225 



same maimer as those of a connecting canal of varying cross section, 

 illustrated in the second example (pars. 392-401). 



MIDSTREAM CURRENTS 



433. The computations developed in this chapter should afford 

 substantially as reliable an indication of the mean velocity at a given 

 cross section of a tidal canal as is to be expected of a computation of 

 the mean velocity set up by steady flow with a constant head. In 

 both cases, the reliability of the results depends upon the completeness 

 of the data on the actual widths and depths in the canal, and on the 

 selection of the coefficient of roughness. The procurement of the 

 data for the computations generally entails much more effort than do 

 the computations themselves. 



434. It should be recollected that the currents which will be en- 

 countered in the navigation of the canal are those in midstream and 

 that their velocity considerably exceeds the mean velocity in the 

 cross section. An analysis of the detailed meter measurements made 

 in the Cape Cod Canal in 1915, when its designed depth was 25 feet 

 at low water and its bottom width 100 feet, shows that the average 

 velocity in a vertical section at the middle of the canal was 25 percent 

 in excess of the average velocity in the entire cross section. While 

 the ratio of the midstream velocity to the mean velocity must depend 

 upon the contour of the bed of the channel in the vicinity of the cross 

 section, available data indicates that in general the strength of the 

 midstream current in a channel of regular dimensions should be taken 

 as 1.3 times the mean strength. 



435. The midstream current also turns later and reaches its maxi- 

 mum velocity after the mean current in the cross section. In a canal 

 of regular section this difference in timing usually does not exceed a 

 few minutes. In a wide natural channel differences of half an horn* 

 or more in the time of the turning of the current near the shore and 

 at midstream are quite common (Manual of Current Observations, 

 Special Publication 215, U. S. Coast and Geodetic Survey, 1938). 



