Formation of the Transition Zone

The sand-mud transition zone may be described in terms of the net flux of sand over the accreting deposits of marine mud. Under excitation by the currents sand moves westwardly from the eastern Sound by ad-vection and turbulent diffusion until it is incorporated into the accreting 

The concentration of sand within the layer of mobile sediment is governed by a mass-balance in a control volume hdxdy  

The instantaneous sand-grain velocities may be written as the sum of three components the mean value over many tidal cycles, the tidal component, and a fluctuating component. 

Since the typical sedimentation rate is less than about a millimeter per year, the mean components of U and c represent averages over about 10 yr. Sediment deeper than h is here assumed to be part of the permanent mud deposit the fraction of this sediment that in fact may be mobilized by occasional very severe storms or bioturbation is neglected. The formation of the mud deposit occurs at a rate of - IVo and IVo is the velocity at which sand grains pass into the permanent deposit of marine mud and are removed from transport. 

Substituting (5.2) and (5.3) into the mass-balance equation and taking the time average over many tidal cycles (i.e., years), a simplified mass-balance equation may be written to represent the steady-state situation 

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