The power in sea, on the other hand, increases gradually from January to July, and then 

 decreases during the remainder of the year. The sum of sea and swell also is shown in 

 Figure 5. This latter curve shows that total wave power is least during July, August, and 

 September. 



To transpose deepwater wave power shown in Figure 5 to nearshore points in Bolinas 

 Bay requires the use of refraction and diffraction diagrams for a large range of wave periods 

 and directions. Unfortunately such diagrams are not available. However, to obtain some 

 measure of relative intensity of wave power in Bolinas Bay, the visual wave observations (at 

 Bolinas and Stinson Beach State Park) and the data from the wave gage in the bay were 

 used. Wave data for these three locations (Fig. 4) were available for the following periods: 



Bolinas, 1 May 1968 to 23 February 1969 



Stinson Beach, 1 May 1968 to 30 November 1969 



Bolinas Bay wave gage, 20 February 1969 to 23 August 1969 



For each of these stations the wave height and period were tabulated for each day of 

 record. The data from the Bolinas and Stinson Beach Stations were breaker heights 

 (Szuwalski, 1970); data from the wave gage were the heights recorded offshore over the 

 underwater pressure unit. Values of H 2 T were computed for each day for each location. To 

 smooth the data and provide a visual comparison of the relative magnitude of wave power at 

 the three localities, 5-day moving averages were computed, tabulated, and then plotted in 

 Figure 6. The effect of wave exposure is seen by comparing the plots from Stinson Beach 

 State Park and Bolinas. The Stinson Beach station is more exposed to the prevailing waves 

 from the west-northwest sector than the Bolinas station which is near the Bolinas Lagoon 

 inlet, and consequently receives protection from Duxbury Reef. (See Figure 3.) 

 Unfortunately the Bolinas observation program was terminated before the installation of the 

 Bolinas wave gage; therefore, a comparison of wave power at the gage can be made only 

 with the Stinson Beach data. The wave gage is more protected from wave action by 

 refraction effects than is Stinson Beach. 



A further comparison of relative magnitude of wave power is shown in Figure 7. In this 

 figure the monthly wave power (sum of sea and swell) for the offshore station (Station 3) 

 summarized in Figure 5 is plotted along with the monthly wave power as computed from 

 the wave data observed at Bolinas and Stinson Beach. The figures show wave power is least 

 at Bolinas, somewhat higher at Stinson Beach, and both are significantly less than the 

 exposed deepwater station. 



3. Longshore Currents. The strength and direction of longshore currents generated by 

 waves breaking at an angle to the shoreline are necessary data in evaluating such shoreline 

 processes as littoral transport and seasonal beach and bottom changes in Bolinas Bay. The 

 only known measurements of longshore currents in Bolinas Bay were the surf observations 

 made at Stinson Beach State Park and at Bolinas. Printout data for these stations were 

 obtained from CERC. Plots of the observations on longshore currents (strength and 



20 



