| Abstract | The longitudinal dispersion coefficient in inland waterway was calculated using experimental data from various natural streams. It was found that the predictive equation proposed by LIN (1977) gave results with high degree of consistency and more convenience to apply. Moreover,
when applying these data to the equation in the form proposed by HARLEMAN (1964), the results indicated that the equation in this form should be
Dx = 119 R 0·98.
For longitudinal dispersion, in the zone of salinity intrusion in
estuary, the relationship between various forms of dimensionless dispersion coefficient and Densiometric Estuary number were investigated. It turned out to be that no functional relationship can be drawn from these
attempts. Therefore, the relationship between dimensionless dispersion
parameter and Densiometric Estuary number, proposed by THATCHER and HARLEMAN
(.1972). , were than tested using data from 5 estuaries. Instead of using the maximum tidal velocity and length of salinity intrusion were used to normalize the dispersion parameter. The equation describing this relation
ship was found to be K/UfLi =
0.084 ED 0·32
Thereafter, the horizontal diffusion coefficients at Ao Phai, in the Gulf of Thailand, were calculated from the equation proposed by TAYLOR (1921) which had been transformed from Lagrangian to Eulerian system. The Lagrangian-Eulerian transformation factor, 8, was determined from experimental results of McQUIVEY, KEEFER, and SHIRAZ! (1971). It was
found that the horizontal diffusion coefficients were in the order of 10 4 cm2 /sec (3-8 m2 /sec).
The dimensionless diffusion coefficients in the ocean were found to be at the same order of magnitude as that proposed by Harleman (1964.
The Reynolds number, was calculated by using the mean water depth as the
characteristic length and the velocity was the average over half a tidal period of the maximum velocity of the whole period.
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