TIDAL SCOUR.
In the report, of 1814, on Dundee we find the following remarks on tidal scour:—
“To put this matter in a clearer point of view, let us see what nature does upon the great scale, as for example in the extensive basin forming the Firth of Tay. We there find that in consequence of the rapidity of the current at the narrow passage in the neighbourhood of Broughty Castle, which may be viewed as the scouring aperture of the basin of the Tay, the water is from forty to eighty feet in depth, and moves with a velocity which carries a great quantity of sandy particles along with it. But no sooner are the waters of this current allowed to spread and cover the basin of the Tay, than the velocity ceases, and the foreign matters fall to the bottom and form the various sandbanks which appear at low water. In a similar way the deposition of silt and earthy particles brought down the river in speats is accounted for. Now, this view of the case is equally applicable to the harbour of Dundee, for so long as the water preserves the velocity it acquires in the scouring apertures or arches in the quays, it carries all its foreign matters along with it; but the moment it is allowed to expand over the extent of the harbour the deposition of these earthy particles begins. And in every case the well-known law in hydraulics holds good, that the scouring effect of a fluid is in the ratio of the square of the velocity.”