EWRI
ASCE
Vegetation Hydrodynamics
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Figure 1. Model vegetation used in Nepf’s Environmental Fluid Mechanics Laboratory at MIT to explore the changes in mean flow and turbulence within and around aquatic vegetation.  Different levels of model complexity are used, from (a) a patch of circular cylinders that can represent reeds and emergent grasses, to (b) a flexible model dynamically similar to seagrass [photo by Jeff Rominger and Marco Ghisalberti].

by Heidi Nepf, Department of Civil and Environmental Engineering, MI

For over a century vegetation has been removed from channels and coastal zones to facilitate navigation and development.  In more recent decades, however, we recognized the ecologic and economic benefits of aquatic vegetation.  Aquatic vegetation removes nutrients, such as nitrogen and phosphorus, so that vegetated stream channels provide a buffer against coastal eutrophication.  Vegetation promotes biodiversity, directly by creating refugia and indirectly by producing spatial heterogeneity in the flow field that provides habitat diversity.  Marshes and mangroves provide coastal protection by damping waves and storm surge.  The importance of mangrove forests, in particular, was highlighted by the heterogeneous impact of the Asian tsuami in 2004.  The death toll and property destruction in villages protected by intact forests was a small fraction of that in villages around which forests had been cut down to make way for aquaculture and resorts.  Finally, by reducing bed-stress, vegetation inhibits erosion and may alter the morphological development of the bed.  That vegetation inhibits resuspension is also key factor in stabilizing clear water conditions in shallow lakes, such that the restoration of vegetation is considered an essential step in lake rehabilitation.  Because of the ecosystem services it provides, including nutrient cycling, habitat, erosion control, and coastal protection, aquatic vegetation contributes economic benefits worth over ten trillion dollars per year (Costanza et al. 1997).

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Figure 2. The Outdoor StreamLab is a research facility developed by the St. Anthony Falls Laboratory (SAFL) and the National Center for Earth-surface Dynamics (NCED) at the University of Minnesota. It will bring together river scientists, engineers, and managers to explore river flow, river ecology, and restoration strategies. Artist rendering by Edward Hukck and Damon Sidel.

The examples above describe important ways that vegetation shapes the form and function of an aquatic ecosystem.  One of the key physical questions we must answer is, at what density of planting and for what area of vegetation are the physical changes in flow and transport sufficient to induce the ecological benefits listed above?  That is, how much vegetation is needed to make a difference in nutrient removal or coastal protection?  Studies in sea grass beds suggest that the biomass of a meadow must surpass a threshold before re-suspension is influenced by the presence of the meadow, and once surpassed the re-suspension decreases with increasing biomass (Moore 2004).  Young plants or sparsely distributed plants may not reduce the flow and bed-stress enough to inhibit resuspension and promote growth, and these patches may simply be erased by erosion.  Restoration efforts must take this into account by providing temporary shelter from high-energy flows until plantings can take hold, and by planting in sufficient density to ensure long term stability.  With funding from the National Science Foundation, we have used scaled laboratory experiments to construct and test models that predict the velocity and turbulence structure within meadows and canopies of different morphology (Figure 1).  The models predict the threshold biomass (or stem density) above which flow and bed-stress are significantly reduced.  Extrapolation from laboratory to field conditions will begin this summer with an exciting new research tool, the Outdoor StreamLab (OSL), located at the Saint Anthony Falls Laboratory in Minnesota (Figure 2).  The OSL is a field scale channel in which flow can be controlled and measurements acquired with laboratory precision.  In the inaugural summer we will investigate how vegetation alters flow in meander bends, and whether or where vegetation may enhance sediment retention sufficiently to alter morphological evolution of the channel.  For more information on the OSL please visit http://www.safl.umn.edu/facilities/OSL.html

References

Costanza, R., R. d’Arge, R. de Groot, S. Farber, M. Grasso, B. Hannon, K. Limburg, S. Naeem, R.V. O’Neill, J. Paruelo, R.G. Raskin, P. Sutton, M. van den Belt, 1997.  The value of the world’s ecosystem services and natural capital, Nature, 387, 253-260.

Moore, K.A., 2004. Influence of seagrasses on water quality in shallow regions of the lower Chesapeake Bay, J. of Coastal Res., 20 (Special Issue), 162-178.