1841.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL 



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a bank of pebbles beyond the reach of the previous tules,) the surface 

 of the pUine corresponds very nearly with a straight line, which, con- 

 sidering that it is a natural formation, is a fact worthy of notice. 



I think this plane may be considered as representing the average 

 dimensions and inclinations of the surfaces over which the beach tra- 

 vels along this coast, and I have therefore generally assumed such an 

 one for the present purposes. Upon such an inclination, the loose 

 pebbles are in contact with each other; and although their depth upon 

 the plane is constantly varying, yet, for the sake of conveying a gene- 

 ral idea, we may assume tlie average to be about six inches, extending 

 between hich and low-wafer mark*. Wlien, however, the plane is 

 less inclined, the same quantity of beach is spread over a larger surface, 

 and its depth is diminished ; and the pebbles are in some places so far 

 separated as to exhibit the appearance of a diminished quantity. In 

 fig. 3, this is illustrated geometrically: 



Fis. S. 



Let A B represent a plane on which all the pebbles are in contact, 

 C B a plane considerably more inclined. If, from the centre of each 

 pebble on the plane A B, a horizontal line be drawn to the plane C B, 

 the position of the pebbles on the latter will be respectively at the 

 various points of intersection. 



Section -1. 



There are numerous points on the coast at which the line of beach 

 is apparently intercepted and its continuity destroyed, and the rock 

 washed bare. Having sufficient evidence that the motion of the beach 

 was continuous, I thought it important to ascertain in what manner the 

 pebbles escaped past those places, and was happy in finding, upon in- 

 vestigation, that a valuable deduction could be made. 



In the description of the accumulative action, it was remarked that 

 the waves having struck the pebbles upwards, became dispersed, and 

 were incapable of returning them to the level from which they were 

 forced. But I now observed, that the surface of the rock, being very 

 irregular, constituted numerous channels ; so that the waves, instead 

 of returning in a dispersed and weakened form, moved back in columns, 

 which were of sufficient power to return every pebble that liad been 

 thrown up ; and as these channels offered no impediment to the angular 

 progressive motion of the pebbles, it was more rapid than on the ordi- 

 nary plane surface. Here, then, was pointed out hy nature a principle 

 on which the shingles might be hastened forward, and their accumula- 

 tion about any particular place prevented ; and by simply reversing 

 that principle, a method of accumulating or retaining the shingles, 

 where they are wanted, is also suggested, viz. by the reduction of the 

 descending force of the breakers. 



The etFect of confining the retiring breakers to a column is also ex- 

 emplified in another manner, when the waves are driven directly upon 

 the beach by a moderate wind, or such as would produce the accumu- 

 lative action. A succession of waves, acting over the same lines of 

 the beach, soon forms a slight depression, which continues increasing 

 until it becomes a definite channel. The whole line of beach being 

 thus acted upon, it assumes the form of a series of banks parallel with 

 each other. The waves do not then recoil in a dispersed form, but, 

 having broken, are again collected and returned through the channels, 

 and remove all loose matter from them. While in this state, the 

 beach has no progressive motion, but continues (to use a military termj 

 "marking time," until, from the change of wind, an oblique direction 

 is given to the motion of the waves. 



Section 5. 



The progressive motion of the beach may be easily traced along 

 the coast as far as the bay called Sandwich Flats. The general 

 character of the motion during its progress is that which is most fa- 

 vourable, under everv circumstance, to the chances of becoming securely 

 deposited. Every part of the coast is attempted by every variety of 

 motion in its turn, until a place of final security is discovered. 



The locality of Romney Marsh appears to liave afforded the sought- 

 for shelter, and now exhibits an extraordinary example of the accumu- 

 lation, which, having been combined with sand, silt, and vegetable soil 

 derived from other sources, has long been considered an acquisition to 

 our surface of considerable value. 



Although this tract has continued increasing to the present day, yet 

 a great quantity of the beach travels past it, and we do not find any 

 other accumulation of much extent between that and Sandwich Flats, 

 beyond which there is no further trace of the shingle which we have 

 so far followed, the pebbles to the northward of these flats being evi- 

 dently those derived from the cliff's near about them. 



On the approach of the shingle to the Sandwich Flats, it becomes 

 gradually dispersed, owing to the increasing inclination of the plane, 

 until it seems to disappear. A considerable extent of these flats has 

 attained a height very little inferior to that of the high-water mark 

 of spring tides; and it is so nearly horizontal, that the water does not 

 partake of that undulating motion upon it which has before been ad- 

 verted to. 



On the Sandwich Flats there is a continual deposit of soil and silt, 

 bro\ight there from the interior of the country by the river Stour, and 

 which, after its exposure to salt water, is particularly suitable for per- 

 manently uniting all the coarser or larger fragments with which it may 

 become intermixed. So much of the materials which have composed 

 the beach as may be conveyed to the higher parts of tti3se flats are 

 not likely to be again disturbed, because many days may intervene 

 before another tide may reach them ; and they thus become united to 

 the surface on which they rest, and gradually contribute to its height. 



The greatest motion of the pebbles being where they are exposed 

 to the action of the greatest number of waves, we must look to the 

 lower levels of these flats to trace the further course of the greater 

 portion of the shingle. But even the slope of the surface of the lower 

 levels is so very gradual, that the undulating motion of the water is 

 ]3roportionally diminished ; the action of the water then becomes greatest 

 in the direction of the land. While, then, we bear in mind the nature 

 of the soil over which it acts, we find an almost insurmountable im- 

 pediment to the further progress of the shingle, and are enabled to 

 account for the rapid extension of the Sandwich Flats towards the sea, 

 which, in fact, is only the continuation of that process which has been 

 for ages in operation, and which has formed a large portion of those 

 extensive marshes between the Isle of Thanet and the main land of 

 Kent. 



Sectijx 6. 



Having described those chief principles which regulate the motion 

 of the shingles on this coast, and having traced their progress to a 

 final destiny, I shall now proceed with some further general remarks 

 referring to the application of the foregoing observations. 



So much effect has been attributed to the motion of the tidal cur- 

 rents, that vast sums have been expended in attempts to divert the 

 motion of the shingles to a distance from the general line of the shore, 

 from whence, by the increased depth and velocity of the current, it 

 has been expected they would be carried past a particular spot, through 

 which a permanently open channel has been required. Such attempts 

 liave been made at various periods during upwards of two centuries 

 at Dover, and more recently at Folkstoue in the same neighbourhood. 

 It is hardly necessary to observe, that such attempts have not been 

 successful, and from the principles which I have laid down, their failure 

 may be easily accounted for. 



if a wall or pier be extended from the shore into the sea, it is evi- 

 dent that such erection w-ill in the first instance impede and prevent 

 the progressive motion. It is also evident, that the progressive is not 

 necessarily combined with the accumulative action, but, on the con- 

 trary, where the former is impeded the latter is assisted. The accu- 

 mulative action, therefore, continues until the angle formed by the pier 

 and the line of the shore is occupied, and the pier being no longer an 

 impediment to the progressive motion, that motion is again restored,, 

 and the general mass proceeds as if no impediment had existed. 



The most perspicuous evidence of these results is exemplified at 

 the harbour of Folkstone. Previously to the commencement of this 

 exclusively artificial work, the beach travelled along the line of clift" 

 in the ordinary way. 



By extending the walls a sufficient distance into the sea, it was ex- 

 pected that a commodious harbour would be formed, and the shingles 

 diverted so far into deep water, that they could not again appear 

 above the surface until they were removed beyond the harbour's 

 mouth. 



The accumulation, however, immediately commenced, and continued 

 as the work advanced until it became apparent that no other eftect was 

 produced upon it than a comparatively slight change of direction. 



