D7.5 Conference proceedings ‘Novel approaches to assess and rehabilitate modified rivers’
Page 133 of 423 Distinct patterns of interactions between vegetation and river
morphology Van Oorschot M et al.
MIJKE VAN OORSCHOT1,2, MAARTEN KLEINHANS2, HANS MIDDELKOOP2, GERTJAN GEERLING1, TOM BUIJSE1 & ERIK MOSSELMAN3,4
1Department of water quality & ecology, Deltares, PO Box 177, 2600 MH, Delft, the Netherlands
2Faculty of Geosciences, Universiteit Utrecht, PO Box 80115, 3508 TC, Utrecht, the Netherlands
3Department of river dynamics and inland water transport, Deltares, PO Box 177, 2600 MH, Delft, the Netherlands
4Faculty of Civil Engineering and Geosciences, Delft University of Technology, PO Box 5048, 2600 GA, Delft, the Netherlands
Riparian vegetation interacts with morphodynamics by influencing river banks and causing hydraulic resistance. Modelling vegetation and morphodynamics is often one-way traffic that either takes into account the effect of vegetation on morphodynamics or vice versa. We coupled a morphodynamic model to a novel dynamic vegetation model to test the hypothesis that dynamic vegetation creates more realistic patterns in vegetation and fluvial morphology as opposed to the ‘old fashioned’ static vegetation. We find that dynamic vegetation as opposed to static vegetation predicts more natural patterns and dynamics in vegetation and fluvial morphology.
INTRODUCTION
Riparian vegetation interacts with morphodynamics to create a biodiverse landscape mosaic (Tockner and Stanford, 2002). Vegetation can reinforce or destabilize river banks and alter the flow field and sediment balance through hydraulic resistance (Simon and Collison, 2002; Rinaldi and Casagli, 1999; Zong and Nepf (2011).
Despite of the numerous conceptual models, there is a discrepancy in process-based modelling of these dynamic interactions. Advanced physics-based models include complex morphodynamics, but simplistic vegetation, while cellular automata include more advanced ecological processes, but simplistic morphodynamics (Camporeale 2013). Here we present the results of a novel dynamic vegetation model coupled to an advanced morphodynamic model. The aim of our work is to investigate the emergent patterns in vegetation and river morphology at the river reach scale by dynamically modelling the processes and their interactions. We want to test the hypothesis that dynamic vegetation creates more realistic patterns in vegetation and fluvial morphology as opposed to the ‘old fashioned’ static vegetation
We compared three different scenarios; a scenario without vegetation, a scenario with commonly used static vegetation and a scenario with dynamic vegetation containing all advanced vegetation processes.
D7.5 Conference proceedings ‘Novel approaches to assess and rehabilitate modified rivers’
Page 134 of 423 METHODS
We have coupled the morphodynamic model Delft3D to a novel dynamic riparian vegetation model. The morphodynamic model was designed to represent average morphodynamic characteristics and hydrology of the Allier river in France (Figure 1). The vegetation model interacted with the morphodynamic model through hydraulic resistance every two weeks.
The vegetation model includes colonization, growth and mortality. Colonization takes place depending on the timing of seed dispersal and the water levels during that period. Growth of vegetation shoot, root and stem diameter is calculated with a logarithmic growth function based on age. Plant properties can change at different life stages and multiple vegetation types with different ages can reside in one grid cell. Mortality of vegetation depends on days of subsequent flooding, days of subsequent desiccation, high flow velocities, burial and scour.
Three scenarios were tested: 1) no vegetation, which is the control run of the morphodynamic model without vegetation, 2) static vegetation, where vegetation could colonize and cause flow resistance but did not grow or die, and 3) dynamic vegetation, including all dynamic processes. The vegetation types in the dynamic scenario are loosely based on Salicaceae species with ecosystem engineering properties. The vegetation type in the static scenario is based on an average Salicaceae bush.
Table 1. Model parameters compared between all scenarios.
RESULTS
The three scenarios show clear differences in river morphology after 300 years (Figure 2). The scenario without vegetation develops towards a straight channel with low sinuosity (Figure 2A). Both scenarios with vegetation show a dynamically meandering river with chute cut-offs, oxbow lakes and vegetation patterns comparable to the Allier river (Figure 2B and Figure 2C). However, the static scenario has a broader floodplain and sharper meander bends with a bigger amplitude than the dynamic scenario.
The scenarios with static and dynamic vegetation show large differences in the vegetation cover and dynamics. The scenario with static vegetation has a dense cover (Figure 2B) which is not very dynamic, while the scenario with dynamic vegetation has a much lower vegetation cover with higher dynamics (Figure 2C).
1 km
km
Flow
D7.5 Conference proceedings ‘Novel approaches to assess and rehabilitate modified rivers’
Page 135 of 423 Discussion and conclusions
The results show distinct differences in river morphology between the scenarios after 300 years. Without vegetation, the river develops into a straight channel, while the scenarios with vegetation show a dynamically meandering system. This confirms that inclusion of vegetation creates a single thread, meandering river, which is in line with other modeling studies (Crosato and Saleh, 2011; Nicholas, 2013) and flume experiments (Van Dijk, 2013; Tal & Paola, 2010). Additionally, we find that river morphology is very sensitive for the way vegetation is defined, either static or dynamic, which is a direct effect of vegetation location and density. Furthermore, the vegetation cover of the static scenario is very dense, while the vegetation cover of the dynamic scenario is less dense and more dynamic, which is in the same range as vegetation cover and age distribution derived from field data by Geerling et al (2006). This shows that dynamic vegetation creates natural vegetation patterns, leading to a more realistic interaction with morphodynamics and consequently a more natural river morphology.
References
Camporeale, C., Perucca, E., Ridolfi, L., & Gurnell, A. M. (2013). Modeling the interactions between river morphodynamics and riparian vegetation. Reviews of Geophysics, 51, 1–36.
Figure 2. River morphology and vegetation pattern after 300 years for A) the scenario without vegetation, B) with static vegetation and C) with dynamic vegetation.
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