Karst hydrology and solution in the Waitomo district, New Zealand

Abstract

The research described in this thesis is the first detailed investigation of karst hydrology and solution in the New Zealand Oligocene limestones. The study area, which is located 7 km west of Waitomo Caves, lies within the most extensive limestone area in New Zealand, the King Country karst. Within this area the landscape is dominated by closed depressions (dolines), which, when delimited on the basis of their topographic divides, form a cellular polygonal karst network. The evolution of the Waitomo karst is examined by considering the physical and spatial relationships between surface landforms and subterranean landforms and drainage. The results of these investigations are used to amplify previous research on the karstification of drainage and to develop an evolutionary model which is similar in some respects to the descriptive, sequential, model of interstratal karstification proposed by Cvijic (1918). The hydrological function of solution dolines is examined and a model of depression hydrology incorporating six flow components (overland flow, interflow, subcutaneous flow, shaft flow, vadose trickles and vadose seepage) is proposed. This model is shown to be useful in interpreting the hydrology of depressions in the Waitomo polygonal karst and it is suggested that it may be adopted as a general model for the hydrology of solution dolines and cockpit depressions. The small experimental basin approach and the analytical techniques of surface hydrology are adopted as a basis for an examination of the discharge characteristics of subterranean streamflow. Two instrumented basins, one under pasture (Glenfield) and one under native forest (Cymru) are examined and compared and it is shown that stormflows and peak discharges are higher in the forested basin, although the overall storage per unit area is lower. These differences are attributed to land clearance operations which partially blocked many depression outlets in the Glenfield basin. The overall hydrological behaviour of the two study streams is shown to be comparable to that of small non-karst surface streams in the eastern United States, Devon and British Columbia. Simple parametric models for the prediction of the major output characteristics (mean daily discharge, stormflow and peak flow magnitude) from the precipitation input are also developed. Limestone solution in the two experimental basins is established by estimation of inputs, throughputs, and outputs of water and dissolved calcium and magnesium. The best estimates of the solutional erosion rate are 69+17.8 -7.3m3/km2/y for the Cymru basin and 69+18.0 -7.2m3/km2/y for the Glenfield basin. In both basins 67 per cent of the annual load is transported by flows greater than the mean, 18.5 per cent (Cymru) and 15.2 per cent (Glenfield) being transported by flood flows equalled or exceeded only 5 per cent of the time. Almost half of the annual solution takes place during the three winter months (June-August), but no one month accounts for more than 18 per cent or less than 2.75 per cent of the annual total. Approximately 37 per cent of the solution occurs on the surface or within the soil profile and the remainder is concentrated in 5-10 m of weathered bedrock (the subcutaneous zone) beneath this. Thus, it is likely that at least 85 per cent and possibly 90 per cent of the total solutional erosion contributes to the surface lowering of soil and bedrock.