INTERTIDAL SAND BARS IN COBEQUID BAY (BAY OF FUNDY)

Abstract

The largest tides in the world, with an average perigee spring range of 15.4 m and a maximum measured range of 16.3 m, occur at Burntcoat Head, on the south shore of Cobequid Bay, The tides generate currents with speeds of 1 to 1.5 m/sec in the centre of Cobequid Bay. These currents, and waves approaching mainly from the west, have reworked sand derived from glaciofluvial outwash and cliff erosion of bedrock into a major sand body which occupies the eastern part of the Bay. The sand body is 30 km long and 6 to 25 m thick. Most of it is subtidal, but close to shore and at the east end of the Bay there are several intertidal sand bars, which reach 6 m or more in relief and thickness, and have dimensions up to 4 kms in length. Maximum speeds of tidal currents over the bar surfaces generally range from 0.5 to 1 m/sec; the strength and direction are largely determined by shore and bar topography. In many areas either flood or ebb currents dominate, producing strongly asymmetrical patterns of sand dispersal. Bars are covered by sand waves (with wavelengths of the order of 30 m; these are not found on all bars), megaripples (with wavelengths in the range 1.5 to 5 m) and ripples. Many megaripples can be observed on depth recordings to reverse their orientation during each ebb or flood. On Selmah Bar, sand waves are flood-oriented and covered at low tide by ebb-oriented megaripples. The sand waves are found only on the south side of the bar in a low area (not affected by wave action) that is strongly dominated by flood currents, and they migrate about 20 cms per tidal cycle.