152 HARBOUR CONSTRUCTION. 



No sooner has the river broken through this barrier and 

 formed a new channel, than the waves again begin to baffle and 

 deflect it, and the whole process is repeated over and over again, 

 the direction of the channel being the resultant of the forces at 

 work. 



Where a river-mouth is wide and exposed to gales from 

 various quarters, the routine above described is less regular, and 

 a bar, intersected by various channels, is usually formed. 



In designing an estuary harbour, the aim of the engineer 

 must be to direct and aid the natural forces tending to remove 

 the bar and keep the channel open, and to protect them from 

 those having a counter effect. 



This will be best accomplished by (a) so concentrating and 

 directing the currents, more especially on the ebb, that their 

 whole scouring power shall be utilized in forming and maintain- 

 ing a channel so far seaward that the matter carried down by 

 the outgoing tide shall be held in suspension until it is brought 

 under the influence of cross littoral currents, or into such deep 

 water as will ensure its dispersion before it can be carried back 

 into the harbour again. 



(6) To increase, as far as possible, the volume of water 

 passing over the bar at each tide by enlarging the tidal com- 

 partment within the estuary or harbour by means of dredging, 

 and by the removal of such sharp bends or other obstructions in 

 the channel as might retard the tidal flow and prevent the tidal 

 compartment, especially in its upper reaches, receiving its full 

 complement of water. 



It has been proved to demonstration over and over again that 

 reclamations or other works reducing the capacity of the tidal com- 

 partment, and consequently the quantity of tidal water passing 

 through a harbour-entrance, injuriously affect the depth of water at 

 the entrance, in proportion to the amount of water thus excluded, the 

 converse being equally true. 



(c) To protect, by means of breakwaters, the outgoing current 

 from the deflecting effect of prevalent heavy seas for such a 

 distance as will enable it to carry the matter in suspension well 

 out to sea. 



These are the broad principles upon which estuary harbours 

 must be treated, but only careful study of local conditions and 



