The Clutha Delta is exposed to a range of natural hazards, and the ORC is already responding through flood protection, drainage infrastructure, and hazard monitoring and assessments.

As the landscape and climate continue to change, many of these hazards are expected to intensify over time.

Many of these hazards are expected to worsen with ongoing landscape changes or climate change:

• Increased flooding hazard: caused by changes to the form of the river mouth, or through climate change affecting river flows, sea-levels, and groundwater levels.
• Worsening groundwater and drainage issues: following an earthquake, liquefaction may cause the ground to sink which could impact groundwater and effectiveness of drainage systems. Sea-level rise is likely to increase groundwater levels.
• More coastal hazards: further sea level rise is likely to worsen storm surges and rates of coastal erosion. Ongoing coastal erosion may erode dune systems, reducing natural protection from storm surges and tsunamis. 

The ORC is already undertaking a range of hazard management responses to protect against natural hazards through the operation of the Lower Clutha Flood Protection and Drainage Scheme and natural hazards monitoring and assessments.  

Because natural hazards and their risks are expected to change over time, with many likely to have greater impacts due to changes in the landscape or climate, the responses we rely on today may need to change in decades to come. The Clutha Delta Natural Hazards Adaptation Programme will further our understanding of current natural hazards, how these are likely to change and work with the community to explore options for the future. Working with programme partners, the community, and stakeholders, a strategy will be developed, identifying approaches for the future management of these risks. 

Explore the sections below for further information on current natural hazards, how they are anticipated to change over time, and the ORC’s existing management actions. You can also visit the Investigations and Reports page.

The Clutha Delta has a history of flooding, primarily from the Clutha River/Mata-au, but also smaller tributaries including the Pūerua and Waitepeka rivers.  

Past flood events have varied in severity. Significant flood events occurred in 1878, 1957, 1978 and 1999, with some causing overtopping and breaching the flood banks. 

There have also been moderate flood events, with recent events in 2019 and 2020. While less severe, these flooding events caused some damage to the flood protection scheme and have required significant repair works.  

Groundwater is the water stored beneath the land surface. The water table can rise due to heavy rainfall, high river and sea levels. When the water table rises close to or above the ground surface, it can cause problems including ponding and waterlogging.

Much of the Clutha Delta is low-lying and sits close to sea level. In these areas, a system of drains and pumps, as part of the flood and drainage scheme, helps to keep the ground dry.

As sea levels rise, groundwater levels are likely to rise too, putting more pressure on this system. To keep the land dry, the scheme is likely to need to pump out more water, more often. The delta will depend more heavily on the scheme to maintain the viability of current land uses. 

Earthquake shaking can have direct and immediate impacts such as damage to buildings and infrastructure.

Strong shaking can also trigger cascading hazards – when a primary hazard triggers secondary hazards. In the Clutha Delta that could include:

• Landslides on the surrounding hillslopes near the delta
• Liquefaction and lateral spreading – where the ground loses its strength and temporarily behaves like a liquid. Liquefaction can also cause the ground to sink, or to slump horizontally towards riverbanks (lateral spreading)
• Tsunamis – earthquakes in offshore locations can generate powerful waves.

All these hazards can cause damage to buildings and infrastructure, including the Clutha Delta’s floodbanks and drainage infrastructure. Some hazards may have potential to result in loss of life.

There are several active faults in coastal South Otago, including the Titri, Akatore, Settlement, and Otanomomo faults. The Akatore and Settlement faults are relatively active, with a fault rupture occurring around every 2000 years, on average. The delta is also susceptible to liquefaction due to the soft sediments making up the ground.

Coastal hazards include erosion and inundation.

Coastal erosion is the gradual loss of land along the shoreline. This process is influenced by waves, tides and currents and sediment supply from rivers. Erosion is also influenced by river mouth structures that alter the flow of sediment. Over time, erosion reduces the natural protection provided by sand dunes, worsening the rate of erosion. In some locations along the Clutha Delta, coastal erosion (shoreline retreat) is occurring at rates of up to 10 metres every year, with the highest rates south of the Koau Mouth.

Coastal inundation is the flooding of low-lying coastal areas by seawater due to storm surges, high-tides or as a result of sea level rise. 

At the Clutha Delta, recent significant storm surges occurred in 2022 and 2023, with the 2023 event damaging the Koau Mouth training line structure and causing about 100 meters of shoreline retreat. The training line is an engineered structure that helps to keep the Koau River mouth open and flowing freely.

Clutha Delta Koau Mouth map showing changes between 2020 and 2023

Over time, coastal hazards are expected to continue to impact the performance of the flood and drainage scheme, which could affect land use, agriculture, and coastal infrastructure. Use the slider to see the changes. 

A range of physical processes are driving or will drive changes to the natural hazards of the Clutha Delta. These include climate change, sea level rise, changes to the Earth’s surface (landscape evolution), and cascading impacts – where a hazard can cause a flow on hazard such as a landslide caused by an earthquake.

These all have implications for future risk and resilience.

Climate change is increasing the frequency and severity of flooding by altering rainfall patterns and intensifying extreme weather events. This can increase the frequency and intensity of flooding, by reducing the effectiveness of current flood defences, and drainage systems. 

Sea level rise refers to the gradual increase in the average height of the world's oceans. This is primarily caused by two factors: the melting of ice sheets and glaciers, and the expansion of seawater as it warms.

Rising sea levels is likely to worsen other hazards including groundwater levels, rates of coastal erosion and storm surges.

Sea levels are projected to increase by up to 1 meter by 2100 (NZ SeaRise).

Over time, the physical form of river channels and mouths change through movement of sediments. 
Sediment build-up occurs when sediment such as sand and gravel accumulate in the riverbed, causing the channel to rise and potentially become shallower. This can affect water flow and increase flood risk. Changes at the river mouth, known as offsetting, occur when the location where the river meets the sea shifts over time, often due to sediment buildup or coastal forces like wave action. These changes can reshape estuaries, influence ecosystems, and impact human infrastructure and land use near the river and coastal margin.

The current Clutha Delta landscape is heavily modified and constantly managed to support human activities. For example, the rivers alignments are kept in place by flood banks.  The Koau and Matau mouths of the Clutha are kept open and in the same place with training line structures.  Keeping the land drained requires pumping support.

Hazards have direct impacts but can also trigger other impacts. These include:

• Liquefaction can cause ground subsidence (sinking ground) or damage to ​floodbanks
• Subsidence (sinking ground) may affect groundwater and drainage
• Coastal erosion reduces sand dunes. This further reduces the coastline’s natural protection from storm surges and tsunamis

The ORC is already undertaking a range of hazard management responses to protect against natural hazards through the operation of the Lower Clutha Flood Protection and Drainage Scheme and natural hazards monitoring and assessments.

Coastal hazards forecasting provides early awareness of sea conditions that may cause inundation and erosion.

The ORC carries out a range of natural hazard-related environmental monitoring around different sites in the Clutha Delta. Most of the data is publicly available through our Environmental Portal, but please reach out to the team if you can't find information for a site you know is monitored. Monitoring includes:

• Rainfall Monitoring 
  • Measuring how much rain falls in specific areas, using MetService’s rainfall radar data.
  • Locations include Balclutha, Pūerua and Inch Clutha.
• River Level and Flow Monitoring:
  • Tracks how high and fast rivers are flowing.
  • Multiple sites around the delta including at Balclutha, Lovell’s Creek, Lake Tuakitoto, and pump stations at Inch Clutha, Kaitangata, and Paretai.
• Groundwater Level Monitoring:
  • Observes changes in underground water levels, giving us baseline information so we can better understand changes.
  • Monitoring is from multiple sites and different levels at locations including Paretai Hall, Settlement Road, Matau Hall, Inch Clutha (Lawson Road), Inch Clutha (Lakeside Road) and Kaitangata. 
• Coastal Monitoring: 
  • Monitors erosion and changes along the coastline.
• Riverbed Profile Monitoring:
  • Measures riverbed levels and the shape of the river to assess whether bed levels may be building up and the potential impact on flood hazard.

ORC Natural Hazards-related Environmental Monitoring Sites on the Clutha Delta

If there are problems using this map, please open the interactive map