Page Banner

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: Phase I Report for SERRI Project No. 80037: Investigation of surge and wave reduction by vegetation

Authors
item Wu, Weiming -
item Ozeren, Yavuz -
item Wren, Daniel
item Chen, Qin -
item Zhang, Guoping -
item Holland, Marjorie -
item Ding, Yan -
item Kuiry, Soumendra -
item Zhang, Mingliang -
item Jadhave, Ranjit -
item Chatagnier, James -
item Chen, Ying -
item Gordji, Leili -

Submitted to: Laboratory Publication
Publication Type: Other
Publication Acceptance Date: March 1, 2011
Publication Date: March 1, 2011
Citation: Wu, W., Ozeren, Y., Wren, D.G., Chen, Q., Zhang, G., Holland, M., Ding, Y., Kuiry, S.N., Zhang, M., Jadhave, R., Chatagnier, J., Chen, Y., Gordji, L. 2011. Phase I Report for SERRI Project No. 80037: Investigation of surge and wave reduction by vegetation. Laboratory Publication. 1:315.

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. Hence, the main objective of Phase I of this research project was to conduct 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, that will be used to predict the behavior of waves in coastal marsh areas; field data on waves in coastal marshes collected in Louisiana coastal marshes; many field measurements of plant properties; numerical models, with improvements based on field and laboratory data collected during this project, that can used to predict wave behavior in coastal marshlands. The improved understanding of wave attenuation in grasses can also be extended to use in other areas, such as levee protection.

Technical Abstract: 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. Hence, the main objective of Phase I of this research project was to conduct laboratory experiments, field measurements and computational modeling to investigate the effectiveness of wetland vegetation in mitigating hurricane and storm surges. Extensive laboratory experiments were carried out to investigate wave attenuation by rigid and flexible model vegetation as well as live vegetation under monochromatic and random wave conditions in a flat-bottom wave flume. The live vegetation species include Spartina alterniflora (dormant and green) and Juncus roemerianus (green). The total number of wave and vegetation combinations was 1,041 for regular waves and 476 for irregular wave experiments. A total of 320 experimental configurations were utilized, with each configuration repeated three times for regular waves and up to five times for irregular waves to obtain more reliable data sets. Drag coefficients of all the tested vegetation species were derived from the collected wave gage data and video images, and regression equations were derived for the drag coefficient as functions of the Reynolds number, Keulegan- Carpenter number and vegetation submergence ratio. Laboratory experiments were also conducted to assess the effect of rigid model vegetation on wave setup over a sloping beach. The experiments demonstrated the reduction of wave setup and runup by vegetation over a sloping beach. Field investigations of surge and wave attenuation by vegetation included two campaigns under tropical storm and winter cold front conditions. Considerable effort was devoted to select sites at Terrebonne Bay, LA, where permission was obtained to access a privately-owned wetland suitable for the project. An array of instruments, including 9 wave gages and one water level gage, was developed and deployed at a fixed location in Terrebonne Bay to measure wave attenuation over shallow water and salt marshes during the hurricane and cold-front seasons of 2009 and 2010. In addition to the instrument array at the fixed location, five portable, self-recording wave gages were successfully deployed twice in Breton Sound and Terrebonne Bay in rapid response to Tropical Storm Ida in 2009 and Tropical Storm Bonnie in 2010, even though no hurricanes made landfall on the Mississippi and Louisiana coast in the project period of 2009-2010. The data collected at the fixed site at Terrebonne Bay was not conclusive because of unexpected factors which could not be controlled. However, valuable surge and wave attenuation data during the two tropical storms were collected. Field investigations also included measurements of vegetation and soil properties. In addition to the field sites at Terrebonne Bay and Breton Sound on the Louisiana coast, eight transects were established at Graveline Bayou in Gautier, MS and the Grand Bay National Estuarine Research Reserve in Pecan, MS. Biomechanical properties Spartina alterniflora and Juncus roemerianus and related soil properties were measured at the selected marsh sites. The relationship of the stiffness and height/diameter ratio of these vegetation species was established using the measured data. Vegetation height, diameter, coverage, and biomass in the lower and higher marshes as well as in coastal and inland marshes were compared. Seasonal vegetation growth was quantified. Four typical numerical models were developed or adopted for quantifying the effects of vegetation on surge and waves. The models included a depth-averaged two-dimensional shallow water model for long waves such as tsunami wave, a one-dimensional Boussinesq model for short waves such as wind waves, a vertical two-dimensional model based on the Navier-Stokes equations for both long and short waves, and a wave-action model for wave energy spectra. The first three models are phase-resolving models that compute the wave dynamic deformation processes, and the fourth model is a phase-averaging model that determines the characteristic wave height, period and spectrum distribution. All four models can handle both non-breaking and breaking waves. The models were tested using data from the present laboratory experiments and from the literature. The model results demonstrated the reduction of surge and waves by vegetation. In the course fulfilling the goals of this project, the research team has devoted tremendous efforts that have resulted in the completion of all the planned tasks for Phase I. The laboratory and field data, the empirical formulas developed from the data, and the numerical models are all significant contributions to better understanding and ability t predict surge and wave attenuation by vegetation. It is anticipated that these results will find broad application in coastal management and planning for areas with existing coastal marshlands as well for marsh restoration and establishment efforts.

Last Modified: 10/1/2014
Footer Content Back to Top of Page