Abstract
Saline wedge invasion in the Gonokawa River was observed over lengthy periods in summer
and winter in 1998. Here we discuss the observed hydrological data from an hydraulic analysis
viewpoint. Saline wedge behavior was captured precisely using newly developed observation
systems which measured salinity, temperature and identified the interface between fresh and salt
water by sonic echo. According to these observations, the length of the saline wedge decreased
when the river discharge increased. In other words, the saline wedge moved downstream as the
fresh water velocity increased. This showed that the river comprised a two-layer laminar flow
with a viscous interface. Wecalculated the length of the saline wedge using specific parameters
including depth to the interface, temperature and salinity. The results accorded well with the
observed data, and a formula to evaluate the interfacial resistance coefficient incorporating the
Keulegan number was proposed. We have also measured the invasion speed of the saline wedge.
Simulations were carried out to estimate the invasionspeed, but the results did not agree with the
observed phenomena, because the simulation model assumed behavior as a perfect fluid.
