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United States Department of Agriculture

Agricultural Research Service

Research Project: INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS

Location: Watershed Physical Processes Research Unit

Title: Experimental investigation of wave attenuation through model and live vegetation

Authors
item Ozeren, Yavuz -
item Wren, Daniel
item Wu, Weiming -

Submitted to: Journal of Waterway, Port, Coastal and Ocean Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 20, 2013
Publication Date: N/A

Interpretive Summary: Hurricanes and tropical storms often cause severe damage and loss of life in coastal areas. It is widely recognized that wetlands along coastal fringes reduce storm surge and waves. Yet, the potential role and primary mechanisms of wave mitigation by wetland vegetation are not fully understood. Knowledge of the reduction in wave height and energy as the waves encounter obstacles such as marsh grasses is essential for assessing the ability of vegetation to limit wave damage. It is also important for coastal marsh restoration design, where the width of the restored or established marsh may be based how the waves will be reduced as they cross the marsh. The goal of this study was to quantify the reduction in wave height and energy as they relate to vegetation type, density, and height through laboratory experiments. Wave attenuation characteristics for rigid and flexible model vegetation as well as live Spartina alterniflora and Juncus roemerianus, two common species along coastal Mississippi and Louisiana, were investigated under both artificial waves with unchanging characteristics and random waves that better represent the range of wave sizes encountered in coastal areas. Each type of model and live vegetation were used to calibrate drag coefficients, which can be used to predict the reduction of wave height in coastal marshlands. Electronic wave gauge data and video image processing were used to determine how the waves changed when they encountered model and live vegetation. The most important results from this work are: (1) the drag coefficients that will be used to predict the behavior of waves in coastal marsh areas and (2) the finding that variable diameter and leaf properties along the length of the plants affects the drag coefficient, which means that the water level will affect the drag coefficient.

Technical Abstract: Hurricanes and tropical storms often cause severe damage and loss of life in coastal areas. It is widely recognized that wetlands along coastal fringes reduce storm surge and waves. Yet, the potential role and primary mechanisms of wave mitigation by wetland vegetation are not fully understood. Knowledge of wave attenuation is essential for assessing the ability of vegetation to limit wave damage. The goal of this study was to quantify the attenuation of waves as a function of vegetation type, density, and height through laboratory experiments. Wave attenuation characteristics for rigid and flexible model vegetation as well as live Spartina alterniflora and Juncus roemerianus, two common species along coastal Mississippi and Louisiana, were investigated under monochromatic and irregular wave conditions. Drag coefficients of each of the tested vegetation species were calibrated based on the collected wave gage data and video images, and regression equations were derived for the drag coefficient as functions of the Reynolds number and Keulegan-Carpenter number. The results showed that vertical variation of plant density strongly influences the bulk drag coefficient.

Last Modified: 8/31/2014
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