Abstract:
Field observations are made of the formation of backwash ripples
on the beachface, formed by undular hydraulic jumps generated by backwash
down the beach face colliding with wave bores. Measured ripple
wavelengths range from set averages of 48 to 70 cm. Within a particular
set of ripples there is a tendency for the spacing to decrease in
the offshore direction. These field observations are compared with
laboratory experiments where undular jumps are generated in a flume,
and with a computer simulation model which models both the flow within
an undular hydraulic jump and the resulting sediment transport which
gives rise to the backwash ripples. The computer model involves a
numerical solution of the Boussinesq equations which govern the fluid
flow, and sediment transport equations which relate the sand transport
rate to the local mean flow velocity. The computer model permits a
study of the detailed time-history of the undular jump development and
the formation of the backwash ripples. This model shows good agreement
with the field observations of backwash ripples, predicting an offshore
decrease in their spacing as observed. The laboratory experiments
showed a similar result so long as the Froude number of the supercritical
flow before the jump occurs is small, on the order of 1.4. Differences
between the computer model and experiments were small and arose
principally from the neglect of internal friction and surface tension
in the model. The study demonstrates the usefulness of the simultaneous
application of computer simulation models and laboratory experiments to
understand complex flow and sediment transport conditions such as occur
on beaches.
