Paul, Maike; Betker, Timo: Critical shear velocity of kelp surrogates under unidirectional flow [dataset]. PANGAEA, https://doi.pangaea.de/10.1594/PANGAEA.971269 (dataset in review)

Surrogates of juvenile kelp were used to assess how they affect the critical shear velocity of the pebbles they are attached to. For this purpose laboratory experiments were carried out in April 2023 at the Ludwig-Franzius-Institute of Hydraulic, Estuarine and Coastal Engineering at Leibniz University Hannover, Germany. Pebbles were exposed to increasing unidirectional flow and the experiment was terminated when the pebbles started to move. Kelp surrogates comprised of an artificial kelp attached to individual pebbles. For the pebbles all three dimensions were measured with a digital calliper gauge (Kraftixx) and their weight was determined with a balance to the nearest 0.01 g (Sartorius Practum). Their volume was determined by water displacement (Paul 2023). The artificial kelp was produced from a geotextile (thickness = 0.8 x 10⁻³ m, density = 258.1 kg/m³, flexural rigidity = 110.55±45.26 x 10⁻² Nm²). They were cut in rectangles of 2 x 10 cm and one short side was glued to the pebbles with superglue. The kelp surrogates were individually exposed to unidirectional flow in a circulating flume with a straight working section of 150 cm length and 7.5 cm width. Flow was controlled via an inlet valve and a sharp crested weir at the end of the working section. Surrogates were placed in the centre of the working section and an Acoustic Doppler Profiler (ADV, Vectrino Profiler by Nortek) was used to measure flow velocities 45 mm in front of the surrogates. It was set to measure flow velocities at 100 Hz in 1 mm bins within the first 10 mm above the bottom. The flow was increased incrementally by adjusting the weir height and discharge; each flow velocity was maintained for 3 minutes during which the motion of the kelp surrogate was observed. If it remained stable during this time, the velocity was increased. If it moved, the previous velocity was defined as critical. Consecutively, the length of the artificial kelp was reduced by 2 cm and the experiment started again with a low flow velocity. This process was repeated until only the pebble remained and its critical flow velocity was determined. Velocity data associated with the respective critical flow velocities was analysed in MATLAB. All values with a Signal-to-noise ratio <15 dB and/or a correlation of <90% were removed from the dataset. For the remaining data, the mean was calculated for each bin and used to calculate the shear velocity according to the law of the wall (Zanke 2013). The individual shear velocities were then averaged to obtain the critical shear velocity for the respective kelp surrogates.