Mike Marden

Gully erosion in Mangatu Forest, New Zealand, estimated from digital elevation models

The methodology and errors involved in determining the amount of sediment produced during two (19·5 and 33·2 year) periods by 11 (c. 0·01 − >0·20 km2) gullies within a 4 km2 area in the headwaters of the Waipaoa River basin, New Zealand, using sequential digital elevation models are described. Sediment production from all gullies within the study area was 0·99 ± 0·03 × 106 t a−1 (2480 ± 80 t ha−1 a−1) during the period from 1939 to 1958. It declined to 0·62 ± 0·02 × 106 t a−1 (1550 ± 50 t ha−1 a−1) during the period from 1958 to 1992, when many of the smaller gullies were stabilized by a programme of afforestation, which commenced in 1960. Both figures are very high by global standards. The two largest (the Tarndale and Mangatu) gully complexes together generated 73 and 95 per cent of the sediment in the specified time periods, but the latter amount is equivalent to only c. 5 per cent of the total annual sediment load of the Waipaoa River.

Gully erosion and sediment production: Te Weraroa Stream, New Zealand

conversion to pasture early in the twentieth century, was ameliorated by reforestation that commenced in 1962. Estimates of sediment production were made using the change in gully area observed in sequential aerial photographs. Channel storage was assessed from stream cross-section surveys. At its peak, gully erosion affected � 6% of the total catchment area. The amount of sediment contributed from gullies declined by 62% as the forest became established, but of the 28.7 Mt of sediment generated by gully erosion between 1950 and 1988, 48% was stored in the channel along the lower 8 km of Te Weraroa Stream. Even if the amount of sediment generated by gully erosion continues to decline, it likely will be many decades before the gravel is released from storage. INDEX TERMS: 1815 Hydrology: Erosion and sedimentation; 1824 Hydrology: Geomorphology (1625); 1878 Hydrology: Water/ energy interactions; KEYWORDS: channel storage, gully erosion, sediment production Citation: Gomez, B., K. Banbury, M. Marden, N. A. Trustrum, D. H. Peacock, and P. J. Hoskin, Gully erosion and sediment production: Te Weraroa Stream, New Zealand, Water Resour. Res., 39(7), 1187, doi:10.1029/2002WR001342, 2003.

Stabilising characteristics of New Zealand indigenous riparian colonising plants

This paper presents selected results on the above- and below-ground growth performance of twelve indigenous woody species commonly found growing naturally in unstable riparian slope and/or bank environments throughout New Zealand. This study was needed because little information exists on the effectiveness of New Zealand’s indigenous riparian plant species for slope and stream bank stabilisation. By examining the growth performance of selected riparian species during the first 5 years following establishment, we provide valuable insights into the likely strengths and limitations of individual species at maturity and, therefore, into their overall potential usefulness, singularly and/or as mixed plantings, for future riparian stabilisation projects. For all species, their root systems are typically shallow and confined to the uppermost 31 cm of soil. Root spread (mean maximum diameter) increased with increasing age with interspecies differences, by age 5 years, ranging from between ~1 and 2.5 m. At age 5 years the mean root biomass, for all species combined, was 1.2 kg/plant, and averaged ~23% of total plant biomass. Changes in the allocation of biomass for root and shoot growth appear to be species and age dependent. The results of this study indicate that most have above- and below-ground growth attributes well suited to colonising steep and unstable riparian slopes where shallow soil failure is prevalent and/or where stream banks are rocky with skeletal soils. All form part of the early plant succession. Once established, and in the absence of grazing, they are relatively fast growing. The effectiveness of riparian restoration programmes using indigenous species, though potentially high for low-order stream, will be limited by their relatively shallow-rooted habit for bank stabilisation on larger rivers without the prior installation of structural protection works.

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.

Root site occupancy modelling of young New Zealand native plants: implications for soil reinforcement

Plants are widely used in soil conservation to control and prevent erosion on hillslopes and on riverbanks. Previous research has shown the mechanical root reinforcement on soil stability can be considerable. However, land and forest managers still require information and simple tools to enable them to determine how and when a species becomes effective in terms of soil stabilisation. This paper uses root length data from a trial of young New Zealand trees and shrubs to develop a simple model to account for the spatial occupancy of a planting site by roots, and by implication their potential strength contribution to soil reinforcement. It is developed by calculating root surface area in contact with the soil to obtain an effective radius of the root spread about the stem. The approach generates a set of coefficients that are unique to a species for a given site which can then be used in the generalised model to predict root site occupancy, which is taken as a proxy for when soil reinforcement is attained. This information can then be used to assess effectiveness of different species mixes in planting plans.

Foraminiferal record of Holocene paleo-earthquakes on the subsiding south-western Poverty Bay coastline, New Zealand

Foraminiferal faunas in 29 short cores (maximum depth 7 m) of estuarine and coastal wetland sediment were used to reconstruct the middle–late Holocene (last 7 ka) elevational history on the southern shores of Poverty Bay, North Island, New Zealand. This coast is on the southwest side of a rapidly subsiding area beneath western Poverty Bay. Modern Analogue Technique paleo-elevation estimates based on fossil foraminiferal faunas indicate that the four study areas have gradual late Holocene (<3.5 ka) subsidence rates that increase from the southwest (mean c. 0.5 m ka–1) to northeast (mean c. 1.0 m ka–1). Only two rapid, possibly co-seismic, vertical displacement events are recognised: (1) c. 1.2 m of subsidence at 5.7 ± 0.4 ka (cal yr BP), which may have been generated by a subduction interface earthquake centred offshore and recorded in other published studies in northern Hawkes Bay, c. 35 km to the south; and (2) c. 1 m of uplift (relative sea-level fall) at c. 4.5 ± 0.3 ka, which might have been generated by rupture on an offshore upper plate fault that also uplifted coastal terraces at Pakarae and Mahia, 40 km to the north and south of the study area, or by rupture on the subduction interface penetrating beneath Poverty Bay. No sudden displacement events are recognised during the last 4 ka although subsidence, possibly aseismic, has continued.

Development of a landslide component for a sediment budget model

Most erosion models focus on overland-flow erosion with fewer incorporating landslide erosion although it is common on hillslopes. Landslide models are typically dynamic, spatially distributed simulations with large data requirements for parameterisation and are often computationally intensive. The Australian SedNet model represents a middle ground between process-based and empirical models and is being modified for New Zealand conditions by incorporating shallow landsliding.We describe a method for implementing a model within SedNetNZ to provide the long-term annual average sediment contribution from shallow landsliding and its spatial distribution. The mass of soil eroded over a defined period is calculated from the landslide probability for each slope class, slope class area, failure depth, soil bulk density, and sediment delivery ratio. Landslide probability is derived from mapping a time series of landslides intersected with DEM-derived slope. The conceptual approach and methodology for parameterisation are suitable for landslide modelling where rainfall-triggered shallow landslides occur. A method for modelling long-term shallow landsliding within a sediment budget model.Time series of historical landsliding mapped from aerial photography.Landslide-slope relationships derived from landslide distribution and slope.Landslide-slope relationships used to spatially model sediment generation.Applicable to any landscape subject to rain-triggered shallow landsliding.

Ophioliticlastic Ihungia igneous conglomerate (Early Miocene), North Island, New Zealand: evidence for an island source related to subduction initiation and deposition within a slump-generated submarine slope re-entrant

In the eastern North Island, the Early Miocene Tolaga Group contains some small isolated outcrops of distinctive basaltic–clast conglomerate and serpentine-bearing sandstone, the Ihungia igneous conglomerate. The shape, texture and composition of ultramafic to mafic gravel and sand components of the conglomerate indicate that they were likely delivered via submarine continental slope canyons from emergent island(s) on which ophiolitic peridotite, gabbro and basalt of the East Coast Allochthon were exposed. We speculatively propose that, following subduction initiation and ophiolite emplacement, and just prior to deposition of these units, eastward (downslope) displacement of part of the allochthon formed a large re-entrant in the continental slope in which the gravel subsequently accumulated. This Miocene Ihungia re-entrant has similarities to modern re-entrants along the continental shelf edge of the Raukumara Peninsula. Thus, subduction-related submarine mass transport processes may have been active along the New Zealand continental margin for the last 20 Ma. This study documents the sedimentary signature of an ophiolite likely created, uplifted and subaerially exposed in association with subduction initiation.