Estimating Wave Attenuation through Diverse Floodplain Forests in the Netherlands
E.A. van Boxtel (TU Delft - Civil Engineering & Geosciences)
S.A. Kalloe – Mentor (TU Delft - Industrial Design Engineering)
B.K. van Wesenbeeck – Graduation committee member (TU Delft - Civil Engineering & Geosciences)
R.C. Lindenbergh – Mentor (TU Delft - Civil Engineering & Geosciences)
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Abstract
Vegetated foreshores are increasingly applied as nature-based flood defence measures, yet field evidence quantifying wave attenuation capacity of structurally diverse natural forests under flood conditions remains limited. Furthermore, complex vegetation structure is often overly simplified in spectral wave models.
This study investigates how diverse floodplain forest structure governs wave attenuation and evaluates Terrestrial Laser Scanning (TLS) as a method for deriving vegetation structural parameters across contrasting forest stands. The performance of TLS was evaluated using a reliability framework that defined the maximum distance over which vegetation structure could reliably be extracted from the point clouds.
Within this reliable domain, frontal surface area profiles (a(z)) were reconstructed and implemented in the phase-averaged wave model SWAN to simulate wave attenuation under varying water levels and wave forcing. Attenuation was governed by the interaction between submerged vegetation structure (a(z) Cd(z)) and wave orbital velocities (u(z)), resulting in dissipation proportional to a(z) Cd(z) u(z)³.
Pioneer and managed stands were characterised by concentrated low vegetation structure, limited horizontal patchiness, and structurally similar trees. Under moderate inundation conditions (1.6–4.0 m water depth above the forest floor), these stands produced the strongest wave attenuation with the smallest range of outcomes, with a median of approximately 40% and an interquartile range of 25–55% for a forest width of 100 m.
Late-successional stands, in contrast, were characterised by vertically distributed vegetation structure, pronounced horizontal patchiness, and structurally complex, diverse trees. Under the same conditions, these stands produced lower and more variable attenuation, with a median of approximately 20% and an interquartile range of 5–55%.
These results indicate that vertical vegetation structure primarily controls the magnitude of wave attenuation, whereas horizontal patchiness governs the variability of attenuation within forest stands. However, attenuation varied across hydraulic conditions and vegetation types, indicating that wave attenuation is not a fixed property of forest structure.
Compared to overly simplified, vertically uniform vegetation representations, TLS-derived a(z) profiles improved structural realism and captured depth-dependent attenuation behaviour. By linking high-resolution TLS-derived vegetation structure to wave modelling, this study provides a quantitative framework for evaluating structurally diverse floodplain forests as nature-based flood defences.