CHAPTER IX. 



ESTUARY HARBOURS GROUND PLANS OF HARBOURS- 

 ENTRANCES. 



Estuary harbours" Bars " Lagoons Removal of " bars " Concentration of ebb 

 current Importance of large tidal compartment Protection of ebb current 

 Site of entrance Maintenance of depth Relative value of upland and tidal 

 waters Borings on site of proposed entrance, etc. Physical and nautical 

 requirements often opposed to each other at harbour entrances Ground plans 

 of harbours "Wave expansion Direction of main entrance Parallel jetties 

 Isolated breakwaters Harbours with two entrances Roadsteads Advantages 

 of deep water at entrance Strong current across harbour entrance objectionable 

 Widths of entrances Reduction of waves Relative advantages of embayed 

 and salient sites for harbours. 



WORKS undertaken with the view of improving estuaries, or the 

 entrances to rivers, constitute one of the most important, and at 

 the same time difficult, sections of harbour engineering. To deal 

 with the subject in an exhaustive manner would involve a fuller 

 inquiry into river engineering than it would be convenient to 

 introduce into this volume; nevertheless, its importance is such 

 as to call for somewhat more than a passing notice. 



A river-mouth in its natural state will, with but few excep- 

 tions, be found " barred " by banks of silt, sand, or shingle. 

 This condition of things is brought about by natural causes, 

 which may be briefly stated as follows : 



The material carried in suspension or caused to travel 

 along the bed of a channel by the river waters, upland or tidal, 

 requires a certain velocity of current to enable it to remain in 

 suspension or be rolled along the bottom. 



When the outgoing current encounters the ocean, or other 

 large body of water, its velocity is checked, and at the point 

 where it becomes so much enfeebled that it can no longer hold 

 the material in suspension or maintain its motion along the 

 bottom, the matter settles down and accumulates, thus forming 

 a bank or " bar." 



