THE PHYSIOGRAPHY OF BOTANY BAY. 1 25 



In another paper 2 it has been shown that, with the exist- 

 ing rise and fall of tide on the New South Wales coast, a 

 ratio of at least one foot width of entrance to four acres 

 of tidal compartment in any estuary, is necessary in order 

 that the velocity of the flood tide at the entrance shall be 

 less than the velocity of the ocean current. When such 

 exist, no sand is brought into the estuary, but is all con- 

 veyed past in the littoral current. 



When the ratio is greater than four acres to one foot, 

 sand is carried in, the amount varying as the velocity of 

 the current necessary to fill the larger area. At present 

 the area of the tidal compartment of Botany Bay is 14,246 

 acres, and the least width of entrance is 3,467 feet, the 

 ratio being 1 : 4.11, and very little sand is at present enter- 

 ing the bay. In proof of this the soundings taken by the 

 author in 1910, as compared with those taken by Captain 

 Cook in 1770, show no general shoaling of the bay. 



In years gone by however, when the waters of the bay 

 occupied the stippled area on the western and northern side 

 of the existing bay, the area was twice the present size, 

 and consequently the current velocity of the tide at the 

 entrance was about twice the present speed. Large quan- 

 tities of sand were thus carried into the estuary and 

 deposited in the still water along the eastern shore of the 

 bay. The largest quantity of sand, would, of course, be 

 carried in during times of strong southerly gales when the 

 ocean current was to a certain extent retarded, and the 

 difference between its speed and that of the flood tide was 

 greatest. The southerly winds at the same time would 

 tend to create a northerly current in the still waters of the 

 bay, thus help to convey the sand to the northern end 

 where Centennial Park and Randwick Racecourse now 



2 G. H. Halligan, " Bar Harbours of New South Wales/' Min. and Proc. 

 Inst, of C.E., Vol. clxxxiv, p. 144. 



