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Harleman Lecture - 2008

'How vegetation alters water motion, and the feedbacks to environmental system structure and function'

Dr. Heidi M. Nepf
Department of Civil and Environmental Engineering
The Massachusetts Institute of Technology
Thursday, October 16th, 4:30 pm
102 Thomas Building
The Pennsylvania State University
University Park, PA  16802

Heidi Nepf

Speaker Biography
Heidi Nepf has been a Professor of Civil and Environmental Engineering at the Massachusetts Institute of Technology since 1993.  Her 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.  By altering fluid motion at many scales, aquatic vegetation provides coastal storm protection, facilitates nutrient cycling, promotes sediment retention, provides habitat, and supports fisheries.  All together, these ecosystem services are valued at 13 trillion dollars annually.  By providing physical models of how vegetation impacts flow, Prof Nepf’s research guides the design of treatment wetlands, informs river and coastal restoration, and describes how land-use changes may impact ecosystem services.  For her expertise in vegetation hydrodynamics, Prof. Nepf was recently appointed to serve on the National Research Council panel charged with the review of the Army Corps plans for restoration and protection of the Louisiana coastline.  She is also a member of the Fluid Mechanics Steering Committee of IAHR and serves on the editorial boards of Water Resources Research, Environmental Fluid Mechanics, and the soon-to-be launched journal, LnO - Fluids.  Prof. Nepf is also a dedicated teacher and has received eight departmental and institute awards for her teaching.  In 2001 she became a Margaret MacVicar Fellow, the highest teaching honor at MIT.  In addition, Prof. Nepf is active in educational outreach.  Previously [1996-2003] she was the Co-Director for Educational Outreach for MIT’s Center for Environmental Health Sciences.  Currently, she is a technical advisor for the event and educator guides for the PBS program Design Squad: Inspiring a New Generation of Engineers.  Prof. Nepf earned a BS in Mechanical Engineering from Bucknell University (’87) and a PhD 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.

Abstract
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.  It removes nutrients, such as nitrogen and phosphorus, providing a buffer against coastal eutrophication.  Vegetation also promotes biodiversity, directly by creating refugia, and indirectly by producing spatial heterogeneity in the flow field that provides habitat diversity.  Further, marshes and mangroves provide coastal protection by damping waves and storm surge.  Finally, by reducing flow speed near the bed vegetation can change the pattern of erosion, or stop it altogether.  Through the above ecosystem services, aquatic vegetation contributes economic benefits worth over ten trillion dollars per year (Costanza et al. 1997).

After giving a general overview of the impacts of vegetation on aquatic ecosystems, this talk will summarize some basic concepts in vegetation hydrodynamics, i.e. the physical way vegetation changes the flow field.  Using these concepts we will explore two case studies.  In the first case, we consider the changes in flow as the density of plants within a seagrass meadow increases. In sparse meadows suspended sediment concentration is comparable to that in unvegetated regions.  In dense meadows suspended sediment is reduced, promoting sediment retention and improving light conditions, two feedbacks that promote meadow survival.  The second case study will examine flow feedbacks that impact seagrass meadow spatial structure, described by the fraction of bed area occupied by plants.  Physical reasoning explains why two configurations, 100% coverage and 40% coverage, are the most stable and thus most commonly observed in the field.