Wave hydrodynamics in segmented wetlands with application to hurricane damage reduction and wetlands management and preservation

2010-2013 - $100,000

Jennifer L. Irish
Zachry Department of Civil Engineering
Texas A&M University, College Station
Texas Engineering Experiment Station

Rusty Feagin
Department of Ecosystem Science & Management/Forest Science
Texas A&M University, College Station

Abstract

Flooding by hurricanes is the primary natural threat to communities along the Texas coast, and fully understanding the risk of coastal flooding is paramount for sustaining these communities and economies. This project addresses a critical need to quantify the benefit of coastal vegetation in coastal storm protection along the Texas coast, to not only better define flooding risk, but also to demonstrate the value of these rich coastal habitats. A two-tiered plan for determining the benefit of coastal vegetation on flooding reduction is proposed. First, the impact of vegetation on wave setup (increased water level by wave breaking) will be quantified in the laboratory because this process dominates initial flooding inundation and is not yet fully understood. Second, a methodology for determining flooding reduction, by both wave setup and wind surge, as a function of vegetation characteristics will be developed by synthesizing laboratory results with existing theories governing steady flow through vegetation.

Accomplishments reported February 1, 2008, through January 31, 2009:

  • Laboratory experiments, planned in summer and fall 2008, were conducted in October 2008 in the 3D shallow water basin at Texas A&M University to investigate vegetated wave dynamics through plant fields.
  • To determine actual plant geometries for the laboratory testing, a site visit to Galveston Bay was conducted in August 2008, during which a number of one-meter-square plots in areas of healthy and unhealthy marsh were sampled, and plant stem spacing, diameter and elasticity were quantified. Natural channels through the marsh were also observed.
  • A synthetic plant field was constructed for use in the laboratory experiments consisting of random arrays of wooden dowels representing plants inset in plywood sheets. Hydrodynamic conditions tested were primarily regular gravity waves, but some tests of solitary waves were also conducted.
  • During the reporting period, the researchers began to analyze data from the laboratory experiments. Preliminary results from visual observation, however, show that when waves first reached the “vegetated” area, wave-crest turbulence and a rapid reduction in wave height were observed, and the effect appeared to be more significant when the plant stems were partially above the surface of the water versus fully submerged. Also, the data indicate a change in wave speed in the presence of vegetation, with higher density plant fields causing a more pronounced change.
  • The researchers also observed differences in the behavior of the flow as illuminated by dye injection in the mixing and dispersion process within plant fields in the presence of waves. The dye plume after injection into the vegetated flumes was quickly dispersed and spread throughout the vegetation as the waves propagated, while in the control flumes the dye plume remained relatively together. At the interface between the control channel and vegetation, the dye plumes in some cases moved into the control channel and in other cases moved entirely into the vegetation and dispersed.