UAT: Child page of a section page
UAT: Child page of a section page
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.
A debris flow is like a flash flood, but it carries mud, rocks, and even broken trees, making it much more powerful.
Debris flows usually happen after:
Heavy rain from thunderstorms is the most common cause of debris flows in Roxburgh.
Reservoir Creek, Blackjacks Creek and some other creeks have had debris flows in recent years, including the event in 2017.
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.
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:
In the diagram:
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.
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 |
|
1978 | Reservoir Creek, Blackjacks Creek, Pumpstation Creek, and Slaughterhouse Creek at least |
|
1993 | Slaughterhouse Creek, Pumpstation Creek at least |
|
2015 | Blackjacks Creek |
|
2017 | Pumpstation, Reservoir, Golfcourse, Blackjacks; Stevensons Creek (no direct impact) |
|
The image above shows the impacts of the November 2017 debris flow event at Reservoir Creek, showing the extensive deposition of sediment debris at the confluence with the Clutha River
The image above shows the impacts of the November 2017 events at Reservoir Creek, looking upstream of the SH8 bridge
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 |
Different organisations help manage debris flow risks in Roxburgh: