BOLINAS LAGOON INLET, CALIFORNIA 



by 



J. W. Johnson 



I. INTRODUCTION 



The hydraulic and sedimentary characteristics of tidal inlets on sandy coasts have long 

 been of interest to engineers involved in harbor design and maintenance. It was not until 

 1928, however, that E. I. Brown (1928) expressed the hydraulic conditions at an inlet in a 

 rational manner and discussed in general terms some of the sedimentary problems. Later 

 M. P. O'Brien (1931) made a study of west coast inlets and proposed a relationship between 

 the minimum inlet area below mean -sea-level and the tidal prism. Several investigators 

 (Brunn and Gerritsen, 1960; Wiegel, 1964; Frautschy and Inman, 1954; Inman and 

 Frautschy, 1965), in recent years, used data from inlets on many sandy coastlines and 

 proposed relationships between inlet area and tidal prism, but all of these relationships agree 

 relatively closely with that of O'Brien (1931). 



Recently O'Brien (1967) reanalyzed all available data from inlets on the west, east, and 

 gulf coasts of the United States, and proposed a relationship which was accurate, between 

 ±10 percent in flow area and ±15 percent in tidal prism. O'Brien (1971) later observed that 

 the relationship seemed questionable since, (a) there appeared to be no effect of the size of 

 sand forming the channel and adjacent shores, (b) the range of tide enters only as it affects 

 the tidal prism, (c) the net and gross rate of transport of littoral drift is not effective, 

 (d) both unimproved and improved inlets are included, (e) lagoons with two inlets 

 conformed if the summation of the flow areas was used, (f) the inlets to both lagoons and 

 estuaries followed the same relationship, and (g) there appears to be no effect of freshwater 

 flow or of sediment transport. 



O'Brien (1971) further stated that he found it difficult to visualize how it is possible that 

 these influences would be completely ineffective in altering the throat area. He believed that 

 the equilibrium relationship between inlet area and tidal prism as he proposed would seem 

 to be a first approximation only, and that the next effort should be directed towards 

 quantative understanding of deviations from it. If the flow area is determined by the tidal 

 prism, it would follow that this area is in a constant process of adjustment, because the tide 

 range, and the related tidal prism, varies continually. To adequately define the importance 

 of the variables involved in the hydraulic and sedimentary characteristics of inlets on sandy 

 shorelines, data are necessary on such items as (a) simultaneous tides in the ocean and in the 

 lagoon, (b) bottom changes of the inlet throughout the year, (c) friction losses, and 

 (d) bottom sediment size distribution and sand waves. Such data are difficult, expensive, 

 and often hazardous to obtain. 



