PARTICLE-ASSOCIATED BACKSCATTER IN CHESAPEAKE BAY:
HURRICANE ISABEL, BEFORE AND AFTER
T. Clayton1, C. Hu2, and J. Brock1
1
U.S. Geological Survey
Ctr for Coastal and Regional Watershed Studies
600 4th Street South
St. Petersburg FL 33701
2
University of South Florida
Institute for Marine Remote Sensing
700 1st Street South
St. Petersburg FL 33701
Abstract
Hurricane Isabel made landfall near Drum Inlet, North Carolina, on September
18, 2003, then proceeded on a north-northwesterly [westerly or easterly?] track up the
mid-Atlantic seaboard. Among the storm’s major impacts was a great deal of sediment
mobilization in the estuarine, barrier island, shoreface and shelf environments of North
Carolina and the Delmarva region. This effect can be clearly seen in satellite imagery of
the area in the wake of Isabel.
Preliminary estimates of particle-associated backscatter, bbp, in the Chesapeake
Bay region before and after Isabel’s passage have been derived from radiance values
obtained by the SeaWiFS ocean-color satellite. Preceding Isabel (Sept. 16), apparent bbp
values are generally lowest [value range?] near the western shore of the bay, increasing eastward
to maximum values near the Pocomoke. In the first post-storm image (Sept. 19), bbp values have
markedly increased bay-wide [value range?], indicating that bay waters are particle-laden. Over the
subsequent week (by Sept. 24), as suspended solids sink out of the water column, bbp conditions
return approximately to their pre-storm state. These results are consistent with field
observations of a temporary, subtstantial decrease in water clarity associated with storm
surge and wave effects. A persistent low-bbp feature near the mouth of the Potomac River
is also evident in the derived maps. Yet the ecological impact of the episodic events, such
as Hurricane Isabel, remains to be studied.
Hurricane Isabel
On September 18, 2003, 13:00 EDT, Hurricane Isabel made landfall near Drum Inlet, NC
(Fig. 1), as a category 2 storm with sustained winds of approximately 85 knots (National
Climatic Data Center, 2003). The effects of the hurricane were felt along much of the
eastern seaboard, from eastern North Carolina to western Pennsylvania. For Chesapeake
Bay residents, the most visible immediate effects included significant flooding (Fig. 2)
associated with a record-high storm surge (Fig. 3).
[Insert Figure 1 ~ here]
http://rapidfire.sci.gsfc.nasa.gov/gallery/?20032610918/Isabel.A2003261.1555.2km.jpg
[Insert Figure 2 ~ here]
[http://mddnr.chesapeakebay.net/eyesonthebay/isabel_impacts.html , Fig. 2]
[Insert Figure 3 ~ here]
http://www.usna.edu/PAO/isabel/images/HillBridgeWestEnd.jpg
Multi-platform Estuarine Monitoring
An optimal approach to estuarine characterization and monitoring combines automated,
real-time in situ sensing with spatial wide-field-of-view/synoptic sensing. Information
about Chesapeake Bay water quality and Isabel storm effects is available from a variety
of sources, including:
 Long-term monitoring stations along the mainstem of the bay and in many
tributaries (Fig. 4); sampling frequency ~ once per month + special-event cruises
 Continous monitoring stations in selected bay tributaries (e.g., Fig. 2); sampling
frequency ~ once every 15 mins.
 Ocean-color satellites (e.g., SeaWiFS, MODIS); sampling frequency ~ once per
day if cloud-free
 Spatially intensive sampling in selected bay tributaries
 Occasional field reports from selected site visits (e.g., SAV study sites)
[Insert Figure 4 ~ here]
http://www.chesapeakebay.net/images/baydots.gif
[This figure can be deleted if necessary due to space constraints.]
The View from SeaWiFS
In ocean-color satellite (SeaWiFS) imagery of the Cheapeake Bay region, one of the most
noticeable Isabel impacts was the suspension of marine sediments in the coastal estuaries
and on the adjacent shelf (Fig. 5).
[Insert Figure 5 ~ here]
http://daac.gsfc.nasa.gov/www/gallery/isabel/seawifs/
turbidity.jpg
[This figure should be large.]
The source of the shelf sediments is believed to be local (as opposed to terrestrial, as was
observed in the wake of Hurricane Floyd, 1999). Within Chesapeake Bay, some
sediment arrived via rivers, where turbidity spikes were observed in association with the
storm’s passage (e.g., Fig. 6). Some of the sediment is due to resuspension from the
bottom, while most of the sediment delivery to the bay seems to have come from
shoreline erosion (Fig. 7) due to the combined effects of wave action and storm surge.
[Insert Figure 6 ~ here]
[http://mddnr.chesapeakebay.net/eyesonthebay/isabel_impacts
.html, Figure 4]
[Insert Figure 7 ~ here]
http://www.nps.gov/hurricane/photos/gewa1.jpg
Water-quality Measurements from Space? A Preliminary View
Based on radiative transfer modeling and fieldwork in Tampa Bay, Florida, Lee et al.
(1999) developed an algorithm to derive a number of water-quality parameters from
satellite-derived measurements of water-leaving radiance (Figs. 8 and 9).
[Insert Figure 8 ~ here]
fig2reflectance_rev.tif
[Insert Figure 9 ~ here]
modelfFlowChart_SeaWiFS.ppt
Hu et al. (2002) adapted this algorithm for application to coastal satellite imagery, and
their method was applied to SeaWiFS data available for the Chesapeake Bay region
immediately preceding and in the wake of Hurricane Isabel. Estimates of particleassociated backscatter at 400 nm, bbp, are shown in Fig. 10. These data should be
considered preliminary, offering a qualitative first look at general patterns.
Preceding Isabel (Sept. 16), apparent bbp values are generally lowest near the western shore (e.g.,
near the mouth of the Potomac River), increasing eastward to maximum values near the
Pocomoke (Fig. 10A).
In the first post-storm image sufficiently free of cloud cover (Sept. 19), SeaWiFS-derived bbp values
have markedly increased bay-wide, indicating that bay waters are particle-laden (Fig. 10B).
Lowest values are again observed near the mouth of the Potomac. Elevated bbp values are also
observed on the shelf and within the “jet” seen in the true-color image (Fig. 5).
Over the subsequent week (by Sept. 24), as suspended solids sink out of the water column, bbp
conditions return approximately to their pre-storm state. As before, lowest bbp values are observed
near the Potomac River, and highest values are observed near the Pocomoke. Relative to prestorm conditions, backscattering remains slightly elevated in the mainstem area off Mobjack Bay.
Over the subsequent days (Sept. 26 and Sept. 27), changs are small relative to those observed
immediately post-storm (Figs. 10D and 10E). Although an increase in particle backscatter is
observed near the mouth of the Potomac, the general pattern is one of continued general clearing.
[Insert Figure 10 ~ here]
http://imars.marine.usf.edu/~hu/river/cbay/imgs/Isabel
(A)
(B)
(C)
(D)
S200325917_map_cbay_opt_bbp400.png
S200326217_map_cbay_opt_bbp400.png
S200326717_map_cbay_opt_bbp400.png
S200326917_map_cbay_opt_bbp400.png
(E)S200327018_map_cbay_opt_bbp400.png
color bar:
http://imars.marine.usf.edu/~hu/river/cbay/imgs/month/color
bars/bbp_colorbar.png]
[These figures are the point of the poster – should be large.]
[Figs. 10D and 10E can be eliminated if need be.]
These results are consistent with field observations of a temporary, substantial decrease in water
clarity in tidal portions of the estuary during and immediately after the storm. These effects are
attributed to sediment resuspension and shoreline erosion associated with storm surge and wave
effects (Maryland Department of Natural Resources, 2003).
Acknowledgements
This work was funded primarily by the U.S. Geological Survey (USGS) Coastal and Marine
Geology Program’s portion of the Atlantic Estuaries Project, in association with the USGS
Chesapeake Bay Science Program. Additional support was provided by the USGS Mendenhall
Postdoctoral Fellowship Program and the National Aeronautics and Space Administration (NASA
grant # NAG5-10738). SeaWiFS data acquisition and processing were made possible by the
SeaWiFS Project at NASA Goddard Space Flight Center and ORBIMAGE. Laurinda Frye
(USGS) was our valued partner in poster production.
References
Hu, C., Z.P. Lee, F. E. Muller-Karger, and K. L. Carder (2002). Application of an optimization
algorithm to satellite ocean color imagery: A case study in Southwest Florida coastal waters.
SPIE proceedings 4892 (Ocean Remote Sensing and Applications, edited by R. J. Frouin, Y.
Yuan, and H. Kawamura), pp 70-79. (http://imars.marine.usf.edu/~hu/papers/SPIE/)
Lee, Z., K. Carder, C. Mobley, R. Steward, and J. Patch (1999) Hyperspectral remote sensing for
shallow water: 2. Deriving bottom depths and water properties by optimization. Applied Optics
38(18): 3831-3843.
Maryland Department of Natural Resources (2003) Hurricane Impacts on the Chesapeake Bay.
http://www.dnr..state.md.us/bay
National Climatic Data Center (2003) Climate of 2003: Atlantic Hurricane Season.
http://www.ncdc.noaa.gov/oa/climate/research/2003/hurricanes03.html
Figure 1. Hurricane Isabel landfall, September 18, 2003. This image is from the Moderate
Resolution Imaging Spectrometer (MODIS) sensor ~700 km above the Earth’s surface.
Image courtesy of NASA MODIS Program.
Figure 2. Preliminary water level data, Annapolis, MD, 15 - 22 September 2003. Data courtesy
of NOAA National Ocean Service CO-OP.
Figure 3. Chesapeake Bay flooding, courtesy of Hurricane Isabel’s record-high storm
surge. U.S. Naval Academy, Annapolis , MD. Photo courtesy of U.S. Naval Academy, Public
Affairs Office.
Figure 4. Chesapeake Bay Program water quality monitoring stations. Map courtesy of the
Chesapeake Bay Program.
Figure 5. The left image, acquired by SeaWiFS on September 2, 2003, shows the relatively
clear waters characteristic of normal (calm) conditions. The right image, acquired on
September 19, one day after Isabel landfall, indicates a large area of suspended marine
sediments. Some of these sediments have been captured and transported northeastward by
the Gulf Stream, a strong ocean current. Images provided courtesy of the NASA SeaWiFS
Project and ORBIMAGE.
Figure 6. In the few places where continuous monitoring stations are established, spikes in
turbidity were typically observed in association with Hurricane Isabel’s passage. These
data are from Stonington on the Magothy River. Typical “background” levels for this station
during the year 2003 are ~ 10 NTU. Data courtesy of the Maryland Department of Natural
Resources.
Figure 7. At the George Washington Birthplace National Monument, the shoreline receded
approximately 50 feet overnight (the equivalent of 15-20 years of erosion in preceding
years). Image courtesy of the National Park Service.
Figure 8. Total radiance measured at the satellite sensor, LT, includes not only
contributions from the water column and the benthos – targets of primary interest in
Chesapeake Bay – but also from the atmosphere. An area of active research is the
development to algorithms to remove this atmospheric “contamination.”
Figure 9. Schematic representation of the multiparameter optimization scheme of Lee et al.
(1999) and Hu et al. (2001). Inputs to the model include satellite-derived estimates of waterleaving radiance plus a number of model assumptions. Output parameters include total
absorption at 440 nm, a(440); particle-associated backscatter, ap(440); gelbstoff-associated
absorption, ag(440); and particle backscatter, bbp(400).
Figure 10. Particle-associated backscatter (bbp) as preliminarily derived from data collected
by the Sea-viewing Wide Field-of-View (SeaWiFS) sensor orbiting ~700 km above the Earth’s
surface: (A) two days before Hurricane Isabel landfall, (B) one day after landfall, and (C) six
days after landfall
Logos:
USGS
University of South Florida