Measurements of wave attenuation due to a soft bottom: The SWAMP experiment

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Abstract
The Sea Wave Attenuation Measurement Program (SWAMP) was designed to provide measurements of the attenuation of waves as they travel from deep water to relatively shallow water off the Mississippi Delta. The soft bottom was suspected of causing strong attenuation in the area. This effect has now been measured. A wave staff and electromagnetic current meter were used to estimate directional wave spectra at the Cognac platform in 312 m (1025 feet) of water and at platform VV in South Pass Block 27 in 19 m (63 feet) of water. In addition, measurements of vertical bottom motion were made at platform VV. Seven storm periods from Sepember 1979 to February 1981 were studied. The directional spectral comparisons showed that the theoretically calculated refraction and shoaling can explain the changes in the spectra when the wave height is low. However, as the wave height increases, a nonlinear attenuation mechanism becomes increasingly strong. The attenuation is a strong function of deep‐water wave height and a weak function of wave frequency. The bottom motion measurements showed that the bottom moved downward with small amplitude under wave crests.

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