Scaled versus real-scale tests: Identifying scale and model errors in wave damping through woody vegetation.

Vegetation in front of levees, dikes and seawalls can decrease wave energy and therefore contribute to the safety against flooding. However, wave damping predictions by vegetation are still inaccurate due to measurement and modelling uncertainties. Many studies focused on finding reliable predictive tools using scaled flume tests with vegetation mimics. Scaling down vegetation can however lead to discrepancies with realistic scales, known as scale errors. In this work scaled tests were conducted on 3D-printed elastic replicas of willow trees and compared with real-scale experiments. We identified differences in measured wave dissipation between the scaled hydraulic model (1:10) and its real-scale prototype with 5 m high live willow trees under storm conditions (1:1). The maximum measured wave damping (30%) was roughly 1.5 times higher than the real-scale experiments (20%). Following the same trend of the real-scale experiments, this amount of wave height damping declined for larger water levels. Largest effects are argued to be due to increased viscous damping (smaller branch Reynolds numbers) and non-exact flexibility scaling. These significant deviations illustrate that full-scale experiments, although expensive, are still needed to validate the results of scaled experiments for woody vegetation. Alternatively, accounting for these discrepancies can make scaled experiments more reliable and expensive real-scale experiments less needed for wave damping studies on woody vegetation. • A forest with Froude and Cauchy scaled mimics of real pollard willow trees with detailed multi-scale branch structure was 3D printed. • Wave damping for the specific tested scaled pollard willow forest increased by a factor 1.5 compared to a real-scale 40-m long forest: 30% instead of 20% wave height reduction. • The major scale and model effects are inferred to be viscous effects, and imperfect elasticity scaling, respectively, both having the same order of magnitude. • Branch deflection was observed for the real-scale and scaled tests with the scaled forest appearing too stiff. • For realistic multi-scale trees, turbulence processes are different for the different elements, and dimensionless numbers have a range of values for different parts. In this paper the main dimensionless numbers, Reynolds and Cauchy, were calculated for different parts of the trees. [ABSTRACT FROM AUTHOR]