Seed-sediment Dynamics

Abstract

In response to the sharp decline of seagrass globally, various restoration techniques have been developed and applied; many at limited scale and with limited success. This research focuses on the fluid mechanics and behaviour of seagrass seeds in a sand mixture through physical laboratory experiments. The objective is to establish a technique that incorporates seagrass seeds with sand-capping to develop seagrass restoration methods suitable for upscaling. Due to the limited amount of available seeds, laboratory experiments were conducted to find a suitable proxy for seeds from both intertidal and subtidal Zostera marina populations by tracking and analysing the particle's terminal settling velocity. The average terminal settling velocity of the seeds was 5.97 cm/s, slightly larger than that of sand particles with a diameter of 0.35 mm with a settling velocity of around 5.2 cm/s. Subsequently, the distribution of proxy seeds in the settled sediment of the "sand-capped layer" was determined after dispersal in five sediment-water mixtures with a volumetric concentration ranging from 5 % to 30 %. Mixtures with three-grain sizes grain sizes were released (d_{50} = 0.175, 0.35, and 0.52 mm) in a column of 1m and subsequently in a column of 1.5 m, where only the top 0.5m had a sediment-water mixture, while the area below consisted of a column of still water. Results show a general trend: seeds segregate to the top of the settled layer, further enhanced by larger grain sizes and higher sediment concentrations. Tests using the lowest concentrations (5-10 %) showed the best result, for which the largest number of proxies were located at an optimal depth (1 cm-5.5 cm) for protection and germination when using medium coarse and fine-sized sand grains. The results provide valuable insights into the dynamics of seed segregation in sediment mixtures and the hindered settling effect of a bimodal mixture, showing that seeds move differently to sand in a mixture, experiencing more rotation and drifting within a mixture not observed when following trajectories in a still water column. The larger seed particles segregated to the top of the settled layer due to the difference in behaviour with the sediment mixture. Even though the terminal velocity of the seeds in a still water column was larger.