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482 . Observed relations between the volume of the tidal prism and the 

 cajMcity of inlets on the Pacific coast oj the United States. — A compilation, 

 made by Prof. M. P. O'Brien of the University of California, printed 

 in Civil Engineering, May 1931, shows that the area, at mean tide, of 

 the cross section at the throat of the entrances to the estuaries and 

 bays on the Pacific coast of the United States, conforms quite closely 

 to the relation: 



in which M is the area of the entrance in square feet, and V is the 

 volume of the tidal prism of the basin between MLLW and MHHW, 

 in square mile-feet. 



It should be observed that the tides on this coast are of the mixed 

 type, whose sequence is such that lower low follows higher high water. 

 The diurnal range, from MLLW to MHHW, therefore affords a measure 

 of the stronger ebb currents. 



483. A study made in office of the Pacific Division, United States 

 Engineer Department, by Mr. Grimm, principal engineer, shows that 

 the area of the cross section over the ocean bars of the larger estuaries 

 of the Pacific coast of the United States, at MLLW, is from 1.04 to 

 1.26 square feet per acre-foot of tidal prism in the basin between 

 MLLW and MHHW. The corresponding average strength of the 

 ebb currents is about 2 feet a second. 



484. Overcurrents in inlets. — The currents in some inlets are much 

 distorted by the overcurrents produced by the variation in the area 

 of the water surface in the basin, and in the area of the cross section 

 of the inlet, with the rise and fall of the tide. The curve of the flood 

 velocities in such an inlet may rise rapidly to a maximum, fall off, and 

 again rise to a second maximum, before turning to the ebb. The ebb 

 currents may go through a similar variation. 



