WIND-WAVE PROPAGATION OVER FLOODED, VEGETATED LAND 



by 

 Frederidk E. Cornfield 



I. INTRODUCTION 



An important factor in the planning and design of works to protect 

 upland property during periods of storm surge is the prediction of the 

 wave height and period that will prevail at and seaward of the protec- 

 tive works (i.e., levee, dike, seawall) for a selected design storm. 

 Although improvements are needed, guidelines are available for predic- 

 tion of the water levels in upland areas resulting from storm surge; 

 however, no guidelines are presently available for computing the wave 

 attenuation for conditions when storm- generated waves travel a distance 

 across shallow flooded areas where the bottom characteristics include 

 vegetation which causes a moderate to high frictional stress. Therefore, 

 it is necessary to estimate the heights and periods of these storm- 

 generated waves. 



Theoretical methods and field measurements are currently unavailable 

 to provide an estimation procedure. This report presents a yreZimlnaay 

 (approximate) method for estimating the growth or decay of waves travel- 

 ing through shallow water over areas with a high frictional resistance 

 from vegetation. The method is based on previously developed equations 

 (U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 1975) 

 for wave growth over areas with low bottom friction, and an equation for 

 the decay of gravity waves over areas with a constant water depth and 

 high bottom friction. This method uses existing shallow-water wave fore- 

 casting curves by adjusting fetch lengths to account for higher bottom 

 friction. Simplifying assumptions are used. The water depth is assumed 

 to have only gradual variations, and the frictional resistance is treated 

 as bottom friction. The procedure discussed in this report has not been 

 verified in the field and may not be applicable to other problems relat- 

 ing frictional resistance to wave development. 



Only limited data are available on the effects of high values of 

 bottom friction on wind waves. Friction factors are estimated by compar- 

 ing vegetation to similar conditions in river channels and on flood plains, 

 The effect of the vegetation on wind stress and the possible effects of 

 motion of the vegetation are not considered. Dense vegetation effects 

 near the water surface which will dampen short-period waves much faster 

 than long-period waves are also not considered. The results obtained 

 are considered to be conservative; i.e., the predicted wave heights are 

 expected to be slightly higher than the wave heights which actually occur. 



II. WAVE FORECASTING CURVES 



The Shore Protection Manual (SPM)(U.S. Army, Coastal Engineering 

 Research Center, 1975) discusses a procedure for predicting the growth 

 of waves over shallow water, which is a modification of a method 



