Investigation of Surge and Wave Reduction by Vegetation
Watershed Physical Processes Research Unit
2010 Annual Report
1a.Objectives (from AD-416)
Conduct laboratory investigations to assess the dissipation of wave energy and water level changes induced by vegetation.
1b.Approach (from AD-416)
Use the wave tank facility housed at the NSL to investigate the effectiveness of vegetation for dissipating wave and storm surge energy. First, rigid elements in regular spacings will be subjected to monochromatic waves, and the results will be compared to currently established theory to ensure proper measurement and operation of the wave tank facility. Successful completion of this phase of work will lead to experiments with flexible elements and live vegetation being subjected to both monochromatic and random waves. Vegetation type and flexible model scales will be based on native vegetation in coastal areas.
Both inland and coastal shorelines are susceptible to erosion by waves and storm surges, but mechanical shore protection can be prohibitively expensive. Vegetative bank protection is self-sustaining and is a much more ecologically sound alternative. Adequate protection depends on both the type of vegetation and the amount available. It is essential for planning to be able to predict the performance of vegetation based on measureable quantities such as stem diameter and the number stems in a given area. The project is focused on measuring the performance of marsh grasses for attenuating waves in relatively shallow water. The improved wave flume at the National Sedimentation Laboratory (NSL) was used to collect wave interaction data over three different densities of rigid model vegetation and one density of flexible model vegetation. Live specimens of Spartina alterniflora and Juncus romerianus were collected from the Louisiana coastal area and were subjected to testing in the wave flume. The wave generator was configured in both flap and piston-type arrangements in order to allow a greater range of depth and wave amplitude to be used in the experiments. Video analysis techniques were developed so that wave profiles before, during, and after wave interaction with vegetation elements could be measured; this step provides data needed for validating numerical models. Monthly reports, frequent emails, and occasional meetings provided opportunities to keep cooperators informed of progress.