A Treatise On The Principles And Practice Of Harbour Engineering

224 HARBOUR ENGINEERING. has acquired a sinuous or spiral motion, deflecting it from side to side, while the flood-tide sets inward in a straight line, curbed by none of the influences which control the river. The tendency of each is to obliterate the traces of the other where they diverge, and to accentuate the common bed where they coincide. At certain points, the river follows a course along one bank, while the main tidal stream favours that opposite, with the result that there are intermediate zones of slack water conducing naturally to the formation of shoals. A typical example of this, if it be necessary to select one, is furnished by the redoubtable “James and Mary ” shoal, which constitutes the most dangerous obstacle to navigation in the River Hooghly, and which has the evil reputation of being one of the most fruitful sources of shipwreck and disaster of any river in the world. The late Professor Vernon-Harcourt, who made a special and exhaustive study of the River Hooghly in 1901, thus describes the circumstances of the formation of the shoal 1. “The descending current of the freshets in the rainy season scours out a deep channel alongside the concave left bank, and, diverging only slightly from this bank on passing Nurpur Point (figs. 204-206), it goes straight across the river into the very deep channel along the concave right bank below the bend a little beyond Gewankhali; whilst the deep flood-tide 1 Vernon-Harcourt on the River Hocghly, Min. Proc. Inst. C.E., vol. clx.