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.
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