A sediment budget for the Heruheru basin, Mohaka Forest

Abstract

The sediment budget approach was used to quantify the geomorphic effectiveness of two rainstorms in early 1996 within the 400ha Heruheru basin, Hawkes Bay. The volume of sediment generated, stored and discharged from the basin was assessed. These events generated approximately 59,000m3 of sediment and 85% of this sediment was discharged from the basin. Debris flows generated 83% of the sediment eroded from the slopes. Secondary processes operating since the rainstorm events have had minor impact being significant only on upper road cuts. Determining the impact of the two rainstorms on soil loss was complicated by the presence of large volumes of forestry "slash" scattered across the slopes and left on the hauler sites. Modification of the slopes for forestry land use and the impacts of clearfell harvest are assessed and the more heavily modified landforms generated a disproportionate amount of sediment. The two rainstorms have had a major influence on land use and the future viability of forestry within the basin by reducing the plantable area of the basin. This is exacerbated by the low volume of sediment storage remaining on the slopes. The findings of this study have major implications for land management throughout the East Coast of the North Island as many acres of marginal land are put under forestry.