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|Title:||Sediment Dynamics of Macrotidal Sand Bars, Bay of Fundy|
|Authors:||Dalrymple, Walker Robert|
|Abstract:||<p>The Minas Basin and Cobequid Bay, Nova Scotia, possess the highest tides in the world: maximum measured range = 16.3 m; mean range = 11.9 m. Numerous large sand bars, with exposed lengths ranging from 1 to 10 km, and widths from 0.2 to 4.25 km, are developed in the area. The total relief reaches 15 m in Cobequid Bay, but only the upper 6 to 7 m is exposed at low tide. The bars are generally asymmetric with the gentler side facing the shoreline, which is commonly bordered by a gravel-veneered, wave-cut platform. The sedimentation dynamics of five major bars has been studied in detail.</p> <p>Reversing tidal currents are the dominant process operating on the sand bars, wave action is limited by the small fetches. The flood tide is of shorter duration than the ebb throughout the area, because the turn of the tide at low water is delayed up to 1.5 hours by bottom friction and the ebb hydraulic gradient. The delay increases toward the head of Cobequid Bay.</p> <p>Time-averaged mean current speeds range from 0.2 to 0.9 m/s, and shear velocities from 1.0 to 5.0 cm/s, with the higher values generally occurring in the channels. Because the bars lie at a slight angle to the flow, their upstream sides experience stronger currents than the downstream sides as the currents flow over the bars. These zones alternate position with every flow reversal, producing an inequality between the ebb and flood currents at most locations. This process, and not helical flow, is responsible for the origin of bars.</p> <p>Fluorescent tracer experiments reveal that residual sediment transport occurs almost everywhere. Residual unit discharges range from 0.38 to 1793 kg/tidal cycle, and average approximately 350 kg/cycle. (Particle transport speeds average only 1 to 2 m/cycle.) On individual bars, sediment moves in opposite directions on either side of the crestline, with upslope components on both sides. Within Cobequid Bay, ebb-dominated transport is largely confined to the axis of the bay, whereas flood transport occurs along either shore. A sediment circulation system analogous to that of ebb-tidal deltas is produced, with clockwise sediment circulation in the northern half of the bay, and counterclockwise transport in the southern half. The swatchways break these large circulation cells into smaller units.</p> <p>Sediment dispersal from a point source is fundamentally similar to that occurring in unidirectional flow because of the residual transport. The tracer distributions are skewed in the direction of residual transport, but become more symmetrical as time passes. The longitudinal dispersion is positively correlated with the centroid migration speed and flow strength, and averages 3.5 times the lateral dispersion.</p> <p>Two distinct types of large-scale bedforms are recognized: megaripples (dunes) and sand waves; the sand waves are subdivided into rippled and megarippled varieties, depending on the superimposed features. Megaripples are more irregular in appearance than sand waves, and possess scour pits which sand waves lack. Sand waves are generally larger but flatter than megaripples, and rarely possess an avalanche face, so that the internal stratification is composite and produced by the superimposed bedforms. Ripples sand waves form where peak mean speeds are less than 0.8 m/s. Both megarippled sand waves and megaripples exist in faster flows; megaripples where mean grain sizes are finer than 1.7 phi (0.31 mm), and megarippled sand waves in coarser sediment.</p> <p>On average, the sediments are medium sands with moderately well sorted, nearly symmetrical and leptokurtic size distribution. The coarsest sizes and poorest sortings occur near areas of gravel lag, and the channels tend to be coarser than bar crests. The finest sands are found in the ebb-dominated parts of Cobequid Bay.</p> <p>There are three fundamental, overlapping-normal size populations: the traction (C), intermittent suspension (A), and continuous suspension (B) populations. The average mean size and sorting of each are: C: -0.18 phi and 1.15 phi-units; A: 1.70 phi and 0.44 phi-units; and B: 2.56 phi and 0.93 phi-units. The fine-grained character of the sediment in the ebb-dominated part of Cobequid Bay is retained during transport (inherited) because the flow energy increases along the transport path. Elsewhere, both A and C populations (and the total sample) tend to be finer where the currents are weaker.</p> <p>Tracer experiments show that the transport and dispersion rates are almost independent of grain size within the traction population, whereas the rates increase rapidly as the size decreases for sizes travelling in intermittent suspension. The transition in behaviour occurs at the grain size whose fall velocity is equivalent to the average maximum shear velocity of the currents. The "break" between the C and A populations, as determined by the point of equal overlap, bears a similar, statistically significant relationship to the shear velocity. The point of intersection of straight-line segments gives less satisfactory results.</p>|
|Appears in Collections:||Open Access Dissertations and Theses|
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