planting is completed, the more effectively afforestation will minimise gully erosion.