In the Stinson Beach State Park studies, the dyed sand moved in both the southeasterly 

 and northwesterly directions; some material was detected about 1.5 miles northwesterly up 

 the beach 2 months after initial placement. This might be expected upon examination of 

 Figure 8 which shows that a northwesterly longshore current prevailed over most of the 

 period, (August and September), following the start of the tracer tests. 



b. Beach Changes. Beaches exposed to wave action undergo profile changes throughout 

 the year, because of seasonal changes in wave characteristics. One of the most important 

 factors in determining the character of a beach profile is the ratio of wave height lo wave 

 length— defined as wave steepness. During extended periods of low wave steepness, low 

 berms usually are built beyond the foreshore with very steep profiles on the beach face, and 

 bars immediately offshore tend to disappear or become discontinuous. During periods of 

 high wave steepness (storm or winter conditions) the beach face becomes less steep and the 

 offshore bars become more pronounced. 



On the beaches along Bolinas Bay, the seasonal changes in characteristics are less well 

 defined than the changes in the offshore area. This lack of definition is primarily in the surf 

 zone, and is the result of the difficulties and hazards of making accurate bottom surveys in 

 the surf zone— especially during winter storms. The hydrographic surveys used in preparing 

 the comparison maps (Fig. 9) were made no closer to shore than about the 8-foot depth 

 contour. The data on seasonal beach changes are therefore confined to the beach profiles 

 made by; (a) Trask (1959), and Trask and Snow (1961), and (b) the Corps of Engineers 

 (1965) and repeat surveys of these ranges by the California State Lands Division in 1969. 

 The location of the Corps' ranges is shown in Figure 4. Surveys by the State Lands Division 

 to determine the position of the mean high tide lines over a limited length of the Stinson 

 spit for the period 1948 to 1970 are important in evaluating beach changes in Bolinas Bay. 



To provide an accurate measure of the seasonal fluctuations of the width of the beach at 

 Stinson spit, the data on the position of the mean high tide line, as determined by the State 

 Lands Division and other sources on 27 occasions from 1948 to 1970, are of value. To 

 obtain information on the position of the mean high tide line, the distance from a base line 

 to the high tide line was measured and plotted as shown in Figure 11 for the appropriate 

 day of the month. (Johnson, 1971.) There is an obvious cyclic pattern of position of the 

 high tide line with the seasons, but no precise relationship exists because of the variation of 

 intensity of wave attack that undoubtedly occurred from year to year. An upper and lower 

 envelope has been drawn in Figure 11 to enclose the plotted points. This plot shows that the 

 position of the mean high tide lines is more variable from year to year during the spring 

 months than during late summer and fall. The distance between envelopes during the spring 

 may amount to as much as 50 feet, and the beach at the mean high tide line is about 150 

 feet wider in summer than in winter. The surveys of the five Corps of Engineers ranges 

 showed an average difference in width of about 100 feet between winter and summer 

 conditions of the mean high tide level. (Corps of Engineers, 1965.) 



27 



