Sediment processes team celebrate Marsden success

Two emerging NIWA scientists in the sediment processes team were awarded Marsden Fund Fast-Start research grants in the 2018 funding round.

Dr Hamish Biggs will investigate the significance of stone rolling for sediment transport processes in rivers. His project will use in situ sensors to measure stone dynamics and impact accelerations during floods. The work will provide fundamental insights into the transport, abrasion and deposition of large sediment material.

Dr Gu Stecca’s project has a focus on managing riverbed levels and flood risk in braided rivers. His research aims to develop guidance about optimal river width design using an innovative two-dimensional numerical modelling approach.

Marsden Fund Council Chair Professor David Bilkey says: “The Marsden Fund is designed to enable our top researchers to develop their most ambitious and exciting ideas. This ‘blue-sky’ funding is vital to ensuring a vibrant research culture in our country, and the resulting work will help us better understand our environment and society. Some of these fundamental discoveries will also lead to new, and sometimes unexpected, solutions to current problems, in areas as diverse as health care, sustainability and social policy.”

Grants to early career researchers rose from 49 last year to 53 in 2018. Support for early career researchers will enable these talented individuals to establish their careers in New Zealand and build momentum in their areas of research.

Dr Hamish Biggs (left) and Dr Gu Stecca (right) were both awarded Marsden Fund Fast-Start research grants recently. [Photo: Simon Hayes, NIWA]

The full title of Dr Biggs research is “The rolling stones: are rolling dynamics a critical component missing from conventional sediment transport?”

Earth’s rivers transport over 10 billion tons of sediment each year, providing level land for agriculture and human habitation. Measuring or accurately modelling the movement of Earth’s sediment is critical for humanity. However, sediment transport formulae are primarily empirical, and the physics of sediment transport has not been rigorously researched. Rolling is currently ignored in sediment transport formulae, which results in a 20-40% underestimate of the total kinetic energy of rolling stones (among other issues). This omission is mainly due to a lack of high resolution data on stone dynamics.

The full title of Dr Stecca’s project is “Managing bed levels and flood risk in braided rivers: does conventional channel confinement theory fail by neglecting key sediment transport processes?”

The river engineering practice of confining braided rivers between embankments to reduce their bed levels and decrease flood risk has been observed to fail in a significant number of cases in New Zealand and worldwide. The underlying traditional bedload transport theory fails to explain why artificial narrowing of river channels can exacerbate pre-existing bed level rise (with consequent increased flood risk) rather than inducing bed down-cutting as expected.

This project hypothesises that the underlying theory fails to capture the key mechanisms of reach-scale sediment transport in braided river settings by neglecting the effect of transient morphological events (TMEs) – processes typical of braided river behaviour that induce localised surges in sediment transport and contribute significantly to overall transport efficiency. As the occurrence of TMEs is controlled by the river width, retaining a width sufficient to permit TMEs may be crucial to stabilising or reducing bed levels.