Author(s): Maike Paul; Pierre-Yves T. Henry
Linked Author(s): Pierre-yves Henry
Keywords: Laminaria digitata; Plant surrogate; Bending modulus; Buoyancy; Hydraulic forcing
Abstract: This research forms part of the eco-hydraulic work package (PISCES) of the HYDRALAB IV project. Within PISCES, results obtained using physical experiments are compared to those obtained in the field. To achieve this, the hydrodynamic conditions in and around plants in the field and the laboratory are measured using identical instruments (see Thomas et al., this volume). Flow and substrate conditions and configuration of plants are reproduced in the laboratory as closely as possible, but some simplifications have been necessary. One set of these simplifications addresses the vegetation in terms of spatial arrangement, number of species and individual plants. Keeping vegetation in laboratories poses problems with respect to plant health, vitality and adaptation to different and changing environmental conditions. One way to avoid these problems is to use surrogates made of inert materials to simulate the effects of plants. While surrogates are widely used to investigate the interaction between vegetation and hydrodynamics, little is known about how well such experiments replicate field conditions. This study, therefore, uses the comparative setup within the PISCES project and develops surrogates for the dominant species at the field site, the brown macroalgae Laminaria digitata. Earlier work has identified plant shape, stiffness and buoyancy as key parameters that drive the interaction between vegetation and hydrodynamic forcing. Consequently, this study focuses on these parameters when designing a surrogate that reproduces the hydraulic roughness of L. digitata as closely as possible. The sampled population is characterised by relatively small specimens with a projected area of 0. 21 m² and a stem length of 9. 3 cm. This atypical morphology could be due to either young age of the individuals, the site's environmental conditions or a combination of several factors. L. digitata at the field site has a mass density of 1001. 5 kg m-3 and is hence almost neutrally buoyant. It shows a distinct difference in bending modulus E between stem (6. 55 MPa) and blade (1353. 14 MPa). Moreover, the difference between blade base (471. 15 MPa) and blade tip (2279. 22 MPa) is statistically significant (p<0. 01), emphasising the spatially inhomogeneous nature of the blade tissue. However, this difference is not apparent in the flexural rigidity J which is 6. 18 × 10-4 N m² for stems, 8. 47 × 10-4 N m² for the blade bases and 8. 42 × 10-4 N m² for blade tops. As J is defined as the product of E and second moment of area, this similarity implies that differences in E can be balanced by differences in the plant’s cross-sectional area and form. Moreover, it highlights the importance of a plant's shape for its rigidity. Based on these values, various artificial materials were identified and a range of surrogates were developed that can be considered similar to real L. digitata. Comparison of the flow fields around, and the behaviour of these surrogates with prototype plants under laboratory and field conditions reveals the extent to which L. digitata's hydraulic roughness can be modelled by such surrogates.