A Treatise On The Principles And Practice Of Harbour Engineering

228 HARBOUR ENGINEERING. route; but so long as the depth of water is adequate, this step need not be taken. Bars. The rectification and improvement of harbour approaches involves not only the training of channels, but in many cases also the removal of a bar—in part, at least. A bar is a ridge or narrow plateau, or even a series of several ridges or plateaus, lying across the entrance to a river or coastal inlet, and rising up above the general level of the sea or river floor in its immediate neighbour- hood, on both sides of it. When the altitude of the bar is sufficiently great to reduce the depth of water over its summit to an extent exceeding the limits imposed by the requirements of vessels using the entrance, it becomes an obstruction to navigation, and, in any case, it acts as an impediment to the development of the port or ports to which it is the threshold, and detracts from the navigable possibilities of the inlet. Bars are to be found mainly in connection with tidal rivers; less often Fro. 210. —Seetion of the Bar of the River Mersey, showing improvement due to dredging operations. in connection with non-tidal rivers. On the other hand, some tidal rivers and many non-tidal rivers possess channels which, while they may be cumbered and rendered tortuous by shoals, are entirely unobstructed by bars The Mersey, the Dee, and the Rhone, for example, have bars of a very pronounced and indubitable character. The Thames, the Humber, and the Severn have channels which enter the sea without any marked obstruction at all. The orig’in of bars has been the subject of some controversy. It was formerly pretty generally held that a bar was due to the detritus brought down by inland waters, and deposited at a spot where the effluent, by reason of its reduced velocity, was no longer able to retain the material in suspension. This argument may indeed hold good in the case of non-tidal rivers, where it has also been advanced to account for the formation of deltas; but in tidal waters the fluctuation of ebb and flow at the river’s mouth should obviously result in a dispersai of any such deposit as soon as it had formed, or even before the material had time to settle. Another view was, that the source of the material being the same, its deposition is brought about by the meeting of conflicting currents, which