Investigation of Surge and Wave Reduction by Vegetation
Watershed Physical Processes Research Unit
2012 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 more ecologically sound alternative. Adequate protection depends on both the type of vegetation and the amount available. For planning marsh restoration or establishment, it is essential 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 with a sloping beach with and without scaled vegetation. Video analysis was used to measure wave runup and wave setup. It was found that wave setup and runup were significantly reduced by the presence of scaled vegetation elements. In this phase of the work, experiments with flexible scaled vegetation on a sloping beach were completed, as were experiments with waves moving over submerged rigid and flexible vegetation. The final phase of the work is underway as of the writing of this report. Live samples of marsh edges were carefully cut from a marsh near the Louisiana coast and transferred to the wave flume. The samples contained intact mature rhizomes and grass. They were allowed to time to root into a packed silty loam substrate that approximately matched the soil characteristics common to coastal marshlands. Video and acoustic measurements were used to monitor the process of wave erosion over time. Monthly reports, frequent emails, and occasional meetings provided opportunities to keep cooperators informed of progress. Monitoring activities included site visits, emails, meetings, and telephone calls on at least a monthly basis.