ON THE NONLINEAR HELMHOLTZ RESPONSE OF ALMOST-ENCLOSED TIDAL BASINS WITH SLOPING BOTTOMS

Short relatively deep tidal basins, which are connected to the open se a by a narrow strait, may exhibit either an amplified (resonant), or d amped (choked) response to the tide at the entrance. Here particular a ttention is given to the lowest mode of response, which is the Helmhol tz or pumping mode, for which the sea level within the basin executes a spatially uniform oscillation. When the basin's sidewalls slope, the restoring process of this oscillator is nonlinear. Possible consequen ces of this nonlinearity are that the basin may exhibit either long-la sting high-waters and short, peaked low-waters or long-term regular mo dulation of its tidal amplitude, or a chaotic modulation of its tidal amplitude, or 'bent resonance horns', implying that resonant and choke d tidal responses may exist simultaneously for the same parameter regi me. Related field observations will be discussed.