Cnoidal and sinusoidal wave reflection from a laboratory sand beach

This study focuses on the evolution of accretional beach profiles under cnoidal and sinusoidal waves and on the reflection of those waves from the sand beach. The SUPERTANK test, STi0, conducted at Oregon State University in 1991 was reproduced at three different scales, 1:10, 1:8.5 and 1:11. Two tests were run at each scale by using sinusoidal and cnoidal waves. The results are compared and differences discussed for: energy spectra, reflection coefficients, phase angles between incident and reflected waves, reflection points and calculated bottom velocities. Beach profiles evolved differently under sinusoidal and cnoidal waves. Sinusoidal waves changed the beach profile part by part sequentially. Changes were first observed nearshore as berm building. After the berm was established, the offshore profile changed. Offshore bars grew under the sinusoidal waves. Cnoidal waves affected the whole profile and changes were observed over the entire “body of the beach” simultaneously. Changes were similar at the beginning of the profile for both kinds of waves, however. The initial profile for all tests was a barred profile. Sinusoidal and cnoidal waves both flattened the beach profile by removing the initial bar.; The variation in reflection coefficient with time during the evolution of the beach profile shows that equilibrium was attained differently for the two types of waves. The reflection coefficient reached a constant value when the berm face slopes was established. The berm slope developed early in the sinusoidal tests but developed later in the cnoidal tests.; Reflected wave spectra showed an energy shift between the fundamental and the first harmonic for both sinusoidal and cnoidal waves. This energy shift from the fundamental to the first harmonic was more pronounced in cnoidal waves, causing deeper bar-troughs on the beach profile. Frequency shifting is observed as beach profile changes. As profile changes, multiple harmonics are affected. First harmonic increases as berm crest builds up. After beach face slope is established, fundamental frequency is observed being sensitive to the slope changes.