Representing vegetation in experimental models of river systems.

Bodewes, Bas

Abstract

The physical behaviour of fluvial systems have been studied in detail and as have their representations of the impact and interaction between hydraulic and sedimentological features within these river systems. However, there is limited understanding of the interaction and impact of organic features like vegetation. Vegetation, in particular riparian vegetation on the banks and floodplains of rivers, is closely intertwined with river behaviour. Rivers can be growth enhancing, by deposition of nutrient rich sediments and the supply of water, or growth inhibiting during periods of low flow or erosive floods. Furthermore, vegetation itself influences the river system by, for example, increasing bank strength and flow resistance.
Vegetation is a living organism interacting with the fluvial system, and its behaviour is dynamic over time (both in terms of growth and decay). Vegetation not only strengthens itself and the substrate it grows in, but it also evolves over time and thrives differently over the seasons in a year. In systems that exhibit dynamic equilibrium this temporal variance of vegetation adjusts into the resulting river morphology and the vegetation itself follows the dynamics of the fluvial system as well. However, present-day predictions of climate change can significantly change river systems. Firstly, flood events may increase in magnitude and frequency; and secondly droughts may increase in length. Simultaneously, changes in temperature and rainfall will affect vegetation growth and decay and may change species types within a given area. These predicted effects will change the behaviour of systems over the next decades, a timescale that is significantly faster than most ‘natural’ changes in fluvial systems. Hence, it is essential to be able to model these fluvial systems and understand their changes over the next decades.
Physical modelling offers a solution to modelling these systems, and enables time to be compressed by reducing the scale of the river systems. In analogue physical models, surrogates are often used to represent vegetation in small-scale models. Surrogate vegetation enables modellers to incorporate vegetation density, growth and decay into models of fluvial environments since the surrogate vegetation represents the cohesive effect of plant roots, introducing the biotic forcing produced by vegetation in scaled physical models of hydraulic and sediment behaviour. However, despite the rapid growth that can be achieved with surrogate vegetation, it still takes a significant time to representative vegetation in an experiment which has a significant financial cost.
This research consists of a number of different experiments that: (i) elucidate how different stages of surrogate vegetation (Alfalfa) affects bank stability and the dynamics of a braided river system; and (ii) demonstrate how chemical surrogates that have instantaneous effects on sediment cohesion can be used as an alternative to growing surrogate vegetation. These experiments are conducted across different scales, with small bank erosion experiments to determine erosion rates for different ages and densities of surrogate vegetation followed by larger scale braided river experiments to demonstrate how the dynamic behaviour of the system is dependent on threshold ages of vegetation. These experiments include the novel use of chemical surrogates such as xanthan gum and sodium alginate which can be used in different concentrations to represent the behaviour of surrogate vegetation in both controlled bank erosion experiments as in dynamic braided systems.
Finally, this research introduces a new method to control the cohesive strength of these chemical surrogates. This method enables experiments to mimic the growth and decay of surrogate vegetation without the need to alter the sediment itself, thereby maintaining the characteristic existing morphology of previous stages. Experiments demonstrate that the chemical surrogates can be used to simulate sequences of vegetation growth, simulating either seasonal or longer-term climate induced changes in vegetation impacts of fluvial systems. Therefore this method significantly extends analogue scale modelling of complex fluvial systems.

Citation

Bodewes, B. (2020). Representing vegetation in experimental models of river systems. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4443326

• Thesis Type: Thesis
• Deposit Date: Nov 14, 2023
• Publicly Available Date: Nov 15, 2023
• Keywords: Geography ; Environmental sciences
• Public URL: https://hull-repository.worktribe.com/output/4443326
• Additional Information: Department of Geography, Geology & Environmental Sciences; University of Hull
• Award Date: Oct 26, 2023