The Influence of Vegetation on Channel Transport
The drag-discontinuity at the top of a submerged canopy creates a shear-layer that
generates coherent vortices. These vortices control exchange between the canopy and the
adjacent open water. Unlike free-shear-layers, the vortices in a canopy-shear-layer do not
grow continuously downstream, but reach and maintain a finite scale determined by a
balance between shear-production and canopy dissipation. This balance defines the
length-scale of vortex penetration into the canopy, e, and thus the region of rapid
exchange between the canopy and the overflow. Deeper within the canopy transport is
constrained by smaller turbulence scales. In this lecture, the physics of the canopy vortex
is used to explore the retention of water within a submerged canopy. This retention
influences both the longitudinal dispersion in the channel (i.e. the canopy acts as a deadzone), as well as the nutrient retention. The predicted canopy retention-times, which
range from minutes to an hour, are consistent with canopy retention inferred from tracer
observations in the field, and comparable to retention times for some hyporheic regions.
Dr. Heidi M. Nepf is a Professor at Massachusetts Institute of Technology, Department of
Civil/Environmental Engineering, Parsons Laboratory. Her research focuses on shallow aquatic
systems, including lakes, rivers, wetlands and coastal waters, with an emphasis on the influence
of vegetation in mean and turbulent transport. Nepf did her academic work (Bucknell University,
PhD, Stanford University and Woods Hole Oceanographic Institution Postdoctoral Scholar
Fellowship) in engineering and environmental hydraulics. Nepf was Co-Director, with Prof.
Culligan, Educational Outreach for the MIT Center for Environmental Health Sciences [19972003] and received several awards including National Science Foundation CAREER Award,
MacVicar Faculty Fellow - Institute award for outstanding teaching, Bose School of Engineering
Teaching Award, MIT, and Samual M. Seegal Prize - for inspiring student in pursuing excellence.
A longer biography can be found at:
http://web.mit.edu/nepf/www/