How Vegetation Alters Water Motion, and the Feedbacks to Environmental System Structure and Function

Heidi Nepf

Civil & Environmental Engineering

Friday, Mar. 9, 2018, 3:10pm
Chase 130

For over a century vegetation has been removed from channels and coastal zones to facilitate navigation and development. In recent decades, however, we have recognized the ecologic and economic benefits of aquatic vegetation. For example, it removes nutrients, providing a buffer against eutrophication, and it provides coastal protection by damping waves and storm surge. This seminar will summarize some basic concepts in vegetation hydrodynamics, i.e. the physical way vegetation changes the flow field, including the coherent turbulent structures formed in the shear layers at vegetation boundaries and within the wakes behind finite patches of vegetation. These concepts will be used to explore two case studies. First, I will consider the change in flow and sediment resuspension as the density of plants within a seagrass meadow increases. Second, I will consider the wake behind a finite patch of vegetation. Because of its porosity, some flow passes through the patch, and this delays the formation of the von-Karman vortex street, leaving a region of low velocity and low turbulence directly behind the patch [clear region in photo]. Fine particle deposition is enhanced in this region, providing a positive feedback for patch growth. A numerical model is used to explore the impact of this patch-scale feedback on the landscape-scale evolution of aquatic vegetation.


Heidi Nepf has been a Professor of Civil and Environmental Engineering at the Massachusetts Institute of Technology since 1993. Prof. Nepf earned a B.S. in Mechanical Engineering from Bucknell University (’87) and a Ph.D. in Civil Engineering from Stanford University (’92). She was a Post-Doctoral Scholar at the Woods Hole Oceanographic Institution (1992-93), where she studied estuarine circulation in the Hudson River. In 2001, she became a Margaret MacVicar Fellow, the highest teaching honor at MIT. Prof. Nepf's research focuses on fluid motion in environmental systems, and she is internationally known for her work on the impact of vegetation on flow and transport in rivers, wetlands, lakes and coastal zones. At MIT, Prof. Nepf directs the Nepf Lab which focuses on the interaction of flow with aquatic vegetation and the feedbacks to sediment transport, chemical flux and ecosystem function, developing models for physical processes that determine how vegetated habitats (green infrastructure), such as seagrasses, salt marsh, and mangroves, provide coastal protection, mitigate anthropogenic nutrient and pollutant loads, and provide blue carbon reservoirs, with the goal of applying these models to improve the management of natural resources and the design of green infrastructure.