The wave base is defined as half the wavelength.

The wave base is defined as half the wavelength.

• A. One quarter of the wavelength
• B. Half the wavelength
• C. Equal to the wavelength
• D. Twice the wavelength

Answer: (B)

Understanding wave base starts with how a surface wave moves the water column. The motion produced by a wave is strongest at the surface and diminishes with depth; the depth at which that motion becomes essentially negligible is tied to the wavelength. A practical way to express this is that the wave base sits at about half the wavelength. At that depth, the orbital motion has decayed enough that the seabed experiences little to no wave-induced movement—the decay of velocity with depth is strong enough that the bottom feels almost nothing from the wave. This is why the rule of thumb uses half the wavelength: longer waves push their influence deeper, while shorter waves stop influencing the bottom much shallower. If the base were a quarter of a wavelength or a full wavelength, the bottom motion would not match the observed cutoff for negligible wave influence; twice the wavelength would push the base even deeper than needed. Keeping the base at λ/2 provides a consistent threshold for when bottom processes like sediment transport become largely unaffected by the wave motion.

Understanding wave base starts with how a surface wave moves the water column. The motion produced by a wave is strongest at the surface and diminishes with depth; the depth at which that motion becomes essentially negligible is tied to the wavelength. A practical way to express this is that the wave base sits at about half the wavelength. At that depth, the orbital motion has decayed enough that the seabed experiences little to no wave-induced movement—the decay of velocity with depth is strong enough that the bottom feels almost nothing from the wave. This is why the rule of thumb uses half the wavelength: longer waves push their influence deeper, while shorter waves stop influencing the bottom much shallower. If the base were a quarter of a wavelength or a full wavelength, the bottom motion would not match the observed cutoff for negligible wave influence; twice the wavelength would push the base even deeper than needed. Keeping the base at λ/2 provides a consistent threshold for when bottom processes like sediment transport become largely unaffected by the wave motion.