160 HARBOUR CONSTRUCTION. 



open channel. A narrow entrance also, by admitting less sea 

 than a wider one, produces quieter water within the harbour. 



Care must, however, be taken not to so contract an entrance 

 as to injuriously check the flow of the ingoing current, other- 

 wise the volume of water entering the tidal compartment may 

 be so reduced as to react upon the bar. 



It would also be bad engineering to so increase the velocity 

 of the current at a harbour-mouth as to interfere with naviga- 

 tion. Much inconvenience has been experienced, and many ship- 

 wrecks have been caused, by the outgoing current from a harbour 

 so checking the way of approaching vessels I refer especially to 

 sailing-vessels and fishing-boats -just as they have been on the 

 point of entering, that they have broached to, missed the entrance 

 altogether, and been driven ashore. Furthermore, a strong 

 current running counter to storm waves sets up a dangerous sea, 

 in which vessels are likely to become unmanageable. 



Experience shows that the velocity of the outgoing current 

 from a harbour should not exceed 3J to 4 miles per hour. 1 



The widths of entrances of existing harbours vary to so great 

 an extent say from under 100 feet to over 3000 feet that, 

 without stating all the physical and other conditions which, in 

 each particular case, had to be taken into consideration in deter- 

 mining such widths, a list of them would be of little use, and 

 might even mislead. 



For some close harbours of magnitude, where the problem is 

 simplified by the absence of upland water and abnormal condi- 

 tions, entrances varying in width from about 500 feet to 850 

 feet have been adopted or proposed. Thus, at Madras an 

 entrance of 550 feet in width is proposed for a harbour having 

 an area of 220 acres ; at Peterhead harbour of refuge an entrance 

 of 600 feet is proposed, the area being 340 acres ; at Kingstown 

 the width of entrance is 750 feet, the area of the harbour being 

 about 250 acres ; and at Ymuiden the entrance is 850 2 feet wide, 

 the area being also 250 acres. 



Miles 

 per hour. 



1 A current with a velocity of 0*4 will move fine sand. 



- 5 coarse sand. 



0-6 fine gravel. 



0*7 gravel the size of beans. 



1*6 v shingle 1 inch diameter. 



2 to 2'5 shingle 2 to 3 inches diameter. 



2 This entrance has a strong current about 4 knots per hour running past it. 



