Managing gravel and the effects of this is part of Environment Southland’s overall approach to river management
There are a range of parties who hold differing and valid perspectives on issues relating to gravel management. In some places, a build-up of gravel can create risks of flooding, erosion and damage to infrastructure, but in other cases, provides benefits such as habitat for aquatic species and birds. As well as localised build-ups of gravel, we have also seen a deficit of gravel in the lower reaches of some rivers, particularly the Oreti River.
A more holistic approach to river management is key to better environmental outcomes, however there are measures that can be taken in the short-term to support gravel management in particular locations. A key aspect is an understanding of the volume and distribution of gravel in Southland rivers.
We are undertaking several actions to progress the
development of a strategic approach to gravel management. These include:
- A collaborative approach
- Science-based decisions
- Technology helping
build a clearer picture.
1.
A collaborative approach
Southland Gravel Working Group
The Southland Gravel Working Group (SGWG) will meet four times
during the first half of 2024. The group will clarify the gravel problem
definition, discuss whether there is consensus on science-led gravel
management, and create a roadmap toward a solution.
SGWG outcomes:
1.
A clear gravel management problem definition.
2.
An explanation of 'good' gravel management
practices for Murihiku Southland, ideally using the science-led principles in
the
Towards Strategic Gravel Management working document.
3.
Identification of ‘the gap’ between current
practices and desired future state.
4.
The SGWG will consider information and feedback
received from the stakeholders and community meetings. This will be provided as
written notes.
5.
Feedback on the long-term approach and possible
further opportunities to collaborate.
6.
Clear short-term pathway for the following steps to
achieve operational certainty in gravel management.
2.
Science-based decisions making
Towards Strategic Gravel Management
Towards Strategic Gravel Management is a
technical working report to inform the discussion on Southland’s gravel
management approach. It ‘outlines a series of scientifically led strategic
principles and recommendations that should underpin and inform gravel
management in Southland’s rivers’.
Find a copy of version 1 of the report here.
Currently, the document is a geomorphological perspective. Over
time, this report will become one of several documents that will provide a fundamental understanding and possible methods for Southland’s gravel
management.
The report makes several suggestions, upon which we have already made some progress:
Recommendation | Environment Southland (ES) action |
The need for reliable information on the gravel load of Southland’s rivers to create gravel budgets.
| Data gathering for gravel budgets is underway. Additional funding is required and has been requested through the 2024/24 Long-term Plan.
|
The need for a holistic approach to gravel management.
| ES is transitioning work to have an integrated catchment focus in accordance with our proposed Southland Water and Land Plan.
|
Further studies on how channels respond to vegetation lock-up in Southland and further trials assess the viability of using gravel management options intended to ‘unlock’ channels, including bar top (beach) skimming.
| The report highlights several trials already carried out by ES. Further trials are being planned, including a focus on ecological values.
|
In the long term, allowing the river room to erode will increase the geomorphic and habitat diversity in the river corridor and improve resilience in the face of increased flood magnitudes.
| Floodplain management will create the
opportunity to explore options for giving the river more ‘room to move’.
|
A collaborative approach to problem-solving.
| See above.
|
A phased approach is needed, and communities will need time to appreciate and understand the changes in practice.
| Staged approaches are being planned for freshwater and floodplain management plans. Community engagement is fundamental to this approach, and ES is committed to working with the community to identify and implement solutions.
|
3. Technology helping build a clearer picture
We are using the latest technology to improve our understanding and that of our community.
Environment Southland is undertaking a resource assessment using bathymetric LiDAR (Light Detection and Ranging) surveying along the length of the Mataura and Waikaia rivers – the first of its kind in New Zealand. This uses water-penetrating light to measure seafloor and riverbed elevations. The data collection for this project is complete, and the data processing is underway. We plan to extend this work to cover all of our major rivers. LiDAR elevation mapping is also nearing completion, with the lower Mataura, Ōreti and Rakiura areas already available.
Among their many functions, the maps help further understand landscape changes and catchments’ hydrological processes, such as stream flow estimation and catchment size. This includes locating gravel build-up in rivers. Two web applications have also been created for people to view and use both the LiDAR data, and the 2023 rural and urban imagery collection. These datasets cover the majority of Southland, with more being added as they become available – they can be found here and here.
Smaller floods helping causing gravel build-ups
Larger flooding events will naturally move more gravel, and during the past twenty years, Southland has experienced fewer large flooding events. Smaller floods tend to disperse sediment across gravel bars without moving the gravel. This can make the gravel bars suitable for weed and vegetation growth, and we see more root mats and stems forming, making the gravel more resistant to river flows. The quality of the gravel can also be affected by silt.
We know that flooding in Southland occurs more frequently during El Niño Southern Oscillation conditions, particularly during summer, and we’ve recently moved into this phase. El Niño is when rainfall totals are highest. Snowmelt can contribute significantly to flooding over winter and spring, and the September 2023 event was a good example of this, particularly in the Aparima catchment.
NIWA has indicated that maximum daily rainfall totals are projected to rise in the various climate change scenarios outlined by the Intergovernmental Panel on Climate Change, resulting in increased river flows and flood magnitudes. This presents a greater risk for our communities.
