Observations of Seiche Forcing and Amplification in Three Small Harbors

Extensive field observations are used to characterize seiches (periods 0.5–30 min) in three small harbors with similar surface areas (∼1 km ² ), water depths (5–12 m), and swell wave climates. On the continental shelf just offshore of each harbor mouth, the energy levels of waves in the infragravity frequency band 0.002– 0.03 Hz (periods 0.5–10 min) vary by more than a factor of 200 in response to comparably large variations in swell energy levels. Energy levels in this swell-driven frequency band also vary (less dramatically) in response to changes in the swell frequency and with tidal stage. Motions at longer seiche periods (10–30 min) are primarily driven by meteorological and other processes (a tsunami-generated seiche is described). As has often been observed, the amplification of seiche energy within each harbor basin (relative to energy in the same frequency band outside the harbor) varies as a function of seiche frequency, and is largest at the frequency of the lowest resonant harbor mode (i.e., the Helmholtz or grave mode). At all three harbors, the average amplification of the grave mode decreases (by at least a factor of 2) with increasing seiche energy, a trend consistent with a nonlinear dissipation mechanism such as flow separation in the harbor mouth or sidewall and bottom friction.