27 



ceived to study very rapid changes in turbulent flow on a small scale. Since 

 oceanographic observations are more generally concerned with large scale proc- 

 esses, clumsier methods may be more suitable. For example, the broad fea- 

 tures of the bottom circulation in clear, shoal water can often be discerned from 

 a study of the "sediment shadows" behind obstacles in the paths of the swiftest 

 currents. These are often most clearly shown on aerial photographs of the 

 estuary. Dye marker or mimeograph paper dropped from aircraft can be photo- 

 graphed to measure the surface currents. This technique permits large areas 

 to be surveyed in a few hours and is especially valuable when wind stress con- 

 stitutes an important driving force. It has been found that current measure- 

 ments can be made with dye dropped from heavy aircraft in weather that is too 

 boisterous for current measurements to be made successfully fronn anchored 

 ships. Fluorescein dye charges must be increased when the light is poor or 

 when the water is turbid. If fluorescein does not show plainly enough to be 

 photographed, mimeograph paper can be used. .Mimeograph paper has the 

 property of soaking rapidly without curling and will usually drift just under the 

 surface. It is not capable, however, of being anchored to produce streamers 

 and reference points in the photographs, as is fluorescein in cloth bags. In clear 

 water on bright days it is possible to drop a weighted charge of fluorescein 

 through the water column and in that way study the change or velocity with depth. 

 Stereoscopic pairs of photographs can be made and if the depth of water is known 

 the significant current velocity changes can be measured at several levels. 



Models have also been used in connection with oceanographic studies of 

 estuaries and coastal circulations. Tully (1949) and Barnes and Lincoln ( in 

 preparation) have built models of fjord type estuaries for which they found Rey- 

 nolds scaling most suitable. For coastal plain estuaries, Froude scaling has 

 been used with adjustment of the density ratio of the mixing sea and river water 

 components by approximately the reciprocal of the vertical exaggeration (von 

 Arx, 1950c). Models serve to guide field work in that they reveal the manner 

 in which observations made at several points are related and often suggest where 

 more critical observations are needed or may best be made. Aside from the 

 field program, however, models may also serve to reveal the contribution to 

 the whole complex circulation pattern made by each of the principle groups of 

 forces. The effect of tides may be isolated as may be the effects of river dis- 

 charge, wind force and direction, and the offshore circulation. Combinations 

 of these may be produced experimentally to study the circumstances which pro- 

 duce particularly favorable or unfavorable ecological conditions or influence the 

 flushing rates in certain restricted areas where sewage, industrial wastes or 

 other contaminants may be pocketed. 



It is interesting to note that while very great advances have been made 

 in recent years providing statistical means for calculating the flushing rate of 

 estuaries and the dispersal of contaminants in coastal waters, there have been 

 no great improvements in methods for studying the circulation itself. Thus, 

 even in relatively small scale problems of oceanography, it is difficult to recon- 

 cile the broad or statistical properties of the circulation with its active mech- 

 anisms. 



