Impact of vegetation on bank erosion in a planform braided physical model

BAS BODEWES; FERNANDEZ, ROCIO LUZ; STUART MACLELLAND; DANIEL PARSONS

Auckland

Simposio; River, Coastal and Estuarine Morphodynamics ISBN978-0-473-50422-9; 2019

IHAR and NZ River Group

In fluvial systems, vegetation is considered to be one of the most significant factors influencing the behaviour of river morphology (Gurnell, 2014). Vegetation increases bank stability and therefore can reduce bank erosion; However the interrelationship between vegetation and bank erosion throughout a river reach is poorly understood. This lack of understanding can be attributed to a number of complexities in the system, for example the variability of vegetation over time; both in terms of seasonality as well as growth and decay of vegetation. Such complexities in the behaviour of the system, over different hydrodynamic and morphological time scales, make field research a challenge. Moreover, it makes it difficult to predict inter-relationships between vegetation and bank erosion under future climate change scenarios.It may be possible to improve our understanding of this from modelling; and although numerical modelling can be used to investigate future scenarios, there is still a need for validated vegetation parameters to improve the accuracy and robustness of such predictions. Physical modelling has the potential to both improve parameters and also validate change over longer periods of simulated time. In the laboratory scaled surrogate vegetation can be used to induce similar effects on sediment stability as vegetation at the field scale, but can be grown much faster than prototype examples. Physical modelling therefore offers a route to parameterise vegetation in numerical models, particularly under a range of forcing scenarios.The use of surrogate vegetation in flume experiments has previously been demonstrated by Tal and Paola (2007) to stabilise fluvial systems. However, there is limited research on the impact of vegetation age and complexity on the behaviour of fluvial systems. Research presented in this paper addresses this through a focus on identifying the effect of vegetation age (linked to its stabilising influence), on bank erosion rates in a braided system.