Mike Page

Magnitude and frequency of landsliding in a large New Zealand catchment

Knowledge of long-term average rates of erosion is necessary if factors affecting sediment yields from catchments are to be understood. Without such information, it is not possible to assess the potential influence of extreme storms, and, therefore, to evaluate the relative importance of various components of a sediment budget. A study of the sediment budget for the Waipaoa catchment, North Island, New Zealand, included evaluation of long-term rates of landsliding for six landslide-prone land systems in the catchment. The number of landslides per unit area generated by each of several storms was counted on sequential aerial photographs and correlated with the magnitude of the corresponding storm. The resulting relationships were combined with magnitude–frequency relationships derived for storms from 70- to 100-year rainfall records in the area to estimate a long-term magnitude–frequency relationship for landsliding for each land system. The long-term average values of the areal landslide frequency (number of slides per unit area per unit time) were then calculated from these relationships. The volumes of a sample of landslide scars were measured in the field, and the proportion of slides that deliver sediment to channels was determined from aerial photographs. These measurements then allowed calculation of the long-term average rate of sediment production to streams from landslides for different land systems and types of vegetation. Results suggest that shallow landslides currently contribute about 15±5% of the suspended sediment load in the Waipaoa River above the Kanakanaia gauging station, and that 75% of the sediment production from the landslides occurs during storms with recurrence intervals of less than 27 years. Reforestation of 6.3% (93 km2) of the slide-prone lands in the catchment between 1990 and 1995 resulted in a calculated decrease in slide-derived sediment of 10%. Calculations suggest that reforestation of an additional 3% (66 km2) of the catchment in areas with the most sensitive combinations of land system and storm regime could decrease the total sediment inputs from landsliding by about 20%.

Sediment budget to assess the geomorphic effect of a cyclonic storm, New Zealand

Abstract A short-term sediment budget was constructed to assess the erosion response of a soft-rock hill country watershed to an intense rainstorm event. The watershed is located in a landslide-prone area on the east coast of the North Island, New Zealand. During March 1988, 753 mm of rain was recorded over a four-day period with 320 mm and 329 mm on successive days. Known as Cyclone Bola, this was the largest rainstorm event in the Tutira watershed in the 93-year rainfall record. The budget quantifies the total sediment generated during the storm, the relative contribution of erosion processes involved, the amount of sediment held in storage and the amount discharged into two lakes within the watershed. A landform map of the watershed was constructed and the contribution of each landform type to the total budget was calculated. A total of 1.35 (±0.13) million m3 of sediment was generated during the storm at an average of 420 m3/ha. This is equivalent to a denudation value of 42 mm for the total watershed (3208 ha) and 83 mm for the landforms severely affected by landsliding (1427 ha). Of the sediment generated, 21% remained on hillslopes, 22% was deposited on valley floors, 51% was deposited on the lakebeds and the remaining 6% was discharged from the watershed via the lake outlet. Most of the sediment generated during the storm was from primary source areas on hill slopes, with sediment in secondary storage providing only a small contribution. Landslide erosion was the main process, accounting for 89% of the sediment generated. Channel, tunnel gully and sheet erosion were only minor contributors to the budget. Six hillslope landforms, which occupy only 44% of the watershed generated 90% of the sediment. The results of this sediment budget, when put in context with the storm magnitude-frequency history being analysed from lake cores, contribute to the identification of sustainable land use and management of soft-rock hill country.

Palaeoclimatic implications of a storm erosion record from late Holocene lake sediments, North Island, New Zealand

Abstract A ca. 2250 year storm history has been identified from a high-resolution lake sediment record from the east coast of the North Island. New Zealand. This event-based chronology identifies 340 storms in the pre-European record. The record is largely one of natural variability without human impacts. Layers of minerogenic sediment representing the products of individual storm events are clearly visible in lake cores. These storm sediment pulses, derived mainly from landslides, record the frequency and magnitude of storms. Clusters of pulses identify six periods of high sedimentation and associated erosion before European settlement (A.D. 1878). At least five of these are interpreted as periods of higher rainfall and probably warmer temperatures, since historic records show that most large storms are derived from the subtropics/tropics. Most of the storm periods correspond to warm climate intervals previously identified from New Zealand and from Southern Hemisphere palaeoclimatic evidence. The Mapara 2 period (ca. 2090-1855 cal. yr B.P.) was the stormiest. Other periods were of shorter duration. The Mapara 2 period occurred at times of sustained warmth in the Tasmanian and Chilean tree-ring records, which might suggest hemispheric warming at this time. One storm period (Burrell), which occurred during the historical ENSO record, was at a time of moderately high ENSO activity. Three earlier periods also appear to show responses to ENSO. Two storm periods correspond to previously proposed New Zealand-wide periods of increased erosion and sedimentation. Estimates of palaeo-storm rainfalls from relationships between storm rainfall and sediment thickness of historical events suggest that storm pulses ca. 12 mm thick represent palaeo-storms of ca. 450 mm rainfall.

A multiple-source Holocene tephra sequence from Lake Tutira, Hawke's Bay, New Zealand

Abstract A core drilled in a swamp at the northern end of Lake Tutira, northern Hawkes Bay, New Zealand, as part of a programme to determine the erosion history of the Lake Tutira catchment, contains 14 tephra layers (10 rhyolitic and 4 andesitic). The mineralogy of each layer was determined, together with electron microprobe chemistry of glass shards, and of hornblendes from two of the andesitic tephras. The rhyolitic layers are identified from their stratigraphic position, mineralogy, and glass/mineral chemistry as: Taupo (1850 conventional radiocarbon years old), Mapara (2160 yr), Whakaipo (2685 yr), Waimihia (3280 yr), Hinemaiaia (4510 yr), Whakatane (4830 yr), and Motutere (5430 yr), but three additional rhyolitic layers (aged c. 3700, 4100, and 4300 yr) could not be correlated with known tephras. Two are possibly reworked layers, but one (c. 4300 yr) may represent a new tephra which has not been previously recognised. The mineralogy of three of the andesitic tephras (aged c. 3100, 4900, and 6000 yr...

Self-organized criticality in layered, lacustrine sediments formed by landsliding

Landsliding is the dominant mass-wasting process in humid-temperate uplands and an important regulator of sediment yield from steep-land drainage basins. Information about the magnitude and frequency distribution of landslides has been derived from aerial photography, but it has proved difficult to set limits on the long-term scaling behavior of landsliding because the requirements of spatial and temporal coherence and the large number of observations necessary to undertake magnitude versus frequency analyses are not easy to fulfill. We use a 2250-yr-long record of hillslope erosion associated with extreme hydrologic events preserved in sediments from Lake Tutira, New Zealand, to investigate scaling in landslide deposits. Both the magnitude versus frequency distribution of sediment layers attributed to landsliding and the distribution of time intervals between landsliding events take the form of power laws, the former with an exponent b = 2.06 and the latter with an exponent b = 1.4. These results suggest that the erosional events originate from a self-organized critical process, and are in agreement with observations of scaling in turbidite deposits and grain flows in controlled laboratory experiments. The implications are that the aggregate behavior of landsliding at the catchment scale is orderly and that the stratigraphic record preserves a unique, long-term perspective on a fundamental geomorphic process and the extreme hydrologic events that trigger it.

13 Changes in basin-scale sediment supply and transfer in a rapidly transformed New Zealand landscape

Abstract Society has an ever-increasing need to manage landscapes. To do this effectively requires improved understanding of the way landscapes behave, and the controls on that behaviour. This is certainly the case where sustainable resource use and hazard mitigation involve the management of the generation, transport, and storage of sediment. Landscapes are complex systems, consisting of a mosaic of landforms. At the broad regional level these landforms are arranged in characteristic patterns, reflecting environmental conditions and associated processes. At the catchment level, assemblages of landforms have a unique configuration, forming an interactive functioning system through which water and sediment are passed. It is this unique, catchment-based assemblage that we seek to manage. Controls on the way landscapes behave are numerous and operate at a variety of scales both spatial and temporal. The way these controls interact on a complex and unique arrangement of landforms is difficult to predict. The East Coast of the North Island of New Zealand is a dynamic landscape. High natural erosion rates have been augmented by recent and rapid anthropogenic activity. Several studies in the Waiapu catchment, involving a range of spatial and temporal scales, are used here to illustrate the impact of natural and anthropogenic controls on basin-scale sediment supply and transfer. In this landscape, the use of vegetation, specifically targeted reforestation, is the most effective method of sediment management. This will be enhanced by improved understanding of stability thresholds and hill slope–channel connectivity.