Quantifying Frontal-Surface Area of Woody Vegetation: A Crucial Parameter for Wave Attenuation

The last years, capacity of vegetation to reduce wave impact is receiving considerable attention. To predict wave attenuation processes within vegetation fields reliable estimates of vegetation parameters are needed. This proves to be difficult for woody vegetation as it consists of complex branch structures, characterized by varying branch densities, diameters and angles. State of the art physical and numerical models effectively use a single value for the diameter, b(v) and density, N of vegetation, which is unrepresentative for complex vegetation, such as trees. Trees can be better described by the projected frontal-surface area, A(v). Hence, this work compares methods to quantify the A(v) in space for a pollard willow forest, and determines suitability of these methods for predicting wave attenuation using a spectral wave model (SWAN). We use data from manual measurements and Terrestrial Laser Scans (TLS), to estimate the vertical distribution of A(v); and data from large-scale flume experiments performed on a willow forest to verify model sensitivity to A(v) inferences. As a baseline for comparison, tree models that describe the structure of the trees in various degrees of complexity are compiled. The most realistic tree model is used to quantify potential errors in TLS and basic manual measurements of N and b(v). An initial comparison shows that the TLS data underestimates A(v), which indicates that conducting manual measurements is more suitable to quantify a homogeneous forest. We found that the TLS suffers from shadowing effects (i.e., blockage of laser beams) and we recommend to apply a correction factor to improve its measurements. Furthermore, we identified the impact that the different methods to determine A(v) have on the estimation of wave attenuation using SWAN; in addition we verified the model results with data from large-scale flume experiments performed on the willow forest. The modeled sensitivity tests indicate large differences in wave attenuation and, consequently, a wide range (0.94-1.70) of bulk drag coefficients, C~D, for the various methods applied. This shows the variation of outcome between measuring methods and highlights the importance of stating the selected method for reliable frontal-surface area estimations, and consequently for reliable wave attenuation predictions.

Automated summary

In recent years, there has been considerable research on the capacity of vegetation to reduce wave impact. This study compares different methods to quantify the projected frontal-surface area (A(v)), and investigates their impact on wave attenuation. The findings suggest that terrestrial laser scans (TLS) underestimate A(v) and recommend applying a correction factor. Furthermore, the study demonstrates large differences in wave attenuation depending on the method used, highlighting the importance of selecting the appropriate method for accurate frontal-surface area estimations and reliable wave attenuation predictions.