Fluvial processes in the upper Waitomo catchment: a study in applied karst hydrology and geomorphology

Abstract

This dissertation presents the results of research undertaken on the fluvial processes occurring in a drainage basin containing similar proportions of calcareous and non-calcareous lithologies, a combination which has previously received little attention. The catchment selected for study, the Waitomo drainage basin in the King Country of the North Island of New Zealand, may be taken as typical of those which occur in the region, and affords the opportunity to apply the results of the study in the formulation of stratagems for the management of both the catchment and the Waitomo Glowworm Cave, a major tourist resource through which the Waitomo Stream flows. The fluvial processes operating in the catchment are discussed by an examination of the hydrologic regime of the basin and by a consideration of the inter-relationships between the storm responses of each lithological grouping in the production of the composite response of the basin. Attention is devoted to the application of hydrograph separation techniques as a tool enabling characterization of the storm response of drainage basins, and modifications to the technique are described which ensure a greater reliability in the results achieved by the use of these techniques. The analysis demonstrates that the response of the stream to storm events may be reliably predicted from a consideration of the relative areas and distributions of component lithologies within the catchment. The storm response of the Waitomo stream is seen to be dominated by the rapid runoff components of the non-calcareous (relatively impermeable) lithologies, while the delayed flow response is dominated by runoff from the calcareous materials of the basin. Attention is further devoted to the transport of materials by the stream, the date showing two features worthy of note. The transport of materials in suspension by the stream demonstrates the influence of variation both in the transporting power of the stream and in the supply of transportable materials to it, resulting in the occurrence of hysteresis effects in the relationship between suspended sediment concentration and discharge. Similar effects are also demonstrated by the relationship between solute concentration and discharge. These data enable an assessment to be made of the erosional state of the catchment, indicating that it is presently experiencing only mildly accelerated conditions. Critical to the operation of the Waitomo stream is the role of the Glowworm Cave in modifying the flow of water from the catchment. This study therefore presents an examination of the processes of sedimentation occurring within the cave system and of the hydraulics of operation of the sumped (piped) portions of the stream channel within the cave. The results indicate that erosion and sedimentation of the alluvial bed of the stream are influence by the morphologic characteristics of the channel, and that the flow is not impeded by the sumps since their cross-sectional area responds rapidly to variation in discharge and the velocity of flow. Finally the results of the analyses conducted in the course of this study permit the formulation of proposals to enable management of both the catchment and the Glowworm Cave in a manner which does not increase the volume of materials transported by the stream or impede its flow through the basin's outlet, the Waitomo Glowworm Cave.