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Characterizing Live Vegetation Response to Wave Forcing

Author(s): Acacia Markov; Ross Henteleff; Jacob Stolle; Ioan Nistor; Enda Murphy; Andrew Cornett

Linked Author(s): Acacia Markov, Ross Henteleff, Jacob Stolle, Ioan Nistor, enda murphy, Andrew Cornett

Keywords: Wave-vegetation interactions; Coastal marsh; Physical modeling; Nature-based solutions

Abstract: The limitations of conventional ‘hard’ coastal protection structures (e.g. seawalls, breakwaters) in adapting to climate change effects, as well as their adverse environmental impacts, have spurred innovation in the field of coastal engineering with a recent paradigm shift to the use of nature-based solutions (NBS). It has been widely demonstrated in literature that marsh vegetation delivers ecosystem services related to coastal flood and erosion risk management in low- and medium-energy wave climates. This includes wave attenuation associated with vegetation-induced flow resistance, and erosion protection resulting from reduced bed shear stresses within vegetated canopies and root stabilization. Previous physical modeling studies have investigated the relative influence of various plant biophysical parameters (stem flexibility, stem width, stem height) and hydrodynamics (wave height, period) on coastal protection function. However, few studies have been conducted at full-scale with live vegetation, owing to logistical and plant husbandry challenges with live vegetation in laboratory settings, and the significant facility requirements. There are uncertainties surrounding the accuracy of wave-vegetation interactions captured by small-scale studies using vegetation surrogates, which exhibit morphological simplifications and a lack of intraspecies heterogeneity in comparison to their live counterparts. To address this critical knowledge gap, prototype-scale physical modeling tests were conducted with live vegetation in the Large Wave Canal at the Institut National de la Recherche Scientifique (INRS) in Québec, Canada. Three species of saltmarsh vegetation native to Canadian coasts were investigated: Spartina alterniflora, Spartina pectinata and Spartina patens. The latter two species have never been tested in a coastal protection context to the authors’ knowledge. Juvenile plants were sourced from a local field site, directly planted into a sandy slope (~1:18) and allowed to establish in the outdoor facility for several weeks before commencement of wave testing. The response of individual stems and plant hummocks to irregular wave forcing was characterized through investigation of plant motion under wave action, using submerged cameras and a novel video tracking algorithm coupled with wave height and velocity measurements. Various hydrodynamic conditions were considered, categorized as calm (Hs < 0.2 m, Tp = 2.5 s), boat swell (Hs < 0.2m, Tp = 10 s), and swash zone conditions (0.1 < Hs < 0.3 m, 3 < Tp < 10 s). The use of a sloped marsh allowed for novel consideration of wave transformation, breaking, and simultaneous consideration of various plant submergence ratios (emergent, transition, submerged) representative of real-world tidal marsh conditions. The results of prototype-scale testing demonstrate the viability of outdoor physical modeling for investigating wave-vegetation interactions, which will pave the way for future experimentation with full-scale marshes. Additionally, species-specific responses to wave-forcing are demonstrated, yielding critical knowledge required to enhance the development of vegetation surrogates for small-scale experimentation, and to guide design of NBS.


Year: 2022

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