The Effect of the Tay Bridge on the Harbour. Dundee Courier, Wednesday 24 January 1866, p.2 col.6 (pdf image)
(Original letter date and publication date unknown)
An excerpt from a longer letter written by Matthew in the Daily Review on the Forth and Tay Bridges, concerning the possibility of reduced tidal flow and sandbanks.
THE EFFECT OF THE TAY BRIDGE ON THE HARBOUR.— Mr Patrick Matthew, of Gourdiehill, in a long letter to the Daily Review on the Forth and Tay Bridges, thus refers to the probable effect of the construction of the latter, as proposed, on the harbour:— “In estimating the obstructive effect of the 100 piers to reduce the momentum of the tidal current flowing and ebbing, and to diminish the amount of scour, as well in the upper as in the lower portion of the firth, we shall find that in most of the spaces, and especially where the line of bridge is at right angles to the stream, that the interstices betwixt the piers is scoured out a little deeper; but the full volume of the current is broken, is divided into as many different currents there are open spaces, and thereby weakened. There is not only the friction upon the piers diminishing the momentum, but the divided streams in passing the piers produce slight backward eddy, and as they meet farther on, having different directions and velocities, impinge upon each other, causing slight whirlpools on which the momentum is lost. The consequence is that sandbanks, both above and below the line of piers, will be formed, still further to distract the full undivided tidal stream upward and downward and diminish the general scour. It is especially necessary that no obstruction be placed to lessen the velocity of the tidal current in the Tay Firth, in order that a volume of water sufficient to fill the firth may sweep over the bar and past the harbour every six hours. The proposed bridge, acting as a breakwater, breaking the wave upon the firth, would still further prevent the general scour. Altogether, the bent form, position, and construction the bridge appears as if devised on purpose to effect deposit in the Tay Firth, and especially in front of Dundee Harbour. Certainly, when natural causes are now operating to abridge the area of the firth instead of as formerly enlarging it, any further diminution of the area, and consequent reduction of the velocity of the tidal current and scour of the Firth by a bridge at Dundee, would be in the highest degree injudicious, and cannot be submitted to by the people of Dundee.”