Field measurements of boat wake attenuation in salt marshes of coastal Alabama

Abstract

Four pressure gauges were deployed along natural marsh transects in Mobile, Alabama, to assess the ability of marsh vegetation to reduce the height and energy of oncoming boat wakes. The present study explores the influence of vegetation species, marsh density, wave height, wave frequency, and seabed terrain on the degree of attenuation. The pressure data were translated into instantaneous free surface position and wave height measurements, and the influence of shoaling and wave breaking was accounted for. It was found that attenuation is nonlinear, with an increased concentration of wave height and energy loss occurring in the first few meters, and that the seabed plays a significant role in wave transformation. The highest 1% of wave heights was most effectively reduced due to the combined influence of terrain and vegetation while the mean wave heights were the least reduced wave height statistic. Determination of accurate decay coefficients was complicated by the high degree of variability observed within the study. The results showed a correlation between wave frequency and energy dissipation, with the most substantial loss occurring at the peak frequency. Collectively, the findings highlight the variability between marsh sites and reinforce the recommendation that there be further exploration of wave energy attenuation by marsh grasses.