In reporting on the defence of the lands of Trinity, on the Firth of Forth, Mr. Stevenson recommended the adoption of a cycloidal talus wall, which was executed under his direction in 1821:—
“In giving an opinion relative to the best mode of defending and preserving this property, the reporter observes that it fortunately happens that the beach is pretty closely covered with large boulder stones, which now form a kind of chevaux de frise in breaking the force of the sea, and making it fall more gently towards high water mark. Were it not that these stones are proposed to be employed in the erection of a more effectual barrier against the waves, the reporter would not fail to disapprove of their removal for any other purpose.
Fig. 20.
“The reporter proposes that a Talus wall or bulwark should be built of these boulder stones, roughly dressed and laid so as to form a cycloidal curve in the central part, as nearly as may be, as represented in the section with its tangents ([Fig. 20]). The properties of the cycloid as applicable to a sea wall in an exposed situation are very important. In particular, if compared with any other curve, in the same vertical line and down through the same points, it will be found of swiftest descent under similar circumstances, therefore the water in its rise must be proportionally retarded. The lower tangent to the curve alluded to also forms a wall towards low water, best adapted for admitting the sea to flow gently over it, while that connected with the upper extremity of the cycloidal part, tending towards the perpendicular, brings gravity into action against the rise of the waves. The practical execution of a wall upon this construction is simple, while the aggregate quantity of materials is less than for any of the curves of the conic sections of similar extent, and it seems upon the whole to be peculiarly applicable for the defence of the sea beach in question.
“If we examine the numerous works of this kind erected for similar purposes along this coast, we shall find that the general process or action of the waves is to undermine the seaward courses of the walls. In some cases, however, where due attention has not been paid to making up the backing of the face wall in a compact and firm manner, the central parts have been found to sink and give way. But the more common mode of failure is by the undermining of the seaward courses, arising from too sudden a slope being given to the face wall, which has a direct tendency to produce additional agitation in the waters at the bottom of the wall, by which the beach is excavated, and the foundation, being exposed to the wash of the sea, its destruction soon follows. If we attend to the distribution which nature makes of the matters composing a sea beach, unless where special local causes occur, we find them laid with a very gradual descent towards low water mark. The sands of Portobello, in this neighbourhood, form a striking example of this. Here small quartzose grains mixed with light sea shells prove, in their effects, a more effectual barrier against the overwhelming force of the waves than perpendicular and massive walls of masonry.”
CHECKING DRIFT SAND.
Mr. Stevenson recommended Lord Palmerston to introduce the Pinus maritima major, as a check for sand drift, on his estate of Mullaghmore, in the following report, dated 21st July 1835:—
“During the reporter’s visit to Mullaghmore, his advice was also asked regarding the operations at present going on for the improvement of the land. He had then much satisfaction in viewing the interesting improvements of reclaiming bog lands, and checking the inroads of the sand flood or drift, by planting ‘bent’ grass upon the shores of this estate. The system of dibbling the bent grass, pursued by Mr. Lynch, is in the best style which the reporter has anywhere met with; and he has been so impressed with the national importance of this scheme, from the success already experienced at Mullaghmore, that he has already taken the opportunity of recommending this system as applicable to the entrance of Ballyshannon, and in other quarters, particularly to the Highland and Agricultural Society of Scotland.
“The question chiefly submitted to the consideration of the reporter, in regard to these operations, was the best mode of defending the margin of the bent grass towards the sea. For such purposes, buildings or fences of any kind are not only expensive in their formation, but are also in constant need of repair. Mr. Lynch seems so much at home in all planting operations that the reporter begs simply to bring under your Lordship’s notice the French mode of planting a species of fir (Pinus maritima major), which was originally suggested to the Government by the late M. Bremonteuil, Ingénieur des Ponts et Chaussées. This system has been extensively tried along the stormy shores of the Bay of Biscay, particularly in the district of Grave, at the entrance of the Garonne, where the arid and sterile sands have been covered with extensive forests, which thrive quite close to the water’s edge. From the climate and exposure of the shores at Mullaghmore, the reporter has no doubt of the success of similar plantations in arresting the progress of the sand flood. It is believed that Mr. Lawson, seedsman to the Highland and Agricultural Society of Scotland, is taking measures to import the seeds of the Pinus maritima major, with a view to trying it on some of the exposed sandy districts of Scotland.”