Stephen Henderson
Research interests
Three-dimensional flow in tidal channels
During 2008 and 2009, together with Jullia Mullarney and Kassi Dallavis, I will contribute to a
multi-investigator experiment studying the tidal flats of Skaggit Bay.
Our component of this project aims to determine how three-dimensional flows might cause erosion and migration of tidal channels. We will also study tidal propagation
along the channels, and small scale (order 10 m) motion associated with very sharp dentsity gradients around the seaward boundary of the tidal channels.
We'll deploy Instruments that measure water salinity, temperature, and velocity in several arrays near a tidal channel.
Nearshore pollution transport in the Great Lakes
During April-July 2007 I worked as a Joint Research Investigator with
Dave Schwab and
Dmitry Beletsky at the
Cooperative Institute for Limnology and Ecosystems Research,
which is a Joint Institute between the Univeristy of Michigan and the
National Oceanic and Atmospheric Administration's
Great Lakes Environmental Research Laboratory. I investigated (and am still investigating) the possible
importance of wave-generated currents to pollution transport near the shores of the Great Lakes
Here's a JGR comment on mixing by breaking waves which I completed while working at CILER:
Forcing of Alongshore-Variable Currents by Breaking Surface Gravity Waves
In 2006, I started studying the
wave-generated currents on Black's Beach, California. Observations from the
recent Nearshore
Canyon Experiment indicate that the direction of alongshore currents
often reverses along this beach, with Southward currents at the North end of
the beach, and Northward currents at the South end. Current reversals seem to
result from alongshore variability of incident waves, which in turn results
from refraction over a submarine canyon. My colleagues in this work are Prof. R.T. Guza of the Scripps Institution of Oceanography, William
C. O'Rielly, Prof.
Tom Herbers of the Naval Postgraduate
School, and Steve
Elgar and Britt
Raubenheimer of the Wood's Hole
Oceanographic Institution. So far, we've developed a simple model for
nearshore waves and currents (the fact that the wave field varies much more
slowly alongshore than across-shore leads to a boundary-layer-like model for
nearshore currents). In the few cases considered so far, the model predicts
reversals in wave-generated forcing of alongshore currents roughly where the
observed currents reversed. I'm now testing the model using the full NCEX
data set. Here's a poster I presented at the 2006 Ocean Sciences conference
with some preliminary (very preliminary) results on this work:
Directional Spreading of Breaking Surface Gravity Waves
Working at the Scripps Institution of Oceanography with Prof. R.T. Guza in 2004, I studied the
refraction of swell frequency (0.05-0.15 Hz) surface gravity waves by low
frequency (0.001-0.05 Hz) surf zone motions such as shear and infragravity
waves. With help from Steve
Elgar and Prof.
Tom Herbers, we found that this refraction accounts for much of the
anomalously high directional spread of surface gravity waves observed in the
surf zone during the Sandyduck
field experiment. Here's a paper on this work:
Nonlinear Boundary Layers and Beach Accretion
During 2002 and 2003, I worked at Oregon State University's College of Oceanic and Atmospheric
Science with Prof.
John S. Allen. We found that an eddy-diffusive model of water and
sediment motion near the seabed predicts the shoreward and seaward migration
of a shore-parallel sandbar observed during the Duck94 field
experiment. According to the model, sand was carried seaward by the seaward
mean flow. More interestingly, sand was driven shoreward by the combined
effects of several nonlinear wave processes: wave skewness, wave asymmetry,
wave-generated vertical momentum fluxes (Eulerian streaming), and the Stokes
drift. Surprisingly, horizontal pressure forces on sediment particles were
not required to predict shoreward bar migration. Here's a paper on this work:
The model code, which solves boundary layer
equations for nearbed water and sediment motion using a k-epsilon turbulence
closure, is available here.
Loss of Infragravity Wave Energy
Before 2002, I was a Ph.D. student at Dalhousie University's Department of Oceanography. My thesis, completed under the excellent supervision
of Prof. A.J. Bowen,
dealt with the surprisingly strong effects of energy losses on low frequency
(0.005-0.05 Hz) surf-zone surface gravity waves (known as `infragravity
waves' or `surf beat'). Here are a couple of our papers showing evidence of
rapid loss of infragravity energy during the Duck94 and Sandyduck
experiments:
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Henderson, Stephen M. and A.J. Bowen (2002),
Observations of surf beat forcing and dissipation, Journal of
Geophysical Research, 107, (C11), 3193,
doi:10.1029/2000JC000498.
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Henderson, Stephen M., Elgar, S. and A.J. Bowen (2001),
Observations of surf beat propagation and energetics, Proceedings of the
27th International Conference on Coastal Engineering, 1412--1421.
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Energy losses scaled as would be expected
for bottom friction. Here's a paper on the theory of dissipative infragravity
waves:
Shameless self-promotion: Here's an award I got for this work on the theory of
dissipative infragravity waves.
During 2005, with help from Bob Guza, Steve
Elgar, Tom Herbers, and Tony Bowen, I revisited this problem. We found that
most of the infragravity energy lost in he surf zone is not dissipated by
bottom friction, but instead is transferred to higher frequencies by
conservative nonlinear triad interactions. Here's a JGR paper on this work:
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Henderson, Stephen M., R.T. Guza, Steve Elgar,
T.H.C. Herbers, and A.J. Bowen (2006), Nonlinear generation and loss of
infragravity wave energy, Journal of Geophysical Research, 111, C12007, doi:10.1029/2006JC003539.
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