The purpose in writing these notes has been to present a discussion of a few topics central to a physical understanding of the mechanics of sediment movement. Discussion has been confined to unidirectional flows (excluding waves) of relatively small scale (excluding Coriolis effects). A large number of topics have been not considered at all or only in passing, including one of the most important problems in sediment mechanics: theories for the prediction of bed-load and suspended-load discharge. It seemed more important to try to develop some physical insight about the elementary processes of sediment movement than to attempt to elaborate any comprehensive quantitative theories of sediment movement.

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Turbidity currents were formed by releasing suspensions of plastic beads (density 1.52, median diameter 0.18 mm) from a lock into a horizontal water-filled flume. Graded beds were formed; the mechanism of deposition was studied by motion photography and the size grading by 150 size analyses.Deposition of sediment took place behind the head even at a time when there was no deceleration of the head: the greater part of the thickness of the bed was deposited during a period of rapid decline in velocity of flow within the body of the current. The mechanism of deposition and the type of grading differed for beds deposited from suspensions with concentrations less than and greater than about 30% by volume. Low concentration suspensions formed 'distribution grading' in which all percentiles showed vertical grading and at least the coarser half of the distribution showed lateral size decrease away from the gate. High concentration suspensions formed 'coarse-tail grading' in which there was almost no lateral size variation and the vertical grading was shown only by the coarsest few percentiles (except at the top of the bed).In high concentration flows the bed did not accumulate layer by layer, as it did in low concentration flows, but was deposited first as an expanded 'quick' layer, which was deformed by shearing and waves produced by the entrained water flowing over the still plastic bed.In both types of graded beds the sorting coefficient (standard deviation of the logarithm of the settling velocity) decreased upward within the bed, and to a lesser extent also laterally away from the gate. The skewness reached a maximum near the center of the bed and became negative at the top.

Two series of experiments were performed in a lucite flume 5 meters long, 50 cm deep, and 15.4 cm wide. In the first series saline density currents were formed by pumping salt solutions at constant discharge into the tilted flume. In the second series, the flume was horizontal and turbidity currents were formed by the releasing of suspensions of plastic beads from a box at one end.In both series of experiments a characteristic head was formed at the front of the flow. It was found that the motion of the head in the turbidity current experiments was closely described by laws developed by Keulegan (1958) for saline surges, and it is concluded that certain aspects of the motion of turbidity current heads can be investigated indirectly by means of experiments on density currents formed from clay suspensions or salt solutions.The salt-solution experiments were designed to investigate the effect of bottom slope on the motion of density current heads. It was found that the velocity of density (and by inference, turbidity) current heads on slopes up to 4% is adequately expressed by Keulegan's formula[Formula: see text]where v is the velocity of the head, Δρ is the difference between the density of the current (ρ) and that of the overlying water, d2 is the thickness of the head, and g is the acceleration due to gravity. The numerical coefficient is approximately constant, but may increase slightly with increase in slope. The form of the equation differs greatly from that of the Chézy equation which has previously been used for the analysis of the movement of turbidity currents.Observations were also made regarding the shape of the head and the motion within and in front of the head.

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The basic theory for the average velocity of uniform flow of a density current is now well established. The resistance at the bottom may be estimated from reasonable assumptions regarding the roughness of the bottom and the size of the current. The principal problem remaining is quantitative estimation of the resistance of the upper (fluid) interface. A review of the literature suggests that this resistance increases with increase in Froude number and decreases with increase in Reynolds number, and the writer's experiments support this hypothesis.As many turbidity currents are large scale and flow over low slopes of relatively small roughness it seems probable that both the bottom resistance and the resistance at the upper interface are small.

The 40‐km‐long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high‐tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel‐shaped geometry is developed, and by erosional or non‐depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single‐channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary. Longitudinal progradation will produce a 20‐m‐thick, upward‐fining succession, the lower 1/2–2/3 of which will consist of cross‐bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb‐dominated portion of this unit will be finer grained than the flood‐dominated part, and will contain trough crossbedding produced by 3‐D megaripples; the flood‐dominated areas, by contrast, will consist mainly of compound cross‐bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb‐ and flood‐dominated areas) will create packages of ebb cross‐bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral‐accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper‐flow‐regime parallel lamination. The succession will be capped by a 4‐m‐thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability.