Transport characteristics of large Wood in headwater streams: insights from a restoration field study and physical modelling

Abstract

Fluvial systems and their effective functioning are of the utmost importance. Large wood, although recognised as a potential hazard, is also an essential component in the majority of headwater streams. The amount of large wood in many watersheds is being purposefully increased due to rewilding, restoration or natural flood management projects, however, the relationships between the key drivers that determine large wood processes are not fully understood. The relationships between large wood loading and geomorphic complexity, as well as the effects of large wood loading, large wood configuration and large wood piece characteristics on large wood transport, are explored in this thesis. The approaches that have been used to investigate the research questions include original empirical field research, original scaled physical modelling and published empirical field research. Reach scale geomorphic complexity in relation to large wood loading has been quantified (positive correlation). The effect of configuring large wood as structures mimicking logjams (post-assisted log structures) has been shown to increase large wood retention and decrease median large wood transport distances. Additionally, the large wood piece characteristics dimensionless length and dimensionless diameter are related to large wood movement status and large wood transport distance. The large wood near functional immobility threshold has been refined, and very short dimensionless length (0–0.35) and small to moderate dimensionless diameter (0.1–0.4) large wood pieces are transported the greatest distances. The implications of these findings are that large wood processes are now better understood, particularly in a restoration context. As there are feedbacks between key large wood process parameters including large wood loading, geomorphic complexity and large wood transport, good understanding of these processes is vital to minimise uncertainty and error propagation so that meaningful estimations can be achieved. Better forecasting of system state changes in relation to large wood can be utilised to predict how a system may respond to large wood manipulations, thus enhancing decision-making relating to large wood. In particular, this is crucial for rewilding, restoration or natural flood management projects, especially as specific configurations of treatment wood can promote greater large wood retention and geomorphic complexity. Additionally, understanding the likelihood of movement and the potential transport distances for individual large wood pieces helps to quantitatively assess large wood hazard.

More information

Identifiers

ORCID for David Sear: orcid.org/0000-0003-0191-6179

ORCID for Julian Leyland: orcid.org/0000-0002-3419-9949

Catalogue record

Export record