An evaluation of the Rouse theory for the suspension of sand in a tidal inlet

Abstract

The Rouse theory for the suspension of sand in the Oka estuary has been evaluated. The Rouse profile, which is often used to represent the vertical distribution of sand in suspension, is a function of the ratio of the sediment fall velocity to bed friction velocity. This ratio, ws/u*, varied with height above the bed, as it is dependent on grain diameter. The grain size of sampled suspended sand was finer near the surface and coarser near the bed. This was evident at the three anchor stations occupied at the Oka estuary, where the field work was carried out. The values of the Rouse parameter showed that the sand transport in suspension took place throughout the benthic boundary layer. The suspension threshold of Bagnold (1966) was found to discriminate accurately between bedload and suspended load. The critical Shields parameter for suspension of sand, derived from the field measurements, defines the suspension threshold better when the friction velocity is assumed constant. Laboratory work carried out to evaluate the vertical distribution of the friction velocity throughout the benthic boundary layer result in a constant distribution with height above the bed in the lower 10%. This supports the assumption made in the Rouse theory. The sand concentration profile is more complex than just Roussian, as three layers have been recognized: a Roussian layer; a buffer layer; and an inner layer. The majority (90%) of the mass flux of sand took place within the inner layer. The concentration in this layer is characterized by an exponential increase towards the bed. This profile was not asymptotic to the normally accepted bed concentration (0.65). A new simplified equation to predict sand concentration in the inner layer was generated whereby: , where z is height above bed and h is water depth. The reference concentration, Ca, of the Rouse profile is predicted using the inner layer equation and solving at the upper limit of the inner layer (z/h = 0.1). The concentration gradient and magnitude (at a given height) were not related to shear stress or near bed flow velocity. Therefore, it is recommended that a near-bed measurement be used to define concentration at the bed, a, from which the concentration profile may be constructed.

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