Fluvial Geomorphology



Fluvial geomorphology is how rivers shape the world, and in its most simplistic form is the interaction between sediment, water and vegetation throughout a river catchment. It is a specialist technical area which focuses on understanding and explaining river processes and how they change through time.

An understanding of river character and behaviour is key to understanding how rivers may behave in the future and under changing conditions (such as climate change or land use changes). This information provides a holistic and long-term foundation for river management plans, flood risk assessments, erosion mitigation, gravel resource management, ecological assessments and enhancements, as well as fish passage design, stream diversion design, and stream enhancement projects.

Our fluvial geomorphologists at T+T regularly collaborate with water engineers, river managers, ecologists, freshwater scientists, landscape architects and geotech engineers across a wide range of disciplines and industries to get the best outcome for our varied clients in New Zealand and Australia.

Example projects include:

Sediment management options:

  • Whirinaki Arm Catchment: Stream characterisation and sediment reduction options assessment
  • Kopurererua Stream: Sediment reduction options assessment
  • Lower Manawatū Sediment Study

Erosion assessments:

  • Lower Burdekin River (Queensland): Streambank stability condition assessment
  • Erosion mitigation options assessment: Te Awa o Katipaki Stream
  • Erosion mitigation options assessment: Kirikiriroa Stream

Stream enhancement projects:  

  • Kopurererua Stream Realignment: Preliminary geomorphic and ecological investigation
  • Mangaohoi and Mangapiko Stream Enhancement: Stage 1 – Catchment Assessment
  • TDC Natural Channel Design Guideline
  • Daylighting project in urban North Shore, Auckland
  • Naturalisation of ponds for amenity and ecological purposes in South Auckland

Input into river/catchment management plans:

  • Waiohine River: Geomorphic trends assessment

Input into fish passage and in-stream habitat design:

  • Taranaki rivers stream geomorphology and fish passage design advice
  • In-stream structures for habitat enhancement guideline

Compliance monitoring for geomorphic conditions:

  • New Chum Creek Diversion Operations Monitoring Report
  • Walker Creek operations monitoring report

Case Studies

Lower Manawatū case study:

Horizons Regional Council’s long-term plan and 30 year infrastructure strategy identified sedimentation effects on the level of service of several flood schemes as being a significant issue in terms of flood protection. Horizons Regional Council engaged T+T in collaboration with Waikato University to investigate sediment processes with the Manawatū catchment, and to identify a range of options to reduce or manage sediment in the long-term.

Through an assessment of geomorphic processes and river character, a total of 13 river types were identified in the Manawatū and Ōroua Rivers. These different river types display unique behaviour, each responding differently to changing conditions, which has implications for management.

A desktop analysis of geomorphic trends over time suggests the Manawatū River is incising over long time periods (hundreds of years). But, over short timeframes (10 years) some river types aggrade in response to some large rainfall events. This has implications for flood risk, and river management.

Changes in climate are likely to increase the amount of sediment delivered to rivers. Changing flow conditions will see some upstream reaches aggrade as sediment supply exceeds the rivers ability to transport it, with potential incision in downstream reaches in bankfull events. But, in low frequency, high magnitude events, this material will be periodically transported downstream. Some river types have the ability to capture and store some of this sediment before it reaches the flood schemes.

Suggestions of future sediment management included supporting and enhancing naturally occurring sediment trapping and storage functions, or reducing bed incision if this was having an effect on downstream flood risk. Options included increasing floodplain engagement, increasing wetland engagement, increasing river length, implementing two-stage channels, relocation of stopbanks as well as sensitive and river type appropriate bed and bank erosion mitigation measures.

Service provided:

  • Assessment of historic sediment regimes in response to changing climatic conditions
  • Assessment of current sediment regimes in response to river management
  • Assessment of existing and historic geomorphic trends
  • Reach typing of rivers based on geomorphic processes and sensitivity to change
  • Review of climate change assessments
  • Assessment of potential future river response to changing climatic conditions
  • Identification of short, medium and long-term sediment reduction or management options for each river type.

Waiohine River case study:

Our investigation found the Waiohine River had a change in river behaviour following the end of the last glacial maximum, switching from a braided system to a wandering system. As the climate warmed, and there was an increase in precipitation, hillside vegetation cover increased, reducing sediment supply. Increases in precipitation also increased sediment transport. The Waiohine River is still displaying a slight incision trend, which suggests it may still be responding to paleo‑climate changes. Infrequent episodic events have also triggered changes in river behaviour. Following the 1855 rupture of the Wairarapa fault, the Waiohine River avulsed, and occupied numerous channel locations on the true right floodplain. The earthquake also induced large-scale landsliding in the upper catchment, and head-cut erosion where the fault scarp crossed the bed of the Waiohine River, creating a short‑term pulse of sediment into the system.

Today, the Waiohine River is managed using a combination of rock groynes, gravel management and willow planting. It is likely the gravel management is removing the coarse surface armour layer on gravel beaches, enabling more frequent transport of coarse material into downstream reaches, and an increase in fine grained sediment supply. The three reaches identified during the investigation also show different responses to river management and sediment inputs. A tendency towards aggradation in one of the reaches, and the numerous flood channel on the true right floodplain, has created an elevated risk for channel avulsion. This risk will increase under predicted climate change flood scenarios.

Climate change also poses additional challenges for the Waiohine River. Predicted seasonal reductions in precipitation, but increases in temperature may reduce vegetation condition on the hillslopes. This may increase sediment inputs into the river. Sea level rise may change the erosional and depositional areas in the Ruamāhunga River, which could lead to bed aggradation in the lower reaches of the Waiohine River.

High magnitude events may also have different impacts on slope processes, and delivery of sediment. The frequency and timing of these events will largely determine the channel response. Some events may trigger aggradation in upper reaches, and a potential ‘sediment starvation’ incision response downstream, while other events may be more successful at transporting sediment pulses into downstream reaches.

Our findings highlight the importance of understanding landscape scale behaviours in order to effectively manage New Zealand rivers at a reach scale.

Services provided:

  • Assessment of historic river response to changing climatic conditions
  • Assessment of current geomorphic trends
  • Review of historic and current management actions
  • Reach typing based on geomorphic processes
  • Review of climate change assessments
  • Review of climate change flood risk modelling results
  • Assessment of potential future river response to changing climatic conditions
  • Assessment of flood risk, including risk of major avulsion
  • Recommendations for future monitoring and data collection
  • Recommendations for consideration in the Floodplain Management Plan
Key Project Contacts

Selene Conn Selene Conn