Literature Summary #2
Ed Pritchard
Deng, Yang, Solo-Gabriele, Helena et al. 2010. Impacts of hurricanes on surface water
flow within a wetland. Journal of Hydrology (2011) Vol. 392, Issues 3-4, 164-173
Y. Deng is with Montclair State University’s Department of Earth and Environmental
Studies in Montclair, NJ, USA. H.M. Solo-Gabriele, along with another author, M. Laas
are with the Department of Civil, Architectural, and Environmental Engineering at the
University of Miami in Coral Gables, FL, USA. Other authors include L. Leonard with
the University of North Carolina-Wilmington’s Department of Geography and Geology
based in Wilmington, NC, USA, D.L. Childers with the Department of Biological
Sciences and Southeast Environmental Research Center at Florida International
University in Miami, FL, USA, G. He with Zhejiang University’s College of Civil
Engineering and Architecture in Zhejiang, China, and V. Engel from the South Florida
Natural Resources Center at Everglades National Park in Homestead, FL, USA.
1) Research problem or question: What are the hydrologic impacts of hurricane
wind and rainfall conditions on water velocity and water elevations in the Florida
Everglades?
2) Background knowledge leading up to study: Due to recent advancements in
technology and proliferation of monitoring systems, researchers have been able to
evaluate impacts of hurricanes on wetlands. These studies have been limited,
however, in their scope and have focused on nutrient releases, changes in
microbial and plant communities, impacts of sediment deposition and salinity, and
changes in wetland size. So far there is a limited amount of research available on
water flows and levels during hurricane conditions in wetlands. This study looks
to evaluate the impact of extremely strong winds on water elevations and surface
water velocity, two factors which are critical in the processes of particulate
settling and re-suspension of wetlands. The Everglades serve as a good test site,
as the ridge and slough ecosystems that make up the landscape are very dependent
upon the processes mentioned above.
3) Methods used by investigators: Water velocity measurements for this study
were collected at sites within sawgrass marshes in the vicinity of two tree islands
within the Shark River Slough of Everglades National Park, known as Gumbo
Limbo Hammock and Satin Leaf Hammock (hardwood hammock communities)
during and after two major hurricanes, Katrina and Wilma during the 2005
hurricane season. Surface mean wind velocity data were available from several
weather observation stations, the two closest of which were within Everglades
National Park at a height of 2 m from ground surface, at stations known as
Tenraw, and Chekika. Hourly rainfall and water elevation data were obtained
from three monitoring stations in ENP, monitored by park officials.
4) Results: Results of the study suggest that during each hurricane wind speeds
increased by over 10 m/s at both SL and GL testing sites. The maximum mean
wind speeds at GL and SL during Hurricane Wilma (31 m/s) were almost twice as
that during Katrina (ca. 17 m/s), and, accordingly, the water flow was much more
strongly affected in magnitude and direction by Hurricane Wilma than by
Hurricane Katrina. Thus, critical wind strengths appeared to exist, above which
the water flow was altered, with stronger winds causing greater impacts on water
flow. The researchers’ interpretation of the cause of the shift in flow speed and
direction were attributed to changes in the combined effects of wind shear,
differential rainfall, shift in hydraulic gradients, and changes in the structure of
submerged vegetation. Moreover, the characteristics of water flow after the two
hurricanes were significantly different and local vegetation likely played another
role, by minimizing the extreme changes in velocity during peak wind conditions.
5) Weakness of study: One weakness of this study lies in the lack of quantitative
data and discussion related to the relationships between the various factors of
hydrologic impacts. This study was highly qualitative and relationships were not
described in detail, therefore, future studies should look in to disaggregating the
different factors that influence water flow during hurricane conditions through
improvements in wind measurements. Studies should also focus more on the
changes in vegetative structure during these hurricane conditions, which might
help explain some of the changes in velocity that were observed after hurricane
conditions. Other suggestions would be to analyze wind velocities and impacts at
various vertical scales such as different water depth gradients and at different
hydroperiods throughout the hurricane season and across the Everglades
landscape.
6) What was learned: From the results of this study, I was able to gather that
baseline wind conditions (<10 m/s) were not a major factor influencing water
flow at depth. During the periods of hurricane force winds produced from
Hurricanes Katrina and Wilma, flow speed and direction were radically altered
due to the combined effects of forces applied at the surface of the water (wind
shear, rainfall, hydraulic gradients) plus forces placed on vegetation that is
emerging above the surface of the water. The longer lasting effects of hurricanes
(time scale of a few days) resulted in altered flow speeds that changed by 50% or
less with flow directions very close to those observed during non-hurricane
conditions. These longer lasting altered conditions could be a result of the
redistribution and removal of emergent vegetation and sediments across the
landscape.