Hydrology and geomorphology of the Paparoa karst, North Westland, New Zealand

Abstract

The Paparoa karst covers an area of approximately 130 km2 in the north-west of the South Island of New Zealand. The region is characterised by a mild climate, and frequent and intense rainfalls in all seasons. Most of the karst retains its original rainforest cover, which together with the climate and well-jointed, relatively pure limestones, has resulted in the development of a distinctive karst landscape. This study attempts to provide the first detailed investigation of the salient aspects of the regions karst hydrology and geomorphology. Geological structure has had a major influence on karst development, with the asymmetrical Barrytown Syncline dividing the karst into three distinctive morphological units. On the shallow dipping western limb of the syncline, an extensive area of polygonal karst has developed. The polygonal karst exhibits a strong fluvial character, and does not appear to possess the uniform dispersion pattern evident in the wet tropics or other New Zealand examples. This is considered to be the result of both comparative immaturity and the exacerbation of fluvial incision by a strong joint pattern. Karst development on the eastern limb of the Barrytown Syncline has been significantly retarded by steep slopes. Surficial karst development is minimal and is largely restricted to a few solution runnels and/or grikes oriented downslope. Nevertheless, drainage is predominantly karstic, with springs emerging along the base of the slope. An area of interstratal karst has developed adjacent to the karst plateau, and parallel to the axis of the syncline. Dye tracing has revealed the existence of major strike-oriented trunk drainage systems underlying the interstratal karst. Water balance estimates suggest that these trunk drainage systems are the focus for much of the subterranean drainage from the karst plateau. The largest trunk drainage system is that feeding the Cave Creek springs. Catchment delineation shows that recharge of the Bullock Creek karst drainage system is complex. Although a number of autogenic and allogenic sources are identified, Bullock Creek is the dominant source of recharge and a major influence on spring flow behaviour. Specific discharges of over 5 m3 s-1 km2 from the 29 km2 catchment were recorded during this study. The capacity of the streamsink zone is estimated to be 15 to 20 m3 s-1. Initiation of the Bullock Creek karst drainage system probably began during the Oturian interglacial. The subsequent capture of the highly dynamic, alpine-sourced Bullock Creek permitted the rapid development of conduit drainage. The hydrological function of the Bullock Creek karst drainage system has been strongly influenced by the effects of, firstly, the landslide damming of Bullock Creek and subsequent formation of the Bullock Creek polje, and secondly, by the periodic infilling of streamsinks by alluvial sedimentation. Uranium series dating of speleothems suggests that infilling was related to proglacial increases in the sediment load of Bullock Creek during the Otiran glaciation. The complementary application of direct observation, measurements of physical hydrology and water quality, artificial tracing, pulse-wave analysis and conduit hydraulics has revealed a conduit system whose structure and function significantly reflects rapid recharge processes, and the highly dynamic and regular flooding regime. Tracer recovery patterns reveal an integrated conduit network receiving recharge from numerous perennial and intermittent tributaries, and discharging to a spring hierarchy in Cave Creek. Status within the spring hierarchy is demonstrated by behaviour as overflow, overflow-underflow, or underflow springs. Peak collective spring discharge is estimated to be in the order of 30-40 m3 s-1, comparable to, and possibly larger than the discharge from New Zealand’s largest known spring. Flow in the conduit network is rapid, ranging from 50 to 680 m hr-1. Pulse wave analysis and repeated tracing of various segments of the conduit system has revealed the existence of both phreatic and vadose elements, with considerable dynamic storage associated with the latter. A simple conduit model is proposed which provides a best-fit to the data. A surprising result of the pulse wave analysis is that flood wave celerity in the vadose segment decreases with increasing discharge. This is interpreted as overflow into a wide, hydraulically rough, bedding plane passage. Data are presented which indicate substantial in-line transient storage within the conduit system, especially under low-flow conditions. Data also indicate that exchange of tracer occurs between the conduit and the adjacent fissured zone. A model is presented which allows for the discrimination of releases of tracer from these two contrasting zones. Attention is drawn to limitations in a number of investigative techniques, in particular those which seek to establish parameter relationships with discharge, commonly used to elucidate conduit structure and function. This study demonstrates the interpretative advantages which can be gained through the concurrent application of multiple techniques.