AN OCEANOGRAPHIC MODEL OF PUGET SOUND 693 



Density Structure 



In the past, most tidal model studies have been concerned with 

 silting in an estuary or harbor rather than the processes involved in and 

 resulting from a variable density structure. Little work on the dy- 

 namics of an inhomogeneous system resulting from the interchange of 

 oceanic water and river discharge in tidal models has been reported in 

 the literature, and the theoretical requirements for similitude in this 

 respect are not well resolved. It is thus necessary to determine the ef- 

 fects and interrelationships of the variable parameters in a model before 

 representation of the prototype may be achieved. A series of studies 

 was initiated to investigate the density structure as it is one of the ma- 

 jor factors influencing the oceanography of Puget Sound. When the 

 model structure under various conditions is known, it can be correlated 

 with nature to establish any major differences and demonstrate possi- 

 ble corrective measures. For the purpose of this work, the salinity 

 structure in each of the four main sections of Puget Sound was typified 

 by measurements at Point Jefferson, Camano Head, Green Point, and 

 Hood Point, (Fig. 7), under controlled conditions of tide, river run- 

 off, and source salinity. (Repeating tides with an average range and 

 diurnal inequality were used throughout this series of studies.) 



In the first experiment, the equilibrium situation was obtained for 

 mean river flow with a source salinity of 16°/(,o, which, for the purpose 

 of simplifying comparisons, has been related to a prototype salinity of 

 32°/oo. The results (Fig. 8) in general show a fresher surface layer 

 with sharp gradients to a depth of 10-30 meters, below which there is 

 a practically homogeneous more saline layer extending to the bottom. 

 At first, it might appear that the homogeneous bottom layers were due 

 to a lack of mixing of fresh water down from the surface; but that 

 this is not the case is shown when the salinity of this bottom water is 

 compared with that of the source water. At Point Jefferson and Ca- 

 mano Head, the bottom salinity is about P/oo less than the source, 

 while for Green Point the difference is about 1.5°/oo, which com- 

 pares favorably with the differences obtained in nature. It must be 

 that the mixing in these lower layers is sufficient to destroy any initial 

 gradient. 



The influence of the fresh water runoff on the salinity structure 

 is best seen from the results of raising the flow of the Skagit River to 

 the normal flood discharge rate of 50,000 c.f.s. (Fig. 8). After 1,000 

 consecutive days of flood, the effect of fresh water was noticed only to 

 a depth of 80 meters at Point Jefferson although the surface layer had 

 definitely freshened and deepened. At Camano Head, on the other hand, 

 the deep water had freshened somewhat with no perceptible change in 



