The Fluid Mechanics Webinar Series

Speaker: Stephen Monismith

Date/Time: Friday 7th January 4pm GMT/11am EST

Title: Flows through Living Roughness

Abstract: Kelp forests, seagrass beds, and coral reefs are among the most productive, bio-diverse, and beautiful of marine ecosystems. Hydrodynamics plays a central role in their function, controlling mass transfer between the organisms and the overlying flow, setting patterns and rates of transport of larvae, and affecting the extent to which the environment (temperature, pH, etc.) in these nearshore systems are different from that of the adjacent ocean. Central to this interplay of physics and ecology is the fact that the resistance to flow provided by kelp plants, seagrasses, or corals is determined by the geometry and density of these “ecosystem architects”, behavior that is described by the dynamics of complex, canopy flows. I will present field observations of these flows drawn from a coral reef in American Samoa, a seagrass bed in Palau, and a kelp forest in Baja. In the first case, drag can be related explicitly to the reef geometry at cm scales. In the second case, drag varies strongly with velocity reflecting the re-configuration of the drag elements due to bending, behavior that can be modeled with some accuracy. In the final case, I will show velocity measurements taken over the span of several years during which time the condition of the kelp forest varied between non-existent and dense. During this time rms depth-averaged velocities varied by a factor of 3, showing the importance of kelp-dependent drag to flow. However, this last case seems substantially more complicated in that drag on flow through a kelp forest is strongly influenced by surface waves, has multiple drag sources, and involves substantial inhomogeneity in the distribution of the kelp plants themselves. Nonetheless, I will present a simplified approach to inferring drag through kelp.