Date of Award

1-1977

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Geology

Supervisor

Professor G. V. Middleton

Abstract

Macrotidal environments, such as Cobequid Bay which has a mean tidal range of 11.7 m (maximum measured range of 16.3 m) near its mouth, are not uncommon in the world's coastal ocean. Such areas are, however, the least studied compared to other areas with smaller tidal ranges.

Cobequid Bay is a large, funnel-shaped estuary located at the head of the Bay of Fundy. The bay's waters are well-mixed due to the large tidal range and because river input is small compared to the tidal exchange. The morphology and distribution of sediments and bedforms is mainly the result of the tides. The bay is flanked by an intertidal bedrock platform that is either exposed or covered with a veneer of sediments. Suspended sediment concentrations are high (up to 2700 mg/l), but mudflats occur only in the small estuaries and bay. Sand deposition is localized in the centre of the bay as an intertidal to subtidal channel-bar complex. The sand bars are elongate features, 1 to 5 km long, asymmetrical in transverse section, oriented parallel to the currents, and covered with megaripples (length = 1-12 m) and sand waves (length = 10-30 m ). The bars are composed of well-sorted, medium-grianed sands that range from 5 to 15 m thick (decreasing in thickness and relief towards the head of the bay), and that are underlain by local accumulations (up to 10 m) of premodern (possibly Pleistocene) sediments and bedrock.

The tidal currents are semidiurnal, reversing and time-velocity asymmetrical. Maximum near bottom (0.5 m ) speeds ranged from 0.3 to 1.0 m/s over the bars, and up to 2.0 m/s in the channels. Flow durations averaged 4.5 and 3.5 h for the ebb and flood over the bars, and 7.0 and 5.5 h for the ebb and flood in the channels. Current strengths and durations decreased towards: the shore, the bar crests and the head of the bay. The ebb and flood are dominated by a high-water 'sheet-flow' and a low-water 'channel-flow' relative to the low water emergence of the bar crests. The currents flow across the bars at an oblique angle to the crestlines during the 'sheet-flow' phase. The strength, duration and asymmetry of the currents are controlled by topographic shielding on and by the bars, the different hydraulic geometries and hypsometries of the channels and the variation of tidal range during the lunar month. The gently sloping sides of the bars are flood-dominated and the steep sides are ebb-dominated.

The measured vertical velocity profiles indicate that velocities are proportional to the logarithm of the depth despite the presence of large bedforms and large depths of flow. Three types of velocity profile shape are related to the location of the measured vertical over the bedforms and to time during the tidal cycle.

Bedform scale, internal and external geometry, and net migration rates differ with sediment size, and flow strength, duration and asymmetry. Megaripples move faster (0.2 to 0.8 m/tidal cycle) than sand waves (0.1 to 0.3 m/tidal cycle), and become larger and more three-dimensional with increasing flow strength and duration. The sequence of bedform development (plane bed→ripples→sand waves→megaripples) with increasing flow strength on the bars is similar to that observed in flumes and other sedimentary environments. Seven different bedform facies are defined from the external morphology and scale of the intertidal bedforms on the bars and their areal distribution is described related to a typical macrotidal sand bar. Megaripples reverse their asymmetry during each ebb and flood, but sand waves do not. Most of the preserved sedimentary structures in the intertidal bedforms are flood-oriented and are from ripples and megaripples. Sets of cross-bedding are separated by 'reactivation surfaces' and surfaces of discontinuity, and "herringbone cross-bedding" is uncommon.

The flow friction factor changes during the tidal cycle with different stages of bedform development and reversal, and suspended sediment concentrations. The friction factor at the time of maximum ebb speeds varies from about 0.032 for small megaripples to 0.050 for larger megaripples.

In winter, the presence of the frozen crust and the grounding of drift ice on the bars effectively stops the development of large bedforms, and obliterates or reduces the size of the bedforms from the summer.

The asymmetry of the currents results in the residual circulation of currents and sediment around the bars, or parts of the bars, within closed, or nearly closed, "elliptical loops." Coriolis effects are unimportant with respect to the pattern of general circulation within the bay which resembles a large "ebb tidal delta." The general properties of the sediments, bedforms and current hydraulics of the sand-body complex in Cobequid Bay are similar to those of other tidal environments, despite dissimilarities of tidal range and the scale, relief and topographic complexity of the elongate sand bars in a macrotidal environment.

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