142 



not, the estuary will be stratified. Since both the tidal range and runoff 

 vary; the former principally on a synodic basis while the latter 

 seasonally, many estuaries shift between fully stratified and well -mixed 

 conditions over a year period (Dyer, 1973). In some cases, very high 

 runoff can virtually "clean out" the estuary of salt water (e.g., during 

 and immediately following storm runoff or during the monsoon season) (Rao 

 et al., 1976; Mehta and Hayter, 1981). 



During a tidal cycle, the water level rises and falls, and this 

 variation directly influences the length of penetration of salt water; at 

 high water, salt water penetrates further than at low water. It is 

 instructive to look at the basic governing equations and their behavior in 

 this context. 



Ippen and Harleman (Ippen, 1966) considered the basic unsteady, 

 one -dimensional (x,t) mass conservation for salt transport-diffusion, and 

 derived the following expression for the resultant profile of salinity, 

 s(x,t) in a non- stratified estuary: 



^^^■t) ^ exp ^_ _U£_ [N- (N-x)expr^ (1-cos at)j + B]^}- (9.1) 



f 



where N = hQUQ/aQcr. Here, Uq = freshwater outflow velocity, Dq = diffusion 



coefficient, B = an empirical, diffusion-related coefficient, a^ = tidal 

 amplitude, o = tidal frequency, Uq = maximum tidal velocity at the mouth 

 and hQ = mean water depth. 



If the end of the intrusion zone is specified at s/Sq = 0.01, the 

 maximum intrusion length, Ljnax' occurs when crt = tt, and the minimum, L-min' 

 when at = . Thus 



W = » [1 - exp[- ^) 1 . B p Hi- - IJ exp(- ^ ) (9.2) 



Imin - BO 52_ - 1) (9.3) 



