Tidal elevation data were presented for places along the Fleet (Abbotsbury, Morkham=s Lake, Moonfleet, Chickerell, Bridging Hard, Smallmouth, Portland). Harmonic analysis of the data was not able to represent the observations adequately, particularly at the inner end of the lagoon. Careful inspection of the data shows that the tidal regime is capable of being understood in terms of non-linear propagation of long waves in very shallow water. Distortion of the tidal wave by unequal progression speeds of high and low water, and the set up of mean level by frictional effects, are shown to be the important physical mechanisms controlling the observed water level fluctuations. A one-dimensional numerical model which incorporates these processes is able to reproduce the observations satisfactorily. Whilst the model predicts strong effects of wind stress, the meteorological influences in the observed data appear to be largely due to external surges in the English channel which propagate into the lagoon through its entrance.

Although the sea level variations in the Fleet do not have the regular tidal oscillation pattern typical of most UK ports and estuaries, it has been possible to account for the observed pattern in terms of the non-linear propagation of long waves. The tidal regime in the East Fleet is typical of a shallow estuary with extreme tidal asymmetry resulting from the large range to depth ratio. The strong frictional effects result in a much stronger set-up of mean level at spring tides than at neaps, and consequently the tides of West Fleet have a strong fortnightly component. The semi diurnal tide propagates into the West Fleet only weakly when the mean level is high at spring tides, and not at all when the mean level is low at neaps. The very shallowness of the lagoon appears to damp all higher frequency oscillations, both tidal and wind driven, and acts as a low-pass filter enabling the longer period tides and external surges to penetrate to Abbotsbury. Viewed in this light, the apparently irregular tidal regime is much more comprehensible and more readily predicted.

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