Some 3 ears ago, when Mr. Beatson was on duty as an engineer at a fort near Portsmouth, built on a point of land much exposed to the sea. the waves made, such havoc, that the walls on that side were constantly giving way, although built in the must substantial manner; and having bulwarks ut large heavy stone besides, to protect the foundation : however, all would not do; those bulwarks Were soon knocked to pieces. and several times the wall itself. At length it was proposed to drive a number of piles at about •10 or 50 yards from the fort. These piles were P2 or 15 inches in diameter, and driven about one diameter from each other nearly in a straight parallel to the %rail where the waves did NIP touch dalnage. They were driven into the ground with a pile-engine till perfectly firm. perhaps S or feet deep. and about :2 feet ut the top of them left above the level of high-water mark. After this was done, the wall received no Collier injur3, the space between the piles and • the fort being always perfectly smooth, however tempestuous the waves might be without. The same simple method might, it is supposed, sometimes protect the banks of large rivers, it exposed to the waves, w hen other methods might but it is suggested, that the most common cause of rivers encroaching on their banks, is the resistance occasioned by a sudden bend. In flat countries, apt sometimes to be over flow n, where there are any such bends or windings in the rivers, it would be of great advantage to straighten the course as touch as possible; for, as every impediment or obstruction w ill naturally cause the water to rise higher than it otherwise would do, and as such bends have that effect, consequently, in the time or a flood the waters will overflow a greater extent of country, and to a greater depth, than it' the river had a free and uninterrupted course straight forward. If the windings of the river cannot be altered. and encroach ments are making on some part of the banks, it must first be considered, whether the force of the water can be driven to another place where no injury can be done. If, for example, a ricer is encroaching on its banks at a., Figure I 3, a jetty of stone, a little way up the river, in the direction y z, would throw oil' the current towards w, and might totally prevent any farther encroachment. On the river ' it h, in Dumfries shire, it is stated, that a good deal has been done in this way by Mr. Millar, of Dalswinton, a gentleman of the most enter prising genius and most liberal mind, who has paid more attention, and laid out more money, in making important and useful experiments, than almost any other private individual. The course of the river, where Ir. AI illar has been carrying on his operations, is said to be neatly as shown at 14, by r s t u ; at 1, it was encroaching most rapidly, and seemed inclined to take a new course towa•ds V, Which would have destroyed some very fine land. and done a great ilea! of mis chief in that part of the country. To prevent this, Mr.Millar made a large cut, about 400 yards in length, from u' to r, and threw in a great quantity of stones quite across the river at 5, to direct its course in a straight line from r to tr. This had, in a great measure, the desired effect, by totally preventing its progress at t, hut now it began to encroach on its banks at u. Ile at first endeavoured to prevent this by driving in, at a considerable expense, a number of piles at a little distance from the bank, and wattled them with willow branches, &c., thinking thereby to protect the bank. The piles were driven in with heavy mallets, apparently litre into the ground ; they continued so for some months, till a heavy fall of rain came on, which swelled the river, undermined the piles, and car ried them all away. But, indeed, it is in vain to think of piles doing any good in such a situation, unless firmly in by a pile-engine ; tor it is not possible to drive them in properly mallets; this must cause way so soon. The piles Ina, succeeding, r. Millar was resolved to try another p'an ; several of his adjacent fields being covered with an immense quantity of stones. he ordered them to be gathered and thrown into the river, so is to form a jetty at a., a little way above the injured bank. Being obliged to go from home about that time, and to leave the execution of the work to some out this jetty too much at right angles to the stream. It had not. therefore, the desired of•et. but rather made the matter than for, it' a jetty is carried out at right angles, as at a, in Figure 4, the current will he forced from u to the opposite side of the river at 1.), and from thence it will rebound towards e, more violently that it (lid bel'ore. But it' a jetty be placed obliquely. as at (t, it will force the current graduall3 towards e. in which position one jetty may do more good than several placed improperly at right angles. Mr. 'Millar was, therel'ore, under the necessity of making other jetties in this way, and at had the satisfaction to find that they answered the purpose intended. Those he made laterally tiirined a surt of convex s,ope. the convexity being
Parallel to the current. Strong planks were also firmly set on edge among the stones, their ends pointing, towards the river. so that if ever any current came so rapidly as to move any of the it must move them all in a bo(ly the whole length ..1 the plank. Perhaps this precaution was unneces sary ; for although stones are thrown into a river loose in this manner, the slush, sand, &e., that come down the river will soon fill up all the cavities, and render it as firm and solid as a regular-built wall. Beatson has been the Wore particular in this description, he says, in order to show the errors that Mr.sIli11ar at first fell into, and the great expense they oecasioned, whereas, had lie been on the spot r, and got the work executed as he intended, it would have saved ri great deal of ut necessary labour as well as money.
It is stated by the same writer, that the next sort id em bankments against rivers, are those to prevent them over flowing their banks, and inundating large tracts of co retry. This may be considered as the simplest :rod easiest of all sorts of embankin;r. if judiciously executed. It is, therefore, the more inexcusable to see, in some places. extensive tracts of the richest meadows completely overflown by every flood, Ibr want of them.
Two ordinary-sized rivers 11501141 inure, even iti the greatest flood, than live 4)r six feet above their common level, unless when they meet with some considerable interruption or con finement in their course. lint if interrupted or confined, they will rise twenty feet or more, as is the case with some parts of the river Mersey, already mentioned. If, for example, a given opiatitity of crater is six feet deep, when running oVer it slitloe twenty feet wide, it is clear, if that space were only made tell feet wide, the water would rise to twelve feet, and if it were made forty feet wide, the same quantity of :ter would only rise to the height of three feet. It is, therelo.•. of great consequence, in preventing inundations, to give the ri as much width as every narrow place. All kinds of obstructions should also be reuioyM, whether /lied by windings, slum's, tree', bushes, or anything else. In some eases this may even precinde the necessity of 12 inhanking; but where embanking is necessary, let the banks by till means be at a sufficient distance from each other, to contain with ease, between them. the largest contents of the river in great floods. The dis noi•e and height of the banks may easily lie ascertained by a section of the river when at its highest, or when the flood-mark is visible. By not attending to this, a great dear! of money has been thrown away on the embankments on the river Mersey, and after all they do effttetually answer the intended purpose ; a great part of the being still overflown every time the river rises to any con s,,tera de height.
Where a sufficient distance is allowed between the embank in lit, their height need not exceed from four to six feet.
rre.wivoalde obstacles are in the way, which cause the riv,.r to rise higher, the bank must be higher in proportion. In either ease. however, the slope of these kinds of banks on each side nay be equal to its perpendicular height, and the bre l Ith im the top about one-third of that height, a hich, supp.ising the hank six feet high, the base would be fourteen feet. and the breadth or the top two feet, as shown at Fiyare 16. in the Plate.
The materials for making these banks should be taken as numb as po-siftle from the sides of the river, which will have the double (Arcot of widening the river and forming the em bankments; and there should be a trench on the inside (from which materials may also be got) with some sluices, :Uri fer inerly directed, to drain off any water from within ; also slu:ees to let in water from the river, if required, which would very much fertilize the meadows, if properly laid out for that purpose.
Such farms as are situated on the borders of rivers are frequently, it a as observed by a late writer, liable to much and inconvenience from them : 1st. Front pint of the soil being carried away em times of flood, ''.2nd. From their overflowing their banks. :3111. From their flowing back in times of flood into the channels of the rivulets and streams that comicact the water limn the more elevated and distant grounds to the rivers, whereby these rivulets and streams are made also to overflow their banks, In respect to the first, the danger of the soil being carried away in time of floods, it is increased or deoreasol atTonling to circumstances, as the form of the banks, the nature of the soil, the rapidity of the river, and the quantity of water thAt lodges on the margins of the banks, or falls over them into the river. Whore the banks of a river are perpendicular, espeeially it' the soil be of a rich mouldering nature. the danger of part of them being carried away by floods is much greater than where they slope gently from the surface of the field to the bed of the river, as has been already fully seen.