Surrogates of juvenile kelp were used to assess how they affect the critical shear stress of the stones they are attached to. For this purpose, laboratory experiments were carried out in June 2024 at the Ludwig-Franzius-Institute of Hydraulic, Estuarine and Coastal Engineering, Leibniz University Hannover, Germany. Specimens were exposed to increasing orbital velocities and the experiment was terminated when the majority of stones had moved. Specimens comprised of an artificial kelp attached to individual stones. For the stones all three dimensions were measured with a calliper gauge. The artificial kelp was produced from a geotextile (thickness = 1.13±0.1 x 10⁻³ m, wet tissue density = 747.4 kg/m³, flexural rigidity = 0.071±0.029 x 10⁻⁴ Nm²). They were cut in rectangles of 2 x 10 cm and one short side was glued to the stones with superglue. The weight of the resulting specimens was determined with a balance to the nearest 0.01 g. The experiments were conducted in a wave flume (110 m long, 2.2 m wide) and the test area within the flume was positioned at the observation window starting 70 m from the wave maker. Euro pallets were laid across the test section as the foundation for the experimental setup, allowing for substructure water flow. On top of the pallets, two layers paving slabs (40 cm x 40 cm x 5 cm) were installed. Overall, the setup consisted of three sections: (1) Flat concrete bed: A flat concrete area was created with additional paving slabs laid with their rough sides up, forming an 80 cm wide and 180 cm long area. (2) Inclined concrete bed: Another section was constructed with tilted slabs, using wooden chocks to create slopes of 5°, 10°, and 15°. Each sloped section was 60 cm wide and 80 cm long and with a slope increase in the direction of wave travel. (3) Gravel bed: An 80 cm wide gravel layer was placed alongside the inclined slabs. The gravel, mixed with epoxy resin to prevent movement, was evenly distributed over geotextile support. For wave management, limestone blocks formed a transition slope to guide wave flow effectively. Specimens were laid out across the test section in marked circles 13.3 cm apart (5 cm diameter). A Microsonic MIC130-4 ultrasonic sensor (accuracy: 0.18 mm) was mounted from the channel wall 30 cm in front of the first specimen positions. It recorded water surface elevations to determine wave heights and periods at a sampling rate of 300 Hz. Data acquisition was synchronised with video recordings from two Logitech webcams, capturing lateral views focusing on a single stone and providing an overhead perspective from 6 m above the channel floor. Water level was set to d = 0.5 m and regular waves were generated with a constant wave period of T_m = 5 s and varying wave heights (0.01 < H_m < 0.22 m). Each wave train consisted of 24 waves to minimise interaction with waves reflecting from the flume's end. Experiments were divided into two groups based on the stone classes (11.2-16 mm and 16-22 mm). For each stone class testing began with maximum surrogate area (A_k = 20 cm²). Tests began with a 1 cm wave height, increasing in 1 cm intervals until most specimens moved. Then, surrogate height was clipped and tests repeated until only evaluating the stone. The onset of specimen displacement was derived from the video footage starting systematically with the smallest wave height and progressing through the conditions. A specimen was marked as moved if it was fully or partially outside the predefined starting circles at the end of a wave sequence. Once it was marked as moved, it kept this label for all following wave conditions. Time series of water surface displacement corresponding to the wave conditions at which individual specimens were marked as moved for the first time were used for analysis. Mean wave height H_m was calculated for the 24 waves in the wave train using the distance between the mean height of wave crests and troughs, respectively. Linear wave theory was then applied to calculate the resulting critical shear velocity u_wave: u_wave = H_m * w / (2 * sinh(k * d)) Where w = 2 pi / T_m is the angular frequency, k = 2 pi / L_m is the wave number, d is water depth (0.5 m) and L_m is mean wave length, which was derived from mean wave period T_m via the dispersion relation. The critical wave shear stress was then derived as: tau_wave = 0.5 * rho_f * f_w * u_wave^2 with f_w = exp[-5.977 + 5.213 * (u_wave * T_m / (2 pi * k_b))^-0.194] Where k_b is the absolute bed roughness which was set to 8 mm for the concrete bed and to 35 mm for the gravel bed and rho_f is density of the fluid (1000 kg/m³).