• 3. The questions connected with the interferences likely to be produced by the erection of a pier upon the outline of a coast are often of extreme importance, not only to the harbour it is desired to create, but also to the whole of the surrounding district; and they must, therefore, be very carefully and attentively studied before such works are commenced. Unless there should exist some local cause of disturbance, it will, in fact, be found that the alluvial matters detached from cliffs and sea-shores by the action of the sea, will be carried forward by the flood-tide until they meet with some impediment able to check their advance ; they then begin to accumulate behind this shelter until the angle is filled in, when the onward progress of the alluvions is resumed. if the littoral current should sweep across the mouth of the pass between the piers, it may carry the sand and ehingle past the latter ; but under almost all circumstances a large portion of the finer matters in suspension will be carried into and serve to silt up the harbour. The same thing will 'occur with a few trifling modifi cations when single solid piers are carried out from a shore, whether their plan be straight or curved, or whether they be constructed in the sea or in rivers ; the alluvial matters will accumulate on the upside to the current, and it often happens (as it did lately at Dover) that, until the advance of the shingle again brings some light moveable materials to protect the foreshore, the newly-created counter-current will wear away the original outline of the coast on the downside of the pier.
The Roman engineers appear at an early period in the history of engineering, to have recognised the importance of these laws, and they constructed their ports habitually upon a series of open arcades, with the avowed intention of interfering as little as possible with the advance of the littoral currents. In modern times the use of wooden or of iron open jetties has been recommended for the same purpose ; but all these methods of palliating the evils of the interference with the progress of the alluvions are open to the serious objection that they leave the Interior of the harbour without protection from the undulations of the open sea ; and it therefore follows that if the com merce of the harbour should be important, the piers should be rendered sufficiently solid to intercept the normal deposition of the alluvions, even on the condition of constructing defence works for the shores on the down side. Any materials which, under these circumstances, may be carried round the beads of the piers must be removed, either by means of dredgers or by periodical sluicing. [Sr.mea.] The practice of the most acientifia engineers of the present day, it may be added, tends towards the use of dredgers in preference to sluicing; both on the score of economy and of efficiency. When the alluvial matters are removed by sluicing they are in fact only carried to the distance which the transporting power of the water can command; and then they almost always are thrown down in the form of a bar, across the mouth of the harbour. It would really appear that, under those cir cumatancea, It is wiser to provide for the extraction of the alluvions by machinery, proportionate to the quantity brought in within certain definite periods ; especially as It would then be easy to construct the piers in plan, solely with reference to the physical conditions of the locality as affecting the navigation, instead of (as in the case of resorting to sluicing) building the piers for the purpose of guiding the outpouring waters.
Lainblardie is the engineer who has studied the question of the advance of alluvial matters on a sm-shore with the most skill, attention, and succors; and It is to his works, or to those of Zendrini, or of Sir II. Delabeche, that the student of this subject must be referred. The leading principles of this branch of the science of hydraulic engineering, ao far as they are likely to affect the construction of piers or harbours, may, however, here be stated briefly as follows—allowance of course being made for local causes of disturbance.
At the base of steep cliffs the alluviens follow the outlines of the coast, and they have, under ordinary circumstances, but little tendency to change their positions ; in deep bays they will assume a curvilinear outline, whose concavity will be turned towards the sea in the directiou of the prevailing wind, unless where modified by the line of the current. If the directions of the prevailing wind and of the current should be parallel to the shore, the transporting power of the current to carry forward alluvial matters will bo less than when the direction of those forcea assumes an angle of 45 to the coast ; and when the wind blows perpendicularly to the shore, the waves may exercise a great effect iu abrading the latter, but the movement of the alluviona will be almost imperceptible. A bay, or a harbour, whose axis may be in the line of the prevailing wind will, therefore, be exposed to become rapidly silted up; and this result will take place with increased rapidity if any land waters bringing down matters in suspension from the uplands should discharge themselves into the bay. One important remark to be made with respect to the deposition of alluvial matters in the embouchures of rivers receiving the flood-tide, under circum stances which should allow the latter to carry forward the shingle it previously detached, ia, that the alluvions so carried in are of far moro serious importance in their effects upon the accumulations around piers or harbours constructed in those rivers than are the alluvial matters brought down from the uplands. There is, in fact, every reason for believing that in such rivers as the Thames, the Seine, or the Rhine, more alluvial matter is carried in from the sea than the land floods can transport to their respective menthe ; which, it may be added, is another reason for constructing open piers in such situations, because they offer less resistance to the natural LAWS of such localities. After all, engineering is essentially the science of adapting the means to the end desired ; and economical conditions of first outlay may often render it expedient to depart from the strict application of even strictly demonstrated laws ; but the attempt to run counter to those laws will always be productive of disastrous results ; and in no case is this more true than with regard to the effect of piers upon the outline of a coast. The histories of Ramsgate, Dover, Calais, or Dunkirk harbours, or of the ports, so called, of refuge on our eastern coast may be referred to as illustrations of this remark.
The Brighton chain-pier may be referred to as an illustration of an ingenious method of constructing works of this description without producing any interference with the movement of the alluvions ; but piers with ouch wide intervals between the points of support give no protection from the agitation of the waves.