Approximate layer-averaged equations describing the mechanics of turbid underflows are derived. Closure of the equations describing the balance of fluid mass, sediment mass, and mean flow momentum provides for the delineation of a three-equation model. A description of sediment exchange with the bed allows for the possibility of a self-accelerating turbidity current in which sediment entrainment from the bed is linked to flow velocity. A consideration of the balance of the mean energy of the turbulence yields a constraint on physically realistic solutions to the three-equation model. It is shown that the self-acceleration predicted by the three-equation model is so strong that the energy constraint fails to be satisfied. In particular, the turbulent energy consumed in entraining new bed sediment exceeds the supply of energy to the turbulence, so that the turbulence, and thus the turbidity current, must die. The problem is rectified by the formulation of a four-equation model, in which an explicit accounting is made of the mean energy of the turbulence. Sediment entrainment from the bed is linked to the level of turbulence in the four-equation model. Self-acceleration is again predicted, although it is somewhat subdued compared with that predicted by the three-equation model. The predictions of both models are summarized over a wide range of conditions.

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Experimental results are reported concerning the nature of reflected flows generated when density currents are incident upon ramp‐type flow obstructions. The reflected flows are bores (moving hydraulic jumps that transport mass) with flow characteristics in common with either a group of solitary waves (weak Type A bores) or the original density current (strong Type C bores). Alternatively, the bore may have attributes in common with both of these end‐member forms (intermediate Type B bores). Bore strength is positively correlated with the ratio of reverse flow thickness to that of the residual tail of the forward flow. The largest values of this ratio occur when ‘proximal’reflections arrive at the steeper ramps. Measured particle paths in the bores indicate that natural examples will have the potential to transport and deposit sediment. Strong bores have velocity characteristics very similar to the original current and thus in nature the generated sequence of sedimentary structures will resemble those of the original depositing current. The train of solitary waves that make up a weak bore sequence exhibits a pulsating velocity profile at a point. Such flows may thus generate repeated sequences of structures separated by fine ‘drapes’that are distinguishable from the deposits of the original turbidity current. These conclusions are applied to examples of reflected turbidites described from the Palaeozoic to Quaternary sedimentary record.

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Pickering & Hiscott, (1985) have demonstrated amply the presence of reverse‐flow units within the thick‐bedded calcareous wacke (TCW) beds of the turbiditic Cloridorme Formation (Middle Ordovician, Gaspé Peninsula, Quebec, Canada). These reverse‐flow units are underlain and overlain by units which reveal flow in the primary (obverse) direction. In this paper, a model is proposed for this reverse flow, based on the probable nature of the primary turbidity flow. It appears that the initial flow was highly elongated (thickness h≪ length L), with h∼ 500 m, velocity U∼ 2 m s‐1 and sediment concentration C∼ 1·25%o. The rate of momentum loss of the flow is estimated by means of a useful parameter which we call the ‘drag distance’, symbol dD, defined by where h and L are the thickness and length of the flow, respectively; cCd is a combined drag coefficient representing friction on the bottom and at the upper interface; and fCd is a form‐drag coefficient related to the shape and size of the head. dD is the distance travelled by a current of constant h and L, flowing over a horizontal bottom and obeying a quadratic friction law, for an e‐fold reduction in velocity. Simple considerations, confirmed by our own experiments (described in this paper), show that such an elongated turbidity current cannot be reflected as a whole from an adverse slope: when the nose of the current reaches the slope, it forms a hump, which surges backwards and sooner or later breaks up into a series of internal solitons. The latter, probably numbering 4–7, will cause reverse flow at a given point as they pass by, provided that the residual velocity in the tail is not too great. Flow in the original (obverse) direction will be re‐established after the passage of the solitons. Quiescent periods in front of, between and behind the solitons, when soliton‐associated currents cancelled out the residual obverse flow, would allow the deposition of thin mud‐drapes. Additional flow reversals observed in a few of the TCW beds cannot be explained readily by the re‐passage of solitons, since wave breaking at the ends of the basin would cause massive energy loss; internal seiches are the preferred explanation for these later reversals.

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