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Investigations and reports
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This page brings together technical studies, Council reports, community presentations as well as other useful resources that can help you to learn more about debris flood hazards management and planning.
Note: 'debris flow' is a generic term for this hazard, the latest (2025) risk assessment specifically looks at the likelihood of debris flood in the Roxburgh area.
Several technical studies and works (led by ORC and technical consultants) have been completed to help better understanding the nature and characteristics of alluvial fans associated natural hazards including debris flood in Roxburgh area.
These studies reports do not give decisions but are useful information resources that help inform the councils, stakeholders and the community to undertake appropriate actions as well as to decide for next steps to manage the hazards effectively both now and in the future.
Please also let us know:
- If you think the reports have missed anything?
- Do you have any questions, or is any information unclear?
- Do you agree with the expert assessments – if not, why not?
- Which of the approaches discussed in the report do you think should be investigated further – and why?
Detailed debris flood hazard and risk assessment 2025:
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- The study assesses comprehensively and technically debris flood hazards and associated risks.
- Report: It’s expected to be available in mid-2026.
Plain language of the Detailed debris flood hazard and risk assessment 2025
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- This report summarises key findings of the Detailed debris flow hazard and risk assessment 2025 report and uses plain language to help understand the findings.
- Report: It’s expected to be available in mid-2026.
Geomorphological assessment:
Engineering options:
Managing ongoing debris flow risk in Roxburgh:
These alluvial fans projects were implemented between 2007- 2009 to map all alluvial fans (larger than 0.5 km2) in the Otago region using GIS (Geographic Information System). Each fan was classified as active or inactive, and by its main depositional process: floodwater, debris or a mix of both.
June 2011
Richard Woods, Natural Hazards Analyst
Otago Regional Council
PDF | 25 MB
An alluvial fan is an accumulation of river or stream (alluvial) sediments that form a sloping landform, shaped like an open fan or a segment of a cone. The principal hazards on alluvial fans are inundation by flood water, debris-flow and debris-flood deposits, channel migration, deposition and erosion. The unpredictable and variable nature of alluvial- fan hazards means that they are potentially very hazardous. Despite that, their elevated profile, with good drainage, makes them attractive places for people to live. The report focuses on 11 active fans with both high hazard potential and existing or planned development, detailing their characteristics and associated risks for communities including Roxburgh.
April 2009
D.J.A.Barrell, S.C. Cox, S. Greene, D.B. Townsend
PDF | 32 MB
This report assesses the nature, extent and hazards of alluvial fans across the region including Roxburgh to inform community safety and land-use planning.
March 2009
Opus International Consultants Ltd
1205 – Version 2
PDF | 7 MB
This report explains what alluvial fans are, exemplifies the different types of alluvial fan that occur in Otago, shows how alluvial fans can be identified. It also discusses the hazards posed to Otago communities from alluvial fans and outlines some of the ways that the risk can be mitigated.
September 2025
Ann Conroy (Team Leader Natural Hazards Adaptation), Jean-Luc Payan (Manager Natural Hazards), Julion Wright (Natural Hazards Analyst), Toan Nguyen (Senior Natural Hazards Adaptation Specialist), Tim van Woerden (Senior Natural Hazards Analyst), Pete Weir (Team Leader Natural Hazards)
PDF | 8 MB
Report updating the Science and Resilience committee on the Roxburgh Debris Flow Hazard Management Programme progress, engagement and programme planning.
November 2024
PDF | 11 MB
Updates Council on progress with the Roxburgh debris flows hazard management programme.
September 2019
EHS1860
PDF | 7 MB
Includes updates on Lake Hayes consultation, Stock Truck Effluent Disposal (STED) Facilities, Executive Leadership Team rural community outreach, Roxburgh debris flow update, and Leith Flood Protection Scheme
November 2017
PDF | 3 MB
Summarises observations and preliminary assessments following the debris flow event and flooding and describes the alluvial fan hazards in the area. Prepared for: Technical Committee | Activity: Safety & Hazards - Natural Hazards | Prepared by: Dr Jean-Luc Payan, Manager Natural Hazards; Dr Ben Mackey, Natural Hazards Analyst. Flooding caused by the 26 November 2017 intense thunderstorm in the Roxburgh Township area resulted in extensive sedimentation in the Township. The purpose of this report is to present the observations and preliminary assessments made after the event and associated flooding. The report also describes the alluvial fan hazards for this area.
