Location: Watershed Physical Processes ResearchTitle: Phase II Report for SERRI Project No. 80037: Investigation of surge and wave reduction by vegetation (Phase II) Author
Submitted to: Government Publication/Report
Publication Type: Other
Publication Acceptance Date: 9/24/2012
Publication Date: 9/24/2012
Citation: Wu, W., Ozeren, Y., Wren, D.G., Chen, Q., Zhang, G., Holland, M., Marsooli, R., Lin, Q., Jadhave, R., Parker, K., Pant, H., Bouanchaud, J., Chen, Y. 2012. Phase II Report for SERRI Project No. 80037: Investigation of surge and wave reduction by vegetation (Phase II). Government Publication/Report. SERRI Report No. 80037-02:1-397. Interpretive Summary: Surge and waves generated by hurricanes and other severe storms can cause devastating damage of property and loss of life in coastal areas. Vegetation in wetlands, coastal fringes and stream floodplains can reduce storm surge and waves while providing ecological benefits and complementing traditional coastal defense approaches such as permanent levees, seawalls and gates. However, little is known regarding the necessary scales and arrangements of vegetation needed to maximize surge and wave reduction benefit. Existing storm surge and wave models utilize the conventional quadratic law for bed shear stress and cannot realistically account for the mechanism of surge and waves through vegetation. Thus, it is highly desirable to develop more realistic parameterizations of the vegetation- dependent bottom drag coefficient. The main objective of Phase II of this research project was to expand on Phase I by continuing to conduct additional laboratory experiments, field measurements and computational modeling to investigate the effectiveness of wetland vegetation in mitigating hurricane and storm surges. The results of this project include: drag coefficients, based on laboratory experiments for both emergent and submerged vegetaton, that will be used to predict the behavior of waves in coastal marsh areas. Wave runup data on clear and vegetated sloping beach sections was measured. Field data on waves in coastal marshes were collected in Louisiana coastal marshes along with many field measurements of plant properties. Numerical models were developed, with improvements based on field and laboratory data collected during this project, that can used to predict wave behavior in coastal marshlands. A laboratory study of marsh embankment erosion was performed so that processes involved could be studied.
Technical Abstract: To better understand and quantify the effectiveness of wetland vegetation in mitigating the impact of hurricane and storm surges, this SERRI project (No. 80037) examined surge and wave attenuation by vegetation through laboratory experiments, field observations and computational modeling. It was a collaborative endeavor of the National Center for Computational Hydroscience and Engineering of The University of Mississippi, the USDA- ARS National Sedimentation Laboratory, the Civil and Envionmental Engineering Department of Louisiana State University, and the Biology Department of The University Of Mississippi. In Phase I dated from January 2009 to March 2011, a large amount of measurement data were collected and a series of empirical formulas and numerical models were developed. These efforts have been continued and expanded in Phase II from April 2011 to August 2012. The laboratory experiments of Phase I considered model and live vegetation at emergent and nearly emergent conditions in a flat-bed wave flume, as well as wave setup with rigid model vegetation on a sloping beach. In Phase II, the team extended the experiments by including the interaction of the regular and irregular waves with rigid and flexible vegetation with higher submergence (i.e., lower ratio of plant height to flow depth). The water levels were measured by both wave gages and video camera. The gage resolution was increased by adding two wave gages in the vegetation zone and a wave runup gage. Additionally, wave setup experiments with rigid model vegetation were extended to flexible model vegetation. The new datasets show that there is no apparent dependency of the drage coefficient on relative plant height for both rigid and flexible model vegetation used in these experiments. The rigid model vegetation had a slightly higher drag coefficient and performed better in reducing wave setup than the flexible one. As part of the field investigations in Phase II, the team collected wave data in a two-day period (Sept. 3-4, 2011) at a salt marsh wetland in Terrebonne Bay on the Louisiana coast during Tropical Storm Lee, and measured waves and bottom currents near the marsh edge on a vegetated wetland in south Louisiana in April 12–20, 2012 during a cold front passage. These field campaigns provided very useful data of wave attenuation by vegetation in high- energy wave environments, which are sorely needed in the literature. The datasets revealed the presence of bimodal wave spectra in the study site, consisting of low-frequency ocean swell in addition to the wind sea. The bulk drag coefficient was observed to decrease with increasing stem Reynolds number and Keulegan-Carpenter number and to be smaller for the longer-period waves than the shorter-period waves. The data was also used to develop a method to determine the frequency-dependent drag coefficient. In continued investigation of the vegetation and soil properties in LA and MS marshes, the team conducted vegetation damage testing using a newly designed plant tension testing device and measured the critical shear stress of coastal marshland soils using a cohesive strength meter combined with a laboratory testing program. Two sites in Terrebonne Bay and Barataria Bay of the LA coast were selected. The data collected by eight field visits showed that the tensile damage forces range from 17 to 76 lbs, with an average and standard deviation of 32.41 ± 11.56 lbs. The plants exhibit more failures at the stem in winter and early spring, but damage occurs in both stem and root randomly in summer when the plant reaches its maturity and hence maximum strength. The critical shear stress of the soils at both sites ranges from 0.4 to 1.9 Pa, and the average values are 1.03 to 1.16 Pa. The critical shear stress generally increases with root content, indicating that the roots play an important role in controllin