Publications

The Manukau Harbour is located on the West Coast of New Zealand’s North Island in the Auckland Region. It has been identified as a potential greenfield port location to replace the cargo operation of the Ports of Auckland, which is located on the East Coast within the Waitematā Harbour. Exposure to large and persistent swell from the Southern Ocean and Tasman Sea, high tidal range, fast tidal currents, and a very large and shallow ebb tide delta at the Manukau Harbour entrance present several challenges to opening and maintaining a navigation channel in this location. Show more…To test the feasibility of the proposal, an in-depth understanding of the ebb tide delta dynamics and sediment transport mechanisms was needed. Using data collected from the field, satellite imagery, and a combined analytical and numerical modelling approach we have estimated capital dredge volumes and importantly rates of sediment infill to the channel at differing temporal scales from individual storms to annual rates and decadal evolution. It has been shown that the ebb tide delta has a rotational morphology over an approximate 20 to 25-year cycle which can affect the capital dredge volumes by up to 10M m3 depending on the position of the channel and banks within the cycle. A net movement of sediment from the south to the north through longshore processes have been shown to contribute 1.5M m3 / year to the system, however, it is the sediment recycled around the ebb tide delta through wave and tidal flows that look to govern the maintenance dredge requirement and therefore the viability of the harbour serving as a port location. Work is still progressing to finalise the technical feasibility of this. Show less…

Many state-of-practice methods for predicting liquefaction manifestation, such as the Liquefaction Severity Number (LSN), are known to suffer from significant overprediction in regions characterised by complex soil profiles comprising interbedded sands, silts, and clays. These methods typically analyse discrete soil layers, and sum results layer-by-layer, which does not properly account for system effectsa. Show more…We demonstrate that machine learning techniques can be used to identify cases where simplified layer-bylayer liquefaction vulnerability indices give overestimated surficial liquefaction manifestation. Specifically, we have developed a convolutional neural network model. With the aim of better accounting for system effects, the model considers the full length of a CPT profile simultaneously to capture system effects of soil profiles such as interbedding, rather than processing and then aggregating information in discrete layers.
A database of over 47,000 case histories was used for training and model evaluation, spanning ten New Zealand earthquakes. Special techniques were used to address sampling bias and class imbalance. Finally, an adjustment procedure is proposed, which uses the machine learning model to improve the accuracy of LSN for specific site categories, resulting in significant accuracy improvements.
This research has been funded by Toka Tū Ake EQC to advance liquefaction science, and support a range of applications in New Zealand, including local government planning, public engagement and education, and loss modelling. Show less…

Variable or unknown ground conditions are a major contributor to uncertainty in a geotechnical project and arise as a result of the geomorphological processes that shaped the land. The planning of a ground investigation programme can be thought of as a sampling optimisation problem, seeking to balance precision, accuracy, and costs through selection of an investigation density that achieves a degree of residual uncertainty that is sufficient for the intended purpose. Show more…This study draws on detailed observations of the patterns of liquefaction-induced land damage mapped across Christchurch City during the 2010 – 2011 Canterbury Earthquakes, and uses the variability in the observed damage as a proxy for variability in the underlying shallow ground conditions. The correlation between investigation density and residual uncertainty in ground conditions is examined using a Monte Carlo approach, varying size of the study area and the investigation spacing. The hypothesis is that as the total area investigated increases, the spacing between investigations can be increased while still achieving the same degree of residual uncertainty in the ground conditions. The results of this study were used to set the minimum investigation density recommendations for land use planning and subdivision in the New Zealand guidance on geotechnical investigations for earthquake engineering. Show less…

The Resource Management Act 1991 (RMA) protects wetland and river features through section 13. How these features are defined depends on which piece of legislation is relied upon, with slightly different definitions included in the RMA, the National Policy Statement for Freshwater Management 2020 (NPS FM) and Regional Plans. While many of these definitions are similar, small nuances can wreak havoc – both in what to call something and subsequently what the consenting implications are. Show more…Since the introduction of the NPS FM, defining these features has become a hot topic, mostly, in our opinion, because of the prohibited activity status applied to many activities affecting natural inland wetlands in the original version of the National Environmental Standards for Freshwater (NES F) (September 2020).
Consequently (and despite having been subject to protection in some form since 1991), delineation of wetlands and whether something was a stock-pugged river or a degraded wetland became a point of contention. ‘Is it a wetland’ has become as triggering to some as the concept of nuclear power (of note as it is one of the widely recognised prohibited activities in Aotearoa New Zealand).
One theme all these definitions have in common is an implied state of mutual exclusivity and stationarity – that if something is a river, it can’t be a wetland (and vice-versa) and that they will remain static through time. What these definitions, policies and rules fail to appreciate is that, in reality, rivers and wetlands are aquatic ecosystems that exist on a continuum of form and function.
From a scientific point of view, aquatic systems have significantly more nuanced characters, behaviours and values (e.g. types such as intermittent, perennial, wandering, braided, valley fill, marsh, swamp…). How these systems behave is driven not by a definition in a piece of legislation, but rather by the prevailing landscape and ecological controls. River and wetland systems may fluctuate between different forms and states depending on numerous climatic, hydraulic and landscape factors.
Policy aside, it is the science, form, and function of these aquatic systems that tells us what we need to do to maintain their ecological and morphological integrity, not the definition introduced by the government of the day. Understanding the character and behavior of these systems in their landscape context is essential to inform decision making, ensuring that water sensitive urban design and nature-based solutions are implemented in a way that respects the processes and functions of these systems, and ensures a measure of success.
Within this paper we describe the science behind the characterisation of these aquatic systems, reflecting on their ecological and morphological function, and what this means for designing nature-based stormwater and flood management solutions that work with the landscape and ecological processes and controls. We explore the implications of the statutory framework and what this means in practice in respect of stormwater management. Lastly, we highlight how wetlands and streams provide an opportunity to integrate ecological, amenity, stormwater and flood management outcomes, while enabling future development. Show less…

Cycline Gabrielle severly damaged a large portion of the Palmerston North to Gisborne line (PNGL). The damage included large embankment washout failures, blocked/damaged culverts, underslips/overslips, scour damage to tracks, loss/reduction of drainage, silt inundation and structural damage to bridges/tunnels. Show more…Tonkin & Taylor Ltd were engaged by KiwiRail to provide specific geotechnical field support during the initial ‘Make Safe’ works on the Napier to Wairoa section of the PNGL.
The premise of the ‘Make Safe’ works is to undertake works that reduce the likelihood of further damage and deterioration of the line under normal annual rainfall conditions and across the next two winters. This was considered to be a suitable timeframe to allow the ‘Make Safe’ requirements to still be satisfied based on the level of engineering being undertaken across the line. Importantly, the focus was not necessarily to reinstate the track for train movements but rather to allow , where possible, rail enabled plant to access sections of the line safely. Show less…