ESTUARY HARBOURS, ETC. 155 



Stiff clay and similar material may be readily removed by 

 ordinary dredging, with permanently good results ; but dredging 

 on a bar composed of sand, gravel, or mud, will, as a rule, prove 

 fruitless, unless aided by training and protective works, inasmuch 

 as the natural forces at work will generally replace the material 

 almost as fast as it can be removed. 



It is often difficult, if not impossible, to so design a harbour- 

 entrance that it shall meet all exigencies, seeing that the physical 

 and nautical requirements are usually opposed to each other. 

 The relative importance of the several requirements must there- 

 fore be duly considered, and the entrance be designed accordingly, 



As an illustration, assume that, with the object of obtaining 

 very smooth water within a harbour, avoiding a "run" up a 

 river-channel, and increasing the scour upon a bar, an entrance 

 should be made unduly narrow, and its direction be such as to 

 cause heavy seas to run across it instead of into it. A vessel 

 entering such a harbour during a gale would be exposed to a 

 " beam sea ; " and, should the tide be ebbing at the time, she 

 would also probably meet a strong outward current, which, 

 coupled with the narrowness of the entrance, might easily 

 involve her in destruction. 



On the other hand, when an entrance is so arranged as to 

 afford easy ingress to vessels during heavy gales as that of a 

 harbour claiming to be a refuge harbour undoubtedly should be 

 it almost of necessity admits heavy seas, not only by reason of 

 its direction, but also of its width. 



Waves, after passing the pier-heads, may, however, be quickly 

 reduced by so designing the ground plan of a harbour as to 

 afford ample area, and especially width, for their expansion 

 (Fig. 22, p. 156), and by a judicious arrangement of spending- 

 beaches. 



When very still water is required, an inner harbour with a 

 well-sheltered entrance is often constructed. 



There can be little doubt that the main entrance to a harbour 

 should face in such a direction as to admit of vessels taking it 

 with ease during the worst gales. 



Much will naturally depend upon its width and form, but it 

 should be so planned as to freely admit vessels when running on 

 a course coincident with the direction of the heaviest seas, or not 

 deviating from it to a greater extent than from two to three 

 points. 



