Abstract:
Tidally forced circulation can cause a net near-bed transport of sediment when
the tidal velocity is asymmetric about a zero mean (flood or ebb dominant) and the
transport rate is nonlinearly related to velocity. The relationship between elevation
and velocity is elucidated here to enable one to determine from tide gauge data
and sediment transport relations whether tidal asymmetry may cause net sediment
transport. Tidal elevation and tidal velocity are related through the equations of
motion of the fluid. If the estuary is shallow, the change in cross-sectional area of the
channel with the tide is significant with respect to total area: the equations become
nonlinear and an exact solution does not exist. A relationship between elevation and
velocity in a nonlinear system is derived through the continuity equation and shown
to be significantly different than the linear relation. Finite difference numerical
solutions of the one dimensional, shallow water nonlinear equations are compared
to the continuity relation and are in good agreement.
The relationship between elevation asymmetry and ratio of flood-to-ebb bedload
transport is calculated for both the linear relation between elevation and velocity
and the nonlinear relation. Results show that the ratio of flood·to-ebb bedload
transport as calculated from the nonlinear relation between elevation and velocity
is similar to the flood-to-ebb ratio calculated from the linear relation.
