The risks we face

Roxburgh is known for its strong sense of community, commitment to safety and stunning natural landscapes. The orchards, rolling hills, river and surrounding ranges are all part of what makes this area unique. 

But living close to steep hills and fast-flowing streams means the Roxburgh community can face natural hazards. One of those hazards is called debris flows

Did you know?

A debris flow is like a flash flood, but it carries mud, rocks, and even broken trees, making it much more powerful. 

What are debris flows?

A debris flow is a fast-moving mix of water, mud, rocks and sometimes broken trees that rushes down hills and through stream channels. It can move quickly and with great force, causing damage to anything in its path. 

When do debris flows happen? 

Debris flows usually happen after: 

  • Very heavy rain, 
  • Sudden snowmelt, or 
  • A landslide that releases loose material into a stream.  

Quick fact

Heavy rain from thunderstorms is the most common cause of debris flows in Roxburgh. 

Local insight

Reservoir Creek, Blackjacks Creek and some other creeks have had debris flows in recent years, including the event in 2017. 

Why this matters for Roxburgh

Roxburgh sits on alluvial fans and is surrounded by steep, bare hills that erode easily. 

During heavy rain, water runs quickly down these slopes into gullies and creeks. If the slopes have loose soil or rock, the water can pick up this material and create a fast, powerful flow known as a debris flow. 

Debris flows are likely to happen in some  parts of Roxburgh, including  areas near Reservoir, Pumpstation, Golfcourse and Blackjacks Creeks. These areas have steep slopes and loose ground that can become unstable during wet weather. 

While debris flows don’t happen very often, when they do, they can cause serious damage to important services and infrastructure, buildings and businesses, and put people’s safety at risk. 

What is an alluvial fan?

To understand why debris flows are a risk in Roxburgh, it helps to know about alluvial fans.  

Alluvial fans are landforms built up over time by material washing down from the hills. The diagram below shows an alluvial fan: 

An alluvial fan is formed when: 

  • Streams and creeks flow down steep hills, they carry rocks, gravel, sand and soil. 
  • The water reaches flat land, it slows down and drops this material. 
  • Over many floods and heavy rain events, the material spreads out in a fan shape. This spread of rock and sediment what we call an alluvial fan. 

In the diagram: 

  • The red dot shows where the water flows out of the steep hills. 
  • The blue line is the stream or creek channel. 
  • The yellow fan shows where rocks and sediment have spread out over time. 

What causes debris flows in Roxburgh?

The shape of the land and the type of rock in the Old Man Range and the hills, west of the Clutha River, produce a lot of loose material. 

The creeks that run down the eastern side of the Old Man Range carry a lot of loose soil and rock, and their catchments (the areas where rainwater gathers and flows into streams or creeks) are small and steep. 

This means that during heavy rain, water can pick up this material quickly, creating powerful flows that travel downstream and cause damage to everything in their path. 

Historical debris flow events in Roxburgh area

Debris flows have been recorded in the Roxburgh area since at least 1938, some have caused major disruption. 

In the past, these events have swept into parts of the township, damaging homes, roads and important services like power and water. 

The table below summarises key debris flow events: 

Date Catchments Key triggers and impacts
1938 Slaughterhouse Creek and several adjacent catchments
  • Intense rainfall
  • 9 m high debris flow
  • 2.4 km section of SH8 inundated; orchards severely damaged
1978 Reservoir Creek, Blackjacks Creek, Pumpstation Creek, and Slaughterhouse Creek at least
  • 116 mm rain in 24 hours
  • Direct debris-flow impact to dwellings; extensive property damage at Reservoir Creek
  • Impact to SH8 likely
  • Prompted construction of the concrete chute
1993 Slaughterhouse Creek, Pumpstation Creek at least
  • Rainfall. 40-100 mm rainfall in 1 hour
  • Substantial damage to property, orchards, SH8 network (30-ton boulder deposited in middle of road), and power supply cut at Pumpstation Creek
2015 Blackjacks Creek
  • SH8 inundated with debris.
2017 Pumpstation, Reservoir, Golfcourse, Blackjacks; Stevensons Creek (no direct impact)
  • Rainfall. 40-100 mm rainfall in 1 hour
  • Damage to water and power supply, extensive inundation of SH8, property damage by debris and flooding associated with the debris flows.

What has been done to manage debris flows hazards?

Several agencies have worked together over the years to manage and reduce debris flow risks in Roxburgh.

The table below summarises the key work by Otago Regional Council (ORC), Central Otago District Council (CODC), New Zealand Transport Agency Waka Kotahi (NZTA), Civil Defence emergency Management Otago (CDEM) and other stakeholders:

Nu.

Actions and objectives

Timeframe

Responsible agency

1

Built a concrete chute along a section of Reservoir Creek.

Early 1980s

 

Otago Catchment Board

The Borough of Roxburgh

2

Conducted regional-scale alluvial fan hazards mapping with more detail for the Roxburgh area.

2009

ORC

3

Commissioned GNS Science to carry out preliminary debris flow hazard assessments.

2017

ORC

4

Conducted a series of mitigation works to remove the excess sediment deposited by, and since, the 2017 events, as well as some channel armouring. These included emergency works immediately following the 2017 events in addition to further works since the events.

2017–present

As required

ORC

5

Installed a rain gauge to monitor rainfall.

2018

ORC

6

Commissioned Golder to carry out two further investigations to provide preliminary geomorphic mapping, culvert capacity calculations, and risk-to-life estimate and a review of possible hazard mitigation concepts for further study.

2019

ORC

7

Implementation of an Interim Channel Monitoring and Maintenance Plan to maintain the capacity of the five creeks (Stephensons, Blackjacks, Golfcourse, Reservoir and Pumpstation) to convey high flows during a debris flow event.

On-going

As required

ORC

8

Undertook helicopter surveys and flew LiDAR and aerial imagery.

2019, 2022, 2024

ORC

9

Upgraded the SH8 culverts at Pumpstation, Golfcourse, and Blackjacks Creeks.

2021-2023

NZTA

10

Installed Otago weather radar.

2020

MetService

11

Rain radar tool implemented.

2023

ORC

12

Roxburgh Community Response Group established.

2024

CRG

13

Emergency management training and planning.

Ongoing

CDEM

Roles of councils and stakeholders

Different organisations help manage debris flow risks in Roxburgh:

  • ORC is responsible for 24/7 flood monitoring, which includes notifying CDEM of high-risk weather forecasts for Roxburgh area and rainfall monitoring.
  • ORC is responsible for management of five creeks (Stephensons, Blackjacks, Golfcourse, Reservoir and Pumpstation).
  • ORC conducts technical investigations to understand debris flows and ways to manage their impacts.
  • CODC oversees land use and spatial planning, local roads management and Three Waters, as well as owns and maintains the concrete chute at Reservoir Creek.
  • NZTA is responsible for State Highway 8 (SH8) maintenance and management as infrastructure provider.
  • CDEM is responsible for regional emergency management planning and leading the ‘4Rs’ (reduction, readiness, response and recovery).
  • Community Response Group (CRG) prepares a Community Resilience Guide (with CDEM support) to support the Roxburgh community before, during and after an emergency. CRG also helps communities become more prepared and resilient to disasters. It has the ability to work autonomously in a crisis by using pre-planning to establish a safe Community Emergency Hub and operate it. 

Potential impacts and risks for the Roxburgh area

  • To be updated once the new technical findings are available (around September 2025)