143 



In the above equations, the parameters Dq and B must be known. In a 

 real estuary, these parameters can be determined by measuring the average 

 salinities at two points in the estuary at low tide. 



Examples of average salinity distribution at high and low water slack 

 as shown in Figs. 9.2a,b are based on the experiments in a model tidal 

 channel at the Waterways Experiment Station (Harleman and Abraham, 1966). 

 This illustration of the difference in salinity intrusion between high 

 water slack and low water slack (tidal range 3 cm) is essentially analogous 

 to what would occur under an equivalent sea level rise. Note that 

 Figs. 9.2a,b clearly show that salinity intrusion is a highly dynamic 

 phenomenon, and that there is a 21.4 m difference in the distance of 

 penetration between the high and low water events. It is thus evident that 

 only a small head is required to cause a significant horizontal movement of 

 the salt water front. Raising the sea level (increasing ho in Eq. 9.1) 

 would amount to pushing both curves up the estuary. The same would occur 

 if ho were increased by dredging. Likewise, decreasing Uq , the outflow 

 velocity, would cause further penetration. This can be easily shown via 

 Eq. 9.2, for example. A possible secondary effect in the event of a sea 

 level rise could be higher tides at the mouth. In this case a^ would 

 increase, thereby reinforcing the penetration effect produced by increasing 



The aforementioned simple, demonstrative theory of course has major 

 liiiiitations by virtue of the assumptions under which the stated equations 

 were derived. In general, however, many of the restrictions can be relaxed 

 via solution of the fully three-dimensional transport equations 

 numerically. Furthermore, by following a hybrid approach involving a 

 combination of field data, physical model-based data and numerical 

 simulation, it is presently feasible to arrive at realistic descriptions of 

 salinity distributions in the estuary, and such effects as those due to sea 

 level rise can be simulated with a reasonable degree of confidence, 

 particularly in the tide-dominated (as opposed to waves) environment. It 

 should, however, be noted that the costs involved in "full blown" studies 

 remain high, with the result that salinity intrusion effects have been 

 investigated so far, in detail, mainly in highly urbanized estuaries. 



