TIDAL AND CURRENT SURVEY CANADA.
INVESTIGATIONS IN HAMILTON INLET AND IN THE NARROWS AT RIGOLET.
SUPPLEMENT TO APPENDIX D ” OFREPORT ; BEING AN EXPLANATION REGARDING THE INCREASE IN THE RANGE OF TIDE AT THE INNER END OF LAKE MELVILLE.
In making a minute comparison between the amount of tide at different places, the “ range ” and not the “ rise ” of tide must be examined, and this must be done preferably with simultaneous tides. The “ rise ” is unsuitable because it is a height measured from the chart datum plane at each place and these planes may be slightly at variance one with another relatively to the tide by an amount quite inappreciable with regard to the purposes of a chart, but preventing a true comparison being made when we are considering such small differences as we have under discussion in this instance. By “ range of tide ” is meant the amplitude of the tidal undulation.
The tidal record at Caravalla, Epinette Point and Rabbit Island was simultaneous, and thus the figures in the second table of the appendix “ d ” to the report, referring to the amplitude of the tidal undulation at these places, are strictly comparable. The first table of “ rise ” is one to use for practical purposes the height being given only to the nearest quarter foot from the datum plane of the chart, and the slight exaggeration of the figures for Epinette Point and Rabbit Island given in this table for rise, is due to the fact that these datum planes, from which the rise is measured are a little lower at these places than at Caravalla.
To explain this greater amplitude at the inner end of the lake we can refer to the fact that tidal waves progressing between converging shores develop in this manner for the reason that the water is crowded by the narrowing confines, causing it to mount up and when a decrease occurs in the depth of water through which the wave is travelling the same effect is produced. The fact that the spring tides increase more than the neaps is significant because this is according to the behaviour of confined tides generally, and substantiates the explanation. In Lake Melville the width is much less at Epinette Point, and the depth is much less as well in Goose Bay, where the observations were taken at Rabbit Island, so this characteristic is to be expected. The apparent increase at Epinette Point as shown by the figures, can scarcely be regarded as indicating that the change begins here the amount is so small but being near the end of the lake it probably does.
Lake Melville is hardly large enough to have any noticeable tide of its own, but the oceanic undulation entering the lake has an amplitude of 1.30 feet for springs and 0.67 feet for neaps, at Caravalla. It may be that this flattens out somewhat in the expanse of the lake (there is no record in the central part of the lake to show this), but certain it is that when the undulation nears the inner end, the water piles up slightly giving a range of 1.63 feet and 0.90 feet at Rabbit Island, an increase of 0.33 feet and 0.23 feet at springs and neaps respectively over the amount at Caravalla. (It may be pointed out that the difference in rise from the mean tide level of the lake is only half this amount.)
Senior Tidal and Current Surveyor.
Ottawa, 11th April, 1924.