Stream channel impacts from extreme events, Mohaka Forest, Hawkes Bay

Abstract

A severe landsliding episode occurred in Mohaka Forest in 1996. Data from the worst affected catchment allowed the development of a model of the controls on the type, and magnitude, of impact on the stream channel. It also allowed the identification of when and where regolith failures occur. The slope failures caused diverse effects on the stream channel system. Impacts ranged from complete scouring of all bed and riparian sediments, to channel infilling with debris. The subcatchments within the basin each responded differently and with different magnitudes. These impacts have altered the timing and volume of subsequent storm events. A classification system was developed for the landslides with respect to their erosional and depositional impacts on the stream channels. Field survey and aerial photographic interpretation were used to map the distribution of landslides and to classify the slides on the basis of their impact. The various classes were then analysed using a Geographic Information System. Analysis showed that it was the topographic features that control the moisture content of the soil which also affected the magnitude and type of impact. These features included; flow accumulation, curvature, regolith thickness, and aspect. Other topographic features such as; gradient, channel morphology, angle at which the landslide track intersects the channel, and logging operations affected the impact by controlling the velocity and nature of material involved in the failure. Topographic indices emphasised the importance of moisture content in controlling differences between landslide classes. A comparison of the hydrological characteristics of a number of monitored subcatchments has shown significant differences in the timing and amount of runoff generated after the landsliding. The subcatchments characterised by reamed slopes and scoured channels, exhibited; sharp 'peaked' flood hydrographs with short lag times; a greater frequency of small flood events; a lower and a faster return to baseflow; and a more variable flow. Subcatchments characterised by the deposition of material, either on the slopes or in the channel, showed an increase in the water detention capacity, which delayed and reduced stormflow, moderated the flow regime, and increased baseflow. Large temporal and spatial variations in water quality have occurred as a result of landsliding. During extreme low flows, seasonal and diurnal variations of temperature dependant parameters (e.g., temperature, pH, dissolved oxygen) were most pronounced in the landslide effected subcatchments. Generally water quality decreased during wet weather because of elevated suspended sediment concentration from the numerous sediment sources. Unaffected subcatchments showed improved water quality as a result of dilution during wet weather. The channel impacts may lead to future changes in channel morphology, hydrological parameters, and water quality characteristics. These will be a response to changes in sediment supply and discharge regime. Increases in sediment storage by debris jams and sediment infills, and the accelerated weathering of bedrock, will supply large amounts of sediment downstream during storm events. This will have adverse effects on sediment related water quality parameters. A change in channel geometry may occur as a result of the altered flow regime and decrease in sediment supply between storm events.